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Technical Training Programme

Rotating Equipment

CHAPTER 5 RECIPROCATING COMPRESSORS

TriStar

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CHAPTER 5 RECIPROCATING COMPRESSORS CONTENTS

Page Number

SECTION - 5.1 Reciprocating Compressor Working Principle & Components 5.1.1 Compressor component ……………………………………………... 5.1.2 How does it work ……………………………………………………. 5.1.3 Compressor cycle in the pressure volume diagram …………………. 5.1.4 Single acting compressor and double acting ………………………… 5.1.5 Trunk compressor and cross – head compressor ……………………. 5.1.5.1 Trunk design ………………………………………………… 5.1.5.2 Crosshead design …………………………………………….

5 6 7 8 9 9 10

SECTION - 5.2 Reciprocating Compressor Component 5.2.1 Drive end group …………………………………………………….. 5.2.2 Compression cylinder group ……………………………………….. 5.2.2.1 Cylinder & cylinder liner …………………………………… 5.2.2.2 Valves and unloading system ………………………………. 5.2.2.3 The piston and piston rings ………………………………… 5.2.2.4 Piston rod and piston rod packing ………………………….

11 13 13 14 16 18

SECTION - 5.3 Reciprocating Compressor Cooling Systems 5.3.1 Cylinder cooling ……………………………………………………. 5.3.2 Cooling methods …………………………………………………… 5.3.2.1 Air cooling method ………………………………………… 5.3.2.2 Water cooling method ………………………………………

20 20 21 21

SECTION - 5.4 Multi – Stage Compression 5.4.1 Theory of multi – staging ………………………………………….. 5.4.2 Advantages of multi – staging ……………………………………... 5.4.3 Cooling systems for gas between stages and after the last stage ……

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SECTION - 5.5 Reciprocating Compressor Lubrication 5.5.1 Function of lubricants ………………………………………………. 5.5.2 Methods of lubricant distribution …………………………………… 5.5.2.1 Forced circulation system …………………………………... 5.5.2.2 Splash lubrication system …………………………………...

24 24 25 25

SECTION - 5.6 Classification of Reciprocating Compressors 5.6.1 Classification of compressors into single or double acting ………… 5.6.2 Classification into single stage or multi – stage …………………… 5.6.3 Classification into trunk type and compressor with crosshead …….. 5.6.4 Classification with respect to cylinder layout ……………………….

27 28 28 29

SECTION - 5.7 Prim Movers for Reciprocating Compressors 5.7.1 Function of a driver …………………………………………………. 5.7.2 Driver selection ……………………………………………………… 5.7.2.1 Electric motor driven compressors …………………………. 5.7.2.2 Internal combustion engine driven compressors …………… 5.7.2.3 Reciprocating steam engine and steam turbine driven compressors ………………………………………………… 5.7.2.4 Gas turbine driven compressors …………………………….

30 30 30 31 32 32

SECTION - 5.8 Reciprocating Compressor Operation 5.8.1 Starting a new reciprocating compressor ……………………………. 5.8.2 Cleanliness of the suction line ………………………………………. 5.8.3 Break in period ……………………………………………………… 5.8.4 Preparation for extended shut down ………………………………… 5.8.5 Routine checks ……………………………………………………… 5.8.6 Air cooled compressor routine checks ……………………………… 5.8.7 Troubleshooting ……………………………………………………..

33 35 37 38 39 40 43

SECTION - 5.9 Reciprocating Compressor Maintenance 5.9.1 Strip down the compressor …………………………………………. 5.9.2 Piston rod maintenance …………………………………………….. 5.9.3 Cylinder & cylinder liner maintenance (inclued water jackets) …… 5.9.4 Reciprocating compressor valves maintenance …………………… 5.9.5 Driving end maintenance ………………………………………….. 5.9.6 Inspection and maintenance of auxiliary equipment ………………. TriStar

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CHAPTER 5 RECIPROCATING COMPRESSORS Introduction Chapter 4 was an overview of different types of compressors and centrifugal compressors. In this chapter further details of reciprocating compressors will be covered. This chapter is nine sections, they are: 1- Reciprocating compressor working principle. 2- Reciprocating compressor components. 3- Reciprocating compressor cooling systems. 4- Multi – stage compression. 5- Reciprocating compressors lubrication. 6- Classification of reciprocating compressors. 7- Prime movers for reciprocating compressors. 8- Reciprocating compressor operation. 9- Reciprocating compressor maintenance.

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SECTION – 5.1 RECIPROCATING COMPRESSOR WORKING PRINCIPLE & COMPONENTS 5.1.1 Compressor Component The basic reciprocating compressor consists of: 12345-

Piston (s). Piston rings. Cylinder (s) Valves (suction valve & discharge valve). Driving mechanism (crank shaft, connecting rod, cross head and piston rod). 6- Suitable frame. Figure 5.1 shows these components

Figure 5.1 Component of reciprocating compressor

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5.1.2 How Does it Work Refer to figure 5.2

Figure 5.2 Compressor cycle

1- Suction Stroke 1- The piston moves from position (A) to position (B). This create vacuum inside the cylinder. The differential pressure exists across the suction valve (inlet valve). Inlet valve open. 2- Gas (or air) is drawn into the cylinder via (through) the suction valve. In the suction stroke the suction valve is open, the discharge valve is closed, the piston movement from (A) to (B). 3- When the piston reach position (B) it stop before change the direction of movement from (B) towards (A). At this moment both valves (suction and discharge) are closed.

