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• Nilesh Gohel

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• Ingeniería Química
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1.

COLUMN PIPING: STUDY LAYOUT, NOZZLE ORIENTATION & PLATFORMS REQUIREMENTS

1.1. 1.0 Sequence of Column Piping Study 1.1 All available information / data from Equipment specification and P&ID shall be written on the elevation view of the column as illustrated in Fig.1, 2 & 3. 1.2 The designer now starts thinking about the proper orientation of nozzles and provisions for access to the points of operation and maintenance. 1.3 Considerations of the pipeline leaving the tower area and the adjacent piping shall be visualized. 1.4 The first step is to orient the manholes preferably all in same directions. Normally, manholes shall be oriented towards dropout area within a 30° segment of column as this facilitates the lowering of tower internals to the main access way. The manhole segment of platform should not be occupied by any piperack. 1.5 A break in ladder rise (normal 5m, maximum 7m) will occupy another segment of column for platform. 1.6 The levels of platforms are to be decided on the elevation view based on the manholes and access to relief valves, instrument for viewing. 1.7 All platform levels in the proper segments of the tower with ladder location should be drawn on plan view. The manhole shall be shown in proper segment with the angle of orientation, and the space for the swing of manhole cover taking davit hinge as centre. 1.8 Layout should be started from the top of the column with the designer visualizing the layout as a whole. There will be no difficulty in dropping large overhead line straight down the side of a column, and leaves the column at a high level and crosses directly to the condenser. This clears a segment at lower elevations for piping or for a ladder from grade level to the first platform. 1.9 Flexibility and thermal load connected with the large-dia overhead lines to the condenser at grade level or higher level shall be considered. The relief valve protecting the tower is usually connected to the overhead line. A relief valve discharging to atmosphere should be located on the highest tower platform. In a closed relief-line system, the relief-valve should be located on the lowest tower platform above the relief -system header. This will result in the shortest relief-valve discharge leads to the flare header. The entire relief-line system should be self-draining. 1.10 From layout point of view, it is preferable to space the platform brackets on the tower equally and to align the brackets over each other for the entire length of the tower. This will minimize interferences between piping and structural members. 1.11 Nozzles and piping must meet process requirements while platforms must satisfy maintenance and operating needs. Access for tower piping, valves and instruments influence placement of ladders. 1.12 In routing pipelines, the problem is faced to interconnected tower nozzles with other remote points. The tentative orientation of a given tower nozzle is on the line between tower centre and the point to which the line is supposed to run. Segments for piping going to equipment at grade e.g. condenser and reboiler lines are available between ladders and both sides of manhole. See the Fig.4 / 5 for overall orientation of a distillation column. Line approaching the yard/piperack can turn left or right depending on the overall arrangement of the plant. The respective segments of these lines are between the ladders and 180°. The segment at 180° is convenient for lines without valves and instruments, because this is the point farthest from manhole platforms. The sequence of lines around the tower is influenced by conditions at grade level. Piping arrangements without lines crossing over each other give a neat appearance and usually a more convenient installation. 1.13 The correct relationship between process nozzles and tower internals is very important. An angle is usually chosen between the radial centreline of internals and tower-shell centrelines. By proper choice of this angle (usually 45° or 90° to the piperack) many hours of work and future inconvenience can be saved. Tower piping, simplicity of internal piping and manholes access into the tower are affected by this angle. After this, the information produced by the designer results in selecting the correct orientation of tower nozzles. 1.14 A davit usually handles heavy equipment such as large-size relief valves and large-diameter blinds. If the davit is at the top of the tower, it can also serve for lifting and lowering tower internals to grade. Clearance for the lifting tackle to all points from which handling is required, and good access should be provided. 1.15 Very often, interpretation of process requirements inside a tower is more exact than for exterior piping design. The location of an internal part determines, within strict physical limits, the location of tower

