Inline VFD duplex booster pump to house pumps

[arena7724]

There is a situation at this building where they installed new duplex booster pumps(VFD) in series (Pumps A) with these old booster pumps (Pumps B). Pumps B pump the water to a roof tank. Pumps A pump water to these Pumps B and to the first 6 stories of the building. The fuse keeps failing in the new pumps (Pumps A). The pump manufacturer thinks the suction line between pumps A & B is not getting enough volume and/or pressure when pumps B turns on and pumps A cannot satisfy the demand so it is overworking itself causing the fuse to fail. I have come up with these possible solutions:

1. There is an electrical/controls problem causing the fuse to fail requiring a simple electrical fix.

2. I noticed they installed a by-pass line with an isolation valve closed so the water only travels through the new booster pumps (Pumps A). I think maybe installing a check valve in lieu of or next to this valve and opening this valve could certainly resolve this issue.

3. The new pumps (Pumps A) are simply not large enough to handle the demand of both the old booster pump (Pumps B) and the the demand of the first 6 stories of the building. The new pumps (Pumps A) either need to be replaced or the old pumps (Pumps B) will need to be replaced with a new dedicated pipe line running to them prior to the inlet of pumps. This way pumps A only feed the first 6 stories as maybe originally intended.

Personally I am leaning towards 2 or 3 but I would like to see from other engineers eyes what other causes and solutions are out there that I may be overlooking. The pumps are centrifugal and pump domestic cold water (55 deg F).

 

[DubMac]

I could not open your link, so I don't know what was in there. Hopefully it was full of juicy data; pump curves, flows, heads, motor sizes, flow diagram, etc........

With just the generic, somewhat abstract situation you've presented, anything more than a good guess just isn't possible.

As a guess..... Pumps A could be flowing "off the end of the curve", or as you state "are simply not large enough to handle the demand of both the old booster pump (Pumps B) and the the demand of the first 6 stories of the building".

Then again it could be #1 or #2, or a myriad of other reasons.

 

[LittleInch]

Try using the upload to engineering.com option.

It should be fairly simple though to see if the rated flow of the A pumps is bigger than the B pumps. If you can't monitor current though it will be difficult to see if 1) is happening or not..com

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[arena7724]

I attached the photo. Hopefully you can see it now. It is just a flow diagram.

 

[rotw]

Seems that adding up pump A and pump B, lead to too much head capability versus the system curve at some operation condition (level of the tank, flow rate in the building, combination) or even at stable operating condition, which may lead to running at the right of the total curve (total curve of pumps in series) whereby if you check the flow of each pump individually, it might be still okay for one pump but not the other which would be operating at its right limit exceeding its motor capability.

Do you have a clear understanding of your system curve (min case, max case) versus the different operation conditions ?

Difficult to not speculate without some diagram or sketch.

 

[LittleInch]

Sadly the diagram adds little to the information supplied. To make any sense of this we need the pump curves, the system curve from pump b to the tank and some idea of what the new vfd pumps are being controlled by, pressure or flow?

Inherently this looks wrong and if the old pumps were doing their job, then adding head by means of the discharge of pumps A, then they will start to pump a lot more water, maybe more than pumps A can handle. Who knows?, supply some data and we might be able to give some help.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Compositepro]

A blown fuse is not a fuse failure. The fuse is doing exactly what it is designed to do. This appears to be a case of design failure.

Pump motors are generally overloaded due to too much flow through the pump. Throtle the discharge of pump B. It is supplying a storage tank so you do not have to fill it so fast.

 

[DubMac]

Also keep in mind that your B pumps are seeing the A pumps discharge pressure on their suction side. You could have a lot of seals/packing leakage and problems on B pumps if the pressure is too great.

Usually a sign of system design flaw when you see centrifugal pumps in series...not always...just mostly.