WHAT IS AN IPF?

Instrumented protective functions (IPFs) are commonplace in the oil and gas industry, known variously as process safeguarding, emergency shutdown (ESD), process shutdown (PSD), etc. Here, we are looking at individual functions rather than the whole system:  an example of an IPF is "high pressure in the first stage separator detected by pressure switch PSH-101 closes inlet ESD valve ESDV-001 via the PSD system".  An IPF comprises an initiating element function, a logic solver function and a final element function.  A typical IPF is shown
below

Note that the initiating element includes any associated trip amplifiers or input modules, and the final element includes output modules, relays, solenoid valves, etc.
An instrumented protective system (IPS) is the logic solver element of the IPF, plus the input and output modules.  In the past, much of the engineering effort to improve reliability was associated with the IPS, but of course successful operation of the function requires all of the elements to operate on demand.