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End Caps (Heads)

4 min read

A domed (dished) head resists pressure as tension in its curved skin, so it can be much thinner than a flat plate, which is bent by the pressure and must be far thicker and stiffened. Use a domed head wherever you can form one, install it with the pressure on its concave side, check the thickness after forming because the stretching thins it, and take the actual thickness and shape from a recognised pressure-vessel code.

Part of Vessel Construction

The closures at the ends of a cylindrical pressure vessel: why shape matters, how heads are formed and joined, and what goes wrong.

A Curved Head Is In Tension; A Flat Head Is In Bending

The stress article (Hoop and Longitudinal Stress) shows that a hemispherical skin carries stress P × r / (2t), half of the hoop stress in a cylinder of the same radius and thickness. A dome spreads the load over a curved surface as simple stretching, which steel handles well.

A flat plate cannot do that. Pressure on a flat plate bends it like a beam supported at the rim, and bending stress concentrates at the centre and at the rim. For the same pressure and diameter, a flat plate must be several times thicker than the shell it closes. The carrier article says a flat head must be much thicker than a dished one and gives a minimum multiple of the wall thickness. Treat that as a lower bound, because the right thickness for a flat head depends on its diameter, the pressure and how the rim is held, and the code formula for it is not reproduced in this corpus.

The practical ranking, as the carrier gives it: a dished head is best, a 2:1 elliptical head is strongest per unit of weight, and a flat head is easiest to make but heavier and needs stiffening.

What You Need

  • A code drawing that specifies the head type, thickness and straight flange (skirt).
  • Forming capacity: a press and die, a spinning lathe, or a forge and a set of hammering forms.
  • A thickness gauge (callipers, or a drilled and measured test coupon) to check the head after forming.
  • Welders and inspection as for the shell (Cylindrical Body).

Method: Forming And Fitting A Dished Head

Step 1: Start with a plate thick enough to survive forming. Pressing or spinning a disc into a dome stretches the plate, and the crown and the knuckle (the curved transition to the straight edge) thin the most. A head cut from plate of exactly the finished thickness will finish thinner than the design allows.

Step 2: Form gradually. Hot or cold forming in stages avoids cracking and wrinkling. Check each stage against a template.

Step 3: Measure the thickness after forming, at the crown and around the knuckle, and compare to the minimum on the drawing. A head that has thinned below the minimum must be rejected, not rated lower.

Step 4: Leave a straight flange at the rim of the head, so the weld to the shell sits on straight, uniformly stressed metal and not in the knuckle.

Step 5: Weld to the shell with a full-penetration circumferential weld and inspect it. This seam carries the longitudinal stress, half the load of the longitudinal shell seam but still a pressure-bearing joint.

Step 6: Install the dished head with the pressure on its inside (concave) face. A dome under pressure on its convex face tends to buckle inward at a much lower pressure than the same dome loaded from the concave side.

One end of the carrier’s autoclave is permanent and welded. The other end is the door, which has its own requirements (Door and Closure Mechanism).

Common Mistakes

  • Using a flat disc “because it is easy”. A thin flat head deflects and eventually cracks or leaks. It is the one place a builder is most tempted to under-build.
  • Not measuring after forming. Thinning is invisible from outside.
  • Welding at the knuckle. The knuckle already carries the highest local stress of the head.
  • Mounting the dome the wrong way round. Pressure on the outside face of a dome is the weak case.
  • Cutting an opening in the crown without reinforcement.

Safety

A head failure releases the whole vessel at once

A head that blows off turns the vessel into a projectile and releases the steam in a single event. Never use a head whose thickness after forming is below the drawing minimum. Never stand in line with the axis of a vessel being pressure-tested.

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