Pressure Vessel & Boiler Fabrication

Pressure vessel work has a characteristic that changes how prep should be judged: the weld will be volumetrically inspected, and a rejected weld on a fabricated shell is expensive to excavate. That moves the economics away from “cheapest way to get an angle on the edge” and toward “prep that makes the weld pass the first time”.

Where the joints are

A code vessel is mostly four joint families, and they fail differently:

• Longitudinal and circumferential shell seams — long, heavy, and fully radiographed. Bevel consistency along the length is what keeps the RT clean.
• Nozzle-to-shell — compound geometry, restricted access, frequently the joint that holds up the schedule.
• Head-to-shell — thick section, often with a transition.
• Heat exchanger tube-to-tubesheet — a different problem entirely, small bore and high count.

ASME BPVC Section VIII and EN 13445 set the acceptance criteria; EN ISO 9692-1 the geometry. None of them care how the bevel was produced — but the inspection regime effectively decides it.

Shell plate: machined, not thermal

On plate above roughly 20 mm the argument for machining is not surface finish, it is repeatability. A milled bevel holds its angle and root face along a 6 m or 12 m seam; a flame-cut bevel does not, and the welder absorbs the variation with filler metal and time.

• DMM heavy-duty hydraulic edge miller — single-sided prep to 150 mm plate.
• DMM-900 — self-propelled along the plate edge for steep and vertical bevels.
• DMM-90X — self-propelled; flips 180° to take X and K joints on plate to 80 mm without turning the plate over.
• DMM-1232 — bevels top and bottom simultaneously on 4 – 55 mm plate, which removes the crane cycle from double-sided prep.

The trade-off between double-sided prep and single-V on heavy plate — filler volume, distortion, and where X-prep starts paying for itself — is worked through in plate beveling and edge milling.

Nozzles and pipe-to-shell

Nozzle prep is where portable machining earns its place, because the work has to come to the shell rather than the other way round. The split-frame clamshell mounts on the OD and cuts, bevels and counterbores in one setup; the internal-expansion beveler works from the bore where the OD is obstructed. Both cut cold, which matters on the alloy grades that turn up in vessel work.

Material makes the method

Vessel fabrication rarely stays in one material. Carbon steel shells with stainless cladding, P91 / P22 chrome-moly in high-temperature service, duplex in aggressive process duty — each has its own reason to avoid a torch:

• Chrome-moly is heat-treatment sensitive; an uncontrolled HAZ from flame cutting compromises the PWHT plan. See beveling P91 and chrome-moly.
• Stainless and duplex lose corrosion resistance in the chromium-depleted zone behind a thermal cut — detail here.
• Clad plate needs the cladding preserved right up to the weld face, which a thermal cut cannot do.

Cold machining sidesteps all three, which is usually the whole reason a vessel shop buys the machine.

Boiler and power work

Boiler drums, headers and water-wall panels sit in the same code family but with tube-scale geometry and access constraints — covered on the power generation page and in boiler tube beveling.