Partial stroke testing (or PST) is a technique used in a safety instrumented 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.
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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 (or IEC) and the Instrument Society of America (or ISA). The following are the standards appropriate to these bodies.
The Partial Stroke Test is used to check the function of the safe position of ESD (emergency shutdown) valves. The partial valve stroke prevents unexpected failure of the safety function by breaking down solid masses or the onset of corrosion, for example. Furthermore, a successfully executed partial stroke demonstrates that certain unresolved errors that would otherwise go undetected, such as spring fractures in the spring chamber of the pneumatic actuator, are not present. Consequently, the interval for testing for these undetected errors can be extended.
The test can be started both locally on the device in a time-controlled manner or from remote. The positioner evacuates output 1 until the position change defined in advance occurs. If this does not happen within the set time (timeout value), an alarm can be output.
Additionally, monitoring is performed to establish whether the valve has moved out of its end position within a defined period of time (dead time). If this has not happened, the test is cancelled as a "failed" test and an alarm is output. This behavior prevents a blocked valve from suddenly freeing itself from the end position and thereby disrupting the process.
At the end of the test, the positioner moves the valve to the last valid position and reverts to the most recently active control mode.
For documentation purposes, the test result is saved in the non-volatile memory.
Example for an electro-pneumatic positioner with partial stroke:
Manufacturer: ABB Type:PositionMaster EDP300
IEC61508 adapts a Safety life cycle approach to the management of plant safety. During the design phase of this life cycle of a safety system the required safety performance level is determined using techniques such as Markov analysis, FMEA, Fault tree analysis and Hazop. These techniques allow the user to determine the potential frequency and consequence of hazardous activities and to quantify the level of risk. A common method for this quantification is the Safety integrity level. This is quantified from 1 to 4 with level 4 being the most hazardous.
Once the SIL level is determined this specifies the required performance level of the safety systems during the operational phase of the plant. The metric for measuring the performance of a safety function is called the average Probability of failure on demand (or PFDavg) and this correlates to the SIL level as follows
| SIL | PFDavg |
|---|---|
| 4 | ≥10−5 to <10−4 |
| 3 | ≥10−4 to <10−3 |
| 2 | ≥10−3 to <10−2 |
| 1 | ≥10−2 to <10−1 |
One method of calculating the PFDavg for a basic safety function with no redundancy is using the formula
Where:
The diagnostic coverage is a measure of how effective the partial stroke test is and the higher the DC the great the effect the test.
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.[1][2]. These are summarised as follows
Gains can be made in the following areas by the use of PST.
There are a number of areas where production efficiency can be improved by the successful implementation of a PST system.
If the gains of the SFF are of an appropriate level the need for costly redundant valves may be eliminated
There are a number of different techniques available for partial stroke testing available and the selection of the most appropriate technique depends on the main benefits the operator is trying to gain.
Mechanical jammers are devices where a device is inserted into the valve and actuator assembly that physically prevents the valve from moving past a certain point. These are used in cases where accidentally shutting the valve would have severe consequences, or any application where the end user prefers a mechanical device.
Typical benefits of this type of device are as follows[3]:
However, opinons differ whether these kind of devices are suitable for functional safety systems as the safety funciton is off-line for the duration of the test.
Modern mechanical PST devices may be automated.
Examples of this kind of device include direct interface products that mount between the valve and the actuator and may use cams fitted to the valve stem. Other methods include adjustable actuator end stops.
The basic principle behind partial stroke testing is that the valve is moved to a predetermined position in order to determine the performance of the shut down valve. This led to the adaptation of pneumatic positioners used on flow control valve for use in partial stroke testing. These systems are often suitable for use on shutdown valves up to and including SIL3
These are however limited to use on pneumatically actuated valves
Timer control systems use a configurable electronic timer that connects between the supply from the ESD system and the solenoid valve. In order to perform a test the timer de-energises the solenoid valve to simulate a shutdown and re-energises the soleniod when the required degree of partial stroke is reached. These systems are fundamentally a miniature PLC dedicated to the testing of the valve.
Due to their nature these devices do not actually form part of the safety function and are therefore 100% fail safe. With the addition of a pressure sensor and/or a position sensor for feedback timer systems are also capable of providing intelligent diagnostics in order to diagnose the performance of all components including the valve, actuator and solenoid valves.
In addition timers are capable of operating with any type of fluid power actuator and can also be used with subsea valves where the solenoid valve is located top-side