VFD and Minimum Flow Valve 1

[Techcitizen]

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?

 

[monaco8774]

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.

Hope this experience might help

 

[bulkhandling]

monaco8774,
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.

 

[ozmosis]

techcitizen
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.

 

[Techcitizen]

Thanks for the replies. To take the discussion further I give some relevant details. we have 3 x 50% (2 running & 1 standby) centrifugal pumps feeding a common header and then individual lines feed the Boilers thru control valve (CV). With the existing set-up, we can not directly transfer the CV controls to the VFD as the inputs could be different for different Boilers. This also means that there can be a case where all the three control valves get closed, the flow requirement becomes nearly zero, the VFD will still run the pp at its min speed, but no flow as the min flow valve will not operate. This further means that we can not altogether get rid of the min flow valve as suggested by 'monaco8774' and have to have some way to protect the pp in the worst case scenario of no flow.
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!

 

[bulkhandling]

It looks to me the control will be very difficult or even not possible for all the scenarios required. VFDs and control valves working together will likely spoil the control system. Just think what signals used to control the valves and what to control the pump speed. How do they match? It's not the matter of one VFD or two.

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.

 

[LAsludge]

Why don't you just put a pressure transducer on the pump discharge and just use the VFD to control feed pressure?
That should smooth out all your controls.

 

[bulkhandling]

I agree with LAsludge. It probably works.
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.

 

[checman]

A VFD on a centrifugal pump for boiler feed water may not be a good investment unless the boiler is sometimes operated at a lower pressure. Some systems run at high pressure in the winter and low pressure in the summer on these systems a VFD can pay for itself. With most boiler feed systems the discharge head is mostly constant and the flows can vary significantly. So to get water into the boiler you need to overcome the boiler pressure it does not matter if it is 10 gpm or 100 gpm the discharge pressure is normally slightly above boiler pressure line loss is normally small. You will need to run at a higher RPM just to build enough pressure to over come the boiler pressure. A VFD works best when both the flow and the pressure fluctuate together. I would say that the operating window for a boiler feed pump is small and there is not a payback on the average boiler feed system.

 

[thewellguy]

I've never worked on boilers, However how about putting a pressure transducer and a flow switch in the dischage side of the pump. The transducer would control speed and the flow switch could turn the pump of if you do not hit minimum flow.

 

[LAsludge]

thewellguy -

How would you turn the pump back on?

 

[itdepends]

How about installing a VFD and flowmeter on the system. Control the boiler makeup based on flow controller that receives a cascade setpoint from the drum level controller. You can also incldude a feed forward signal from the system steam demand.

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.

 

[ozmosis]

Agree to a degree with checman, the operating window is small. To say there is no payback is not strictly true as it depends on how long you expect your return on investment. Even a reduction from 50Hz to 40Hz or 60Hz to 45Hz (the tests we have done show this to be a typical range) will show significant savings as the majority of time it is either the lower part of the band or stopped.
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.

 

[Ashereng]

LASludge,

Start-up bypass.

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