Dished heads are the curved end caps welded to the cylindrical shell of a tank, boiler or pressure vessel. Their shape — torispherical, semi-elliptical, hemispherical or shallow — decides how much pressure they can take for a given thickness, how deep they are and how much they cost to form.
A cylinder under internal pressure needs an end that carries the load without bending. A hemisphere does it best — the stress in it is half that in the shell — but it is deep and expensive to form. A torispherical head joins a shallow spherical crown to the shell with a tight curved knuckle, which makes it much shallower and cheaper, at the cost of a high bending stress in the knuckle. The semi-elliptical head lies between the two: deeper than a torispherical head and good for almost twice the pressure of a torispherical head at the same thickness.
The common proportions have names. The Klöpper head (DIN 28011) has a crown radius equal to the outside diameter and a knuckle radius of one tenth of it; the American flanged-and-dished (F&D) head is similar, with the knuckle no less than 6% of the crown radius. The Korbbogen head (DIN 28013) has a crown radius of 0.8 × D and a knuckle of about 0.154 × D, and is practically a 2:1 semi-ellipsoidal head, with a depth close to a quarter of the diameter. Shallow heads with a large crown radius and a small knuckle are used for atmospheric tanks, where pressure is not the design case.
Heads are formed from hot-rolled plate: the crown is pressed or spun from a circular blank, then the knuckle and the straight flange are rolled on a flanging machine; thick and large heads are hot pressed, and very large ones are welded from segments (petals). Forming thins the plate in the knuckle and work-hardens it, so pressure codes require a minimum thickness after forming and, beyond certain strains, a post-forming heat treatment. The straight flange keeps the head-to-shell weld away from the knuckle, where the stress is highest.
Pressure-vessel heads are designed to the code the vessel is built to — ASME Section VIII Division 1 (UG-32 and Appendix 1) in most of the GCC's oil, gas and petrochemical plants, EN 13445 or AD 2000 in European-designed plant. Tank, air receiver and LPG vessel fabricators in the region buy heads from specialist head makers in Europe, Asia and the region itself; large or thick heads are often imported formed. Four size tables give typical dimensions, masses and capacities of torispherical, semi-elliptical, small-knuckle and medium-radius heads.
A head is ordered by type, diameter (inside or outside — say which), minimum thickness after forming, straight flange length, material and code. Capacity of a 2:1 semi-ellipsoidal head is about 0.1309 × Di³ (Di in metres, result in m³) plus the straight flange; its inside depth is Di/4.
Full dimensions: Torispherical dished heads (R = D, r = 0.1D) — dimensions, mass and capacity · Semi-elliptical (Korbbogen) dished heads — dimensions, mass and capacity · Small knuckle radius dished heads (shallow tank ends) — dimensions, mass and capacity · Medium radius dished heads for tanks — dimensions, mass and capacity
| Region | Standard | What it covers |
|---|---|---|
| USA | ASME BPVC Section VIII, Division 1 ASME |
Rules for construction of pressure vessels — UG-32 formed heads under internal pressure, UG-79/UG-81 forming tolerances, Appendix 1 supplementary formulas |
| Europe | EN 13445-3 CEN-CENELEC |
Unfired pressure vessels — design; clause 7 covers dished ends (torispherical, ellipsoidal and hemispherical) |
| Europe | EN 13445-4 CEN-CENELEC |
Unfired pressure vessels — fabrication, including forming of dished ends and heat treatment after forming |
| Germany | DIN 28011 DIN |
Dished ends — torispherical (Klöpper) form; dimensions and tolerances |
| Germany | DIN 28013 DIN |
Dished ends — semi-ellipsoidal (Korbbogen) form; dimensions and tolerances |
| Germany | AD 2000-Merkblatt B3 | Design of domed ends under internal and external pressure |
| Europe | EN 10028-2 CEN-CENELEC |
Flat products for pressure purposes — non-alloy and alloy steels with elevated temperature properties (P265GH, P355GH), the usual head material |
| USA | ASTM A516/A516M ASTM |
Pressure vessel plates, carbon steel, for moderate- and lower-temperature service (Grades 55 to 70) |
Grade equivalents and the standards bodies: Steel grades & standards
Steel Mills - Agents & Traders · Stockist - Distributor · Fabrication · Other Steel Users · Dubai
Steel Mills - Agents & Traders · Stockist - Distributor · Fabrication · Other Steel Users · Dubai
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