246 Steels: Metallurgy and ApplicationsFigure 3.30 The production of seamless steel gas cylinders (After Irani 18)where fe is the maximum permissible equivalent stress at test pressure.
In general, fe is equal to 0.75 x minimum yield strength. However, a value
of 0.875 x minimum yield strength is allowed in special portable containers,
e.g. those used in aircraft, underwater breathing apparatus and life raft inflation,
thereby facilitating the use of lighter containers. For a Cr-Mo steel cylinder
(230 mm OD), used for the transportation of oxygen at 200 bar (equivalent to a
test pressure of 300 bar), equation (2) provides a minimum wall thickness t of
5.39 mm.^18
BS 5045: Part 2 provides the same equations for the calculation of minimum
thickness as those shown earlier for seamless containers (equations (1) and (2)).
However, as indicated previously, the proof strength values of steels specified
in Part 2 are significantly lower than those included in Part 1. Thus for a given
test pressure, welded containers would require a greater thickness than seamless
containers but, in effect, the former operate at lower pressures.
Rather strangely, BS 5045 Parts 1 and 2 make no reference to impact test
requirements although Oldfield 2~ reports that leak-before-burst philosophies are
growing in acceptance and that fracture toughness tests are being developed for
use with gas containers. This is particularly pertinent when handling aggressive
gases, as illustrated in the following section.
Hydrogen gas containers
During the 1970s and early 1980s, almost one hundred failures occurred in Europe
(20 in the UK) in gas containers that were used for the delivery of hydrogen by
road trailer. Failure was due to the propagation of fatigue cracks from small