Determining pump flow from pump curve and pressure gauge

[mae1133]

We have an existing pump and pump curve. We know the physicals on the suction side of the pump, but the discharge is buried with no records of the physical layout, sizes, etc. We do have a discharge gauge installed about a foot above the pump discharge reading 16 psi. Knowing this, together with the pump curve, can the flow be estimated? Don't we also need a suction gauge to determine the differential?

 

[LittleInch]

So can you post the pump curve?

You clearly have 14psi diff head. Depending on the shape of the curve you can see what accuracy level when you apply say a +/- 2psi margin to account for all those things Artisi states.

I must say this sounds like a very low diff head pump. For a 3" pipe I can't see this being accurate to any degree for that size of pump or that the actual pump was tested, but a lot depends on the shape of the curve. For low power small pumps even things like types of seal and losses in the shaft power can be significant which they wouldn't be on a smaller pump.

Also any voltage and current measurements for the motor?



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

 

[mae1133]

The pump curve is attached. I have asked for additional info, however what I stated above is all I have received. So, I'm at a bit of a loss. Using the info I have does not even seem to fit with the pump curve. If I trust the discharge pressure, the pump would be running off the curve.

 

[RVAmeche]

What's the fluid SG?

 

[mae1133]

Water

 

[RVAmeche]

Static Head = 5 ft
Assume Inlet Losses = ~5 ft
Suction Head = ~0 ft
Discharge Head = 16 psig = 37 ft

Based off those assumptions it'd appear to be ~175 GPM so almost off the curve.

 

[LittleInch]

Ok, you're at a steep part of the curve so even taking a few psi one way or the other it looks like you're between 160 to 190 US GPM.

but you are right at the far end of the curve. Not a good place to be, but the end of the curve is just where they kind of give up measuring. At this size your motor is probably over sized so is happy to run within it's max amps / temperature and the pump just merrily pumps away, maybe making a little bit of noise, but will work.

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

 

[danschwind]

mae1113,

The new system you are planning to install will have any sort of flow measuring device? Couldn't you get away with oversizing your new pumping system by some margin and installing some sort of flow control device (even a manual valve to introduce some pressure drop)?

Edit: nevermind, I didn't read your previous comment regarding the VFD. I think that with the current estimatives you could maybe draw two system curves (considering that you know your static head) and have some sort of operational range for your new pump.

 

[mae1133]

Dan: My plan is to put the new pump on a VFD to allow for some flexibility. If needed, we will throttle a manual valve. The new system will have a magmeter for flow measurement and pump control.

 

[Artisi]

I believe you are floundering around snatching at straws.
To start guessing what the flow might be you certainly need to collect more data before making any definite plans, or just duplicate the pump you have and leave it at that.

Where did / what is the curve showing the various head notations - where did it come from?
What is the power input / motor maximum power rating, in amps?
What is the service life of the pump, has it even been overhauled or impeller diameter changes?
What is the internal condition of the pump?

The pump could he operating any where within its capability in regards to flow at the pressure you have advised.
It is only an assumption that the pump is operating near the end of curve, the impeller could be worn, corroded or similar.

A late thought:
Where does the supply water come from, can the inflow be measured or a draw-down test on the supply tank undertaken to establish the flow rate.

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.)

 

[mae1133]

I think you have all provided everything you can based on the information I currently have available to me. Thanks for all your assistance and thoughts.