section ii - equipment piping and assembly applications
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Section II - Equipment Piping and Assembly Applications
Back To Main
A: General Guidelines for Equipment and Piping Location, Spacing, Distances and Clearances-
By: James O. Pennock
This article is intended to aid both the novice and experienced piping designer with guidance for plot
plan development.
C1: Introduction to Vessels and Vessel Orientation - By: James O. Pennock
C2: Vertical Vessel Orientation - By: James O. Pennock
(BACK TO TOP)
Section - II
A: General Guidelines for Equipment and Piping Location, Spacing, Distances
and Clearances
By: James O. Pennock
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)
Compressors, Centrifugal
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.
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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.
Compressors, Reciprocating
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.
Control Valves
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.
Cooling Towers
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 towerequally. 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)
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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 pulledout. 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 thebundle 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
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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 equipmentfor 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.
Furnace Piping
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.
Flare Stacks
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.
Future Provisions
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.
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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 todetermine 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)
Level Instruments
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.
Loading Racks
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)
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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)
Personnel Protection
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.
Pipe
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
Platforms
Minimum width for ladder to ladder travel: 2'- 6" (800mm)
Headroom: 7'- 0" (2.1m)
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Headroom from stairwell treads: 7'- 0" (2.1m)
Minimum clearance around any obstruction on dead end platforms: 1'- 6" (500mm)
Pressure Instruments
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.
Process Units
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.
Pumps
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, andshould 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.
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Maintenance and operating aisles with a minimum width of 2'-6" (800mm) should be provided on three
sides of all pumps.
Pump Strainers
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.
Railroads
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.
Roads
Major process plants normally have three classes of roads. They might be called Primary roads,
Secondary roads and Maintenance access ways.
Clearance or Vertical Width Shoulder Side or off road
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distance
requiredRoad type
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.
Safety Access
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 l ieu of cage
protection and landing platforms.
Sample Connections
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.
Spectacle Blinds
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.
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Steam Traps
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.
Tankage
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.
Temperature Instruments
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)
Utility Stations
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
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Clearance between the outside of hand wheel and any obstruction (knuckle clearance) should be 0'- 3"
(80mm)
Valve Operation
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.
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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
Walkways should have a 2'-6' (1m) horizontal clearance (not necessarily in a straight
line) and headroom of 7'- 0" (2.1m)
(BACK TO TOP)
Section - II
C: Introduction to Vessels and Vessel Orientation
By: James O. Pennock
The question on many minds may be "Why does Piping do Vessel Orientation?" We can answer that
question two ways. The first answer would be, because of the traditional role of Piper and the content
of the vessel orientation activity itself. The traditional role of the Piper has always been the bringing
together of multi-discipline information to create the plant layout and piping plans. The activity of vessel
orientation has the same multi-discipline focus.
The second way to answer the question is to ask "If not the Piper, then who?" Civil? Structural?
Electrical? Instrumentation? No, they are not logical candidates. Structural? The structural engineer
does engineer the support for some vessels but they do not truly design the support. Process? While the
process engineer does have a great deal of interest and input in the workings of a vessel, their interest is
more from a function and performance focus. Vessels? Why doesn't the vessel engineer do the vessel
orientation? Or better yet, why doesn't the Vendor do the vessel orientation? The response to that is in
all of the non-vessel factors that influence the vessel orientation activity. What are non-vessel factors?
Non-vessel factors include:
A. Site -- Vessel orientation is influenced by where the vessel is located on the site
B. Relationship to related equipment -- Proper vessel orientation must consider the location and method
of connection to related equipment
C. Support -- Vessel orientation of many vessels includes the method of support
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D. P&ID interpretation -- The person responsible for vessel orientation must be very proficient in reading
and understanding a P&ID
E. Internals to external object relationships -- Internals effect the nozzle locations that in turn connect to
the piping. The piping is subject to thermal expansion, and must be supported. The piping must meet all
the process requirements from the P&ID, and must be in compliance with the Plant Layout DesignSpecification. The piping must also be supported, and must meet the all the applicable Code criteria, etc.
F. Operations and Maintenance -- Vessel orientation must be compatible with the requirements of the
operators and the people who must maintain the vessels.
This brings us back to answer number one. Vessel orientation requires the bringing together of and the
coordination of data and requirements from many disciplines. Piping in their Plant Layout role is already
functioning in this mode. Most major engineering and design firms (in our Industry) have found that
Piping Design is the most logical and most efficient group for developing complex vessel orientations.
The ideal scenario for the development of a vessel orientation is like a chain. The links of the chain arelike the steps required completing the finished design. With the ideal scenario you would not start step
two until step one is completed and so on. The ideal circumstances means that the Plot Plan has been
firmed up and approved, the P&IDs have been developed, reviewed, and issued approved for design
(AFD). It means that the unit piping transposition has been developed. It means that Process has
completed their input to the vessel datasheet and Vessels has completed their preliminary work.
Occasionally, the piping designer has been required to initiate a vessel orientation under other than the
most ideal of circumstances. In some cases the vessel orientation has been started before the P&IDs
were ready for the first Client P&ID review. Starting Vessel orientation before the source documents are
ready will expose the job to risks, errors, recycle and increased costs.
As much as we try to avoid this situation, it can still happen. Premature starts in vessel orientation are
due to the requirement for early purchase of vessels identified as long delivery. The Construction
schedule of any project is based on the delivery of key equipment and materials. The construction
schedule in turn will impact the start-up schedule. Once the Client has awarded the project, they are
anxious to get their plant "on-stream" as soon as possible. The sooner they get on-stream, the sooner
they can recover the capitol investment and see the expected profits.
The delivery time for vessels such as: alloy reactors, heavy wall high pressure vessels, or crude vacuum
columns often take more than a year from PO (purchase order) release to shipment. In the past, one
way to expedite the overall schedule, the Client has pre-purchased the vessels prior to the award of theproject. There is a potential risk for increased cost in this scenario also.
