Partial Stroke Test for Emergency Shutdown Valve

During normal plant operation most SDVs/BDVs remain in static full open/close for long period time.
There is concern about operation failure caused by Valve failure.

Current ESD Test weakest points in my Company: 

• Not covered valve testing such as actuator, stem/shaft, body valve, and etc
• Sources of error may occur: broken shaft or stem, broken actuator springs, leaking piston seals, damaged mechanical feedback, and etc.

Partial stroke test is a function test to check periodically that the SDV is operated securely without any non-conformities such as sticking of valve and actuator in the event of emergency, by closing the valve slightly and slowly to such range as not to interfere with the process during plant operation. With partial stroke test, we will actuate the shutdown valve (ball valve) up to a certain travel percentage (generally it is 15% travel or 15 degree closing) of its full travel.

Advantage of Partial Stroke Test:

• Detectable damage of actuator, stick valve and valve stem
• Reduce the Probability of Failure on Demand (PFD), Improved Safety Integruty Level (SIL)
• Extended interval of full stroke test.

Weak point of Partial Stroke Test:

• Not covered all valve test, e.g. fully valve travel

Pictures below is example of ESDV hydraulic Panel with Partial Stroke Key Switch Facility:

Relation of Probability of Failure on Demand and partial Stroke Test

Probability of Failure on Demand (PFD) is statistical probability or chance that a system will not perform its intended function when required to do so.

Failure Rate (λd) :  The average rate at which a component could be expected to fail.
Test Interval (Ti): Time Interval between test to test
Simplified PFD equation for 1oo1 configuration (ISA TR 84.0.02): PFD= 1/2 λd.Ti
Diagnostic Coverage (DC): The diagnostic coverage of a component or subsystem is the ratio

of the detected failure rates to the total failure rates of the component or subsystem.

PFD with Partial Stroke :

PFD  = PFDp + PFDf

        = [DC. ½ λd .Ti]p + [(1-DC). ½ λd .Ti]f

Full Stroke Compare to Partial Stroke

Full Stroke Test:

• 100% Covered valve component testing, so it will give better reliability performance
• Difficult to implemented in some units that live continuously and have no bypass or switching line.

Partial stroke test:

•  Not covered all valve component testing
• Easier to implemented since it is not interrupted the production.