Towers, Reactors, Drums, Tanks & Spheres
There is one aspect of design that is common to all of the types of vessel described below. It is that all of the piping nozzles or manways are positioned and orientated by a piping designer. The nozzle locations are based on Company standards, good engineering practice and the process information shown on the P&ID. In addition the platforms and ladders required to access some of these nozzles are also laid out by the piping designer based on industry standards and the clients maintenance and access requirements.
Tower is the generic name given to the iconic tall slender vessels always associated with refineries and other process facilities.
They are more accurately described by their process function as Distillation or Fractionation Columns, Absorbers or Contactor Columns as used in glycol dehydration or amine sweetening, but while they can have different process parameters their function and operation are similar
The layout of a tower and its associated piping and platforms is arguably the most complex design challenge a piping designer can face. It should only be undertaken by the most senior of piping designers.
Towers are rarely more than 2.5 M in diameter and can be up to 120M high, they operate at relatively low pressures and temperatures.
It is impossible to write a definitive guide to tower layout as each stage of the layout process is interdependent on the others and there are many different tower variables. This interdependence and complexity means that the moving of a single nozzle late in the design can cause a complete redesign of the whole tower. What follows are some operational info and design guidelines common to all towers.
Distillation occurs when heated liquid passing down the tower by gravity comes into contact with vapours flowing up the tower by convection, this contact occurs on the many bubble trays which are usually placed at approximately 600-800mm equal spacings up the tower. Some towers use packed beds rather than trays and in this case the nozzle orientation is simplified by the absence of tray downcomers, the following guide is based on trayed towers as these are the most complex.
The tray consists of multiple bubble caps across the flat surface of the tray, at the edge or edges of the tray a small lip or weir retains about 50-75mm of liquid on the tray before it spills over into the downcomer and onto the next tray down. The vapour travels up through the bubble caps and interacts with the liquid on the tray due to the liquid seal created by the weir.
At various levels in the tower different petroleum products with different boiling points are drawn off. The lighter petroleum products such as aviation fuel near the top, the heavier such as bitumen near the bottom.
The heated crude feed inlet is usually between the middle and the top of the tower, any un-distilled liquid that makes its way down the trays to the bottom of the tower (Bottoms) is drawn off and re-heated to a vapour through a kettle type re-boiler located on a structure close to the bottom of the tower.
The vapour produced is recycled to the side of the tower below the trays to flow up and react with the down-flowing liquid on the trays. The vapour (Overheads) exits at the top of the tower and after being condensed to liquid through an aerial condenser it flows down to a Relfux drum, some of this condensed liquid is pumped back up to the top of the tower to again flow down and react with the up flowing vapour in a continuous reflux process. Some of the condensed fluid is pumped away as petroleum product.
At the start of the design process the only information a Senior Piping Designer will have or require is a P&ID, Plot Plan and preliminary tray vendor drawings. He/She will not have, or need, a vendor vessel drawing as most of the information is flowing from the designer to the vendor not vice versa. The P&ID will show the tower diameter, tan to tan length, skirt height, location of diameter transitions, number and type of trays and nozzle size and spec and their elevation locations relative to each tray. The Plot Plan will show the tower location relative to the pipe rack and maintenance access area.
All nozzle and platform orientations should always be quoted in positive clockwise degrees from 0 to 360. 0 is always pointing towards Plant North.
Location of the process nozzles is closely tied to orientation of the trays, they are interdependent on each other and must be orientated together.
There are basically two types of tray design, single or double downcomer. In the single design the downcomers ( about one third of the towers diameter forming a chord across the diameter) are placed on alternate sides of the tower on alternate trays.
In the double design there is a downcomer each side on the first tray (each about one quarter of the towers diameter) on the next tray down there is a single downcomer in the middle of the tray. Then at the next tray the downcomers are each side again and so on alternately down the tower.
Once the orientation of the downcomers is established they cannot be individually changed or rotated within that bank of trays in that section of the tower. If there is a diameter transition in the tower or a break in the trays a different rotated tray pattern can be established in the next bank of trays but it must remain consistent in that bank. Sometimes a diameter transition will necessitate a change from single to double downcomers. The types of tray and their locations will always be shown on the P&ID.
The fundamental rule in tower design is that no nozzle or manway shall ever be placed in a downcomer. Even vapour TI’s should be placed above the tray liquid level, not in the downcomer area.
As all of the feed and draw off nozzles are best located in an arc facing the pipe rack this tends to orientate the downcomers perpendicular to the rack. However it is always possible that rotating the trays by a few degrees may assist in the nozzle and piping layout.