Under normal circumstances a Vessel fabricator will not normally do any rolling and cutting of plate until
the order has reached a certain milestone. They will need the final checked, corrected and approved
vessel drawings. This includes all the nozzles, pipe supports, pipe guides, ladders, platforms, etc. The
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Vendor's fabrication and delivery performance clock does not start ticking until they get the drawings
back approved.
A project with a fast track schedule or pre-purchased vessels will put a lot of pressure on the piping
design group. Piping should normally have time to properly develop the Plot Plan, the P&ID
transposition, the other related piping layouts, in order to come up with the best vessel orientations foreconomics, operability, and maintenance.
As piping designers you owe it to the Client, your company, as well as to yourself to do the best job you
know how. This philosophy is true when doing vessel orientations as with any other piping design
activity. You should check into all aspects of the vessel piping and the orientation. You need to start by
collecting, verifying, and using the proper information.
During Plot Plan development, the piping designer must take into consideration many items that can
also have a bearing on the vessel other than the orientation itself.
Such items include:
Lay-down space -- Prior to erection, tall columns require space for final assembly
Erection equipment -- The cranes (or other lifting devices) planned to lift and set the vessels require vast
amounts of space
Plant road limitations; Rack heights, shoulder clearances, logistics
Special vessels such as Reactors have several factors, which should be kept in mind. The most important
one, of course, is to keep the alloy piping as short as possible by locating the Reactors near the Heaters.
Catalyst handling facilities is another important consideration. This is true whether the catalyst is to be
loaded by crane or by vessel mounted monorail. The removal of spent catalyst, usually by tote bin, truck,
or conveyor, is another space consideration.
We all need to remember space is money to the Client. Wise use of plot space can save the Client
money by reducing installation costs and operating costs.
Vessel Configurations
Vessels come in a wide variety of configurations. The variety is expressed in their sizes, shape, and
function. They also will have a wide range of pressure, temperature and metallurgy. This list is only
intended to highlight the main examples.
Vertical Vessels with no internals
(A.k.a.: Tanks, Drums, and Pots)
Example: Mix Tank, Air Receiver, Volume Bottle, Flash Drums, Fuel Gas K. O. Pot, Feed Surge Drum, and
Dump Tank
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Discussion: This type of vessel will normally be small (< 24" diameter x 3' - 0" T-T) to medium sized
(24"dia to 48" diameter x < 10" - 0" T-T). They may be mounted to the support surface (grade, floor, or
platform) via a traditional vessel skirt, attached legs, or lugs. When located at grade this vessel may be
mounted directly on the concrete paving or floor depending on vessel weight and soil conditions.
Vertical Vessels with simple Internals
Simple internals such as Demister Pads
Example: Feed Knockout Drum, Separator Drum, Filter, and Coalescer Drum
Discussion: This type of vessel will normally be medium (24"dia to 48" diameter x < 10" - 0" T-T) to large
sized (Over 48" diameter and over 10' - 0" T-T). They may be mounted to the support surface (grade or
platforms) via a traditional straight vessel skirt, a flared skirt, attached legs, or lugs. When located at
grade this vessel will normally be mounted on an octagon foundation.
Vertical Trayed Vessels with straight sides
Example: Fractionator, Contactor, and Stripper
Discussion: This type of vessel can be as small as two or three feet in diameter or may be very large at
20' - 0" or more in diameter. The diameter, height, number of trays, type of trays along with the other
related items depends on the function. These vessels will normally be supported at grade via a
traditional vessel skirt. This vessel will normally be supported on the traditional 9" to 1' - 0" high octagon
concrete foundation.
Vertical Trayed Vessels - Coke Bottle (two diameters w/ transition)
Example: Splitter, Stabilizer, Lean Oil Still, and Absorber Column
Discussion: This type of vessel will have two diameters. The Coke Bottle Vessel is a multi purpose vessel.
The larger section will have different internals and function differently than the smaller section. The
bottom of the Column will normally be the larger diameter with a conical transition piece to join the
two. This type vessel will normally be mounted at grade via a traditional vessel skirt and be supported
on an octagon foundation.
Variation: A variation of this type vessel is the Inverted Coke Bottle. The Inverted Coke Bottle Vessel will
normally have a short skirt at the transition point and be mounted on an elevated platform in a
structure. The smaller (lower) section will hang down inside the structure.
Vertical Packed Tower Vessels
Example: Dryers, Feed Purifiers,
Discussion: these types of vessel will normally be medium sized. Packing may be a manufactured mesh
or a granulated natural material. The location and orientation of this type of vessel must consider the
loading and removal of the packing. These vessels may operate at ambient, temperatures, the lower
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Horizontal Vessels - Elevated without Boots
Example: Steam Drum, and Feed Surge Drum
Discussion: these types of vessel will normally be medium to large sized. They will be mounted to the
support surface (foundation or platforms) on traditional vessel saddles. When located near grade this
vessel will normally be mounted on an elevated foundation. The NPSH requirements of the related
pumps are critical to setting of the support elevation.
Horizontal Vessels - Elevated with Boots
Example: Stripper Receiver, Accumulator, Interstage K. O. Drum, and Flare K. O. Drum
Discussion: these types of vessel will normally be medium to large sized. They will be mounted to the
support surface (foundation or platforms) on traditional vessel saddles. When located near grade this
vessel will normally be mounted on an elevated foundation. Access is normally required for the Boot
operating valves and instruments. The NPSH requirements of the related pumps are critical to setting of
the support elevation.
Horizontal - Underground or Pit Vessels
Example: Dump Tank, Kill Tank, and Hazardous Material Storage Tank
Discussion: This type of vessel may be small, medium, or large in size. They will be mounted to the
support surface on traditional vessel saddles. When located at grade this vessel will normally be
mounted on a low foundation. When located in a pit, the pit size must allow for safety, operation, and
maintenance. Pit mounted installations may also require sumps and drainage pumps. Underground
(buried) installations may require double wall tanks with leak detection provisions.
