Volume XLVI.II: Design operations & maintenance friendly pressure vessels - Part 2

  • Volume XLVI.II: Design operations & maintenance friendly pressure vessels - Part 2


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.

Note: Readers are advised to make their own engineering judgments on the validity of the design improvements suggested here, and to develop their own conclusions.

Design operations-and-maintenance-friendly pressure vessels—Part 2

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 prevailing industry design requirements, recommendations and practices.

Avoid fireproofing and flanged joints inside the skirt

Many company specifications ask for fireproofing inside the vessel skirt due to the presence of flanged joints. These features create problems for field maintenance. Corrosion under insulation (CUI) and corrosion under fireproofing (CUF) have assumed greater importance and have been known to cause equipment collapses. API’s first publication in 2014, API-RP-583, “Corrosion under insulation and fireproofing,” underscores the importance of tackling this problem (FIGS. 1A and 1B). 

Table 2 of API-583, “Locations for corrosion under insulation and fireproofing,” clearly identifies the insulated zone at the skirt weld and fireproofed skirts as areas to be watched for accelerated corrosion and leaks. API Standard 2510, “Design and construction of LPG installations: Section 10.8.4,” states, “When a vertical vessel is supported by a skirt, the exterior of the skirt shall be fireproofed.” This implies that even for highly flammable services like liquefied petroleum gas (LPG), the interior of the skirt need not be fireproofed.

It is recommended to cover large skirt openings with a vented removable cover, preventing plant site waste materials from accumulating inside. Typical site waste materials include plastic bags, pieces of fireproofing/insulation pads, pieces of gaskets, etc. Stray animals have been known to occupy unattended vessels with large skirt openings. The practice of storing small tools and consumables inside vessel skirts should be discouraged from a safety perspective—keeping skirt openings closed is the best option.


Code supports covering skirt openings

API 2510A, Section, “Fire protection considerations for the design and operation of LPG storage facilities,” states, “The interior should be fireproofed if there is more than one access opening in the skirt that is not covered with a plate.” The section implies that fireproofing is not required if skirt openings are minimal and covered. The code indirectly infers that if there is only one opening, it may be left open. However, this should be discouraged. Skirts should always be gas-tested before manual entry.

Adequate sizing of vessel’s process nozzle at bottom

Always ensure that the bottom process nozzle is large enough. The recommended minimum is 4-in. nominal pipe size (NPS), regardless of what a computer program dictates. Small nozzles eventually clog, particularly in the upstream industry, even if the fluid is classified as “clean” on data sheets. FIG. 2 shows a typical vessel where the bottom nozzle was enlarged to 4-in. NPS as a last-minute change. The rest of the piping was left as 2-in. NPS, to be replaced later. Any hot work to a vessel is a herculean task and very difficult for operating plants, but piping changes are comparatively easy and can be accomplished in a routine plant shutdown.

Adequately sized gravity drain nozzle

All vessels that undergo maintenance must be hydrotested and completely drained of water. The potential drainage time for large vessels is an issue in delaying plant startups. Some large vessels take 24 hr–48 hr for a gravity drain. Most company specifications/data sheets do not specify a maximum drainage time under gravity flow. It is recommended to restrict drainage time to 3 hr–4 hr. This information is rarely found on vessel drawings or data sheets, and it is vital for planning of other maintenance activities in tandem.

Literature is available to estimate drainage time under gravity for horizontal and vertical vessels and spheres, and to account for pressure loss in drain piping.1,2

Reinforcement plate for 150# flanged opening

Code calculations do not dictate reinforcement pads for small-diameter openings, particularly for low-design pressure vessels. Reinforcing pads may not be needed for small-diameter openings, even in high-pressure vessels. A practical example of a water-seal drum is illustrated in FIG. 3A. The bottom drain nozzle is 6-in. (150-mm) NPS and welded directly to the vessel without a reinforcing pad, as dictated by the code calculation. Water and a hydrocarbon mixture accumulated inside a skirt, as shown in FIG. 3A, and a leak path shown in FIG. 3C. As the skirt opening was covered, no dirt was accumulated, reducing the fire hazard and enabling easy cleaning.


It is recommended to provide reinforcement pads for bottom nozzles, thereby superseding code calculations (FIGS. 3C and 3D). Advantages include:

  1. Reinforcement pad provides secondary protection from leaks
  2. Leaks can be detected at early stages from “tell-tale” hole
  3. Hot work can be carried out from the outside using low-heat electrodes, without the need to make the vessel gas-free
  4. If leakage is a persistent problem, a pressure gauge can be installed for early warning, as shown in Fig. 3E.


This recommendation has been subsequently clarified under the ASME-VIII chapter, “Best practices for the installation of pressure relief devices.”

Although this recommendation is not retro-effective, it should be possible to modify non-complying vessels by relocating relief valves on the side inspection openings available on most vertical vessels. If installed upstream, the relief valve size must be reconfirmed due to possible liquid carryover to the relief valve. If valves are installed sideways and discharge in the open, then care should be taken to strengthen the side nozzle per API-520, Part 2, Section 4.4.1. Recalculating noise at grade per API 521, Section is appropriate due to the slightly higher noise level at grade caused by the lower elevation of the relief valve. A vessel nozzle that is one size larger than the relief valve nozzle is preferred for possible future upsizing of the relief valve. Using a reducing elbow is preferred from a stress and lower pressure-drop point of view, as compared to a standard elbow and a reducer combination.

