We are looking into installing a Variable Frequency Drive for a Boiler Feed Pump. One of the issue is what to do with the minimum flow valve at the pump discharge. With VFD the pump may run at less than the discharge pr setting of the minimum flow valve in case of very low flow requirement, and the minimum flow valve may not operate at all. Anyone has experience with such a modification? Any suggestions?

I have experience of taking out the minimum flow protection completely from a VFD driven pump, the pump was controlling level in a crude seperator on the crude side so similar low NPSH requirement as per a boiler feed pump and we were getting problems with the control of the valve so we got back to the vendor who basically said tahth since the speed reduces to control flow there was no way of going below the minimum flow required of the pump. We had no problems after the min flow was removed even at low flows because the pump speed reduced to the point where the minimum flow required was always less.

In another aplication we used a logic block to take the measured speed of the pump and simply factor it to get the minimum flow set point on the min flow control valve, again this worked fine in that case it was a water injection pump so no NPSH problems but very high differentials up to 300 bar.

We've been looking at VSD's on steam boilers over the past few years or so but concentrated on some specific features required on the FD fan. However, we are starting to look at the boiler feed water pumps and can add a few comments that may help:
Note: I assume a modulating feed water valve is fitted..?
 
1/ From what we have seen so far, the range of the VFD when controlling boiler feed water pumps is very small - around 40 to 50Hz (you can use the same relative drop on 60Hz).  Anything below approx. 40 Hz it is not possible for the pump to overcome the back pressure of the boiler including the pressure drop of the associated valves and fitting etc and therefore no water will enter the boiler.  So essentially you could either ramp the pumps down to turn off or operate them at a minimum frequency whereby the flow rate is low enough not to harm the pump and or below the internal 10% relief designed in the pump ( Grundfoss )  -  potentially No minimum flow valve required....!
2/ Alternatively - rather than a minimum flow valve ( which is usually controlled by a spring & diaphragm ) perhaps try an orifice - the hole size is calculated to meet the minimum flow rate of the pump.  Basically the pump operates at a frequency that is sufficient to force the pumps minimum flow rate through the orifice back to the feed tank when the modulating valve is closed.  The VFD is controlled by a pressure sensor on the feed water line so as to maintain a constant pressure up stream of the the valve at all times and governed by a minimum frequency set point.

I hope this helps. Would be interested in your comments on this as it is something we are now looking at as we (Siemens Building Technologies) design/manufacture and supply control products to (mainly)OEM burner companies in the form of burner management control, VSD's and other equipment.

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