Different Size Pumps in Parallel 2

[mls1]

I've recently integrated a new pump at a water treatment plant that is in addition to two existing pumps. All three pumps are VFD driven with speed control from a PID loop in a local PLC controlling discharge pressure at a constant value. The existing pumps ramped together and worked well but the new pump is twice as big so it really can't ramp at the same speed. So we operate in two general modes:

1.--The smaller pumps coming on first and ramp to maintain pressure. If they reach 100% and can not maintain pressure, they are fixed at 100% and the big pump is started with the PID output assigned to it.

2.--The big pump comes on first and ramps. If it gets to 100% then it is fixed at 100% and the small pumps come on and ramp with the PID output.

When switching the PID from small pumps to big and vice versa I manually correct the control variable to smooth out the transition. So far the system is stable and the transitions work well.

Here is the problem, if the plant is in mode 1 and the operators want to switch to mode 2 or vice versa there is a serious pressure transient while the pumps adjust to the new mode. I can see several ways to try to smooth this out but most are pure brute force. Has anyone else had experience with a system like this and what was your solution?

 

[EdStainless]

Valvecrazy, how about a link to the type of valve you are talking about. I still consult a bit with pump guys, this could come in handy.

= = = = = = = = = = = = = = = = = = = =
Rust never sleeps
Neither should your protection

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[Valvecrazy]

http://www.cyclestopvalves.com/ppt/Photos-2006.htm

 

[ccfowler]

quark,

The problem is the stability of pumps with relatively flat head vs. flow curves operating in parallel. A single pump with such a characteristic curve can still be a bit of a stability problem where system transients can be troublesome, but these are likely to be trivial compared to those in a system involving parallel pumps.

System transients can start some very interesting, substantial instantaneous changes in the flow distribution between such pumps when they are simply running across the line. Even if each ASD controller can anticipate these instantaneous load changes perfectly, they cannot cause instantaneous changes in the speeds of their respective pumps. The net result is multiple drive controllers hunting around amplifying the instabilties following a system transient.

 

[quark]

BigInch and ccfowler,

Thanks, I got it now. It is pretty clear with BigInch's explanation of dH/dQ.

To all,

I would like to wrap up my part of the discussion. I strongly opine that, a fluid handling system is to be carefully analyzed interms of system requirement, type of fluid moving device and the means of transportation, as you all do. The combined study can, only, give the optimum solution.

I can't agree totally that VFD manufacturers are misleading people. For example, the Danfoss energy savings calculator asks you to fill in the minimum head requirement (or the dP if you have a recirculation system) and then calculates the savings accordingly. The package supplied by ITT also has this feature. I am satisfied with the kind of systems I have.

mls1,

I wish you to overcome your problems. Do let us know about the proceedings.

 

[Valvecrazy]

You have to have something to compare to when figuring energy savings. Most pumping systems require a certain amount of head. When compared to a correctly sized pump running at BEP or even a pump being throttled with a valve, variable speed pumps do not save energy. The only time a variable speed can be more efficient is when the head varies considerably, which is not that common. Even then a pump system could be more efficient by utilizing different size pumps running at or close to BEP. Without considering these things, an energy savings calculator for variable speed pumps can be very misleading.

Of course, that is my point. Varying the speed makes sense that it would lower energy consumption. Throttling with a valve is counter intuitive. So, only those who really understand pumps would know that throttling reduces energy consumption the same as varying the speed.

Anytime you vary the speed of a pump to match the system curve, I can find a constant speed pump that will produce that head and flow while using less energy. With this in mind, I do not see how anyone can say a variable speed saves energy when used for constant head applications.

 

[ccfowler]

Quark,

Adjustable speed drives cannot fix the problem of poorly sized pumps and the resulting excessive power consumption. The best applications for gaining the greatest energy savings using adjustable speed drives involve properly sized pumps operating in a system involving simple circulation where large variations in the flow rate are common. As the shaft speed changes, the BEP flow follows a locus of points that will keep the pump working very near its BEP under all conditions, and the energy savings can be dramatic.

Now, consider an identical system with the exception that the pump sizing is well away from ideal. This can happen very easily where too much margin is allowed for uncertainties, or more likely, an ASD is applied as a retrofit to save energy using the old pump and motor. In this situation, the pump always operates well away from its BEP under all circumstances. Even though some energy savings may be measured, they will probably on be a small fraction of the savings that would be available.

Where the pump is working against a very substantial head in applications such as boiler feed pumps or reactor charge pumps, the potential energy savings of an ASD are often very hard to find.

The heart of the problem is the simplistic assumption that is commonly made: "adjustable speed drives always save energy." That is just simply not true, and it is not unusual for an adjustable speed drive system to consume more energy than a fixed speed pump with a control valve. It all depends upon the details of the specific application.

 

[BigInch]

It all depends upon the details of the specific application.

http://virtualpipeline.spaces.msn.com

 

[BigInch]

Whoops, not through yet.

More about pump curves and how they affect selection.

http://www.pump-zone.com/article.php?articleid=390

http://virtualpipeline.spaces.msn.com

 

[Valvecrazy]

Good article Biginch. Here is one I like.

http://www.cyclestopvalves.com/comparisons_13.html

 

[BigInch]

You're right, but I beat you to it. I already checked out that URL reference you gave and downloaded all of that good stuff I could grab... a few days ago.

http://virtualpipeline.spaces.msn.com

 

[itdepends]

I agree with BigInch above RE "details of the specific application."

e.g. If you're dealing with abrasive slurries then VFD's can offer advantages in reduced wear compared to dropping the pressure across a control valve.

