The above figures show the combination of function blocks to execute the flow control. As it can be seen,
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When the process is started, the first pump runs at the “MinimumSpeed” for the time defined in the “TimeToRunMinSpeed” duration. This is to gradually start filling the pipe. After this the normal operation of PID control takes over.
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The PID acts as a comparator: the PID compares the required flow and the actual flow to generate output in terms of percentage. This needs to be connected to the “PmpFlowDistributor”.
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“PmpFlowDistributor” takes the “PidOut”. Then converts this output in terms of speed for the master pump. The information of master pump comes from the “MasterPump” of the “PmpSequenceGen”. The scaling of the “PidOut” in terms of speed is done in following ways. Speed reference “MasterPump” = (“NominalSpeed” of “MasterPump”) * (“PidOut”/100)
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This speed reference is given to the master and the follower (if Follower Mode = 1). If Follower Mode = 2, then follower pumps will run at their fixed speed given in the input “FollowerSpeed” [1..8].
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The distributor then decides about the number of pumps to operate based on the following formula, Number of pumps = “FlowSetpoint”/“NominalFlow”, here it is assumed that all pumps are of same rating.
It is assumed that all the Pumps are of the same capacity.
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As the flow set point increases, the demand of pumps increases.
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Example: if each pump is of capacity = 100 cubic meters per hour and the flow set point is 400 cubic meters per hour, then four pumps will start together. This is unlike the pressure control where the pumps do not start together but based on speed reference of the master pump.