Page 263 - Valve Selection Handbook

250 Valve Selection Handbook

out in a manner that assures the proper relief function of the pressure
relief valve. The following discussion is restricted to installations in gas
or vapor service.

Rupture disc device at inlet of pressure relief valve. The operational
requirements are given below:

• The pressure relief valve shall pop open in unison with the bursting of
the rupture disc.
• Leakage across the corroded rupture disc must be prevented from pressur-
izing the space between the rupture disc and the pressure relief valve.
In the majority of installations, the operating differential amounts to
only 10% as is common for pressure relief valve installations. For this
operating ratio, rupture discs with zero manufacturing range, a burst
tolerance of plus/minus 2%, and an allowable operating ratio of 90 or
95% are available.
• To ensure that the rupture disc opens fully, rupture disc and pressure
relief valves should open simultaneously. But taking into account a set
pressure tolerance of the pressure relief valve of plus/minus 3% and a
burst tolerance of plus/minus 2%, the opening pressure of the pressure
relief valve could, in the worst case, be higher than the burst pressure
by 5%. To compensate for this possibility, the set pressure of the pres-
sure relief valve may have to be 3% lower than the rated burst pressure
if this does not lower the blowdown below the permissible level. Set-
ting the valve not more than 5% higher than the disc has been reported
to achieve full opening of the disc, 1 however, setting the valve lower
than the rupture disc is the preferred option.
The pressure relief valve used in this combination should be of the
pop-open type and may be unbalanced unless back-pressure conditions
demand a balanced type.

2
The ASME Code requires a telltale indicator assembly to be fitted in
the space between the pressure relief valve and the rupture disc to moni-
tor any leakage into this space across a corroded disc. Any pressure
build-up in this space would raise the burst pressure of the disc by the
amount of leakage pressure build-up.

1
Teledyne Fanis Safety and Relief Valve Catalog No. FE-80-100 page 7.26
2
ASME Code (1992), Sect. VIII, Div. 1, UG-127 (3)(4)