Volume XLVI.I: Design Operations & Maintenance Friendly Vessels - Part 1

  • Volume XLVI.I: Design Operations & Maintenance Friendly Vessels - Part 1


The scope of this presentation is to present basic information and understanding of the ASME code for the design of pressure vessels for the chemical and process industry as applicable in the United States and most of North and South America. For more information about our productsheavy plate & custom fabrication services or fabrication capabilities contact us today! 

Design operations-and-maintenance-friendly pressure vessels—Part 1

Many scholarly articles have been published on the design, selection and fabrication of pressure vessels.1–3 The articles, books and training materials published to date focus on the requirements of vessel designers and the manufacturers. None of the published literature appears to address the needs and concerns of the ultimate beneficiaries—the end users.

For vessel designers, fabricators and quality control inspectors, this is a onetime responsibility. They probably never look back once a vessel is out of their territory. The reason is simple: almost all pressure vessels are designed per specific requirements of the asset owners. A few proprietary designs are exceptional cases, such as reactors, desalters and coalescer vessels, etc. However, only the internals would be proprietary, and the base vessel would still be required to comply with the owner’s general specifications. In such cases, it is important that the end users do not feel overwhelmed by the proprietary designs and adhere to their company’s basic vessel specifications.

Note: This article is the first in a series of design articles intended to increase awareness of operational and maintenance related concerns and what design engineers can do to provide user-friendly and fit-for-purpose equipment for the hydrocarbon processing industry (HPI). This series is based on the actual implementation of what is narrated and the satisfactory experiences drawn by the end users.

Readers are advised to make their own engineering judgment on the validity of the design improvements suggested herein and to develop their own needs. If engineering or other professional services and judgments are required, then the assistance of a competent professional authority should be sought.

Throughout this article, the terminology “vessel” is used to represent pressure vessels, drums, columns, towers, heat exchanger shells and any equipment designed using pressure vessel codes such as ASME-VIII, EN 13445, PD 5500, etc. The terms “codes, standards, specifications, regulations and recommended practices” are used to broadly define the overall design requirements, recommendations and practices prevailing in the industry. The terms “vessel fabricator” or “manufacturer” have same meaning, as do the terms “owner” and “end user.”

Ensuring functional safety and long-term service. Provided here are useful ideas to ensure that a vessel meets functional safety requirements and provides operation-and-maintenance-friendly service to end users over the long term. The tips provided are simple to implement, do not interfere with proprietary designs and do not violate any of the code requirements—rather, they exceed them. The tips are also generic and do not require any code-specific calculations. Therefore, they are applicable to pressure vessels built to any code. This article essentially covers what the codes and design books would not reveal. The tips would lower ownership cost if implemented at design stage and assist the end users to meet their health, safety and environmental regulations, as well as reduce or eliminate field modifications during service life of the vessel. Carrying out field modifications, which invariably requires hot work, is one of the most painstaking exercises in operating plants.

There are many reasons why pressure vessels should be ergonomically designed. Pressure vessels are probably the longest serving equipment in the HPI. Their life often exceeds the working life of plant personnel, and the vessel can be passed on to the next generation. Even though a facility may cease oil and gas production, a well-maintained vessel would survive. Good pressure vessel design in the initial stages is also important, as there is practically no involvement of vessel designers and fabricators in subsequent field modifications, if any are ever undertaken. For pressure vessels, there are no performance tests to be conducted prior to dispatch. This is true even for proprietary designs. During plant commissioning, vessel manufacturers’ representatives are generally not needed—unlike rotating machinery, where the designs are proprietary and machinery manufacturers are usually involved in site performance tests, troubleshooting, modifications works, etc. Many recommendations exist to enhance vessel design.

Design vessels to match outer diameter to piping specs. With the advancement of computer-added design and drafting, the subjective visualization of the actual size of vessels has diminished. Computer printouts are exchanged, and the design is prepared with minimal manual intervention. A typical process software would carry out the inside diameter calculation of a vessel by taking various process parameters into consideration. The software is not programmed to standardize the vessel’s outside diameter. The mechanical design software would determine the inside diameter, calculate pressure wall thickness (including allowances) and pick up the next commercially available plate thickness.

The selected plate thickness is added (twice) to the vessel’s inner diameter, and the vessel’s outer diameter is established and passed on to a vessel fabricator. The thickness formulae in ASME-VIII, Div. 1, Section UG-27, “Thickness of shells under internal pressure” is based on vessel inside dimensions. Tubular Exchanger Manufacturers Association (TEMA) standard, Section N-1.1 1, defines the nominal diameter as the inside diameter of the exchanger shell.