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nozzles, instruments, piping and the steelwork. The layout designer has to concentrate on a large-scale drawing of tower-internal details and arrangement of process piping to finalize the piping study. 1.16 Access, whether internal or external is very important. This includes accessibility of connections from ladders and platforms and internal accessibility through shell manholes, handholes or removable sections of trays. A manhole opening must not be obstructed by internal piping. 1.17 Reboiler-line elevations are determined by the draw off and return nozzles and their orientation is influenced by thermal flexibility considerations. Reboiler lines and the overhead lines should be as simple and direct as possible. 1.18 Fig.6 shows the segments of tower circumference allotted to piping, nozzles, manholes, platform brackets and ladders as normally recommended to develop a well-designed layout. 1.2. 2.0 Nozzle Orientation and Level Nozzles are located at various levels on the tower to meet the process and instrumentation requirements. 1.2.1. 2.1 Manholes Nozzles are to be oriented keeping provision for maintenance and operation needs. Manholes are usually located at bottom, top and intermediate sections of tower. These access nozzles must not be located at the downcomer sections of the tower or the seal pot sections of the tower. Where internal piping is arranged over a tray, manhole shall be provided but it should be ensured that the internals do not block the maintenance access through the manhole. Possible location of manhole and handholes within the angular limits of b° are illustrated in detail-2 of Fig.4

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1.2.2. 2.2 Reboiler Connections Reboiler connections are normally located at the bottom section of the tower. Detail-1 of Fig.4 shows reboiler draw-off connections for single-flow tray. This connection can be very important for arranging tray orientation. The simplest, most economical location for reboiler connections with the alternative location within the angular limits of a° is shown. The angle a° depends on the size of reboiler draw off nozzle and the width of the boot (dimension ‘b’) at the tray down flow. The return connection from the thermosyphon reboilers is shown in detail-1 of Fig.4. These lines should be as simple and as direct as possible, consistant with the requirements of thermal flexibility. For horizontally mounted thermosyphon reboiler, the draw off nozzle is located just below the bottom tray and for vertically mounted recirculating thermosyphon reboiler, the draw off nozzle is located at the bottom head. For both the systems, the return nozzles are located just above the liquid level as shown in Fig.7.

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1.2.3. 2.3 Reflux Connections Reflux nozzles are provided with internal pipes that discharge the liquid into the sealpot of the tray below. Detail 3 of Fig.4 shows the reflux connections. Care must be taken that the horizontal leg of the internal pipe clears the tops of bubble caps or weirs. It must be ensured that the internal pipe can be fabricated for easy removal through a manhole or can be fabricated inside the tower shell. 1.2.4. 2.4 Overhead Connections The vapour outlet nozzle is usually a vertical nozzle on the top head of tower. In addition, the vent and relief valve could be located on the top head with a typical platform arrangement for access to vent, instrument connections and top manhole. In a closed relief line system, relief valve should be located on the lowest tower platform above the relief system header. This will result in the shortest relief valve discharge leads. The entire relief line system should be self draining. 1.2.5. 2.5 Bottom Connections The liquid outlet is located on the bottom head of the tower. If the tower is supported on skirt, the nozzle is routed outside the skirt as shown in Fig.8. The elevation and orientation of this line is generally dictated by the pump NPSH requirement and the pump suction line flexibility. (see Fig.9)

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1.2.6. 2.6 Temperature & Pressure Instrument Connections / Level Instruments The temperature and pressure instrument connections are located throughout the tower. The temperature probe must be located in a liquid space and the pressure connection in a vapour space as shown in Fig.10.

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The level instruments are located in the liquid section of the tower usually at the bottom. The elevation of the nozzles is decided by the amount of liquid being controlled or measured and by standard controller and gauge glass lengths. Level controllers must be operable from grade or platform and level gauges / switches may be from a ladder if no platform is available. Fig.11, 12, 10, 13 & 14 illustrates a few instrument connections on tower.

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1.3. 3.0 Access and Maintenance Facility 3.1 Access whether internal or external is very important. This includes accessibility of connections from ladders and platforms and internal accessibility through shell manholes, handholes or removable sections of trays. 3.2 Tower maintenance is usually limited to removal of exterior items (e.g. relief or control valves) and interior components (e.g. trays or packing rings) Handling of these items is achieved by fixed devices (e.g. davits or trolley beams) or by mobile equipment (e.g. cranes). When davits or beams are used, they are located at the top of the tower, accessible from a platform and designed to lower the heaviest removable item to a specific drop out area at grade level. When mobile equipment is used, a clear space must be provided at the back (side opposite to piperack) of the tower that is accessible from plant auxiliary road.

