This specification discusses pressure vessel engineering and design in compliance with the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1.

This specification in combination with general pressure vessel requirements including fabrication and Inspection, Testing, Painting, Marking, Preparation for Shipping and Storage will define the minimum acceptable requirements of unfired pressure vessels suitable for installation in energy industry.

Pressure Vessel Engineering and Design

1. General Engineering

Pressure Vessel Supplier should be responsible for the detailed mechanical design of pressure vessels in accordance with the ASME Code and other Technical Requirements.

The minimum acceptable internal corrosion allowance should be 1/8" (3 mm) for all “wetted” components, including but not limited to the vessel shell, heads, nozzles, internals, etc., unless specified otherwise.

  • If the vessel is to be clad, no corrosion allowance is necessary
  • Internals that are fabricated from stainless steel or corrosion resistant high alloy steels should be exempt from this corrosion allowance requirement
  • Internal rings, supports, baffles, vortex breakers, miscellaneous plates and structural shapes, piping supports, etc., should have a minimum thickness of 1/4" (6 mm) exclusive of corrosion allowance

For internals "wetted" or exposed on both sides, the corrosion allowance should be at least 50% greater than the single side corrosion allowance specified for the shell.

Internals that are bolted and easily removable from the vessel should be exempt from the 150% corrosion allowance requirement, i.e., they should be provided with the same corrosion allowance as the shell.

The minimum thickness of all pressure containing components, including shells, heads, nozzle necks, piping, etc., should be 1/4" (6 mm), including corrosion allowance. The minimum thickness of internals should be 1/4" (6 mm), including corrosion allowance.

A corrosion allowance of 1/8" (3 mm) should be added to the calculated thickness of skirts or support legs attached to vertical vessels and to saddles that support horizontal tanks or drums.

If provided, General Arrangement drawing should be considered preliminary and Buyer will finalize arrangement details during the approval process. Buyer reserves the right to revise nozzle locations, at no additional cost, up to the point of actual fabrication, i.e., until the shell or head penetrations are cut.

Pressure vessels should be designed and fabricated to facilitate maintenance, repairs and alterations, in particular on adjustable or removable internals.

When specified on the vessel data sheet, vessels designed for internal pressure should be stamped for external pressure, as noted.

The design pressure of equipment in vacuum service or which may be subjected to vacuum during reasonable start-up, operating, shutdown or upset conditions, should be full vacuum except where the Supplier can demonstrate that an economical alternative design will not allow full vacuum to occur.

The following Minimum Design Metal Temperatures (MDMT) and coincident pressures should be indicated on both the vessel nameplate and the Manufacturer's Data Report.

Vessels subject to steam-out should be designed to withstand the steam-out pressure/temperature condition and external pressure of 7.5 psi (0.5 bar) at 450°F (232°C).

2. Design Requirements

Pressure Vessel Supplier's design should accommodate all of the following requirements, as applicable.

  • Fabrication/Handling Loads
  • Internal Design Pressure
  • External Design Pressure
  • Ocean Transportation Accelerations
  • Wind Pressure Loads
  • External Nozzle Loads
  • Support Clip Point Loads
  • Thermal Expansion
  • Hydrostatic Test
  • In-Service Flooded Condition
  • Sand Loadings

Static head pressures should be included in the design pressure.

The MAWP should be based on the actual metal thickness less corrosion allowance.

The maximum allowable pressure should be limited by the shell or heads, not by minor parts such as flanges, nozzle necks, reinforcing pads, piping, fittings, or manways. The design report should clearly identify the limiting component.

Vessels in vacuum service should be designed for a minimum internal pressure of 50 psig (3.45 bar).

MDMT should be as shown in vessel datasheet, which should apply to both pressure containing and vessel support components. MDMT should not be warmer than 50°F (10°C).

The shells of larger diameter horizontal vessels should have sufficient thickness and/or adequate stiffness to be structurally stable when full of water at atmospheric pressure.

Temporary and permanent stiffening should be provided to prevent distortion of the vessel during manufacture and transport.

Vessel supports should be designed for wind, seismic and transportation loads.

