Newer submarines utilize large, such as 84 and 167 cubic foot, air vessels (flasks). Typically, they are made up from a short cylindrical midbody section, capped by curved, generally hemispherical end pieces. Each flask contains a supply and discharge pipe fitting, an internal drain assembly, and an access plug in one of the hemispherical end pieces. The access plug is necessary for inspecting and maintaining the flask and also for installing the internal drain assembly. Currently, a 151/2 inch clear opening has been selected for the access plug because it is the smallest diameter through which a workman can enter and is large enough for tooling. The access plug is threaded into the curved end portion of the flask, and then welded in place to prevent leakage through the threads.
The flasks are designed to withstand an internal operating pressure of 5,000 psi and a test pressure of 8330 psi. The access plug threads react loads normal to the plug, but the curved interior of the vessel results in forces directed otherwise. Consequently the weld, or seal, is subject to tensile forces which can lead to failure thereof.