Introduction to Piping General Arrangement Drawings

This post gives introduction to piping general arrangement drawings, like inputs required for generating any piping plans, presentation of piping plans, civil, structural details, instrumentation trenches, underground piping, supports, pipe racks etc.

Piping shall be routed so that the optimum piping layout is achieved in terms of process requirements, ergonomics, operation, inspection and maintenance. Having considered these factors, the number of flanges, fittings, valves and welds shall be minimized.

Piping General Arrangement Drawings

Piping GA drawings describe the arrangement of piping and may be drawn as plans and elevations. The plan and section method is the conventional or classical drawing procedure of orthogonal projections. Plan view at different elevations are prepared. Sections are taken through the piping and elevations are drawn if so required by the client. The drawing shall contain all the piping and equipment in the area for which it is prepared.

On the plot plan the piping routing is sketched free hand before beginning the drawings. The rack is sketched first. Then the lines from racks to equipment are drawn based on the best layout of lines.

Some terminology used in piping drawings are explained as follows.

• The battery limit is defined as the boundary of a plant area. It may be the boundary of in plant area or an offsite.
• The matching line of boundaries of two drawings is called the match line. Match lines are represented by co-ordinates in the horizontal plane and by elevation/ elevation ranges in the vertical plane.
• The term equipment is given to the items listed in the equipment list and includes vessels, heat exchangers, reactors, tanks, columns, pumps, blowers, compressors, etc.
• The connecting details of equipment input & out put to other equipment or environment is called the piping. The piping should satisfy the requirements of PEFS/ P&ID.
• The area in which main equipment are located is called the plant area. It is the area, within the unit plot plan where some process is taking place.
• The area apart of the plant area is called the offsite. Offsite behaves as a part of plant itself or an auxiliary service area for the plant area. Usually, Tanks farms and cooling towers are located in the offsite area.

Input required for generating piping General Arrangement drawing

The following are the data required to complete a GA drawing.

1. Overall plot plan and Area / Unit plot plan(s)
2. PEFS/ P&I Diagram
3. Process Equipment specifications and Equipment drawings
4. Line list/ schedule
5. Piping specification
6. Vendor drawings for instruments
7. Civil drawings like layout & details of buildings, columns, beams, foundations and drainage system.
8. Structural drawings of platforms, equipment supports & pipe bridge.
9. Interface with Vendor scope
10. Seal flushing & quenching arrangements
11. Standards of piping symbols
12. Design standards, typical standards & connecting details of the instruments to measure flow, pressure, temperature, level etc.
13. Piping arrangements for pump suction and discharge.
14. Dimensions of pipe fittings, valves, flanges etc.
15. Design standards of blanks, spacers, spectacles, vents & drains, etc.
16. Engineering standards for pipeline spacing & branch connections.

Presentation of Piping General Arrangement Drawings

1. Drawing scales shall be 3:1 00 except those meant for off-site, tank farms and pipe bridges for which the scale shall be decided depending on the area to be covered.
2. Plant North direction shall be marked on the top-right corner.
3. Direction of north shall be same for all the GA drawings in a project.
4. Battery limit / match line shall be indicated with co-ordinates on all four sides. Each match line shall be accompanied by the Continued on Drawing (COD) number.
5. The number of GA drawings will depend on the plant area and the extend of detailing required to take care of elevations.
6. The number of detail sheets required to cover the complete detailing of the area represented in a GA drawing will depend on the complexity of piping, the no of platforms / structures to be shown at various elevations, the volume of information notes to other departments, etc.

Equipment & Nozzle

1. Equipment location shall be from Plot Plan. Portable equipment shall be located by indicating the area specified for the purpose.
2. Nozzles shall be drawn to scale, as projected from the equipment with their orientation.
3. Mark flags to nozzle & indicate the nozzle number. Do not mark size, rating etc.
4. All the nozzles shall be oriented, even if there is no connection to it.
5. Size, rating, description, orientation and elevation of the nozzles shall be indicated in the orientation chart.
6. Indicate davit near manholes with hinge location where required.
7. Indicate starting & ending orientation for spiral staircases.

Civil details

1. Building & pipe bridge columns with center lines shall be marked in the GA drawing.
2. Pedestal for the equipment shall be shown along with the equipment.
3. There shall be a clear idea about the size & layout of beams, columns & foundations.
4. Location & size of insert plates required shall be shown in the drawings.
5. Location & size of cut outs required for the passage of lines and erection of equipment shall be shown in the GA drawings.

