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Bumpless pump sequencing
Process control authority Bela Liptak brings in specialists from his cadre of co-authors to answer a reader's question regarding problems with the control scheme for two variable speed pumps.
03/08/2005
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When switching from 2 to 1 pump operation, if “under-pressuring” the system for a few seconds is acceptable, I have allowed the supply pressure to drop a bit, until PC-01 brings it back up. In that connection, you are absolutely right that if the process can not tolerate a short pressure dip either, a “y” signal is also needed.
The importance of the role of external reset in preventing reset windup in cascade loops was already explained by Greg Shinskey in connection with Q3, so I will not repeat it, but would recommend it that you add it to your control scheme.
My reason for showing “start on speed, stop on flow” is not very theoretical and it comes from George Matousek. He was this Czech engineer, who knew everything there was to know about pumps (and beer), but was not very talkative. So when I asked: “Why George?” His answer was: “Because it works!” Naturally, you are right that if the “minimum system curve” is as you have assumed, it would not, but I have not run into such case yet. On the other hand, if Murphy is right and anything that can happen will, than one day maybe I will.
Béla Lipták:
FROM MY own personal experience, I have found feed-forward schemes difficult to implement and tune. In the applications like the one described, I have had better success playing around with the maximum ramp rate adjustment in the VFD's and allowing the top level PID controller to compensate. If the VFD's are restricted in how fast they can ramp up, and the pressure control PID has enough gain and derivative action to stay ahead, the loop will take care of itself whenever a lag pump jumps in.
In the ideal case, the VFD's are configured with a fast ramp rate, but the PID sends two seperate speed command signals to the VFDs. With both pumps running the signals to the two pumps are identical. When a lag pump is kicked on, the signal to the lag pump is immediately forced to zero percent at the beginning of the cycle. The signal to the lag pump then floats up to match the signal to the lead pump, but it is subject to a slow ramp rate restriction. This gives the pressure control PID time to adapt to the higher flow rate. After a few seconds, the ramp rate restriction to the lag VFD is turned off.
That's my opinion.
David Gardellin, Onyx Valve Co.
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