Page 264 - Valve Selection Handbook

Rupture Discs 251

The ASME Code permits the capacity of the combination of rupture
disc and pressure relief valve to be taken at 90% of the rated capacity of
the pressure relief valve or at the value of the combination factor as certi-
fied by test. 1

2
Rupture disc device at outlet of pressure relief valve. The purpose of
this combination is to protect the valve internals against the ingress of
corrosive fluids that might be present in the discharge system.
This pressure relief combination permits the entry of leakage past the
valve seatings into the chamber between the seatings and the rupture
disc. To prevent a pressure build-up in this chamber, the upstream side of
the rupture disc must be provided with a telltale indicator assembly.
When the valve is called upon to open, pressure builds up in the cham-
ber between the seatings and the rupture disc that amounts to the burst
pressure of the rupture disc. In assessing the possible burst pressure, the
maximum rated burst pressure plus burst tolerance and any superimposed
back pressure should be taken into account. The valve and piping compo-
nents upstream of the rupture disc must be designed to withstand this
pressure.
The pressure build-up thus assessed could impede the opening of a
conventional pressure relief valve. For this reason, the use of a balanced
pressure relief valve is suggested.

Rupture disc devices at inlet and outlet of pressure relief valve. The
design may follow the design recommendation for the above rupture
disc/pressure relief valve combination, choosing a balanced pop-action
pressure relief valve.

Rupture disc/pressure relief valve combination for use in liquid ser-
vice. Rupture disc/pressure relief valve combinations with the rupture
disc mounted to the valve inlet can be used also in liquid service if a sub-
stantial gas volume between the rupture disc and the liquid level can be
assured so that the pressure relief valve opens in unison with the bursting
of the rupture disc.

1
ASME Code (1992), Sect. VIII, Div. 1, UG-129 (c), UG-132
2
ASME Code (1992), Sect. VIII, Div. 1, UG 127 (c)(3)