This article should only be used as a guide. It's intended purpose is to help the piping designer who is responsible for placement of one specific item in a typical refinery, chemical or petrochemical process plant or someone who may need help in developing a total plot plan for a complex unit.
The guidelines given here are based on my many years of experience with one of the world's largest engineering, design and construction companies along with the U. S. OSHA Part 1910 and the NFPA (National Fire Protection Association) Code No. 30.
The latest editions of these codes and any other applicable national, regional and local codes should be referred to and used because they may be more stringent.
The subjects covered in this article have been arranged in alphabetical order in the hope it will make them easier to locate.
Access (See Maintenance)
Columns (See Vertical Vessels)
Locate centrifugal compressor as close as possible the suction source. Top suction and discharge lines either should be routed to provide clearance for overhead maintenance requirements, or should be made up with removable spool pieces.
Support piping so as to minimize dead load on compressor nozzles; the load should be within the recommended allowance of the compressor manufacturer.
Centrifugal compressors should have full platforming at operating level. Heavy parts such as upper or inner casing and rotor should be accessible to mobile equipment. Review the equipment arrangement for access and operation.
Locate lube and seal oil consoles adjacent to and as close as possible to the compressor. Oil return lines from the compressor and driver should have a minimum slope of 1/2 inch per foot to the inlet connection of seal traps, degassing tanks, and oil reservoir. Pipe the reservoir, compressor bearing, and seal oil vents to a safe location at least 6 feet above operator head level.
Locate reciprocating compressors so suction and discharge lines that are subject to vibration (mechanical and acoustical) may be routed at grade and held down at points established by a stress and analog study of the system.
Accessibility and maintenance for large lifts such as cylinder, motor rotor, and piston removal should be by mobile equipment if the installation is outdoors or by traveling overhead crane if the installation is indoors (or covered).
Horizontal, straight line, reciprocating compressors should have access to cylinder valves. Access should be from grade or platform if required.
Depending on unit size and installation height, horizontal-opposed and gas engine driven reciprocating compressors may require full platforming at the operating level.
Locate control valve stations accessible from grade or on a platform. In general, the (flow, level, pressure, temperature) instruments or indicators showing the process variables should be visible from the control valve.
Locate cooling towers downwind of buildings and equipment to keep spray from falling on them. Orient the short side of the tower into the prevailing summer wind for maximum efficiency. This means that the air flow (wind) will travel up the long sides and be drawn in to both sides of the cooling tower equally. When the wind is allowed to blow directly into one long side it tends to blow straight through and results in lower efficiency. Locate cooling towers a minimum of 100 feet (30m) from process units, utility units, fired equipment, and process equipment.
Cradles (See Insulation Shoes and Cradles)
Equipment Arrangement (General)
Arrange equipment, structures, and piping to permit maintenance and service by means of mobile equipment. Provide permanent facilities where maintenance by mobile equipment is impractical.
Group offsite equipment, pumps, and exchangers to permit economical pipe routing. Locate this equipment outside of diked storage areas.
Exchanger, Air Cooler (Fin Fans)
Air Coolers are in typically used in the cooling of the overhead vapor from tall vertical vessels or towers such as Crude Fractionators and Stripper Columns. The natural flow tends to follow gravity, where the tower overhead is the high point then down to the Air Cooler, then down to the Accumulator and finally the Overhead Product transfer pumps. With this in mind the Air Coolers are normally located above pipeways. This conserves plot space and allows the pipe rack structure with it's foundation to do double duty with only minor up grade to the design. If the pipe rack is not used then plot space equal to the size of the Air Cooler is required. In addition a totally separate foundation and stand alone structure is required.
Exchanger, "G" Fin (Double Pipe)
These exchangers can be mounted almost anywhere any they can be mounted (with process engineer approval) in the vertical when required. A G-Fin Exchanger is recognizable by its shape. One segment looks like two long pieces of pipe with a 180 degree return bend at the far end. It is one finned pipe inside of another pipe with two movable supports. This type of exchanger can be joined together very simply to form multiples in series, in parallel or in a combination of series/parallel to meet the requirements of the process. This exchanger is not normally used in a service where there is a large flow rate or where high heat transfer is required. The key feature with this exchanger is the maintenance. The piping is disconnected from the tube side (inner pipe). On the return bend end of this exchanger there is a removable cover. When the cover is removed this allows for the tube (inside pipe) to be pulled out. This exchanger is normally installed with the piping connections toward the pipe rack.
