Rotating Equipment Chapter 3 Pumps
Technical Training Programme Rotating Equipment CHAPTER 3 PUMPS TriStar T.S – M - RE – (Rev. 3) May 2004 Page 1 of
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Technical Training Programme
Rotating Equipment
CHAPTER 3 PUMPS
TriStar
T.S – M - RE – (Rev. 3) May 2004
Page 1 of 125
Technical Training Programme
Rotating Equipment
CHAPTER 3 PUMPS
Objectives: At the end of this chapter the trainee will be able to: Understand pump types, components, application, performance, auxiliaries, operation and maintenance.
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CHAPTER 3 PUMPS CONTENTS
Page Number
SECTION – 3.1 The Function of the Pumps SECTION – 3.2 Classification of Pumps 3.2.1 Positive displacement pumps ……………………… 3.2.1.1 Reciprocating positive displacement pump .. 3.2.1.2 Rotary positive displacement pumps ………
9 10 13
3.2.2 Dynamic pumps …………………………………… 3.2.2.1 Pump theory ……………………………….. 3.2.2.2 Centrifugal pumps ………………………….
15 15 15
SECTION – 3.3 Centrifugal Pump Components 3.3.1 Centrifugal pump components ……………………..
17
3.3.2 Function of the components …………………………
18
3.3.3 Stuffing boxes & mechanical seal ………………….. 3.3.3.1 Packed stuffing box ………………………… 3.3.3.2 How does it work …………………………… 3.3.3.3 Lantern rings ……………………………….. 3.3.3.4 Arrangements of the lantern ring to meet specific services ……………………………. 3.3.3.5 Packing selection …………………………… 3.3.3.6 Conventional packing draw – backs ……….
20 21 22 23
3.3.4 Mechanical seals …………………………………… 3.3.4.1 Mechanical seals overview …………………. 3.3.4.2 Mechanical seals construction ……………… 3.3.4.3 Sealing points for mechanical seal ………… 3.3.4.4 How does it work ………………………….. 3.3.4.5 Advantages of mechanical seals …………… 3.3.4.6 Comparison between conventional packing and mechanical seals ……………………….
26 26 26 28 28 29
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SECTION – 3.4 Classification of Centrifugal Pumps SECTION – 3.5 Pump Performance Curves, Pump Power and Efficiency 3.5.1 Factors affecting pump performance …………………. 3.5.2 Effects of specific gravity ……………………………. 3.5.3 Effect of viscosity ……………………………………. 3.5.4 Specific speed ………………………………………… 3.5.5 Typical characteristic curves for a centrifugal pump … 3.5.6 Pump power ………………………………………….. 3.5.6.1 Definition …………………………………….. 3.5.6.2 Pump power …………………………………. 3.5.6.3 Pump power and efficiency …………………..
42 42 43 43 44 48 48 48 49
SECTION – 3.6 Pump Operation 3.6.1 Safety ………………………………………………… 3.6.2 Priming ………………………………………………. 3.6.3 Starting ………………………………………………. 3.6.4 Running ……………………………………………… 3.6.5 Stopping …………………………………………….. 3.6.6 Operation against closed discharge ………………….
50 50 50 51 51 51
SECTION – 3.7 Operating Difficulties 3.7.1 General ………………………………………………..
52
SECTION – 3.8 Cavitations 3.8.1 What is the cavitation? ………………………………… 3.8.2 The main reasons of cavitation ……………………….. 3.8.3 What is the effect of cavitation on the pump …………. 3.8.4 Symptoms of cavitation ……………………………….
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SECTION – 3.9 Pump Auxiliaries 3.9.1 Pump drive …………………………………………… 3.9.2 Couplings ……………………………………………. 3.9.3 Strainers ………………………………………………
57 58 58
SECTION – 3.10 Pump Maintenance 3.10.1 Safety ……………………………………………….. 3.10.2 Lubrication …………………………………………. 3.10.3 Gland packing ………………………………………. 3.10.4 Mechanical seal …………………………………….. 3.10.5 Coupling ……………………………………………. 3.10.6 Overhauling overhung pump shaft …………………. 3.10.6.1 General instructions ………………………. 3.10.6.2 Dismantling ………………………………. 3.10.6.3 Inspection of components ………………… 3.10.6.4 Assembly ………………………………….
59 59 60 61 61 61 61 62 66 68
3.10.7 Maintenance of centrifugal pump (in between bearings pumps) ……………………... 3.10.7.1 Dismantling the pump ……………………. 3.10.7.2 Inspection ………………………………… 3.10.7.3 Reassembling the pump …………………..
71 71 72 74
SECTION – 3.11 Reciprocating Pumps 3.11.1 How it works ………………………………………… 3.11.2 Reasons for using reciprocating pumps …………….. 3.11.3 Disadvantages of reciprocating pumps ……………… 3.11.4 Pump classification ………………………………….. 3.11.5 Liquid end components ……………………………… 3.11.5.1 The liquid cylinder …………………………. 3.11.5.2 Pumping element ………………………….. 3.11.5.3 Stuffing boxes ……………………………… 3.11.5.4 Valves ………………………………………
76 76 76 77 80 80 80 83 91
3.11.6 Drive End Components ………………………………
94
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3.11.7 Flow Characteristics …………………………………. 3.11.8 Power – Pump Drive Systems ……………………….. 3.11.9 System design ……………………………………….. 3.11.9.1 The suction vessel should ………………… 3.11.9.2 The suction piping should ………………… 3.11.9.3 The discharge piping should ………………
95 98 98 98 99 99
3.11.10 Remedies for low NPSHA ………………………… 3.11.11 Unloading the pump ……………………………….. 3.11.12 Slurry applications ………………………………… 3.11.12.1 Stuffing box area (packing) ……………. 3.11.12.2 Pump valves ……………………………. 3.11.12.3 Plunger or piston rod (in case of piston pump) ………………..
101 101 103 103 104 104
3.11.13 Reciprocating Pump Maintenance …………………… 104 3.11.13.1 Liquid end components maintenance ……. 104 3.11.13.2 Drive end components maintenance .……. 106
SECTION – 3.12 Pulsation Dampeners 3.12.1 The function of pressure pulsation dampener ………… 3.12.2 Installation ……………………………………………. 3.12.2.1 Mounting ……………………………………. 3.12.2.2 Precharging ………………………………….
108 108 108 109
3.12.3 General precharging instructions …………………….. 3.12.4 Maintenance …………………………………………… 3.12.4.1 Precharge …………………………………….. 3.12.4.2 Troubleshooting ……………………………… 3.12.4.3 Diaphragm removal …………………………. 3.12.4.4 Diaphragm installation ……………………….
109 110 110 111 111 112
SECTION – 3.13
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SECTION – 3.1 FUNCTION OF PUMPS A wide variety of pumps are used in petroleum industry. A pump is used to increase the total energy content of a liquid in the form of pressure increase. Pumps transfer liquids, for example, between vessels. They are the fluid movers of liquids. The pumps are used to perform one of the following jobs: 1- Move liquids from low level to high level (figure 3.1) 2- Move liquids from low pressure location to high pressure location (figure 3.2) 3- To increase the flow rate of a liquid (figure 3.3)
Figure 3.1 Figure 3.2 Move liquid from low level to high level Move liquid from low pressure location to high pressure location
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Figure 3.3 To increase the flow rate of liquid
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SECTION – 3.2 CLASSIFICATION OF PUMPS The most common classification of pumps is based on the way energy is added to the liquid and pump geometry. They are classified into two main categories: Positive Displacement Pumps Dynamic Pumps
3.2.1 Positive Displacement Pumps Energy is added to the liquid by the application of force that moves the liquid from the low-pressure side (suction) to the high-pressure side (discharge). See figure 3.4
Figure 3.4 Positive displacement pump They are classified as follows: Positive displacement Pumps
Reciprocating Pumps
Reciprocating Piston Pump
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Reciprocating Plunger Pump
Rotary Pumps
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1- Gear pump 2- Lobe pump 3- Screw pump Page 9 of 125
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3.2.1.1 Reciprocating Positive Displacement Pumps Reciprocating positive displacement pumps include three designs: 1- Reciprocating piston pump (figure 3.5 and figure 3.6) 2- Reciprocating plunger pump (figure 3.7) 3- Diaphragm pump (figure 3.8)
Figure 3.5 Reciprocating piston pump (single acting)
Figure 3.6 Double acting reciprocating piston pumps
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Figure 3.7 Plunger pump
Fig. 3.8 Diaphragm pump
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Figure 3.9 Suction and discharge strokes of diaphragm pump
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3.2.1.2 Rotary Positive Displacement Pumps Single rotor Screw pumps, sliding vane pumps, etc. Multiple rotor Gear pumps (figure 3.10, 3.11), lobe pumps (figure 3.12) and screw pumps (figure 3.13).
Figure 3.10 External gear pump
Figure 3.11 Internal gear pump
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Figure 3.12 Lobe pump
Figure 3.13 Screw pump
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3.2.2 Dynamic Pumps The liquid velocity is increased inside the pump to a value higher than the discharge velocity. Velocity reduction within or after the pump is converted to pressure.
3.2.2.1 Pump Theory The rotating impeller imparts a centrifugal force and kinetic energy in the form of velocity to the liquid. They are classified as follows: 3.2.2.2 3.2.2.3
Centrifugal Pumps Special Pumps
3.2.2.2 Centrifugal Pumps How it Work? Figure 3.14 shows the impeller and pump casing of centrifugal pump. Let us see how it works?
Figure 3.14 Centrifugal pump 1. Liquid flows through the pump inlet and into the eye of the impeller. 2. The impeller whirls the liquid around in a circle. The liquid is forced from the center to the outside of the impeller. Centrifugal force pushes the liquid outward from the eye. TriStar
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3. Liquid enters the pump casing when it leaves the outer edge of the impeller. When the liquid enters the casing, speed decreases, as the speed of the liquid decreases, its pressure increases. 4. As centrifugal force moves the liquid away from the impeller eye, a lowpressure area (zone) is formed in the suction eye. This low pressure area in the suction eye causes liquid to flow into the suction eye. A typical centrifugal pumps is show in figure 3.15.
Figure 3.15 Typical centrifugal pump
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SECTION – 3.3 CENTRIFUGAL PUMP COMPONENTS 3.3.1 Centrifugal Pump Components Pump consist of rotating components (rotor) and stationary components. Figure 3.16 show centrifugal pump components.
Figure 3.16 Horizontal single stage centrifugal pump
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The following table shows the correct name for each item.
Table 3.1 Recommended names of centrifugal pump parts
3.3.2 The Function of Pump Components 1- Impeller An impeller is the part which imparts energy to the liquid being pumped. Energy is added to the liquid as it moves through the rotating vanes of the impeller (figure 3.17)
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Figure 3.17 The impeller
2- Shaft The impeller is firmly attached to the shaft and rotates with it. The shaft performs two jobs: Carry the impeller (s) and all other rotating parts and keep them in their correct position with respect to the pump casing. Transmit the required driving power to rotate the impeller (s)
3- Shaft Sleeve To protect the shaft from wear in stuffing box area. As spacer between different impellers in multi-stage pump.
4- Coupling Transmits the required power to drive the pump shaft and all other rotating parts.
5- Wear Rings One wear ring is fixed to the impeller and rotate with it (impeller wear ring). One wear ring is fixed to the pump casing and does not rotates (case wear ring). These two wear rings together work to minimize the internal leakage inside the pump. TriStar
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6- Pump Casing It contains all rotating parts (shaft, impeller, impeller wear ring) etc. Pump casing directs the liquid which leaves the impeller to the discharge nozzle (pump discharge).
