Pump curve

[AgSilver]

In front of me is a pump curve with some specs.
Duty flow: 1600 gpm
Duty head: 120 psig
NPSHR: 11.3 psi

Now that I have a different flow rate, I want to check if this pump is still ok. The only information I have is that the water tank is exposed to atmospheric pressure, its elevation relative to the pump, and the operating temperature (to get the relevant vapour pressure). Since I know nothing about the velocity and piping configuration, I decided to lump (Hv - Hf) = K*Q^2.

NSPHA = Hatm + Hz + Hv - Hf - Hvp

Hatm = head developed from atmospheric pressure
Hz = elevation head (surface of liquid is above pump)
Hv = velocity head
Hf = friction losses
Hvp = vapour pressure of water at specified temperature

Thus,
(120 psig) + 14.7 psi = 14.7 psi + 13 psi + K*1600^2 - 0.26 psi

The 13 psi comes from the fact that there is 30 ft of head (water) and the 0.26 psi is the vapour pressure of water at the operating temperature.

Rearranging the above equation,
I have K = 107.25/1600^2

Again, since I know nothing about the piping configuration, I will assume that K is the same at the different flow rates.

To find to the new NPSHA for a different flow rate,
NPSHA = 14.7 psi + 13 psi + K*(Q/1600)^2 - 0.26 psi

Am I doing this wrong?

 

[AgSilver]

I consulted a colleague just now and I was told that I could just move across the pump curve and just put the operating point at 1000 gpm (our new flow rate going through each pump) and 120 psi because our system is essentially the same, only the flow rate changed. Does this work?

 

[SNORGY]

I would approach it a bit differently.

Assume about 15% rise in head to shut-off from the first point. Then the increase in head at the new duty point would be:

[0.15 x (1-1000/1600)] x 3/2 x 120 psig = 10.125 psig

("3/2" to adjust for "2/3 Rule" for concave down parabola)

Checking for sanity:

15% x 120 = 18 psi rise to shut off
going to 5/8 x original flow, if rise to shut off was linear, would be 3/8 x 18 = 6.75 psi; I estimated 10.125 psi; seems believable to me...

Thus:

New P = 130 psig; NPSHR has gone down as well, since you have moved towards the left on the characteristic curve, but is more difficult to quantify. I think the NPSHA stays the same.

With great hesitation, I hit the "Submit Post" button...

Regards,

SNORGY.

 

[JohnGP]

AgSilver,
I'm confused with what you are trying to do with the maths, but then I'm easily confused.

If the pump was operating successfully with the duty point at 1600gpm at 120psig head, then NPSHa is generally not going to be any worse at a lower flow. If the "system is essentially the same", then to have the pump operate at 1000gpm you would need to increase the system head as SNORGY is intimating, by shutting in a valve for example, or alternatively reduce the pump speed, or reduce the impeller diameter.

The pump will always want to operate at the point where the system resistance curve meets the pump curve.

 

[vk1975]

AG silver

Hope the changed flow conditions are above the minimum flow values specified by the manufacturer. As I understand that flow operating point is shifted towards left and hence NPSHR should not be a problem. Lower flow requires less NPSHr for the same pump model

Regards

VK

 

[rmw]

First, as vk1975 states, moving to the left on a centrifugal pump curve is moving towards a more favorable NPSHr point, so you are good there.

My question is how did you get there? You don't just move on a curve. The pump flow changes because the system resistance curve changed. So something will have or will have to change to move you to the different operating point on the curve.

As long as that change isn't on the suction side of the pump, you should be OK.

Your pump curve should also have a corresponding line on it for NPSHr. If it doesn't, ask your vendor for it, they have it. And if you don't already, require a NPSHr curve as one of the test curves produced when they produce and test your pump.

rmw

 

[LeoChem]

AG Silver

If you have the pump curve you already have everything. As you realized by yourself you just need to move on the pump's curve to discover which will be your new pump Head (I believe is what you call NPSHA).
But you need to observe some details like:

1) Is suction head below NPSHr?

Pump curve must have a NPSHr curve as rmw said (wrote)

2) Is the head too close from pump shut off?

If you are working with a flow near the shut off you may have some operational problems tha may danificate the pump

3) Is the pump working efficiently?

Every pump has a efficiency curve and is better for the operation if it's working in the best efficiency.

Maybe a pump working with a flow of 1600gpm is working efficiently, but the same pump is not efficient with 1000gpm or 2000gpm. The solution in this case is to change the impeler diameter, the number of pump stages (someone said this but didn't find who).



Hope I could help

 

[AgSilver]

Thanks for your help guys. My boss confirmed that all I had to do was move along the pump curve (impeller size was determined from the last phase of study) to the new flow rate and determine how much pump head we have and see if it is above the expected head loss across our system.

 

[JohnGP]

AgSilver,

You would know exactly (or at least somewhere near) where you would be operating by determining the system resistance curve for your system - see something like

http://www.idc-online.com/pdf/Papers/Bernard_DaCruz.pdf

By plotting system resistance on the pump curve that you have, you will see where the pump is likely going to operate, and you can see whether you need to introduce some form of control if it wants to provide more flow than you want.
Cheers,
John

 

[Artisi]

Just a quick look at the problem, what I don't understand at this point is how the 120psi remains the same with a drop in flow rate - please explain.

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.)

 

[SNORGY]

Sounds a bit like some discharge throttling going on, with the pressure measured downstream of the valve. System pressure control loop?

Regards,

SNORGY.