Mechanical Partial Stroke Test that is Guaranteed to Stop Actuators / Valves
14 May 2008 15:01
In order to perform a partial stroke test on an ESD package during production it is essential to have a 100% guarantee that during this test the predetermined angle is not exceeded.
The test angle is determined by calculating how many degrees you can close the valve before it will influence the flow / process during production. There are many different types and ways to perform a partial stroke test but not many use a mechanical stop to physically block the movement of the actuator / valve at a predetermined angle to be 100% sure it will stop.
Recently BASF in Antwerp performed a partial stroke test with a limit switch assembly, meaning the movement of the actuator is detected by a limit switch at the top of the actuator and set at a predetermined angle. The limit switch steers the solenoid back up (to re-open the actuator) when the test angle is reached.
Unfortunately the valve was stuck and after releasing the air slowly out of the air cylinder, the actuator didn’t move. After a certain amount of air was released, the actuator started to move, passed the limit switch and gave a signal to the solenoid to re-pressurise the air cylinder. As the valve did not move at the beginning of the test, most of the air was gone out of the air cylinder already and it took a long time to get re-pressurised. The actuator overshot the test angle and there was a disturbance in the flow which lead to a total shutdown.
MECHANICAL PARTIAL STROKE SYSTEM
This could have been prevented by using a mechanical partial stroke system, which fiscally blocks the actuator at the predetermined angle. Many types of diagnostic (software-controlled) partial stroke testing system give a lot of useful information and can be used to do a partial stroke test, but none of them block the actuator mechanically and they will always leave the possibility the valve will pass the partial stroke angle, resulting in a flow disturbance and even a shutdown.
FAITH MECHANICAL PARTIAL STROKE SYSTEM
The Netherlocks FAITH system is designed to mechanically block the actuator and therefore guarantees a 100% safe test system, as it is impossible to exceed the angle regardless of unexpected influences or malfunction of the control system.
For more information on this company:
Netherlocks Safety Systems - Valve Positioning and Access Control Locks, Lockout Devices and ESD Test Units
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