The first nozzles to be located and orientated are the liquid draw off nozzles. This is due to the fact that their elevation is critical to the tray level where the draw off is to occur, the ID of the nozzle bottom must be as close as possible to the top of the tray.
Next the liquid feed and reflux nozzles should be located and orientated, while their elevation still needs to be at the location shown on the P&ID the exact elevation and orientation is not as critical as they will often have an internal distributor above the tray to feed the liquid on to the tray parallel to the downcomers. This internal distributor pipe can have a 45 deg. , or less, elbow to achieve this from a nozzle that is not parallel to the downcomer.
The manways are orientated on the back arc of the tower opposite the piperack, facing the maintenance area. Again it is important not to overlap the downcomer area with these large (20”- 24”) nozzles. There is usually only one manway for 5 or 6 tray levels. The trays have a removable panel for access between the trays.
Finally the re-boiler vapour return inlet and the level bridle nozzles can be located. Their orientation is not dependant on internals as they are both located below the last tray at the bottom of the tower.
The lower bridle nozzle needs care in its elevation as it needs to be as low on the shell as possible while avoiding the lower head seam line. The seam line is usually 50mm above the lower head tan line but on heavy wall vessels this distance can be larger. The upper bridle nozzle is always located directly above the lower nozzle and above the high high liquid level (HHLL). The vessel bridle will support all of the level instrumentation but pressure and temperature instruments (PI’s & TI’s) are located at other locations shown on the P&ID. PI’s are always in vapour spaces away from downcomers. TI’s are usually positioned in the liquid level of the tray, their height relative to the tray is critical. However they are sometimes required to measure vapour temperature, and will need to be well above the tray, if in doubt check with the process engineer.
Ideally no nozzle should intersect a seam and usually all vertical and horizontal vessel seam lines will be designed to avoid nozzles. However, where it is unavoidable the vessel vendor will place a reinforcing pad around the nozzle. Seams and re-pads will be designed by the vessel vendor once they receive the nozzle layout
The Overheads vapour nozzle and the Bottoms liquid nozzle are both located respectively in the centre of the top and bottom semi elliptical heads. The top head will also contain the PSV nozzle offset from center. The bottoms nozzle drops down vertically, then is elbowed and orientated horizontally to its flange outside the vessel skirt as no flanges should ever be located inside the skirt.
After positioning all the nozzles and manways the only other wall penetration items requiring orientation are the skirt access hole and the 4 equi spaced skirt vents. The access hole should be no more than 600mm above grade towards the back of the tower and the skirt vents, which are short 2” pipe sections, as high as possible in the top of the skirt.
With only the arc of the tower facing the rack between downcomers available for all nozzle orientations it is impossible to place each nozzle in an orientation where there will be a clear run for piping to run straight down the tower to or from every nozzle.
It is important at this stage to make allowances for pipe supports and guides. If no thought is given to their impact on line spacing down the tower it can result in much rework rearranging the piping layout and even reorientation of nozzles. Pipe and vessel insulation thickness also need to be taken into account when positioning lines on the tower
While some lines can drop straight down from their nozzles others will need offsets at the nozzle to avoid nozzles and piping below them. These offsets are usually a 90 deg elbow rolled at 45 deg after the nozzle mating flange and then a 45deg elbow to go vertical. They can be fitting to fitting but some will require a pup piece between the elbows. Offsets are always made at the nozzle, never on the run down the tower.
Once the piping is laid out the pipe supports can be added. Any line dropping from a nozzle must be supported as close to the nozzle as possible. It is acceptable to support immediately below the offset described above. Larger lines (4” and above) are usually supported by trunnions found in your company’s pipe support standards or under the Standards section on this web site.
Trunnions need a fairly substantial support structure so care must be taken during nozzle orientation and initial piping to allow for them not to interfere with other supports or lines.
After the supports are positioned the guides can be placed. Some guides can be relatively small and simple and can be selected and placed by following your company’s vessel guide standards. Others can be even larger than trunnion supports, these are usually two or four way guides required at the request of the stress engineer.
All steel required for pipe supporting is usually supplied by the Pipe Support Vendor. The vessel vendor only supplies the welded support clips at the vessel shell locations specified by the piping designer.
Ladders & Platforms
Unlike most other platforming on the plant, which is designed by the EPC Company’s structural group, vessel platforms are usually laid out by the piping designer but engineered and supplied by the vessel vendor.