7- Stuffing Box It is a cylindrical cavity where the shaft passes into the casing. The packing material presses around the shaft in this cylindrical cavity to minimize the leakage of liquid to outside the pump. A mechanical seal may be used instead of packing.
8- Bearings Its function is to carry the pump rotor and keep it in its correct position with respect to the casing.
3.3.3 Stuffing Boxes Any pump converts the energy of a prime mover, such as an electric motor, into velocity or pressure energy of the liquid being pumped. In a centrifugal pump, the product enters the suction of the pump at the center of the rotating impeller. See Figure 3.18
Figure 3.18 Fluid flow in a centrifugal pump TriStar
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As the impeller vanes rotate, they transmit motion to the incoming product, which then leaves the impeller, collects in the pump casing, and leaves the pump under the pressure through the pump discharge. Discharge pressure will force some product down behind the impeller to the drive shaft, where it attempts to escape along the rotating shaft. Pump manufacturers use various design techniques to reduce the pressure of the product trying to escape. Such techniques include: 1. The addition of balance holes through the impeller to permit most of the pressure which acting behind the impeller to escape into the suction side of the impeller. (figure 3.19) 2. The addition of small pumping vanes on the back side of the impeller. (figure 3.20). However, as there is no way to eliminate this pressure completely, sealing devices are necessary to limit the escape of the product to the atmosphere. Such sealing devices are typically either compression packing or mechanical seal.
Figure 3.19 Back wear ring and balancing holes
Figure 3.20 Back vanes
3.3.3.1 Packed Stuffing Box Stuffing boxes have the primary function of protecting the pump against leakage at the point where the shaft passes out through the pump casing.
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If the pump handles a suction lift and the pressure interior stuffing box end is below atmospheric, the stuffing box function is to prevent air leakage into the pump. If this pressure is above atmospheric, the function is to minimize liquid leakage out the pump.
3.3.3.2 How Does it Work? 1- Early attempts to control the leakage of the product around rotating shafts consisted of merely restricting the clearance between the shaft and the wall of the pump casing by packing a soft, resilient material around the shaft within an extension of the pump back head called a stuffing box. 2- Figure 3.21 Shows a typical stuffing box sealed with square rings of compression packing. 3- The compression packing rings, which must be carefully installed in a clean stuffing box, are held in place by a gland. 4- As the gland bolt nuts are tightened, pressure applied to the gland is transmitted to the compression packing, forcing it against the shaft or shaft sleeve and effecting a seal. Because this pressure is not evenly distributed throughout the packing, most of the sealing and consequently most the wear occurs in the first few rings adjacent to the gland. (Figure 3.22) 5- Frictional heat, which develops where the compression packing contacts the rotating shaft or shaft sleeve, is reduced by permitting the product to leak to the atmosphere at a controlled rate. This leakage is essential to carry away the frictional heat and as lubricant between the shaft (or shaft sleeve) as rotating element and the packing rings as stationary element.
Figure 3.21 Stuffing box with compression packing TriStar
Figure 3.22 Pressure distribution
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3.3.3.3 Lantern Rings
Figure 3.23 Lantern ring
Figure 3.24 Stuffing box with lantern ring
The lantern ring (figure 3.23) is a device made from a rigid material such as bronze, stainless steel, nylon or TFE, and is of open construction to allow free passage of sealing liquid ( or lubricant). Normally, the sealing liquid (or lubricant) enters the outside of the ring, and flows to fill the space between the packing rings and the shaft ( or shaft sleeve). The lantern ring usually has packing rings on either side (figure 3.24)
3.3.3.4 Arrangements of the Lantern Ring to Meet Specific Services 1- When a pump operates with negative suction head See Figure 3.25 a: The inner end of the stuffing box (product side) is under vacuum, and air tends to leak into the pump. For this type of service, packing is usually separated into two sections by a lantern ring (seal cage). Sealing fluid is introduced under pressure into the space, causing flow of sealing fluid in both axial directions. This construction is useful to assure liquid for cooling and lubrication between the packing rings and the shaft or shaft sleeve. TriStar
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Figure 3.25 Arrangement of lantern ring to meet specific services 2- See Figure 3.25 b This construction is useful for pumps handling flammable or chemically active and dangerous liquids since it prevents outflow of the pumped liquid. 3- If the product being pumped is too contaminated with abrasives See Figure 3.25 c: Clean liquid flush to lantern ring to prevent dirty liquid to enter the stuffing box area. If the abrasives lodged between the packing rings and the shaft (or shaft sleeve) it will act completely like a cutting tool against the shaft or shaft sleeve.
3.3.3.5 Packing Selection Factors that must be considered in selecting a packing involve: The fluid's conditions, such as temperature, lubricity and pressure. All equipment parameters: Shaft speed. Shaft size (in stuffing box area). Stuffing box dimensions. Continuous or intermittent service.
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3.3.3.6 Conventional Packing Draw-Backs The drawbacks of conventional packing are: Packing operates on the principle of controlled leakage. They never attempt to totally prevent fluid from leaking from the equipment. This leakage will cause: a. Waste of product,
b. Pollution.
It requires regular adjustment of the gland. Pressure limits: Packing not suitable selection for high pressure working conditions like water injection pumps. Power consumption: The packing consume more power. Packing rubbing on a shaft (or shaft sleeve) similar to driving an automobile with the handbrake engaged. This relatively high power consumption will increase the running cost.
Maintenance cost: Most of the time, the shaft ( or shaft sleeve) should be changed due to damage. The rubbing between the packing rings and the shaft will cause score marks and rough surface on the shaft in the stuffing box area. That means extra maintenance cost and more downtime. Beside this, most bearing failure is caused by contamination rather than overloading. The easiest way to contaminate a bearing is from the leakage coming through the packing.
Speed limits: Packing have limited speed, if you try to use it in speeds higher than its limits, the failure will happen. The argument for packing usually centers around four statements: 1. 2. 3. 4.
You don't have to take the pump apart to change packing. In an emergency, you can always add a ring of packing. Packing is cheaper. Packing is less complicated.
Let's look at each of these statements if it is true:
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Statement 1: You do have to take the pump apart to change sleeves and bearings. Shaft sleeve replacement is a normal part of repacking a pump. The fact of the matter is that you will have to dismantle a packed pump more than a sealed pump. Statement 2: If you need reliability, use a mechanical seal with an auxiliary packing gland. Statements 3: Packing is cheaper if you consider the packing alone. Bicycles are also cheaper than automobiles. Statement 4: Packing is less complicated only to an inexperienced man. If you have ever tried to teach an apprentice how to inspect a stuffing box and shaft, cut packing, install it so as to align the lantern ring, tamp it in place, and adjust it properly so as to keep leakage to a minimum and not generate excessive heat (you have to do it by feel), then you know just how complicated packing really is.
3.3.4 Mechanical Seals 3.3.4.1 Mechanical Seals Overview The mechanical seals was developed to overcome the disadvantages of compression packing. Leakage can be reduced to a level meeting the environmental standards.
3.3.4.2 Mechanical Seals Construction All mechanical seals are constructed of four basic sets of parts. As shown in figure 3.26, these are: 1. A set of seal faces which are called sometimes primary sealing device. One that rotates (rotating face) and one that is stationary (stationary face). 2. A set of secondary seals known as secondary sealing device or gaskets such as 0-rings, wedges, U-cups and V-rings.
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3. Spring (s). 4. Mechanical seal hardware including seal flange (gland ring), shaft sleeve, etc.
Figure 3.26 A simple mechanical seal
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3.3.4.3 Sealing Points for Mechanical Seal There are four main sealing points, (see figure 3.26) 1. The primary seal is at the seal face, point A. The primary seal is achieved by two very flat, lapped faces which create a difficult leakage path perpendicular to the shaft. Rubbing contact between these two flat mating surfaces minimizes leakage. As in all mechanical seals, one face is held stationary in a housing (stationary face), and the other face is fixed to, and rotates with the shaft (rotating face). These two faces are made from two dissimilar materials, one of them is softer than the other. For example carbon – graphite (as soft face), the other is usually hard material (as tungsten – carbide as hard face). The seal faces are made from two different materials in order to help prevent adhesion of the two faces. 2. The leakage path at point B (between the floating seal face and the shaft or shaft sleeve) is blocked by floating seal face gasket (either 0-ring, Ucup, a V-ring, or a wedge). 3. Leakage path at point C (between the seal flange and stuffing box face) is blocked by seal flange gasket which could be 0-ring or any other shape of static gaskets. 4. Leakage path at point D (between the seal flange and the stationary face) is blocked by stationary face gasket or seat gasket.
3.3.4.4 How Does it Work? 1. The two flat seal faces are pushed together by axial force from the closing mechanism (spring or metal bellows) and by product pressure in the stuffing box cavity. 2. When the seal is in operation, the two seal faces are lubricated by the same product inside the stuffing box. It is known that, for the seal to work efficiently, it is necessary for a stable fluid film to exist between the seal faces. In the majority of cases this film is a liquid. The function of this liquid film between the seal faces is for cooling (carry away the frictional heat) and lubrication. If this film stability is destroyed, excessive wear takes place leading to rapid seal failure. TriStar
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3. Improperly positioned seals could allow a wide gap between the faces, causing a leak path. The faces could also be squeezed so tightly together that no lubrication is present, causing rapid seal failure.
3.3.4.5 Advantages of Mechanical Seals Mechanical seals replace packing in stuffing boxes where the liquid must be contained. These seals offer: 1. Reduced friction power loss. 2. Eliminate the wear on the shaft or shaft sleeve in the stuffing box area. 3. Invisible or minimum leakage. 4. Ability to function in relative extremes of shaft deflection and end play. 5. Suitable for high working pressures and high running speeds.
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3.3.4.6 Comparison Between Conventional Packing and Mechanical Seals Packing 1. How Does it Work?
Figure 3.27 Packing (figure 3-27) forced into a stuffing box around the shaft. It seals by throttling the fluid trying to leak between the packing and shaft. Packing wears the shaft and increases the power needed to rotate the shaft.
2. Shaft Run-Out Shaft run-out is one costly enemy of conventional packing. It beats out packing, making sealing problem tough. If the shaft run-out is over 0.003 inch, it's impossible to seal properly, especially at high speeds.
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Mechanical Seal 1. How Does it Work?
Figure 3.28 Mechanical seal (figure 3-28) has two seal faces at right angles to the shaft. One seal face is fastened to the shaft and revolves with it, while the other is stationary and is held against the machine casing. The wearing faces that seal have a small area compared to the area conventional packing seals against. Because of this small area, and the preloaded spring(s) forcing the two faces together, there's less friction at the seal faces. And of course there is no wear on the shaft because seal faces take it all, they can relapped or replaced when needed.
2. Shaft Run-Out Mechanical seals can take more shaft run-out without leaking. Reason is that sealing faces are at right angles to shaft. The elastomeric gaskets and the spring(s) allow for some misalignment between the seal faces which could happen due to shaft run-out.
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Figure 3.29 Effect of shaft run-out
3. The Effect of Shaft Axial Float on Packing
Figure 3.30 The effect of shaft axial play
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End play (shaft axial play) is common with most shafts especially when starting up or shutting down. Such shaft movement does not affect the packing if shaft has no grooves in packing area. But usually shafts or sleeves do groove after a short while. Then shaft end play disturbs packing, open it up and causes leakage.
The Effect of Shaft Axial Float on Mechanical Seal
Figure 3.31 The effect of shaft axial play on the mechanical seal
Shaft end play (shaft axial play) does not affect the mechanical seal if this end play within certain limits (about 0.003" for rolling element bearings as a thrust bearing and about 0.015" for slide surface bearing as a thrust bearing). The spring (s) will keep the seal faces close.
4. Power Consumption It is relatively high in case of packing (about three times the power consumption in mechanical seal for the same shaft size and speed).
5. The Required Time for Replacing:
Figure 3.32 Replacing packing and mechanical seal TriStar
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In case of Packing: Can be packed in place. It needs adjusting several times after start up until it reach the normal running conditions.
Mechanical Seal: Installed over shaft end. It needs more time for installation. It does not need any additional adjustment after the installation.
6. Pollution Is relatively high in case of packing because the packing must leak for cooling and lubrication. In case of mechanical seal in normal running conditions, less leakage i.e. less pollution. Double seals are able to stop product leakage 100%.
7. Cost of Product In case of packing, the cost of product is high- due to high leakage rates. In case of mechanical seal, the cost of product is low- due to very small leakage rates.
8. High Pressures and Big Shaft Diameter Services The packing not suitable sealing device for big shaft diameter or high pressures. The mechanical seal is suitable sealing device for big shaft diameters and / or high running speed.
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SECTION – 3.4 CLASSIFICATION OF CENTRIFUGAL PUMPS Centrifugal pumps can be classified with respect to the following parameters: 1- With respect to the impeller design. 2- With respect to the flow of liquid after it leave the impeller. 3- With respect to the pump case design. 4- With respect to the split of the casing. 5- With respect to the number of stages. 6- With respect to the shaft position.
1- With Respect to the Impeller Design Single suction impeller (figure 3-33) or double suction impeller (figure 3-34)
Figure 3.33 Single Suction Impeller
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Figure 3.34 Double suction
2- With Respect to the Flow of Liquid After Leave the Impeller Radial flow – Axial flow – mixed flow (figure 3.35)
Axial flow
Mixed flow
Radial flow
Figure 3.35
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3- The Pump Case Design The pump case could be volute design (figure 3.36) or double volute (figure 3.37) or diffuser – type (figure 3.38)
Figure 3.36 Volute design pump case
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Figure 3.37 Double volute pump case
Figure 3.38 Diffuser – type pump
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4- The Split of the Casing Could be axial split casing (figure 3-39) or radial split casing (figure 3-40)
Figure 3.39 Axial split casing
Figure 3.40 Radial split casing TriStar
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Figure 3.41
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5- Number of Stages The pump could be single stage (one impeller) figure 3-16 or multi – stage (figure 3-41).
6- With Respect to the Shaft Position The pump could be horizontal shaft (figure 3-41) or vertical shaft (figure 3-42)
Figure 3.42 Vertical pump TriStar
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SECTION – 3.5 PUMP PERFORMANCE CURVES The centrifugal pump is now the most widely used type of pumps in the petroleum industry. It must be correctly sized, fitted, and installed to operate satisfactorily. This section is a guide to the selection, application, and limitations of centrifugal pumps.
3.5.1 Factors Affecting Pump Performance The following factors does affect the performance of centrifugal pump: 12345-
Running speed. Impeller size (diameter). Liquid specific gravity. Liquid viscosity. NPSH (net positive suction head)
At constant speed the characteristics of centrifugal pumps are: 1- The capacity varies directly as the diameter of the impellers; if the diameter of the impeller is increased 10% the capacity is increased 10%. 2- The head or pressure developed by the pump varies directly as the square of the diameter of the impeller; if the diameter of the impeller is reduced to 90% of its original diameter, the head developed is reduced to 90% squared, or 81% of the original head. 3- The horsepower required varies as the cube of the impeller diameter, if the impeller diameter is reduced to 90% of its original value, the horsepower required reduced to 90% cubed, or 72.9% of its original value. Under ideal conditions, a change in speed of the pump changes the capacity, head and horsepower in the same ratio or proportions – directly, square and cube – as does the same percentage change in impeller size.
3.5.2 Effects of Specific Gravity Because the discharge pressure and horsepower required to drive the pump are a function of the specific gravity of the liquid, both are affected in direct proportion to changes in specific gravity. TriStar
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Centrifugal pump can push a heavy liquid and a light liquid to the same height, even if water is piped to the pump instead of lighter oil, it continues to raise the liquid to the same elevation. The pressure on the discharge line would increase as a result of the higher specific gravity of water. The pump would have to exert more force and would automatically require more horsepower.
3.5.3 Effect of Viscosity Viscosity is a measure of the friction between the particles in a liquied, molasses has a high viscosity, water a low viscosity. A change in viscosity will change the capacity, head, efficiency and brake horsepower (input to the pump shaft) requirements of a pump. These effects are hardly noticeable up to about 70 SUS; however, above viscosities of 100 SUS, these effects become quite pronounced, causing a reduction in efficiency and a drop in head at a given flow rate. SUS (Saybolt Universal Second) is the standard unit of measurement for viscosity of oil in the American petroleum industry. It is the number of seconds required for a measured quantity of oil at a constant temperature to run through a small hole in the tube of a standard saybolt viscosimeter. An oil which takes 60 second to run through this hole is said to have a saybolt universal seconds viscosity of 60, or 60 SUS. A change in temperature greatly affects the viscosity of some crude oils; usually the higher the temperature, the lower the viscosity.
3.5.4 Specific Speed Specific speed is a term used to compare the performance of impellers, irrespective of their size. Specific speed (usually designated by the symbol NS) is the speed, in revolutions per minute, at which a geometrically similar impeller would run if it were or such size as to discharge one gallon per minute against one foot head. The specific speed equation is
NS =
NQ H
3
1 2
4
Where N = impeller speed, revolutions per minute. Q = capacity. Gallons minute H = total head in feet per stage Specific speed should be thought of as an index or type number referring to the performance or general proportions of an impeller, but not to its actual size or revolutions per minute. TriStar
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3.5.5 Typical Characteristic Curves for a Centrifugal Pump Two methods are commonly used for plotting the characteristic curves of a centrifugal pump. Figure 3-43 shows the method used in presenting pump performance for a single speed and a fixed impeller size, these curves result from a test of a pump at a constant speed and are curves a manufacturer commonly uses to certify the performance of a pump. Figure 3-44 shows the method used to express more fully the entire range of performance of a pump with maximum and minimum diameters of impellers at a given speed. These curves are commonly used in the selection of a pump for a specified service. The curves in figure 3.44 are made-up from the average performances of a number of tests for various diameter impellers that have been ploted in the form shown in figure 3-43. Figure 3-45 shows a third method of plotting characteristic curves for a centrifugal pump driven at variable speed but with fixed impeller diameter. Characteristic curves of the pump and the system through which it pumps should be plotted on the same chart. These curves will assist in the selection of the proper pump for the desired service, and will show the effect of operating conditions, such as changes in speed, viscosity or system characteristics. Practically, all performance curves furnished by manufactures are for pumping water; if the pump is to handle some other liquid, proper corrections must be made for viscosity and specific gravity.
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Figure 3.43
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Figure 3.44
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Figure 3.45
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3.5.6 Pump Power There are three factors does affect the power consumed. These factors are: 1- The flow rate (Q). 2- The discharge pressure. 3- The specific gravity of the liquid.
3.5.6.1 Definitions Capacity: The pump capacity Q is the volume of liquid per unit time delivered by the pump. In English measure it is usually expressed in gallons per minute (GPM) and for large pumps, in cubic feet per second (ft3/sec). In metric measure the units are liters per second (L / Sec.) and cubic meters per second (M3 / Sec) Head: The pump head (H) represents the net work done on a unit weight of liquid in passing from the inlet or suction flange (S) to the discharge flange (d). Power: There are liquid horsepower and mechanical horse power. Liquid horsepower = Lhp =
QSH 3,960
Q in GPM S specific gravity H in feet
3.5.6.2 Pump Power B.H.P. =
BPH differential pressure Psi Horsepower 2450 effeciency
B.H.P. = Breake horsepower B.P.H. = Barrels per minute Differential pressure = in Psi In metric system TriStar
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3.5.6.3 Pump Power and Efficiency The power is sometimes called the liquid power or hydraulic power. The input power, P (in kW), represents the driving shaft mechanical power and is sometimes called brake power if we allow for friction losses. The pump hydraulic power = .g.Q.H The pump efficiency = .g.Q.H / P Where, Q pump capacity in m3 / s, H total pressure head in m, liquid density in kg/m3, g is the acceleration due to gravity = 9.81 m/s2.
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SECTION – 3.6 PUMP OPERATION 3.6.1 Safety Ensure that all protective guarding is in position and securely fixed.
3.6.2 Priming The pump must not be run dray at any time. The pump casing and inlet line must be completely filled with liquid before the pump will operate.
3.6.3 Starting In general, the following procedure should followed: 1- When fitted ensure that flushing and / or cooling liquid supplies are turned on. 2- Close the outlet valve (on the discharge line of the pump). 3- Be sure that the suction valve (on the suction line of the pump) is open. 4- Prime the pump 5- Start the motor and immediately check outlet pressure. If the gauge does not register positive pressure, stop the motor and check for air leakage or any other possible cause. 6- If the pressure is satisfactory, slowly open outlet valve. Do not operate the pump with valve closed for more than a few minutes.
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3.6.4 Running 1- Packed gland should leak, and leakage should take place soon after the stuffing box is pressurized. Until steady leakage takes place, the pump may overheat. If this happens, the pump should be stopped and allowed to cool and when re-started, leakage should take place. In general, gland nuts should not be slackened but when hot liquids are being pumped, this may become necessary. After the pump has been running for ten minutes with steady leakage, tighten the gland nuts. Continue to tighten gland nuts until leakage is reduced to an acceptable level. When adjustment is completed there should be drip leakage from the gland, ensuring that overheating does not take place. 2- With a mechanical seal, no adjustment is necessary and any slight initial leakage will disappear when the seal is run in.
3.6.5 Stopping Close outlet valve and switch off motor.
3.6.6 Operation Against Closed Discharge Closing the discharge valve on an operating centrifugal pump reduces the flow rate even though the pressure is a maximum. However, the power consumed by the pumps is not zero, it is about
1 the rated power because of friction of parts 2
and the churning of the enclosed liquid. This condition requires that precaution be taken. No centrifugal pump should be operated against a closed discharge valve. It may be necessary to hold the valve closed or nearly closed for few second when starting the unit. Power absorbed in rapidly churning the liquid results in a dangerous temperature increase. This condition is usually provided for by means of a small by – pass with a check valve around the discharge valve. Another protective measure is the installation of by – pass with valve to the pump sump or the suction side of the pump.
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SECTION – 3.7 OPERATING DIFFICULTIES 3.7.1 General This section gives information on fault diagnosis and possible remedies to operating difficulties. The matrix in next page details a list of ten possible symptoms to which the possible cause or causes can be ascertained by reading off composite the black rectangles.
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Figure 3.46 Fault diagnosis matrix TriStar
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SECTION – 3.8 CAVITATION 3.8.1 What is the Cavitation? The formation and subsequent collapse of vapor-filled cavities in a liquid due to dynamic action are called cavitation. The cavities may be bubbles, vapor-filled pockets, or a combination of both. Cavitation happens if the local pressure becomes equal to or below the vapor pressure of the liquid at this temperature, and the cavities must encounter a region of pressure higher than a vapor pressure in order to collapse. Dissolved gases are often liberated shortly before vaporization begins.This may be an indication of impending cavitation, but true cavitation requires vaporization of the liquid. Bubbles which collapse on a solid boundary may cause severe mechanical damage. All known materials can be damaged by exposure to bubble collapse for a sufficiently long time. This is properly called cavitation erosion or pitting.