Supplemented by the American Society of Mechanical Engineers (ASME) and TEMA, and assured by the fabricators that they can build a vessel to match any inside or outside diameter, no effort is made to standardize a vessel’s outside diameter. A typical plate bending machine (FIG. 1A) can roll a plate into a cylindrical shell with any inner diameter, as long as it is higher than the roller diameter. Plate thickness is the only limitation.

ASME-VIII and TEMA do not encourage standardization of vessels based on the outer diameter. This is in contrast to the piping codes to which a vessel is invariably attached. For example, ASME B31.3, Section 304.1.2 and ASME-B31.1, Section 104.1.2 perform calculations based on the outer diameter.

It is recommended that efforts be made to round off the vessel’s calculated outer diameter to match with the corresponding pipe outer diameter, e.g., up to 80-in. outer diameter (OD). Beyond 80 in., it may be rounded off in multiples of 6 in. ASME piping standards B36.10 and B36.19 cover pipes up to 80 in. NPS. Seamless pipes are available up to 24 in. Submerged arc-welded pipes (SAW) are available from 16 in. to 48 in. in North America and up to 64 in. in Asia. Use of pipes in lieu of plates is recommended wherever available.

The problem was referred to the plant engineering team. The fabricated shell was replaced with 16-in. (nominal pipe size) NPS seamless pipe, Sch. 80. Saddle and nozzle projections were adjusted to ensure no changes to plant piping. Using seamless pipe of appropriate specification avoided HIC test and saved fabrication costs, and units were ready within two weeks. Similarly, another exchanger with an original shell OD of 440 mm, 22-mm thickness was redone using 20-in.-NPS seamless pipe, 508-mm OD and 26-mm WT (Sch. 80). These examples show the importance of engineers reviewing and standardizing the shell dimensions produced by the computers, where possible.

The advantages of vessel OD standardization are:

  • Vessel fabricators can acquire seamless and welded pipes for vessel fabrication, wherever available. Short lengths of pipes of approved specifications are usually readily available at fabricators’ works and with operating companies. Such pipes with traceable documentation can be approved for fabrication of vessels and exchangers.
  • Less time and effort, and lower cost at the vessel manufacturer’s shop. Standard templates can be used to verify the OD of rolled plates, and there is no need to fabricate custom-made templates for each ordered vessel.
  • Saddle design is standardized, as the outer curvature of vessel is standardized.
  • Synchronized pressure/temperature rating of the connected piping and the piping components welded to the vessel, such as nozzle necks and welding-fittings.
  • Extra-long vessels can be fabricated with one piece of pipe. If plates are used, then a limitation of plate width exists. FIG. 1C shows a typical pig launcher barrel fabricated using plates with many longitudinal and circumferential welds.
  • If using pipes, a host of time consuming and expensive quality tests can be avoided. Such tests have already been carried out as part of pipe quality testing procedures.

Use standard materials where possible. In a new grassroots project, it is easy to adhere to exotic specifications, even for small items. The delivery periods for such materials get overlapped by big-ticket items. However, when it comes to maintenance replacements, procurement of items with exotic specifications in small quantities is a bottleneck. In the example given earlier, forged tubesheet to SA-266 was used. Ordering new sheets per SA-266 would take up to four months for delivery. Recovering old tubesheet was not an option as the diameter changed. The tubesheet was replaced with SA-516 plate, which is widely available, easy to fabricate, permitted by the code and posed no quality compromise. SA-516 should have been used in the first instance.

Do not provide side manways on small vessels. Many company specifications require at least one manway on a vessel, regardless of the vessel dimensions. Such manways do not serve the purpose and have many disadvantages:

• Technicians cannot easily get inside

• Fabrication issues: excessive welding heat-input on the self-reinforcing pad tends to distort the vessel

• Extra fittings, such as weldolet and debit arrangement, may be needed

• Projection of manway adds extra dead volume to vessel and may be a surprise to unsuspecting process engineers

• Crevice corrosion in the dead volume stagnant area, in FIGS. 2A and 2B (left).

NOTE: Always refer to ASME Section VIII Division 1 and 2 for vessel requirements. Statements of a general nature may imply there are no restrictions involved. ALWAYS check the Code first.

One recommendation is for small vessels, perhaps up to 42-in NPS, to be provided with either a flanged dished end or a blind flange, depending on the availability. FIG. 2B (Option 1 and Option 2) show the suggested configurations.

