Rupture Disc vs PSV: Selection Guide & Combination Installations

Introduction to Overpressure Protection

Process engineers primarily rely on two devices for overpressure protection: the Pressure Safety Valve (PSV) and the Rupture Disc (RD). While both prevent catastrophic vessel failure, their mechanics, operational behaviors, and ideal applications differ significantly.

The Pressure Safety Valve (PSV)

A PSV is a mechanical, spring-loaded (or pilot-operated) valve designed to open at a set pressure and reclose once normal pressure is restored.

• Pros: Recloses after relief (saving product and allowing continuous operation); set pressure can be tested in situ; established sizing standards (API 520).
• Cons: Susceptible to plugging or corrosion from process fluids; can chatter if undersized; potential for minor seat leakage during normal operation.
• Best For: Most general process applications (steam, air, clean hydrocarbons, water).

The Rupture Disc (RD)

A Rupture Disc is a non-reclosing, sacrificial membrane (usually metal or graphite) engineered to burst at a specific pressure and temperature.

• Pros: Zero leakage (hermetic seal); instantaneous opening response (< 2 ms); immune to plugging; can be made from exotic alloys (Hastelloy, Tantalum) cost-effectively.
• Cons: Does not reclose (entire vessel contents may be lost); requires shutdown to replace; burst pressure is highly temperature-dependent.
• Best For: Highly corrosive, toxic, highly viscous, or polymerizing fluids; rapid deflagration protection; extreme high/low pressures.

Operating Ratio and Burst Tolerance

When specifying a Rupture Disc, two critical parameters must be understood:

1. Burst Tolerance: The acceptable variation around the specified burst pressure. Standard manufacturing tolerance is typically $\pm 5\%$.
2. Operating Ratio: The maximum allowable ratio of normal operating pressure to the disc's marked burst pressure to ensure long life and prevent premature fatigue failure. Conventional discs require an operating ratio of $\le 70\%$, while modern reverse-buckling discs can operate up to $90 - 95\%$ of their burst pressure.

Combination Installations: RD in Series with a PSV

A very common API 520 configuration is installing a Rupture Disc directly upstream of a PSV. This combines the advantages of both devices:

• The RD provides a perfect, zero-leakage seal during normal operation, isolating the PSV internals from corrosive or polymerizing process fluids.
• If an overpressure event occurs, the RD bursts, the PSV opens to relieve the pressure, and then the PSV recloses to save the remaining inventory.

API 520 Rules for Combination Installations

When an RD is installed under a PSV, strict design rules apply:

1. Non-Fragmenting Disc: The rupture disc must be of a non-fragmenting design (e.g., scored reverse-buckling). If it fragments, pieces will jam the PSV open.
2. Tell-Tale Indicator: The space between the RD and the PSV must be monitored (via a pressure gauge or switch) and vented. If the RD develops a pinhole leak, pressure will build in this cavity. Because the RD bursts based on differential pressure, pressure in the cavity will prevent the disc from bursting at the intended vessel pressure.
3. Capacity Factor ($K_c$): The presence of the RD creates flow resistance. When sizing the PSV, a combination capacity factor $K_c = 0.9$ must be applied (unless the specific RD/PSV combination has been tested and certified by the ASME National Board for a higher factor).

Try the Calculator

When sizing a PSV in the PSV Sizing Calculator, remember that if you plan to install a rupture disc upstream, you must manually apply the $0.9$ combination capacity factor ($K_c$) to ensure the valve area is adequately oversized to compensate for the flow resistance!