2- Discharge Stroke 1- The gas is trapped in the cylinder. 2- The piston moves from (B) to (A). The volume of gas decrease and its pressure increases and the temperature also. TriStar

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3- When the gas pressure inside the cylinder becomes higher than the pressure of the gas in the discharge manifold, the discharge valve open and the gas passes through the discharge valve to outside the cylinder. 4- The piston keep going pushes the gas outside the cylinder until it reaches again position (A). When the piston reaches position (A), it completes one complete cycle.

5.1.3 Compressor Cycle in the Pressure Volume Diagram P-V diagram (figure 5-3) represent this compressor cycle:

Figure 5.3 P-V Diagram 1- At point (1) the piston is at extreme position towards the valves. 2- From point (1) to point (2) on P-V diagram – there is vacuum inside the cylinder but the suction valve still closed. 3- At point (2) the pressure across the suction valve become enough to open the valve. The valve open and the gas (or air) enters the cylinder and keep enter to point (3).

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4- At point (3) both valves (suction & discharge) are closed, the compression stroke from point (3) to point (4). Until point (4), the compression stroke still going. 5- At point (4) the discharge valve opens, the gas leaves the cylinder via discharge valve and the piston keep going pushing away the gases to point. At point (1) the compressor complete one complete cycle. The area 1 2 3 4 represent the power consumed to perform compression.

5.1.4 Single Acting Compressor and Double Acting Compressor The major difference between single acting and double acting compressors is how many compression strokes per one crank shaft revolution. Refer to figure 5-4: In single acting compressors the suction valve (s) and discharge valve (s) on one side of the piston head (figure 5-4-a). In double acting compressor the suction valve (s) and discharge valve (s) on both sides of the piston head (figure 5-4-b) i.e one valve set on the head end, one valve set on crank end.

(a)

(b)

Figure 5.4 Single acting and double acting compressors

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Comparison between single acting compressors and double acting Single acting compressor

S. No.

1

Number of valves

2

Number of compression strokes in one crank shaft revolution

Double acting compressor One set of valves on One set of valves each side of the piston (suction valve (s) and head. discharge valve (s)) on (one set on head end one side of the piston & one set on crank head (head end) end.) one

Two

3

The amount of gas to be pumped in one complete revolution.

once

Twice single acting for the same cylinder size and the same stroke.

4

Power consumption per unit volume

Higher

Less

Mechanical construction

No cross head (single acting compressors the piston is connected directly to the crank shaft using connecting rod)

Needs cross head, piston rod, and piston rod packing.

5

5.1.5 Trunk Compressors and Crosshead Compressors Reciprocating compressor can be classified into: 1- Trunk design (figure 5.4 a) 2- Crosshead design (figure 5.4 b)

5.1.5.1 Trunk Design In this design the piston head is connected direct to the connecting rod to the crank shaft. This design is always single acting it never be double acting.

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This design is common used in the following cases: 1234-

Single acting compressors. Small sizes. Lubricated cylinder. High speed compressors.

5.1.5.2 Crosshead Design In this design the piston is connected to piston rod. Piston rod is fixed with the crosshead which is driven by connecting rod and crank shaft (figure 5.4 b). This design is common used in the following cases: 1234-

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Double acting compressors. Big size compressor. Oil free compressors (the compression cylinder does not lubricated). Medium and low speed compressors.

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SECTION – 5.2 RECIPROCATING COMPRESSOR COMPONENT In this section further details of reciprocating compressor components will be covered. The common parts of all reciprocating compressors are grouped into two groups: 1- Drive end group. 2- Compression cylinder group.

Drive end group (some times called crank mechanism) consists of: Crank shaft & crank shaft bearings Connecting rod Crosshead.

Compression cylinder group consists of: Cylinder & cylinder liner. Valves (suction valve & discharge valve). Piston & piston rings. Piston rod & piston rod packing.

5.2.1 Drive End Group (Crank Mechanism) Drive end group changes the rotary motion of the crank shaft into reciprocating motion of the piston (trunk design) or reciprocating motion of cross – head (cross – head design). The prime mover drive the crank shaft to rotate. The crank shaft, connecting rod and crosshead change this rotary motion into reciprocating motion (figure 5.5)

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Figure 5.5 Rotating and oscillating components

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5.2.2 Compression Cylinder Group

Figure 5.6 Cylinder assembly The piston moves back and forth inside the cylinder to perform compression The compression cylinder consists of: 1234-

Cylinder and the cylinder liner. Valves & un loading system. The piston and piston rings. Piston rod and piston rod packing.

5.2.2.1 Cylinder & Cylinder Liner The piston moves in reciprocating motion inside the cylinder. Cylinder liner are inserted into the main cylinder body to make repair easir. If wear happen in the cylinder liner due to piston head movement, it is cheaper to replace the liner than replacing the cylinder. TriStar

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The liners are pressed or shrunk into place. It must be sufficiently thick enough to withstand the pressure load during the compression stroke.

