Partial stroke testing is an accepted petroleum industry standard technique and is also quantified in detail by regulatory bodies such as the International Electrotechnical Commission (IEC) and the Instrument Society of Automation (ISA).
These techniques allow the user to determine the potential frequency and consequence of hazardous activities and to quantify the level of risk.
The metric for measuring the performance of a safety function is called the average probability of failure on demand (or PFDavg) and this correlates to the SIL level as follows One method of calculating the PFDavg for a basic safety function with no redundancy is using the formula Where: The proof test coverage is a measure of how effective the partial stroke test is and the higher the PTC the greater the effect of the test.
Finally, a PST cannot always differentiate between different faults or failures within the valve and actuator assembly thus limiting the diagnostic capability.
These are used in cases where accidentally shutting the valve would have severe consequences, or any application where the end user prefers a mechanical device.
An example of such a mechanical PST system:[4] Other methods include adjustable actuator end stops.
This led to the adaptation of pneumatic positioners used on flow control valve for use in partial stroke testing.
The main benefits are : The main benefit of these systems is that positioners are common equipment on plants and thus operators are familiar with the operation of these systems, however the primary drawback is the increased risk of spurious trip caused by the introduction of additional control components that are not normally used on on/off valves.