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Fig. 15, 16, 17 & 18 illustrates the access and maintenance facilities to be considered in the piping arrangement around a tower.

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On free-standing columns, access for major maintenance to insulation or painting will usually require the erection of temporary scaffolding. Space for scaffolding at grade level and provision of cleats on the shell to facilitate scaffold erection should be considered. 3.3 Utility stations of two services viz. steam and air are usually provided on maintenance platforms. Steam and air risers should be located during piping study to keep adequate cleats for support. (see Fig.19)

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1.4. 4.0 Platforms and Ladders 4.1 Platforms on towers are required for access to valves, instruments, blinds and maintenance accesses. Platforms are normally circular and supported by brackets attached to the side of the tower. Generally, access to platforms is by ladder. Fig.20 illustrates the platform requirements.

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4.2 Platform elevations for towers are set by the items that require operation and maintenance. The maximum ladder run should not exceed 7m. 4.3 Platform widths are dictated by operator access. The clear space on platform width shall be min.900mm. For platforms with control stations, the width of platform shall be 900mm plus the width of control station. The platform for manholes and maintenance access, adequate space for swing the cover flange flange must be provided. 4.4 Top-head platforms for access to vents, instruments and relief valves are supported on head by trunions. 4.5 Access between towers may be connected by common platforming. 4.6 It is preferable to space platform brackets on tower equally and to align brackets over each other over the entire length of shell. This minimizes the structural design and interferences from piping. 4.7 On very wide platforms or those that support heavy piping loads, knee bracing is required in addition to the usual platform steel. The potential obstruction immediately under the knee brace must be kept in mind during platform design. 4.8 Fig. 3, 15, 21, 22, 20 & 19 illustrates a few platform considerations.

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2.

TOWER PIPING: CENTER FOR EQUIPMENT LAYOUT

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Tower Piping: Center for equipment layout

A tower is usually a major part of a designer’s area. It is advisable to treat it as a central piece of equipment and extend the design around this center. Columns, towers and vertical vessels are to be arranged in a row with a common centerline if of similar size. If, however, diameters vary considerably, lining up with a common face will be found to be beneficial. During the initial stage of piping studies, piping designer should investigate in co-operation with vessel designer about the preference of lined up towers with interconnecting platforms, for convenient operation and maintenance access. The platforms are supported from towers. In such cases, slight alterations in tray spacing, internal piping arrangement, skirt height and tower length can help to put all tower manholes on same elevations. In turn lined up manholes will help platform arrangement, providing also common access to valve instrument. When arranging common platforms for several towers in line, allowance must be made for the differential expansion between towers. Suitably arranged hinges or slots in the platforms between towers, which introduce flexibility into the platforms shall be provided. All these feature shall be decided at the early stages of design because they affect good piping arrangement. Main work of tower piping is connected with the proper orientation of nozzles and provision of access to points of operation and maintenance. Generally, platforms of manholes shall be utilized for operating and maintenance access for valves and instruments. Small valves and instruments are usually arranged outside the platforms and are operated from the ladder. Additional platforms are required for operating valves, line blinds, relief valves (3” and above, orifice plates, transmitter of a level controller and handling davit. The operating aside is usually under pipe rack, so first ladder on tower should be on pipe rack side. To handle heavy equipment (large size relief valves, large diameter, line blinds), a davit is usually needed. The davit should be on the side of the vessel away from the rack. The area at grade should be kept clear for a dropout. for bigger diameter vessels, two davits shall be furnished. If it is located at the top of the tower, it can serve as well for lifting and lowering the top internals to grade. Clearance for the lifting tackle to all points which require handling is essential, as also sufficient access and removal space. For reactor feeding catalyst, a permanent trolley beam over the filling manholes is usually provided with adequate access at grade for lifting and removal of the catalyst. Fig.1 shows plan with segments of its circumference allotted to piping, nozzles, platforms and ladders, in a pattern which leads to a well-designed layout. The complete circumference of the tower is theoretically available to arrange the items. Piping should be located radially as far as possible. Fig.1 also shows the principal features such as manholes, platforms and pipe runs typically applicable to Tower piping arrangement. Nozzle elevations are determined by process requirements and manhole elevations by maintenance requirements. For economy and easy supports, piping should drop or rise immediately upon leaving the nozzle and run parallel and as close as possible. To make the orientation, follow the following steps:

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2.1.1. Tray Orientation Right relationship between process nozzles and tower internals is very important. This is often influenced by reboiler draw off and return nozzles and orientation by flexibility considerations. Changing from one pass to two pass, the two pass trays shall be rotated through 90 deg. to upper trays. 2.1.2. Nozzles & Manholes Orientation of nozzles depends on the type of distributors and process requirements. Before detailing, details and type of distributors must be known to the designer. Then he could produce right orientation of nozzles shall be located on tray area and must be accessible from ladder or platform. Temperature connections are usually located in the liquid space of tray downcomers. In some cases, it could be also in vapor space. In front of thermowell nozzles, a clearance of approximately 600mm is required to remove thermowell. Pressure connections are usually located on the vapor space just below the trays. All instrument locations are to be confirmed by process department. Care should be taken with interference such as between two reinforcing pads, one near the other, nozzle baffle and down comer and weir dams. Manholes should preferably be placed on road side on tray area so that it is convenient for removal and lowering to grade of tower internals. Accessibility whether internal or external is very important and is often not given enough consideration. A balance must be made between the external accessibility of connections from ladders and platforms and internal accessibility from shell manholes, handholes or removable section of trays.

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For example, a shell manhole opening must not be obstructed by internal piping unless the piping is removable through the manholes or can be slung clear from an internal hitching point. In either case, the break flange bolts must be accessible from the manhole. The following considerations must be made at the initial stage of design as they bear directly on the external arrangement of tower. 1. Analyze the functions of the internals. 2. Determine the desirable location of the shell connections relative to external requirements (piping platforms). 3. Layout of the internal piping required to satisfy preferred location shell nozzle and the preferred tray orientation, and if necessary adjust these to make a workable adjustment. 2.1.3. Platforms, Ladders and Davit Platforms are considered as work area for manholes and rest area when an intermediate area is added, if the height between two work platforms exceeds 9 meters.  Generally, layout analysis should be started from the top of the tower and those having reboilers should be started from the bottom, but with the designer visualizing the layout as a whole. There will be no trouble in dropping the large lines (such as overhead vapour lines) straight down the side of the column. The lower spaces can then be laid out with piping and nozzles’ knowing what space is already occupied by these large vertical lines. Condensers are often located at grade. In such cases, a large overhead lines drop right alongside the tower to the condenser at grade. Condensers can also be elevated. An elevated condenser is more convenient from a tower piping layout standpoint because the large overhead line leaves the immediate vicinity of the tower at a high level, leaving the lower section open, say, for a ladder from grade to the first platform. Whether the condenser is at grade or at an elevated level, the flexibility and thermal load problems connected with large diameter overhead lines must be considered.  For valves and blinds, the best location is directly at tower nozzles. Valves in branch connections or at nozzle should be in a position where the line will be self-draining on both sides of the valve. A dead leg over closed valve collects liquid or solids. The trapped liquid can freeze, or when opening the valve, without draining the leg, can upset process conditions. All instruments should be oriented so as not to obstruct the passage way at the ladder exits or entrance. Convenient access and groupings of instruments and valve will help inspection and plant operation. Instruments should not be located adjacent to manholes. The manhole cover can damage instruments when being swung open during maintenance.  The tower elevation is governed by the following: 1. Net positive suction head requirements if the tower bottom line is a pump suction line. This can elevate the tower bottom tangent line. 2. Thermo siphon type reboiler circuit can also elevate a tower. 3. Gravity flow from tower bottom or from an elevated nozzle can also elevate a tower. 4. Head room requirements. To support the tower at the chosen elevation, a steel skirt down to grade or a combination of a short steel skirt and concrete plinths will be required.  Piping around tower shall be spaced taking into consideration the structural design of the supporting arrangement. Special care should be taken to see that supports for cold lines do not interfere with the other pipes. For supporting tower piping from tower shell, designer should select proper type. While locating clips, care shall be taken to ensure that these clips are not located on the circumferential and longitudinal weld seams indicated in the vessel date sheet.

