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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

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 Design Specification. 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 are like 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 the project. 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 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 for economics, 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
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 normal process temperatures, or at high temperatures. These vessels may be mounted to the support surface (grade or platforms) via a traditional vessel skirt, attached legs, or lugs. When located at grade this vessel will normally be mounted on an octagon foundation.

Vertical (Refinery Type) Reactor Vessels
Example: Reactor, Converters
Discussion: This type of vessel will normally be medium to large sized, high pressure (> 500 psig) and high temperature (> 600o F). These vessels will be filled with one or more layers or beds of various materials that will act as a catalyst. The sidewalls and heads on this type of Reactor may be five to seven inches thick. Refinery Reactors may be mounted to the support surface on a short vessel skirt, on lugs, or on legs. The bottom head and nozzle must be elevated to allow for removal of the catalyst. The location and orientation of this type of vessel must consider the loading and removal of the catalyst. These vessels will normally operate at very high process temperatures and will be located in close proximity to fired heaters.

Vertical (PharmBio & Fine Chemical Type) Reactor Vessels
Example: Reactor, Mix Tank, and Cook Tank
Discussion: This type of vessel will normally have a diameter and height of similar dimensions. The ratio of nozzles to vessel size will be very high. These vessels will have added complexities with the requirements for mixers and jacketing. These vessels will normally be mounted to the support surface on lugs, a collar, or on legs. These vessels are normally located on an upper level of an enclosed structure or building. The bottom head and nozzle must be elevated to allow for operator access, gravity flow to other equipment, or critical pump NPSH requirements.

Vertical Vessels - Bins and Silos 
Example: Agricultural Product Storage, Dry Chemical Storage 
Discussion: Bins and Silos are used for dry material storage. These vessels are normally thin walled, operate at atmospheric pressure, and made of materials other than carbon steel. These vessels will normally have a cone bottom. The configuration of the cone is based on the angle of repose of the commodity to be stored. These vessels may be supported via skirt, legs, or lug mounted in an elevated structure. These vessels may have flat, cone, or dome roofs.

Horizontal Vessels at grade 
Example: Condensate Collection Drum, Separator, and Settler Drum
Discussion: This type of vessel will normally be small to medium sized. They may be mounted to the support surface (grade or platforms) on extended vessel saddles. The extended saddle allows for clearance for bottom connections at a lower cost. When located at grade this vessel may be mounted on a foundation or the paving (depending on vessel weight and soil conditions).

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.

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
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
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.
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:
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

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.


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
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 more complex 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. 
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 flow 
c) 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 sump
f) 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
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:

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 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. 
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.


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
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 downcomer. This tray type is fairly new. It will most likely be used on most new vessels in the future. It is also the type of tray that is favored on revamp projects to get more out of an existing tower.

Tray hardware devices

The normal trays inside the typical vertical vessel will contain openings (or holes) and may be fitted with a fractionation or separation device. This device is what will accomplish the purpose of the vessel. If these devices are not present or do not function properly then the product is not made. 
The common tray devices are:

a) Bubble Cap (Used mostly on revamps) -- Simple, and common method to facilitate the separation process. The Bubble Cap will normally be a round (cup shaped) cap inverted over a short and smaller diameter chimney. The skirt area of the inverted cap may be plain or have (open or closed) slots.
b) Box Cap -- This cap is very much like the common Bubble Cap except it is square.
c) Tunnel Cap -- This will be a long narrow rectangular shape
d) Uniflux Tray -- This is a series of overlapping and interlocking plates. In cross section the Uniflux tray will have the shape of a reclining squared off "S". 
e) Valve (Most common) -- The valve tray will have small flat metal plates fitted over the holes in the trays. The plate is loose to move up and down, but is retained in position by a clip type device. Vapor pressure under the "Valve" plate causes it to rise and gravity brings it back down.
f) Sieve (2nd most common) -- The Sieve tray will have holes and nothing else. The hole size is calculated to provide a fragile balance between the liquid head above the tray and the vapor pressure under the tray.


There may be a number of places where weirs are used. The simple weir to provide proper tray flooding will normally not cause any design problems. There are also some special purpose weirs that may effect the location of nozzles. In most cases the existence of special purpose weirs will not be known at the start of the Vessel orientation activity. It is however, a good idea to ask the question anyway.

Down comers

Down comers can come in a verity of shapes also. They straight across in the horizontal direction, or they can be bent. They can be straight up and down in the vertical direction, they can be sloped or slanted (tapered), or they can be a combination. These variations will all impact the orientation to some extent. The major impact, by the downcomer on the orientation is the geometry or location of the vertical plane itself. The orientation of the down comers will have a direct relationship to the orientation of certain nozzles and manholes.

Other Tray Terms

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 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
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 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
a. The ladder approach at grade should be free of obstructions and easily accessible (Verify preferred location with Project requirements). 
b. The Manhole orientation should be oriented in the back half of the vessel toward the access way. The manholes should be arranged with consideration to the type of load handling device (One centerline if monorail, one or two centerlines if davit, no specific restriction if crane). 
c. Load drop area should be located on the main access side
d. Level instruments should be located on or near the front half of the vessel and visible from the main operating aisle
e. The piping risers to and from the vessel should be located to the front half of the vessel for easy routing to the pipeway and equipment

a. Manholes will influence the entire vessel orientation to a certain degree. The location of the manholes must be compatible with the location of the tray down comers. The down comers in turn influence the location of the process and instrument nozzles. 
b. The preferred elevation of manholes above the platform is 2' - 6" from the centerline. The limits are; 6" minimum from the top of the platform to the bottom of the flange, or 4' - 0" maximum from the top of the platform to the bottom of the flange (Verify preferred location with Project requirements).
c. Platforms may not be required for manholes that are 15' - 0" or less above grade, unless a platform is required for another reason such as an instrument (Verify preferred location with Project requirements).
d. Space and clearances are important around manholes. Check flange swing and tray lay down space.

Ladders and Platforms
a. Check to see that the approach to the ladder at grade is clear of all obstructions and hazards. 
b. Check to see that the entry onto each platform is clear and not blocked by level or other instruments. 
c. Check to see that the entry onto each platform is clear and not blocked by an open manhole flange.
d. Check to see that there is a clear path from one (down) ladder to the next (down) ladder for unobstructed travel during emergencies.
e. Platforms may need to be added or extended for access to operating valves, spec blinds, or instruments.
f. Special platforms are often required at the channel end of a thermo-siphon reboiler or other equipment that is mounted directly into (or onto) the vessel.
g. Investigate lining up and connecting platforms servicing equipment (Reboilers or Accumulators) located in adjacent structures but related to the vessel.
h. Maintenance criteria at Reactors often require platforms large enough and strong enough for large flange or head lay down in addition to catalyst storage and handling.
i. Check the location and size of the pipe penetration holes through platforms. The opening is to be one inch larger (in diameter) than the flange or pipe plus insulation, which ever is greater (Verify preferred location with Project requirements).
j. Provide proper routing and support for all lines regardless of size. Do not route small lines vertically 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).

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.

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

a. The HLL, NLL, and LLL need to be carefully considered because they will set the elevations of the level instruments
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

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.

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

Some vessels will be lined. Linings may be metallic, plastic, or glass. Welding to the vessel shell after initial fabrication is not allowed.
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).

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.

Mr. Pennock can be contacted via E-Mail at

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