simple sump pump system - pressure 1

[tercalla]

please help this newbie out:

visualize a lift station/ sump pump system with bleed off above check valve (i.e., pump turns on/off & all water in FM is discharged back into sump so we dont have to run heat trace). given the following information, how do i come up with the operating pressure, long-term pipe pressure, and short-term pipe pressure (for waterhammer & surge)

velocity: 1.95 fps (i know, kinda low)

Qpump: 45 gpm (@ TDH=30-35')

elevation diff from pump to discharge: 35 ft

discharging into gravity fed main.

pipe type: 3" sch 40 pvc

pipe length: 40' (pipe runs mostly vertically, small portion horizontally)

TDH: 32' (static about 30', all other losses: 2')

thanks SO much for your help!

 

[BigInch]

What you have given, I think, are the Best Operating Point (BEP) data for the pump, 45 gpm @ 30-35 ft. It is probably not the greatest differential head that the pump can develop. that head you will find at a flow of 0 gpm (when discharge is closed with the pump running). That head is also known as the "shut-off" head.

Pumps develop a total discharge pressure by adding the differential head to the suction pressure, so you must know the suction pressure and then add the pump's maximum differential pressure to get the maximum total discharge pressure.

With the info you have given, I calculate the differential pressure of the pump (at BEP operating condition) as
35 feet of water head = 35 * 62.4/144 = 15.0 psig,
to which you must add the suction pressure.

To find the maximum piping pressure you could have,
you should make the same calculation when you know the both the pump's shut-off head and also the maximum suction pressure that you will ever have feeding the pump.

Max Pipe Pressure = Maximum_Suction_pressure + Shut-Off_Pump_Head (ft) x 62.4 / 144

Going the Big Inch!

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[tercalla]

BigInche - Your a lifesaver!

Since the pump shuts on/off and the pipe bleeds out after each cycle, there is no water head in the pipe. Therefore, that means the shut-off head is also 35 feet, correct? How is the suction pressure determined?

Also, as for BEP - the TDH 30-35 & 45 gpm is EXACTLY how i need the pump to perform in order to fullfill my design objective. If the pump should supply a higher head, I would be screwed...

Thanks again!

 

[BigInch]

Not exactly... the term "shut-off" head refers to the head when the water in the discharge line is shut off, not when the pump is shut off.

Confused?

The simple explanation goes like this..

A running pump has a certain amount of energy that it can deliver to the water. At low flows (or "shut-off flow"), not much friction is developed, therefore almost all of the pump's energy can be converted directly into head (or the pressure equivalent). As flows in the pump and pipe increase, the energy needed to overcome friction also increases, leaving less energy available for making head.
So, as flowrate increases, the head produced gets lower and lower.
OK got it?
So, if your discharge water piping CAN be shut off somehow, your running pump will then be making "shut-off head". If there is any possibility of your discharge line getting clogged or the end blocked or run over by a truck or something, its best to design your pipe for that higher pressure. Most of the time shut-off head will be around 20% to 30% higher than BEP head. There is some possibility that starting the pump will develop that pressure for a short time, even though the pipe is not actually closed off. That is due to the water accelerating in the pipe. When the acceleration has finished and velocity is constant, the pressure will reduce to the BEP head (or hopefully somewhere near there).

Last question was "How to determine suction pressure?". If the system is existing now and you can close a valve near where you will place this pump, close that valve, drain any water from pipe to the downstream side (your future discharge line), then put a pressure gauge on the upstream side of the valve (your future suction line). That pressure you read on the gauge is the suction pressure when there is no flow . Suction head = suction pressure (psi) X 144 / 62.4

Suction pressure will tend to reduce when you eventually have flow going to the pump, but you will have to calculate how much that reduction will be, using the pipe flow formulas, the suction line size and the 45 gpm flowrate.

That should do it for now. Let me know how it goes.



Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[BigInch]

Rereading your original statement of the problem, I see it is probably taking water from a sump.

In which case suction head is calculated as follows,

If the pump is dry mounted (motor) above the sump with a shaft inside a pipe going down to an impeller located in the water or if its a totally submersible pump located underwater, the suction head is basically the height of the water level above the impeller or the submersible's intake.

If the pump is mounted totally above the sump, with only a suction pipe going down into the sump, you will have a negative (gauge) suction head = to the elevation of the pump (centerline) - the water level elevation.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[tercalla]

thanks so much! i didnt get a chance to go through your responces on Friday. i'll be getting to them today.

it is a submersible pump. all these different heads are killing me!

i really appreciate all your help!

 

[BigInch]

No worries. If you still need any additional help, you might be able to find my e-mail address on my webspace, bottom of profile tab. Feel free to write to me directly.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com