EMERGENCY SHUTDOWN VALVES:
OPTIONS FOR PARTIAL
STROKE TESTING
As a plant operator, you want to
maximize revenue by running
your plant 24/7 for as many months
or years as you can—and you want
to do it safely. To achieve this, you
need to periodically cycle your automated emergency shutdown (ESD)
valves. However, if you close the
ESD valve, you shut down the
process flow. If you shut down the
process flow, revenue also ceases to
flow. If only there was a means by
which you could confirm ESD valve
operability without shutting down
your process flow.
Enter the increasingly popular
partial stroke testing (PST) of the
ESD valves to enhance system safety while allowing process and revenue flow to continue.
PARTIAL STROKE TESTING DEVICES ADD
SIGNIFICANTLY TO PLANT SAFETY WHILE
HELPING TO KEEP PROCESS AND
REVENUE FLOWING. HERE, THE AUTHOR
DESCRIBES A VARIETY OF OPTIONS THAT
PLANT OPERATORS CAN USE TO TEST
EMERGENCY SHUTDOWN VALVES.
BY ED HOLTGRAVER
operability without stopping the process flow. While partial stroke testing does not
confirm the sealing ability of the valve seat, it does verify that the valve and the actuator remain capable of motion and have not been subject to corrosion or other factors
that would prevent movement. Assurance of proper operation and, therefore, safety
integrity levels (SILs), improve with more frequent partial stroke testing.
Why Partial Stroke
Testing?
In keeping with modern economic
demands, industrial processes and
related equipment have been
improved such that they are capable
of running for extended periods with
minimal maintenance. Continually
active components are often monitored to predict pending failure,
thereby enabling maintenance with
minimal impact on revenue.
Seldom-used components such as
ESD valves, however, may suffer
corrosion and blockages that are
not evident until an emergency
requires they perform their safety
function. Failure to operate as
intended is unacceptable, thereby
mandating functional testing.
Full closure testing of ESD
valves is preferable but unrealistic,
except during increasingly rare
plant shutdowns. But the industry
has recognized a need to at least
partially stroke ESD valves to verify
PST Defined
Generally, the actuator for an ESD valve contains a spring to cause valve closure in the
event of an emergency. Pressure acts upon a piston to compress the spring and to open
the valve. The pressure remains applied to the piston until such time that the valve is
called upon to close. If all is in working order, release of the pressure allows the spring
force to overcome the valve resistance and to cause the valve to fully close (Figure 1).
In the simplest terms, a Partial Stroke Test Device (PSTD) serves to limit actuator
(and valve) travel to a predefined travel percentage as chosen by the end-user (nor-
Figure 1 – Spring is extended, valve is closed. Applying supply pressure to port forces piston to
compress spring while causing counter-clockwise motion to open valve. Valve will remain open
until supply pressure is removed.
