ASD savings based on pump motor loading?? 13

[pumpedup]

I was recently told that if the actual load on a pump motor were less than 50% of rated, there would be great potential for energy savings through the use of VFD’s. Is there a “rule of thumb” or an equation that would help predict savings by using VFD’s on synchronous pumps?

 

[buzzp]

Dadfap, I used to install and troubleshoot your brand of equipment for the same company you work for. These were almost all used in oil wells or water wells. I agree with your comments. On a new well, especially, a drive can save lots of dollars. The best estimates for well production are often incorrect and the drive allows the well to be optimized for production eliminating the need to pull the motor/pump/seal by a workover rig that costs $500/hr plus. If a well proves to have a repeatable production rate, then when the motor or cable or whatever needs to be reworked then maybe a switchboard should be installed with the properly sized motor/pump. Besides, when you work in the oil industry, money is not that big of a deal but production is.

 

[sparksski]

As all the previous comments suggest, VSD's are not the answer for everyone. If you are using throttling devices on pumped liquids or damper control for air movements and theses devices are continually varied to change output then yes VSD's may provide some good energy savings, and cut down on pipework shock/damage etc.

If, however, you continually run your plant at 50% speed then you should think about changing your motor to a lower speed variety.

By removing throttling devices from systems and allowing the VSD to match demand to output I have measured my own systems and found that the manufacturers figures are accurate.
At 100 speed power = 100%.
At 80% speed power = 50%.
At 50% speed power = 13%.

% speed cubed = power is the rule of thumb I have seen quoted on many occassions.

The pump experts will have more knowledge than I do and I will happily stand corrected.

 

[PUMPDESIGNER]

Just another "little" thought.
I have seen where those making the decisions to spend the big bucks on VSDs will spend the rest of their lives if necessary to prove their decision was correct. They are the only ones who could tell us if they thought the VSD was a good buy, but they will not give us that information accurately just because they must defend the money they lobbied for and obtained at a risk to their own career.

My biggest need is for unbiased information. Are there are University sponsored studies or research that results in usable information? I have looked hard and long and have found little to none as far as really useable stuff.

dadfap - You like most of the guys commenting seem reasonable enough and I could enjoy working with you. But I do have one comment about "choke" valves. That is not an accurate or correct word to use. It reveals a lack of appreciation for the true beauty of centrifugal pumps and incuctive motors. They do not have to be run at BEP to be highly efficient machines that are very easy to use, simple, cheap, and better whenever they are used correctly because they are a simple and beautiful thing. Run a centrifugal at 10, 15%, 20% under BEP with a "choke" valve and just sit back and enjoy the savings, highly efficient and very simple compared to a VSD.

PUMPDESIGNER

 

[jraef]

I'm jumping into the fray on this a bit late, but here is my 2 cents worth.

Like Dadflap, I too sell VFDs. I cannot speak for the entire industry but there are some general misconceptions throughout the above posts that I feel need clearing. I have read many other posts from d23 and Pumpdesigner and respect your opinions, but it appears to me that you have passionate feelings about this issue, possibly created by inexperienced or unscrupulous vendors. This is most unfortunate but, I admit, all too common in our industry. Regrettably there is no equivalent of an engineering license, journeyman card or operating permit for the sales profession. If there were, there would be a lot less salesmen P-)

1) Most if not all payback calculation programs DO in fact take the normal energy savings of a throttled system into account. True, many vendors who use them do not understand this, but it is done in the algorithms nonetheless. The same holds true for initial cost analysis. In the instructions we are specifically charged with including all aspects of the installed cost, including labor and harmonic mitigation. Do some vendors selectively leave that out in order to make their case more spectacular? Absolutely. Do buyers have a responsibility to protect themselves from said vendors by reviewing the accuracy of the statements? I think this question predates VFDs by a few thousand years. The Romans used the term Caveat Emptor!.

2) While true that as speed goes down, efficiency goes down and heat loss (rate) goes up, this is relative to the fact that energy consumption went down drastically at the same time. To use the early example of 50% speed, energy consumed at that speed went down to 12.5% of that consumed at full speed. So even though throughput efficiency dropped to 95%, that is 95% of the 12.5%, not 95% of the 100%! The same holds true for heating effects as well. The motor is getting more heat PER UNIT OF POWER CONSUMED, but that number just went down drastically so the net efect is that at 50% speed, the motor is seeing less heat than at 100% speed with a throttling valve. Drive losses, same issue.

I think ccfowler summed it up very well in his first response and I voted him a star. VFDs do not work in every application and in fact, I sell quite a few Soft Starts for that very reason. That does not mean that we need to throw the baby out with the bathwater. Energy savings are real and proven, but only when the application alows VFDs to be used at their fullest potential. If you apply them just because a salesman told them to, you get what you desrve.

Final thought: As our friend the Rooster would say, "Just because the fox isn't in the henhouse it doesn't mean the hens are going to be left in peace.... Cock-a-doodle-dooooo!"

