HI FRIENDS IHAVE QUESTION 1.WHAT IS PARTIALSTROKE TEST?WHY
IT IS USED ON ESD ON OFF VALVES?
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Answer Posted / haridass
In normal running plant the ESDVs will not be operated
unless there is an emergency situation, it happens rarely.
Sometime those valves will get struck in open position and
it will not help the situation to protect the plant.
So to avoid this kind of problem, the PARTIAL STROKE
TESTING (PST) system is applied. Periodically, the ESDV
will be closed 20% from full open to close and immediately
open fully, which will not affect the process flow. This
movement makes the valve free from struck-up.
This PST function takes place only when the valve is full
open. During the valve close PST FAIL alarm will be
occured. and recorded in the event summary.
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A PLC is programmed to drive three motors A, B and C as follows : After running motor A for an hour, motor B should get ON and motor A should get OFF. And after running motor B for an hour, motor C should get ON and motor B should get OFF. And after running motor C for an hour, motor A should get ON and motor C should get OFF. (That is, a cyclic repetition with time period one hour) Now, the second condition is: If any motor gone faulty and is tripped, the consecutive motor should get ON and the faulty (tripped) motor should be eliminated from cyclic repetition. The remaining two motors should bear the cyclic repetition of one hour each. (For healthy motor take logic as high and for tripped motor take logic as low) Now, the third condition is: When motor A load(Amps) exceeds 60% of its rated load, motor B should get ON in parallel to motor A. And when both motors A & B are running in parallel, and if their load exceeds 60% of their rated load, motor C should also start in parallel to A & B. [Here the cyclic repetition is eliminated, until their loads become normal(40%)] [Take logic high(1) for load at 60% and logic low(0) for load at 40%] Now draw a flow chart or a ladder diagram to fulfill the above condition.
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