ASME has allowed dual PRD systems ever since I started in process safety back in 1993. These systems usually have a Rupture Disc (RD) BEFORE the Pressure Safety Valve (PSV). This is usually done to protect the more expensive and more complicated PSV from the corrosive process gases. Although I never managed a PR system where a RD was placed after the PSV, this too is allowed by the code. This Releif System design used to be covered in Section VIII, UG-127, but since 2021, ASME has moved the requirements to it’s NEW(er) Section XIII – Rules for Overpressure Protection, Part 8.
BIG TIP: these dual PRD systems now have to be identified by a metal plate or plates securely fastened to the pressure relief valve or rupture disk device that contains specific information about the RD and PSV!
A rupture disk device may be installed between a pressure relief valve and the pressurized equipment, provided the following conditions are met:
- The flow capacity of the combined pressure relief valve and the rupture disk device shall meet the maximum permissible overpressure requirements of the referencing Code or Standard.
- The combined capacity of the pressure relief valve (nozzle type) and rupture disk device shall be the rated capacity of the valve multiplied by a factor of 0.90.
- The space between the rupture disk device and the pressure relief valve shall be provided with a pressure gage, try cock, free vent, or other suitable telltale indicator. This arrangement permits the detection of disk rupture or leakage.
- Users are warned that a rupture disk will not burst at its marked bursting pressure if back pressure builds up in the space between the disk and the pressure relief valve, which will occur should leakage develop in the rupture disk due to corrosion or other causes.
- The opening provided through the rupture disk after the disk bursts shall be sufficient to permit a flow equal to the capacity of the pressure relief valve and there shall be no chance of interference with the proper functioning of the pressure relief valve.
- In no case shall this area be less than the inlet area of the pressure relief valve unless the capacity and functioning of the specific combination of rupture disk device and pressure relief valve have been established by test according to 9.5.
- The use of a rupture disk device in combination with a pressure relief valve should be carefully evaluated to ensure that the fluid being handled and the valve’s operational characteristics will result in the opening action of the valve coincident with the bursting of the rupture disk.
- The installation shall ensure that solid material will NOT collect in the inlet or outlet of the rupture disk
- accumulation of such material could impair the relieving capacity of the relief system.
- Fragmenting-type rupture disks shall NOT be used upstream of a pressure relief valve.
A rupture disk device may be installed on the outlet side of a pressure relief valve, provided (a) through (i) are met.
This use of a rupture disk device in series with the pressure relief valve is permitted to minimize leakage through the valve of valuable material or of noxious or otherwise hazardous materials to accommodate the use of rupture disks on pressurized equipment for which a rupture disk alone or disk located on the inlet side of the valve is impracticable, or to prevent corrosive gases from a common discharge line from reaching the valve internals.
- (a) The pressure relief valve shall NOT fail to open at its proper pressure setting regardless of any back pressure that can accumulate between the pressure relief valve disk and the rupture disk. The space between the pressure relief valve disk and the rupture disk shall be vented or drained to prevent accumulation of pressure or suitable means shall be provided to ensure that an accumulation of pressure does not affect the proper operation of the pressure relief valve.
- Users are warned that many types of pressure relief valves will NOT open at the set pressure if pressure builds up in the space between the pressure relief valve disk and the rupture disk device.
- A specially designed pressure relief valve such as a diaphragm valve, pilot-operated valve, or a valve equipped with balancing bellows above the disk may be required.
- (b) The valve and disk combination shall meet the maximum permissible overpressure requirements of the referencing Code or Standard.
- (c) The marked bursting pressure of the rupture disk at the coincident temperature plus the additional pressure in the outlet piping that will occur during venting shall not exceed the design pressure of the outlet portion of the pressure relief valve and any pipe or fitting between the pressure relief valve and the rupture disk device. In addition, the marked bursting pressure of the rupture disk at the coincident disk temperature plus the pressure developed in the outlet piping during venting shall not exceed the set pressure of the pressure relief valve.
- (d) The opening provided through the rupture disk device after the disk bursts shall be sufficient to permit a flow equal to the rated capacity of the attached pressure relief valve without exceeding the allowable overpressure.
- (e) Any piping beyond the rupture disk shall be designed so that it will not be obstructed by the rupture disk or its fragments.
- (f) The contents of the pressurized equipment shall be clean fluids, free from gumming or clogging matter, so accumulation in the relief system will not interfere with pressure relief valve function.
- (g) The system shall be designed to consider the adverse effects of any leakage through the pressure relief valve or outlet-side rupture disk device, to ensure system performance and reliability. Some adverse effects resulting from leakage may include obstruction of the flow path, corrosion of pressure relief valve components, and undesirable bursts of the outlet-side rupture disk.
- (h) The design pressure of the pressure relief valve’s bonnet, bellows, if any, and exit connection to the rupture disk shall be greater than or equal to the burst pressure of the disk.
- (i) The bonnet of a balancing bellows or diaphragm type pressure relief valve shall be vented to prevent accumulation of pressure in the bonnet.
The combination of devices as described above shall be identified by a metal plate or plates securely fastened to the pressure relief valve or rupture disk device. The marking shall include the following:
- name of the Manufacturer of the valve
- design or type number of the valve
- name of the Manufacturer of the non-reclosing pressure relief device
- design or type number of the non-reclosing pressure relief device
- capacity or combination capacity factor
- name of the organization responsible for this marking
- This shall be the pressurized equipment user, pressurized equipment Manufacturer, non-reclosing pressure relief Manufacturer, or pressure relief valve Manufacturer
Source: ASME Section XIII