![]() It is advised to do each valve PST individually in the auto configuration, so the timer settings need to be appropriate.įigure 3: Typical PST pass report. In an auto configuration, it may be better to configure different test timer setting for each valve so not all valves are started at the same time and thus may create unexpected process upsets. In the auto-initiated option, the software/program (from the DCS or AMS/PDM) can be configured for the time interval for each PST test. In a manual-initiated option, the user chooses when to start the test. These can be any of the conditions where process upset would be higher and thus plant shutdown or safety could be compromised. This is easy if the test is realized with the help of the DCS application program. The PST setpoint and time interval must be tested and fine-tuned before the plant is put back into full service.Ĭertain process conditions may be used as interlocks for the PST test and thus inhibit valve movement. The PST time interval setting depends on the valve reaction time, which can be obtained from manufacturer’s documentation. Manufacturer’s recommendations can help here. During sizing of the valves (which will go through PST testing in the future), a sizing scenario should be considered to ensure how much process upset can be tolerated during the testing for the PST setpoint. The PST setpoint is defined as the final open position during the test. There also should be procedures in place to address situations when a PST fails without compromising plant safety. If the valve does not reach the setpoint within the defined PST interval, the test fails. If the valve reaches the setpoint within the defined interval, the test passes. Regardless of the implementation methods, the concept involves initiating valve movement, reading its movement and bringing it back to the original position (see Figure 1).Īs the flowchart in Figure 1 indicates, the procedure is a controlled valve movement to a PST setpoint within a certain time interval. The selected implementation concept depends on considering various factors such as cost, field environmental conditions, remote operated plants, etc. Some manufactures offer smart field devices that work with HART commands. The test also can be performed remotely from a control room through application software through a distributed control system (DCS) or asset management system/product device manager via HART commands if the field device is intelligent enough. Some manufactures offer field-based small panels with pushbuttons and lamps to test the function directly in the field. ![]() The PST implementation methods can be categorized as field initiated, remote initiated, auto-initiated and manual initiated. ![]() There are various ways to implement PST and they involve hardware and software in varying degrees. ![]() The PST requirement arises in plants where turnaround time (TAR) is high, and it is not possible to do a full stroke test for an extended time. This means all other means have been tried and are not feasible, or the cost to achieve the desired SIL target is prohibitively high. PST increases the SIL, but because the implementation is expensive, it should be a last resort to achieve the SIL level targets. Benefits of performing a PSTĪ PST is necessary to achieve higher safety integrity level (SIL) (typically SI元) where probability of failure on demand (PFD) calculations of the safety instrumented function (SIF) loop do not achieve the desired targets by any other means. The setpoint for the PST depends on the process upset it will create, and thus, the sizing of the valve and manufacturer recommendations. The alternative is a full stroke test (FST), where the valve is completely (100%) closed/opened during the test the typical range of a PST is 10% to 20% of valve movement. It also is referred to as a partial-valve stroke test (PVST). A partial-stroke test (PST) is a procedure/test used to stroke emergency shutdown (ESD) valves partially.
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