Structural

1. Indicate all the platforms including equipment platforms & operating platforms.
2. Indicate T.O.S. (top of steel) elevation for all the platforms.
3. Locate the corners of the platforms for rectangular/ square type platforms and indicate the inside & outside radius for the circular/ octagonal type platforms. Also indicate the starting & ending orientation for a sector of ring type platforms.
4. Ladders for the platforms/ equipment shall be located by specifying their orientation. Also specify the starting &ending elevations.
5. When equipment like air coolers, deaerators, desuperheaters are located over the pipe bridge, provide operating and maintenance platforms.
6. To get accessibility to manholes & hand holes provided with an equipment.
7. Orientation of ladders shall be indicated.
8. Indicate the load on platforms due to piping.

Piping – General arrangement

1. Piping shall be drawn with standard symbol representation. Symbols shall be drawn to scale according to the dimension of the piping item.
2. Layout of piping shall be drawn as per the PEFS/ P&ID and piping specification.
3. While making a piping drawing give primary importance to accessibility to operation and necessary clearances.
4. Provide necessary facilities for supporting the lines and the other piping materials.
5. Minimize the number of fittings and length of pipes for a good piping layout.
6. Avoid over crowding as well as wide spreading of pipelines which will create supporting problems.
7. Design standards and Engineering specifications for piping materials & instrumentation shall be strictly followed.
8. Provide necessary high point vents & low point drains, even if it is not mentioned in the PEFS/ P&ID.
9. Care to be taken to see that laying of pipe will not interfere the approach of crane/ trolley to the equipment, if it is required.
10. As far as possible routing of pipelines shall be near the beams, columns, and underneath of roof slab, such that it can be supported easily from those items.
11. As far as possible laying of pipe lines on the ground/ floor level shall be avoided.
12. For overhead lines maintain minimum headroom clearance.
13. If necessary, detailing of overcrowded lines and complicated connections can be done as an enlarged view in the free space available.
14. Pipelines shall be represented in single line up to 300 NB size and in double line above 300 NB size.
15. Reduction in size and branch connections to be clearly indicated by location, change in size and type of fitting-used.
16. Line identification number, co-ordinates of starting & ending points, BOP elevations to be marked for each fine.
17. Provisions to be made for supports to be anchored at certain locations and guide/ sliding supports at other locations as suggested by piping mechanical.
18. Before finalizing the routing of high temperature lines, stress analysis to be done to see that the expansion and supporting are within safe limits.

Piping –Pipe Rack/ Bridge

1. Spacing for pipelines over a pipe rack/ bridge shall be as per the relevant standards. Small lines unable to span the normal spacing shall be supported by intermediate pickups attached to the larger lines.
2. When there is a change of size for the line over the Pipe Bridge provide eccentric reducers, keeping the flat side on bottom to get the bottom of pipes at the same elevation.
3. As far as possible keep the utility lines over the top deck of the Pipe Bridge.
4. Vent header and lines to the flare stack shall be laid on the top deck.
5. When tapping are to be taken from a header, it shall be from the bottom side for liquids and from the topside for steam / gas lines.
6. Keep the lines having smaller size in the middle and higher size in the outer edges of the Pipe Bridge.
7. Keep the hot lines to the outer edge of the pipe bridges in order to support the expansion loop and permit maximum expansion movements in a minimum space.
8. As far as possible locate expansion loops near the pipe bridge columns for easiness of supporting.
9. Provide operating platforms with cat-ladders for valves located over pipe bridges.
10. The lines coming out or towards the Pipe Bridge shall have the elevation such that it can be supported from the tie beam provided.
11. Interference of piping in the space provided for the cable tray shall be avoided.

Piping – Underground

• Underground lines shall be shown in dotted lines. If the lines are in trenches, the trench dimensions also to be indicated.
• If there are a number of underground lines and there exists interconnections between them or with equipment, a separate drawing shall be prepared.
• The lines and equipment in an under-ground GA drawing shall be in firm lines.
• The match line elevation for underground GA drawing shall be at a height of 500 mm above the grade elevation.
• When underground pipelines are routed it shall be checked to avoid fouling with foundation and basement of equipment and buildings.
• When valves are to be erected in underground lines, the same shall be kept safely within a pit in such a way that valve operation and access are possible.

Piping – Tank farm

For tank farms, in addition to the tanks, pumps and other equipment the following items are also to be shown.

1. Dyke wall with stairways and landing and fencing.
2. Low point drains in the oil lines, which are flammable, shall not be connected to an open system.
3. Atmospheric venting is not recommended for flammable liquids and gases.
4. As far as possible make the interconnections between the tanks within the dyke walls, so that any leakage can be contained within the dyke wall.