API Storage Tanks
Example: Feed Storage, Intermediate Product Storage, Off-Spec Product Storage, Finished Product
Storage, Batch Storage, Fire (or other) Water Storage
Discussion: These are the traditional Tank Farm tanks. There are a number of sub-types, which include
Cone Roof Atmospheric; Cone Roof with captured venting, Open Floating Roof, Enclosed Floating Roof,
and Double Wall LNG Storage Tanks. These tanks have specific location, support, piping connection,
safety, and access criteria based on the commodity to be stored.
Special
Example: Spheres, Spheroids, and Bullets
Discussion: These vessel types have special location and orientation criteria and should be handled on
an Ad Hoc basis.
Vessel Supports
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There is a wide variety in the methods used to support vessels.
There include:
a. Skirts
b. Saddles
c. Ring Girders
d. Lugs
e. Legs
f. Portables on Casters
g. Pads
h. Direct Bury
Each of these support types may also have variations
Vertical Vessel Components
The pressure containment elements of the vessel are based of the process requirements for pressure,
temperature, commodity, corrosion rate, plant life criteria, and the applicable Codes.
The Pressure containment components include the following:
a) Shell
b) Heads
c) Boot
d) Transitions (Coke Bottle Vessels)
e) Nozzles
The other components include the following:
a) Trays
b) Internal piping
c) Support
d) Load Handling Devices
e) Pipe supports and Guides
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f) Platforms, Ladders, and Cages
g) Code Name Plate
Vertical Vessel Terminology
Normally vessel components are described using common terms such as shell, head, nozzle, and
support. Some vessels will also have special terms based on function.
Typical special terms include the following:
a) Flash Section -- The area or zone of the fractionation vessel where the primary feed enters the vessel.
b) Fractionation Section -- The portion of the vessel that includes the trays.
c) Stripping Section -- A place in the vessel that includes the introduction of supplementary heat such as
high temperature steam
d) Surge Section -- The bottom portion of the vessel that normally includes the main outlet nozzle which
is connected to the bottoms pumps.
Shell
The shell of the vertical trayed vessel will have many variables including the following:
a) Wall thickness
b) Metallurgy (May have different material at top vs. bottom)
c) Layers (single layer vs. multiple layer or cladding)
d) PWHT (Post weld heat treat) requirements for all or part
e) Vacuum reinforcement rings
f) Insulation support rings
Heads -- Top and Bottom
Heads for vessels will include the following shapes:
a) Dished -- The Dished head is a flatter version of the Semi-Elliptical
b) Semi-Elliptical -- The traditional type used on process plant pressure vessels (2:1 SE Head)
c) Spherical -- This head is sometimes referred to as a round head or Hemispherical-head
The top head and the bottom head may be the same shape but they will have some differences.
The differences for the top head include:
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a) Same material as top of Shell
b) May be thicker material for reinforcing
c) May be thinner material
The differences for the bottom head include:
a) Same material as bottom of Shell
b) May be thicker material for reinforcing
c) May be thinner material
Transitions
The cone or transition piece for regular and inverted Coke Bottle vessels may come in the following
shapes:
Flat side -- The cone is cut from flat plate and formed to a simple cone. There is no knuckle radius at the
top or bottom of the cone. The connection to the straight shell of the vessel is an angled weld. Usually
there is a reinforcing ring on the shell very close to the shell/cone junction.
Shaped side -- The cone is cut from flat plate and rolled to a shaped cone. There is a knuckle radius at
the top and bottom of the cone. The cone has a straight tangent at the top and bottom to match the
shells. The connection to the straight shell of the vessel is a common butt weld.
Nozzles
Overhead Vapor Outlet Nozzles
The overhead vapor outlet nozzles on a vertical vessel can have some latitude when it comes to
attachment location. The attachment connection can be direct to the top head of the vessel or may be
from the side. When the connection is from the side there will normally be a pipe inside the vessel
angled up to the top head area. Small vapor outlet nozzles from small diameter vessels can be located
out the side of the vessel and still be cost effective. Large diameter vapor outlet nozzles on large
diameter vessels will be more cost effective if attached to the top head. The line is then looped over to
the selected pipe drop position to go down the vessel.
Heater/Vessel Feed Transfer (Feed Inlet) Nozzles
All vertical fractionation vessels will have a feed inlet nozzle. This feed nozzle is special and critical on
some vessels. Refinery Crude columns and Vacuum columns are examples that have this type of nozzle.
This nozzle installation is characterized by the following:
a) Attached line originated at a fired heater
b) High temperature
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c) High velocity
d) Mixed phase flow
e) May require internals such as a distributor pipe or impingement plate
A Feed Transfer nozzle will normally be the "Key" (Genesis) nozzle for any large fractionation vessel.
Normally any side inlet orientation is possible but in most cases this will then dictate the tray
orientation.
Liquid (secondary) Inlet Nozzles
A normal liquid feed nozzle will not have the same complexities as the Feed Transfer type. This nozzle
installation is characterized by the following:
a) Attached line originated at an exchanger
b) Hot but not overly high on the temperature scale
c) Some may have potential for mixed phase flow
d) Normal line velocity
e) May require vessel internals such as a distributor or inlet pipe
f) Watch Instrument connections in relationship to Inlets and reboiler returns.
Reflux Nozzles
A normal reflux nozzle will not have the same complexities as other nozzles.
This nozzle installation is characterized by the following:
a) Attached line originated at a pump
b) Low on the temperature scale
c) All liquid flow
d) Normal line velocity
e) May require internals such as a distributor or inlet pipe. Multiple pass trays will require a morecomplex distributor or inlet pipe than a single pass.
Draw-Off Nozzles
The purpose of this nozzle is to draw-off or remove the primary product. They are also used to Draw-off
a secondary product to side stream stripper. May be installed with a sump to remove unwanted water in
the process stream.
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This nozzle installation is characterized by the following:
a) Located in the downcomer area of the column
b) May be in a sump
c) May be a larger size than the normal attached line size (Some of the initial vertical drop will be the
larger size)d) All liquid flow
e) Normal line velocity May require internals if multiple pass trays
Bottom Reboiler Feed Nozzles
The liquid outlet nozzle will normally be in the center of the bottom vessel head.
