Definition:
Response Time (or SIF Response Time) refers to the total elapsed time from the moment the process is at a trip condition to completion of the protective action.
Note that some will state this is when the instrument senses the trip condition, but that is not the definition we will use here.
Key Points:
- Must be less than the Process Safety Time (PST).
- Affects sizing of instrumentation and final elements.
- Unfortunately, people do use this term differently.
- Rule of thumb is this would be <1/2 of the PST.
Example:
A tank reaches 150F which initiates a SIF. There is a temperature element in a thermowell which then repositions a large valve with a 2 sec valve closure time. The time delay of the temperature instrument is 0.5 sec due to the mass in the thermowell. The logic solver (PLC) operates at essentially 0 sec delay. The total SIF Response time would be 2.5 sec.
See also: Process Safety Time (PST) for a very similar term
Cited Source:
- IEC 61511-1:2016, Clause 11.2.7.