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Speed of Centrifugal pumps installed in series.

WyElCo (Electrical)
10 Jul 11 0:35
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?
hydromech (Mechanical)
10 Jul 11 6:26
There is no real need to have them in constant sync during start up. Depending on the particular specs of the pumps, you might find the second pump start to turn as it is fed by the first.

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   
MikeHalloran (Mechanical)
10 Jul 11 12:01
Since the first pump's purpose is to supercharge the inlet of the second, you might want to set up the first pump to control that intermediate pressure.

 

Mike Halloran
Pembroke Pines, FL, USA

WyElCo (Electrical)
10 Jul 11 12:30
Hydromech; The pumps will be providing water to an irrigation pipeline that has a number of different zones. The flow rate is always going to be changing but the pressure has to remain constant. I will set the suction pump to start before pump 2 to ensure that pump 2 is primed before it starts (pump1 is manually primed). After start up should they be in constant synch or does it matter as far as efficiency and performance are concerned.

Mike; can you elaborate on you comment. Do you mean installing a pressure transmitter in between the 2 pumps?
MikeHalloran (Mechanical)
10 Jul 11 12:54
A transmitter, or at least a switch.

Mike Halloran
Pembroke Pines, FL, USA

BigInch (Petroleum)
11 Jul 11 0:52
The best way to run this is probably to eliminate the vfds.  Then you will always have constant speed and constant pressure with any given flowrate.  Irrigation systems are very convenient to run at a constant flowrate, BECAUSE you can vary the time that water is pumped to each section and so get precisely whatever water volume delivered to each section that you need.

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

Artisi (Mechanical)
11 Jul 11 1:31
Without having all the detail available re the pump performance curves, the system curve for the "system" it is a bit hard to make any firm recommendations, however - if you REALLY want to use VFD I would think that having pump 1 fixed speed and pump 2 variable would be more logical - unless the flow / pressure variation negates this arrangement - but this is something we don't know.  

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.)  

WyElCo (Electrical)
11 Jul 11 2:41
The primary reason that I am using VFD's is because there is only single phase service available in the area; so I am using the drives to create the 3 phase needed for the size of motors that are required. I could still set them up to run 60Hz all the time but i assumed there would be some energy savings if I utilized the variable speed. Also, the pumps will automatically stop when the customer turns off all of the zones and vice versa; the pressure is available as soon as he opens the next zone valve. If this process took any matter of time, and the pump was left running, the pump would heat up and do some internal damage would it not?
Oh, in case you're wondering, there is a small pressure tank to maintain psi while the pumps are off.
BigInch (Petroleum)
11 Jul 11 4:37
Fantastic use for a VFD.  smile
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

DubMac (Petroleum)
11 Jul 11 10:21
Watch that you don't blow the seals out of the second pump; always a concern in series pumping.

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.   
WyElCo (Electrical)
11 Jul 11 11:15
The reason for the 2 pumps in series is because the pump supplier wanted to provide the customer with 2 readily available smaller pumps vs. 1 large not so readily available pump. The first pump is sucking (lifting) water from a ditch, feeding into the second pump, and the second pump is pumping into the pipeline.  
BigInch (Petroleum)
11 Jul 11 12:10
Read as, "The supplier wanted to do at least twice the maintenance that he would normally be called out to do".

 

Let your acquaintances be many, but your advisors one in a thousand'  ...  Book of Ecclesiasticus

BigInch (Petroleum)
12 Jul 11 0:19
Your additional info indicates that your pipeline may not be going directly onto a field, but is a supply pipeline to an irrigation system downstream.  In that case it may be of some use to have pumps in series w/o VFD, if the pipeline is not running at a constant pressure, your pump station is acting as pressure source for the pipeline, and you can deal with a variable flow corresponding to the variable head your pumps are delivering, and you have no to little static head.  If you have significant static head, the VFDs probably arn't going to be part of the solution, if you have low flow pumping requirements.  If you must spend a hell of a lot of time pumping between 70 and 85% of your pump's design flowrate, Qo, the VFDs might work, but then you should probably have a smaller pipeline and design for Q1 that would be 85% of that old Qo, then you're back to eliminating the VFD again.  

Let your acquaintances be many, but your advisors one in a thousand'  ...  Book of Ecclesiasticus

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