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# VFD pump

## VFD pump

(OP)
All, I have a feed pump offer with VFD Installation which is with 50Hz and 2 poles , synchroneous Speed is 3000rpm. Querries are
1. To what minimum frequency or rpm i can go down ?
2. How can i calcualte the Minimum rpm or frequnecy (Hz)? because if i have any load lower than this rpm , i need to decide upon the requirement of control valve or Orifice.
3. I dont see a stall Region for the VFD offer. Can somebody enlighten this?
4. How abt the start up?

### RE: VFD pump

1) No telling with the available info. Down to whatever head the systems still needs to do anything.

2) You calculate it knowing what your system operating point needs to be against your pump's performance curve. Knowing what you need for pressure and flow and what the pump can provide at the needed flow/pressure tells you the minimum speed.

3) Unclear. Never heard of a 'stall region' for a VFD. They can dish out any speed all day long. What they are driving may not take it though.

4) Once the operating region is understood 2-above one sets the VFD parameters to meet those needs and to prevent head based dead-heading.

Keith Cress
kcress - http://www.flaminsystems.com

### RE: VFD pump

(OP)
itsmoked, thas for ur immediate Response.
1. i see a min rpm of 1800. What happens if go down or if i have an operating Point below 1800rpm? at the Moment 70% load is at 2400 rpm
2. Point 1
3. 1st time we are offering different Speed pump. I am Aware of stall in constant Speed pumpimg. any way i understood there is no stall possibility for variable Speed.
4. during start up or till reaching the min rpm , How does this System works?

### RE: VFD pump

A bit more information on what you are trying to achieve with slowing the pump would help with you question/s.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

### RE: VFD pump

(OP)
Ja, I have the pump operating curve from 3000rpm (50Hz) to 1800rpm (30Hz). I heard from electrical engineers that below 40Hz, curve prediction may not be reliable . BFP connected to the Boiler, Main discharge pipe has no control valve and the Bypass line has the control valve. Need to decide upon from which load this Control valve Needs to be operated. So, lets say as per my electrical enginner saying of below 40HZ Performance of the pump is no good then i will Limit the Speed to 40 Hz and use the Control valve for any load below this. So my question is what is th min RPM or HZ?

### RE: VFD pump

For boiler feed pumps remember that head / pressure varies by the square of the pump speed. Hence 50 to 30 htz gives you roughly 35% of the head at 50 htz. For a feed pump?? Really??

I don't understand the stall thing either but maybe you mean the speed at which you get no flow??

start up just ramps to the speed you want in a certain time. This is normally short enough not to be an issue for the pump or motor.

You need to look at the overall system design here which we can't see - no diagram or data

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: VFD pump

(OP)
Yes , 35% of the head at 50HZ . But unfortunately i am evaluating the Price for fixed pressure Boiler.

### RE: VFD pump

(OP)
Yes , 35% of the head at 50HZ . But unfortunately i am evaluating the Price for fixed pressure Boiler.

### RE: VFD pump

Boiler feedwater pumps are often a poor application for a VFD which never offers a payback because almost all of the input energy goes into developing head, not creating forward flow. If your boiler has a wide drum operating pressure range then a VFD may make sense, but on a fixed-pressure boiler it will likely never pay back. Don't forget to factor in lifetime costs of a VFD, not just the purchase cost.

### RE: VFD pump

With the feedback posted so far, it is clear you have to use a positive displacement pump on VFD if power savings at part load is to be realised for this boiler feed pump application. This pd pump alternative may most likely be not a good idea from a machine reliability perspective. At the moment, the bypass control valve for a centrifugal pump in this service will have to handle almost the entire minimum flow seen at 3000rpm. Take a look at the centrifugal pump curve and see what is the minimum possible speed to achieve the min required boiler pressure.

### RE: VFD pump

Raai,

You don't seem to be answering the questions raised. you seem fixated on the motor part and ignoring the pump part
To repeat a bit itsmoked.
1) Min Rpm will be whatever RPM is required to create the min pressure you need to get water into your fixed pressure boiler.
You haven't provided any information ( pressure, pump curves etc) for anyone to work that out
2) See it smoked
3) Stall I assume you mean the speed below which there is no flow because the pressure isn't high enough?
4) Start up you just ramp up as fast as possible to your min RPM you found in item 2.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: VFD pump

(OP)
Hello Gentlemen, I am bit busy in daily routine, will come with some answers posted withink this week!