Exchangers, Reboiler (Kettle Reboiler)
Locate kettle reboilers at grade and as close as possible to the vessel they serve. This type of reboiler is identifiable by its unique shape. It has one end much like a normal Shell and Tube exchanger then a very large eccentric, bottom flat transition to what looks like a normal horizontal vessel. You could also call it a "Fat" exchanger. The flow characteristics on the process side of a kettle reboiler are the reason for the requirement for the close relationship to the related vessel.
Reboilers normally have a removable tube bundle and should have maintenance clearance equal to the bundle length plus 5 feet (1.5m) measured from the tube sheet.
Exchangers, Shell and Tube
Shell and tube exchangers should be grouped together wherever possible. Stacked shell and tube exchangers should be limited to four shells high in similar service; however, the top exchanger should not exceed a centerline elevation of 18 feet (5.5m) above high point of finished surface, unless mounted in a structure. Keep channel end and shell covers clear of obstructions such as piping and structural members to allow unbolting of exchanger flanges, and removal of heads and tube bundles.
Exchangers with removable tube bundles should have maintenance clearance equal to the bundle length plus 5 feet (1.5m) measured from the tube sheet to allow for the tube bundle and the tube puller.
Maintenance space between flanges of exchangers or other equipment arranged in pairs should be 1'- 6" (0.5m) (min.). Exchanger maintenance space from a structural member or pipe should not be less than 1'- 0" (300mm) (min.).
Furnaces (Fired Equipment)
Locate fired equipment, if practical, so that flammable gases from hydrocarbon and other processing areas cannot be blown into the open flames by prevailing winds.
Horizontal clearance from hydrocarbon equipment (shell to shell) 50'- 0" (15m) Exception: Reactors or equipment in alloy systems should be located for economical piping arrangement.
Provide sufficient access and clearance at fired equipment for removal of tubes, soot blowers, air preheater baskets, burners, fans, and other related serviceable equipment.
Clearance from edge of roads to shell 10'- 0"(3m)
Pressure relief doors and tube access doors should be free from obstructions. Orient pressure relief doors so as not to blow into adjacent equipment.
The elevation of the bottom of the heater above the high point of the finished surface should allow free passage for operation and maintenance.
Locate snuffing steam manifolds and fuel gas shutoff valves a minimum of 50 feet (15m) horizontally from the heaters they protect.
Burner Valving for a Floor Fired Furnaces: Combination oil and gas firing valves should be operable from burner observation door platform. For those fired by gas only, the valves should be near the burner and should be operable from grade.
Burner Valving for a Side Fired Furnaces: Locate firing valves so they can be operated while the flame is viewed from the observation door.
Locate the flare stack upwind of process units, with a minimum distance of 200 feet (60m) from process equipment, tanks, and cooling towers. If the stack height is less than 75 feet (25m), increase this distance to a minimum of 300 feet (90m). These minimum distances should be verified by Company Process Engineering.
Space for future equipment, pipe, or units should not be provided unless required by the client or for specific process considerations. When applicable this requirement should be indicated on the plot plan and P&IDs.
Insulation Shoes and Cradles
Locate Insulation shoes anywhere a line crosses a support for hot insulated piping when the piping is 3 inch (80mm) and larger carbon and alloy steel lines with design temperatures over 650 degrees F (350C).
Large diameter lines (20 inches (500mm) and over), stainless steel lines where galvanic corrosion may exist, lines with wall thickness less than standard weight, and vacuum lines should be analyzed to determine if shoes or wear plates are needed.
Provide cradles at supports for insulated lines in cold service and for acoustical applications.
Ladders & Cages
Maximum height of a ladder without a cage should not exceed 15'-0" (4.5m)
Maximum vertical distance between platforms 30'- 0" (9m)
Cages on ladders over 15'-0" (4.5m) high shall start at 8'-0" (2.5m) above grade.
