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1. Staged set pressures

Avoid identical set pressures. Set one valve lower (primary), and the others slightly higher (secondary). For example, at 1.0 MPa MAWP, set primary at 0.9 MPa and secondary at 0.95 MPa. This prevents all valves from popping simultaneously, reducing seat wear. API 520 recommends a 5–10% pressure difference.

2. Combined capacity

Total capacity is not simply additive. If one valve alone meets the required relief load, the others serve as standby. If multiple valves must open together, confirm that system accumulation pressure (≤10% of set pressure, 21% for fire case per ASME) is sufficient to lift all of them.

3. Separate inlet pipes

Each valve must have its own inlet pipe from the vessel or main header. A common branch pipe causes backpressure interaction, making secondary valves open early or late. Inlet pressure loss per valve must be ≤3% of set pressure.

Bottom line: For parallel safety valves – stage set pressures, verify combined capacity, and keep inlet pipes independent. Follow these three points for reliable multi-valve system performance.

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