Before starting to lay out any ladders or platforms the designer should familiarise him/herself with the regulatory health and safety standards at the plant’s location, the engineering company’s standards and the clients requirements.
The regulatory health and safety standards will dictate, handrail height, ladder egress, cage height and the maximum vertical ladder run allowed, longer than maximum allowed vertical ladder runs require an intermediate platform. In most jurisdictions this maximum run is 30ft or 9M but it can be less in some jurisdictions. Adhering to the correct standard is critical as it would drastically change the platform and ladder arrangement if the wrong standard were initially used.
The company/clients standards should specify which nozzles need permanent access. It is a given that all manways should have permanent platform access if they are higher than a given distance from grade. Also the level bridle and its instruments always need good access although operator access can often be achieved directly from a ladder with no actual platform required.
Process nozzle access can vary greatly from client to client. Some require all nozzles accessed, some require access only to those fitted with spectacle blinds or valves. It is imperative that the designer checks the nozzle access requirements before starting the layout.
Except for the top rectangular platform all other platforms are semi circular with varying circumferences and usually positioned on the back arc of the tower. They are usually 1.5M wide and accessed by side step ladders, not step through. Always remember several platforms can be accessed from the same single ladder providing its total vertical run does not exceed the maximum and that the ladder rungs can line up with the top of each platform.
The platform levels are mostly determined by the manway levels as there is a minimum standard distance between the platform and the underside of the manway. Any other nozzles in the same general location that require access can be reached from the manway platform if they are within the height restrictions for good access. If not they will require a dedicated platform
Platform layouts are infinitely variable and is difficult to define what constitutes good layout other than to say that any platform should be efficiently sized to service as many items as possible from a single level while not being larger in circumference than necessary. Ladders should be positioned such that the access or egress can never be obstructed by an open manway cover or any other obstruction. Be aware of overhead clearance of anything projecting above the platform. Be aware that the vessel vendor will sometimes require support braces approximately every 15 deg of shell circumference underneath each platform, which could also pose a headroom issue on the platform below or an interference with pipe supports.
Always check for this when you receive the vendor’s platform details for squad check. Also check that they have correctly incorporated holes for any piping platform penetrations, the holes should have a circular kick plate and be 2” larger than the insulation O.D.
The last item on the tower check list is the main tower davit for lifting or lowering items to and from the tower. It is positioned to be accessible and operated from the top platform but must be able to rotate to pick up any object from any platform. A reserved space envelope of at least the same diameter as the tower must be modeled from grade to the full height of the tower to ensure this space is kept free from obstructions.
It is the experienced Senior Piping Designers skill that makes this extremely complex piping and platform arrangement look well designed and deceptively simple. Perhaps one day A.I. might be able to attempt something of this complexity, I’d love to be the checker on its first attempt.
Reactors, like towers, are usually vertical and cylindrical. But unlike towers their diameter can be as large as 10M with a height of up to 50M. They also operate at high temperatures and pressures which is why their top and bottom heads are usually hemispherical not semi-elliptical and their wall thickness can be 50mm or more.
The process operation of a reactor is fairly simple. The process fluid is passed through a catalyst bed where a chemical reaction takes place to change the fluids properties. It is usually a once through process without the complex piping and internals associated with a refluxing distillation tower.
While the general principles of layout and orientation are the same as distillation towers, the numbers of nozzles, platforms and pipe supports are relatively small, although due to the higher temperatures involved pipe stress mitigation maybe more complex.
The major layout consideration for a reactor is the catalyst removal and loading, which has to be done at regular intervals. As no two catalytic reactors are exactly the same it is up to the piping designer to liaise with the mechanical engineer and the reactor vendor to ensure that adequate access and catalyst handling facilities are provided adjacent to the reactor.
Drums are always cylindrical and usually horizontal, they are rarely more than 5M diameter but their length can vary considerably. Drums up to 10M long will have 2 support saddles, longer drums can have multiple saddles depending on the vessel wall thickness. For layout purposes in the 2 saddle arrangement they can be positioned 1/5 of the drum length in from each end, but their final position must be determined by the vessel vendor.
Most drums are completely horizontal but sometimes the P&ID will call for the drum to be sloped. This is achieved by having one saddle higher than the other for 2 saddles or continuous increasing saddle heights on multiple saddles.
Like all horizontal equipment one support must be fixed and anchored while all the others are allowed to slide. It is the piping designers job to determine which is the fixed end. Generally speaking if a drum is located perpendicular to a pipe rack, like exchangers, it is the end closest to the piperack that is fixed. However this can vary depending on the stress analysis of the connecting piping. A stress sketch should always show the location and the distance from the connected nozzle to the fixed end.
