Partial Stroke Testing - What is it?
Partial stroke testing is used to test a safety integrity function within a control system while a shutdown valve is online without impacting the process, in other words – partial stroke testing assists in determining that the safety integrity function (SIF) will operate on demand.
Why Use Partial Stroke Testing
SIFs mandate that high integrity shutdown valves are tested at regular intervals to maintain Safety Integrity Level (SIL). To minimise impact on the process, operators usually choose to perform an offline full stroke test during a scheduled shutdown. If however, a SIF requires more frequent testing then many users perform a partial stroke test (PST).
Standards for Partial Stroke Testing
Although PST is an accepted standard technique, especially in the oil & gas industry, there are no international standards detailing how a partial stroke test should be carried out. However, various standards define the requirements for safety related systems and describe how to quantify the performance of PST systems, e.g.
IEC61508– r 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
Disadvantages of PST
PST does come with disadvantages, the most cited objection to using it is
the increased probability of causing an accidental trip of the safety system resulting in a plant shutdown. Different methods of performing a PST, as described below, mitigate for this, but in the end they all hava a residual inherent risk.
Even a succesful PST introduces a disturbance into the process or system, and for some processes or systems that are sensitive to disturbances it may not be an appropriate choice.
Although PST can be used diagnostically, it cannot always differentiate between different faults or failures within the valve and actuator assembly thus limiting its diagnostic capability.
Partial Stroke Test Methodology
There are three basic types of partial stroke test equipment: mechanical limiting, position control, and solenoid valves. Each type involves different levels of sophistication and risk.
Mechanical limiting methods involve the installation of a mechanical device into the valve and actuator assembly to physically limit the degree of valve travel. A limit switch or visual inspection is used to confirm valve movement.
When mechanical limiting methods are used, the valve is not available for process shutdown, thereby lending itself to use in cases where accidentally shutting the valve would have severe consequences. However this prompts some to question if these devices are suitable for functional safety systems as the safety function is offline for the duration of the test.
The mechanical devices used for partial stroke testing include collars, valve jacks, and jammers.
- Valve collars are slotted pipes that are placed around the valve stem of a rising stem valve. The collar prevents the valve from traveling any farther than the top of the collar.
- A valve jack is a screw that is turned until it reaches a set position. The valve jack limits the actuator movement to the screw set position. Valve jacks are available for both rising stem and rotary valves.
- Mechanical jammers are integrated into the design of a rotary valve. They are essentially slotted rods that limit valve rotation when placed in position using an external key switch.
Position control method uses a positioner to move the valve to a pre-determined point. A limit switch or position transmitter can then be used to confirm valve movement.
Position control is suitable for performing a partial stroke test on both rising stem and rotary valves.
The positioner does contribute to the spurious trip rate during normal operation, since the positioner can fail and vent the air from the valve. If the solenoid used for valve actuation is installed between the positioner and the actuator, the safety functionality is never lost during the partial stroke test. De-energising the solenoid valve will shut the valve, regardless of the positioner action.
Solenoid Valve Methods rely on de-energising the coil of the solenoid used to actuate the valve. When the valve moves, confirmed by using a limit switch or position transmitter, it can be inferred that the solenoid valve successfully vented. The solenoid coil is then re-energised preventing total closure of the valve.
It is rare that for a PST to rely on an operator manually de-energising and re-energising the coil. Instead, various automated systems have been developed, including electrical relay systems, electronic control systems, and integrated solenoid valve systems.
- Electrical systems use an electrical switch to de-energise the solenoid valve and use an electrical relay attached to the actuator to re-energise the solenoid coil when the desired PST point is reached.
- Electronic control systems use a configurable electronic timer that connects between the supply from the ESD system and the solenoid valve, usually very close to the valve actuator. To perform a test the timer de-energises the solenoid valve and then re-energises the solenoid when the required degree of partial stroke is reached.
- Integrated solenoid valve systems have the control electronics embeded into a solenoid valve enclosure removing the need for additional control boxes. The integrated unit combines a high-integrity solenoid valve with an electronic control system to provide simple pass/fail reporting, and increasingly, high-level diagnostics.