Part types / Pressure vessel and tank components
Machining for Pressure Vessel and Tank Components
A welded fabrication first, and a machined part second.
Most large pressure vessel and tank components arrive at the machine shop already welded, as a rolled and welded shell, a formed head, or a fabricated nozzle assembly. Machining comes after fabrication, which means the machining plan has to work around whatever the weld sequence left behind, including distortion, residual stress and a code-driven heat treatment step that may still be ahead of it.
Sealing faces on flanges, nozzle bores and gasket surfaces are usually the only precision features on an otherwise fabricated part, so they carry a disproportionate share of the tolerance and surface finish requirement relative to the size of the component.
What decides the tolerance
The features that carry most of the risk on this part type, before you get to the machine that cuts it.
- Post-weld heat treatment sequencingWhether PWHT happens before or after finish machining, since a part can move during the cycle.
- Flange facing and nozzle boresThe precision features on an otherwise fabricated part, usually the reason the job needs a machine shop at all.
- Surface finish for sealingA stated roughness on gasket and O-ring faces, tighter than the rest of the component.
- Code traceabilityMaterial certificates, weld procedure records and NDT reports that travel with the part regardless of who machines it.
- Nozzle orientation and projectionPosition and angle relative to the shell, often the dimension a piping designer cares about most.
Where this work usually goes
Typical envelopes and table loads for each machine class are on the machine class reference.
Vertical turning lathe
Facing large heads, flanges and shell ends where diameter dominates.
Horizontal boring mill
Nozzle bores and flange faces on an assembled or partially assembled vessel.
On-site or field machining
Where the vessel is too large to move, or the work is a repair on an installed unit.
Materials
Carbon and low alloy steel plate for most process vessels, stainless and duplex grades for corrosive service, and clad or overlay materials where the pressure boundary and the wetted surface need different properties.
Before you send the RFQ
- State where PWHT falls in the sequence relative to machining, and whether any features are machined only after it.
- Give the applicable code and edition, since it decides inspection and documentation requirements the shop has to plan for.
- Call out sealing faces separately from structural tolerances so they are not lost in a general dimensional callout.
- For a field or on-site job, describe access and whether the vessel can be rotated, since that changes which machine route applies.
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