Minimum toe clearance behind a ladder 0'- 7" (200mm)
Minimum handrail clearance 0'- 3" (80mm)
Locate liquid level controllers and level glasses so as to be accessible from grade, platform, or permanent ladder. The level glass should be readable from grade wherever practical.
Wherever possible, orient level instruments on the side toward the operating aisle.
Locate loading and unloading facilities that handle flammable commodities a minimum of 200 feet (60m) from away from process equipment, and 250 feet (75m) from tankage.
Maintenance Aisles (at grade)
Equipment maintenance aisle for hydraulic crane (12T capacity) should have a horizontal clearance width of 10'- 0" (3m) (min.) and a vertical clearance of 12'- 0" (3.5m) (min.). Where a fork lift and similar equipment (5000 lbs / 230kg capability) is to be used the horizontal clearance should be 6'- 0" (2m) (min.) and the vertical clearance should be 8'- 0" (2.5m) (min.).
Where maintenance by portable manual equipment (A-frames, hand trucks, dollies, portable ladders or similar equipment) is required the horizontal clearance should be 3'- 0" (1m) (min.) and the vertical clearance 8'- 0" (2.5m) (min.).
Operating Aisle (at grade)
Minimum width 2'- 6" (800mm)
Headroom 7'- 0" (2.1m)
Orifice Runs and Flanges
Locate Orifice runs in the horizontal. Vertical orifice runs may only be used with the approval of Company Control Systems Engineering. Orifice flanges with a centerline elevation over 15 feet (4.5m) above the high point of finished surface, except in pipeways, should be accessible from a platform or permanent ladder.
Locate orifice taps as follows:
Air and Gas
-Top vertical centerline (preferred)
-45 degrees above horizontal centerline (alternate)]
Liquid and Steam
-Horizontal centerline (preferred)
-45 degrees below horizontal centerline (alternate]
(Note: The piping isometrics should show the required tap orientations)
Locate eye wash and emergency showers in all areas where operating personnel are subject to hazardous sprays or spills, such as acid.
Personnel protection should be provided at uninsulated lines and for equipment operating above 140 degrees F (60 C) when they constitute a hazard to the operators during the normal operating routine. Lines that are infrequently used, such as snuffing steam and relief valve discharges, may not require protective shields or coverings.
Clearance between the outside diameter of flange and the outside diameter of pipe to the insulation should not be less than 0'- 1"* (25mm)
Clearance between the outside diameter of pipe, flange, or insulation and structural any member should not be less than 0'- 2"* (50mm)
*With full consideration of thermal movements
Minimum width for ladder to ladder travel: 2'- 6" (800mm)
Headroom: 7'- 0" (2.1m)
Headroom from stairwell treads: 7'- 0" (2.1m)
Minimum clearance around any obstruction on dead end platforms: 1'- 6" (500mm)
Locate all local pressure indicators so they are visible from grade, permanent ladder, or platform. Those located less than 15 feet (4.5m) above high point of finished surface should be accessible from grade or a portable ladder. Those located in a pipeway should be considered accessible by portable ladder. Those over 15 feet (4.5m) above high point of finished surface should be accessible from a platform or permanent ladder.
The relation of units, location of equipment, and routing of pipe should be based on economics, safety, and ease of maintenance, operation, and construction requirements. The alignment of equipment and routing of pipe should offer an organized appearance.
Process Unit Piping
Locate all pipe lines in major process units on overhead pipeways. In certain instances, pipes may be buried, providing they are adequately protected. Lines that must be run below grade, and must be periodically inspected or replaced, should be identified on the P&IDs and placed in covered concrete trenches.
Cooling water lines normally may be run above or below ground, based on economics.
Domestic or potable water and fire water lines should be run underground.
Locate pumps close to the equipment from which they take suction. Normally, locate pumps in process units under pipeways.
Design piping to provide clearance for pump or driver removal. Similarly, on end suction pumps, piping should permit removing suction cover and pump impeller while the suction and discharge valves are in place.
Arrange suction lines to minimize offsets. The suction lines should be short and as direct as possible, and should step down from the equipment to the pump. Suction lines routed on sleeperways may rise to pump suction nozzle elevation.
Orient valve handwheels or handles so they will not interfere with pump maintenance or motor removal. Valve handwheels or handles should be readily operable from grade.
Maintenance and operating aisles with a minimum width of 2'-6" (800mm) should be provided on three sides of all pumps.
Provide temporary conical type strainers in 2 inch (50mm) and larger butt weld pump suction lines for use during startup. Arrange piping to facilitate removal.
Use permanent Y-type strainers on 2 inch (50mm) and smaller screwed or socket weld pump suction piping.
Headroom over through-railroads (from top rail) 22'- 6"** (7m)
Clearance from track centerline to obstruction 10'- 0"** (3m)
(** Verify conformance with local regulations)
Relief Valves (Pressure, Safety and Thermal)
Locate all relief valves so they are accessible. Wherever feasible, locate them at platforms that are designed for other purposes. Relief valves with a centerline elevation over 15 feet (4.5m) above high point of finish surface (except in pipeways) should be accessible from platform or permanent ladder.
Pressure relief valves that discharge to a closed system should be installed higher than the collection header. There should be no pockets in the discharge line.
Safety relief valves (in services such as steam, etc.) that discharge to the atmosphere should have tail pipes extended to a minimum of 8 feet (2.5m)above the nearest operating platform that is within a radius of 25 feet (7.5m). This requirement may be waived, provided a review of the proposed arrangement indicates that it does not present a hazard. Review all pressure and safety relief valves discharging flammable vapors to the atmosphere within 100 feet (30m) of fired equipment for vapor dissipation.
Pressure and Safety relief valves, 1-1/2 inch (40mm) and larger, should only be installed with the stem and body vertical position.
Thermal relief valves, 1 inch (25mm) and smaller, may be installed with the stem and body in a horizontal position when it is impractical to install it in the vertical position.
Major process plants normally have three classes of roads. They might be called Primary roads, Secondary roads and Maintenance access ways.
Clearance or distance required
|Road type||Vertical||Width||Shoulder||Side or off road|
|Primary||21'-0" (6.5m)||20'-0" (6m)||5'-0" (1.5m)||20'-0" (6m)|
|Secondary (*)||12'-0" (3.7m)||12'-0" 3.7m)||3'-0" (1m)||10'-0" (3m)|
|Maintenance access||10'-0" (3m)||10'-0" (3m)||(not req'd)||5'-0" (1.5m)|
(*) Normally secondary plant roads may be used as tube pull areas.
Provide a primary means of egress (continuous and unobstructed way of exit travel) from any point in any building, elevated equipment, or structure. A secondary means of escape should be provided where the travel distance from the furthest point on a platform to an exit exceeds 75 feet (25m).
Access to elevated platforms should be by permanent ladder. Safety cages should be provided on all ladders over 15'-0" (4.5m)
The need for stairways should be determined by platform elevation, number of items requiring attention, observation and adjustment, and the frequency of items.
Ladder safety devices such as cable reel safety belts and harnesses, may be investigated for use on boiler, flare stack, water tank, and chimney ladders over 20 feet (6m) in unbroken lengths in lieu of cage protection and landing platforms.
Locate all sample connections so they are readily accessible from grade or platform.
In general, where liquid samples are taken in a bottle, locate the sample outlet above a drain funnel to permit free running of the liquid before sampling.
Hot samples should be provided with a cooler.
Sleeper Pipe Supports
Normally, route piping in offsite areas on sleepers. Stagger the sleeper elevations to permit ease of crossing or change of direction at intersections. Flat turns may be used when entire sleeper ways change direction.
Locate spectacle blinds to be accessible from grade or platform. Blinds located in a pipeway are considered accessible. Blinds that weigh over 100 lbs (45kg) should be accessible by mobile equipment. Where this is not possible, provide davits or hitching points.
Closely grouped flanges with blinds should be staggered.
Locate all steam traps at all pocketed low points and at dead ends of steam headers. Also, provide traps periodically on excessively long runs of steam piping, for sufficient condensate removal, and to ensure dry quality steam at destination. Steam traps should be accessible from grade or a platform. Steam traps located in pipeways should be considered accessible by portable ladder.
