Pump in parallel - Fluctuations in flow 1

[Liad]

Hey all,
We are running a system with 3 positive gear pumps in parallel that installed at the outlet of a tank, like the picture below:

The left pump is the closest pump to the tank.

The outlet of the tank is 1.5", going up to suction of the pump is 3/4" for each pump and the discharge pipe is 1/2".
On each pipe coming out of the pump we installed an very accurate mass flow meter.

We are facing an issue when we are working in 2 or 3 pumps. when running a single pump the flow is stable.
When working with two pumps, the pump that closer to the tank having fluctuations in flow, while the pump that is further away has a very stable flow.
When we are working with 3 pumps, the pump that closest to the tank has fluctuations and the other 2 are very stable.
The flow is the same in one/two or three pumps.

I would be happy for your opinion on the subject, I assume that the reason is losses in head suction, but from calculations I have made there should be no problems.
Thanks in advance.

 

[LittleInch]

Mr 44 - keep up. He's already said that the total flow stays the same but he reduces the speed of his gear pumps (servo motor) to adjust the flow.

So its 10lpm for one pump or 5 lpm each for two pumps.

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

 

[1503-44]

Oh. They want to do maximum maintenance. Right.

A black swan to a turkey is a white swan to the butcher.

 

[Liad]

Hey all,
I will appreciate your opinion on adding booster pump as I mentioned above:

Beside some mechanical changes, do you think adding a booster pump at the outlet of the tank on the 1.5" pipe will help?
If yes, I will be happy to know what do you think are the parameters required to take to select the appropriate pump.
I was thinking of taking a centrifugal pump with a flow beyond the maximum required in the system against max pressure in this line. In addition, to connect a return line to the tank.

Thanks
Liad

 

[LittleInch]

I actually don't think it will do much for you. But if you do then yes a pump sized for max flow at whatever head you want is a good idea. Unless you have long periods of no flow then the return line can cause issues as well in terms of control and passing fluid when it shouldn't. Also this then adds tot he head of the gear pumps.

The easiest and cheapest by a long way is simply to make the flow path into the branch much smoother and see if it solves your problem.

Have a really close look at the branch and send us a photo. Mt suspicion is your branch pipe extends a long way into the header pipe.

Fix that first.

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

 

[1503-44]

Right. Don't add a pump.
You'll probably get the same problem, just at higher pressure, or worse.
Better to work on fixing what you have already.

A black swan to a turkey is a white swan to the butcher.

 

[bimr]

At 68 F, water has a specific gravity of 0.9982, a viscosity of 1 centipoise (cP) and a vapor pressure of about 0.3 pounds per square inch absolute (psia). At 80 C, water has a specific gravity of 0.9716, a viscosity of 0.355 cP and a vapor pressure of nearly 7 psia. Hot water is lighter, thinner and less lubricating and vaporizes to a greater degree.

Link

 

[Snickster]

The only thing I can think of is that there is air in the fluid which enters the suction line of the first pump pumping.