3. Shell

Pressure vessel shells should be fabricated from rolled and welded plate.

For vessels 24" (610 mm) and smaller in outside diameter, seamless pipe in accordance with ASME Code may be utilized.

Vessels made from pipe should have wall thicknesses that account for the mill under thickness tolerance allowed by the piping Code and ASTM specifications.

Longitudinal seams on horizontal vessels should be located a minimum of 30 degrees above the horizontal plane through the centerline of the vessel. Long seams should be staggered about the vessel vertical centerline between adjacent shell cans.

4. Heads and Transitions

Unless specified otherwise, all vessel heads should be 2:1 ellipsoidal type with straight flanges a minimum of 1-1/2" (38 mm) long.

For vessels 24" (610 mm) and smaller in outside diameter, weld caps in accordance with the ASME Code may be utilized.

Tori-conical transition sections are preferred. Conical transitions are permitted only when agreed to by the Buyer.

When conical transitions are used:

  • Joint efficiency of 1 is not permitted, due to difficulty in examining the joints
  • Stiffening rings should not be located closer than 6" (150 mm) from the weld seam of conical transitions

5. Nozzles, Manways, Bosses and Other Openings

Unless specified otherwise by Buyer, all vessel nozzles should be flanged, with a minimum size of 1-1/2" NPS (DN 40).

Flanged nozzles smaller than 1-1/2" NPS (DN 40) should not be allowed unless specifically approved in writing by Buyer, and should never be smaller than 3/4" NPS (DN 20).

Threaded connections should not be allowed unless specified or specifically approved in writing, by Buyer.

  • If approved, threaded connections should be 6000# forged steel full couplings as a minimum
  • Threaded connections should be limited to either 1/2" or 3/4" NPS (DN 15 or 20)
  • Threaded connections should have their threads chased after installation or postweld heat treatment
  • Threaded connections should not be acceptable for use on internally coated or clad vessels or stainless steel vessels

Weld-o-lets, Thread-o-lets, and Sock-o-lets should not be acceptable for vessel connections.

“Set-on" nozzle designs are not acceptable.

All shell and head attachments, including nozzle necks, manway necks, threaded couplings, reinforcing pads and clips should be located a minimum of 2" (50 mm) from vessel longitudinal and circumferential seams.

  • When unavoidable and approved in writing by the Buyer, attachments may cover a welded joint
  • However, prior to covering the joint, the seam weld should be ground flush and 100% radiographed to a minimum of 6" (150 mm) beyond each side of the attachments

Nozzles should be either long weld neck flanges, or of built-up construction from pipe nozzle necks and flanges.

  • Long weld neck flanges are preferred by Buyer for all nozzles 3" (DN 80) and smaller
  • Flanges used in built-up construction should be forged steel weld neck type, bored to match the inside diameter of the pipe nozzle neck
  • Socket welded or slip-on flanges should not be acceptable, except on manways
  • Studded pad-type nozzles should not be permitted

Unless specified otherwise, ANSI Class 150 through Class 600 series flanges should be raised face (RF) type, and Class 900 and above should be ring type joint (RTJ) flanges.

On nozzles and manways having tongue and groove facing, the groove should be on the vessel unless the flange face is directed downward, in which case the tongue should be on the vessel.

Vessel nozzle neck thickness should be in accordance with the ASME Code, but with the following minimum thickness requirements:

Nominal Diameter (DN) Nom. Pipe Size: Min. Schedule:
40 1-1/2" XXS
50 to 80 2" to 3" 160
100 4" 120
150 to 250 6" to 10" 80
300 to 400 12" to 16" 60
450 to 500 18" to 20" 30
600 24" 40

Nozzle outside projections should be sufficient for the removal of a properly sized stud from the back side of all flange bolt holes, i.e., between the back of the flange and the vessel shell, head or reinforcing pad. As a minimum, all nozzles should project at least 6" from the vessel outside diameter to the flange face.