3.8.2 The Main Reasons of Cavitation Centrifugal pumps begin to cavitate when the suction head (NPSH) is insufficient to maintain pressures above the vapor pressure throughout the flow passages. This could happen due to one or more of the following reasons: 1. The unfavorable inlet flow conditions, believed to have been the cause of the cavitation, were at least partly due to elbows in the approach piping. Elbows in the suction side of the pump will cause additional pressure drop beside the pressure drop due to the flow of liquid inside the suction line (frictional pressure drop). Modification to the approach piping and the pump inlet passages reduced the cavitation. 2. In between/bearing pump double-suction impeller, cavitation can happen in this type of pumps, in addition to the reason #1, due to a bend (elbow) in suction line in the horizontal level figure 3.47
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This bend (elbow) may cause uneven distribution of flow to the impeller. The liquid will flow toward the outside of the elbow and result in an uneven flow distribution into the two inlets (suction eyes) of the double suction impeller. One side of the impeller will get enough liquid and there is no enough liquid on the other side, (figure 3-47). In such cases, there is a great probability of cavitation to happen in one suction eye of the impeller which does not get enough liquid. There is another thing that will happen prior to cavitation, one stuffing box-on the side where there is no enough liquid - will suffer from pressure drop, which can change completely the hydraulic forces on the seal faces. When such elbow cannot be avoided, it should be in a vertical position if possible. Where it is necessary for some reason to use a horizontal elbow, it should be a long radius elbow and there should be a minimum of two diameters of straight pipe between the elbow and the pump suction as shown in figure B.
Figure 3.47 Horizontal elbow in suction line
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3. Blockage in the suction side also can cause cavitation. If the suction valve is not fully opened or strainers on the suction side is partially blockaged additional pressure drop will occur. This may lead to cavitation. 4. Another reason can cause cavitation in deep-well pumps. Deepwell pumps are usually provided with a check valve close to the discharge. However, this valve cannot prevent the liquid elevated in the column from flowing back into the well and creating a vacuum between the liquid level and the check valve. When this pump is started again, vaporization due to this vacuum may lead to cavitation shocks.
3.8.3 What is the Effect of Cavitation on the Pump? 1. One of the effects of Cavitation is almost always to cause the pump to run unevenly with strong radial and axial vibrations. If the axial vibration due to cavitation increased and becomes as strong axial vibration, this can cause rapid breakdown of the. sealing faces of the mechanical seal. 2. The large pressure pulsation in the stuffing box can have a determintal effect on the performance and life of a mechanical seal. If the pressure in the stuffing box reduced to certain limit, in this case there is no enough hydraulic pressure to create a liquid film between the seal faces and then the seal running dry. 3. The bubbles which collapse when reaching a zone of higher pressure may exert enormous local stresses on the surfaces on which they collapse, causing damage.
3.8.4 Symptoms of Cavitation 1- Simmering noises and cracking are heared. 2- Severe axial vibration in the pump shaft. 3- Unsteady state of flow of liquid. Liquid is mixed with air and gas bubbles.
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SECTION – 3.9 PUMP AUXILIARIES 3.9.1 Pump Drive Electrical Motors Motors are the major drivers to supply energy to rotating pumps. The motor power should be greater than the pump input power in order to allow for friction and other types of losses. Steam Turbines The availability of steam may suggest a turbine drive. A steam turbine drive is usually chosen where exhaust or high-pressure steam is available. Gas Turbines A gas turbine drive may be used to power a rotating pump. Gas turbines are usually available in the large size power.
Gas Engines
A gas engine drive may be used when gas is available at a low cost. Gas engines are used to provide power to drive reciprocating or centrifugal pumps. Diesel engine This drive is similar to the gas engine drive. The difference is the fuel used. The compression ratio of a diesel engine is greater than that of a gas engine.
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3.9.2 Couplings On most centrifugal pumps, couplings (Figure 3.48) join the shafts of the drivers and pumps. Alignment of shafts is very important for good and smooth operation of the pump. Misalignment will cause the shaft and other pump parts to vibrate. Serious vibration can generate enough stresses to break the shaft and coupling. Also, vibration can cause bearings to wear, internal parts to rub so they become unbalanced, etc. All of these conditions require maintenance and result in equipment downtime.
Figure 3.48 Couplings
3.9.3 Strainers The primary function of a strainer is to protect the equipment. Normally strainers are placed in the line at the inlet to pumps, control valves or any other equipment that should be protected against damage. The strainer is selected for the design capacity of the system at the point where it is to be inserted in the line.
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SECTION – 3.10 MAINTENANCE 3.10.1 Safety The following safety precautions should be observed before commencement of maintenance. 1. Do not attempt any maintenance on the pump whilst it is in operation. 2. Before carrying out dismantling or maintenance, isolate the power supply to the pump driving unit and/or automatic starting devices. 3. Ensure inlet and outlet isolating valves are closed and holding. 4. Drain pump casing to a safe area. Wear the correct protective clothing to suit the pumped liquid when removing drain plug.
3.10.2 Lubrication 1- Pump and Motor Bearings – Grease Lubrication Machines are supplied with bearings pre-packed with grease and ready to put into service. For re-lubrication, a lithium based grease such as Shell Alvania R3 is recommended. As a guide to quantity of grease, the actual bearing should be filled and then one third of the housing.
Note that Excessive Grease Can Cause Overheating Units without grease nipples. Clean out and recharge housings with fresh grease at 6000 hours operation or 3 years whichever is earlier. Some motors have "sealed for life" bearings which cannot be re-lubricated.
2- Pump Bearings – Oil Lubrication Oil lubricated units are supplied without oil. To fill the reservoir either remove the breather or hinge back the constant level oiler. When the oil level can be seen in the bend of the oiler, cease adding oil and fill the oil bottle. Hinge forward to allow oil to run into reservoir. Repeat as necessary until level in oil bottle remains constant. Top up bottle as necessary during operation. TriStar
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When a pump has been initially operated with a particular grade of oil, the bearing temperature should be checked. You must use the oil which is specified by the manufacturer. The oil should be changed every six months when the pump is operating for 8 hours per day. When conditions are more severe, such as hot service, damp or corrosive atmosphere or continuous service, the oil should be changed more frequently.
3.10.3 Gland Packing (When Fitted) 1- New gland packing has to be run-in and it is normal practice to start the pump with the stuffing box gland relatively loose i.e., gland nuts only finger – tight. There should ALWAYS be a slight leakage of liquid from the gland to keep an efficient seal and to lubricate and cool the packing. Should the gland need re-packing;, it should be noted that some pumps have a split gland which can be completely removed to make packing a simple task. Other pumps have a pressed stainless steel gland which allows space for re-packing.
Figure 3.49 Stuffing box with lantern ring TriStar
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2- stuffing boxes are equipped with a lantern ring (Figure 3.49) which can be used to feed water or a compatible fluid to the packing. This is useful, when the inlet pressure is less than atmospheric, as air leakage into the pump is prevented. Also, when an independent fluid source is used, it is possible to flush the packing of any grit or solids handled by the pump. 3- The gland should be inspected at frequent intervals to check that operation is correct.
3.10.4 Mechanical Seals A mechanical seal, whatever type, when correctly selected for type of liquid and application, should give a long period of service without any attention. No adjustment is necessary and any slight initial leakage will be eliminated when the seal is run-in. The seal and any auxiliary flushing should be examined frequently for correct operation.
3.10.5 Coupling The coupling should be examined at frequent intervals to ensure that correct alignment is maintained and that the driving elements are not worn.
3.10.6 Overhaul 3.10.6.1 General Instructions 1- The frequency of a complete overhaul depends upon the hours of operation of the pump, the severity of service and the care the pump receives during operation. DO NOT open the pump for inspection unless there is evidence of trouble inside the pump or in the bearings. 2- Should dismantling prove necessary, great care must be taken. For ease of re-assembly, lay out all parts in the order in which they are removed. 3- Protect all machined faces against metal to metal contact and corrosion. Do not remove the bearings unless they are to be replaced.
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3.10.6.2 Dismantling The sequence of steps to strip down the pump depends mainly on the type of the pump. The pump could be overhang pump or in between bearings pump. For this reason this section divided into two parts. Each one covers one of these two designes. One important thing should taken into consideration: The manufacturer’s instructions should be followed when dismantling and assembling the pump. For this pump (overhang) – please check figure 3-50 and spare parts list on the next two pages. 1- The pump is of the "back pull-out" type which enables the pump casing to remain secured to the baseplate and pipework when dismantled. The spacer coupling enables the motor to remain secured to the baseplate and consequently no further alignment should be necessary after re-assembly. Care must be exercised during dismantling, to prevent damage to internal components.
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Figure 3.50
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Pump Parts List
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The dismantling procedure should be carried out in the following order: 1. Close inlet and outlet valves, and drain liquid from pump. 2. Disconnect auxiliary piping (cooling, flushing, etc). 3. Drain oil from the bearing housing, and remove the correct level oiler 4. Remove the coupling spacer. 5. Remove the bearing housing support foot from the baseplate and remove the nuts which secure the integral frame/bearing housing and adapter to the pump casing. NOTE: On larger units do not remove the support foot. Fit a bolt (hand tight) into the tapped hole in the frame to act as a support. 6. Remove the rotating element together with the housing as one unit for further dismantling. 7. Prise open impeller nut lockwasher and remove impeller, nut (right-hand thread). If an inlet inducer is fitted this should be removed first (righthand thread). 8. Pull off impeller. 9. For pumps fitted with gland packing, remove the gland nuts securing the gland, and remove the gland from the studs. Unscrew the stuffing box cover/bearing bracket bolts (where fitted), before removing stuffing box cover. Remove gland packing, lantern ring and packing seating ring. Remove shaft sleeve. 10.For those pumps fitted with mechanical seals, the manufacturer's instructions should be followed when dismantling and assembling. For standard mechanical seals the following procedures give general guidance:
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Remove the nuts securing the seal plate (seal flange) to the casing and slide the seal flange away. Unscrew the stuffing box cover/bearing adapter bolts (where fitted) and remove the stuffing box cover. The inboard seat on a double seal will come away in the stuffing box cover.
Mark the position of the seal drive collar to the shaft. Loosen the drive screws in the seal drive collar and remove the rotating element of the seals, from the shaft sleeve. For double seals, smooth any marks on the sleeve made by inboard seal before removing outboard seal. Remove stationary seat, from the seal plate. This should only be done if the stationary face or its seating ring are being replaced. Remove shaft sleeve.
Dismantling Bearing Frame (Bearing Housing) This operation should only be carried out when bearings are being replaced: 1. Pull off the pump half coupling and remove coupling key. 2. Remove liquid thrower and both bearing covers. Note that metal labyrinth throwers are secured by socket head setscrews. 3. Press shaft, with bearings, out of bearing housing, removal direction is towards the coupling end. In some designs the outer race and roller of pump end bearing will stay in the housing. These can now be removed. 4. Remove bearing locknut (and lockwasher where fitted) and drive off bearings. 5. Check the condition of the oil thrower which is fitted between the bearings when bearing cooling feature is supplied.
3.10.6.3 Inspection of Components When the pump has been dismantled the components should be examined to fined out the parts which should be replaced.
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1. Shaft Examine the shaft carefully. Its condition should be checked where the impeller, shaft sleeve and bearings fit. The shaft may become damaged by corrosion or pitting, caused by leakage along the shaft at the impeller end of the shaft sleeve. Check the shaft keyway for distortion. Excessive thermal stresses or corrosion may loosen the impeller on the shaft and subject the keyway to excessive shock. Replace a shaft that is bent or. Check the shaft for possible runout.
2. Bearings Extreme care is to be taken when removing the bearings as they may be damaged to such an extent that they are no longer usable. Always check the bearings immediately after removal for any imperfections, or for any play between the races. It is recommended that new bearings are installed, because very often damage caused by removal cannot detected until the pump is put back into service.
3. Impeller After removal, the impeller should be checked for corrosion, blocked waterways and worn spots. Impellers should be statically balanced after any machining work is carried out.