This nozzle installation is characterized by the following:
a) Located in the bottom of the surge section of the column
b) May be a very large size and all liquid flowc) Normally very low line velocity
Side Reboiler Feed Nozzles
This is also a potential Key Nozzle. The liquid outlet nozzle must be oriented in the same quadrant as the
bottom downcomer.
This nozzle installation is characterized by the following:
d) Located in the downcomer area of the column
e) Will be in a sumpf) May be a larger size than the normal attached line size (Some of the initial vertical drop will be the
larger size)
g) All liquid flow
h) Normal line velocity
i) Relationship to elevation of associated Reboiler is critical to nozzle elevation and internals
Side Reboiler Vapor Return Nozzles
One of the primary issues with this nozzle is the orientation relative to the other internal items and
nozzles. If not placed in the right place the velocity of the return can blow liquid out of a seal pan or can
affect the readings of any instruments attached to the far wall.
This nozzle installation is characterized by the following:
a) Attached line originated at a thermo-siphon or kettle type reboiler
b) High temperature
c) Moderately high velocity
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d) All vapor flow
e) May require internals such as a pipe or impingement plate
f) Relationship to elevation of associated Reboiler is critical to nozzle elevation and internals
Bottoms Out and Drain Nozzles
The bottoms-out nozzle is normally a pump suction source. The standard type is located in the bottom
head then piped through the skirt with a drain nozzle off the bottom out line nozzle. This would be a
combination nozzle. A variation of the bottoms nozzle is the siphon or winter type. This type may be
used (with process approval) when bottom clearance is a problem.
Note: It is common industry practice to avoid locating any flanged connections inside the vessel support
skirt. All flanges are subject to leaks, and vessel skirts are classified as a confined space.
Level Instrument Nozzles
Extreme care must be used when locating level instrument nozzles. There are access and clearances
problems that must be considered on the outside of the vessel. There are sensing location and
turbulence problems associated with the inside of the vessel.
These nozzle installations are characterized by the following:
a) Must be attached in the same pressure volume of the vessel
b) Lower nozzle in liquid of the surge section, upper nozzle in vapor space
c) Located in static area (or with stilling well)
d) Requires external access for operation and maintenance
Pressure Instrument Nozzles
Pressure readings are normally taken in the vapor area of a vessel. Pressure connections shall be located
in the top head area, 3" to 6" under a tray, or well above any liquid level in bottom section.
These nozzle installations are characterized by the following:
a) Located in a vapor space of the vessel
b) Requires external access for operation and maintenance
Temperature Instrument Nozzles
Temperature readings are normally taken in the liquid area of a vessel. Temperature connections shall
be located 2" to 3" above the top surface of a tray, in the downcomer, or well below any liquid level in
bottom section.
These nozzle installations are characterized by the following:
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a) Located in liquid in the downcomer area
b) Requires external access for operation and maintenance
c) Interference with internals
Vapor temperature readings may be required for some situations. When required the preferred location
is in the downcomer area half way between the two trays.
Tangential or Hillside connections may be required due to the thermowell length or to accommodate
access from the ladder and platform arrangement. With the Process Engineer's approval investigate the
possibility of raising or lowering the temperature point one tray for better ladder and platform
arrangement.
Steam-Out Nozzles
Process plant vessels that contain hydrocarbon or other volatile fluids or vapors will normally have a
Steam-Out Nozzle. This nozzle has a number of options such as:
a) A simple blind flanged valve on the nozzle -- After the plant is shut down by Operations, the
maintenance group would remove the blind flange from the valve. They then attach a temporary flange
fitted with a hose coupling and proceed to steam out the vessel by connecting a hose from a utility
station.
b) A blind flanged valve and hard piped steam line configured with a steam block valve and a swing ell.
c) A fully hard piped connection from a steam source. This method would have double block valves, a
bleed, and a spec blind for positive shutoff.
The vessel steam-out nozzle should be located near the surge section (bottom) Manhole on vertical
vessels.
Manholes
Manholes are also considered a nozzle. They just do not have any pipe attached to them. They are
however, a very complex piece of the vessel orientation puzzle. The types of manholes normally relate
to the method of cover handling provided.
Manholes come in the following types:
a) With Hinge -- A Manhole may be hinged for side mount, for top mount, or for bottom mount
b) With Davit -- A Manhole may have davits for side mount or top mount only
c) Plain -- A Plain Manhole may be for side mount, for top mount, or for bottom mount
The manhole orientation in top or non-trayed section of a vertical vessel is somewhat flexible. Normally
any orientation is possible; however, the orientation of the manhole should be checked to insure that
the entry path is not blocked by any internals.
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The Manhole may be located in the top head on large diameter vessels if there is a platform that is
required for other items. Top Manholes on large diameter vessels have their built in good points and
bad points. The good point is that during shutdown the open manhole provides for better venting. It
also allows for a straight method for removal and reinstallation of the trays. The bad point is that ladder
access must be provided down to the top tray, and the manhole is competing with the other nozzles for
the space on the vessel head.
Orientation for manholes that are located in the trayed section of the vessel is more complicated. The
location of between the tray manholes has a number of restrictions. These restrictions include the type
of trays and the tray spacing. The first choice for the location of a manhole is between the down comers.
The last choice is in the downcomer space, but behind the downcomer. The downcomer would be fitted
with a removable panel to allow further access into the vessel. The location to be avoided is above a
downcomer where there is the potential for falling down in the downcomer space and injury. It would
be better to seek approval to move the manhole up or down one tray than placement over a
downcomer.
Manhole orientation in the surge section of a vessel is not as restrictive. The surge section of a vessel is
the bottom portion that, during operation will contain a large volume of liquid. Any orientation is
possible for a manhole in this section. However, the location of all manholes should be in the back half
of the vessel away from the pipeway. The surge section may have a large baffle plate bisecting the
diameter of the vessel and extending vertically many feet. A removable plate or hatch may be installed
in this baffle (by vessels) to allow access to the far side. The vessel orientation of the manhole should
not hit the baffle or be located so close to the baffle that entrance is obstructed.
Trays
The type of trays, the number of trays, and the number of passes are not the specific responsibility of
the piping layout designer. However, there is the need to know factor. A common understanding of
terminology will improve communications and prevent errors. The common tray parts are:
a) Tray (support) Ring -- The tray support ring (or Tray ledge) is technically not a part of the tray itself.
The tray support ring is only there to support the tray. If there are no trays, then there is no need for
tray support rings, therefore tray rings are linked to the trays. Tray support rings are normally a simple
donut shaped strip welded to the inside of the vessel. They could also be in the shape of an inverted "L"
welded to the vessel wall. Problems arise when the Designer does not allow for the tray support device.
b) Trays (or Tray Deck) -- One or more sections, consisting of plates, forming a horizontal obstruction
throughout all or part of the vessel cross section. The trays will normally be constructed to form flow
patterns (one or more) called passes. The purpose of tray deck is to provide a flow path for the process
commodity and contain the fractionation or separation device.
c) Weir -- A low dam (on a tray) to maintain a liquid level on the tray
d) Downcomer -- The primary liquid passage area from one (higher) tray to another (lower) tray
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e) Valves -- Tray hardware device
f) Bubble Caps -- Tray hardware device
g) Draw off - A way to remove liquid from the vessel
h) Trough - A way to collect and move liquid from one point to another
i) Riser - A device to channel vapor from one lower point to a higher point
j) Seal Pans - A device (with a liquid seal) that prevents vapors from passing
k) Beams & Trestles - Devices that support trays (or other types of internals) in very large diameter
vessels
l) Baffles - A separation device inside a vessel
m) Chimneys - (See Riser)
Tray Pass Patterns
The trays and the related down comers can be arranged in a wide verity of patterns.
Typical Tray arrangements are:
a) Cross Flow, Single Pass -- (Common) this tray pass arrangement has one feed point, one flow
direction, and one downcomer. The single pass tray will normally be used on small diameter vessels and
the smaller diameter of a Coke Bottle vessel.
b) Cross-Flow, Multiple Pass -- (Common) the multiple pass trays will come in two pass, three pass, four
pass, and on and on. These will normally be found in the larger diameter vessels. Multiple pass trays
require multiple feed and draw off arrangements. The more passes, the more complex the orientation
problems.
c) Reverse Flow, Single Pass -- (Rare)
d) Radial Flow -- (Rare)
e) Circumferential Flow -- (Rare)
f) Cascade Flow -- (Rare)
The single pass tray will have a single downcomer. The 2, 3, or 4-pass tray will have the same number of
down comers as passes. The number of passes (number of down comers) will have a big effect on the
orientation. Some towers may have more than one Tray pass configuration. They may have single pass
in the top Trays and two-pass Trays in the bottom. The change from one pass configuration to another is
chance for error. The alignment of the single pass tray will normally be perpendicular to the two pass
trays.
Tray Types
There is what would be considered "Standard" Trays, and there are also "High efficiency Trays".
a) "Standard" Trays -- This tray will have an open downcomer with no separation occurring in the
downcomer area. This tray is the old stand-by and has been used for many, many years.
b) "High efficiency Trays" -- This tray will have a sealed downcomer with separation occurring in the
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Some other terms that will be found relating to trays.
a) Sump -- This is a sealed downcomer type area that is designed to provide a retention volume for some
purpose.
b) Seal Pans -- This is a portion of a tray that is set deeper than the rest of the tray to form a seal for the
downcomer from the tray above.
c) Side Draw Tray -- A tray arrangement that allows the removal of a specific liquid product
d) Chimney Tray -- A full circumference tray fitted with long open pipes to allow vapor to pass from
below the tray to the space above.
e) Baffles -- Plates installed in the vessel for a specific purpose
f) Impingement Plates -- Somewhat like a baffle but normally a plate installed in the vessel at the inlet to
prevent blowout to devices located on the opposite side of the vessel.
g) Tray manholes -- Most, if not all, trays will have a removable panel (somewhere in the tray) to allow
inspection passage without dismantling the total tray
Vessel Support
The method of vessel support depends on various factors. These factors include process function,
operation access, maintenance clearances, ease of constructability, and cost. Meeting the positive
criteria for all or the majority of these factors will drive the support method.
The primary methods of support are:
a) Tall Skirt on foundation at grade (Most common)
b) Short Skirt on elevated pier foundation, table support, or structure
c) Legs on foundation at grade
d) Lugs on elevated pier foundation, table support, or structure
Each of these vessel support methods has their own good points and bad points. The Tall Skirt is the
most common because it meets more of the "preferred criteria" than the others do.
Skirt Vessel Support
The minimum height of the skirt is normally set by process based on the NPSH requirements of the
pumps or for the reboiler hydraulic requirements. The designer may need to increase the skirt height
due to:
a) Vertical distance required by pump suction line geometry
b) Vertical distance required by reboiler line geometry
c) Operator aisle headroom clearance
d) Suction line entering the pipe rack without pockets
The approval of the Process engineer, Project Manager, and the Client will be required for any increase
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to the skirt height.
The skirt will have one or more access openings and will have skirt vents.
Skirts of vessels in refineries or other plants processing flammable commodities will normally be
fireproofed. The fireproofing is normally a two-inch (2") thick layer of a concrete type material applied
to the outside of the skirt. Check for the specific type. Some materials may require up to 6" to obtain the
required fire rating.
Load Handling Devices
Load handling devices are required for Vertical Vessels if:
a) The vessel is over thirty feet (30') tall
b) The vessel has removable trays and internals
c) The vessel has components that require frequent removal for routine maintenance (PSV, control
valves)
d) The components weigh 100 pounds or more
Methods of load handling include:
a) Davit -- A small somewhat inexpensive device used for lifting and supporting heavy objects up and
down from elevated platforms. Limited to a fixed reach.
b) Monorail -- A more expensive method
c) Crane -- A far more expensive method and is dependent on availability
If a davit or monorail is not installed then a crane with the required reach and load rating must be
rented or an alternate method must be jury-rigged. Any jury-rig method will have a high potential for
accident and injury.
