Rupture disks vs safety valves: when to use each

Rupture discs and safety valves are the two main primary pressure relief devices in the process industry. Although they are often presented as alternatives, they are complementary technologies with very different profiles. The correct decision is rarely "one or the other": it is usually which one goes in front and which one behind.

How they work

A safety valve (PSV/PRV) opens when the pressure exceeds its set point, discharges the fluid and closes again once the pressure has recovered. It is a reusable, calibratable device and, in its pilot-operated version, capable of maintaining the set point up to 100% of the flow rate.

A rupture disc is a metallic membrane calibrated to burst at a specific pressure. Once burst, the relief is total —it does not close again— and the disc must be replaced before resuming production.

Comparison table

Response speed

The rupture disc opens in milliseconds, with no mechanical inertia. It is the only valid option against deflagrations or very rapid pressure rises. The safety valve needs tens to hundreds of milliseconds depending on size and spring.

Tightness

The disc is metallurgically tight: there are no leaks until it bursts. Conventional safety valves show leakage near the set point, which is why codes require a minimum margin between operating pressure and set point (typically 10%).

Reusability

The valve is reusable; the disc is not. This means that after an overpressure event the disc must be replaced before restarting, whereas the valve simply needs to be bench-tested.

Aggressive fluids or fluids with solids

Polymerising, corrosive, viscous or solids-laden products can block or foul a valve's seats. The disc —which is isolated from the process by its very nature— withstands these conditions better.

Series combination

The most widespread installation is disc upstream + valve downstream, with an inter-stage space monitored by a pressure gauge or pressure switch. This combination provides:

• Absolute tightness of the disc against the process fluid, extending the valve's service life.
• Reuse of the valve after a relief event (the disc is replaced, the valve is overhauled).
• Compliance with ASME VIII Division 1 §UG-127 when the combined capacity factor is justified.

The reverse configuration —valve in front, disc behind— is used to prevent back pressure from the common flare header from acting on the valve's spring.

The decision is not disc versus valve, but where to place each one so that each technology does what it does best.

Selection criteria

• Disc only: relief against deflagrations, hazardous fluids where leakage is unacceptable, minimum maintenance costs.
• Valve only: clean processes, possibility of multiple events without shutdown, need to modulate the relief.
• Disc + valve: most critical applications in chemicals, pharmaceuticals and petrochemicals.

Conclusion

Rupture discs and safety valves do not compete: they cooperate. Understanding their physical behaviour allows them to be combined to obtain the best of both technologies: the immediacy and tightness of the disc, and the reusability and precision of the valve. Tecnovent designs both devices and their combinations according to ASME, API 520/521 and EN ISO 4126.