Speed of Centrifugal pumps installed in series.
Speed of Centrifugal pumps installed in series.
(OP)
I am setting up Variable Speed Drives's that are running 2 centrifugal pumps that are plumbed series (Discharge of Pump 1 piped to suction of Pump 2). Both pumps are identical. Pump1 is sucking water out of an irrigation ditch and the discharge is plumbed directly into the suction of pump2. The pump supplier did all of the head calculations and this configuration is supposed to be adequate for the customers demands. My goal is to set up the drives to maintain a certain pressure on the pipeline. Should the speeds of both pumps be synchronized with each other or should pump 2 ramp up to and assist only after pump 1 has reached maximum speed?





RE: Speed of Centrifugal pumps installed in series.
In any event, it is the seals that you need to protect. They will be damaged if you run them dry for too long.
If it is possible to start them together, then do that. If you need to delay a pump starting, then start the lower pressure pump first, then start the second pump.
Hydromech
RE: Speed of Centrifugal pumps installed in series.
Mike Halloran
Pembroke Pines, FL, USA
RE: Speed of Centrifugal pumps installed in series.
Mike; can you elaborate on you comment. Do you mean installing a pressure transmitter in between the 2 pumps?
RE: Speed of Centrifugal pumps installed in series.
Mike Halloran
Pembroke Pines, FL, USA
RE: Speed of Centrifugal pumps installed in series.
VFD use with irrigation is pretty much a scam, unless you have some SUPER UNIQUE scheduling requirements. Exotic plants, or the Hanging Gardens of Babylon? It's no surprize that the pump supplier has I suspect pretty much done all of this design, you being electrical and all, right?
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus
RE: Speed of Centrifugal pumps installed in series.
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: Speed of Centrifugal pumps installed in series.
Oh, in case you're wondering, there is a small pressure tank to maintain psi while the pumps are off.
RE: Speed of Centrifugal pumps installed in series.
I'm not electrical, so I'm not speaking to that, but it sounds like that doesn't come for free. What's the catch for doing that?
Most people assume there will be energy savings using a VFD, but usually it isn't true. I'd advise you to just open the valves to various sections and let whatever flow that wants to go there go then turn off the section valve when enough has been delivered. You can check later whether you can save any money by operating it with a VFD. I would tend to doubt it, but not to say it's totally impossible.
An operator should open the next segment valve first, then close the previous segment's valve. Always better to not stop flowing completely when making transitions between modes of operation. Put that in the system operating manual to avoid damaging the pump from overheating!
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus
RE: Speed of Centrifugal pumps installed in series.
Wish you could enlighten me on why a series configuration was selected over a parallel setup; which is almost always used in "constant pressure" variable flow booster systems.
Is there an issue getting suction feed to the 2nd pump?
The flow rate of pump 1 will always be exactly equal to the flow in pump2 (mass in, mass out). Adjusting the speeds independently seems like it would have the pumps continually "searching" up and down the performance curve.
Artisi is right, you should provide some flows, heads, performance curves if you want a meaningful analysis.
RE: Speed of Centrifugal pumps installed in series.
RE: Speed of Centrifugal pumps installed in series.
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus
RE: Speed of Centrifugal pumps installed in series.
Let your acquaintances be many, but your advisors one in a thousand' ... Book of Ecclesiasticus