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1.

DISTILLATION COLUMN PIPING: ABSORPTION STRIPPING, FRACTIONATION

1.1. 1.0 Introduction to Column Piping Study for the column piping should start after complete understanding of the following document: a) Technical specification of the column b) P&ID c) Unit Plot Plan d) Basic Engineering document highlighting the specific process requirement, platform requirement and guidelines for the general arrangement of piping around the column. e) Details of internal arrangements e.g. for packed type – the packing height, packing support and manhole / hand hole locations, and for tray type – the nos. of tray, type of tray, downcomer location, manhole location etc. f) Instrument data sheet. g) Line list with operating / design conditions of the fluid. Some understanding of the process function will facilitate the column piping study to meet the requirements of operation, maintenance, safety and the aesthetics. Various types of column with their varying functions are in use for refinery and Petrochemical industry. Generally they are distinguished based on the specific operation for mass transfer viz. Distillation, Absorption – stripping or Fractionation etc. 1.2. 2.0 Distillation Column Piping The distillation is separation of the constituents of a liquid mixture via partial vaporization of the mixture and separate recovery of vapour and residue. Various kinds of devices called plates or trays are used to bring the two phases into intimate contact. The trays are stacked one above the other and enclosed in a cylindrical shell to form a Distillation Column. The feed material, which is to be separated into fractions, is introduced at one or more points along the column shell. Due to difference in gravity between liquid and vapour phases, the liquid runs down the column, cascading from tray to tray, while vapour goes up the column contacting the liquid at each tray. The liquid reaching the bottom of the column is partially vaporized in a heated reboiler to provide reboil vapour, which is sent back up the column. The remainder of the bottom liquid is withdrawn as the bottom product. The vapour reaching the top of column is cooled and condensed to a liquid in the overhead condenser. Part of this liquid is returned to the column as reflux to provide liquid overflow and to control the temperature of the fluids in the upper portion of the tower. The remainder of the overhead stream is withdrawn as the overhead or distillate product. The typical distillation process tower is illustrated in Fig.1 and crude distillation of products across temperature range is illustrated in Fig.2

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1.2.1. 2.1 Absorption And Stripping Many operations in petrochemical plants require the absorption of components from gas streams into lean oils or solvents. The resultant rich oil is then stripped or denuded of the absorbed materials. The greatest use of this operation utilizes hydrocarbon materials, but the principles are applicable to other systems provided adequate equilibrium data is available. A typical flow diagram of absorption-stripping system for hydrocarbon recovery from gaseous mixture is illustrated in Fig. 3.

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1.2.2. 2.2 Fractional Distillation A fractionation column is a type of still. A simple still starts with mixed liquids, such as alcohol and water produced by fermenting grain etc. and by boiling produces a distillate in which the concentration of alcohol is many times higher than in feed. In petroleum industry, mixtures of not only two but a lot many components are dealt with. Crude oil is a typical feed for a fractionation column and from it, the column can form simultaneously several distillates such as wax distillate, gas oil, heating oil, naptha and fuel gas. These fractions are termed cuts. The feed is heated in a furnace before it enters the column. As the feed enters the column, quantities of vapour are given off by flashing due to release of pressure on the feed. As the vapours rise up the column, they come into intimate contact with down flowing liquid. During this contact, some of the heavier components of the vapour are condensed and some of the higher components of the down flowing liquid are vaporized. This process is termed refluxing. If the composition of the feed remains the same and the column is kept in steady operation, a temperature distribution establishes in the column. The temperature at any tray is the boiling point of the liquid on the tray. ‘Cuts’ are not taken from every tray. The P&ID will show cuts that are to be made, including alternatives. Nozzles on selected trays are piped and nozzles for alternate operation are provided with line blinds or valves. The fractionator tower is illustrated in Fig.4

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The typical vacuum tower and stripper is illustrated in Fig.5. Stripper is used to strip lighter materials from bottom of a main or a vacuum tower distilling crude bottom residue under vacuum.

