4 replies to this topic

[felderosfelder101021]

Hello, I am trying to size a pump system but this results to a negative discharge. I am trying to pump a water at 25 C to 50ft above grade. This results to a negative differential head. The path is that it must go up the piperack of 50ft then passes through it then goes back to grade level for some reason. Can you please help me what missing concepts I am here? Please see the result of the sizing using a software below (see attached file)
 

Attached Files

•  Sketch.pdf   38.01KB   7 downloads

[breizh]

try to reduce head losses, let say increase pipe diameter and or change the pump with higher TDH.
Note:PIPE SUCTION LINE SHOULD BE BIGGER OR EQUAL TO PIPE DISCHARGE LINE.
my 2 cents
Breizh

[felderosfelder101021]

try to reduce head losses, let say increase pipe diameter and or change the pump with higher TDH.
Note:PIPE SUCTION LINE SHOULD BE BIGGER OR EQUAL TO PIPE DISCHARGE LINE.
my 2 cents
Breizh

Tried your suggestion, I still have it my suspect is that it is due to the static head recovery as it goes to downward? Isn't that accounted in pump sizing as well? I am on sizing case, I cannot change the pump TDH as it is the TDH suggested by the system.

Attached Files

•  Sketch2.pdf   39.63KB   1 downloads

[Pilesar]

The only reason you might have negative pressure is if the last downleg is liquid full. Consider putting an atmospheric vent at the top of the last downleg. Then the pressure at the J6 junction will be atmospheric. You might need a larger pipe for the last downleg. Alternately, consider putting a control valve or restricting orifice at the discharge sink to hold backpressure in the line.

[felderosfelder101021]

The only reason you might have negative pressure is if the last downleg is liquid full. Consider putting an atmospheric vent at the top of the last downleg. Then the pressure at the J6 junction will be atmospheric. You might need a larger pipe for the last downleg. Alternately, consider putting a control valve or restricting orifice at the discharge sink to hold backpressure in the line.

Thanks for your suggestion. It somehow worked, if my understanding is correct, the downward piping shall be sized to self-venting application (PD Hill Equation , Fr<0.3) is that correct? 

Now, I just find it weird, for example during typical pump operation during start up, this scenario will happen (the on in sketch). This scenario seems like taking no credit to the downward (pressure recovery). I am abit confused.

Also, say we have a vacuum breaker there at high point, if the fluid is not water and any toxic hydrocarbon, wouldn't that be not advisable to do?

Attached Files

•  Sketch3.pdf   39.6KB   1 downloads