4. Joints It is recommended that new joints (gaskets, O-rings) are installed after the pump has been dismantled. The joints should be of the same material and thickness as the original joint, so that they will compress to the same thickness.
5. Stuffing Box The packing rings should be replaced by new set with the same size, same material.
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3.10.6.4 Assembly General 1. To assemble the pump, reverse the dismantling procedure previously described. Consult the exploded-view illustrations and take note of the following procedures which detail special considerations of assembly which must be observed. 2. Ensure all gaskets are assembled correctly. Clean the inside of the integral frame/bearing housing and adapter and the bores for the bearings. 3. The inner races of the bearings are an interference fit on the shaft. These should be fitted by either heating or by using hydraulic press. NOTE The method described in the previous paragraph is preferred. Heat the bearings in an oil bath or electric oven to uniform temperature and mount it quickly on the shaft. If the alternate method (using of hydraulic press) is used, apply the force using an arbor press (hydraulic press), in forcing the bearing onto the shaft, ensure that the race is never misaligned. The inner race should be checked with a feeler gauge to ensure it is right up against the shaft shoulder. Heat the bearing to expand it so that it can easily be placed in position and allowed to shrink to grip the shaft. To avoid damage to bearing shield and grease, care is to be taken that the temperature is not raised above 100 deg C. Force the bearing onto the shaft using equipment that can provide a steady, even load. Care is to be taken to avoid damage to the bearing and the shaft. 4. On grease lubricated pumps, pack the bearings with grease and pack the bearing cover cavity approximately one third full with grease.
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5. When fitting labyrinth thrower deflector the groove in the pump end bearing cover should be filled with grease and the labyrinth thrower positioned to give a clearance of 0.5 to 0.65 mm in front of the bearing cover. Screws for the drive end bearing cover should be tightened uniformly. 6. When refitting the shaft sleeve, ensure the joint or "0"-ring is correctly fitted. 7. The stuffing box should be packed with good quality packing, suitable for the liquid being handled. It is important to fit the lantern ring and the packing" seating ring in their correct positions to ensure that the lantern ring is situated in line with the gland seal connections. The packing scarf joints should be staggered by 90-180 degrees from the previous ring. 8. When replacing a mechanical seal, extreme cleanliness is required. The two sealing faces of the seal and the surface of the sleeve must be free from scratches and other damage. 9. Carefully press the stationary seat into the seal flange ensuring that the seat sealing ring is not deformed, that where an anti –rotation pin is fitted correct engagement with slot is achieved, and that the face is square to housing. 10. The rotating element (rotating face) should be carefully mounted onto the shaft sleeve ensuring that the sealing ring is not damaged. 11. Position the seal into the same position, remembering to check setting dimension that it originally occupied and tighten the drive screws in the seal drive collar. When refitting the seal plate (seal flange), check that the seal is compressed by the action of moving the seal plate into position. It should not be over compressed and locked up solid.
Renewable Rings (Wearing rings) When fitted, these should only be removed from the casing and the impeller when they are to be replaced. To decide if these rings should be replaced or not, you should check the wear ring clearance.
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Wear Ring Clearance Check the conditions of case wear ring, any signs for corrosion, wear due to rubbing. Check if it is fixed in its position or it become loose. Check the conditions of the impeller wear ring, any sings for corrosion, wear due to rubbing with case wear ring. Check its fixation to the impeller. Check the running clearance (wear ring clearance). Compare the observed values with the manufacturer given value. If the clearance is greater than the specified value, the wear rings (one of them or both) should be replaced. They should be prised out using levers behind them. Alternatively, careful drilling followed by chiseling to split the ring. Will facilitate removal. Replacement rings are to be pressed into position (the pump casing wear ring and impeller wear ring). Ensure entry is square to the recess.
Replacement of Oil Lip Seal 1. Oil lip seals, like mechanical seals, are not totally leak free devices. In mechanical seals, leakage is usually in the form of vapour and is not visible. Oil from a bearing housing does not evaporate and can be visually unpleasant in a clean pump room. 2. Very careful fitting practice for oil lip seals is therefore essential, otherwise oil leakage will be unacceptable in any environment. Particular attention must be paid to protecting the seal from keyways, by using shimming or tape and careful handling of the shaft to avoid even fine longitudinal scratches. 3. Perfectly assembled seals can have a leakage rate from almost zero to very small amount. This is the equivalent of approximately 2 drops per hour. At this leakage rate, the constant level oiler would need filling only every 6 months. 4. Refit coupling hub, which should be heated for fitting. It should not be knocked onto the shaft causing loading and damage to the bearings. 5. Rotate pump by hand to ensure there is no binding. 6. Refit pump to baseplate and check coupling alignment. 7. Replace all safety guards.
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3.10.7 Maintenance of Centrifugal Pump (in Between Bearings Pump) The required steps in this case will be as follow The steps which are given here are general guide lines. Before starting to do complete overhaul for this type of pumps read the manufacturer’s instruction. The manufacturer’s instruction should be followed when dismantling, inspection and assembling.
3.10.7.1 Dismantling the Pump In overhauling a pump of this types, it is unnecessary to disconnect the suction or discharge piping, or to move the pump, unless it is necessary to remove the case from the base plate. Necessary steps to dismantle the pump are: (Refer to figure 3-51 – on next page) 1. Open the case drain to drain off any liquid could be inside the pump case. 2. Disconnect the air – vent line. 3. Disconnect the mechanical seal piping system. 4. Put match marks on all mating parts to b sure that no mistakes could happen during reassemble. 5. Open the motor – to pump coupling and remove the spacer (or the spool) if spacer – type coupling is used. 6. Remove caps and top halves of bearings shells from inboard and outboard bearings. 7. Remove cap – screws holding glands to case. 8. Remove nuts from studs holding the upper and lower halves of the case together. 9. Use the jacking bolts to crack off the joint between upper and lower halves of the pump casing. 10.Lift off top half of the case, making certain that the shaft is not lifted, this may occur if the case wear – rings are extremely tight in their groves in the top half.
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11.Lift out the rotor of the pump – including shaft, impeller (s), mechanical seals and ball thrust bearing. 12.Procedure from this point depends almost entirely on the nature of the trouble and the extent of repairs or replacements.
3.10.7.2 Inspection Washing and cleaning is essential before start inspection of the parts. Remove the old gaskets (between upper and lower halves of the pump casing) and use oil – stone for cleaning these surfaces and to make it flat.
1- Rotor Inspection Check the straightness of the shaft on lathe machine – check any signs of corrosion or wear shaft sleeves: Corrosion or wear due to rubbing. Impellers: Any sings of corrosion,
2- Impeller Wear Rings Signs of corrosion, wear due to rubbing, wear ring clearance.
3- Thrust Bearings 4- Throat Bushings 5- Case Wear Rings Any signs of corrosion, wear due to rubbing check the grooves of wearing rings, throat bushing.
6- Pump Casing
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Figure 3-51 Typical 5 stage case
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3.10.7.3 Reassembling the Pump Before reassembly of the unit, all parts which have a machined fit – such as wear – rings, throat bushings, sleeves and bearings should be check separate for fitting clearance with the meeting part (or component). These clearances must be within the specified values. The pump is reassembly in reverse order to disassembly 1-
All gaskets and – O – rings must be replaced, use the same thicknesses and materials.
2-
For mechanical seals (if it is used), the seal faces must be re-lapped and all gaskets should be replaced. Assemble the mechanical seal on the pump shaft.
3-
Re-install the bearing brackets (bearing housing), inboard and outboard sides, install the duel pins for the bearing housing. Re-install the lower halves of the bearing shell on both sides.
4-
Assemble the rotor components. Check the spaces between different impellers.
5-
Lower the rotor in the lower half of the pump, wear rings must fit correct in its position – wear ring lock pins should fit correctly in its positions.
6-
Added some oil between the shaft and the bearings and rotate the shaft slowly. Check if there is any rubbing between the rotor and stationary parts. Make the required corrections.
7-
Put the gasket between lower and upper halves of the pump casing.
8-
Lower the top half of the pump casing. Be sure that it fit properly in its places. Use guide pins. Before seating the top half completely, insert the duel pins. Tight all nuts hand tight. Apply the required fighting torque as sequence given by the manufacture.
9-
Rotate the shaft slowly – to check any rubbing could happen. Check the shaft axial play. Make the required correction if it is needed.
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10- Added the lubricating oil in the bearing housing. Connect all piping either for pump venting or mechanical seal system. 11- Check the shaft alignment between the motor and the pump. Make the required corrections. 12- Re-assemble the coupling. Check the rotation of the system.
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SECTION – 3.11 RECIPROCATING PUMPS 3.11.1 How it Works A reciprocating pump is a positive – displacement machine, i.e, it traps a fixed amount (fixed volume) of liquid at near suction conditions, compresses it to discharge pressure, and pushes it to discharge nozzle. In a reciprocating pump, this is accomplished by the reciprocating motion of a piston, plunger or diaphragm.
3.11.2 Reasons for Using Reciprocating Pumps The justification for selecting a reciprocating pump instead of a centrifugal pumps must be: 1- The total cost of reciprocating pumps is less than centrifugal pumps (total cost including: Initial cost + maintenance cost + power consumption). 2- High efficiency (84% up to 94%). 3- Suitable for high pressures (10,000 Psig). 4- Suitable for abrasive and / or viscous slurries. 5- The reciprocating pump capacity is a function of speed.
3.11.3 Disadvantages of Reciprocating Pumps 1- Pulsating flow. Because of the pulsation, special consideration must be given to system design. 2- In most applications, the maintenance cost is greater than that in centrifugal pump. 3- The direct – acting pump has a low thermal efficiency when driven by a gas such as steam. Big improvement happen if a flywheel and the pump is driven by electric motor.
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3.11.4 Pump Classification Reciprocating pumps are usually classified by their features: Drive end, i.e. power (driven by electric motor) or direct –acting (figure 3-55) Orientation of centerline of the pumping element i.e. horizontal or vertical. Number of discharge strokes per cycle of the crank shaft (drive rod) i.e. single – acting or double – acting. Configuration of the pumping element, i.e. piston, plunger or diaphragm. Number of drive rods, i.e, simples, duplex or multiplex (also it refer to the number of cylinders)
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Number of cylinders
Term
1 2 3 4 5 6 7 8
Simplex Duplex Triplex Quadruplex Quintuplex Sextuplex Septuplex Nonuplex
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Figure 3-53 illustrates this classification in chart form
Figure 3-53 Classification of reciprocating pumps TriStar
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Figure 3-54 shows two examples of power reciprocating pumps use an electric motor for the drive.
Figure 3-54 Power pumps use an electric motor (single – acting horizontal pump) Figure 3-55 Shows direct acting – pump (on the next page)
Figure 3-55 Power pump use an electric motor TriStar
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The size of a power pump is normally designated by listing first the diameter of the plunger (or the piston), and second the length of the stroke. For example a pump designated as 2 x 3 has a plunger diameter 2 inches and a stroke length of 3 inches. For a direct – acting pump, the same convention is followed, except that the diameter of the drive piston precedes the liquid – end – element diameter. For example, a pump designated 6 x 4 x 6 has a drive – piston diameter of 6 inches, a liquid – piston diameter of 4 inches and a stroke length of 6 inches.
3.11.5 Liquid End Components The liquid end is that portion of the pump that does the pumping. Elements common to all reciprocating pump liquid ends are: 1234-
The liquid cylinder. Pumping element. Stuffing boxes and Valves
3.11.5.1 The Liquid Cylinder The liquid cylinder is the major pressure – retaining part of the liquid end, and forms the major portion of the pumping chamber. It usually contains or support all other liquid – end components. A piston pump is normally equipped with a replaceable liner (sleeve) that absorbs the wear from the piston rings. Because a plunger contacts only stuffing box components, plunger pumps do not require liners.
