74
EB 8389-1S EN
Partial stroke test (PST)
During the partial stroke test, the valve
moves from its current operating point to a
defined 'Stop' value and back to the initial
position again.
The change in travel can be performed ei-
ther in steps or in a ramp function (Fig. 18).
For the test in a ramp function, additionally
the ramp times for the increasing and de-
creasing ramps need to be defined.
To perform a partial stroke test, the 'Lower
range value' must be near the current oper
-
ating point ± 'Tolerance limit'.
After being activated, the test does not start
until the 'Settling time before starting test'
has elapsed. Based on the operating point,
the valve moves until reaching the 'Stop'
valve position. The valve remains in this po-
sition for the time defined by the 'Waiting
time after step change' before performing a
second step change in the opposite direction
from 'Stop' to the operating position. The
'Sampling time' defines the time interval be
-
tween which the measured values are re-
corded during the test.
Test cancelation conditions
Various cancelation conditions provide addi-
tional protection against the valve slamming
shut or moving past the end position. The
positioner cancels the partial stroke test
when one of the following cancelation condi-
tions is fulfilled:
Settling time before
starting test
Waiting time after
step change
∆
t
decreasing
∆
t
increasing
Stop
Lower range value
Operating point
Operating point + Tolerance limit
Operating point – Tolerance limit
Waiting time after
step change
Settling time before
starting test
Start PST
Start PST
t [s]
x [%]
Fig. 18:
Course of the partial stroke test with step response (left) and ramp function (right)
∆
t
decreasing
‘Lower range value’ – ‘Stop’
x
‘Ramp time (decreasing)’
100
∆
t
increasing
‘Lower range value’ – ‘Stop’
x
‘Ramp time (increasing)’
100