When a Davit is to be included the following must be determined and furnished to Vessels:
a) The location
b) The swing
c) The clearance height (including lifting device)
d) The reach - the removal items (e.g... PSV, Control Valve, Block Valve, Blinds, etc.) and the drop zone
e) The maximum load of external items (Vessels will determine weight of internals)
When a Monorail is to be included the following must be determined and furnished to the Vessels
engineer:
a) The platform, and monorail support configuration
b) The clearance height (including lifting device)
c) The reach to the drop zone
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d) The maximum load of external items (Vessels will determine weight of internals)
Pipe Supports and Pipe Guides
The Pipe Supports and Pipe Guides (PS & PG) for the piping that is attached to the vessel is the
responsibility of the Piping Group. You're the Piper, that's pipe, and you need to make sure it is properly
supported and guided. The rule is (or should be) that all lines shall be properly supported and guided.
One key element of the PS & PG is the "L" dimension. The "L" dimension is the distance from the O. D. of
the back side of the pipe to the O. D. of the vessel. This dimension should be as small as possible but not
less than required for maintenance. The rule of thumb for the "L" dimension is 12" minimum and 20"
maximum. Dimensions of under the 12" and over the 20" are sometimes allowed. For example, if fitting
make up results in an "L" dimension of 11 13/16" do not add a spool piece and extra weld.
Lines should be supported as close to the nozzle as possible. The type of support is based on the weight
of what is being supported. It may be just a straight pipe dropping down the side of the vessel. Or, it
may be much more.
The requirements for pipe supports attached to a vessel must be evaluated for the following:
a) The shell thickness
b) Orientation
c) Elevation
d) The "L" dimension
e) The weight of the basic pipe and fittings (based on size and wall schedule)
f) The weight of the water during hydro test
g) The weight of the insulation (if any)
h) The weight of any added components (block valves, control valve stations, relief valves, etc.)
i) The clearance to other objects (Seams, Stiffener rings, Nozzles, Clips, Pipe Lines, Platforms)
The requirements for pipe guides attached to a vessel must be evaluated for the following:
a) The shell thickness
b) Orientation
c) Elevation
d) The "L" dimension
e) The size of the line at the point of guiding
f) The distance above the horizontal turn out (allow 25 pipe diameters +/-)
g) The maximum allowable span between guides
h) The clearance to other objects (Seams, Stiffener rings, Nozzles, Clips, Pipe Lines, Platforms)
Pipe supports and guides should be staggered vertically for clearance from supports or guides on other
lines running parallel.
Platforms, Ladders, and Cages
Platforms with access ladders must be provided as required for access to manholes, operating valves,
and instruments as defined in the project criteria. Normally objects below 15' - 0" from grade will not
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require permanent platforms and ladders. These objects are judged assessable by portable means
(Check the Project design requirements).
Platform spacing shall be even foot increments when multiple platforms are serviced from a single
ladder. The platforms shall be arranged to allow the following:
a) Minimum 7' - 0" headroom to underside of any obstruction
b) Minimum 2' - 6" radial width for primary egress path (I. D. of platform to O. D. of platform)
c) Minimum 2' - 6" clear distance between ladders
d) No obstructions in path between primary egress ladders
e) Maximum 30' - 0" vertical travel length of ladder between platforms
f) Side step off at all platforms (Step through ladders are considered dangerous and therefore should be
avoided). This requirement should have been reviewed with the Client and defined in the Design
Criteria.
g) Combining with platforms on other vessels when potential for improved operations or maintenance
exists
h) Flanges of top head nozzles shall be extended to provide access to bolts
i) Minimum 1' - 6" clearance around objects if for maintenance access only
Code Name Plate
Every vessel will have a Code Name Plate. On a vertical vessel the code name plate must be on the
(pressure containment) part of the shell. It cannot be attached to the skirt. The best place for the code
name plate on a vertical vessel is 2' - 6" above the horizontal centerline of the surge section manhole.
Make sure the location selected is accessible on grade or on a platform.
Common problems with vertical vessels
a. Schedule crunch - Vessels scheduled for purchase too early requiring firm orientations with very little
backup information.
- Approved and Issued for Design P&IDs
- Exchanger type and location
- Flare header and PSV location
b. Thin wall vessels not able to support load on pipe supports
c. High wind presence requiring extra guides
d. Late changes to PSV sizing prompting changes to pipe support and guides on line to flare
e. Late change to control valve location criteria (Flashing service now required to be located to elevated
platform on vessel with line downstream of valve self drain to vessel)
f. Reboilers requiring spring mounted supports due to tight piping and differential growth
g. High steam-out temperature requiring extra flexibility in the piping
h. Extra heavy object removal in excess of Davit load capabilities
Vertical Vessel Orientation
Recommendations
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behind the ladders. Do not route small lines vertically between the vessel shell and the inside radius of
the platforms. Do not route small lines vertically up the outside of the platforms in line with or close to
the manholes.
k. Ladder access openings must be fitted with a safety gate. Check for proper clearance for gate swing.
l. Some processes are subject to periods of hazardous operations. Ladders and ladder cages may need to
be designed for operators with self-contained suits and air packs (SCBA).
Skirts
a. The minimum skirt height is set by Process and indicated on the P&ID.
b. The skirt height is normally based on the minimum NPSH of the bottom pumps.
c. The skirt height may be influenced by the physical requirement of a thermo-siphon reboiler.
d. The final skirt height needs to consider and be adjusted for; physical configuration of the bottoms
nozzle, any headroom clearance required over operating aisles, vertical fitting geometry of the piping
configuration, and the pump suction nozzle location.
e. As a general rule no flanged connections are allowed inside the skirt of a vessel. This area is
considered a confined space in most plants and flanges will tend to leak over time.
f. Increasing the Skirt height may be considered when adjacent vessels warrant lining up and connecting
platforms.
