VFD and Minimum Flow Valve
VFD and Minimum Flow Valve
(OP)
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?





RE: VFD and Minimum Flow Valve
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.
Hope this experience might help
RE: VFD and Minimum Flow Valve
Suggest you take off the control valve or keep it fully open for the "another" application. Control valve after the VFD pump only wastes power and conplicates the control.
RE: VFD and Minimum Flow Valve
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.
RE: VFD and Minimum Flow Valve
And thanks 'sed2developer' for your thoughts. In our case the pp discharge pr is 19 bar and drum pr is 7 bar, so we may be running at times at less than 40 Hz also. Regarding orifice, I feel it will always allow some amount of water to pass and hence not efficient. I also note that you have mentioned controlling the VFD so as to hold the pp discharge pr constant, this gives us one way of control. Another way is to take the open feedback from the different Drum Level Control Valves and regulate VFD such that at least one valve is held near to 100% opening (say 90%), do you think this can work?
Another issue is, one of the VFD supplier suggested running one pump at full load and the second one to run with VFD? Is this a good idea?
And yes, I still look forward to some idea from someone to overcome the min flow problem with VFD!
RE: VFD and Minimum Flow Valve
I'm not an expert at boiler feed water, but the variation on feed water shall be re-visited. You should give them a minimum flow and when below that flow, pump shall be stopped.
So use one pump to feed one boiler could be a good way to satisfy the flow control need, with one pump standby for all the three services. VFD pumps, no control valve.
RE: VFD and Minimum Flow Valve
That should smooth out all your controls.
RE: VFD and Minimum Flow Valve
For the minimum flow, you may need a recirculation line.
Do not agree on the idea of one pump fixed speed, the other VFD, except you only run the variable speed when the other pump is stopped. Otherwise, it's not easy to make the two pumps match. In other words, the VFD pump can easily run into below minimum flow or even dead-headed.
RE: VFD and Minimum Flow Valve
RE: VFD and Minimum Flow Valve
RE: VFD and Minimum Flow Valve
How would you turn the pump back on?
RE: VFD and Minimum Flow Valve
Put a minimum flow setpoint on the flow controller- this will prevent the pump flow from dropping below the minimum requirement. You'll need a high drum level interlock to stop the pump.
However- I agree with checman- a boiler feed pump is not the best application for a VFD for cost saving. An appropriate fixed speed pump (with a flat pump curve) and control valve should be better.
RE: VFD and Minimum Flow Valve
We have used a pressure transducer feeding back to the VSD using the internal PID loop of the drive. To answer LAsludge on "how you turn it back on". We have used an internal function of our drive called 'Hibernation' that means the drive itself looks at the operation of the motor/pump and if it runs for a certain time at minimum frequency (therefore maximum pressure) then it automatically switches off and if the pressure demand drops then it automatically starts again. This is not just the Siemens VSD that has this, Danfoss have a 'Sleep' function in theirs, Vacon have a similar and I think ABB in some of their VSD's have a similar function.
RE: VFD and Minimum Flow Valve
Start-up bypass.
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