Page 236 - Valve Selection Handbook

Rupture Discs 223

If the specified burst pressure is 10 barg (145 psig), as before, but the
manufacturing range is stated to be plus 7%/minus 4%, a disc that is
rated anywhere between 10.7 barg (155 psig) and 9.6 barg (139 psig)
meets the disc specification. The maximum rated burst pressure must not
exceed the maximum allowable burst pressure except where permitted by
the code.
Manufacturers are able to offer reduced manufacturing ranges and, in
the case of reverse-buckling discs, also zero manufacturing range.

Operating ratio. This is the ratio between the maximum operating pres-
sure and the minimum burst pressure. The operating ratio is designed to
ensure a satisfactory service life of the rupture disc. The values recom-
mended by manufacturers range between 70% or less and up to 90%,
depending on type of rupture disc and operating temperature. Non-steady
operating conditions, such as cycling and pulsating pressures and fluctu-
ating operating temperatures, may vary these values.

METAL RUPTURE DISCS

There are two major types of rupture discs made of ductile metal:

• forward-acting types, being tension loaded
• reverse-acting types, being compression loaded

Forward-domed and flat rupture discs are the tension-loaded types,
while the reverse-buckling disc is of the reverse-loaded type. The follow-
ing describes a cross-section of these discs as offered by the industry.

Tension-Loaded Types

Solid forward-domed rupture discs. Solid forward-domed rupture
discs are formed from flat discs by applying a fluid pressure to the under-
side of the disc of normally above 70% of the burst pressure. This
method of manufacture gives the rupture disc a hemispherical shape, as
shown in Figure 6-3. When operating pressure grows beyond the pre-
doming pressure, the dome starts to grow. As the operating pressure
approaches 95% of the burst pressure, localized thinning in the region of
the dome center occurs that leads to rupture of the disc. This failure is
accompanied by some fragmentation of the disc.