5.2.2.2 Valves and Unloading System  What is the function of compressor valves? Compressor valves are a mechanical devices placed in the cylinder to permit one way flow of gas either into or out of the cylinder.

 How does it work? To understand how does it work, let us first look at the valve components. Figure 5.7 shows the valve basic components. The compressor valve requires four basic items to do its job. They are:

Figure 5.7 Valve basic components TriStar

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Valve seat. Moving sealing element (valve disc). Certain sort of spring force. A cage (or guard) to contain the travel of the moving sealing elements.

These four basic components are essential components. Beside these components, there are some other to improve or to assist these basic components to perform their functions properly. These valves are opened by difference in pressure across the valve; no positive mechanical device is involved. Springs assist closing.

 Demands on a Compressor Valve 1- The valves are installed in the compressor directly in the gas (or air) stream. That means the selection of valve component materials is critical and very important. 2- The pressure drop across the valve on both strokes – suction and discharge strokes is another parameter which should be taken into consideration. A good valve should be able to perform the required job with minimum pressure drop. 3- Low mass of the moving parts for low impact force. 4- Quick response to low differential pressure. 5- Small outside dimensions to allow low clearance volume. 6- Low noise level. 7- Tightness in closed position. 8- Ease of maintenance and service.

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Valve Designs There are several designes are commonly used to day. The most common types are:    

Plate valve (figure 5-7) Channel valve. Poppet valve (figure 5-8) Reeds

Figure 5.8 Puppet valves

5.2.2.3 The Piston and Piston Rings Piston design and materials depends upon the working conditions like speed pressure, size, and the gas being compressed.

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Piston rings provide a seal that prevent or minimize leakage between the piston and the liner. During operation, the pressure of the gas between the piston ring and the piston pushes the piston ring against the wall of the cylinder (liner). In some other designs, there is a spring ring between the piston and the piston ring to keep the contact between the piston ring and the wall of the cylinder. The function of rider rings (or guide rings) are to carry the weight of the piston (in horizontal cylinders) and to guide the piston during its movement (in vertical cylinders). In non-lubricated compressors (oil – free compressors) the piston rings and rider rings are made from low friction materials like Teflon (figure 5.9)

Figure 5.9 Piston rings & rider rings

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Figure 5-10 is an exploded view of non lubricated piston

Figure 5.10 Exploded view of non lubricated piston & piston rings

5.2.2.4 Piston Rod and Piston Rod Packing The piston rod connects the piston to the crosshead and transmits the piston force (figure 5.11) The Packings and oil wipers slides on the hardened piston rod surface.

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Figure 5.11 Piston rod packing

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SECTION – 5.3 RECIPROCATING COMPRESSOR COOLING SYSTEMS 5.3.1 Cylinder Cooling Heat in a cylinder comes from: 1- Compression of the gas being compressed. 2- Friction of piston and piston rings on cylinder wall, and the rod packing on the rod. Heat can be considerable, particularly when moderate and high compression ratios are involved. This heat must be removed for the following reasons: 1- Lowering cylinder wall and cylinder head temperature reduces losses in capacity and horse power per unit volume due to suction gas preheating during inlet stroke. 2- Reducing cylinder wall and cylinder head temperature will remove more heat from the gas during compression, lowering its final temperature and reducing power required. 3- A reduction in gas temperature and in that of the metal surrounding the valves provides a better operating temperature for these parts, giving longer valve service life and reducing the possibility of deposit formation. 4- Reduced cylinder wall temperature promotes better lubrication, resulting in longer life and reduced maintenance. 5- Cooling, particularly water cooling, maintains a more even temperature around the cylinder bore and reduces warpage or going out of round (thermal distortion).

5.3.2 Cooling Methods There are two methods commonly used: 1- Air cooling method (air cooled cylinders) 2- Water cooling method (water cooled cylinders)

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5.3.2.1 Air Cooling Method In this design fins are added to the cylinder and to the cylinder head to radiate the heat generated. Cooling fan is widely used to force the air across these fins for better cooling. These air – cooled units are not recommended for continuous or heavy – duty service.

5.3.2.2 Water Cooling Method This design is used if the water is available. It consists of: 

Water jacket around the cylinder (s). The cooling water is circulated in the water jacket to absorb the heat from the cylinders. The hot water is pumped to the radiator.



Radiator – it is air cooler. The hot water enter the radiator. The cooling fan push the air across the radiator tubes. The water in the radiator become cooled.



Water pump to circulate the water from the cylinder jackets to the radiator and back again.

The cooling efficiency in water cooling system affected by the following factors: 1- Cylinder size. 2- Running speed of the compressor. 3- Gas characteristics. 4- Wall thickness. 5- Temperature difference between the gas temperature inside the cylinder (at the end of the compression stroke) and the cooling water temperature in the jackets. 6- The ambient temperature.

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SECTION – 5.4 MULTI – STAGE COMPRESSION 5.4.1 Theory of Multi – Staging

Figure 5.12 P-V Diagram for two – stage reciprocating compressor Refer to figure 5-12: Single stage P-V diagram will be 1 2 compression is equal to the area 1 2 3 4.

3

4.

