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Old March 3rd, 2014, 08:57 AM   #1

Charbel is offline
Join Date: Jan 2012
Location: Beirut
Posts: 284
Control based on level and VSD


there was a requirement to control the variable speed pumps based on level and at the same time change their speed.
The system consists of 2 pumps, please see picture below.

See the sequence of operation:

L1 is the level of dry run protection for the pumps at which low low level alarm shall be activated
At L3, pump 1 shall start.
During operation i.e. between level 2 and level 3, the speed of the pump is controlled by the level of sewage as measured by the hydrostatic level sensor such that when the level of sewage is increased after a predefined time, the speed of the pump shall be increased to cater for the increase of the sewage incoming flow
During operation i.e. between level 2 and level 3, when the level of sewage is decreased after a predefined time, the speed of the pump shall be decreased to cater for the decrease of sewage incoming flow.
When the first pump reaches full speed and sewage level reaches L4, the second pump shall cut in and an high high level alarm shall be activated, both pumps shall start running at the same speed to cater for the required demand.
Do you have feedback on the above, is the system workable from hydraulical point of view and is it easy to program?

thanks a lot for your valuable feedback!
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Old March 3rd, 2014, 10:43 AM   #2
Tom Jenkins
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Join Date: Apr 2002
Location: Milwaukee, WI
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You don't show the "hydrostatic level sensor" in your diagram - to me this implies an analog signal. If that is the case then this essentially a PID or similar feedback control with pump speed proportional to level.

On the other hand, based on your diagram, I suspect that what you really want to do is use level switches to control the pump speed, with a different preset pump speed at each level. That is also feasible and, if you set the speeds properly, should do the job.

Neither system is unusual or particularly complex programming. Whether it is easy or not is a function of your skill level.
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Old March 4th, 2014, 10:24 AM   #3
South Africa

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Location: Pietermaritzburg, South Africa
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The Danfoss Aqua drives have a useful pump cascade function for several drives controlling one process.
"Do... or do not. There is no try" Yoda
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