Multiple VFD Pumps to Common Header 2

[KevinAtBurnsRoe]

I have an application where I have 4 variable speed pumps that can supply feedwater to a common header that in turn feeds up to 4 industrial boilers. It takes up to 3 of these pumps operating in parallel to handle the load of all 4 boilers. There is also a smaller, constant speed pump used for very low loads. The pumps are not identical, 2 are motor driven and 2 are turbine driven. Our goal is to maintain a constant header pressure. I would like suggestions on how to control the speed on the pumps over the load range, including cutting in and out pumps as required. Pump starting and stopping will be manual by operators based on some alert.

 

[ccfowler]

KevinAtBurnsRoe,

Since you appear to have three different pump types (four counting the little one), you can expect that they will probably not always "play nicely together." I would want to include individual flow meters to monitor the flow through each pump. If the pump head vs. flow curves do not all rise continuously to shut-off, you should anticipate having problems with instability and with initiating flow at start up of each pump against the existing discharge header pressure.

Another likely problem will be one pump trying to "hog" the load, and the role of being that "proud" pump may change abruptly from one pump to another as flow rates vary. The load sharing problem is the salient justification for adjustable speed drives on the electric powered pumps if their operation is to be biased to optimize steam flow through the turbines serving the other pumps.

If some modest variation in the discharge header pressure can be tolerated and the two electric powered pumps are identical, then your control system can be greatly simplified by eliminating the adjustable speed drives and controlling the speeds of the two turbine driven pumps to control the flow through the electric powered pumps. This would reduce the electric power consumption of the system by eliminating the parasitic lossed of the adjustable speed drives in addition to saving the initial cost of these drives.