Nozzle Design Details and Reinforcement

  • Nozzle design details and reinforcement calculation per current Code is acceptable, provided the design conforms to all the other requirements listed in this specification

Nozzles with Inside Projections

  • Vessel drains and liquid outlets should be flush with the inside surface of the vessel and should be provided with vortex breakers, unless otherwise shown on the vessel drawings. Flush designs may be required to remove internals
  • The projection of process connection nozzles should be established with due consideration of sand or scale accumulation
  • Buyer approval of nozzle projections should be required
  • Inner edges of nozzles, manway necks, and other pressure vessel penetrations should be ground smooth to 1/8" (3 mm) minimum radius

Reinforcing Pads and Saddles

  • All reinforcing pads and saddles should be drilled and tapped for a 1/4" NPT (DN 8) telltale hole
  • Telltale holes should be located on the low axis of the reinforcing pad
  • Two (2) telltale holes should be provided on all nozzles greater than 24" NPS (DN 600), and on all split or segmental reinforcing pads
  • A preliminary compressed-air and soap-suds test should be made in accordance with Paragraph UW-15(d) of the ASME Code

Manways and Manholes

  • Manways should be provided to access each vessel section that requires cleaning or maintenance, and to permit the removal of internal components
  • Unless specified otherwise, manway size should be 20" NPS (DN 500) for vessels with inside diameters of 4'-0" (1220 mm) and less, and 24" NPS (DN 600) for vessels with inside diameters greater than 4'-0" (1220 mm)
  • A grab bar should be provided inside the vessel above the manway to facilitate vessel entry, as well as individual rung ladders below
  • All manways and handholes, as well as nozzles designated as "spare", should be provided with blinds, including studs, nuts and gaskets
  • Handhole blinds should be provided with two (2) handles
  • All nuts, bolts and washers should be carbon steel coated with Type II Class 8 chromated/cadmium plating
  • Manway blinds should be hinged or davited, complete with a handle
  • Nozzle, manway and handhole flange bolt holes should straddle vessel normal centerlines unless noted otherwise
  • Handholes should be circular with an 8-inch (203 mm) minimum interior diameter


  • Davits should be designed such that no adjustments are necessary to align the blind flange with its mating flange
  • Davits should not sag
  • Davits should be fitted with grease fittings in the swivel
  • Davit arms fabricated from pipe should be capped to prevent intrusion of moisture

Nozzles, manways, handholes and couplings should be attached to the vessel with full penetration welds, including reinforcing pad to nozzle neck welds.

The orientations of critical nozzles should be shown on the pressure vessel project datasheet.

The Supplier should be responsible for ensuring that nozzle locations will allow for easy access to various control instruments and valves.

The use of gooseneck nozzles (nozzles with elbows, tees, or any other weld fitting) should be subject to Buyer approval.

Vessel Nozzle Loads

  • Vessel/Nozzle connections should be capable of withstanding reasonable piping loads
  • Buyer reserves the right to supply actual piping loads as they become available
  • Local stress at the nozzle connection should be calculated using WRC 107/297 when applicable, or ASME Section VIII, Div 2, Appendix 4; extrapolation of geometric parameter curves in WRC 107/297 should not be permitted

6. Internals

Internal non-pressure-containing parts, such as trays and catalyst support beams, internal equipment, such as cyclones and grids, and attachments welds to vessel, should be designed using allowable stress for material and temperature specified in ASME Section II, Part D, as applicable.

Supplier should provide all internals in accordance with the following requirements.

  • Internals furnished by others should be installed by the Supplier
  • Baffles, partitions, and other parts should be designed to accommodate differential thermal expansion between vessel shell and internal parts
  • Baffles, partitions, and other parts should be designed to avoid trapping water or flammable liquids that cannot be drained or purged

An inlet diverter should be provided on the inlet nozzles of vessels that handle multi-phase fluids, e.g., separators, scrubbers, knockout drums, etc. In addition, inlet diverters should be provided for erosive fluids and for high velocity fluids such as those downstream of high pressure let-down valves.

