Working Principle of Partial Stroke

Introduction

Partial-stroke tests (PSTs) of emergency shutdown (ESD) valves improve safety instrumented system (SIS) performance; monitor these critical valves to ensure the system’s ability to shut a process down in the event of an emergency.

A partial-stroke test (PST) is a procedure/test used to stroke emergency shutdown (ESD) valves partially. It also is referred to as a partial-valve stroke test (PVST). The alternative is a full stroke test (FST), where the valve is completely (100%) closed/opened 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 will create, and thus, the sizing of the valve and manufacturer recommendations.

A PST is 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. PST increases the SIL, but because the implementation is expensive, it should be a last resort to achieve the SIL level targets. This means all other means have been tried and are not feasible, or the cost to achieve the desired SIL target is prohibitively high. The PST requirement arises in plants where turnaround time (TAR) is high, and it is not possible to do a full stroke test for an extended time.

How can work the Partial Stroke

The standard of Partial stroke on board an actuator is equipped with 3 solenoids on the panels (A, B, C), which C is normally open, means the air or hydraulic oil passes without the solenoid being energized, while A, B are ESV (Emergency Shutdown Valve) and they used for close or open valve, in this case are normally closed, means the air or hydraulic oil pass through only if energize them. Below the photo with the drawing:

The standard procedure the Partial stroke has to work only with the solenoid C because in the event of coil fail, or the fuse breaking, the solenoid returns to the normally open condition and then the actuator / valve reopens, without close the valve with abnormally situation, consider that are critical valves. Closing a production by half sequence is very dangerous.

The standard actuator is equipped with three limit switch and are: ZSH (limit open) ZSL (limit close) and ZSX (limit partial stroke). Sometime the ZSX limit switch which is the limit partial stroke it is replaced with the position transmitter FZI or EZI. below the figure with all details

Working Procedure step by step

1. Considering that the valve is open from PLC, the operator sends the partial stroke sequence which include at first step the check of I/O card, then pressure transmitter of production line, and the check of supply which is air or hydraulic oil, all depend from project
2. The system before active the solenoid, tests the limit switches if they are in normal positionshould be with contact open, and ZSH, means ZSL (Limit close) (Limit open) should be contact close, and last the ZSX (limit partial stroke) should be with contact open like ZSL. Sometime the configuration is changed and limit of partial stroke (ZSX) which is with digital contact is replace with analog feedback (0 - 100% 4-20mA)
3. The PLC supply the solenoid C which discharges the air or hydraulic oil supply (C is opposite to A and B) towards the actuator and then the valve going to closes direction, at the same time the PLC starts a timer which is the waiting time of limit switch ZSX (Partial Stroke limit). The timer is configured according to actuator size. With an big actuator will be a big timer inside PLC (more big is actuator, and more time will be apply). If the ZSX will change status before the time intervenes it is fine, if instead the timer will intervene as first, the system will generate an fault. Anyway, in both cases or the limit switch intervenes first or the timer intervenes first the PLC switches off solenoid C again for return back the actuator to open position, and PLC giving fail message and stop sequence if the timer intervenes first or goes to the next step if the limit switch intervenes first.
   
4. Sometimes it happens that the type of configurations change according to the projects, for example some project configure the partial stroke with all three solenoids, means the valve it will move 3 times, for solenoid A,B,C. I don't agree with this type of procedure at all. The reason is only one and is described below": The solenoid C is classified fail open, means if in case coil fail or fuse damages, the actuator/valve can return in open position without any problem. Regarding the solenoids A, B are fail close, means in Partial stroke test the PLC remove one by one power from Solenoids A and B in case one of them it will damage the coil or fuses the actuator will go in close position fully, and closing the production with a big risk since an anomalous block. The Partial Stroke standard is provided only for the solenoid C or which one will be a fail open.

Conclusion

Below an video with all Partial Stroke details  (Working Principle)

In my opinion where existing two solenoids only and both are fail close It is not advisable to perform the partial stroke , and where are installed three solenoids and one of them is fail open, should be using as Partial Stroke only the fail open.

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