The power consumed for

Two – stage compression the compression process begins at point (3). In the first stage to compression is carried only up to point (3 a). At point (3 a) the gas is withdrawn from the cylinder and is cooled, reducing its volume back to point 3b. Compression in the second stage then occurs along the line 3b – 4a.

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Figure 5.13 Multi – stage compressor

5.4.2 Advantages of Multi – Staging 1- Saving in power (see figure 5-12) 2- Multi – staging with cooling of the gas (or air) between stages reduces the maximum gas temperature in the cylinders. This eliminating difficulties with lubrication, carbon deposits and thermal stresses. 3- In the case of oil lubricated air compressors, where high discharge temperature can present the hazard of discharge line explosion – multi – staging becomes essential. 4- Another advantage of multi – staging is the reduction of the pressure differential across each cylinder; this lightens the load and stresses imposed on valves, piston rings and packing rings and corresponding increases the life of these parts.

5.4.3 Cooling System for Gas Between Stages and After the Last Stage The cooling of gas (or air) between different stages is done by entercoolers. The cooling of gas (or air) after the last stage is done by after cooler. The same cooling method which is used to cool the cylinder, is used also for enter coolers and after coolers (i.e. air cooling or water cooling).

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SECTION – 5.5 RECIPROCATING COMPRESSOR LUBRICATION The most important operational item in compressing gases is proper lubrication. Proper lubrication includes: 1- Selection of high quality lubricants suited to the particular service conditions. 2- Cleanliness in storage and dispensing. 3- Application in correct quantities in a way that permits effective performance.

5.5.1 Function of a Lubricants 1234567-

Reduce friction between the moving parts. Minimize wear. Dissipate frictional heat through cooling and heat transfer. Flush away entering dirt as well as wear debris. Reduce gas leakage. Protect parts from corrosion. Minimize deposits.

5.5.2 Methods of Lubricant Distribution Any lubrication system must be able to deliver lubricating oil to all the points to be lubricated and in proper amount. There are two common methods are used to deliver the lubricating oil to the points to be lubricated. They are: 1- Forced circulation system. 2- Splash system

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5.5.2.1 Forced Circulation System This system consists of four basic component. They are 1- Oil tank – to contain the lubricating oil. 2- Positive displacement pump – to circulate the lubricating oil from the oil tank through the system to all required points. 3- Oil coolers – to make cooling for lubricating oil. 4- Oil strainers and filters to remove any suspended solids from the lubricating oil. Beside these four basic items, there are some control items like non – return valves, relief valves.

Figure 5.14 Force – feed lubrication

5.5.2.2 Splash Lubrication System In this design a portion of the crank shaft or connecting rod normally dips into the oil reservoir (crank case) and produces a spray or mist. Most small single – acting reciprocating compressors use this method. No oil filter or strainer in this system. In a few cases, the sump may contain a water cooler to remove some of the heat developed method.

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Figure 5.15 Splash lubrication

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SECTION – 5.6 CLASSIFICATION OF RECIPROCATING COMPRESSORS There are several ways to classify the reciprocating compressors. It could be classified with respect to; 1234-

Single or double acting. Single stage or multi – stage. Trunk or compressors with crosshead. Cylinder (s) arrangement.

5.6.1 Classification of Compressors into Single or Double Acting Compressors Single – acting Compressors are machines in which compression is effected in one end of the cylinder only (figure 5.16) Double – acting Compressors are machines in which compression is effected in both ends of the cylinder (figure 5-17)

Figure 5.16 Single acting compressor

Figure 5.17 Double acting compressor TriStar

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5.6.2 Classification into Single Stage or Multi – Stage Single stage compressor the compression is done one time. Multi – stage compressors, the gas leaves 1st stage, re-introduced again to the second stage and so on. It is used to provide higher pressures.

5.6.3 Classification into Trunk Type and Compressors with Crosshead Trunk-piston configuration (figure 5.4-a) permits compression only on the top side of the piston (single – acting) Crosshead Configuration (figure 5-4-b) permits compression on both strokes, i.e. double acting compressor, it could also be single acting. Often the purpose is to permit oil – free air delivery because the piston rod can pass through a sealing arrangement that will prevent oil from entering the compression cylinder.

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Figure 5.20 Examples of cylinder arrangement

5.6.4 Classification with Respect to Cylinder Layout

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SECTION – 5.7 PRIME MOVERS FOR RECIPROCATING COMPRESSORS 5.7.1 Function of a Driver A driver (together with any connecting medium between it and the compressor such as a gear and/or coupling) must do more than just drive the compressor at a rated condition. It must first start the compressor from rest, accelerate it to full speed, and then keep the unit operating under any design condition of capacity and power. Following are six important factors should be taken into consideration 1. 2. 3. 4.

Starting torque available and required; Acceleration requirements; (from zero speed to full speed). Avoidance of excessive electrical current pulsation; Avoidance of shaft torsional and lateral resonance and excessive speed variation during each revolution; 5. Power requirements during seasonal changes; and, 6. Power requirements due to upsets from rated intake and discharge pressures.

5.7.2 Driver Selection Motive power for a reciprocating compressor may be provided by one of the following principle driving sources: 1. 2. 3. 4.

