Page 235 - Valve Selection Handbook

222 Valve Selection Handbook

lished by bursting a minimum of two rupture discs. 1 The burst pressure
thus established shall not exceed the maximum allowable burst pressure
of the vessel level as defined by the code. 2

Burst tolerance. 3 The ASME Code allows a tolerance around the rated
burst pressure within which the rupture disc is permitted to burst. For
burst pressures up to 2.76 barg (40 psig), the burst tolerance shall not
exceed plus/minus 0.14 barg (2psig). For burst pressures above 2.76 barg
(40 psig), the burst tolerance shall not exceed plus/minus 5%. For certain
types of rupture discs, manufacturers are able to offer reduced burst tol-
erances down to plus/minus 2%.
The burst tolerance allows the rupture disc to burst above the applica-
ble maximum allowable burst pressure level by the amount of the burst
tolerance.

Manufacturing range. The manufacturing range is an allowable range
of pressures around a specified burst pressure within which the rupture
disc can be rated. The pressure range must be agreed upon between user
and manufacturer. The purpose of the agreement is to permit the econom-
ical production of some types of rupture discs.
The types of rupture discs mainly involved are forward-domed rupture
discs, which by their design fail in tension. Because the tensile strength
of the metals used in the manufacture of rupture discs is fairly high, the
discs must be made of relatively thin foils. Thin foils of uniform thick-
ness and tensile strength, however, are difficult to produce and vary
between heats of material.
Manufacturers must therefore select from different foils until the
desired burst pressure has been achieved. To keep manufacturing costs
within acceptable limits, only a limited number of finding tests can be
carried out.
The manufacturing range may be expressed as a minus or plus/minus
percentage around the specified burst pressure or in pressure units. These
are two examples.
If the specified burst pressure is 10 barg (145 psig) and the manufactur-
ing range is stated to be minus 11%, a disc that is rated anywhere between
10 barg (145 psig) and 8.9 barg (129 psig) meets the disc specification.

1
ASME Code (1992) Sect. VIII, Div. 1, UG 127(a)(l)(a)
2
ASME Code (1992) Sect. VIII, Div. 1, UG 134
3
ASME Code (1992) Sect. VIII, Div. 1, UG 127(a)(l)