Sump pump discharge head 4

[timmyc00]

I am new to centrifugal pumps. I have a pump above a sump open to atmosphere by 12'. There is a pressure gauge on the discharge of the pump reading 40 psi. When I look at the pump curve, do I add in the 12' of head to the 40 psi, or just read the 40 psi?

Also, when the level drops in the sump, the discharge pressure drops also. I thought that was strange. It should remain constant, right? Am I deadheading the pump?

 

[BigInch]

Discharge Head is the sum of Suction Head Plus Pump Head. If suction head drops, your discharge pressure may drop, or may increase. This is because its not only what the pump wants to do. The system curve also has an effect on the final result. The dropping suction pressure and discharge pressure of the pump will also tend to cause the system flow to go lower. With that lower flow, its possible that the pump head increases (look at the pump curve and see that you go towards shutoff head when flow lowers), thereby actually increasing discharge pressure. But, if your system curve also decreases substantially with the reduced flowrate and suction pressue (more of a decrease in system head than the pump wants to increase its head) the net result might be that the pump will go in the other direction, increasing flow and reducing net pump head, or move towards the runout end.

To answer your question, you can't tell what the discharge pressure will be, if you only know the suction head and the pump curve.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[timmyc00]

I have the discharge pressure on a gauge downstream of the pump. I know the suction head at the impeller that also has a gauge and I have the pump curve. So I basically add the discharge and suction head, and that number is where I am on the Y axis of the pump curve...correct?

So if the discharge pressure reading is 40 psig, and the suction is -5 psig, I am at 35 psig on the pump curve?

 

[SNORGY]

I think...

The curve reflects *differential* head.

With your readings, you are seeing 45 psi differential, or about 104 feet of water.

Looking at it another way, 40 psig x 2.31 ft/psi = 92.4 ft

But you are also lifting 12 feet on the suction side...

92.4 + 12 = 104.4 ft; 104.4/2.31 = 45.2 psi differential.

I sure hope I am not wrong. I hate it when I am wrong.

Regards,

SNORGY.

 

[BigInch]

No sorry Snorgy, it is as Timmy calculated.
That is, if there is 12 feet of available suction head.
(by the description so far, I can't really be sure, but it looks like that)

<And neglecting any velocity heads>

Total Discharge Head = 40 psi * 144 / 62.4 = 92.3 ft
Suction Pressure = 12 ft * 62.4/144 = 5.2 psi

Pump Differential Pressure = 40 psi - 5.2
Pump Differential Pressure = 40 - 5.2 = 34.8 psi,
Pump Differential Head = 34.8 * 144/62.4 = 80.3 ft






**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[timmyc00]

right..the pump has to lift 12' of water from the sump below.

thanks biginch, that makes sense

 

[timmyc00]

to clear up, the gauge right before the impeller on the suction side is reading -5 psig

 

[JohnGP]

Well, with that clarification it would be as SNORGY indicated. The pressure differential is 40 - (-5)= 45psi, as you have suction lift and not positive suction head.

 

[timmyc00]

the old switcheroo...stars to johngp and snorgy

thanks guys

 

[BigInch]

OK. I thought it was a submersible pump.

Timmy, Next time you must post an accompayning diagram!

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[Artisi]

Yep, nothing like posting the full detail - save a lot of crystal ball gazing and gets you the right answer a lot sooner.

 

[SNORGY]

I'm so confused...
If I am wrong...
Maybe I should keep my night job (dog trainer)...
Oh well. Sit happens.
(Sadly, sometimes it doesn't...this isn't boding well for me in either career...)
But seriously...
If it's -5 psi on one side and +40 psi on the other side, the differential is 45 psi. That's the extent to which I looked at it initially.
Thanks JohnGP and timmmyc00.
Star to BigInch for the diagram, a whole other "perspective" on things.


Regards,

SNORGY.

 

[BigInch]

Snorgy, You're OK. I was the one that guessed wrong.
Sorry for the confusion. Back to the kennel now. Got a spare cage?

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/