Introduction to Vessel and vessel orientation

7/28/2019 Introduction to Vessel and vessel orientation

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pipingdesigners.com Training Seminar

Section - II

C: Introduction to Vessels and Vessel Orientation

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 roleof 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 andperformance 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 equipmentC. 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&IDE. 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 mustbe 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 bringingtogether 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


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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 ofthe 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 idealcircumstances means that the Plot Plan has been firmed up and approved, the P&IDshave 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 Vesselorientation 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 longdelivery. 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 recoverthe capitol investment and see the expected profits.

The delivery time for vessels such as: alloy reactors, heavy wall high pressure vessels, orcrude 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 forincreased 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 deliveryperformance 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 dothe 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


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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 manyitems 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 spacePlant road limitations; Rack heights, shoulder clearances, logistics

Special vessels such as Reactors have several factors, which should be kept in mind. Themost 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 savethe 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 andmetallurgy. 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, orlugs. 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 InternalsSimple 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 flaredskirt, attached legs, or lugs. When located at grade this vessel will normally be mounted

on an octagon foundation.


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Vertical Trayed Vessels with straight sidesExample: 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 besupported 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 amulti 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 willnormally 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 CokeBottle 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 thestructure.

Vertical Packed Tower VesselsExample: 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 hightemperatures. 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 VesselsExample: 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 onthis 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 andorientation 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 VesselsExample: Reactor, Mix Tank, and Cook Tank

Discussion: This type of vessel will normally have a diameter and height of similar


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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 willnormally 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 carbonsteel. 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 haveflat, cone, or dome roofs.

Horizontal Vessels at gradeExample: 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. Theextended 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 BootsExample: 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 BootsExample: 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 VesselsExample: 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 sizemust allow for safety, operation, and maintenance. Pit mounted installations may also


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require sumps and drainage pumps. Underground (buried) installations may require

double wall tanks with leak detection provisions.

API Storage TanksExample: Feed Storage, Intermediate Product Storage, Off-Spec Product Storage,

Finished Product Storage, Batch Storage, Fire (or other) Water StorageDiscussion: 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.

SpecialExample: Spheres, Spheroids, and Bullets

Discussion: These vessel types have special location and orientation criteria and should

be handled on an Ad Hoc basis.

Vessel SupportsThere is a wide variety in the methods used to support vessels.There include:

a. Skirts

b. Saddles

c. Ring Girdersd. Lugs

e. Legs

f. Portables on Castersg. Pads

h. Direct BuryEach of these support types may also have variations

Vertical Vessel ComponentsThe pressure containment elements of the vessel are based of the process requirementsfor pressure, temperature, commodity, corrosion rate, plant life criteria, and the

applicable Codes.

The Pressure containment components include the following:

a) Shellb) Heads

c) Boot

d) Transitions (Coke Bottle Vessels)e) Nozzles

The other components include the following:a) Trays

b) Internal piping

c) Supportd) Load Handling Devices


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e) Pipe supports and Guides

f) Platforms, Ladders, and Cagesg) 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 mainoutlet 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 parte) Vacuum reinforcement rings

f) Insulation support rings

Heads -- Top and BottomHeads for vessels will include the following shapes:

a) Dished -- The Dished head is a flatter version of the Semi-Ellipticalb) 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 reinforcingc) 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

TransitionsThe cone or transition piece for regular and inverted Coke Bottle vessels may come in thefollowing shapes:


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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 thevessel 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 thetop 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 outletnozzles 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 morecost 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 examplesthat 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 velocityd) 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 exchangerb) Hot but not overly high on the temperature scale

c) Some may have potential for mixed phase flow

d) Normal line velocity


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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 scalec) All liquid flow

d) Normal line velocity

e) May require internals such as a distributor or inlet pipe. Multiple pass trays will requirea 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 alsoused 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 sumpc) 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.


a) Located in the bottom of the surge section of the columnb) 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 samequadrant as the bottom downcomer.


d) Located in the downcomer area of the column

e) Will be in a sump


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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 levelin 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 areab) 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 toaccommodate 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 willnormally 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 byOperations, 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 valveand 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) Manholeon vertical vessels.


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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 ofmanholes 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 onlyc) 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 somewhatflexible. 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 havetheir 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 andreinstallation 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 thelocation 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 removablepanel to allow further access into the vessel. The location to be avoided is above adowncomer 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 ofliquid. 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 extendingvertically 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


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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 ofthe 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 thevessel. 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 willnormally 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) traye) Valves -- Tray hardware device

f) Bubble Caps -- Tray hardware deviceg) Draw off - A way to remove liquid from the vessel

h) Trough - A way to collect and move liquid from one point to anotheri) 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 largediameter 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 morepasses, the more complex the orientation problems.

c) Reverse Flow, Single Pass -- (Rare)

d) Radial Flow -- (Rare)e) Circumferential Flow -- (Rare)

f) Cascade Flow -- (Rare)


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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 efficiencyTrays".

a) "Standard" Trays -- This tray will have an open downcomer with no separationoccurring 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 onmost 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 functionproperly 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 beplain 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 shaped) 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 theholes 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 holesize is calculated to provide a fragile balance between the liquid head above the tray and

the vapor pressure under the tray.

Weirs


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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 somespecial 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 andmanholes.

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 liquidproduct

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 processfunction, 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


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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 othersdo.

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 geometryc) 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 requiredfor 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 aconcrete 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') tallb) 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 havea high potential for accident and injury.


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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 supportsc. High wind presence requiring extra guides

d. Late changes to PSV sizing prompting changes to pipe support and guides on line toflare

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

RecommendationsUniformity

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 theaccess 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 vesselfor easy routing to the pipeway and equipment

Manholesa. 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 downcomers 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 preferredlocation 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 locationwith Project requirements).

d. Space and clearances are important around manholes. Check flange swing and tray lay

down space.

Ladders and Platformsa. Check to see that the approach to the ladder at grade is clear of all obstructions and


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hazards.

b. Check to see that the entry onto each platform is clear and not blocked by level orother 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, specblinds, 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 theoutside 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 laddercages may need to be designed for operators with self-contained suits and air packs

(SCBA).

Skirtsa. 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 ofthe 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. Thisarea 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.

Reboilersa. Reboilers will be one of the following; Fired (Heater Type), Thermosiphon (vertical orhorizontal 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.


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d. The Kettle or Thermosiphon Reboiler elevation is set by Process and indicated on the

P&ID.

Pipe Supports and Guidesa. 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 Flexibilitya. 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 investigatede. Each piping system or line needs to be considered individually

Instrumentationa. The HLL, NLL, and LLL need to be carefully considered because they will set the

elevations of the level instrumentsb. 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

Electricala. Space shall be allocated for conduit runs up the vessel. These conduits will carry powerto platform lights, gage glass illuminators, and in some cases electrical tracing.

b. Conduits are also required for controls (instrumentation)

Piping Valvesa. 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 issuesa. 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.

ConstructabilityAll vertical vessels shall be reviewed for constructability. This review needs to consider


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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. Laddersd. Internals

e. Paintf. Insulation

Fire Protectiona. 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 thevessel 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 connectionsover 24" do not have a single standard and need to be defined for specific type (API or

MSS).

James O. Pennock is a former Piper with more than 45 years experience covering process

plant engineering, design, training, pipe fabrication and construction. He is now retired

and lives in Florida, USA.

Introduction to Vessel and vessel orientation

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