Electrical power (electric motor) Internal combustion engine (gas engine or diesel engine) Reciprocating steam engine or steam turbine. Gas turbine.

5.7.2.1 Electric Motor Driven Compressors Electrical energy is quite economical and in most case, is readily available. Electric motors are efficient and reliable and are the most commonly used prime movers for compressors.

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Motor driven compressors may be further identified by the type of connection that is used between the motor and the compressor. Any one of the following types of drive connection may be used: - Belt (V-belt or flat belt) - Flange mounted motor - Direct connected motor - Flexible coupling - Speed reduction gears. The following should be taken into consideration when you select an electric motor as a prime mover of reciprocating compressors:  The selection of the correct flywheel effect for a motor driven reciprocating compressor is extremely important. This is especially true for direct connected motor driven compressors with partial capacity unloading. The periodically pulsating torque of the compressor can produce harmful pulsations in the current drawn by the driving motor.  It is highly desirable to order the driving motor from the compressor manufacturer, who is ideally qualified to make the best flywheel effect selection. His overall knowledge of the compressor torque characteristics will ensure undivided responsibility.  A reciprocating compressor may be driven by an induction motor with a speed reduction gear interposed between the driver and the compressor. This permits the use of a higher speed and less costly motor, but such savings will be offset by the cost of the gear reduction unit.  It is important that the proper type of couplings be selected for gear driven compressors. These couplings must have suitable elasticity and damping.

5.7.2.2 Internal Combustion Engine Driven Compressors When a suitable supply of fuel gas or fuel oil (liquid fuel) is available, internal combustion engine driven reciprocating compressors may be preferred for economic reasons. These units are often applied to gas transmission, gas distribution, re-injection, gas gathering and process work.

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5.7.2.3 Reciprocating Steam Engine and Steam Turbine Driven Compressors Steam as a motive power is one of the oldest means of driving reciprocating compressors. For many years the reciprocating steam engine was not only the most common but the most efficient compressor drive. For most applications where steam is presently the most economical energy source, the steam engine has been replaced by the steam turbine. Such an arrangement commonly employs a speed reducing gear between the steam turbine, which operates at relatively high speed, and the reciprocating compressor, which runs at relatively low speed. Steam-turbine driven machines, suitable for a wide range of steam conditions, are available in sizes up to 5,000 HP. The operating economy of such units, although good, is frequently less than that of the steam engine driven compressor, particularly when operated under reduced speed or load. When suitable flywheels and couplings having proper damping characteristics are employed, system torque pulsations are reduced to acceptable values, and successful installations result. Governors for steam-turbine driven compressors may be of the manual adjustable, constant speed type or they may be arranged to vary speed automatically in response to changes in system pressures, flows, temperatures, etc.

5.7.2.4 Gas Turbine Driven Compressors As reciprocating compressor horsepower capabilities increase, and with the ever-increasing sophistication of double reduction and high speed gearing systems, it has become practical to consider the use of a gas turbine as a prime mover. Its very high speed and horsepower limit its use to units over 5000HP and to applications where a source of fuel gas is available. At lower horsepowers, internal combustion gas engines (I.C.E.) power units are more commonly used.

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SECTION – 5.8 RECIPROCATING COMPRESSOR OPERATION Reciprocating compressor operation covers five main areas:      

Starting a new reciprocating compressor. Cleanlines of the suction line. Break-in period. Preparation for extended shutdown. Routine checks, and Trouble shooting.

5.8.1 Starting a New Reciprocating Compressor To start reciprocating compressor for the first time, the things to be done are listed in the manufacture instruction book. These should be carefully followed. They include the following, but may vary with type of unit.  Check over the compressor to insure that all parts are assembled properly and securely tightened.  Remove the crankcase covers and clean out the interior thoroughly, be sure that the oil strainer is clean. Check the main and connecting rod bearing cap nuts to see if they are tight and the locking devices are in place.  Turn the compressor by hand and slowly at least two complete revolutions to make certain that the moving parts are free from interference.  Fill the crankcase with the proper lubricating oil to the required level. On units equipped with pressure oiling systems for the frame running gear, the oil pump should be primed.  Be sure all lubricator line are filled and lubricators are functioning properly.  Check to insure that the gas (or air) suction line leading to the cylinders is thoroughly cleaned, free from rust, pipe scale, welding shots, sand, water or condensate and other foreign material which will damage the compressor cylinder if allowed to enter.

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Also, see that the suction and discharge lines are unobstructed and that all valves therein are properly set. Be sure intake filters and screens are in place and charged with oil as necessary.  On multi-stage units it may be advisable, if the unit has been in storage or subjected to questionable handling during the shipment to remove the tube bundles from the inter coolers to make certain that the shell and tubes are thoroughly cleaned and free from foreign material.  The suction ports of the cylinders should also be thoroughly cleaned, removing any material that may have accumulated therein during the erection of the unit.  Check the valves in the cylinders making sure that the valves are placed properly in the cylinder ports, suction and discharge, and securely tightened in place.  A valve or two (dependent on the cylinder size) should be left out of the cylinder during the initial running period. This will allow a light load on the unit during this period and permit ready observation of the lubrication and wearing in of the cylinder bores.  Before starting the compressor, turn on the cooling water to the cylinders and to coolers used with the compressor. Check to insure that the cooling system is filled and that flow is indicated at the outlets. Adjustment of the amount of cooling water should be made after the compressor has warmed up.  Check the direction of rotation of the primover. This check must be done on the primover separately, i.e. disconnect the primover from the unit and then make this check.  Open proper valves in unloader, discharge, and regulator lines. Be sure the unit is unloaded as necessary for starting. After the correct rotation is established (motor driven or otherwise) the unit should be started and run for a period of several minutes. Immediately upon starting the unit, note that the lubricator is feeding properly and that all parts are being lubricated. Also, note that the unit operates without any noise or knocks. After this short run stop the unit and feel all bearings to make sure that there is no tendency of parts to heat too rapidly and that the lubrication to all parts is adequate.

