Pros and Cons of Spherical vs. Cylindrical Pressure Vessels

December 13, 2018

From a purely theoretical viewpoint, spherical pressure vessels are superior. When a fluid pushes outwards, the pressure exerted on that spheroidal void will be uniformly experienced by its rolled metal panels. If that's the case, why aren't all pressure vessels designed to look like squat spheres? Obviously, just by looking at a local fuel depot, that's simply not the case. Cylindrical pressure vessels are everywhere, after all.

A Theory versus Practice Perspective

Granted, at least in the oil processing industry, spheroidal pressure vessels are the logical choice. And they're out there, lined up in globular arrays on concrete aprons. They uniformly distribute immense fluid pressures. Generally speaking, however, it's easier, more economically sensible even, to fabricate a cylindrical pressure vessel. Rolled plates are easier to manufacture and weld. To complete the equipment form, there's even a range of end cap choices. These headers, shaped as hemispherical (half spheres) or torispherical (flattened dish) end elements, imbue cylindrical fluid storage vessels with subtly different pressure handling facilities. Finally, leaning heavily on the practical perspective, it's not easy to manufacture a completely spheroidal storage unit. As a compromise of sorts, rolled cylindrical panels, which are welded into wide-diameter tubes, are mounted with spheroid-like headers.

Single-Dimension Scalability

Here's another important point, one that effects processing plant design. If a cylindrically profiled pressure vessel gains a capacity increase, we just have to return to the fabrication yard to make the alterations. A new replacement unit gains cylindrical length, plus a slight increase in its diameter. It climbs higher in the processing facility, so there's more support framework to add to the design. If spheroidal pressure vessels receive a capacity boost, the matter is so much harder to resolve. The diameter of the sphere widens, for there is no separate tubular section to alter. That entire sphere needs to grow in size to accommodate the larger fluid load. Still, assuming the facility doesn't require scaling, spheres are the superior force spreading solution. And, instead of bringing in larger metal balls, the designer can always add a second equally sized spherical pressure vessel. Of course, that approach only works if there's plenty of concrete plinth space inside the processing network. On oil refineries, chemical processing plants, and fuel storage depots, space is often a precious resource.

Logistically speaking, fabrication floors need more project time when they're building spherical pressure vessels. This is a single asset design process. It moves off in two directions, arcing radially to form a massive sphere. Elsewhere, sectional procedural design methods dominate cylindrically shaped vessels. There's the cylindrically rolled sheets, plus the two headers. It's simply a more fabricator-friendly design, though not the optimal fluid distribution choice.

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