VFD and a centrifugal pump??

[Buhsman]

Hello all,
I am an electrician in a can manufacuring plant with a question. Our requirements for cooling water used to be around 100gpm for our ironers. They have done several modifications and now think that we need to increase the flow. I agree that they do need to increase the flow but our superintendant wants to know if we can regulate the flow of a bigger pump and motor with a Variable Freq. Drive.
Supposedly we are replacing a 100gpm pump and a 5hp motor with a 200gpm pump and 10hp motor and he wants to be able to regulate flow by changing pump speed. Is this something you should do with a centrifugal pump??

Thanks,
Kyle

 

[semo]

It is done all the time. Won't hurt the pump. A centrifugal pump can also be throttled using a valve on the discharge. You can deadhead the pump against a closed valve and not have a great deal of problems as long as you control the temperature of the water in the pump.

 

[Buhsman]

Thank you for the information.

 

[quark]

Don't take it for granted that easily. If your 200gpm pump has a head of 16meters initially, when you reduce the speed by half to get a flow rate of 100gpm, the pump can't generate a head more than 4meters. Secondly, the pump may not be running at the best efficiency point. Your investment cost with respect to bigger pump, bigger motor and the drive will be high.


Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[semo]

I agree with Quark, you will have to look at the hydraulics of the pump you propose with the piping system you have. The whole pump curve will move up and down with the varying speed.

I assumed you/superintendent/someone was looking at the hydraulics. If you talk to a pump supplier, they should be able to help select a pump with a curve that will fit the needs.

 

[PUMPDESIGNER]

Ok, got some bad news about the Wife this week, so my glass is half empty and I find it hard to be completely gracious. But no ill intent is this.

I disagree with much.

Trust a supplier to correctly figure it out?
Only if that supplier is qualified, few are, and to be really frank, some suppliers are far more dangerous because they know a little.

Pump hydraulics come first, a high Ns and high Nss pump would completely be out of question for throttling, big trouble, these pumps cannot be operated far from BEP, and definitely not near shut off. VFD can however work on those type pumps if done correctly.

Why not find some qualified engineer to look at it and give you some advice. You do not have to take his advice, but it might be helpful.

Yeh, it is a "simple" thing in one sense, because centrifugal pumps are forgiving and gentle usually but not always, and you can get away with quite a lot, especially with a VFD.

But it is not simple to achieve excellence.
Your method approaching this problem "works" often, but sometimes a real nasty thing jumps up and bites you and ruins your day and week and month as you are embarassed in front of everyone and lose much sleep wondering what could possibly have gone wrong. Avoid the surprise, take your time, find and even be willing to pay for some good advice.

PUMPDESIGNER

 

[GerH]

You can successfully vary the discharge pressure of the pump using a VFD. As mentioned above the frequency chnage will probably be in the high range end 100% down to maybe 60%.

I think you should also ask what will regulate the speed. Do you want an automatic system the delivers a constant pressure in which case you will need a pressure transmitter and VFD with built in PID controller. On the other hand you may only need to pre-set the speed to a suitable fixed value by watching a pressure gauge.

Another and possibly simpler solution might be to buy a fixed speed larger pump and reduce the impeller diameter to get the delivery pressure you require. You might either swop out the impeller for one of less diameter or take it and turn it down on a lathe. Usually one or at most two steps will suffice, taking care not to go too far down as you cannot go up again without a new impeller.

 

[bimr]

I agree with pumpdesigner on this one. Why not locate a qualified engineer to look at it and give you some advice. Don't you have a plant engineer?

 

[semo]

Good call Pumpdesigner. No ill taken. Hope things get better for your wife.

I should have looked at what I was saying more. I would never in my own business just do something without checking the hydraulics first. No two installations are the same. I guess I was thinking in general terms and not considering the oddball situations; which, you're right, occur more than you think and can end up biting you. A lot of varibles do come into play.

I was also thinking about one supplier in our area that is great for pumps. We always do our own hydraulics; but, on really hard ones, they'll double check us with the factory. The other suppliers, well.

Just a question for all, if you needed dual flow rates, have you ever used a two speed pump. Where the motor had dual windings? I've not; but, heard it can be economical if the pump curves fit the application.

 

[PUMPDESIGNER]

Hello semo,

It is done all the time in the spa industry.
One speed for circulation.
Higher speed for stimulating those tired muscles.

In water circulators they used to do it, but now they use VFD built into the motor.

Low Ns pumps can be great at variable flows, in fact, Low Ns pumps could be almost considered the default choice because simple is better when it works correctly. Nice smooth behavior at low flows, no vibration, power savings because low Ns pumps have current draw proportional to flow rate down to about 70% of capacity by one report, I have seen them draw less power down lower than that. We have to install jockeys, try to be in the 500-800 Ns range (American System obviously), really nice with no VFD required.

PUMPDESIGNER