3.11.5.2 Pumping Element All reciprocating pumps contain one or more pumping elements (pistons, plungers or diaphragms) that reciprocate into and out of pumping chambers to produce the pumping action. A piston (figure 3.56) is a cylindrical disk, mounted on a rod, and usually contains some type of sealing rings (piston rings). Could be two rings or more.
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The number of piston rings and its dimensions, configuration and materials depends on the working conditions (fluid, pressure and temperature) and the cylinder size. The piston rings must be able to move freely in the piston ring grooves and the piston head assembly (piston head and the piston rings) must be able to move inside the cylinder with enough tighting pressure against the cylinder wall. The function of the piston rings is to stop the leakage between the piston head and the cylinder wall (or cylinder liner) during the pumping stroke (discharge stroke)
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Figure 3-56 Piston head, piston rod and piston rings
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A plunger (figure 3-57) is a smooth rod and, in its normal configuration, can only be single acting.
Figure 3-57 Plunger (in vertical plunger pump) A plunger must seal only in the stuffing box, and touches only the packing and possibly stuffing – box bushings.
3.11.5.3 Stuffing Boxes Sealing between the pumping chamber and atmosphere is accomplished in a stuffing box (or packing box). The stuffing box contains rings of packing that conform to and seal against stuffing box I.D. and the piston rod (or the plunger in case of plunger pumps). If a lubricant, sealing liquid or flushing liquid is injected into the center of the packing, a lantern ring or seal cage is required. The lantern ring provides an annular space between the packing rings so that the injected fluid can freely flow to the rod surface. TriStar
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Stuffing – box designs 1- Standard non – lubricated stuffing box The following figure (figure 3-58) shows this design
Figure 3-58 A Standard non – lubricated stuffing box
Figure 3-58 B Pressure gradient across the packing TriStar
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Application For cool water and fluids with comparable lubricity. Total packing length must be less than plunger stroke length to properly wet the last ring of packing with pumpage. 2- Standard lubricated stuffing box
Figure 3-59 Standard lubricated stuffing box Application Most of lubricant migrates into pumpage. Packing may be square, chevron. Suitable for non – lubricated liquids (less lubricity). Suitable for dirty liquids – contaminated liquids.
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3- Alternative lubricated stuffing box
Figure 3-60 Alternative lubricated stuffing box Application Puts lubricant under last ring – where it is needed most. Allows use of low – pressure and drip – type lubricators. Very little lubricant migrates into pumpage. Packing may be square, chevron. 4- Standard box used to bleed off pumpage
Figure 3-61 Standard box used to bleed off pumpage TriStar
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Application High friction causes excess heat. Short life of packing and plungers. Poor application – improper use of standard box. 5- Modified gland follower to allow bleed off
Less friction and lower temperature than unit in figure 3.61 Longer life of packing and plungers. Secondary packing cannot be adjusted to compensate for wear
Figure 3.62 Modified gland follower Application Less friction and lower temperature than unit in figure (3-61). Longer life of packing and plungers. Secondary packing can not be adjusted to compensate for wear. 6- Non lubricated, spring – loaded V-ring packing
Figure 3.63 Non lubricated spring – loaded V-ring packing TriStar
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Application Minimal leakage. Normally limited to intermittent duty. Self – adjusting. 7- Lubricated spring – loaded V-ring packing
Figure 3.64 Lubricated spring – loaded V-ring packing Application Good design, long life, minimal leakage. Puts lubricant under last ring – where it is needed most. Allows use of low – pressure and drip – type lubricators. Self – adjusting.
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8- Lubricated Two – Gland Stuffing Box
Figure 3.65 Lubricated two – gland stuffing box Application The old standard for high – pressure, critical. Provides independent adjustment of primary and secondary packing. (proper adjustment requires skilled mechanic) Full-size secondary packing. Positive packing lubrication. Long packing and plunger life. Negligible leakage to atmosphere. Excellent for volatile fluids.
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9- Tandem spring – loaded packing with bleed off and lubrication
Figure 3.66 Tandem spring - loaded packing Application The best design for most high – pressure critical services. Combines best features of two – gland box and spring – load packing. Negligible leakage, long life, self – adjusting packing. 10-
Glandless stuffing box with spring – loaded square packing
Figure 3.67 Glandless stuffing box TriStar
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Application Lubrication is optional
3.11.5.4 Valves The valves in a reciprocating pump are spring loaded non – return valves. It opened by the liquid differential pressure, and allow flow in only one direction. When the flow stoped the valve will close immediately. They have variety of shapes: 1- Stem – guided disk valve
Figure 3.68 stem – guided disk valve Application For general service and thin liquids. It is used for hot – water boiler feed and general service. It is made of bronze and although other alloys may be used. For lower temperatures and pressures, the valve disk may be made of rubber, which has the advantage of always making a tight seal with the valve seat.
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2- The wing – guided valve
Figure 3.69 Wing – guided valve Application Used for high pressures. The wings on the bottom of the valve to guide it in its seat. The beveled seating surfaces on the valve and seat tend to form a tighter seal than the flat seating surfaces on a disk valve. There is also less danger that a solid foreign particles in the fluid will be trapped between the seat and the valve. This type of valve is commonly made from a heat treated chrome – alloy – steel forging and cast hard bronze and other materials may be used.
Table 1 Material and service specifications TriStar
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3- Semi spherical valve
Figure 3.70 Semi spherical valve It is spring – loaded and can be operated at higher speeds than ball valves. One advantage of this type is it does not have obstructions to flow in valve seat (i.e. less P compared to all other types). Widely used for thick, viscous liquids also used for liquids with suspended solids because their rolling seating action prevents trapping of the solids between the seat and the valve. Materials and service specifications for pump liquid ends are given in table 2
Table 2 – Types of valves and their applications TriStar
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3.11.6 Drive end components The drive end of a power pump is called a power end (figure 3.71). Its function is to convert the rotating motion from a driver to reciprocating motion for the liquid end.
Figure 3-71 The power end in the power pumps Refer to the figure 3-71, the power from the motor to the driving pulley via V-belts. The driving pulley will drive the crank shaft which will drive the crosshead and piston rod (or the plunger). The main components of the power end is: 1- Power frame, which supports all other power – end parts and usually, the liquid end. 2- The crank shaft(sometimes, a cam shaft). The function of the crank shaft to work in conjunction with the connecting rod to drive the crosshead linear motion (reciprocating motion). The connecting rod is driven by the throw of the crankshaft on one end, and drives a crosshead on the other. The crosshead moves in pure reciprocating motion, the crankshaft in pure rotating motion. 3- The main bearings support the shaft in the power frame.
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Figure 3-72 Crankshaft and the main bearings
3.11.7 Flow Characteristics 1- As the pumping element (piston, plunger or diaphragm) is withdrawn from the pumping chamber, liquid within the chamber expands and the pressure decreases. Since most liquids are relatively incompressible, very little movement of the pumping element is required to decrease the pressure. 2- When the pressure decreases sufficiently below suction pressure, the differential pressure (i.e., suction pressure minus chamber pressure) pushes the suction valve open. The valve opens gradually and smoothly as the velocity of the element increases. Liquid then flows through the valve assembly and follows the element on its suction stroke. As the element decelerates near the end of this stroke, the suction valve gradually returns to its seat. When the pumping element stops, the suction valve closes. 3- The pumping element then reverses and starts on its discharge stroke. The liquid trapped in the pumping chamber is compressed until the chamber pressure exceeds discharge pressure by an amount sufficient to begin to push the discharge valve away from its seat. The action of the discharge valve is the same as that of the suction valve. TriStar
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4- The flow velocity for a simplex (single cylinder) double – acting, direct acting pump is shown in figure 3-73.
Figure 3-73 A flow velocities in double – acting, direct – acting reciprocating pumps Refer to figure 7-73 The velocity of liquid entering and leaving the pump falls to zero twice per pumping cycle. The pump accelerates quickly to maximum velocity, then maintains that velocity until decelerating near the end of the stroke.
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The resultant flow velocity for a duplex (two cylinders) double – acting, direct – acting pump is almost constant, as shown in figure 3-73 b. One cylinder starts before the other stops, and the resulting overlap (with properly set valves) results in a relatively smooth flow in both suction and discharge lines. 5- With a power pump, the velocity of the pumping element (piston, plunger or diaphragm) varies approximately as the sine of the angle of the crank throw. Since the velocity of liquid in the piping is proportional to plunger velocity, it can be plotted as a percent of average, as shown in figure 3-74.
Figure 3-74 Flow curves for reciprocating power pumps
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3.11.8 Power – Pump Drive Systems The most common driver for power pumps is: Electric motor. Turbines. Internal combustion engines. Power – pump speeds normally range from 20 to 500 r.p.m. Motor drivers for these pumps typically run at 1000 to 1800 rpm. Therefore, it is necessary to connect these two shafts (driver and driven) with some kind of speed reducer: Pumps below 100 H.P., the V-belt drive is popular. Pumps above 100 H.P., reduction gear box (step – down gear box) is popular. For small diaphragm pumps, variable – speed drivers are some times employed. For pumps 60 H.P. and above, variable speed is commonly obtained with hydraulic couplings and eddy – current clutches.
3.11.9 System Design System design is second only to speed in importance for obtaining a satisfactory installation. Improper design often results in a system that vibrates and is noisy. Pulsations may be severe enough to damage pump components and instrumentation. Field experience and information from the standards are condensed into a list of design guidelines for the (a) suction vessel, (b) suction piping, and (c) discharge piping.
3.11.9.1 The Suction Vessel Should Be large enough to provide sufficient retention time to allow for free gas to rise to the liquid surface. Have the feed and return lines enter below the minimum liquid level.
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Include a vortex breaker over the outlet (pump suction) line. Contain a weir plate to force gas bubbles toward the surface. The top of the weir must be sufficiently below the minimum tank level to avoid disturbance.
3.11.9.2 The Suction Piping Should Be as short and direct as possible. Be one or two pipe sizes larger than the pump suction connection. Have an average liquid velocity less than the values from the curves in figure 3-74. Contain a minimum number of turns, which should be long – radius elbows or laterals. Be designed to preclude the collection of vapor in the piping. (There should be no high points, unless vented. The reducer at the pump should be the eccentric type, installed with the flat side up). Be designed so that NPSHA, allowing for acceleration head, exceeds NPSHR. Include a suitable suction stabilizer, bottle or pulsation dampener that is located in the suction pipe and adjacent to the liquid end if the acceleration head is excessive. Contain a full-opening block valve so that flow to the pump is not restricted. Omit a strainer or filter unless regular maintenance is assured. (The starved condition resulting from a plugged strainer can cause more damage to the pump than that caused by solids.)
3.11.9.3 The Discharge Piping Should Be one or two pipe sizes larger than the pump discharge connection. Have an average velocity less than three times the maximum suction-line velocity. Contain a minimum number of turns, using long-radius elbows and laterals where practical.
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Include a suitable pulsation dampener (or provisions for adding one) adjacent to the pump's liquid end. Contain a relief valve, sized to pass full pump capacity at a pressure that does not exceed 110% of "cracking" pressure (i.e., the opening pressure of the relief valve). The discharge from the relief valve should be piped back to the suction vessel so that gases liberated through the valve are not fed back into the pump. Contain a bypass line and valve so that the pump may be started against negligible discharge pressure. Contain a check valve to prevent the imposition of system pressure on the pump during startup. The features of a good system as described here are shown in figure 3-75.