Reboilers
a. Reboilers will be one of the following; Fired (Heater Type), Thermosiphon (vertical or horizontal shell
& tube), or Kettle type (horizontal shell & tube).
b. Fired Reboilers shall be located a minimum of fifty feet from the vessel.
c. Piping to and from any type of reboiler will be hot, and have sensitive flow conditions.
d. The Kettle or Thermosiphon Reboiler elevation is set by Process and indicated on the P&ID.
Pipe Supports and Guides
a. Piping is responsible for locating the pipe supports and guides on vessels
b. Piping is responsible for defining the size and loads on the pipe supports on vessels
Piping Flexibility
a. Piping must determine the operating thermal growth of the vessel. The vessel will have a series of
temperature zones from the bottom to the top.
b. The differential expansion between the piping risers and the vessel must be checked to prevent over
stressing the piping or the vessel shell.
c. The routing of cooler reflux lines must consider the total growth of the hotter vessel.
d. Potential for differential settlement needs to be investigated
e. Each piping system or line needs to be considered individually
Instrumentation
a. The HLL, NLL, and LLL need to be carefully considered because they will set the elevations of the level
instruments
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b. Orientation of level instrument connections needs to consider the internals
c. All instruments shall be accessible
d. Watch out for space requirements for gage glass illuminators.
e. TI and TW connections will require removal space
Electrical
a. Space shall be allocated for conduit runs up the vessel. These conduits will carry power to platform
lights, gage glass illuminators, and in some cases electrical tracing.
b. Conduits are also required for controls (instrumentation)
Piping Valves
a. Valves are meant to be operated and to be operated they must be accessible.
b. Small valves (2" & smaller) may be considered accessible from a platform or ladder. Large valves (3" &
larger) shall be accessible on a platform.
Misc. Piping issues
a. Lines to and from vessels may be subject to conditions such as 2-phase flow or vacuum.
b. Some PSV relieving to atmosphere will require snuffing steam. The steam pressure (in the line) must
be adequate to reach the top of the vessel.
c. Large overhead lines vs. PSV location require special attention for function and support.
d. Vertical vessel piping needs to be checked for heat tracing requirements. A tracer supply manifold
may need to be added at the top of the vessel.
Constructability
All vertical vessels shall be reviewed for constructability. This review needs to consider receiving logistics
lay down orientation, lifting plan, pre-lift assembly items (piping, platforms, ladders, internals, etc.)
- Pre-lift assembly items may include the following:
a. Piping
b. Platforms
c. Ladders
d. Internals
e. Paint
f. Insulation
Fire Protection
a. Some vessels may require special insulation for fire protection.
b. Some vessels may require fire monitor coverage
c. Some vessels may require sprinkler systems
Misc.
Some vessels will be lined. Linings may be metallic, plastic, or glass. Welding to the vessel shell after
initial fabrication is not allowed.
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Some vessels will have flanged connections that are larger than 24". These connections will occur at
connections for piping, reboilers, or other equipment. Flanged connections over 24" do not have a single
standard and need to be defined for specific type (API or MSS).
(BACK TO TOP)
Section - II
C-II: Vertical Vessel Orientation
By: James O. Pennock
The following article was prompted by questions from a young piping designer. He wrote:
-------------------------------------------
Hi
I am getting ready to do my first vessel nozzle orientation. The vessel is a Stripper Tower (a). Can you
help me? First, what are the things I have to take into consideration? Second, what are the key steps in
the process for doing a vessel nozzle orientation?
Regards
XXXXXXXXXXXX
---------------------------------------------
(a)The name/function of the vessel has been changed.
For your first question: "What are the things I have to take into consideration?"
The answer to this question is very simple; you must take everything in to consideration. Everything is
important! Someone may tell you that some things do not matter but this is not true, everything
matters.
You need to consider the following:
a) Timing: Vessel orientation is normally the only equipment related layout activity that can be done
without specific input from a vendor. All of the information required for vessel orientation is generated
on the project in the form of P&ID data and project standards. It is also one of the few activities that will
feed one or more other downstream groups whose work is critical to the project schedule. With this in
mind this activity can and should be started as soon as the P&ID reaches "Approved-For-Design" (AFD)
status. Te vessel orientation activity can be started manually or on basic 2D CAD before the 3D PDS data
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base is fully loaded and checked. There is some logic to doing this activity manually or in 2D CAD
because of the amount of trial and error required to finally achieve an acceptable and approved
orientation. Once the orientation is approved and the PDS data base is ready the 3D model can be built
with no recycle.
b) The Plot Plan (Note 1): The plot plan is required to identify the location of the vessel and its relatedequipment. The related equipment includes the equipment that feeds the vessel (is up-stream) and also
the equipment that the vessel feeds (is down-stream). It shows and locates adjacent, non-related
equipment. It also shows adjacent structures that may support the related up-stream or down-stream
equipment. It also indicates the plant features such as pipe racks, operating aisles, maintenance access
areas and the direction of Plant North.
c) The project foundation criteria: Vertical vessels normally sit on an octagon pad foundation with the
top of grout at EL101' - 0" (high point of finished paving = EL 100' - 0"). You need to have and
understand the type and elevation of the foundation for this vessel.
d) The P&ID's (Note 1): The P&ID's are required to show the process streams that connect to the
Stripper Tower and its related equipment. In my experience P&ID's are much like the pages in a book.
Some equipment (the heater) starts or shows on sheet one P&ID the story continues with the key item
(the Stripper Tower, Thermosyphon Reboiler and Bottoms Pumps) showing on sheet two and then
continues to some conclusion (the overhead condensers) on sheet three. You will need all three process
system P&ID's. The Stripper Tower P&ID will show a graphic of the column along with all the piping
connecting to the vessel. There will also be a data block at the top of the page. This data block should
include the vessel number, the vessel name and the basic size. It will also indicate the design
temperature and the insulation requirements (if any). The graphic of the vessel should also indicate the
basic type of internals (Trays or Packing). If the internals are Trays then the number of trays should be
indicated. The trays just above or just below where a line is connected should be numbered. If the
internals are some form of packing then the extent of the packing beds should be indicated.
e) The project Line List (Note 1): The line List is required to give you specific and critical key data about
the lines such as the Line Number, line class, maximum operating temperature and insulation
requirements,
f) The project Piping Material Specifications (Note 1): The Piping Material Specifications are required to
give you the data about metallurgy and any specifics about fittings, flanges, valves or requirements for
PWHT (post weld heat treatment).
g) The Vessel Drawing (Note 2): The vessel drawing at this time will most likely be marked "Preliminary."