  • Inlet diverter design should utilize either box or pipe type construction
  • Three-phase vessels should be equipped with inlet compartmental boxes or baffles with downcomer pipes, to pipe fluid to the interface level
  • Compartmental boxes should be adequately sized for gas/liquid separation
  • The inlet diverter should be designed such that no "reflection" of the inlet stream impinges or erodes the vessel shell
  • A wear plate should be provided on the vessel shell or head if diverter construction directs the inlet fluid against the vessel shell or head
  • Wear plates should be a minimum of 3/8" (10 mm) thick
  • Wear plate size should be a minimum of five times the cross sectional area of the inlet nozzle or 12" (305 mm) greater in radius than the nominal pipe size of the inlet nozzle, whichever is greater

Mist extractors should be provided to remove entrained liquids from gas streams to a minimum level of 0.1 gallon per million standard cubic feet (MMSCF) at 10 microns and larger, and should be sized to maximize turndown capacity.

  • The design should base the open area on the vessel supplier's maximum allowable velocity and the governing combination of pressure and flow
  • Materials of construction and all mounting hardware should be 316L stainless steel

Mist extractors should be either parallel plate (vane) type or wire mesh mist pad types as specified. If not specified, mist extractors should be parallel plate (vane) types.

Parallel plate (vane) type mist extractors should be provided with a liquid collection pan, including a liquid seal, to allow proper drainage to the liquid section.

Wire mesh mist pad extractors should have a mist pad density of 11 pounds per cubic foot (lb/ft3) (176 kg/m3) minimum, and mesh thickness of 6" (150 mm) minimum.

Contact the Buyer for additional requirements on vapor/liquid separators and scrubbers.

  • Mist pads should be of 316L stainless steel construction, including mesh, support grid, distribution plates and all mounting hardware
  • Mist pad assemblies should be accessible for inspection and maintenance, and easily removable for cleaning or replacement

Vortex breakers should be provided on all liquid outlets that feed process equipment, in particular pumps and hydrocyclones.

  • The design of vortex breakers should comply with Industry Standard
  • Nozzle inside projection should be flush with the bottom of the vessel

Vessel drains should be flush with the bottom of the vessel and should be provided with vortex beakers unless shown otherwise on vessel drawings.

If specified in the vessel data sheet, Supplier should provide a jetting system for sand removal.

  • The system should consist of a loop-type manifold with VeeJet steel jet nozzles, or Buyer approved equal, directed at the vessel drains from both sides
  • Minimum inlet nozzle and header size should be 2" NPS (DN 50)
  • All piping should be schedule 80 thickness as a minimum and should be adequately supported with stand-offs that are seal welded to the vessel shell
  • All drains should be covered with a removable sand pan a minimum of 18" in length
  • Jetting system piping should use flanged spools of such length that they may be readily inserted and removed through the vessel manway

Internal stiffener rings should be seal welded.

  • Seal welding details should be indicated on the Supplier's approved detailed fabrication drawings
  • Provide a 1-inch (25 mm) skip in the weld at the bottom of an internal pad for a vent
  • Avoid welding over vessel shell welds

Unless specified otherwise, all internal bolting should be 316 stainless steel, and should be double-nut.

Vessels installed on a Floating Production Facility (FPF).

  • Due to the motions of the FPF, Supplier should design all vessels to maintain performance comparable to similar vessels installed on a fixed foundation. This will require Supplier to pay close attention to the design of vessel internals and may require the installation of additional baffle plates or other motion suppression internals to maintain acceptable performance.
  • Motions for which the equipment should be designed should follow Basis of Design

Mole Sieve Vessels

  • Fatigue analysis is typically required for these vessels
  • Structural support beams for mole sieve vessels should be designed on the basis of a 20 psi (1.4 bar) (minimum) pressure drop from gas flow across the bed. This differential pressure should be added to the design live and dead loads from other internals.

7. External Attachments

Supplier should provide all external attachments in accordance with the project requirements or vessel data sheet.

External attachments should be shop-welded prior to any stress relief.

External attachments should not cover vessel weld seams without prior Buyer approval.

Lifting Lugs

  • Lifting lugs should be provided to facilitate handling and installation
  • Horizontal vessels should be provided with two (2) lugs installed on the top centerline of the shell
  • Vertical vessels should be provided with two (2) lugs installed on the top
  • Vertical vessels should be provided with one tailing lug installed at the skirt base plate to facilitate laydown and uprighting of the vessel
  • Lifting lug design should incorporate a safety factor of 2.0 applied to the vessel total dry weight, including all internals
  • Unless specified otherwise, the lug design load should include the weight of external attachments such as ladders and platforms
  • Lug design should be based on a maximum sling angle of 30 degrees from the vertical
  • For vertical vessels, the lug design should account for shipment of the vessel in the horizontal position and uprighting of the vessel


  • All vertical vessels 6'0" (1.8 m) or more in overall height should be equipped with clips on the shell and top head to permit the addition of a ladder and platform
  • Pairs of ladder clips should be provided below the top head seam, above the skirt baseplate and at intervals not exceeding a 12'0" (3.7 m) spacing

Insulation Support

  • Unless specified otherwise, Supplier should furnish shop-welded insulation supports in accordance with the project requirements or vessel data sheet
  • Support rings should be provided on approximately 12'0" (3.7 m) centers projecting to within ½" (13 mm) of the thickness of the insulation

Grounding Lug

  • A grounding lug should be supplied with the vessel

8. Vessel Supports

8.1 General

Vessel supports should be designed in accordance with the AISC Manual of Steel Construction.

  • Support bolting design (size, number, minimum spacing, etc.) should be based on use of ASTM SA-325 bolting
  • Designs based on other bolting should require Buyer approval

Wind, seismic and transportation loads should be in accordance with ASCE 7.

  • Basic wind speed should be per project requirements
    • As a minimum, the design wind velocity should be 100 miles per hour (3-second gust) (160 km/hr)
    • Unless otherwise stated, Exposure Category D (wind from open body of water) should be assumed
    • Unless otherwise stated, importance factor for wind load should be 1.15
  • Seismic coefficients should be as stated in the project requirements
  • Unless otherwise specified by Buyer, transportation design loads should be based on a lateral acceleration of 0.65g acting along both the longitudinal axis and the transverse axis, and a vertical acceleration of 1.5g as a minimum. The allowable stress may be increased by 33% for the transportation design condition.

Vessels should be provided with integral self-supporting supports designed to accommodate the following loads and load combinations, without additional support.

  • Erect Load with Full Wind. Dead load should include installed weight of the vessel, including internals, platforms, insulation and other permanent attachments.
  • Operating Load with Full Wind or Seismic Loads
    • Operating load should include all dead and live loads, full and zero process liquid levels, full and zero pressures and thrust loads from attached piping
    • Full and zero liquid level and pressure combinations are required to determine the maximum longitudinal tensile and compressive stresses
  • Field Hydrostatic Pressure Test Load with Partial Wind

When a field hydrostatic pressure test is required, the vessel should be assumed to be full of water and support should be designed for 50% of the design wind speed (or 25% of the wind load).

When specified by Buyer, vessel supports should be designed to support the vessel completely filled with sand at 3000 pounds per cubic yard (lb/yd3)(1800 kg/m3), and simultaneously subjected to the maximum wind load. As a minimum, production separators and test separators should be designed to meet this requirement.

Base plates and rings should be designed such that the support bearing pressure should not exceed 750 psi (51.7 MPa).

Support steel should be a minimum of 1/4" (6.4 mm) thick including corrosion allowance.

  • A corrosion allowance of 1/8" (3 mm) should be added to the calculated thickness of skirts, saddles and support legs.
  • All support welding should utilize continuous seal-welded construction, both inside and outside.

8.2 Horizontal Vessels

Horizontal pressure vessels should be provided with two (2) structural steel saddles welded to the vessel shell.

Additional supports should not be acceptable without prior written agreement by Buyer.

Saddle design should include a web plate, base plate, end flanges and stiffening webs as a minimum.

