Flowserve MX Electronic Actuator SIL Safety IOM - Manual (Page 18)

Limitorque MX Electronic Actuator FCD LMENIM2350-01 – 9/13
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7
Partial Stroke Testing
(PST)
To ensure that the actuator remains ready to perform its safety function and to detect those faults that the actuator
cannot discover by its automated Built-In Self-Test [BIST], it is strongly recommend that the user should perform
periodic Partial Stroke Testing [PST]. By moving the actuator through a small portion of its stroke, the user can gain
assurance that the unit remains healthy enough to perform its safety function.
The Limitorque MXa actuator supports two operating configurations for PST:
7.1 Basic PST Description
Available as a configuration option in all MXa actuators, basic PST tests and exercises all of the control circuitry needed
to move the actuator and its driven equipment.
Reference Figure 4.1 Safety Block Diagram - Basic ESD and PST on page 11.
7.2 Enhanced PST Description
Available as a purchased add-on, enhanced PST performs all of the tests that are performed by basic PST, but also
extends coverage to the user’s wiring and Safety Instrument System control equipment. Note especially that in this
configuration, the PST signal uses the same user control and actuator inputs as the ESD signals. Therefore, by
performing a PST in this configuration, the user is also explicitly testing his own ESD signaling for this actuator.
Reference Figure 4.2 Safety Block Diagram - Enhanced User Wiring on page 13.
The ESD release signals are set up as redundant signals for safety. There are two signal inputs, and BOTH must be in
the active state. If there is an active ESD and both ESD release inputs are in the active state, the ESD will be unlatched
and the unit will return to normal operation. If the ESD release inputs are in a fault state, an active ESD will NOT be
released. The ESD release inputs will have no effect on a partial stroke ESD test.
The momentary ESD/PSESD input will be ignored if the signal is present for less than 100msec, and is guaranteed to
latch the ESD/PSESD if the signal is present for greater than 800msec. Once the ESD is latched, the unit will perform
the ESD action.
The partial stroke enable signals are set up as redundant signals for safety. There are two signal inputs, and BOTH must
be in the active state. If the partial stroke enable inputs are in the active state, and an input is detected (>800msec) on
the momentary ESD/PSESD input, then a partial stroke ESD test will be run. If the partial stroke enable inputs are not
active or in a fault state, and an input is detected on the momentary ESD/PSESD input, then the ESD will be latched in
and the actuator will do the ESD until the ESD release is given.