Figure 3.75 Good system design for reciprocating pumps
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3.11.10 Remedies for Low NPSHA In designing the suction system for a reciprocating pump, NPSHA may be found to be less than NPSHR. A number of remedies to overcome this problem are avail-able. To increase NPSHA: Enlarge the diameter of the suction line. Reduce the length of the suction line by providing a more direct route or by moving the pump closer to the suction vessel. Install a suction bottle, stablizer or dampener adjacent to the pump's liquid end. A fabricated bottle has often been successfully used at pressure below about 50 psig, but maintenance of a liquid level is required. A section of rubber hose in the suction line, adjacent to the pump, will often reduce the acceleration head. Elevate the suction vessel or the level of liquid in the suction vessel. Reduce temperature of pumpage. Reduce speed of the power pump, or select a larger pump running slower. At the lower speed, it may be possible to operate the pump with light suction-valve, "r springs, or none at all. If the above steps are insufficient, impractical or impossible, a booster pump must be supplied. A booster for a power pump is normally a centrifugal, although direct-acting reciprocating and rotary pumps are sometimes used. The NPSHR for the booster must be less than that available from the system. The head of the booster should exceed the power-pump NPSHR plus suction-line losses plus the acceleration head by at least 20%. The booster should be installed adjacent to the suction vessel, and a pulsation bottle or stabilizer should be installed adjacent to the power pump to protect the booster from pulsating flow.
3.11.11 Unloading the Pump Allowing the pump to be started in an unloaded condition by installing a bypass line yields numerous benefits: 1- The primary one is that all pumping chambers have an opportunity to become primed. Each pumping chamber of a reciprocating pump is an independent entity, operating in parallel with each other chamber.
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2- It is possible for only one of the chambers in a multiplex pump to be primed while all other chambers remain vapor -locked. A reciprocating pump tends to be self – priming. However, the large clearance volume, common to most reciprocating pumps, makes it impossible for the plunger to produce much of a discharge pressure when a chamber is filled with gas. Normally, a pump will be full of air after maintenance, and some chambers gain air that leaks through the packing during idle periods. Upon startup, gas is often draw into the pump from the suction line. For this reason, it is necessary that discharge pressure be kept low during the first half-minute or so of operation. This gives each pumping chamber an opportunity to clear itself of gas, and be operating fully primed when exposed to discharge pressure. With most systems, it is possible to keep the discharge pressure low only by having a bypass line. This line should not connect into the pump suction because the gases again would be ingested by the pump. Other benefits accrue from starting a power pump against negligible discharge pressure. The starting torque will be about 25% of full – load torque – allowing the use of a motor having a normal starting torque, and reducing the time of the inrush current. Also, couplings, V-belts, gears or chains will be lightly loaded. The power end of the pump has an opportunity to establish full lubrication films on all sliding surfaces, and the plungers have an opportunity become wetted by lubricant and / or pumpage. If a pump suddenly begins to run roughly or its capacity drops during normal operation, it is probable that gas has been ingested by one or more pumping chambers. If the system contains a bypass line (as illustrated in figure 3.75, it is only necessary to open the bypass valve until the pump clears itself. If a pump continues to be plagued by gas, the source of the gas should be identified and eliminated. If it is made absolutely necessary by process requirements or emergency conditions for a power pump to be started against system pressure, it is recommended that:
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1. The driver starting-torque capability exceed running torque by at least 50%. 2. Drive-train components be suitable for the driver starting – torque. 3. The pump not be allowed to sit idle for more than 10 h, or provisions be made for prelubricating power-end bearings, and. 4. Pumping chambers be kept primed.
3.11.12 Slurry Applications The standard reciprocating pump is not designed to handle slurries. Modification to standard designs and, in some cases, special designs are required to achieve satisfactory operation and component life. There are three weak points (areas) in reciprocating pumps: 1- Stuffing box area (packing) 2- Valves 3- Piston rings in case of reciprocating piston pumps.
3.11.12.1 Stuffing Box Area (Packing) To achieve satisfactory packing and plunger life, abrasive slurry must be prevented from entering the packing. Methods to achieve this include: A wiper ring between the pumpage and packing. A long – throat bushing, injection of a clean liquid into the throat area for flushing. Insertion of a diaphragm or floating piston between the plunger and the pumpage.
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3.11.12.2 Pump Valves Special pump valves are often required for slurries. Depending on the nature of the solids, they can be ball, bell, bevel seat with elastomers insert, wing – guided type with reduced seating area, or disk with special seating surfaces.
3.11.12.3 Plunger or Piston Rod (in Case of Piston Pump) Presence of abrasive particles in the liquid being pumped will cause rapid damage in the external surface of the plunger (or the piston rod). These solid abrasive particles impede in the packing rings and acts as cutting tool against the moving item (plunger or piston rod). By utilizing of one of these three methods which are covered in 3.11.12.1 we can provide suitable protection.
3.11.13 Reciprocating Pump Maintenance Maintenance problems in this type happen in two main areas:
Liquid end 1234-
Valves. Packing & stuffing box. Plunger and piston rod (in case of piston pumps). Cylinder (or cylinder liner) in piston pumps.
Drive – end 12345-
Crosshead. Connecting rod. Crank shaft. Bearings. Power Frame.
3.11.13.1 Liquid End Components Maintenance 1- Valves There major problems could happen in the valves 1- Bad contact between the disk and the seat. This could be due to: Corrosion in the disk or the seat due to wrong selection of materials. TriStar
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Errosion in the disk or the seat due to high flow speed. This could be due to wrong selection of materials or changed working conditions. Presence of solid particles which will cause damage in both disk and the seat. 2- Broken spring (s) due to corrosion, or spring stiffness may change. 3- Plockaged spring due to presence of foreign materials in the liquid being pumped.
2- Packing & stuffing Box Short packing life can result from any of the following conditions: 1- Improper packing for the application. 2- Insufficient lubrication. 3- Misalignment of plunger (or piston rod) with stuffing box. 4- Worn plunger, piston rod, stuffing box bore or stuffing box bushings. 5- Packing gland too tight or too loose. 6- High speed or high pressure than the prameters which was given to the manufacturer. It could happen due to change in the working conditions. 7- High or low temperature of pumpage. 8- Excessive friction (too much packing in box). 9- Packing running dry (pumping chamber gas bound). 10- Shock conditions arising from entrained gas or cavitation, broken or faulty valve springs, or system problems. 11- Solids from the pumpage, environment or lubricant. 12- Improper packing installation or break – in (where required). 13- Icing caused by volatile liquids that refrigate and form ice crystals upon leakage to atmosphere, or by pumping liquids at temperatures below zero degree (32ºF).
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3- Plunger and Piston Rod After the packing, the plunger is the components of a power pump that requires the most frequent replacement. The high speed of the plunger and the friction load of the packing tend to wear the plunger’s surface. For longer life, plungers are sometimes hardened. A more popular method is to apply a hard coating to the plunger surface. Such coatings are of chrome, various ceramics, nickel – based alloys, or cobalt – based alloys. Desired features for the coatings include hardness, smoothness, high bond strength, corrosion resistance, and low cost.
4- Cylinder (or cylinder liners) in piston pumps Due to continues friction between the piston rings and the cylinder, wear will happen in both piston rings and the cylinder. A piston pump is normally equipped with a replaceable liner that absorbs the wear from the piston rings.
3.11.13.2 Drive End Components Maintenance The following figure (figure 3-76) shows the main components of the drive end..
Figure 3-76 Drive end (power End), horizontal pump
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Drive end compoents includes: 12345-
Crosshead Connecting rod Crankshaft Bearings Power fram
The most common problems related to these components are: 1- Normal wear due to rubbing between the moving components. 2- Up – normal wear due to presence of solide particles in the lubricating oil. 3- Fatigue failure (bearing failure).
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SECTION – 3.12 PULSATION DAMPENERS 3.12.1 The Function of Pulsation Dampeners Multi-cylinders arrangement is used with plunger pump and piston pumps to minimize the pressure pulsation. Pressure pulsation dampeners are used beside multi – cylinder arrangement to smooth out the pressure pulsation. It is installed on the discharge line of these pumps.
3.12.2 Installation 3.12.2.1 Mounting Pulsation dampeners (see figure 3.77) are mounted on standard ANSI and API flanges which can met with ring joint or raised-face connections. Caution: Do not attempt to precharge the dampener prior to mounting the unit onto the piping system.
Figure 3.77 Pulsation Dampener
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3.12.2.2 Pecharging The correct precharge is vital for maximising the efficiency of the pulsation dampener. Generally, precharge pressure is based on the average operating pressure of the system. Other system parameters can affect the precharge pressure.
3.12.3 General Precharging Instructions For normal operating conditions, set the precharge pressure at 50 to 60 percent of the average operating system pressure or at 2000 psig, whichever is lower. The precharge pressure can be as low as 30 percent or as high as 75 percent of system operating pressure. However, these precharge settings may shorten diaphragm life.
Suction Dampeners All units are shipped with zero psig nitrogen precharge. Generally, unless the suction system is being pressurised by a precharge pump, no additional precharging of a suction dampener is required. When using a precharge pump, set the precharge to approximately, but no more than, 50 percent of the available suction pressure.
Precharge Procedure Precharging pulsation dampener requires standard hand tools, a charging hose assembly and a torque tool capable of applying 120-150 lb-in torque. 1. Unscrew the lifting eye counter clockwise in order to expose the charging valve. 2. Connect the nitrogen source to the charging valve. 3. Open the charging valve by turning the top hex nut 3 to 4 full turns counter clockwise (see 3.78). 4. Install nitrogen precharge. 5. After precharge has been installed, close the charging valve and tighten it using 120-150 lb-in torque. 6. Remove the nitrogen source and install the lift eye. TriStar
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Figure 3.78 Pulsation Dampener Precharging
3.12.4 Maintenance 3.12.4.1 Precharge The pulsation dampener is virtually maintenance-free if properly installed and precharged. However, the vessel should be checked periodically for proper precharge and also for leaks. Unless conditions indicate otherwise, checking the unit every six months should be sufficient. Remember: The precharge is based on the average pressure in the system. If this pressure changes, adjust the precharge.
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3.12.4.2 Troubleshooting The fault analysis tree is shown in figure 3.79
Figure 3.79 Pulsation dampener fault analysis tree
3.12.4.3 Diaphragm Removal 1. Remove all pressure from the pulsation dampener. System pressure and precharge pressure must be zero psig before removing the pulsation dampener from the line. To remove precharge pressure, open the charging valve and bleed it off. Warning: Stand clear of escaping pressure to avoid injury. 2. Remove the pulsation dampener from the line. 3. Remove the jam nuts from the studs or the cap screws if applicable. 4. Remove the bottom plate by prying it loose with a medium size screw driver or pry bar. 5. Take care not to damage the sealing surfaces in the throat of the dampener body. TriStar
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6. Remove the diaphragm using one of the two following ways. a.
Cut around the metal insert and remove the top portion of the diaphragm. Remove the remaining portion by folding and pulling it out.
b.
Screw an eyebolt into the diaphragm insert. Invert the diaphragm with a crowbar or a similar tool.
