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SIF design

Final elements: valves, actuators, solenoids and motor trips

The part of a SIF that usually dominates PFDavg, the design choices that matter and the failure modes to cover in tests.

2 min read · First Out AI knowledge library

In most process SIFs the final element contributes more than half of PFDavg. Its design deserves as much attention as the calculation.

Valve assembly

A shutdown valve "subsystem" includes the valve, the actuator, the solenoid valve (or several), quick exhaust valves, tubing and the air or hydraulic supply. Failure rates must cover the whole assembly.

Choices that matter

  • Fail-safe action: spring-return actuators driven to the safe position on loss of air or power. Energise-to-trip designs (motor-operated valves) need monitored supply and circuits, and usually reach lower integrity.
  • Tight shut-off: if the hazard needs the valve to seal (not only to close), seat leakage is a dangerous failure and must be tested.
  • Stroke time: specify it from the response time budget; check it at every proof test.
  • Solenoid arrangement: a single solenoid is a single point of failure; 1oo2 solenoids improve safety, 2oo2 solenoids improve availability.
  • Partial stroke testing: reduces PFDavg and can justify longer full test intervals.
  • Redundancy: two valves in series (1oo2) need separate solenoids and preferably separate air supplies to keep β low.

Motor trips

A pump or compressor trip acts through contactors or breakers. De-energise-to-trip is preferred. Two contactors in series (1oo2) protect against a welded contact. Check that the trip path is independent of the motor control logic.

Failure modes to cover in the proof test

Valve does not move; moves too slowly; does not reach the end of travel; leaks past the seat beyond the limit; solenoid does not vent; spring broken or weak; actuator leaks; position indication wrong.

In First Out AI

Final element groups carry "Action on trip" and "Required stroke time" fields used by the SRS and the proof test draft. The valve steps in the proof test draft follow these failure modes.

Written for First Out AI as engineering guidance. It supports engineering work and does not replace it; check every value against your standards, certificates and company rules with a competent functional safety engineer.