EB 8389 EN
75
Tests
5.4 Partial stroke test (PST)
The partial stroke test (PST) is particularly
suitable for the status-oriented detection of
malfunctions in pneumatic shut-off valves. As
a result, the probability of failure on demand
(PFD) can be reduced and it may be possible
to extend maintenance intervals.
A shut-off valve normally in its end position
can be prevented from seizing up or getting
jammed. The initial breakaway torque must
first be overcome after the valve starts to
move from its end position. The initial break-
away torque depends on the plug/seat seal,
deposits on the plug, the process medium
and friction at the valve trim. After the initial
breakaway torque has been overcome, it
can be assumed that the valve is able to
close completely.
The recording of the test results additionally
allows an analysis of the dynamic control re-
sponse.
The partial stroke test can be performed
once (test immediately started) or, with an
on/off valve in automatic mode, regularly
(time-controlled), provided the start condi-
tions are met (see page 73 and
page 74):
−
A control valve is in the manual mode.
−
An on/off valve is in the manual or auto-
matic mode. In automatic mode, the test
is only started when the Set point w is
greater than the 'Limit fail-safe position'
(Code 49 - h2).
The following listed parameters are activated
while the partial stroke test is being per-
formed:
Fig. 12:
Diagnosis > Tests > Partial stroke test
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