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,

 

[BigInch]

I don't represent anybody except myself and prefer to arrive at the best solution to any problem by optimization of as many relavent variables as can be defined, whether its water, oil, or beer that's in the pump or the pipeline. In this case, I have agreed that a VSD is a possibility, but do propose that the OP evaluate it as one of the possible solutions, not the only one. I have generally found that petroleum companies also usually prefer similar solutions, unless deliverability within the project time constraints is not possible, in which case overall project economics can take precedense over what may be the relatively microeconomic factors of pumping economics. As the net result is more energy delivered to the consumer than could ever be saved from installing the most efficient systems, there is some logic to the madness there too.

And .. valvecrazy is, I gather from past posts, not involved in big oil, but in the lower pressure water distribution applications, where his generalizations are usually more representative of typical situations and economics are much tighter, not to mention the effects of the other VSD associated operational problems that can wreck havoc for a whole lot of people in a very short time. I don't think its irresponsible to talk in the general sense, since there can be many variables, other than cost or variables that are not convenient to reduce to cost, that can influence the ultimate selection of any one solution over another. In that respect, he does draw attention to several factors which could have a bearing on the ultimate decision for this OP or many others.

As for any individual situation is concerned including this OP, for any one application you can produce showing energy savings with VSDs, I could give you two scenarios where valves would perform better in the overall project, but might even possibly use more energy in the process (maybe not), so there can be many facets to consider other than energy cost. (I've been forced to choose electric motors as drivers when diesel would have been better just to defer the emission "cost" to the central power supplier.) So, energy use or cost may not represent the objective for all projects and the rest is 6 of 1 and half-dozen of another, so I see the only solution is to look at your overall project requirements and make selections accordingly. What else can one do?

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[Valvecrazy]

Can I get an Amen for Biginch.

Clicketyclack is right however. If this is an application where the flow remains the same but the head is twice what it needs to be, then a drive will reduce the power required considerably. The drive will reduce the power required by the pump to 70 HP. Add back in the parasitic and other losses contributed to the drive and you will be pulling 75 HP or more. But again the drive is not saving any energy. Because if this is the only design point, then a correctly sized pump will probably only pull slightly above 60 HP.

So the drive is actually wasting power over what really needs to be done, which is change out the pump. Like Biginch said, "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".

I am glad you said that a VSD is not a fix all, because that is what is driving me crazy. Each application is different. What is irresponsible is to assume that VSD will always save energy. There is nearly always a better or at least as good a way of saving energy as a VSD. Correctly sizing the pump is the best way, and many times a simple control valve can be just as good or better.

 

[dcasto]

I like the way a VFD fits a long liquid pipeline. At flowrate = X and pressure drop = y, then at flowrate = 2x pressure = about 4Y.

On a pump at speed = a, flowrate = x and head = y. At speed = 2a, then flowrate = 2x and head = 4y. Nice fit.

 

[BigInch]

Right...but as (I know) you know that's only without significant static head to overcome, which is the ideal case that many specific pipelines of course don't have (see BTC pipeline with two mountain ranges to 2800 m (9186 feet) elevations.) That can have drastic affects on the suitability of VSDs. Boiler feeds are another of what must be many similar systems.

In fact, even VSD have alternates. Other non electrical options. Diesel engines are the original pipeline VSDs, although the VSD benefits were not specifically intended, just a natural result of their modus operandi. They don't require major electrical demands, don't require 3rd party electric power furnished to often remote locations, no power supply harmonics, no shaft currents, have reasonable reliability, but true they arn't as efficient as electric drivers. The number of those installations still in use or still being designed just goes to prove that maximum energy efficiency at the pump-driver isn't always the ultimate consideration.

...but I digress.

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[ClicketyClack]

Sorry Big Inch, I didn't mean to offend you if I did. It was meant to be playful jab.

I would also consider the other benefits that a well programmed drive can offer. There are some out there that offer dry run, dead head, min flow, run out and cavitation protection.

I run hundreds of pumps in my location and about 20 (which only one has caused me any issues) have drives on them. So I fully understand and agree with you both on your valve position.

It sounds like Sam has already purchased a pump since he says trimming the impeller is not an option. Otherwise he would just have another pump selected. My point is simply that in this situation, when you need to reduce pressure from 28 bar to 14 bar, if it looks good on the variable speed pump curve, you should seriously consider a drive.

 

[dcasto]

The bigger pipeline pumps we normally look at have a maximum rpm of 4000. With the VFD we can access the added capacity with off the shelf 3600 rpm motors by just uping the output frequency to 67 hz.

