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.
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: