Answer / s vedpathak

In closed vessals, the vessal pressure may vary.this can
cause erratic level feedback.But if DP LT is used it take
into account this vessal pressure as it acts on both LP &
HP side. So LT feedback is purely based on the head due to
vessal fluid.Wet leg & dry leg method can be used depending
on the application.

Answer / vaishnavi

In the case of closed tank der will pressure acting above the liq also.So in this case inorder to get accurate measurement we need to measure both pressure so we use dpt.But in the case of an open tank there will atmospheric pressure above the liq surface.Hence we need not measure pressure above liq.moreover as radar involves reflection principle this cannot be used to measure both the pressures in the closed tank.

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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