Where EN 378 requires relief valves on refrigeration vessels, how a set pressure is chosen against the design pressure, why dual relief valves on a three-way changeover (Henry SDK type) are fitted on plants that cannot be pumped down, and the catalogue's stocked set pressures from 16.2 to 27.6 bar plus CO2-class devices.
EN 378 (and the PED behind it) requires that any refrigerant-containing vessel that can be isolated full of liquid — receivers, suction accumulators above threshold sizes, oil separators, heat-exchanger shells — is protected against overpressure, normally by a spring-loaded relief valve discharging to a safe location. The classic trigger scenario is a liquid-full receiver isolated by service valves and warmed: liquid expansion raises pressure far faster than any compressor could. Relief valves are the last line after the high-pressure cut-out, and their presence, set pressure and discharge routing are items a class surveyor or AKV inspector checks against the plant documentation.
The set pressure equals the protected vessel's design pressure (PS); the valve must be sized so that flow capacity at 110 % of set pressure exceeds the worst-case vapour generation (fire case per EN 378-2 sizing). The catalogue's Henry Technologies stocked points cover the common HFC vessel classes:
| Set pressure | Valve |
|---|---|
| 16.2 bar | 5230A, 5231A, 5232A |
| 20.7 bar | 5231A 3/8" MPT × 1/2" flare |
| 24.8 bar | 5231B 1/2" MPT × 5/8" flare, 5240 1/2" × 3/4" FPT |
| 27.6 bar | 5231B |
All are CE-marked against the PED category for their size. A relief valve that has lifted must be replaced — the seat is no longer certifiable tight — which is exactly why changeover assemblies exist.
On plants whose receivers cannot be taken out of service (supermarket racks, marine plants at sea), EN 378 anticipates a dual relief valve arrangement on a three-way changeover valve: two full-capacity valves share one vessel connection, the changeover isolates one at a time, and a lifted or leaking valve is replaced with the vessel live. The catalogue stocks these as complete assemblies — SDK2 at 16.2 bar, 20.7 bar, 24.1 bar, 24.8 bar and 27.6 bar, plus the single-valve SDK1 27.6 bar — with 1/2" NPT vessel connections and gauge ports for lift indication.
Relief valves protect vessels; pressure switches protect the running plant first. High-pressure cut-outs with manual reset (e.g. Penn P77BEB 3–30 bar) and dual-pressure controls act long before a relief lifts. Burst discs in series with relief valves (with a gauge between) appear on CO2 and ammonia plants; the gauge indicating pressure between disc and valve is the tell-tale of a pinholed disc. Keep the whole chain documented in the plant log: cut-out settings, relief set pressures, last change dates.