 

[Valvecrazy]

There are lots of good applications for variable speed. I just don't believe that pumping fairly cool and fairly clean water at a constant head or pressure is one of them. Such a large percentage of pumping applications fall into this category, that considerable hype to favor the use of variable speed is widespread. In reality, variable speed pumps are not be the best choice for these applications. However, when stated often and with enough conviction, even people who should know better, start to believe that variable speed pumps have some kind of magical energy saving potential.

One of the few things I know of that is really "counter intuitive", is that when restricting the flow of a constant speed centrifugal pump, the energy consumed is reduced. Because this fact is "counter intuitive", and slowing the RPM of a pump just makes sense that energy used would be reduced, there is much confusion on the subject. Understanding these facts is the best sign of someone who really understands pumps. I am simply amazed at the number of people who continue to falsely state that variable speed pumps save energy. I am even more amazed at how few people truly understand that restricting a pump can reduce energy consumption as much as varying the speed.

 

[BigInch]

The power consumption curve should be down below the H vs Q curve. It should be obvious.

Until then, the one-eyed man is king in the land of the blind.

http://virtualpipeline.spaces.msn.com

 

[stanier]

Great discussion.

I find that VFDs are invariably specified by the process engineer because he is not sure of his numbers. It is akin to the spread of the word processor over a typewriter or PLC over a relay panel. The attention to detail by the process engineeer is not there or is just lack of confidence? How often I have heard "we will sort it out during commissioning". This attitude is so prevalent amongst the youger generation. Perhaps it is a sign of the times where engineers are expected to work 60 hours a week and study too.

In regards to the original posting you could always attend to the surge problem and retrofit a surge accummulator. At least it would protect the system from the operators. But that is fixing the symptom not the problem.

VFDs have a part to play in reducing maintenance from faigue, particularly if pumps stop and start frequently.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au

 

[BigInch]

I think its a lack of knowledge or maybe lack of time to do a proper optimized design, with the philosophy being, "We do any flowrate and save money too. Ha ha ha ha......

http://virtualpipeline.spaces.msn.com

 

[burnt2x]

Hi there!

As far as I can remember, what we did with very variable water demand was to install a tank filled with air to take the spikes off the hydraulics! We call them bladder tanks.

We never had busted lines from then on!

My two cents!

 

[pesho]

There are some unknown parameters such as what are the exact pump duties and where they fall on the pump curves, what is the amplitude of the transients and what would be acceptable regarding that matter? We have to keep in mind any multiple pump system is exposed more or less to transients when pumps are being switched on/shut down. I’m wondering what is the reason for the 2 different control modes? Having in mind each of the small pumps is half of the big one i.e. we have a total of 2 “big” ones, I think Mode 1 should be enough.
Most often similar pump systems are being operated by 2 VFD driven small pumps while the big one is fixed speed. The small ones always comes on 1st and 2nd and after they reach 100% they are “being replaced” by the big one. The shut down is in reverse order. And yes, the system has to have a properly sized diaphragm tank.

Ref to VFD vs throttling:
While the statements that VFD is not always the best (most effective) solution for every single application, it brings some unpleasant “side effects” and it can’t make up for a poor pump selection are correct, I would disagree with all people who simply want to toss all VFD's out from the window. VFD doesn’t necessarily save energy but sometimes it does. On the other hand throttling always wastes energy. To say the opposite is simply incorrect. How big is the energy waste and whether it is acceptable – this is the right question. Sometimes we may decide to pick up a little less efficient but cheaper and simpler system, sometimes we go with higher investment costs and make them up by lower service costs, etc. It always comes down the total cost of ownership i.e. what would be the total amount of all expenses by the end of the service life of a certain piece of equipment including the initial investment (design, installation and start-up), power consumption, maintenance, etc. The perfect, universal product that could be used everywhere has yet to be designed and manufactured and until then we have to make our selections on a case by case scenario.

 

[Valvecrazy]

"VFD doesn't necessarily save energy but sometimes it does. On the other hand throttling always wastes energy. To say the opposite is simply incorrect." Pesho

This statement is simply incorrect. If you choose a pump with good brake horse power characteristics, there is very little (IF ANY) power savings with a VFD over throttling with a valve. When maintaining a constant outlet pressure or TDH, throttling can reduce the power consumption just as much as a VFD. Because you lose head by the square of the speed, the RPM cannot be slowed down enough to save any energy or you cannot maintain the TDH required. See the pump curve below.

When 148' is the required TDH, this pump can only be slowed from 3450 to 3278 RPM. Any slower and it cannot produce 148' of head. Notice that the power curve of this 15 HP pump when used with variable speed, follows very closely to the power curve of the throttled pump. There is only 1 or 2 HP difference anywhere along the curve. When you add back in the parasitic loses of a Drive, and the loss of efficiency for running on a pulsing DC power instead of sinusoidal AC power, there is basically no difference in power consumption between VSD and throttling. So if throttling always waste energy, then a VFD also always wastes energy. Running a pump at BEP is the only way to NOT waste energy. Anytime you slow the pump down or throttle it back, you are using more energy per gallon than if running the pump at BEP.

I have seen throttling reduce energy consumption many times. When compared to cycling rapidly or dumping excess water, throttling can save considerable energy. I have never seen a constant pressure application where VFD could save more energy than throttling. With the many negative side effects and complications of VFD controls, any energy saved usually comes at a higher cost of equipment, repairs, technical assistance, and other things.

The fact that power required decreases when throttling with a valve is "counter intuitive". The fact that power required decreases with a reduction in speed is not "counter intuitive". This makes it hard for many people to understand that throttling with a valve can save as much energy as a Variable Speed Drive, but it is still the truth.

 

[BigInch]

There is a BIG difference between optimization and waste. Nobody argues that throttling costs some energy, but that amount "wasted" just might be less than what the VFD uses in parasitic energy alone, not to mention initial cost and maintenance.

http://virtualpipeline.spaces.msn.com