Rupture Disk on Pressure Relief Valve Outlet
A rupture disk (RD), also known as a pressure safety disk, burst disk, bursting disk, or burst diaphragm, is a non-reclosing pressure relief device that can be used as the sole device as well as in combination with a pressure relief valve (PRV), either in parallel (as an additional or backup device) or in series. While the installation of the RD upstream of the PRV is the most common arrangement for the combination of RD and PRV in series, the RD is permitted downstream of the PRV. The reasons for installing a RD downstream of a PRV may include:1
- Protecting the PRV internals against corrosion, contamination, fouling/plugging or sticking that may be caused by discharge of other fluids into a common relief header
- Allowing the use of a conventional type PRV in installations where it may be exposed to significant variable superimposed back pressures in a common relief header, yet the use of bellows type or pilot-operated type valve is not preferred
- Detecting possible PRV leakage which may be an indication of process upset that could subsequently lead to PRV opening from overpressure
Detailed requirements for a RD on the outlet of a PRV can be found in the following standards:
- API Standard 520, Part 1 (9th Edition, July 2014) §220.127.116.11 2
- ISO 4126, Part 6 (2003) §5.4.3 3
- ASME Section VIII Div 1 (July 2011) UG-127 (a)(3)(c) 4
The requirements of ASME Section VIII are excerpted here for reference, and should be reviewed prior to the use of this design. A few additional design considerations for when RD is installed on the outlet of a PRV include the following:
- The minimum net flow area (MNFA) of RD should be equal or greater than the outlet area of the PRV; the RD manufacturer can be consulted for the MNFA of the installed RD
- Application of combination capacity factor (CCF) is not appropriate in calculating the PRV capacity for RD installed downstream of the PRV
- The RD burst pressure should be set as low as possible and the space between the PRV outlet and RD should be vented, drained and pressure monitored in order to prevent pressure buildup that may affect operation of the upstream PRV
- The outlet line of the PRV should be designed so that the pressure drops in the outlet piping through the RD and fittings do not adversely affect the performance of PRV and do not exceed the acceptable limit for the type of PRV installed; the RD manufacturer can be consulted for the certified flow resistance value KR of the installed RD
- The type of RD selected should be able to withstand the variable superimposed back pressure in the common vent system (e.g., flat forward-acting composite disks that can be furnished with a support to prevent reverse flexing from back pressure)
 American Petroleum Institute. “API Standard 520: Sizing, Selection, and Installation of Pressure-relieving Devices in Refineries; Part I—Sizing and Selection”. 9th Edition, 2014 July.
 International Standards Organization. “ISO 4126-6:2003, Safety devices for protection against excessive pressure – Part 6: Application, selection and installation of bursting disc safety devices”. 2003.
 American Society of Mechanical Engineers. “2010 ASME Boiler & Pressure Vessel Code, 2011a Addenda, Section VIII, Division 1 – Rules for Construction of Pressure Vessels”. 2011 July.