3.12.4.4 Diaphragm Installation The pulsation dampener assembly is shown in figure 3.80 while the charging valve assembly is shown in figure 3.81 1. Clean all interior surfaces of the vessel paying particular attention to all sealing surfaces. Remove all scale and/or rust around the mouth with fine sandpaper if necessary. 2. Clean and inspect the bottom plate for damage and corrosion. Replace or repair if doubtful of the sealing capability of the bottom plate. Remove all scale and/or rust with fine sandpaper if necessary. 3. Lubricate the bottom plate and all interior surfaces of the vessel with a castor oil or a similar device. 4. Take the new diaphragm in the rest position and roll one side toward the other forming a crude football shape. A thin belt or strap may be helpful in holding this shape. 5. Apply lubricant to the diaphragm and insert it long ways into the body of the vessel as far as possible (the diaphragm should go over half way into the body). Fold the remainder of the diaphragm into the vessel and allow it to open inside the body. Position the diaphragm within the neck of the vessel. 6. If the vessel is equipped with a Teflon backup ring, install it over the bottom plate. Insert the bottom plate into the diaphragm neck taking particular precaution not to push the diaphragm back into the body. 7. Thread the jam nuts over each stud and tighten the nuts until the bottom plate is snug against the body. 8. Clean and inspect the mounting flange. Repair if necessary. TriStar
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Figure 3.80 Pulsation dampener assembly
Figure 3.81 Charging valve assembly
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9. Install a new gasket and mount the dampener. Secure flange studs and nuts. 10.Remove, clean and inspect the charging valve and the O-ring seal. Replace valve if damaged or worn enough to be doubtful of its ability to seal at either the O-ring or the stem. 11 Clean and inspect the charging valve adapter on the body. Remove all scale or rust with fine sandpaper if necessary. Install the charging valve using thread lubricant. Caution: Do not use Teflon tape! 12 Tighten the lower hex nut on the charging valve using 50 to 60 lb-ft torque. 13. Precharge the dampener with nitrogen only. Proper precharging is vital for maximising the efficiency and life of the diaphragm.
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SECTION – 3.13 DIAPHRAGM METERING PUMPS Are Positive Displacement Reciprocating Pumps 3.13.1 How it Works? 1- A diaphragm metering pumps is a reciprocating power pump that displaces a predetermined amount (volume) of liquid in a specific period. This pump is also known as a controlled volume, proportioning or chemical – injection pump. 2- The pump is usually driven from an outside source, which may be at a constant or variable speed. 3- The pump contains a flexible diaphragm, one side of which is in direct contact with the process liquid (liquid being pumped). The diaphragm can be driven hydraulically or mechanically (figures 3.82, 3.83).
Figure 3.82 Hydraulically actuated diaphragm
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Figure 3.83 Mechanical actuated diaphragm 4- The pump contains a mechanism for changing the effective displacement. 5- Refer to figure 3.82 (hydraulically actuated diaphragm): The plunger perform pumping for intermediate fluid (hydraulic fluid). This intermediate fluid (hydraulic fluid) makes the flexible diaphragm moves back and forth. This movement of flexible diaphragm perform pumping for the liquid being pumped. The flexible diaphragm makes complete separation between the two fluids. The portion of the pump where the flexible diaphragm moves back and forth and seal the liquid being pumped is called liquid end assembly. Liquid end assembly include all parts that contain, or are otherwise in contact with, the liquid being pumped. The liquid head assembly includes: Diaphragm (s). Diaphragm – displacement chamber. Suction and discharge check valves. (figure 3.82 on the next page). 6- Refer to figure 3.83 mechanically actuated diaphragm, the flexible diaphragm is direct driven by the diaphragm push rod.
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In this design the amount of pumpage liquid can be controlled by either the length of the stroke of the diaphragm push rod or the number of strokes, i.e. the speed.
3.13.2 Areas of Applications Diaphragm metering pumps are selected for applications in which one or more of the following characteristics are important: 1- Leakage or cross-contamination between the pumpage and other fluids must be prevented. 2- Flow must be insensitive to variations in discharge pressure. 3- High accuracy is required for controlling the out put capacity. 4- Pumping chamber must be remotely located from the rest of the pump. 1- Leakage Zero – leakage and cross – contamination are major considerations when pumping liquids that are pure, sterile, toxic, carcinogenic, radioactive, precisely formulated, corrosive, flammable, pyrophoric, explosive, oxidizing or hydroreactive. For such materials, pumps having dynamic seals offer the possibility, and frequently the need, of leakage to assure lubrication of the seal. This leakage requirement of dynamic seals can be avoided through the use of canned, magnetic – drive and reciprocating – diaphragm pumps. 2- Stiffness The average flow must remain constant when the system pressure changes Figure 3.84 shows how the capacity varies for typical centrifugal, rotary and reciprocating pumps against a given system – head curve. When the system head changes, there will be a change in pump capacity. For example, a 10-gpm pump, with a discharge – pressure capability of 100 psi could have the following changes in capacity for a 10-psi change in discharge pressure: Pump type
Rate of change
Reciprocating Rotary Centrifugal TriStar
0.01 gpm / 10 psi 0.3 gpm / 10 psi 3 gpm / 10 psi
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Figure 3.84 Senstivity to changes in capacity for different types of pumps These data indicate that the reciprocating pump is the “stiffest” pump, i.e. has the least sensitivity for changes in capacity when the discharge pressure varies. The major reason is that the leakage past the check valves and plunger seals of reciprocating pumps is comparatively low. 3- Accuracy The third reason for selecting a diaphragm metering pump is the ease with which capacity can be accurately adjusted to meet the required conditions. An example of this capability is shown in figure 3.84. Note that the characteristic curve does not pass through zero. Turndown ratio, linearity, steady – state accuracy and flow repeatability are the features that comprise the overall accuracy of the pump. Let us define these terms: Turndown ratio is the rated capacity divided by the minimum capacity that can be obtained while maintaining the specified flow repeatability, steady – state accuracy and linearity. A typical value is 10:1.
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Linearity is the maximum deviation from the ideal straight line that can be drawn through plotted calibration test points that describe how flow is varying with the capacity setting. This deviation is expressed as a perecentage of the rated capacity. A typical value is 1%. Steady state accuracy is the flow variation expressed as a percentage of rated capacity under fixed system conditions. Steady state accuracy applies throughout the turndown ratios. A typical value is 1%. In some specifications, this defined as a percentage of the mean delivered flow instead of rated capacity. Flow repeatability, expressed as a percent of rated capacity, describes the reproducibility of a pump flowrate under a given set of conditions when the capacity setting is varied and then returned to the set point being tested. A typical value is 1%.
3.13.3 Pump Components Liquid end assembly, valves and stroke – adjustment mechanism are major elements in diaphragm metering pump. Figure 3.85 shows the pump components
Pump Components The pump components can be classified into two groups: Drive end or power end and. Liquid end.
3.13.3.1 Drive End Include 1- The input shaft (from the electric motor). 2- Worm and worm wheel (as speed reduction system) – to reduce the speed (step down gear system) 3- Adjustment – arm assembly – to adjust the length of the stroke of the piston on the fluid end side in order to control the amount of hydraulic fluid which is used to operate the diaphragm. 4- Crosshead. 5- Hydraulic fluid cylinder & hydraulic fluid piston to pump the hydraulic fluid which is used to operate the diaphragm. 6- Pump head assembly. TriStar
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3.13.3.2 Liquid End Assembly (Figure 3.85) The liquid end assembly includes all parts that contain, or are otherwise in contact with, the liquid being pumped. It include the following components:
Diaphragm (s) Diaphragms provide isolation and transmit hydraulic motion from one liquid to another liquid on the other side (liquid being pumped). Diaphragms must withstand maximum flexing, with stresses below the materials endurance limit and should be of sufficient thickness and density to prevent permeation. On both sides of the diaphragm there are dish plates to prevent excessive flexure. The diaphragm moves within these two dish plates back and forth to perform pumping.
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Figure 3.85 Diaphragm pump components
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Valves Refer to figure 3.86 the following valves are widely used in this type of pumps.
Figure 3.86 Check valves for suction and discharge lines These are needed on the suction and discharge strokes of metering pumps. One check valve permits liquid to flow into the liquid end head from the liquid supply on the suction stroke of the plunger or piston. The other valve directs the liquid out of the liquid head into the discharge system on the discharge stroke.
3.13.4 Capacity Controls There are five or more methods commonly used to adjust the capacity of metering pumps. The choice of these is determined by the application for which the pump is intended.
1. Manually Adjustable While Stopped This control is generally found on packed plunger pumps of conventional design. Capacity changes are effected by moving the crankpin in or out of the crank arm while the pump is not in motion. This is the least expensive method and is used where frequent changes in pump displacement are not required. TriStar
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2. Manually Adjustable While Running This feature is most frequently found on mechanically actuated diaphragm pumps, where it is accomplished by limiting the return stroke of the diaphragm with a micrometer screw. On packed – plunger pumps, stroke adjustment while running is relatively complicated. Stroke length is set by some type of adjustable pivot, compound linkage, or tilting plate which is manually positioned by turning a calibrated screw. On hydraulically actuated diaphragm pumps, relatively simple control is provided by manually adjustable valving which changes the amount of the intermediate liquid bypasses at each stroke.
3. Pneumatic Where metering pumps are utilized in continuous processes, it is necessary that they be controlled automatically. In pneumatic systems the standard 3 to 15 Psi gage air signal is utilized to actuate air cylinders or diaphragms directly connected to the stroke – adjusting mechanism.
4. Electric On electric – control systems, stroke adjustment is through electric servos which actuate the mechanical stroke – adjusting mechanism. These accept standard electronic control signals.
5. Variable Speed This method of adjusting capacity is achieved by driving a reciprocating pump with a variable – speed prime mover. Since it is necessary to reduce stroke rate to reduce delivery, discharge pulses are widely spaced when the pump is turning slowly. Surge chambers or holdup tanks are used when this factor is objectionable. In control situations involving pH and chlorinization, two variables exist at once, e.g., flow rate and chemical demand. This is easily handled by a metering pump driven by a variable speed prime mover. Flow rate can be adjusted by changing the speed of the pump, and chemical demand by changing its displacement.
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3.13.5 Service and Maintenance Proportioning pumps utilizing manual capacity controls can be installed, serviced, and operated by plant personnel. Pneumatic capacity controls are more sophisticated, but once their construction and function are understood, maintenance and service should become routine. Electric capacity controls require a basic understanding of electric circuits for installation, operation, and routine service. Modular construction allows repair service by replacement of component groups, thus diminishing the task of trouble shooting. All proportioning pumps utilize suction and discharge check valves. These require regular maintenance and service since they experience high – frequency operation, encounter corrosion, and must be kept in good working order to ensure accurate pump delivery. Service periods are greatly dependent on liquids pumped, pump operating speed and daily running time. Usual service periods run from 30 days to 6 months or longer. Check valves are designed to facilitate service with a minimum of down time, thus allowing replacement of wearing parts at minimum expense. Good design allows this service to be accomplished without breaking pipe connections to the pump. Packed – plunger pumps require periodic adjustments of the packing take-up device to compensate for packing and plunger wear. Packing has to be replaced periodically, and regular lubrication of bearings and wear points is required. Speed-reduction gearings, either separate integral units or built-in type, requires changing of the lubricating oil at six – month intervals. Diaphragm pumps usually require replacement of the diaphragm as part of routine service at six – month intervals. Hydraulically actuated diaphragm pumps also require changing of the hydraulic fluid. Pneumatic and electric controls generally present no special maintenance problems. The frequency of routine cleaning and lubrication is dependent on environmental conditions and should be consistent with general plant maintenance procedures.
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3.13.6 Installation Proper installation of metering pumps is very important if reliable pump operation is to be obtained. NPSH must be kept as high as possible, and manufacturers’ recommendations as to pipe size and length, strainers, relief valves, and bypasses must be observed.
3.13.7 Materials of Construction For the great majority of common chemicals, pump manufacturers publish data on materials of construction. In general, packed-plunger and hydraulically actuated diaphragm pumps are available as standard construction in mild steel, cast or ductile iron, stainless steel, and plastic. Mechanical actuated diaphragm pumps (figure 3.83 page 3.116) are usually available as standard construction in plastic and stainless steel. Almost any combination of materials can be furnished on special order. Diaphragms are available as standard construction in Teflon, chemically resistant elastomers, and stainless steel from various manufacturers.
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