Avoid internal ladder in corrosive service

Internal ladders installed in vertical vessels in corrosive service and in vessels packed with internals serve little purpose. Such ladder rungs create safety hazards and obstruct installation of scaffolding for maintenance works. FIG. 5A shows lower ladder rungs that have corroded and fallen apart, which could be attributed to a higher 

concentration of corrosive fluid in the lower stagnant portion of the column.

The integrity of such ladders, including the rungs, is doubtful. Ladder rungs are made from ¾-in. (20-mm) bars and welded to the vessel wall, sometimes with poor workmanship. Due to corrosive media degradation, the welding is insufficient to sustain a human load. The wear plate design is also questionable (FIG. 5B). Typical wear plates require ¼-in. (6-mm) tell-tale holes to ensure porosity-free welding. The tell-tale hole provided from a good welding aspect only assists corrosive fluid to enter the cavity and expedites the detachment of the wear plate from the vessel wall. What began as a good intention from the design phase could potentially prove fatal for field personnel.

In this particular case, due to severe corrosion, the ladder rungs were removed and the area ground flushed and painted. An aluminum ladder lowered from the manway proved to be a good temporary substitute.

Nozzle openings through welded seams

It is a general impression that opening through welded seams is not permitted by codes. Such openings are often detected only in the late stages of vessel design, when the plate-cutting diagrams have been prepared. An owner’s design engineers are not involved in the review of plate-cutting diagrams, and (at this stage) the vessel general arrangement (GA) and piping GA/isometric drawings are already frozen. To avoid openings on welded joints, nozzles are relocated and related piping is rerouted. This exercise is expensive, time-consuming and unnecessary.


ASME-VIII, Div. 1, UW-14, “Openings in or adjacent to welds,” permits this practice, as does ASME-VIII, Div. 2. The overlap of reinforcement pads of adjacent openings is also permitted. FIG. 6 shows a Div. 2 vessel with a manway located on a circumferential seam weld. The vessel has been in intermittent service for the past 39 years without any leaks.

Proper relief valve nozzle sizing

A vessel relief valve opening one size larger than the relief valve inlet nozzle is recommended (FIG. 4). A reducing elbow design in lieu of a standard elbow and reducer is preferred and helps field personnel avoid hot work in case a larger relief valve is needed in the future. Vessels have a long life expectancy among major process equipment, and should manage increased gas flowrates in aging production fields. 

Separate as-built drawings for each vessel

Some vessel manufacturers provide only one set of as-built drawings if multiple identical vessels are ordered. For two identical vessels, V-101A and V-101B, these manufacturers will provide only one set of as-built drawings tagged as V-101A/B. The mantra, “All work is completed by computer numerical control (CNC) machines, so all vessels are identical,” is a paperwork-saving shortcut approach that does not help the end user. At times, only one vessel may be modified in the field, so one set of as-built drawings for each tagged vessel (V-101A and V-101B) are vital. Third-party inspectors should ensure compliance and insist on signing off on each set separately. This requirement is applicable only for as-built drawings. During design stages, only one set of GAs should be provided tagged as V-101A/B.

Transportation and storage notes

While part of an operating company’s technical support team, the author was asked for a particular grade of mineral oil. It was discovered that the oil was requisitioned to apply on the inside surface of a vessel before it was put back into operation. Further questioning revealed that the vessel GA drawing included the comment, “Mineral oil, grade xxxx, is to be applied inside the vessel.” The mineral oil coating was meant for new vessels as a rust preventive during transportation and prolonged storage. The technicians had unknowingly been following the procedure for vessel maintenance.

The lesson is that transportation notes should not be appended on vessel GA drawings. If appended, the purpose of such notes should be clearly stated to avoid confusion, keeping in mind that maintenance personnel faithfully adhere to all instructions stated on vessel GA drawings. The impression that the role of vessel GA drawings ends once a vessel is commissioned is incorrect. GA drawings are a valuable maintenance aid and necessary throughout the life of a vessel. Providing a set of A-3-size GA drawings and internals, no matter how congested the reduced-size copies look, is suggested. Jumbo-size drawings (A-0, A-1) are difficult to handle in the field; photocopiers are rarely available to make copies of A-0 and A-1, and such drawings usually end up in untraceable custody, albeit with good intentions.

State vessel operating weight

General protocol dictates that vessel GA drawings provide a vessel empty weight and a hydrotest weight simulating a vessel that is full of water. The transporter must know the empty weight. The hydrotest weight is required by the vessel manufacturer to ensure that the shop floor can handle the load, and it is also used to design the vessel foundation onsite. This works well if the fluid is water or lighter than water.

What happens if the vessel is designed for fluids heavier than water—e.g., sulfuric acid (specific gravity = 1.84)? The fluid weight would almost double, but the GA drawing may still state the hydrotest weight. Cases have been reported where the hydrotest weight from a GA drawing was inadvertently used to design a civil foundation for sulfuric acid vessels, resulting in undue foundation settlements. Specifying a vessel weight as full of liquid for heavier fluids, in addition to usual empty steel weight and hydrotest weight, would be a good practice.

Source: Murti, D.G. – The Augustus Group, Montgomery, Texas. Kossik, J., “Draining time for unpumped tanks,” Chemical Engineering, Vol 107, No. 6, June 2000 Loiacono, N.J., “Time to drain a tank with piping,” Chemical Engineering, Vol. 94, No. 11, August 1987.

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