Two Pumps with Auto start/Stop switch through selection

[silebi66]

Process description :
The waste water is transferred from reaeration tank to the clarifier which incorporated gear box and conveyor.
The wastewater with sludge be carried outside and transferred by sludge return pump to two destination
(1).back to aeration basin.(Main route)
(2). Sludge storage tank (accumulated enough sludge, then discharge).

There are two trains of clarifiers, and two trains are running at the same time.

Each train is with 2 clarifiers / 2 sludge returns pump (1 primary/1 backup).

The pump is VSD type. Upon failure of the primary pump in train 1,

(automatic case):
The primary pump is taken out of service (thus becoming the backup one), and backup pump is placed into
service(becoming the primary pump), Selection of the primary and backup is alternated on daily basis
via the scheduling function of a time clock.

(Manual case) :
the primary pump and backup pump is alternated through selector switch which is inside the MCC(motor control center).
The maintenance personnel can manually flip the switch periodically.


1. Why is the VSD control signal always transfer to MCC first instead of interlock ?

2. Is possible to just replace MCC function(in the scheme) with Interlock function and

no more MCC to participate in this P&ID scheme ?


Thank you very much for your help.

 

[rutherford703]

This is more electrical than chemical process. Let me try to explain the function of the process. In the drawing something showed as interlock is not an interlock, actually it is a controller. The controller input is from flow transmitter (normally called PV) with a signal of 0-24V and the outputs (normally called OP)with signals of 0-24V go to MCC.

MCC may include a manual or automatic means for starting and stopping the motor, selecting forward or reverse rotation, selecting and regulating the speed, regulating or limiting the torque, and protecting against overloads and faults. MCC is normally provided by the motor vendor. It is clear that only a controller cannot perform all the above function. Therefore, MCC is necessary for this application.

Thanks