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5.8.2 Cleanliness of the Suction Line Cleanliness of the suction line is an extremely vital consideration particularly on process compressors. The suction lines are usually long and the gas to be compressed is drawn from a closed system, from gas generating or process equipment, towers, heat exchangers, formers, etc. Such suction lines usually contain heat exchangers, separators, knock-out drums and like equipment to clean and dry the gas to be compressed before it is drawn into the compressor cylinders Although every precautions has been taken in the thorough cleaning of the suction piping and vessels therein during the installation and connecting up of such equipment, foreign material may have entered these lines during installation. It cannot be stressed too strongly that if such foreign material is drawn into the compressor cylinders excessive wear and failure of elements of the cylinder, such as valves, pistons, piston rings and severe scoring of the cylinder bore may result. Several methods are used in starting up a unit to prevent foreign material entering the compressor cylinders: Method 1: Blow out the suction pipe line and the equipment therein: This method is considered not fully effective. If such procedure is followed and a considerable amount of foreign material is blown from the line, such may be an indication that all may not be removed by this method. To insure complete removal and cleanliness, sections of the suction lines and equipment therein should be dismantled and again clean thoroughly. Method 2: Install a reinforced conical fine screen with sufficient flow area in the suction line as close as possible to the compressor cylinder suction flange. Such screen must be readily accessible for periodic removal, cleaning and re-installation. The use of this screen in the suction line is suggested also even when the blow back method (Method #1) is used. It is recommended that a pair of pressure gauges or a manometer be connected on either side of the screen for the purpose of checking the pressure drop through the screen. When the pressure drop becomes excessive the screen should be removed, cleaned and replaced.

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On multi-stage units, particularly where the gas compressed flows between stages through long fabricated lines, coolers, separators and the like, similar screens should be installed at the suction flange of all stage cylinders. Generally on a two-stage air compressor the suggested methods apply to the first stage suction only. The valves which have been left out during the idle run-in period should now be installed. Run the unit with the suction being taken from the atmosphere with a free discharge. It may be necessary, on a gas compressor, to temporarily disconnect the suction and discharge lines from the system during the run-in period. Such an open suction line should be screened temporarily to prevent foreign material being drawn therein. The compressor should run in this manner for about ½ hour, after which the unit should be brought up to full load by gradually increasing pressure in increments over a period of 4 to 8 hours. For an air compressor the intake and discharge lines can be connected to their respective systems for the load and pressure build-up. In the case of a gas compressor, it is recommended that the load build-up be made on air, within discharge temperature limitations or with inert gas to equal or equivalent pressure or load before connecting the suction and discharge lines finally to the gas system. When running in a two-stage or multi-stage compressor the valves left out during the initial idle run-in should be replaced in one stage at a time with running periods between each stage valve installation. This will not only permit gradual loading of the unit but the blowing out of any foreign material that may have inadvertently accumulated in the coolers and inter-stage piping during the erection of the unit. Before installing the valves in these stages and between running periods the wearing in of the cylinder bores can be noted and the run-in periods governed accordingly. The valve ports should be thoroughly cleaned at the time valves are installed. On multi-stage unit the load build-up should be accomplished over a larger period than a single stage unit. A run on full rated load of 16 to 24 hours provided all elements are wearing in and operating properly should insure that the unit is properly broken in and ready for service.

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During the speed and load build-up, observe at frequent intervals the bearings and the piston rod temperatures to make certain that these elements are operating properly and are not overheating. It is recommended during the breakin run (except on non-lubricated compressors) that oil be applied to the piston rod to facilitate the running in of the packing. This is specially true when metallic packing is used. The additional time taken to run in the unit to insure that it is operating properly will be compensated for by the increased satisfaction resulting in the subsequent performance of the unit. In the running in of an I.C. engine or turbine-driven compressor, this same procedure as outlined for a motor driven compressor should be allowed. However, in this case, the advantage of the variable speed feature should be utilized by running the unit at the start at slow speeds and gradually building up to full rated speed as the pressure and load is built up.

5.8.3 Break – In Period The advantages of proper break-in of a compressor before putting it steadily under load are seldom appreciated. The break-in period is essential in two cases:  The new compressor, and  After overhauling the compressor and replacing most of the moving parts. Whenever new parts are installed, the break-in procedure must be repeated sufficiently to properly seat the parts. It is not necessary after replacing nonmoving parts. All machined surfaces have asperities, or hills and valleys. The height of the hills or depth of the valleys is a function of many such variables as metal structure, type of finish machining operation, and machine tool operator experience. During break-in, it is necessary that mating parts establish a satisfactory bearing with each other. To do this, there must be a certain amount of wear, rounding off or bending over of the asperities, etc. This is probably the most critical period in the life of such parts and the demands on the lubricant are the most extreme. During this break-in period, the oil must maintain:  Lubrication.  Keep the bearing sufficiently cool, and  Flush away the wear particles and any dirt that comes over. TriStar

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Consult manufacture instruction book for proper lubricant for break-in of cylinders. Most of reciprocating compressor units, are shop tested and normally require only a few hours of break-in, and it should be run on part load a while in the field before going into full-time service.

5.8.4 Preparation for Extended Shutdown Any compressor, when taken out of service for a long period, will deteriorate rapidly through rust and corrosion if not especially protected. The following are basic precautions to be taken with reciprocating compressors.

Air-Cooled Compressor  The crankcase should be drained and refilled with a preservative oil. Such oil contains more inhibitor than normal inhibited oil. The machine should then be operated a minimum of 15 minutes at no pressure for thorough distribution and the driving off of any crankcase condensate. At the same time, fog some of this oil into the compressor intake (in oil lubricated compressors only).  All openings then should be taped or plugged to exclude moisture.  Relieve V-belt tension.  Drain the receiver and after-cooler. Drain after-cooler cooling water, if used.  The driver should be treated in accordance with the manufacturer's instructions. This will permit storage for a year. However, if the unit can be run for 30 minutes every two or three weeks at full load and followed by 15 minutes at no pressure, these precautions may not be necessary. Be sure operational periods are scheduled and that the machine is actually run.

Crosshead Type Compressors  When a three or four days shutdown is contemplated, run the compressor at no load for ten to .fifteen minutes, simultaneously operating the cylinder and packing lubricator by hand to pump extra oil to those parts.

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 For a shutdown of several weeks, do the same, then remove the piston rod packing and the oil wiper rings from the rod, coat the rod with grease, grease rings, and warp them in waterproof paper. Leaving the packing rings on the rod may result in a ring of corrosion that will injure the packing when the unit is re-started.  When a unit is to be shutdown for a lengthy period, write to the manufacturer for complete instructions.  If freezing is possible during shutdown, all water must be drained from jackets, inter-coolers, after-coolers, separators, drain traps, and oil coolers. If this is neglected, cylinder jackets or cooler tubes might burst and major repairs will result.  When putting the unit back into service, go through every check and procedure required for first starting a new machine.

5.8.5 Routine Checks Operating and maintenance routine will vary with the size and type of compressor. The manufacturer's instructions should be followed. Following are general routine maintenance instructions:  Regular inspection should be made at definite intervals at which time any necessary corrections may be made such as replacement of worn parts or packing, adjustment for wear of working parts and cleaning of valves and crankcases. Particular attention, on air compressors, should be paid to the suction filters to make certain that it is clean and unclogged at all times.  The frequency of regular inspections will be dependent upon the conditions prevailing at the installation. It is recommended that the unit be checked very frequently during the first few weeks of operation and extension of the periods between inspection be dependent upon the experience observed during the earlier periods of operation.  Oil used in the crankcase of a reciprocating compressor must be changed at intervals. Here again the frequency of changes will be dependent upon the actual conditions prevailing but the oil should be changed at least once a year. Before refilling, the crankcase should be thoroughly cleaned.

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 Water jackets should be inspected and washed out as frequently as water conditions may require. On multi-stage units particular attention should be given to the inter-cooler water surfaces, which must be kept as clean as possible in order to keep the compressor operating at peak economy. It must be remembered that inefficient inter-cooling will result in an increase in the horsepower to drive the compressor and consequently an uneconomical unit.  It is recommended that the inlet and discharge line from each cylinder be equipped with a thermometer. The normal operating temperature should be observed during the early operating period of the unit. Any increase in the normal discharge temperature from a cylinder can indicate the presence of worn valves, incorrect speed, defective capacity controls, inadequate cooling water quantity, excessive cooling water temperature, excessive discharge pressure, inadequate cylinder lubrication, worn piston rings or scored cylinders. Table will assist in inspecting and observing the operation of the compressor  It is recommended that an operator's log be used to record the operation of the compressor. The log book should show the operating conditions such as temperature, inter-stage pressures, etc. at regular intervals and should report all normal maintenance performed on the unit such as oil changes, oil added, etc. The log should also contain a report of any unusual conditions or events such as power or water failure. The log should make note of manual draining of separators in the event automatic traps are not used and in case automatic traps are used the log book should contain periodic notations as to whether or not the automatic traps are working satisfactorily.  The log book should contain, in the front, instructions on action to be taken under such emergencies as cooling water failure, lubricating oil failure and power failure. The log book should also contain instructions and a list of procedures to be followed when the compressor is to be shutdown for other than a brief temporary period.