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1.2.3. 2.3 Columns based Internals Columns based on internal details are often called as either Plate Columns or Packed Columns. Plate Column: The lighter hydrocarbons vaporize and flow up through the holes in the tray plate, making contact with the liquids on that tray. Tray types are: Bubble Cap trays, Valve trays, Sieve trays Bubble Cap Trays: Bubbling action effects contact. Vapour rises up through ‘risers’ into bubble cap, out through slots as bubbles into surrounding liquid on tray. Liquid flow over caps, outlet weir and downcomer to tray below. Valve Trays: Commonly used valve trays are stamped out by big press and these trays come with small valves attached to them which allow vapour traffic. Sieve Trays: Sieve trays are perforated flat plates. They are inexpensive for small diameter vessels but large diameter towers must have extensive supports for these trays. Sieve trays are used for heavy hydrocarbon fractionation.

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All trays have foam on top of liquid. The height of the foam will vary with the process. Foam may rise a foot or more above the tray liquid. Liquid-gas contacting is made effective through the above trays by cross-flow or counter flow. In counter flow plates, liquid and gas utilize the same openings for flow, thus there are no downcomers. Perforated plate with liquid cross flow (sieve plate) is the commonly specified tray. These two types of flow is illustrated in Fig.8.

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The two most commonly used types of tower viz. the trayed and packed arrangements are illustrated inFig.6 and Fig.7 respectively.

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1.3. 3.0 Required Information for piping The basic document listed shall be studied thoroughly for conceptual arrangement of piping around a column. 3.1 The basic layout and general engineering specifications describe:  The minimum access, walkways, platforms width and headroom requirements.  Handling facilities for tower internals, manhole covers, line blinds, relief valves.  Maximum rise of ladders.  Pipe-system requirements, such as open or closed relieving systems.  Minimum line-size and required hose-stations.

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Access to valves and instruments. 3.2 Design Standards show:  Details of ladder dimensions  Ladder and platform position (Step through or side step landings)  Toe-plate, handrail and safety-gate details. 3.3 P&ID and Technical specification of column provide :  Process data showing interconnected equipment and piping.  Pipe sizes and pipeline components.  Steam tracing and insulation thickness.  Tower elevations and differences in related equipment levels. 3.4 Plot Plan gives:  The physical location of a column and its relationship to other equipment.  Main access.  Main pipe run or pipe rack.  Location of pumps. A typical cross-section of a piperack running through the tower area of a refinery type plant is illustrated inFig.9.

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A typical plan of equipments located in the refinery type of plants highlighting the maintenance access is illustrated in Fig.10.

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3.5 Instrument standard shows:  The location of instrument connections to tower for gauges, level controllers and level alarms.  Location of pressure and temperature connections without orientation.  The instrumentation systems around the tower are depicted in the P&ID. 3.6 Fabrication drawing / detail dimensional drg. of column provides :  Diameter and height of column.  Details and dimensions of internals.  Manhole  Process-piping connections in elevation (without orientation)  Drum, pump, exchanger drawings giving details of adjacent process equipment or equipment supported on column itself. An integrated piping study should be developed from the above information. The piping study should take care of all the general recommendations of piping arrangement around the column and its related equipment and facilities as illustrated in Fig.11

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2.

PIPE RACK PIPING STUDY

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Pipe rack piping study:

