Safety valve selection: set pressure, back pressure and execution types
The three pressures that drive selection
Set pressure is where the valve opens; overpressure is the allowed rise above it (commonly 10%) during discharge; back pressure is the pressure at the outlet — superimposed when the discharge system is already pressurised, built-up when the valve’s own discharge creates it. A spring-loaded valve’s behaviour shifts with back pressure, which is why the execution types below exist. Set pressure, working pressure span and back-pressure capability are execution-level facts: each published model page states its own.
The published executions and where each fits
The catalogue publishes five safety-valve executions as identifier pages. Their design intent, summarised from each page’s own selection guidance:
| Execution | Design intent (from the published selection guidance) |
|---|---|
| Full Lift Safety Valves | Select this execution where a full-lift pop action and maximum discharge capacity are required - steam boilers, air receivers and pressurized systems inside th… |
| Low Lift Safety Valves | Select this execution where liquid or modest gas relief duty calls for a proportional, low-capacity discharge rather than a full pop - small boilers, compresso… |
| Steam Safety Valves | Select this execution for boiler and steam-line overpressure protection inside the PN1 |
| Type 441/442 Balanced Bellows Safety Valves | Choose the DIN EN 1092 PN 10-40 or ASME B16 |
| High-Efficiency Pilot Operated Safety Valves | Size the orifice (D to T+) for required capacity, then select the body — carbon steel or stainless steel — against the process fluid and temperature |
Full-lift vs low-lift vs pilot-operated vs balanced bellows
A full-lift (pop-action) valve opens to full discharge capacity within a small overpressure — the standard choice for steam, air and gas where maximum capacity in minimum overpressure is required. A low-lift (proportional) valve opens in proportion to overpressure, suiting liquid relief and modest gas duties where the system can absorb a wider blowdown.
A balanced-bellows execution isolates the spring and disc back from outlet pressure, so set pressure stays stable under superimposed or built-up back pressure that would otherwise derate a conventional spring valve. A pilot-operated safety valve (POSV) uses a process-driven pilot to hold the main valve closed and open it at set point: it holds operating pressure close to set pressure, and its published pages state large set-pressure spans with high back-pressure ratios — the process-industry answer where conventional springs run out of range.
Selection and inquiry practice
Select in this order: (1) the required discharge capacity and set pressure from the protected system’s design basis; (2) the execution that stays within its published set-pressure and back-pressure envelope; (3) materials against the medium; (4) the connection (threaded NPT or flanged RF/RTJ, per the published end options) and the governing standard set — the published safety pages list API 520/521/526/527 design-and-testing references and ASME B16.5 / DIN 2543 / EN 12627 flange dimensions.
Temperature limits are execution- and model-level facts — the cryogenic and high-temperature spans published on individual pages belong to those pages, not to the family. Send medium, set pressure, back pressure, required capacity, size and connection, and the matching is a confirmation against the published tables rather than an assumption.