Loop checking means inspection of full loop of instruments- junction box- control room wiring after the installation to ensure proper working. Before the start of the plant loop checking ensures that the signal from the transmitters/control passes on line and receives at the receiver end and carries out the commands/instructions.
We can do loop checking
in two ways. In first method we give the actual input in to the instrument, in
the other method we give corresponding signals depends upon the output of the
instrument.
During the loop check we give some
known input and check the corresponding output at other side (control room). If the output is corresponding to input, the
loop is passed OK. If not some fault exists
in the loop. For example, if you want to
check ON/OFF control valve during the loop check, you give 24V DC from control
room, then valve must open fully, and when you give 0V DC the valve must close.
There are two types of loops,
closed loop and open loop. Open loop is
used for indications only whereas closed loop is for control.
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