Both Rupture Discs (also called bursting discs) and Pressure Relief Valves (PRVs) may be used independently as source of pressure relief. Rupture discs are pressure relief devices intended to burst once the intended burst pressure is reached. PRVs are designed to allow for the release of overpressure and to reclose when the pressure returns back to an acceptable level.
Using rupture discs in combination with PRVs often offers the best and most cost-effective solution.
Advantages of Rupture Disc in combination with Pressure Relief Valve (PRV)
Several arrangements exist in which rupture discs and PRVs may be used in tandem together, but perhaps the most common method is installing a rupture disc upstream of a PRV.
- Protection of PRVs against aggressive fluids for corrosion
By installing a fluid-compatible rupture disc upstream of the PRV, as the valve is physically isolated from the process and protected from the potentially corrosive fluid. In other words, the PRV’s exposure to the fluid is limited to an overpressure situation when the rupture disc bursts and opens. Therefore, this assembly allows for the use of “standard” material PRVs and related spare parts, resulting in a substantial reduction of maintenance and replacements. These materials aren’t only necessary for the body of the PRV but also for its internal components, including the spring, poppet, seat, screws, O-rings, gaskets and more. - Prevention of leakages and emissions
In order to achieve leak tightness, most spring-operated pressure relief valves rely on special metal-to-metal sealing surfaces which inevitably results in some leakage that increases as the operating pressure approaches the valve’s set pressure. PRV leakage rates are addressed in industry standards, and acceptable leakage rates are defined in API 576 for example.
Furthermore, it’s common for PRVs to partially open when pressure builds but doesn’t quite meet the PRVs set pressure. PRVs may leak or “chatter,” resulting in unwanted emissions and lost product. Where such leak rates are unacceptable for environmental or safety reasons, rupture discs positioned upstream of the PRV eliminate emissions up to 100 times in a simple and cost-effective manner by providing a leak-tight seal and avoiding PRV chattering and leaking. - Allow In-Service Testing of PRVs without taking them out of operation.
Periodic testing of PRVs is required by ASME, API, EN (European Standards) and various other regulating bodies to ensure proper calibration and to meet regional standards. This process usually requires the uninstallation of each PRV, which depending on the scale of the process, can be a time-consuming and costly endeavor because of the associated downtime. However, using rupture discs upstream of PRVs may allow for testing without the removal of the devices.
Here’s how it works- Pressure is applied in the space via compressed air between the rupture disc and PRV. The pressure is manually or automatically monitored.
- The pressure between the rupture disc and PRV rises to the opening pressure of both the rupture disc and PRV. The rupture disc should not be damaged during the test cycle.
- As the pressure reaches the opening pressure of the PRV, it will open.
- The measured opening pressure can now be compared to the rated nominal opening pressure of the PRV to determine if it is functioning within its design parameters – all without removing the PRV.
Requirements for the combination Rupture Disc with pressure relief valve in acc. to ASME BPVC Sect. XIII Part 8
A rupture disc may be installed between a pressure relief valve and the vessel provided:
- The flow capacity of the combined pressure relief valve and the rupture disc 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 disc 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, trycock, freevent, or other suitable telltale indicator. This arrangement permits detection of disk rupture or leakage.
For Section VIII, Division 3 (UD3 Designator) devices, in lieu of one of the previously mentioned indicators, the series combination can be provided with a second rupture disc device in parallel whose burst pressure is 116% of vessel design pressure. 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 disc and the pressure relief valve, which will occur should leakage develop in the rupture disc due to corrosion or other cause. - The opening provided through the rupture disc after the disc bursts shall be sufficient to permit a flow equal to the capacity of the pressure relief valve [see (2) and (3)], and there shall be no chance of interference with proper functioning of the pressure relief valve.
However, 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 disc device and pressure relief valve have been established by test according to ASME BPVC XIII 9.5. - The use of a rupture disc device in combination with a pressure relief valve should be carefully evaluated to ensure that the fluid being handled and the valve operational characteristics will result in opening action of the valve coincident with the bursting of the rupture disc.
- 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 discs shall not be used upstream of a pressure relief valve.
Marking of this combination
The combination of devices as described shall be identified by a metal plate or plates securely fastened to the pressure relief valve or rupture disc 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 organisation responsible for this marking. (This shall be the pressurised equipment user, pressurised equipment Manufacturer, non-reclosing pressure relief Manufacturer, or pressure relief valve Manufacturer.)
Permitted pressure relief devices
| Used for Pressure Equipment | Overpress Device | Type | ASME Designator |
|---|---|---|---|
| Pressure Vessel Section VIII Div. 1 | Pressure Relief Valve (PRV) | direct spring-loaded or pilot-operated | UV Designator |
| Pressure Vessel Section VIII Div. 1 | Rupture disc | exluded fragmenting-type rupture discs | UD Designator |
Pressure settings
When the required capacity is provided in more than one pressure relief device, only one pressure relief device need be set at or below the MAWP, and the additional pressure relief devices may be set to open at higher pressures but in no case at a pressure higher than 105% of the MAWP, except as provided in pressure vessel exposed by fire or other unexpected sources of external heat. The set pressure tolerance for pressure relief valves shall not exceed ±2 psi (15 kPa) for pressures up to and including 70 psi (500 kPa) and ±3% for pressures above 70 psi (500 kPa).
According to ASME Section XIII 9.5 (d)(1), the marked burst pressure of the rupture disc should be between 90% and 100% of the marked set pressure of the PRV. The burst pressure tolerance for rupture disk devices at the specified disk temperature shall not exceed ±2 psi (15 kPa) of marked burst pressure up to and including 40 psi (300 kPa) and ±5% of marked burst pressure above 40 psi (300 kPa).
Design standards and codes for rupture disc in combination with pressure relief valve
ASME BPVC Section VIII Div. 1 UG-150 to UG-156: Pressure Vessel
ASME BPVC Section XIII: Rules for Overpressure Protection Part 8
EN ISO 4126-3: Safety devices for protection against excessive pressure – Part 3: Safety valves and bursting disc safety devices in combination
EN ISO 4126-6: Safety devices for protection against excessive pressure –Part 6: Safety devices for protection against excessive pressure – Application, selection and installation of bursting disc safety devices