  • The contact angle between the saddle and vessel shell should be between 120 and 150 degrees
  • A reinforcing plate should be provided between the vessel shell and the saddle, if needed to reduce local concentrated stresses in the vessel wall
  • All saddle reinforcing plate corners should have a radius of five times the plate thickness, as a minimum

Unless specified otherwise, saddles should be provided with bolt holes to allow bolted connection to platform steel.

  • Saddle bolt holes should be slotted at one end of the vessel for thermal expansion if the temperature difference between maximum operating condition and minimum ambient condition is greater than 1000F (380C)
  • If thermal growth between the saddles exceeds 3/8" (10 mm), a slide bearing plate should be provided between the saddle base plate and the supporting structural member on the platform
  • The slide plate design should include Teflon slide plates and pipe sleeve-type guides

Horizontal vessel shells should be analyzed in accordance with L. P. Zick's "Stresses in Large Horizontal Cylindrical Pressure Vessels on Two Saddle Supports."

In no case should the distance between the head tangent line and the saddle centerline be greater than 20% of the tangent to tangent length of the vessel.

Internal stiffener rings should not be accepted as a means of stiffening horizontal vessels.

8.3 Vertical Vessels

Vertical vessels should be provided with a circular skirt-type support.

  • Support skirts should be the same outside diameter as the vessel supported
  • The skirt should be butt-welded to the knuckle portion of the bottom head, and the weld should be contoured to blend smoothly into the head
  • The allowable stress value for skirt thickness should be calculated in accordance with the ASME Code using a weld joint efficiency of 0.55

Unless specified otherwise, skirts should be provided with bolt holes to allow bolted connection to platform steel.

Skirt design should include a base ring, access openings, high point vents and pipe openings, as follows:

  • Skirts should be provided with a base ring with a minimum thickness of 1/2" (13 mm) and a maximum thickness of 2" (50 mm)
    • If necessary, gusset plates should be provided to limit the base ring to 2" maximum thickness
    • Gusset spacing should be 18" (457 mm) minimum
  • Skirts should be provided with one (1) access opening
    • The opening should be 18" (457 mm) minimum in diameter
    • Skirt access openings should be reinforced with a pipe or rolled collar-type sleeve along the perimeter of the opening
    • The access opening may be oblong on tall skirts, to provide better access
    • Provisions should be made so that the skirt will not hold water
  • Skirts should be provided with a minimum of two (2) vent holes located as close to the bottom head of the vessel as possible. Vent holes should be 2" (50 mm) in size, evenly spaced around the vessel circumference, and reinforced with pipe sleeves
  • All pipe openings should be reinforced with pipe sleeves approximately 1" (25 mm) larger in diameter than the largest pipe spool flange, as applicable
  • All reinforcing sleeves should project 2" (50 mm) beyond the skirt (both internally and externally) or should be equal in length to the thickness of any fireproofing or insulation to be applied to the vessel, whichever is greater
  • The number of anchor bolts should be in multiples of four and there should be a minimum of four
  • The minimum size of anchor bolts for towers should be 1 inch (25 mm) diameter
  • The minimum size anchor bolts permitted for vessels other than towers should be 3/4" (19 mm) diameter
  • Fillet welds attaching skirts to heads should have a minimum fillet size of the thickness of the skirt
  • If base rings are fabricated from segments, these should be joined with full penetration double butt welds

Vessels with outside diameters less than or equal to 24" (610 mm) and not exceeding 10'-0" (3.05 m) in overall height may be provided with leg type supports, subject to Buyer approval.

Wind Requirements for Vertical Vessels

  • Vertical vessels or columns with H/D ratios exceeding 15 (where H is the length of the vessel from the point of support to the top tangent line and D is the average diameter in the top third of the vessel column) should be checked for vortex shedding vibrations
  • Critical wind velocity of the vessel should be greater than 3 times the maximum 10-minute sustained wind velocity corrected at the top of the vessel

Unless otherwise specified, deflection at the top of vertical vessels or columns should be limited to 6 inches per 100 ft (5 mm per meter) of vessel height.

For additional information on unfired pressure vessels, please see general pressure vessel requirements including fabrication and Inspection, Testing, Painting, Marking, Preparation for Shipping and Storage.