The shell of an autoclave is a rolled plate closed by a longitudinal seam, and that seam carries the highest stress in the vessel, so it needs a full-penetration weld made to a written procedure by a tested welder, inspected for internal flaws, and positioned where it can be seen rather than at the bottom where corrosion collects. Roll the plate to a true circle, because an out-of-round shell is bent by the pressure at the seam, and use only steel of known identity.
Part of Vessel Construction
Rolling, welding and inspecting the main cylinder of a steam pressure vessel, and where the shell is most likely to fail.
The Longitudinal Seam Takes The Hoop Stress
The hoop stress in a cylinder is twice the longitudinal stress (Hoop and Longitudinal Stress). The longitudinal seam is the joint that the hoop stress tries to pull apart, so it works at double the load of the circumferential seams that join a head to the shell. This is why the carrier article asks for a full-penetration weld on that seam and why it must be inspected: a flaw there sits across the largest stress in the vessel.
The carrier’s procedure for the shell is to cut the plate to size, roll it into a cylinder on a plate-rolling machine or by bending over a mandrel, weld the longitudinal seam with full penetration ground flush, and inspect the weld by X-ray or dye penetrant if possible. The notes below explain why each step matters.
What You Need
- Plate of known identity. A mill certificate, or at least a material you can identify. Spark testing sorts steels by carbon content (Metal Type Identification); high-carbon steel welds badly and cracks beside the weld.
- A plate-rolling machine or a rigid mandrel that forms a true curve along the whole length.
- A tested welder and a written procedure for the joint, steel and thickness actually used.
- Dye penetrant and a way to see inside the weld (radiography) where possible. If neither exists, the vessel’s rating should be limited by the inspection that was actually done.
- A clean drawing showing where the seams and every opening go.
Method: Rolling And Welding The Shell
Step 1: Roll to a true circle. Roll the plate gradually, checking against a template cut to the inside radius. Rolling often leaves the two edges flat, which gives a flattened strip at the seam. Pre-bend the edges before rolling, or the finished shell will have a flat spot there.
Step 2: Check for out-of-roundness. Measure diameter in several directions, particularly at the seam. A shell that is not round is pushed toward round by the pressure, which bends the wall at the seam in addition to stretching it. That bending stress adds to the membrane stress and does the damage at the very place the weld is weakest.
Step 3: Fit the seam with an even gap and no step between the two edges. A step (misalignment) concentrates stress at the weld toe.
Step 4: Weld in the procedure’s passes. Full penetration means the weld metal reaches and fuses through the whole thickness, so the joint is solid right through. Surface appearance does not show this. Clean each pass of slag before the next.
Step 5: Dress the weld as the drawing requires. The carrier asks for the seam to be ground flush, which removes surface irregularities that would concentrate stress or hide a crack from penetrant testing. Do not grind below the plate surface; that thins the wall.
Step 6: Inspect before anything else is welded on. Dye penetrant shows surface-breaking cracks. Radiography is the way to find lack of fusion and slag inside. A flaw found now costs a repair; a flaw found in service costs the vessel.
Step 7: Put the seam where it can be inspected. In the finished vessel, orient the seam so that it is not at the bottom, where water settles and corrosion works against a stressed joint, and not in the path of a nozzle or opening.
Common Mistakes
- Welding unknown steel. Without knowing the steel, nobody knows how it behaves beside a weld.
- Cosmetic welds. A bead that looks neat but has not penetrated is the commonest hidden defect.
- Grinding the weld flat and thin. Dressing is for smoothness, not for removing metal below the plate.
- Positioning the seam at the bottom. Out of sight, it corrodes first.
- Cutting an opening through a seam. Combine two stress raisers and invite a crack.
Safety
A weld flaw is a crack waiting for pressure
Never accept a pressure-bearing weld that has not been inspected. Never weld on a shell that held fuel, solvents or unknown contents. Welding fumes and arc light are hazards in their own right, so work in ventilated space with eye protection.
What’s Next
- Learn next: End Caps (Heads) — how the circumferential seams and head shapes close the ends
- Combine with: Failure Modes — how a seam flaw grows into a failure