Partial stroke testing (or PST) is a technique used in a control system to allow the user to test a percentage of the possible failure modes of a shut down valve without the need to physically close the valve. PST is used to assist in determining that the safety function will operate on demand.
PST is most often used on high integrity emergency shutdown valves (ESDVs) in applications where closing the valve will have a high cost burden yet proving the integrity of the valve is essential to maintaining a safe facility.
In addition to ESDVs PST is also used on high integrity pressure protection systems or HIPPS. Partial stroke testing is not a replacement for the need to fully stroke valves as proof testing is still a mandatory requirement.
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). The following are the standards appropriate to these bodies.
IEC61508 – Functional safety of electrical/electronic/programmable electronic safety-related systems
IEC61511 – Functional safety – Safety instrumented systems for the process industry sector
ANSI/ISA-84.00.01 – Functional safety: Safety instrumented systems for the process industry sector (an ANSI standard)
These standards define the requirements for safety related systems and describe how to quantify the performance of PST systems
Advantages of Partial stroke testing
The benefits of using PST are not limited to simply the safety performance but gains can also be made in the production performance of a plant and the capital cost of a plant.
These are summarised as follows
Gains can be made in the following areas by the use of PST.
Reducing the probability of failure on demand.
There are a number of areas where production efficiency can be improved by the successful implementation of a PST system.
- Extension of the time between compulsory plant shutdowns.
- Predicting potential valve failures facilitating the pre-ordering of spare parts.
- Prioritisation of maintenance tasks.
Disadvantages of Partial stroke testing
The main drawback of all PST systems is the increased probability of causing an accidental activation of the safety system thus causing a plant shutdown, this is the primary concern of PST systems by operators and for this reason many PST system remain dormant after installation. Different techniques mitigate for this issue in different manners but all systems have an inherent risk
In addition in some cases, a PST cannot be performed due to the limitations inherent in the process or the valve being used. Further, as the PST introduces a disturbance into the process or system, it may not be appropriate for some processes or systems that are sensitive to disturbances.
Finally, a PST cannot always differentiate between different faults or failures within the valve and actuator assembly thus limiting the diagnostic capability.