PUMPS, VALVES & ACTUATORS
ENHANCING RELIABILITY & SAFETY
Sunil Doddi, senior principal process controls engineer at Air Products, explains how to achieve process safety through partial-stroke testing of emergency shutdown valves
artial-stroke tests (PSTs) of emergency shutdown valves (ESD) improve safety instrumented system (SIS) performance and monitor the critical valves to help ensure the system’s ability to shut a process down in the event of an emergency.
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PST can be applicable for valve actuators or solenoids. This article focuses on valve actuators; however, there is also a brief discussion about the solenoid test. A partial-stroke test (PST) is a procedure/test used to stroke emergency shutdown valves (ESD) partially. It may also be referred to as a partial-valve stroke test (PVST). The alternative is a full stroke test (FST), where the valve is completely closed/opened (100%) 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 may create, the sizing of the valve, and manufacturer recommendations.
A PST may be 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. However, PST implementation may be expensive, hence it should be a last resort to achieve the SIL target level. This means all other means (for example, inherent process design improvements, implementing other protection layers, etc.) have been tried already and are not feasible/practical, or the cost to achieve the desired SIL target is prohibitively high. Also, PST requirements may arise in plants where turnaround time (TAR) is lengthy, and it is not possible to do a full stroke test in between TAR. The primary objective of PST is to verify the valve actuator will move when the process safety demands it, and the actuator is not stuck due to non-movement for long periods. It should also be noted, PST does not detect all valve failures, such as seat failures. Hence, at the end of a TAR, a FST is required. There are many ways to implement PST and they involve hardware and software in varying degrees. The PST implementation methods can be categorised as follows: 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 PST may also be performed remotely from a control room through application
34 NOVEMBER 2024 | PROCESS & CONTROL
software via a Distributed Control System (DCS). Alternatively, an Asset Management System (AMS)/Product Device Manager (PDM) may implement PST via HART or Fieldbus commands if the field device is an intelligent device. Some manufactures offer smart field devices that work with HART or Fieldbus commands.
The selected implementation method depends on a range of factors such as: cost, field environmental conditions, remote operated plants, etc.
Regardless of the implementation methods, the PST involves initiating partial valve movement, reading its movement, and bringing the valve back to the original position (see flowchart - Figure 1). As the flowchart shows, the procedure is a controlled valve movement to a PST setpoint within a certain time interval. If the valve reaches the PST setpoint within the defined interval, the test passes. If the valve does not reach the setpoint within the defined interval, the test fails. Procedures should be in place to address PST fails. In a test fail case, the root cause of the failure needs to be evaluated, mitigations need to be implemented, and final corrective actions need to be pursued. While designing the plant, a design case needs to be considered for PST fail operation, for example a by-pass around the valve may be designed to allow for online maintenance.
Defining the PST setpoint The PST setpoint is defined as the final position during the test (e.g., 10 per cent open). During sizing of these valves, a sizing scenario should be performed to consider how much process upset can be tolerated during the testing. This will help to define the PST setpoint. Manufacturer’s recommendations are applicable here as well.
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 during commissioning and before the plant is put back into full service. Adequate safety margin should also be considered since valves do not typically get faster over time. Certain process conditions (typically defined by the process safety representative) may be used as interlocks for the PST test, and thus inhibit valve movement at certain times. This configuration may be easy to implement if the
test is deployed with the help of the DCS application program. These interlocks can be any of the conditions where process upset would be higher and thus plant process 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 unique test timer settings for each valve. It is advised to perform each valve PST individually as part of the auto configuration, so the timer settings must be validated and appropriate for the process operation.
A solenoid test (if applicable) for PST on the valves, works on a similar principle 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 final deployment. In summary, partial-stroke testing (PST) enhances the reliability and safety of emergency shutdown valves (ESD) within safety instrumented systems (SIS). Various implementation methods for PSTs, including manual, remote, and automated approaches, cater to different operational needs and environments. Ultimately, integrating PST into regular maintenance protocols is essential for sustaining process safety and minimising the risk of valve failures
Air Products
www.airproducts.co.uk
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