Pumps in Parallel Project 4

[packlbn]

I have a parallel pump projet where the application is to supply a packaging plant with general water. The system in place is a parallel systme with one 15kw pump in parallel with a 22kw pump without any monitoring. These pumps are started manually. I have gone though a couple of threads like thread407-191266 and have discoved that pumps in a parallel system is not as straight forward as put out to be. I had the idea of using identical pumps and swiching between them using a signal VSD when ever one fails the other would come online. But with the fluctuating demand, depending on how many machines are running, and only having the option of monitoring the pressure on the discharge side of the pumps to regulate the VSD the pressure transmitter would react to slowly to regulate the VSD - this would cause hammer? How would i go about selecting the best option for this application?

 

[Valvecrazy]

So the VFD’s of just 5 years ago are now completely obsolete, just like the ones produced now will also be obsolete very soon. This same cycle has been happening with Drives for decades, and will continue to happen for decades, because there are things mother nature just won’t let you fix. You are right that the pump specific Drives have more features and are easier to set up than Drives of a year or so ago. However, they still don’t save enough energy or last long enough to justify their use on most pump applications. Drives do allow people who do not understand pumps, to band aid a badly designed system. Fresh water pump systems have been known to last 20, 30, even 50 years without problems. This will never happen with a Drive controlled system. I can’t help but think, that is exactly why pump companies and many other people are pushing Drives like they do. Job Security at the end users expense.

All those protection features can also be had with many different brands of pump protectors. Even with Across The Line controls, flow rate can be calculated by power consumption.

I think inviting a child molester to baby sit is a very good analogy for a Drive on a fresh water pump.

 

[BigInch]

Ya. I don't know where that came from, but it'll probably wind up being the very best analogy I'll ever think of. Well, it'll make the top ten for sure.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[itdepends]

I've said it before and I'll say it again- it depends on what your pumping. Discharge control valves may be good for clean fluids- particularly with high static/low frictional heads- but they don't last very long on abrasive slurries and aren't the best fit on systems with high flow variation (with associated large changes in frictional pressure loss).

Just because a system has a reasonable range of operating conditions- doesn't necessarily mean it's badly designed.

 

[BigInch]

Can't argue with the specific cases, only in the general and trivial manner that VSD seem to be sold these days as the Snake-oiled Holy Grail for all pump problems.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[ScottyUK]

ROFL, that sounds like the way valves are being presented by certain people in another thread...

VFDs are mis-sold into unsuitable applications, valves are mis-sold into unsuitable applications. Selective truths are presented by VFD people and by valve people as 'evidence' that their solution is 'best'. As long as salesmen make their living from commission based on sales this will continue. Both technologies have their place, depending on the application.

Caveat emptor!


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If we learn from our mistakes I'm getting a great education!

 

[BCEagle]

Valvecrazy said: "You are right that the pump specific Drives have more features and are easier to set up than Drives of a year or so ago."

Kudos to Valvecrazy for showing he is keeping up with some of the latest progress in drive design. That is step in the right direction. Or perhaps towards the dark side, depending on your perspective.

 

[MortenA]

Biginch

I think your point is interesting although i accept your reservations regarding certain applications - however i would like to know if your opinion towards use VFD is stricly economical, control optimization or a combination?

Best regards

Morten

 

[sterl]

Somebody got close to saying it but....

Drives need to match system requirements; and controls need parallel system responses.

A high differential pump required because the system has a high static head or contains spring loaded in line components or even thermal control type devices do not make good candidates for VSD's. And a turbo machine that has a "flat" head to flow curve is not going to fit a circuit with a high dynamic component operating over a broad speed range.

A high flow circuit where most of the required pressure difference is dynamic or frictional; along with a pump having considerable slope to the V to h curve, can do well on a VSD providing slowing it pushes it toward the sweet spot (BEP). And the pump is full of impellor to begin with.

Fans often perform a lot like the latter. The characteristics of pump circuits vary a whole lot more.

$0.02 & done.

 

[Artisi]

Just came acros this article regardingh VFD - worth a read

http://www.pump-zone.com/articles/581.pdf

 

[BigInch]

Lotta good stuff on pump-zone. Be sure you scan it clean.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain