There are a number of different techniques ailable for partial stroke testing and the selection of the most appropriate technique depends on the main benefits the operator is trying to gain.
Mechanical Jammers editMechanical 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 he severe consequences, or any application where the end user prefers a mechanical device.
Typical benefits of this type of device are as follows:[3]
The devices assure metal-to-metal prevention of stroke past the specified set point. Unlike some electronic systems, there is no need to commission and calibrate controls or continually train personnel, resulting in additional significant cost sings. The devices are vibration resistant, making them highly reliable. The risk associated with hing an ESD event occur at time of manual mechanical PST may be considered statistically insignificant and allows a rational consideration of the advantages mechanical devices offer. Modular design allows for addition of limit switches, potentiometers, remote control operation, etc. The test is a comprehensive test of the logic solver and all final elements, only the sensing elements of the safety function are not tested. The valve is tested at the designed operating speed as it simulates an ESD event Jammers he a very low probability of causing a spurious trip.However, opinions differ whether these devices are suitable for functional safety systems as the safety function is offline 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. An example of such a mechanical PST system:[4]
Other methods include adjustable actuator end stops.
Pneumatic valve positioners editThe 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. The main benefits are :
Elimination of the cost of manual testing Tracking and records of the PST tests for an optimum Safety monitoring. When the positioner is connected to the Safety System, the date and result of the test are registered in the Sequence of Events, for Insurance purposes. Remote access to valve diagnostics from the control room, with action oriented reports for predictive maintenance.The main benefit of these systems is that positioners are common equipment on plants and thus operators are familiar with the operation of these systems, however the primary drawback is the increased risk of spurious trip caused by the introduction of additional control components that are not normally used on on/off valves. These systems are however limited to use on pneumatically actuated valves.
Electrical relay systems editThese 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 editElectronic control systems use a configurable electronic module 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 solenoid 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.
Integrated solenoid valve systems editAnother technique is to embed the control electronics into a solenoid valve enclosure removing the need for additional control boxes. In addition there is no need to change the control schematic as no dedicated components are required.