Piping – Pumps

1. Pump suction and discharge lines shall be arranged such that it can be supported properly with no load applied on the nozzles.
2. The auxiliary piping details required to indicate the seal flushing / quenching and cooling connections as per API 610, shall be incorporated as per the PEFS/ P&ID.
3. Reducer in the suction line near the suction nozzle of horizontal centrifugal pumps shall be of eccentric type with flat side on top.
4. Temporary cone strainers shall be provided in the suction line.

Piping – Towers

1. Orientation of manholes, hand holes and skirt access openings should face tower access or drop out area. Manholes should be oriented such that they are located over the trays, not over or in front of down comers.
2. Segments around tower should be allotted to platforms, ladders and piping to be done without interfering the passage over the platforms.
3. Provide platforms for the accessibility to hand holes, manholes, locally mounted instruments and valves.
4. Orientation of ladders should be staggered to give necessary provision for safety.
5. Platform clips on bottom tangent line should be avoided.
6. Platform on top head of tower is only required around the top nozzle if there is a blind or valve.
7. Piping connections shall be such that it will not interfere with platforms, ladders and instruments.
8. No pipeline or instruments shall be oriented towards the drop out area.
9. Pipelines shall be grouped and arranged such that it can be supported from the tower itself. Provide suitable clips for supports and guides.
10. Instruments should be oriented so as not to obstructs the passage and ladders. To minimize platform width, level controllers should be located at the edge of platforms.
11. Relief valves discharging to atmosphere shall preferably be located at the highest tower platform pointing away from any other platform. Relief valves discharging in to a closed system shall be located at a platform above the header to which it is discharging without any pockets.

Piping – Heat Exchanger

1. It is necessary to provide crane access to air coolers/ heat exchangers for tube bundle removal and erection. No pipeline shall cross the area provided for tube bundle removal and bonnet removal.
2. Exchanger piping shall be such that there will be provisions for flexibility to accommodate expansion due to temperature, for the line and the exchanger.
3. For stacked exchanger the platform should be located under the bottom exchanger.

Piping – Pipe Supports

1. Pipe supports need not be; but the location and loading shall be indicated in the case of supports requiring Civil Engineering design.
2. If special type of supporting is required, it shall be drawn as a detail in any free space available on the drawing.
3. While detailing tower piping, the location and orientation of clips and type of support required shall be given as notes.

Instrumentation

1. Indicate all the instruments shown in the PEFS/ P&ID which are to be installed on equipment and lines.
2. Mark the location, size, rating and tag number of each on-line instrument.
3. The symbol representation for instruments shall be as per standard for the project. The symbols shall be drawn to scale as per the dimensions indicated by the vendor drawings.
4. The location of instruments shall be such that the connections and indicating dials will not block the passage ways and ladders.
5. The pressure gauge should be in the vertical upright position. For vertical lines, the tapping are taken towards the side in a horizontal plane and then bend upwards by an elbow.
6. Pressure connection in a control loop (PC, PRC, PRCa, etc.) shall generally be located at least 6 pipe diameters downstream from sources of flow disturbance (control valves, manual valves etc.). The location should to checked & confirmed by

Dimensioning

1. The dimensioning of shall conform to the following requirements
2. To be dimensioned fully: horizontally by co-ordinates & dimensions and vertically by elevation.
3. Duplication in dimensioning to be avoided.
4. Pipe elevations shall be to the bottom-of pipe (BOP), grouped to have a common support. For pipe lines running over the Pipe Bridge it shall be to the BOP (Bottom of pipe) elevation.
5. Location for valves shall be to the centreline for socket welded / screwed valves, end point for butt weld valves and face of flange for flanged valves.
6. Instruments to be located to the centre line for pressure instruments, temperature instruments & flow orifice and to the face of flange for rotameter & flanged control valves.
7. Valves on vertical lines shall be located by the elevation of the bottom flange face for flanged valves. BW valves on vertical lines shall be located to bottom end with suffix “B” to the elevation.
8. Grade and Platform elevations shall be boxed-up.
9. The dimensions of insert plates and cut outs can be specified by dimensions.
10. Rectangular / square platforms shall be dimensioned by co-ordinates or dimensions and the circular / sector of a ring type platforms shall be dimensioned by inside diameter and width of platforms.

Issue of Piping General Arrangement Drawings

After review within the department by stress analysis, static and rotating equipment lead engineers the drawing shall be released to all other disciplines, for Inter Discipline Checks and their comments.

Accepted comments and suggestions made by other departments during Inter Discipline Checks shall be incorporated.

Piping Plans issued to construction after Design review/HAZOP reviews completed and subsequent P&ID’s issued for construction.