Have 3 circulating pumps. One of them is connected to a Siemens G110 inverter (it has a analog input) and I would like it to control the pressure in the system when different zones are closed off by varying the speed of the pump. I am not really familiar with differential pressure sensors, but can someone advise which type i need and how to connect it to the inverter and inside the pipe/where? The other 2 pumps should only kick in when needed. How can i automatise this?
thanks

 

Reply,
It's a little hard from your description I assume you are trying to maintain a constant Differential between a supply and return line as more load (users) are added.

First figure out what pressure differential you want then buy a transmitter capable of reading at least twice that. If you don't have a lot of instrumentation make sure you get a transmitter that can be configured with internal buttons and ask the suplier to set it for the span you want.
If your VFD has on-board PID control you could use that otherwise you will need a controller or perhaps a small PLC.

As the load increases the controller will ramp up the pump speed until it eventually runs out of speed to maintain the differential. At this point so a 2nd pump will be required. As the 2nd pump starts your control scheme will need to cut back on the speed of the first.

A simple scheme might monitor the output of the PID controller. When it reaches 100%, start another fixed speed pump at the same time setting the controller for variable speed pump output back to 50%. As the load decreases when the controller output drops to say 40% stop one of the fixed speed pumps and step the variable speed back to 100%. You will need to make sure that the pumps don't start and stop too frequently or more than on starts. Gather as much information as you can on how the pumps are to operate before you start I imagine that you must be doing something like this in manual at the moment.

If you send me a few more details, perhaps a sketch I can recommend a transmitter Make/model

Good Luck
Roy
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