My this post explains the steps required to be performed for carrying out pipe rack piping study. General arrangement of the piperack is finalized during the development of the overall plot plan. The piperack may be an integral part of a process unit located in the middle of the unit or it may be an arterial part connecting several services of other process units. The following data and drawings are required to be studied before starting the detailed design of pipe rack piping study:  Unit Plot Plan / Overall Plot Plan  Piping and Instrumentation diagrams  Plant layout specification  Client specification  Material of construction  Fireproofing requirements Normally, the piperack piping study, with its structural and platform requirements is the first priority item for detail engineering of a process unit. The points which are finalized during the piperack piping study are:  Exact width of the piperack  Numbers of levels and elevations  Access and maintenance platforms requirements There are number of pipelines which are routed through the pipe rack. Before starting the pipe rack piping study it is essential to classify these pipelines. Pipelines in the pipe rack are classified as process lines, reliefline headers and utility headers. 2.1.1. Process lines: (a) Which interconnect nozzles on process equipment more than 20ft. apart (closer process equipment can be directly interconnected with pipelines) (b) Product lines which run from vessels, exchangers, or more often from pumps to the unit limits to storage or header arrangement outside the plant. (c) Crude or other charge lines which enter the unit and usually run in the yard before connecting to exchangers, furnaces or other process equipment e.g. holding drums or booster pumps. 2.1.2. Relief-line headers: Individual relief lines, blow down lines and flare lines should be self draining from all relief valve outlets to knock-out drum, flare stack or to a point at the plant limit. A pocketed relief line system is more expensive, because usually an extra condensate pot is required with instruments, valves and pumps. To eliminate pockets some relief line headers must be placed at higher elevation above the main yard usually on a teesupport on the extended pipe rack column. However, on some non-condensing gas systems self drainage is not so essential. Relief lines can be individual, some with large diameters and occasionally high temperatures. 2.1.3. Utility lines: Utility lines in the pipe rack can be put in two groups: (a) Utility headers serving equipment in the whole plant. Such lines are: low and high pressure steam lines, steam condensate, plant air and instrument air lines. If required, cooling water supply and return and service water can also be arranged on the pipe rack. (b) Utility lines serving individually one or two equipment items or a group of similar equipment (furnaces, compressors) in the plant. Such lines are: boiler feedwater, smoothering steam, compressor starting air, various fuel oil lines, lubricating oil, cooling oil, fuel gas, inert gas and chemical treating lines. Steam header should drain to the steam separator for more effective condensate collection.

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Branch connections to steam headers usually connect to the top to avoid condensate drainage to equipment. 2.1.4. Instrument lines and Electrical cables: Instrument lines and Electrical cables are often supported in the yard and extra space should be provided for these facilities. The best instrument line arrangement eliminates almost all elevation changes between the plant and the control room. This can be easily achieved when instrument lines are supported outside the pipe rack column on a suitable elevation. 2.1.5. STEPS TO PIPE RACK PIPING:  The first step in the development of any pipe rack is the generation of a line-routing diagram. A line routing diagram is a schematic representation of all process piping systems drawn on a copy of pipe rack general arrangement drawing / or on the unit plot plan where the pipe rack runs in the middle of the process unit. Based on the information available on the first issue of P&I Diagram / Process flow diagram i.e. line size, line number, pipe material, operating temperature etc. the line routing diagram is to be completed.  Once the routing diagram is complete, the development of rack width, structural column spacing, road crossing span, numbers of levels and their elevations should be started. Pipe rack column spacing shall be decided based on the economics of the pipe span as well as the truss arrangement to accommodate double the span for road crossing or avoiding underground obstructions. Pipe rack arrangement should be developed to suit the specific plant requirements.  The pipe rack width can now be worked out with a typical cross-section of the rack with the levels. Normally, pipe racks carry process lines on the lower level or levels and the utility lines on the top level. Instrument and electrical trays are integrated on the utility level if space permits or on a separate level above all pipe levels. Any pipe rack design should provide provision for future growth to the extent of 25 to 30% on the rack clear width. When flanges or flanged valves are required on two adjacent lines, the flanges are to be staggered. Thermal expansion or contraction must be accommodated by keeping sufficient clearance at the location where the movements will occur. The clearance of the first line from the structural pipe rack column is to be established based on the sizes furnished by the civil / structural engineers.  After analyzing all the requirements and arrangements, the dimensions are to be rounded off to the next whole number. Based on the economics, the width and the number levels e.g. two tier of 30 ft. wide or three tiers of 20 ft. wide rack will be decided. The gap between the tiers shall be decided on the basis of the largest diameter pipeline and its branching. The difference between the bottom line of pipe in the rack and the bottom of a branch as it leaves the rack shall be decided carefully, to avoid any interference due to support, insulation, size of branch etc. All branch lines from the main lines on pipe rack shall be taken aesthetically on a common top of steel (TOS). With the above considerations, the conceptual arrangement of pipe rack is to be finalized.

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