Quando Omni Flunkus Moritati

 

[PUMPDESIGNER]

jraef - I agree with your statements absolutely. There are very honest foxes that will protect the chickens, in other words the analogy is useless when a good salesman is on hand presenting accurate information.

I am not generally negative because of a bad experience in a job. Fortunately, by the time drives came along in the late '80s I was already seasoned enough to be careful. I have never been hurt on a VSD job and our general experience has been excellent. The last years we have been using the GE AF300 without a single problem.

My general negativity probably comes from two factors:
1. Still too much overall ignorance in this issue, partly because full studies have not been done that fully deal with replacement costs, maintenance issues, etc.

2. The largest problem I have is in the irrigation industry. I can see that VSDs have great use in many industries, but very little value in the irrigation industry. And yet I have to face that issue in the irrigation industry constantly. The nature of irrigation pump and system design is to maximize use of the pump and water the turf/crop as fast as possible. Generally, most irrigation systems use the pump at full BEP almost all the time. And yet many pump station manufacturers promote VSDs like they were the only thing any sensible person would use.

PUMPDESIGNER

 

[jraef]

PUMDESIGNER,
Too bad about the irrigation industry. That has been a powerhouse market for soft starters for the exact reasons that you mention. The only viable application issue that I have seen is matching recovery rates on wells in areas where the water table fluctuates greatly. That is only a handfull of places though. With as many times as I have seen pump panel fuses replaced with pieces of pipe, I shudder to think of a farm worker having to troubleshoot a VFD that has been baking in the hot sun for months.

Quando Omni Flunkus Moritati

 

[PUMPDESIGNER]

Quando Omni Flunkus Moritati - Even though I took Latin in High School, I am very lazy and do not feel like looking it up. What does your sign off mean?
Quando Omni Flunkus Moritati

PUMPDESIGNER

 

[dadfap]

pumpdesigner

I use the term "choke valve" simply because it is an standard term for the oil industry.

The main reason that I don't like them is that where the dynamics of either the well bore or the surface manifold, piping, process plant or seperator dictate a reduction in flow for extended periods of time (months in many cases) I hate the thought of the wasted horsepower that has been installed.

all the best

dadfap

 

[PUMPDESIGNER]

dadfap - I understand exactly what you are looking at. I think that is why VSDs have such a good potential in the oil industry, if only they would last longer, like 20 years without failing, you know, sort of rock solid pieces of iron. I had a former engineer with Pratt and Whitney tell me that in his and others opinions solid state equipment will never achieve that reliability.

I thought it wise to also comment more accurately as to where much of the baloney comes from. Most of what I have seen comes not from the actual drive manufacturers, but the OEMs that market their stuff with the drives.

PUMPDESIGNER

 

[dadfap]

pumpdesigner

problem with many people who market drives is that they lack the understanding of many of the applications.

problem with many people in the pump industry who sell pumps and drives is that they don't understand drives.

oh and on 20 years, give me enough money to design one and get me people who are willing to pay for them once they are designed and I can possibly do it. the other problem with wanting 20 years out of electronics is the dinosaur problem, one small change to the electronics industry and they are obsolete.

all the best

 

[PUMPDESIGNER]

dadfap - I agree with that obsolete concept, sort of stops us in our tracks about building too good at too high a cost. I wonder however if that paradigm will ever change? When paradigms do change the change is slow and often catches us off guard.



PUMPDESIGNER

 

[jraef]

PUMPDESIGNER

"When the going gets tough, play dead!

Quando Omni Flunkus Moritati

 

[ccfowler]

Long life from electronic components is possible but not with the all-to-common cheapest possible alternative equipment selections. Use of all "Mil-Spec" components despite their greater cost is almost always necessary in industrial settings to deal with conditions that are likely to be encountered intermittently, if not continually (high or low ambient temperature, abnormally high voltages, etc.).

Beyond this, generous protection provisions for voltage spikes, high temperatures, hostile ambient materials (corrosive fumes, etc.) are important in assuring a long life for electronic components.

In cases where VSD's can provide significant economic or operational advantages, the cost of a durab VSD and suitable additional provisions is likely to be

 

[ccfowler]

Sorry about the dandy little key stroke error that provided my incomplete entry above!

In cases where VSD's can provide significant economic or operational advantages, the cost of a durable VSD and suitable additional provisions is likely to be relatively trivial in comparison to the benefits that it can provide. If relatively powerful advantages are not available from the installation of a VSD, it is probably best to avoid installing one.

 

[PUMPDESIGNER]

ccfowler - I like the way you put that:
"If relatively powerful advantages are not available from the installation of a VSD, it is probably best to avoid installing one."

If all providers of the VSDs (especially OEMs), would use that approach and speak that way then this discussion would never have taken place and I would have no gripes.

PUMPDESIGNER