Calculating pressure in a pumped pipeline

[arh13p]

If I am pumping a tank of water up to a free outlet (lagoon) I was wondering if I am calculating the pressure in the pipeline correctly.

based on the selected pump curve I figure out the operating point for that pump with the system. For this example say it is 300 GPM @ 30' of total head the pump needs to handle.

would the pressure in the pipe simply be [(30')(62.4 lbs/cu.ft.)]/144 = 13 psi?

Thanks,

 

[chicopee]

Nope, since pressure drop in such a line, assuming full flow, will be a function of pipe dia. and length, flow velocity, and pipe roughness. A fast way is to graph the pump characteristic curve and the system curve. Where the two graphs intersect will be you answer.

 

[bimr]

Not really. You also have to add the flow generated piping headloss.

The 30-feet of static head should be the difference from the bottom of the storage tank (say 3-feet off the bottom) to the maximum water elevation in the lagoon. You also have to add the flow generated headloss (if any) in the suction and discharge piping.

The pressure in the pump discharge will be the difference of the elevation in the lagoon minus the elevation of the pump plus the pressure from the flow generated headloss between the pump and the pipe discharge point. This pressure should be less than the design head pressure of the pump because it does not include the suction lift (if any) nor the pressure from the flow generated headloss between the pump and the suction.

You can find some flow calculators to determine the pressure drop in the piping at:

http://irrigation.wsu.edu/Content/Calculators/General-Calculators.php

 

[arh13p]

that I already did. that is what I meant by figuring out the operation point. where they intersect is at 300 GPM @ 30' of total head. so to figure out pressure is it just based on the head as I calculated above?

 

[arh13p]

the total head includes the static and dynamic head losses.

 

[bimr]

OK then.

But the pressure in the pipe will depend on the elevation where the pressure is taken.

 

[arh13p]

Is the way I did it the highest pressure that will be in the line since the total head is already taking into account the total elevation difference? or am I looking at this all wrong?

Pump is at elev. 100.00
discharge at elev. 125.00
Static head = 25'

dynamic losses is computed to be 5'

total head loss is 30'.

It is probably something simple that I am missing here. Is there a different formula I should be using to calculate pressure?

 

[psmart]

arh13p - I think you got it right. That should be the pressure in the discharge pipeline measured at the pump. As bimr points out, the pressure will be somewhat less as you go up the pipe to towards the discharge end.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[77JQX]

The pump elevation is irrelevant to calculating the TDH of he system.

Static head is the elevation of the lagoon surface minus the elevation of the supply tank surface (or the HGL of the supply pipeline pressure), assuming the pipeline profile doesn't exceed either. Add your dynamic head to static head defined this way to get your total head. Select a pump to overcome the total head at your desired flow rate. Then worry about the pump elevation and suction losses to find out if you have sufficient NPSHA.

 

[arh13p]

I have a small tank with a centrifugal pump in it. to be conservative I was using the bottom of tank (where the pump suction is) but I could use the top of the tank where the water level would be when the pump turns on which would give me a smaller static head. I assume I don't need to look into the NPSHA because this is not a pump system where have a suction pipe drawing water to the pump.

 

[77JQX]

Then you've got it. With such a small TDH, I'd look at all possible elvation combinations between the supply and the lagoon elevations just to be sure the pump doesn't ever run off the curve and burn out the motor. And it's never a good idea to make assumptions about NPSH, especially if you want to pump the supply tank dry.

 

[bimr]

I think you have it correct. I would the minimum elevation in the tank as 2-3 feet off the floor.

 

[arh13p]

That is typically how we design them. the pump is on a float system so the tank will never be completely empty. thanks for all the replys.