7 Pumps in parallel - a bad design???
7 Pumps in parallel - a bad design???
(OP)
I'm involved with a chilled water system that has 7 identical pumps in parallel. It is a primary-secondary chilled water system, and these pumps are on the secondary. They all have VFD's that maintain a differential pressure out in the piping system. I don't know exactly how they are staged on and off. I think once a certain percentage on the drive is reached (e.g. 80%), another pumps in energized. something like that.
To date, no more than 3 or possibly 4 have ever run together. The load was never large enough. Future load is coming online soon, and I had the "privilege" to listen to an outside consultant rant about how poor the installation is. This person had never seen 7 pumps in parallel before and said that it was asking for trouble. He warned that there will most likely be cavitation and other issues as more pumps come on. He didn't offer any backup to these claims.
I fully understand the limitation of parallel pumps as you approach the maximum head. You essentially get no additional flow by turning on an additional pump. But, I don't think he was referring to that. I was supposed to just listen at this meeting, so I did not press him for an explanation.
Any thoughts on what he could be talking about? What are the dangers of too many parallel pumps .... especially with VFDs?
To date, no more than 3 or possibly 4 have ever run together. The load was never large enough. Future load is coming online soon, and I had the "privilege" to listen to an outside consultant rant about how poor the installation is. This person had never seen 7 pumps in parallel before and said that it was asking for trouble. He warned that there will most likely be cavitation and other issues as more pumps come on. He didn't offer any backup to these claims.
I fully understand the limitation of parallel pumps as you approach the maximum head. You essentially get no additional flow by turning on an additional pump. But, I don't think he was referring to that. I was supposed to just listen at this meeting, so I did not press him for an explanation.
Any thoughts on what he could be talking about? What are the dangers of too many parallel pumps .... especially with VFDs?





RE: 7 Pumps in parallel - a bad design???
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: 7 Pumps in parallel - a bad design???
RE: 7 Pumps in parallel - a bad design???
Not a bad system, it should let the pumps always run in an efficient range.
That is if they are programed correctly.
Are they programed to not turn on adjacent pumps?
What is the inlet pressure supplied to the pumps?
And is the inlet piping very large?
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P.E. Metallurgy, Plymouth Tube
RE: 7 Pumps in parallel - a bad design???
http://www.mvn.usace.army.mil/Portals/56/docs/PAO/...
RE: 7 Pumps in parallel - a bad design???
The only real disadvantage is that they probably cost more in space, wire and pipe to lay out 7 as opposed to 2, 3 or 4 larger units, but they might also be small enough that each unit could fit in a submarine, or underground pump station. Where are they located? Is that important to number and size?
If they are all connected to the same circuit, you probably don't need the VFDs, as you will have a pretty fine control of flowrate simply by turning units on or off.
Think of the reliability you have at anything but maximum flow. At 50% flow you could have any of up to 3 units down and still be able to makeup the flowrate.
Reaction to change doesn't stop it
RE: 7 Pumps in parallel - a bad design???
RE: 7 Pumps in parallel - a bad design???
I would say thought that there is no systemic reason why this is a issue, but what appears to be more than 6:1 turndown ratio is quite high and having multiple parallel pumps is often the source of some grief as no identical pump is truly identical or that each path through the pump is equal either.
It also depends where on the pump curve the system normally operates and how flat it is, but that's a different issue
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: 7 Pumps in parallel - a bad design???
Reaction to change doesn't stop it
RE: 7 Pumps in parallel - a bad design???
Given the right information, the operating head for an increased number of pumps in operation can be calculated.
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: 7 Pumps in parallel - a bad design???
A large leaker (or single stuck-open) check valve will cause significant recirculating flow and pressure surges.
Small one will get worse with time.
RE: 7 Pumps in parallel - a bad design???
RE: 7 Pumps in parallel - a bad design???
It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)
RE: 7 Pumps in parallel - a bad design???
Reaction to change doesn't stop it