Understand partial-stroke testing

A partial-stroke test (PST) is a procedure/test used to stroke emergency shutdown (ESD) valves partially. It also is referred to as a partial-valve stroke test (PVST). 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. The setpoint for the PST depends on the process upset it will create, and thus, the sizing of the valve and manufacturer recommendations.

Benefits of performing a PST

A PST is necessary to achieve higher safety integrity level (SIL) (typically SIL3) 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. PST increases the SIL, but because the implementation is expensive, it should be a last resort to achieve the SIL level targets. This means all other means have been tried and are not feasible, or the cost to achieve the desired SIL target is prohibitively high. 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.

The primary objective of PST is to conclude the valve actuator will move when the situation demands it, and it is not stuck due to nonmovement for longer periods because of high TAR time.

It should also be noted PST does not detect all valve failures, such as seat failure. Hence, at the end of a TAR, an FST is needed. PST and FST are both necessary for the valve at the end.

PST implementation

There are various ways to implement PST and they involve hardware and software in varying degrees. The PST implementation methods can be categorized as field initiated, remote initiated, auto-initiated and manual initiated. Some manufactures offer field-based small panels with pushbuttons and lamps to test the function directly in the field.

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 smart field devices that work with HART commands.

The selected implementation concept depends on considering various factors such as cost, field environmental conditions, remote operated plants, etc.

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).

As the flowchart in Figure 1 indicates, the procedure is a controlled valve movement to a PST setpoint within a certain time interval. If the valve reaches the setpoint within the defined interval, the test passes. If the valve does not reach the setpoint within the defined PST interval, the test fails. There also should be procedures in place to address situations when a PST fails without compromising plant safety.

The PST setpoint is defined as the final open position during the test. 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. Manufacturer’s recommendations can help here.

The PST time interval setting depends on the valve reaction time, which can be obtained from manufacturer’s documentation. The PST setpoint and time interval must be tested and fine-tuned before the plant is put back into full service.

Certain process conditions may be used as interlocks for the PST test and thus inhibit valve movement. This is easy if the test is realized with the help of the DCS application program. These can be any of the conditions where process upset would be higher and thus plant shutdown or safety could be compromised.

In a manual-initiated option, the user chooses when to start the test. 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 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. It is advised to do each valve PST individually in the auto configuration, so the timer settings need to be appropriate.

Figure 3: Typical PST pass report. Courtesy: Hydro-chem[/caption]

A solenoid test (if applicable) on the valves, which needs PVST, works on similar principles as outlined in Figure 1, except instead of initiating valve movement, the power supply to the solenoid is momentarily turned off . The timer and solenoid off-on pulse settings need very fine-tuning before implementing.

Sunil Doddi is a senior control systems engineer at Hydro-chem, a division of Linde Engineering North America. Edited by Jack Smith, content manager, Control Engineering, CFE Media, jsmith@cfemedia.com.

MORE ANSWERS

KEYWORDS: Partial-stroke testing, PST

Partial-stroke tests (PSTs) of emergency shutdown (ESD) valves improve safety instrumented system (SIS) performance.

The primary objective of PST is to conclude the valve actuator will move when the situation demands it.

There are various ways to implement PST and they involve hardware and software in varying degrees.

CONSIDER THIS

If called upon in an emergency, will the emergency shutdown valves in your plant operate properly?

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