Partial Valve Stroke Test (PVST)
The Partial Valve Stroke Test is used in the Safety Instrumented Function (SIF) to diagnose valve failures without having to stop the process. The problem of the shut-off valves in low demand SIFs is that they remain in a position (open or closed) for many months, without knowing if they will work properly when the action of the SIF is demanded. What we do with the partial stroke test is to move the valve a small percentage of its stroke (between 5 to 20%), without hardly disturbing the process.
The PVST does not replace the “Full Stroke Test” (FST) that we should perform periodically.
What percentage of failures can we detect with the Partial Valve Stroke Test?
Depending on how we do the test, the detected failures can vary between 15 and 80%. The Technical Report of ISA TR96.05.01 (Partial Stroke Testing of Automated Valves) shows some examples of the effectiveness of this test (the term “Diagnostic Coverage” is the percentage of dangerous failures detected by the diagnostics):
|Level||Type of PVST||Diagnostic Coverage|
(only with PVST)
|1||PVST without instrumentation (only with visual inspection)||15%|
|2||PVST with measurement of stroke time||40%|
|3||PVST with actuator-based instrumentation (the relationship between pressure and position provided by the actuator instrumentation is monitored in order to detect valve degradation).||60%|
|4||PVST with external condition monitoring system (includes sensors and data acquisition devices installed in the valve and actuator, such as torque, pressure, position sensors, as well as acoustic and flow sensors to detect leaks with closed valve). FST (Full Stroke Test) can also be performed automatically without stopping the process if the type of assembly and process allows it.||80%|
The drawback of the Partial Valve Stroke Test is the risk of a spurious failure, that is, a failure that causes the unwanted shutdown of the process. This may be due to different causes, for example:
-Excess stroke of the valve (can be avoided with mechanical stops).
-Excess of torque on the stem (greater than the maximum torque allowed on the stem – M.A.S.T.)
-Failure of any of the components used for the PVST.
Common error when using the PVST
In the following table we show values extracted from SIL Certificates of various manufacturers. In the column “Diagnostic Coverage” we see the percentage of detected dangerous failures if we implement the partial valve stroke test (PVST). It is important to realize that the parameter Cpt (“Coverage of Proof Tests”), which measures the percentage of dangerous failures detected during the Proof Tests, decreases when we perform the PVST automatically, because a part of the DU failures are revealed by the PVST.
Let’s look at this with a simple example. Assume that the rate of dangerous undetected failures is 442 FITs (failures per hour x 10-9 ). If the Proof Tests were perfect (Cpt = 100%) we would detect all these failures. The valve manufacturer recommends that we perform a series of tests every year (FST test, visual inspection and, if necessary, leak test) to detect 55% of the failures, that is, there are 199 failures that we do not detect ( 442 x 45% = 199).
If we implement the PVST automatically then 162 of the DU failures become DD (“Dangerous Detected”), and therefore, after PVST we will have 280 DU failures (“Dangerous Undetected”). If we perform the same tests recommended by the manufacturer every year, then we will detect an additional 29%, that is, 81 failures (280 x 29% = 81). Therefore, we will have 199 dangerous failures (280-81 = 199) undetected neither by the PVST nor by the Proof Tests (FST, visual inspection).
It is a very common error not to reduce, or even raise, the value of Cpt when we calculate the Probability of Failure (PFDavg) with partial stroke test, that is, use 55% instead of 29% in the previous example. It should be noted that if the proof tests are the same, the total failures detected (either with the PVST or with the FST) are more or less the same.
Therefore, the benefit that PVST gives us is small in terms of Probability of Failure, but it gives us an early detection of some failures of the final element, so that they can be repaired to have the SIF available if required. On the other hand, it has a positive impact on the value of SFF (“safe failure fraction”).
In the following image we see the calculations made with SILcet. We have assumed Beta = 5% and TI = 1 year.
–Case 1: PVST is not performed. We use Cpt = 73% calculated with the values provided by the manufacturers of each element (valve, actuator and solenoid).
–Case 2: we perform the PVST every 30 days and consider Cpt = 42% calculated with the Cpt values indicated by the manufacturers of each element. As the fault detection time we assume the worst case (30 x 24 = 720 hours).
–Case 3: there is PVST but we use the Cpt values of case 1. The PFDavg value is much higher and valid for SIL-3.
–Case 4: there is PVST and we assume a Cpt value of 90%. In general, this is not correct if the Proof Tests are the same as in cases 1 and 2.
The Partial Valve Stroke Tests (PVST) is a complement to the Full Stroke Test (FST) recommended by the manufacturers of valves and actuators. Its main benefit is the early detection of problems, and not so much its impact on the Probability of Failure (PFDavg), although each case must be analyzed, especially in the case of severe services. In these cases, the fact of performing any test in real operating conditions is undoubtedly positive and we can detect undetectable faults in another way.
If we implement a more sophisticated partial stroke test, with specific devices for it, and that also improve the full stroke test, then we will reduce the final number of undetected failures (“hidden failures”), and therefore the PFDavg value will be clearly smaller.
PVST can be used as a diagnostic tool (see previous examples), or as part of the “proof tests” to increase the Test Interval TI (example: the PVST is performed every 30 days, and the FST every 2 years).
Video: PVST demonstration