5.8.6 Air-Cooled Compressors Routine Checks  When first started After eight to ten hours of operation, tighten all head, valve cover, cylinder flange, shaft covers (crankcase cover), and foundation bolts to recommended torque. TriStar

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 Weekly check Intake filters for cleanliness. If conditions are bad, clean them. A biweekly schedule may be found adequate later. Drain air receiver and inter-cooler. In humid weather, it may be necessary to drain more frequently. On larger units, this should be a daily procedure. Check the crankcase oil level. Replenish as necessary (toping). Operate all safety valves manually.  Monthly Clean the machine and driver externally. Check time required to pump up the receiver with its outlet shut. A record kept of the time required to increase the pressure from, say, 30 psig to 100 psig, will provide a check on machine efficiency. Any serious difference may indicate leaky or broken valves. Run a pipe leakage check. During the lunch hour, or any other time when the surroundings are quiet, follow closely all distribution lines beyond the receiver while the compressor is shutdown but pressure is still high. Listen for leaks. Fix those found.  Two to three months Change crankcase oil. For definite recommendations, read the instruction book. Tighten all bolts and check V-belt tightness.  Six months maximum Remove and clean all valves. Do this first after only two to three months, then as needed up to six months maximum. Check air ports, pistons, and head for deposits and remove any found.

Crosshead Type Compressor At least once a shift  Drain receiver. Conditions may warrant more frequent drainage.  Check inter-cooler and after-cooler condensate trap operation several times.  Check cooling water flow and outlet temperature every one or two hours.  Check crankcase lube oil pressure several times if forced circulation system is used. TriStar

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 Check cylinder and packing lubricator feeds. Make sure each feed is pumping oil and that reservoir is filled with correct oil.  Check and record inter-stage and discharge pressures. If any substantial change is noted, find and correct the trouble immediately. Inter-stage pressure is normally almost constant for a given discharge. Any abnormality in reading probably indicates leaking valves.  If frame oil filter can be cleaned manually, operate it as directed.

Weekly  Pop all safety valves manually.  Operate regulator manually throughout entire load range to keep free any unloader that may not normally be in regular operation.  Keep the compressor and its surroundings clean.  Check crankcase oil level and add proper oil as needed.  Check cleanliness of intake air filters.  Operate safety shutdown or alarm devices where possible.  After first week, tighten foundation bolts and all other nuts and bolts on the unit to recommended torque. Re-check alignment if there is foundation bolt take-up.

Semi-Annually  Change crankcase oil (on some machines more often). See instruction book.  Check condition of water side of coolers and jackets.  Tighten all foundation bolts and nuts and machine bolts. Re-check alignment if there is any foundation bolt take-up.

Annually  Check clearance in all bearings.  Adjust crosshead or replace parts to restore piston rod alignment as necessary due to wear on crosshead shoe or piston.  As experience dictates (at least annually) check condition of all valves.  Check condition of cylinder bore through valve opening.  Check effectiveness of lubrication by observing the degree of oiliness on the cylinder bore and valves.

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CAUTION: If for any reason the compressor is operated without water and cylinders become overheated, do not turn on cooling water until they have cooled. A sudden application of cool water may crack the cylinder.

5.8.7 Troubleshooting Pages following present charts designed to “tip off” the maintenance man where to look for the cause of some difficulty. These charts, although designed mainly for 100 psiG compressors of all types, apply quite generally. Some items apply only to the vertical air-cooled units, some to water cooled only and some to twostage only. Many categories in the chart refer to normal conditions. There is no way to know what normal conditions are except to keep a record of temperatures and pressures. Other items should also be recorded in certain cases. 1- Temperatures recorded a minimum of twice a day could well include the ambient on air-cooled units a few feet from the compressor and the air discharge temperature. On larger two-stage units, the discharge temperature of the first stage cylinders should be added. Water temperature in and out is desirable on water-cooled compressors. 2- Pressures recorded should include those taken at discharge or receiver and, on a two stage compressor, always at the inter-cooler. It is a particularly valuable norm to have. On smaller compressors operating either start-and-stop or constant-speed, make a one hour study (at least once a month) of the number of starts or full load periods and the total running time. Do this at the same hour each time and select the hour when maximum demand is apt to exist. The purpose of making this study is to discover any trend in operating time that would render a change in control advisable. Also, if the operating time is increasing steadily, there is either increasing system leakage, some compressor deficiency, or an increased demand. The correct reason should be determined, the proper remedy applied. 3- Valves problems are apt to be the predominant maintenance item. Leaking and broken valves and gaskets should be discovered and repaired quickly. As previously discussed, these lead to many added problems and even to dangers if neglected.

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4- A defective inlet valve can generally be found by feeling the valve cover. It will be much warmer than normal. Discharge valve leakage is not as easy to detect since the discharge is always hot. Experienced operators of water-cooled units can usually tell by feel if a particular valve is leaking. This is not for those who frequently check their units. The best indication of discharge valve trouble is the air discharge temperature. This will rise, sometimes rapidly, when a valve is in poor condition or breaks. This is one very good reason for keeping a record of the air discharge temperature from each cylinder. It must be remembered that there will be seasonal variations in temperature, all temperatures rising in summer and falling in winter. 5- The recording of inter-cooler pressure on multi-stage units is valuable because any variation, when operating at a given load point, indicates trouble in one or the other of the two stages. If the pressure drops, the trouble is in the low pressure cylinder. If it rises, the problem is in the high pressure cylinder.

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