Understanding the Different Types of Pressure Vessel Heads and their Applications

August 22, 2019

It's easy to point out a pair of hemispherical pressure vessel heads. They look like half-sphere end caps. Indeed, their perfectly round outlines make fluid storage vessels look like giant pills. Designed to provide a consistent dish radius, this ideal shape is popular because it's good at distributing heavy fluid pressures. However, that's not always going to be the case, so designers have come up with several alternative vessel head profiles.

Fabricating Elliptical Heads

These arcing end dishes look a lot like Hemi heads, except for their lack of radial uniformity. The radius of the dish is set at the head rim, but that value diminishes as it swings towards the centre of the dish. The radial depth of the dish head reaches its smallest point when perpendicular to the vessel rim. In effect, a 2:1 ratio (semi-elliptical shape) produces a low-profile dish. This design is also built to contain higher pressures. Semi-Elliptical vessel heads use thinner walls because their curving outlines are structurally capable of holding back higher internal stresses.

Developing Flanged and Dished (F & D) Heads

An arithmetical progression is taking place. With a hemispherical head, its radius doesn't vary. Those spherical outlines use thin materials because a sphere is a structurally strong shape. Semi-elliptical heads use a 2:1 fabrication ratio. That means, compared to the head's diameter, the centre of a semi-elliptical dish is 4 times shorter than its width. Lower than a hemispherical dish, the arcing shape is almost as strong as a sphere, so it also used thin-walled metals. Next in this logical progression, flanged and dished heads deliver an almost entirely flat end cylinder cap profile. Predictably, these flattened dishes are designed to tolerate moderate fluid pressures, so their walls thicken slightly.

Expect to see Hemi heads in use on larger fluid storage systems. However, because they're constructed out of welded sheets, not single-sheet curved panels, then they're not exactly the cheapest option. If hemispherical heads are prohibitively expensive, a design company will probably offer a low-profile Semi-Elliptical (SE) alternative header configuration. Almost as capable as a pair of Hemi dishes, 2:1 Ellipticals are also made from thin-walled sheets, but they don't cost as much to fabricate. Skipping down to moderate pressure applications, to settings that involve the storage of fuels and chemicals, F & G heads are more than good enough for most industrial and commercial demands. By the way, there are head profiles that combine the best features of all of the above discs shapes. Also, as yet another workable option, flat heads suit low-pressure usage areas, although they require more alloy thickness, for they don't have any geometrical support.

Contact Details

Fusion - Weld Engineering Pty Ltd
ABN 98 068 987619

1865 Frankston Flinders Road,
Hastings, VIC 3915

Ph: (03) 5909 8218

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