Locate any tankage containing hydrocarbon or other combustible fluids or gasses a minimum distance of 250'-0" (115m) from any process unit, rail loading facility or truck loading facility.
The minimum spacing of offsite storage tanks and dike requirements should be in accordance with the latest edition of the National Fire Protection Association, Code No. 30, and OSHA part 1910.106 (b), where applicable.
Locate temperature test wells, temperature Indicators and thermocouples to be accessible from grade or a portable ladder. Those located in a pipeway should be considered accessible by a portable ladder. Those located over 15 feet (7m) above high point of finished surface should be accessible from a platform or permanent ladder.
Locate all local temperature indicators (TI) should be visible from grade, ladder, or platform.
Towers (See Vertical Vessel)
Provide and locate utility stations with water, steam, or air as indicated below:
All areas should be reachable with a single 50 foot (20m) length of hose from the station.
Provide water outlets at grade level only, in pump areas, and near equipment that should be water washed during maintenance.
Provide steam outlets at grade level only in areas subject to product spills, and near equipment that requires steaming out during maintenance.
Provide air outlets in areas where air-driven tools are used such as at exchangers, both ends of heaters, compressor area, top platform of reactors, and on columns at each manway.
Hose, hose rack, and hose connections should be provided by the client or be purchased to match the clients existing hardware.
Valve Handwheel Clearance
Clearance between the outside of hand wheel and any obstruction (knuckle clearance) should be 0'- 3" (80mm)
Locate operating valves requiring attention, observation, or adjustment during normal plant operation (noted on the P&IDs) so they may be within easy reach from grade, platform, or permanent ladder as follows:
- 2" (50mm) and smaller may be located reachable from a ladder.
- 3" (80mm) and larger must be reachable and operable on a platform
Operating valves with the bottom of handwheel is over 7 feet (2.1m)above high point of finished surface or operating platform may be chain-operated.
The centerline of handwheel or handles on block valves used for shutdown only, located less than 15 feet (4.5m) above high point of finished surface, and those located in pipeways, may be accessible by portable ladder.
The centerline of handwheel or handles on block valves used for shutdown only and located over 15 feet (4.5m) above high point of finished surface, except those located in pipeways, should be operable from permanent ladder or platform.
In general, keep valve handwheels, handles, and stems out of operating aisles. Where this is not practical, elevate the valve to 6'- 6" (plus or minus 3 inches) clear from high point of finished surface to bottom of handwheel.
Vents and Drains
The P&IDs should indicate, locate and size all vents, drains, and bleeds required for process reasons and plant operation.
Provide plugged hydrostatic vents and drains without valves at the high and low points of piping.
Provide valved bleeds at control valve stations, level switches, level controllers, and gage glasses per job standard.
Vertical Vessel (Column) Piping and Platforms
Locate vertical vessels in the equipment rows on each side of the pipeway in a logical order based on the process and cost. The largest vessel in each equipment row should be used to set the centerline location of all vertical vessels in that equipment row. This largest vertical vessel should be set back from the pipe rack a distance that allows for; any pumps, the pump piping, an operation aisle between the pump piping and any piping in front of the vessel, the edge of the vessel foundation and half the diameter of this the largest vessel. Set all other vertical vessels in this same equipment row on the same centerline.
Provide a clear access area at grade for vessels with removable internals or for vessels requiring loading and unloading of catalyst or packing.
Provide vessel davits for handling items such as internals and relief valves on vessels exceeding a height of 30 feet (9m) above the high point of the finished surface, and on vessels not accessible by mobile crane. Orient davits to allow the lowering of appurtenances into the access area.
Walkways should have a 2'-6' (1m) horizontal clearance (not necessarily in a straight
line) and headroom of 7'- 0" (2.1m)
About the Author
James O. Pennock has more than forty-five years in the process plant design profession. He has been involved in both home office and job site assignments on refinery, chemical, petrochemical, power and other projects. His experience ranges from entry level designer to engineering manager. Much of this was with Fluor. He is also the author of the book "Piping Engineering Leadership for Process Plant Projects." He is now retired, living in Florida, USA and does only occasional consulting work.