Page 261 - Valve Selection Handbook

248 Valve Selection Handbook

P B1 = rated burst pressure minus burst tolerance of inner disc
P B2 = rated burst pressure minus burst tolerance of outer disc
Example:
Normal operating pressure PI = 140 psig
Operating ratio of rupture disc = 0.7, burst tolerance plus/minus 5%.
p
Fluid pressure in space between rupture discs P 2 = — 70psig

70
Then P B1 = P B2 = — = lOOpsig minimum burst pressure
= 105 psig rated burst pressure

Selecting Rupture Discs

The selection of rupture discs is interdependent on the pressure differ-
ential between the operating pressure and the maximum allowable work-
ing pressure of the pressure system. Manufacturing range, burst toler-
ance, and recommended operating ratio for the selected rupture disc must
be accommodated within this pressure differential.
Figure 6-34 shows the pressure level relationships between the maxi-
mum allowable working pressure and the operating pressure resulting
from the installation of a rupture disc as the sole pressure relief device.
Manufacturing range in this case is plus 10%/minus 5% and the burst tol-
erance plus/minus 5%.
For an installation of this type, the ASME Code 1 requires the rated
burst pressure not to exceed the maximum allowable working pressure
but permits the plus burst tolerance to do so. Under these conditions, the
minimum possible burst pressure could amount to 82% of the maximum
allowable working pressure. Allowing for an operating ratio of 70%, the
operating pressure should not exceed 57% of the maximum allowable
working pressure.
For burst pressures higher than 2.75 barg (40 psig), some rupture disc
manufacturers deviate from the assessment of the minimum burst pres-
sure as shown in Figure 6-34 by neglecting the minus 5% burst tolerance.
When the pressure differential is as short as 10%, as is common in
pressure relief valve installations, the only available choice is a rupture
disc with zero manufacturing range, an operating ratio of 90% or 95%,
and possibly a reduced burst tolerance.

1
ASME Code (1992) Sect. VIII, Div. 1, UG-134