(OP)
@ ScottUK : Would be interesting if you could share something on maintenance cost. ( any thumb rule $perMW...etc) ### RE: VFD pump Yearly clean and filter change, maybe more frequently depending on the environment. Fans at three years, maybe more frequently if you have a lot of abrasive fines in the air (a steelworks for a neighbour, for example). Capacitor change at six years. Drive end-of-lifed and support withdrawn by manufacturer after about 15 years, requiring complete replacement (or run to failure and accept the unplanned downtime). In the UK I'd be expecting operation & maintenance costs of around £30k - £40k over the lifetime of an MV drive, excluding the cost of renewal at end of its working life. The maintenance costs for a 1MW drive won't be massively different to a 2MW drive in the same voltage class. The costs will depend on the individual OEM's planned maintenance schedule, their labour costs and yours, the number of drives you have on site, etc. ### RE: VFD pump (OP) ScottyUK ;thx for the numbers .This VFD is for Pump and the site is in Scottland aswell. For Fixed pressure Boiler I was under the impression of Not saving much power , moreover this utility boilers are operated at TMCR (Turbine max rating). Below are some results of numbers. Motor capacity : 120kW Expected saving : 22kW For one year : 22 * 8760 * 0,9(assumed Plf)= 173448kWh/year Cost of electricity : 15,6 pence / kWh = 173448 * 15,6 /100 = 27058 Pounds / year Motor rating : 120 kW = 120 * 1,35 = 162HP Installed cost assumed : 500$ / HP (https://www.pumpsandsystems.com/motors/july-2015-v...)
Installation cost = 500 *0,8 * 162 = 64800 Pounds
Maintenance cost : 40000 Pounds (Ref Scotty UK)
Fixed cost per year : (64800 + 40000) / 25 (25 years of life time assumed ) = 4192 Pounds per year
Pay back : 27058 / 4192 = 6,5 years approx.
Thats it.
Questions are welcome for further improvisation.

### RE: VFD pump

You need to first explain the 22kW "saving~".

The actual power used by a pump which is being throttled isn't the max motor rating.

Also don't neglect the considerable heat losses associated with a VFD.

Not sure you're going to end up with that much of saving.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: VFD pump

(OP)
thx for the response in improving it further though polite responses are much appreciated -- ''Need'' doesnt play a role!!! apart,
22kW is very basic. For this load calculate the constant speed power consumption and the one with the VFD. difference is the saving. May be i would include the VFD heat loss (around max 3%??) . but in my view adding the VFD heat loss may be increase the payback for 6.5 to let say 7-8 years max. But in overall life time it seems VFD is profitable. Anybody who has evaluated the VFD vs No VFD??.

### RE: VFD pump

Well you are going to "need" to explain where you got the figure from to someone...

"For this load calculate the constant speed power consumption and the one with the VFD" - Well that's what I'm requesting so that we can whether this calculation is in fact correct.

3% is on the low side of VFD losses 4-5% is more likely at this sort of size of drive.

Also this reduces the saving as it add to the total load of a VFD, so your payback period increases.
Now maybe it's me but payback is usually, at a basic level, extra costs divided by savings.

To me it would be base case of fixed speed pump and then what the delta is to purchase and install and maintain a VFD divided by the electricity saving to get an approx. payback period.

Note that the discounted cash flow calcs give a longer payback period.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

### RE: VFD pump

(OP)
"For this load calculate the constant speed power consumption and the one with the VFD" - Well that's what I'm requesting so that we can whether this calculation is in fact- ''Well i am not sure i can share but the principle is correct. Thanks for coming forward to check. this is small capacity pump''

To me it would be base case of fixed speed pump and then what the delta is to purchase and install and maintain a VFD divided by the electricity saving to get an approx. payback period. - ''Saving of 22kW / Installation + Maintenance cost is considered.
Well , If could pick up some value from my above thread and reiterate with numbers would be usefull.

### RE: VFD pump

If you're looking for 25 years of operation you'll almost certainly be replacing the VFD at least once.

Your pump is a lot smaller than I was anticipating, the BFW pumps I'm familiar with are big ones in the megawatt range and the drives are typically supplied at 6.6kV or 11kV. I shouldn't have assumed!

My figures are way off for a little pump with an LV motor - say £1000 a year to cover parts and materials, contracted-in support from an OEM partner, plus your internal labour costs. New drive needed at after 10 - 12 years costing about £7k at today's prices.

Don't forget that you can only save energy that is being wasted - that's not as straightforward as certain VFD manufacturers would like you to believe, and doing the calculation accurately can take a fair amount of effort.

### RE: VFD pump

I'll add that in a baseload application with constant drum pressure, a VFD is likely to be a poor choice of modulating element in the control scheme.

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