Rupture disks and other pressure relieving devices must be of an adequate size if they are to prevent system overpressure. The conditions for which over-pressure protection may be required must be carefully considered. During relief operations, the maximum flow rates necessary to prevent a pressure increase beyond specified limits must be determined.

The ASME Code requires that when a rupture disk is used as the primary relief device, it must be sized to prevent the pressure from rising above 110% of the MAWP (UG-125(c)). If used as a secondary relief device or as multiple relief devices, the size must prevent the pressure from rising above 116% of the MAWP (UG-125(c)(1)). If used as a supplementary relief device for hazards external to the protected vessel or system, the size must prevent the pressure from rising above 121% of the MAWP (UG-125(c)(2)).

The general procedure that should be followed when sizing a rupture disk is as follows:

1) Determine the maximum flow rate that must be vented. This will normally result from a worst case incident. The American Petroleum Institute Recommended Practice 520 and 521 set forth several examples for developing “worst case incidents.”

2) Determine the applicable properties and physical state of the fluid media that will be vented.

3) Select the appropriate equations and verify that the assumptions used for developing them are valid for the “worst case” under consideration.

4) Calculate the vent area required. (To learn how to calculate vent area, refer to the PDF file)

Yes, in either secondary relief or in series:

Secondary Relief – When used in a secondary relief capacity, the rupture disk provides a backup vent to a primary relief device, usually a relief valve. Its purpose here is usually to provide additional protection against an unlikely but possible major event that would exceed the capacity of the primary relief device.

In Series with Relief Valve – When used in series with a pressure relief valve, the rupture disk is usually installed upstream of the valve. The disk will protect the valve from process media that can corrode or plug it. The disk can also act as a seal, preventing any leakage through the valve unless the disk is ruptured.

The space between the rupture disk and the pressure relief valve must have a pressure gauge, try cock, free vent, or suitable telltale indicator. The normal configuration is an excess flow valve in combination with a pressure gauge. This arrangement is to eliminate the possibility of, or facilitate the detection of, a back pressure build-up. Because a disk responds to the differential pressure across it, it will not burst at its rated pressure if a back pressure is allowed to exist in this cavity.

A low-pressure rupture disk can be used on the downstream side of a relief valve that discharges into a common manifold to prevent exposure of the valve to process or corrosive media discharging through the common manifold. The space between the relief valve outlet and the rupture disk must be vented to prevent the accumulation of pressure, which could adversely affect the relief valve set pressure. An excess flow valve will suffice for this feature.

The main codes are ASME Section VIII, Division 1, API Recommend Practice 520, CE Pressure Equipment Directive 97/23/00. There are other industry codes that also have references to rupture disks.