Advantages include:

  • Lack of a stagnant dead area, which is unnecessary from a design point of view and detrimental due to the potential for crevice corrosion
  • Better liner accuracy for volume control
  • Lower costs, as less welding is involved and a davit is not required, as shown in FIG. 2B
  • Time savings: a fabrication shop would produce dished-end in 24 hr to 48 hr.

In the case of tall and slim columns, as shown in FIG. 2C, a split in the middle is very maintenance friendly. FIG. 2D shows the original vessel design (simulated image) with a side manway. The change was incorporated during a mechanical review of drawings by the maintenance team.

Bottom nozzles/boot should stay within saddle height. A majority of company specifications require a 6-in. high saddle for smaller vessels and a 12- in. high saddle for large vessels, as measured at the vertical axis of the vessel. This design does not consider the nozzles and boots projecting beyond the saddle. FIG. 3A shows such a vessel. It is an unnecessary cost-saving exercise and creates subsequent problems onsite.

It is recommended to ensure saddle projection exceeds the bottom nozzles and boot projection. The advantages/disadvantage of tall steel saddles (FIG. 3B) are:

  • Ease of transportation: In the upstream industry, a majority of vessels are generic gas and oil separators and free-water knockout drums. Vessels are moved from one field to another to meet fluctuating production patterns. In such scenarios, vessels with tall saddles are easy to transport and do not require wooden pallets, in case nozzles and boots project beyond the saddles.
  • If the saddle projection is short, it must be compensated with an increased civil foundation height. While this may look like a better option from a cost perspective, it has disadvantages. The piping stresses are transferred to the ground via a concrete pedestal. Concrete is not appropriate for handling piping stresses, as cracks that are difficult to repair can appear. Steel saddles distribute loads much more effectively than concrete. Field preference is for steel saddles that overlap all bottom appurtenances by at least 6 in.
  • The only disadvantage for tall steel saddles is that fireproofing costs are marginally higher and may require fireproofing per relevant specification. Tall concrete pedestals would not require fireproofing, as concrete is treated as inherently fire-resistant.

Avoid inside projection on manways. Vessels are usually designed for full vacuum or half vacuum for steam-out conditions. Under certain conditions of full vacuum, the design may dictate the manway to be strengthened in addition to an external reinforcing pad. The most popular method to provide the required extra strength is to project the neck inside and weld circumferentially, as shown in FIG. 4 (left). This design makes exiting the vessel a very unpleasant, if not impossible, exercise. Another disadvantage of inside projection is that it does not allow complete vessel drainage. The small hole in the projected nozzle does not help effective drainage. An inside reinforcing pad could be a solution, but it would not drain the vessel completely.

Inside projection can be easily avoided by slightly reducing the vacuum required for steam-out conditions, thereby eliminating the projection. The vessel design engineer must adjust the vacuum numbers and communicate back to the process engineer for endorsement. A vessel need not be designed for full vacuum, as it will theoretically not achieve it.

In one case, the vessel design software dictated inside projection due to full vacuum design. The vessel was designed for a high-altitude location where atmospheric pressure was 93.3 kPa. The design engineer used the default software value for an atmospheric pressure of 101.325 kPa. When reduced to 93.3 kPa, the inside projection vanished. It is imperative that the software default values are not used without knowing their implications. What might appear as mundane data at the vessel design stage may be the root cause of an unpleasant experience for operation and maintenance personnel.

Spare nozzles. Fitting new nozzles on an existing vessel is an expensive proposition. It is worth adding a spare nozzle that can be used at a later date. The most useful spare nozzle is at the top of the vessel, preferably of the same size as the main inlet nozzle.

Cost implications. The initial vessel cost should not be evaluated in terms of CAPEX alone. If maintenance costs and possible field modifications (OPEX) are combined, all suggested measures eventually reduce the ownership cost of the vessel. Traditionally, the OPEX for static equipment like pressure vessels has been considered to be very low, as compared to CAPEX. That is not true for poorly designed vessels. Producing the correct products in the first instance is a win-win situation for all.


  1. Heinze, A. J., “Pressure vessel design for process engineers,” Hydrocarbon Processing, May 1979.
  2. Smolen, A. M. and J. R. Mase, “ASME pressurevessel code: Which division to choose?” Chemical Engineering, January 1982.
  3. Pullarcot, S. K., Practical guide to pressure vessel manufacturing (Mechanical engineering), Ed. 1, FACT engineering and design organization, Mercel Dekker Inc., Basel, New York, 2005.

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Fabricated Projects Include:

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The sizes of these projects are up to 200’ in length, 350 tons, 16’ diameter and 4” thick.

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