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Mechanical Engineering/DECREASE IN WATER FLOW DUE TO PIPE FRICTION

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Question
I AM RUNNING A DEEP WELL MULTISTAGE PUMP OF 3 CUSEC CAPACITY   ( 303 M3/HR). PUMP DISCHARGE PIPE IS 10". THIS PIPE IS 500 METER LONG WITH 2 ELBOWS, AND THIS WATER IS DROPPED INTO A UNDER GROUND POOL.

I THINK DUE TO THIS LENGTHY TRAVEL INSIDE A CONCRETE PIPE AND 2 BENDS, I AM NOT COLLECTING 3 CUSEC WATER IN THE POOL. HOW MUCH WATER IS REACHING TO THE POOL. PLEASE GIVE SOME BRIEF IDEA TO SOLVE THIS ISSUE?

Answer
Shahzad - based on the information provided, your system is as follows:

303 Cubic Meter / Hour = 1335 Gal. per Minute +/-
500 Meters of Piping = 1650 Feet of Piping +/-

Based on the following formula:
1250 GPM = 5.67 Ft. Per Second Velocity  w/ 0.929 Feet of Friction loss head per 100 feet.
for arguments sake, we are adding 10% to the numbers above to equate approx. 1350 GPM + s.f.

1350 GPM = 6.30 Ft. Per Second Velocity - this is a decent number, you don't want to go higher than approximately 12 Ft. per Second. - Friction loss is approx. 1.03 Feet of Friction Loss Head per 100 Feet.

1.03 Feet of loss x (1650 Foot of Pipe / 100) = 170 +/- Feet of pressure drop from the piping

Elbows (quantity 2) at S.R. 90* = 14 Feet of pressure drop each = 28 Total feet

Total pressure drop in the system is 198 Feet of head Loss.  

Now, the situation of the low flow in the system at the discharge is a bit confusing to me.  Are you not getting enough quantity or not enough pressure?  Depending on the pump, you should have it sized to move 1350 GPM @ 198 Ft/Hd + discharge pressure you are trying to achieve

I.E. If you are trying to achieve 1350 GPM @ 108 PSI (250 Feet of Head / 2.31) then your discharge pressure would be approximately 52 Feet of Head or 21 PSI (PSI = Ft. Head x 0.433)

Beyond the friction loss, if you are having issues with the flow rate, there may be an issue within the system that the pump is being starved causing cavitation.  This can happen if the feed line isn't large enough, if there is a vortex in the feed system due to the submergence not being deep enough or the necessity of a baffle being installed or even having a concentric reducer installed upside down.

Please provide a sketch of the system and all dimensions and pertinent information (i.e. motor info, pump info, piping info) so I can explore further.

I hope this helps.  Good luck!  

Mechanical Engineering

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Michael Aschenbrener

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I can answer most questions on pumping applications, setups and systems. Repairs and chemical compatibility. Reading pump curves and friction loss calculations.

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I work as a process pump application specialist. I also work in a million dollar per year pump shop repairing everything from centrifugal pumps, grinder pumps, self primers and AODD pumps.

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MI process pumps and equipment online development and training. On the job training in both application design and pump repair. Online eaton hydraulics institute. Baldor/Dodge/Reliance training program.

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