The basic difference
A safety valve is a spring-loaded valve. It opens when the pressure reaches its set point, relieves the excess, and closes again when the pressure falls. The process carries on.
A rupture disc is a thin membrane designed to burst at a set pressure. Once it has burst it stays open until someone replaces it, and the contents of the vessel are released until the pressure source is removed.
Side by side
| Rupture disc | Safety valve | |
|---|---|---|
| After relieving | Stays open; must be replaced | Recloses; process continues |
| Leakage in normal service | None; a solid metal or graphite barrier | Some seat leakage is possible, more as operating pressure nears set pressure |
| Speed of opening | Milliseconds; full bore at once | Slower; lift depends on the spring and flow |
| Loss of contents | Everything, until the source is isolated | Only what is needed to bring the pressure down |
| Moving parts | None | Disc, spring, guide; can stick or foul |
| Operating pressure margin | Typically up to 70 to 90 % of burst pressure, depending on disc type | Typically up to 90 % of set pressure (95 % for pilot-operated) |
| Corrosive, sticky or polymerising fluids | Well suited; wide choice of materials and linings | Prone to sticking and seat damage unless protected |
| Purchase cost | Low, even in exotic alloys | Higher, much higher in exotic alloys |
| Maintenance | Replace at set intervals and after any burst | Periodic testing, recalibration and overhaul |
| Testing in place | Not possible; the device is destroyed by testing | Possible |
When a rupture disc is the better choice
- Very fast pressure rise, such as a runaway reaction or a heat-exchanger tube rupture, where a valve cannot open quickly enough.
- Toxic or expensive fluids where any leakage through a valve seat is unacceptable.
- Corrosive, viscous or solidifying fluids that would foul a valve.
- Very large relief areas, where a disc is far smaller and cheaper than the equivalent valve.
- Secondary relief for an unlikely case, such as external fire, alongside a valve that handles the routine cases.
When a safety valve is the better choice
- Relief events are expected from time to time, and stopping the plant to change a disc each time would be costly.
- The contents should not be lost, or the release must be kept as small as possible.
- Clean service such as steam, air and utility gases, where a valve works reliably for years.
- Boilers, where codes require safety valves.
Using both in series
A rupture disc installed directly under a safety valve gives the advantages of both. The disc seals the process from the valve in normal operation, so the valve stays clean and there is no leakage; if the pressure rises, the disc bursts and the valve then relieves and recloses.
Three rules apply to this arrangement:
- Monitor the space between the disc and the valve. If the disc leaks, pressure builds up behind it and the disc will no longer burst at its marked pressure. The space needs a pressure gauge, indicator or free vent (a tell-tale).
- Derate the valve capacity. Unless the combination has been tested and certified together, the valve capacity is multiplied by a combination factor Kc = 0.9.
- Use a non-fragmenting disc. Pieces of a burst disc must not lodge in the valve, so a reverse-buckling or scored design is used rather than one that shatters.
A disc can also be installed on the outlet of a valve to protect it from a corrosive discharge header, with the same need to keep the space between them at atmospheric pressure.
Sizing either device
The sizing principle is the same for both: find the worst-case relief flow, then the flow area that passes it at the relieving pressure. The rupture disc and safety valve calculators do this, and the safety valve tool applies Kc = 0.9 when you tell it a disc is installed upstream.