VFD retofitting for pumps 5

[Sam993]

I would like to know how difficult is it to replace the induction motor for an existing pump with VFD motor. what are the things which we have to change. Regards,

 

[TenPenny]

It depends on a large number of items. Is the pump close coupled or long/frame mounted? Horizontal or vertical?

If it's a standard horizontal mount motor, long coupled, all you need to worry about is matching the frame size of the motor, and putting the coupling hub on the motor.

If it's close coupled, where the impeller is on the motor shaft, it's more involved.

More details will make for a more useful answer to your question.

 

[EdStainless]

Why are you looking at a VFD? Is this pump called upon to run outside of its recommended range often?

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[Sam993]

We need VFD because the present requirment for head is much lower than the design one. The alternative is impeller trimming where we have to trim too much which is not recommended. It is horizontal with coupling.

 

[dcasto]

It's not difficult. Some induction motors can be used as they are, some may have to be replaced. The VFD will just replace tour stsrter.

 

[Valvecrazy]

What is the fluid being pumped and what is the fluid temperature? If it is just cool water, you can simply valve it back for the same effect.

 

[Artisi]

It might be easier and less complicated to replace the whole pump unit with one that is suited to the application.

 

[BigInch]

What flow and head do you have now and what flow and head do you want to have?

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[Sam993]

The flow is the same which is 90 m3/hr, however the required head dropped from 28 Bars to 14 Bars due some process change parameters.

 

[BigInch]

So then at half the head coming from a 25% speed reduction you get to a "natural" pump flowrate of 67 m3. The problem with VFDs is that a slower speed tends to reduce head and flow too. OK not totally unmanageable. I don't like sticking VFDs on to anything that moves, but I have to admit that they can (at east temporarily) lend themselves to a quick-n-easy solution when system flowrates or heads have had to change for one reason or another. It would be a better flow match to change out the pump, but it appears a VFD could be an alternative possibility if holding close to that 90 m3 is not essential.

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[Valvecrazy]

If you have an isolation valve on the discharge of the pump, you can manually throttle to maintain 14 bars for an easy test. Throttling will achieve the end result, and drop the amps the same as a VFD. Then you will see that a simple pump control valve is all that you need, and you won't be inheriting all the problems that come along with the VFD.

 

[BigInch]

Valvecrazy,
Thanks. I should have said something about both alternatives.

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[Artisi]

Can you achieve the duty needed with a speed reduction and still maintain reasonable hydraulic performance. You need to generate a number of reduce speed H/Q curves to see where the pump will operate in relation to BEP and then calculate the running costs (power consumption) over the life of the installation. You might be surprised the increase in annual running costs for a drop in efficiency of a few points.

 

[Valvecrazy]

What many people do not understand is that the power consumed will be almost exactly the same for throttling with a simple valve as it is for slowing with a VFD. You should weigh the cost of a few points lower efficiency to the cost, maintenance, complications, downtime, and other headaches of using a VFD. It usually turns out that a little lower efficiency cost less overall than adding the expense and side effects of a drive.

 

[ClicketyClack]

True. But many people also do not understand that throttling can destroy an expensive pump (seals, shafts, wear rings, bearing failures, etc). This is a significant reduction in pressure. And how would you throttle and maintain the same flow on a centrifugal?

I don't buy that throttling will save you the same amount of money over time as a VSD in this application.

I would go with Artisi's suggestions. Hopefully you have room to increase impeller diameter and can slow the pump down. But it needs to be a good fit on the variable speed curves.

 

[ClicketyClack]

Here are some numbers using a Goulds 3316 3x4-11H as an example, assuming 70 degree F water and 28/14 Bar differential:

Fixed Speed at 938' TDH
400 GPM @ 938' TDH
10.75" Impeller
3550 RPM
Efficiency 66%
144 HP

Same Pump, Variable Speed @ 469' TDH
400 GPM @ 469' TDH
10.75" Impeller
2650 RPM (25% turndown, do-able)
Efficiency 68%
70 HP <--- BIG savings

No way a valve can do this. Simple first law thermo.

 

[BigInch]

VSD and valve solutions each have their place.
Keep in mind that this is a bit of an unusual case. How many times does flow remain the same with head reducing by 50%?

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[ClicketyClack]

No argument there. I just don't agree with Valvecrazy's power consumption claim. It is terribly inaccurate in this case.

 

[BigInch]

Well... he doesn't call himself "crazy" for nothing.

In most of the typical cases, he's not going to be too far off from his claim. You just have to realize that VSDs arn't usually a viable solution when heads don't change in proportion to the afinity laws. Where they do, VSDs usually can be made to work, where they don't, valves usually work. VSDs work best when heads and flows both change proportionally to the affinity laws, flowrates vary widely and time for each flowrate is short AND tanks are expensive, can't be accomodated or the process MUST have variable flows. If flows and heads experience some variation, but are mostly constant, valves can work well.

In lots [not all] of the everyday typical practical applications that VSDs are being pushed to fill these days, slight benefits of VSDs can be totally eliminated by extraneous considerations and may really be disadvantageous when consideration of such things as shaft currents, adverse power supply harmonics and maintenance costs are included.

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[ClicketyClack]

Each application is different. It is irresponsible to imply that throttling will give you the same result as reducing the speed in each application.

I hesitate to apply the Affinity Laws to every pump. In my hypothetical above, the head is reduced by half and the flow is constant. Yet the same pump can accomplish this (with the same impeller diameter) by reducing speed. The pump is just running further to the right on its curve.

Only a representative of big oil could look past the energy savings a VSD could offer in this application Although I agree with you completely that VSD's are NOT a fix all.