7 Pumps in parallel - a bad design???

[BronYrAur]

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?

 

[LittleInch]

Needs a schematic. What you describe doesn't sound like a primary secondary to me but can't see what you're looking at.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BronYrAur]

It is primary-secondary. Extremely simplified diagram attached. 10 chillers on the primary loop with a decoupling bridge to the 7 secondary pumps in parallel. Each has a VFD to maintain a DP way out in the system.

 

[EdStainless]

So all pumps that are running run at the same speed.
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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[cvg]

I believe there are 8 parallel in this installation.

http://www.mvn.usace.army.mil/Portals/56/docs/PAO/FactSheets/PermanentPumps_Jul13_FINAL2.pdf

 

[BigInch]

First of all, are all the pumps the same, or do they have different flow and head capacities?

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

 

[BronYrAur]

I don't have any sizing information at my fingertips but all pumps are the same. They have been installed for years. The person who made the comments is new to the facility. My question was more of a general one that is there a certain point we're too many pumps in parallel becomes problematic in terms of cavitation? I personally think the guy making the comments just likes to hear himself talk. But it wasn't really the right for him to challenge him on his comments. I don't see a problem especially since they all have drives. Pumps that are operating ramp up and down simultaneously. And then at some point which I think is 80% on the drive the system will bring another pump on. Similar scenario for bringing a pump off. As I mentioned in the original post, we have never had to run more than three or four at the absolute most. The load will be changing such that we will probably need to run 6. I was just trying to do a little research and see what might cause this so-called cavitation problem.

 

[LittleInch]

The devil here might be in the detail about flowrates, pressures and how the pumps are actually connected.

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.

 

[BigInch]

So what's the problem? You going to rebuild it using 3 pumps or something? What's the guy's point squawking off now after it's been running for so many years? Advertising his smarts?

Reaction to change doesn't stop it

 

[Artisi]

So long as the total head on the pumps with 6 or 7 in operation is within the pump capability there is no real problem. As for the comment about cavitation, many people wouldn't know what cavitation is - even if it bit them on the arse.
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.)

 

[racookpe1978]

OPen up and inspect the check valves for EVERY pump BEFORE running the set as a whole.

A large leaker (or single stuck-open) check valve will cause significant recirculating flow and pressure surges.

Small one will get worse with time.

 

[BronYrAur]

Thanks all for your comments. So it sounds to me like there is no inherent stupidity in having 7 pumps in parallel versus only two or three. That's all I was trying to get at regarding this individual's comments. Since he offered nothing to back up his I did not take them very seriously to begin with. Just making sure I wasn't missing something obvious.

 

[Artisi]

What would be / should be of interest to you is the head on the pumps at current condition and the head required at the future (7) pump condition. The head loss due to friction increases as Q^2, double the flow and the friction head increases by a factor of 4. Worth looking into.

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.)

 

[BigInch]

Or there are many (7) circuits all with similar head loss-flow relationships. In which case 7 pumps makes perfect sense.

Reaction to change doesn't stop it