What the calculator is for
The process safety time (PST) is the time from the start of a failure or deviation to the hazardous event if nothing acts. A safety instrumented function has to detect the condition and reach the safe state well within it. Its response time is the sum of the sensor, logic solver and final element times; what is left of the PST is the margin for everything you have not modelled.
- Response = sensor + logic solver + final element
- Remaining = PST − response − engineering margin
Worked example
A pressure trip has a process safety time of 8 s at its trip point. The transmitter responds in 1.2 s, the logic solver in 0.2 s and the shutdown valve closes in 2.8 s: a response time of 4.2 s, 52 % of the PST. With a 1 s engineering margin, 2.8 s remain.
The budget fits, but the response uses more than half the PST, a limit many companies apply. A faster-stroking valve or an earlier trip point would restore the margin.
Where the times come from
The process safety time comes from the process: the hazard analysis, dynamic simulation or a simple mass and energy balance from the trip point to the hazardous condition. The element response times come from device data and site tests: transmitter damping and update rate, logic solver scan time, and the valve's measured stroke time from proof tests. Record each figure and its source in the safety requirements specification, so the budget can be rechecked after any change.
Typical response times
| Element | Typical response |
|---|---|
| Pressure or flow transmitter | 0.1 to 1 s |
| Temperature measurement (thermowell) | 5 to 60 s |
| Safety PLC scan and output | 0.05 to 0.5 s |
| Solenoid valve | 0.1 to 1 s |
| On/off valve, small | 1 to 5 s |
| On/off valve, large | 10 to 60 s |
Common mistakes
- Measuring PST from the wrong point. PST runs from the deviation at the trip point, not from normal operation.
- Forgetting slow sensors. Thermowells and analysers can take tens of seconds and often dominate the budget.
- Using the valve's stroke time alone. Add the time to vent the actuator and for the process to respond to the closed valve.
- No margin. Response times drift with fouling, ageing and filter settings; keep an allowance.
Basis
IEC 61511-1 requires the SIF response time to be specified in the safety requirements specification so that the safe state is reached within the process safety time. The half-PST limit is a widely used company practice, not a requirement of the standard.