Most drums are used for transient (not permanent) liquid storage and are being continuously partially filled and emptied as part of the process flow. Many drums are used for gas/liquid/solid separation and have internal baffles and weirs to achieve this function.
Liquid Inlet, gas outlet or PSV’s are usually on top of the shell or head. Liquid outlet, usually to pumps, is on the bottom of the shell. Just like any vessel feeding a pump suction the drum must be elevated to provide Net Positive Suction Head (NPSH) to protect the pumps from cavitation. This is shown on the P&ID as a dimension from grade (or platform) to the underside of the drum shell.
Some drums are equipped with a liquid boot, this is a cylindrical shell extension vertically down from the liquid section of the shell. They are usually no more than 1M in diameter and 1.5M long. Their purpose is to provide additional liquid capacity to the shell without increasing the shell diameter, they also provide additional NPSH to the pump suction which is often a nozzle on the bottom of the boot head. The lower level bridle nozzle is positioned as low as possible on the side of the boot.
There is also a sub group of drums commonly called Bullets which are used for high pressure gas (Butane or Propane) storage. Due to their high pressure they have hemi-spherical heads, are usually no more that 2.5M diameter and can be more than 20M long hence the name bullets, they are usually placed in groups of 2 or more. While their piping involves only small bore manifolded lines it is their location in the plant that it is important.
High pressure flammable gas is by far the most dangerous fluid stored on any plant. The explosion of a bullet can be catastrophic with the vessel heads travelling hundreds of meters and causing severe damage to anything in their path. The piping designer must therefore always locate Bullets at the edge of the plot with their heads facing a direction least likely to cause damage in the event of an explosion. They are usually surrounded by an earthworks bund in an attempt to contain the heads. It is not only their shape that lends them the name bullets.
Generally speaking the liquid inlet or gas outlet nozzles or are positioned along the top center line of the drum shell or high in one of the semi-elliptical heads. Liquid outlet nozzles are positioned along the bottom center line or low in one of the heads.
Manways can be along the top center line or low on the center line of the heads or even tangentially low on the shell. Some drums will require 2 or more manways depending on the internals.
The actual positions of the nozzles are also dependant on the internals and the piping designer must be in possession of a P&ID and a Vessel Data Sheet before orientation commences.
The level bridle nozzles can either be high and low tangentially on the side of the shell or in similar positions on the centerline of one of the heads or the boot.
It is worth mentioning here that the vessel vendor may not know the purpose of all of the nozzles so it is important that the vessel drawing should show that the level bridle nozzles should be “Jig Set”. This means that the vendor must use a temporary template or jig to ensure that the two nozzles, center to center distance, projection and their flange bolt holes line up exactly.
This applies to any bridle nozzles on any type of vessel the piping designer lays out. Jig Set also applies to each individual pair of instrument nozzles on the bridle piping isometric.
Like tower platforms, drum platforms are usually supplied by the vessel vendor and laid out by the piping designer. They are usually rectangular and run the length of the drum with a single side step ladder at one end. Again it is dependent on company standards or client preference whether access to the top nozzles is required. But as there is often a gas blanket control system or a PSV to be inspected and maintained it usually is.
If a platform is required the only other decision to be made is whether the nozzles should project through the platform or whether the platform is to the side of the nozzles. The through nozzle arrangement is more practical from a maintenance point of view but results in a larger platform and longer nozzle projections and is therefore more expensive.
Tanks are mainly used for relatively long term storage of produced liquid petroleum products or chemicals required by the process system, they are not pressure vessels and are not part of the process itself. Their diameters are often far greater than their height. Diameters can be greater than 50M with heights up to 15M.
As they are not pressure vessels they have flat bottoms resting on a compacted earth tank pad, the roofs are flat, domed or conical and can be fixed or floating. Floating roofs can be Internal or external and are more expensive than fixed roofs but are preferred because they eliminate the potentially explosive vapour space above the liquid in the tank. They are also less environmentally harmful as they eliminate the need to vent off vapour to atmosphere as the liquid rises during filling.
Whether the roof is fixed or floating makes no difference to the nozzle or piping layout except where a vapour recovery or fire suppression system is required, however these systems are not common.
Unlike other vessels which are generally covered under ASME Boiler & Pressure Vessel Code, tanks and their nozzles are covered under API 650, the piping designer should read the relevant parts of this standard before orientation.