There are also automatic transmissions out there that can handle 1000 HP drives with 95% efficency.

 

[BigInch]

I thought so. I saw the smiley but the best defense is a complete explanation in any case.

Ya, I agree its probably not going to be a waste of time to consider a VSD for this case.

http://virtualpipeline.spaces.msn.com

 

[Sam993]

This is an old pump. It is around 20 years old and running Naphtha. We have done a change recently, so the discharge piping will go to a lower pressure vessel. So, we end up with a control valve closing 95% and producing very high noise. If we have to trim it then we have to do so much that may cause suction recirculation problmes. When need 78% of the maximum impeller diameter. So, I thought installing VFD is the easiest which will avoid any changing in the piping which is required with new pump installation. My main cerncern do I have to go for the motor supplier to confirm if his old motor can accept VFD. Or I have to buy new motor at is designed for VSD applications.

Regards for all contributions.

 

[BigInch]

We can't tell. As a general guide, AC induction motors are designed for use at one speed. Why not call an electrical engineer and let him have a look at it. A standard induction motor when run at reduced speeds may have inadequate cooling by the motor fan. Over longer time periods, the motor may get overheated and ultimately fail. Also VFD output power to the motor inherently has power frequency harmonics. These harmonics are likely to present higher voltages at the motor winding for which they are not designed to withstand with the result being failure of the motor winding.

Is your motor standard?

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[BigInch]

More info over at the electrical-motor engineering forum in
thread237-190754

http://virtualpipeline.spaces.msn.com

 

[ClicketyClack]

A VFD vendor can tell you. The motor needs to have a certain insulation class to be VFD rated. Also, it's a good idea to keep the drive as close to the motor as possible, less than 50 feet is suggested by GE, so the VFD enclosure is a consideration. Another consideration is your power quality. Drives don't like "dirty" power.

 

[Valvecrazy]

To run on a drive the insulation class of the motor must be increased. Most newer motors already have this type insulation. Motor manufacturers have gone to higher class insulation on all motors because they do not know if it will be used with a drive or not. They know that running on a drive, a 480 volt motor will see spikes of more than 2,000 volts so they just plan on it.

If the motor is older and still has 600 volt insulation, it will fail very quickly on a drive. If you have to replace the motor to use a drive. It would be less expensive to just purchase a new pump and motor of the correct size. Then you would not need the drive and you would not have to deal with all the negative side effects of VSD, which 2,000 volt spikes it just one of.

 

[ClicketyClack]

New 75 HP drive $7500
New 75 HP motor $8000
Baseplate modifications $2000

OR

New pump assy about $15,000
Piping changes $2000

ValveCrazy makes a very good point.

 

[Valvecrazy]

Thanks Clicketyclack. But, I think the new pump he needs is about 75 HP. To use the existing pump, he would need a 150 HP drive and new motor. That would make the difference even more favor a new pump.

 

[ClicketyClack]

If the new application only draws 75 HP max he could use a 75 HP drive on a larger rated motor. It just depends on the amp draw.

Of course this is an estimate anyways based on my Goulds example. With an older pump he may need a 100 HP drive / motor combination, depending on the pump characteristics.

If the pump is very old and you have to buy a new motor and drive anyways, I would go for a NEW fixed speed pump that better fits the application, even if I had to make piping changes. Why spend 75% of the cost of a new pump assy to bandaid an old pump? Sounds like something an accountant at corporate purchasing would do.

 

[dcasto]

Aw, but save 15% on the KwHr and that's $5000/year or NPV10 = $30,000.

 

[BigInch]

Not including power cost incrementation, but if I can get it up and running 1 month sooner without the VFD, I can make 175,000 clear profit during that time. Great. At least now everybody's thinking about evaluation rather than personal preference. OK, maybe we should add the VFD later when flowrates become variable or... drop.

http://virtualpipeline.spaces.msn.com

 

[Artisi]

dcasto - I agree, unfortunately the power calculations always seem to be forgotten and should form part of any good engineering study.
In many cases the savings in power costs can soon pay for new equipment - but then we have to fight with the "bean-counters" who consider this as capital expenditure for which we don't have the funds - but burn as much power as you like as this is operating expenditure and is your problem.

 

[dcasto]

And if this were a 7500 HP motor, the local power company may say no to starting it without a VFD. Or if it was the only motor and you had a demand meter that captured starts, you'd save too.

 

[itsmoked]

Around this area N. CA motors over 50HP generally are required to have some form of reduced voltage starting. (Really it's reduced current.)

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com