It will give you; the inside diameter (I.D.), the tangent-to-tangent shell length, the shape of the top and
bottom heads and the skirt height. This drawing should also have a table showing all the nozzles with
the basic information such as: identification, quantity, and size, flange rating, the elevation above (or
below) the bottom tangent line for each nozzle, the purpose for the nozzle and any special instructions.
The vessel drawing needs to also indicate where the internals start and end inside the vessel.
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h) The Internals (Note 2) (Trays or Packing) A tower can have a number of different types and
configurations of internals. It may be Trays or it may be some form of Packing.
- Trays: If you have Trays then you need to know: the number of trays, the spacing of the trays, the
number of passes for the trays (1-pass, 2-pass, 3-pass etc.). You also need to know if there are any "draw
sumps," baffles or other special features.
- Packing: You need to know the number of "Beds," the depth of the beds and the method of installing
and removing the packing material. You also need to know and understand about the type of feed
distributor(s) to be used. You need to know about the packing discharge nozzles.
For the purpose of this article we will assume we have 35 single pass trays.
i) The Thermosiphon Reboiler data sheet (Note 1): This will give you the preliminary size and type
information. The P&ID indicates that this vessel has a vertical Thermosiphon reboiler fitted to it. Some
discussion should normally take place to determine the optimum tube length and the proper support
elevation and support method.
j) The project Vessel Platform Standards (Note 1): This will give you the required information about the
minimum vertical spacing between platforms. It will also give you specific details about platform
supports and how to make the openings where pipes must pass through a platform. This drawing will (or
should) also give you specifics about handrails.
k) The project Vessel Ladder Standards (Note 1): This drawing will give you all the required information
about ladder construction and more important the limits for the maximum vertical run for a single
ladder.
l) The project Vessel Nozzle Standards (Note 1): This will give you all the normal options for un-reinforced and reinforced nozzles. It may also show you some options for internal nozzle piping.
m) The project Vessel Davit Standards (Note 1): A davit is a small device permanently mounted on the
vessel that acts as a crane for lifting heavy objects such as tray sections.
n) The project Vessel Pipe Support and Guide Standards (Note 1): These are devices attached to a vessel
that support and/or guide the vertical runs of pipe. This drawing also defines the minimum distance
from the outside of a vessel shell to the back of an adjacent pipe. Where I came from this was called the
"L" dimension. The "L" dimension was normally 12" (adjusted as required for insulation) The maximum
was 20" without a special design. The key was to have a minimum of 7" clear between two co-existing
insulations. These supports and guides also require a wider than normal line spacing in the vertical plane
as the lines go up or down a vessel. This is mainly due to the configuration of the Trunnion (Note 3)
support attached to the pipe and the pipe clamp used for the guide.
o) The project Piping and Vessel Insulation Specification (Note 1): From this document you will get the
thickness of the insulation needed for the pipes and vessel at the operating temperature.
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(Note 1): These items are normally created by your company for the project and should be "Approved
for Design" (AFD) quality documents. This means that they have been through all of the proper in-house
reviews and checks and have then been approved by the Company and the Client for use in the design
of the work.
(Note 2): These documents will initially come from the project Vessel Engineer. They will normally bemarked "Preliminary" until they receive and process your orientation drawings. Later you may receive
the vessel fabricator's detail drawings for "Squad Check" (review and approval).
(Note 3) For more information about a Trunnion support see www.pipingdesigners.com look under
Training and Secondary Pipe Supports
There may be other documents that are required due to a specific company's method of operation.
The next things you need to consider is; functionality, safety, operation, maintenance and
constructability.
Functionality: No matter what, this vessel must do its job. You must know and understand what that
intended job is. You do not need to be a process engineer but you should be involved in the review of
the P&ID for this specific vessel. You need to hear what the critical issues are relating to this vessel and
the connected piping. If your company does not include piping in the formal review of the P&ID's then
you need to seek out the process engineer and ask him or her to explain the function, key points and
any critical issues relating to this vessel.
Safety: This is the other important issue relating to vessel orientation. The operation must be able to be
done in a safe manner. The same must be said for both maintenance and constructability. To achieve
this goal the locations of nozzles relative to the placement and arrangement of the ladders and
platforms must be carefully considered. The travel path (access and egress) must be arranged so the
main travel path cannot be blocked by open manholes, scaffolding, tools, tray parts, valves or piping.
The basic rule here; a: ladder #1 comes up with a side step-off (right or left) on to platform #1. Then b:
there is a minimum rest space equal to one ladder width. Then c: the next ladder (#2) continues up to
the next platform. Platform #1 can continue beyond ladder #2 around the vessel to provide access to
nozzles and manholes. This arrangement does not impede or obstruct the clear path for rapid escape
from the vessel for anyone from a higher elevation. Other safety issues include one or more skirt access
openings located near grade which should be located with clear access. There will also be four or more
skirt vents located high near the skirt-to-vessel attachment which also should not be blocked.
Operation: Process plants need to be operated. Most operation is concentrated around valves andinstruments. These items must be accessible. Accessible means reachable. This reachable is conditional.
Nozzles with a nominal size of 2' (NPS) and smaller can be reachable from a ladder or from a platform.
Nozzles 3" (NPS) and larger shall be reachable on a platform. In this context the from means that the
object is not more than 18" (one arms length) from the ladder or platform and the on means the object
must be fully inside the platform. There is normally only one exception to this rule. That is for valves or
nozzles that are located less than 20 feet from grade and can be accessed w
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