Many of the nozzles on tanks are positioned to face the pipetrack serving the tank, this means most of the nozzles are grouped together in a relatively small area of the tank circumference and are located tangentially in parallel rather than radially. All of the inlet/outlet nozzles are also located as low as possible on the tank shell to maximize the operational capacity of the tank.
Exceptions to the above are shell nozzles for manways, mechanical mixers, electric heaters, instruments or steam coils which can be located anywhere that’s convenient providing there is room to remove large items such as mixers. The only other nozzles are located on the roof and are for level instruments, sampling and PVRV’s, they are accessed from a small, rectangular roof platform which in turn is served by a spiral staircase, with rest platforms, around the wall of the tank.
Piping runs to and from each tank on a branch from the main tank farm pipetrack which runs outside the bund wall. The branch track passes through the bund wall and each line is sealed on both sides of the bund to maintain the integrity of the bund in the event of a tank rupture.
Tanks are unique in the plant in that they are the only equipment where settlement is expected to occur. The significant weight and size of storage tanks makes it uneconomic to pile the bases of the tanks so they are usually erected on a pad which is comprised only of compacted earth.
Once the tank is filled settlement will begin and continue for the first few years of the tanks life until it reaches stability. The amount of settlement can be roughly calculated by the civil engineer and can be as much as 100mm.
This means that all of the piping connected to the tank must be flexible enough to be able to absorb the downward movement of the nozzle it’s connected to. The amount of movement and the space available usually precludes the use of natural flexibility so each nozzle must be fitted with a form of flexible connection.
These connections can vary from simple braided hoses for small bore nozzles to full articulated joints on the larger lines. The position of the pipetrack is offset from the tank nozzles to accommodate the flexible connections.
Tank Farm Layout
Storage tanks are always grouped together in an area adjacent to the main plant known as The Tank Farm. Tank farm layout is governed by company/client/ industry standards based on insurance requirements as there is potential for large tank fires and major asset loss when many tanks full of flammable liquid are grouped together.
The primary safety feature of the tank farm is the berm, bund wall or dyke, this is usually an earthworks embankment completely surrounding the tank with enough capacity to contain the entire contents of the tank in the event of rupture, with some room to spare (freeboard)
When dealing with a single tank this is a fairly simple calculation, the total volume of the tank divided by the proposed height of the bund will give the minimum area required within the toe of the bund. The additional volume created in the triangular space from the toe to the top will give freeboard. The nominal height of the band should not be more than 2M as even this height will require a toe to toe bund base width of approximately 7M. As the earthworks take up a considerable amount of the available area, any bund higher than 2M becomes impractical. Even single small tanks located within the plant for process chemical storage require containment, but due to space limitations it is usually a concrete wall not an earth berm.
Very often multiple tanks are placed within a single bund. This is a more complex calculation and must take into account the types of liquid stored, the distance between tanks and the required volumetric capacity of the bund, which is not the total capacity of all tanks. One basic rule that can be used in initial layout is that the minimum bund capacity should be the greater of 110% of the largest tank volume or 25% of the total volume of all the tanks
Many of these standards are quoted in para. 10.2 of the Piping Design and Layout Manual on this site, but the standards that apply to the specific plant location must also be followed.
Further safety requirements of tank farm layout is that no equipment, particularly pumps, shall be located within the bunded areas and that adequate fire hydrant coverage is located throughout the tank farm. Each hydrant should be served by alternative access and egress routes.
As the name implies these are spherical high pressure vessels used for the cryogenic storage of LNG, methane, ethylene, hydrogen, and oxygen gases in a liquefied state and many others in a gaseous state. They are usually no more than 15M spherical diameter and supported on multiple legs on a concrete base.
They are usually located together in a separate area of the tank farm but are not surrounded by a bund wall.
A spherical shape is the strongest natural shape and can withstand the highest internal pressures, its other advantage is that it has a smaller surface area per unit volume than any other shape. This means, that the quantity of heat transferred from warmer surroundings to the liquid in the sphere, will be less than that for cylindrical or rectangular storage vessels. As cryogenic fluids must be kept cold at high pressures these are the primary reasons for their choice in cryogenic fluid storage.
The primary inlet/outlet nozzles are all located on the bottom and middle of the sphere pointing vertically down, there are a few ancillary nozzles such as level and PSV’s mounted on the middle top served by a spiral stair case and top platform similar to a tank.
It is likely that sphere piping will be subject to cryogenic conditions so the practices and support standards shown in the Special Pipe Support Details section of this site will apply.