Pumps in Parallel Project
Pumps in Parallel Project
(OP)
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?





RE: Pumps in Parallel Project
Two identical pumps generally work well operating together, if the headers are also hydraulically balanced. Don't attach a small diameter header and place one pump on the end and the other in the middle of the headers and things should work out just fine.
You may not need a VSD, if your flowrates do not vary much. If the process flowrate varies 50% or more, a VSD may be the right solution, otherwise a pressure control valves may offer more benefits.
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RE: Pumps in Parallel Project
You can still alternate if you want but, staggering the pressures to set up a lead and a lag pump is a much simpler set up. Alternating usually wears out both pumps at the same time. Staggering will leave less hours on the secondary pump, which makes it more dependable as a spare.
RE: Pumps in Parallel Project
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RE: Pumps in Parallel Project
RE: Pumps in Parallel Project
RE: Pumps in Parallel Project
RE: Pumps in Parallel Project
With two pumps and only one Drive, you still have all your eggs in one basket. It is more likely the Drive will be what trips out instead of a motor. You can use an Across The Line bypass for the Drive. However, when the Drive malfunctions and you are left running on bypass, you will have no control over the discharge pressure.
With an Across The Line starter and a control valve for each pump, you would have a dependable primary system and a dependable backup system. Ether valve would be capable of controlling the downstream pressure while any or every part of the other pump system is removed for repair. Centrifugal pumps can be so dependable when running ATL with Valves as controls, that the entire system can last 20 to 30 years with no problems.
"From past experience, small drives typically fail in eight years and are
replaced rather than repaired."
http://w
I personally find smaller drives to have more like a five year average life. Therefore, you will have replaced the Drive from 3 to 6 times instead of the system having lasted 20 to 30 years without any problems. I don't think a Drive could ever save enough energy to pay out in a case like this, not to mention cost and headaches of the down time, and all the other associated problems that go along with Drive control.
There are those that would have you believe that a Drive is a "perpetual motion machine". I believe a Drive is a "perpetual maintenance machine".
RE: Pumps in Parallel Project
RE: Pumps in Parallel Project
RE: Pumps in Parallel Project
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RE: Pumps in Parallel Project
The drive has its own main contactor and overload. After the dirve each pump has its own contactor and overload. Under the condition that the overload of one pump trips and not the drive then the standby pump will start.
Artisi
Why does the flow return to the source and the impeller turn the wrong direction... because there is a pressure difference but i'm only electrical guy.
RE: Pumps in Parallel Project
High pressure in the discharge header will tend to reverse spin a stopped pump.
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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain
RE: Pumps in Parallel Project
RE: Pumps in Parallel Project
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
RE: Pumps in Parallel Project
You might want to check the valve / VFD argument in thread237-205555: Harmonics and VFDs regarding the pro's and con's of each. There are some sound arguments from both sides, plus some quasi-science from those with a vested interest in selling their products. Draw your own conclusions.
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If we learn from our mistakes I'm getting a great education!
RE: Pumps in Parallel Project
Very simple rule for VFDs VSDs..
IF FLOW VARIES BY 50% OR MORE
A VFD might be more efficient, but ONLY IF
required head varies appx with flow squared.
IF NOT
You don't need to be thinking about VFD.
You will have a more efficient system using
a properly selected pump & control valve.
Be very suspicious of anybody who won't agree with the above. You may find an occasional example that breaks this rule, but you could probably count them all on one hand after you retire.
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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain
RE: Pumps in Parallel Project
"BigInch (Petroleum) 6 Mar 08 3:34
Artisi, if I may,
High pressure in the discharge header will tend to reverse spin a stopped pump.
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"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain"
No argument from me on this one- actually I missread / interperated the posting from - packlbn - I was just pointing out that a NRV has two functions in this particular installation, the main one being to stop flow back to the source and to prevent reverse rotation of the impeller which is realy only a problem if the unit is called to start while running in reverse (as in this installation)
This is what was said by packlbn anyway just my poor reading - bad hair day.
RE: Pumps in Parallel Project
Pump Protection VFDs can add add a whole new level of control, and can provide opportunities to catch small system upsets before they become big and expensive pump problems. If you haven't looked at the latest pump specific innovations several manufacturers have added to their VFDs, it is worth your time to look at them.
And yes, VFDs are not appropriate for all applications.
RE: Pumps in Parallel Project
Packlbn....If the "buffer" tank you are drawing water from is below ground causing a suction lift, and you use the same suction line for both pumps, you should put the NRV or check valves on the suction side of the pumps. Otherwise the pump that is running could draw air from the mechanical seal of the pump that is not running.
RE: Pumps in Parallel Project
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.
RE: Pumps in Parallel Project
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
RE: Pumps in Parallel Project
Just because a system has a reasonable range of operating conditions- doesn't necessarily mean it's badly designed.
RE: Pumps in Parallel Project
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
RE: Pumps in Parallel Project
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!
RE: Pumps in Parallel Project
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.
RE: Pumps in Parallel Project
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
RE: Pumps in Parallel Project
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.
RE: Pumps in Parallel Project
http://www.pump-zone.com/articles/581.pdf
RE: Pumps in Parallel Project
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