6 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   20 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   14 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   17 downloads

[Pilesar]

The new model sketch is incomplete. There is still the downleg remaining. The downleg does not need to be sized for self-venting, although that would certainly be large enough. In this situation with no vent, there would be a performance difference if the final discharge were in a vapor space or underneath a liquid surface. If you install a vent at the top, there is no need for self venting! Is the fluid a hydrocarbon or water? A discharge to atmospheric pressure suggests there would be no problem with a vent to atmospheric pressure! The vent may be as simple as a 1-inch hole cut into the pipe vertical downleg near the top. Air would enter the pipe at the top as needed. Note that this vapor will then take up some of the volume in the piping resulting in additional pressure drop for the liquid flow which might require larger piping -- the AFT Fathom model will have difficulty with two phases so you should check this with the fluid flow calcs in a general purpose steady-state simulator. The smallest piping would be needed if the whole system were full of liquid. This could be achieved with the flow restriction (valve or orifice) at the end of the pipe. The flow rate is controlled some way. Putting the flow control valve at the end of the line might be something to consider. You would also want a device to expel air at the top of the downleg -- perhaps a manual hight-point vent would be adequate if needed for startup. The economics of the system include the piping and the power to operate the pump. Easiest for the designer would be to just use a large enough pipe for the downleg (sized for self-venting) but I encourage you to check out the other options if time allows. Part of engineering is learning so that you can apply what you learn to a future problem.

[katmar]

Referring to the first sketch - with 400 gpm in a 6" pipe there is essentially no friction loss so the static height difference will ensure that the pressure at J6 is always 42 ft (18.2 psi) less than the pressure at J3 (assuming P4 is full of water). If J3 is at atmospheric pressure (14.7 psia) then J6 will try to be at a negative absolute pressure (14.7-18.2) which is impossible and the result will be vacuum, boiling and vibration.

To achieve stable operation without vibration you probably want to keep the pressure at J6 above 10 psia. It seems that you cannot decrease the static height so you have 3 options. 

1. You could raise the pressure at J3 by using a back pressure valve or orifice as already suggested.

2. you could decrease the ID of pipe P4 to 2.4" or less to increase the friction losses, but the loss would change with flow rate and if the flow is variable this is not a feasible option.

3. You could install a vent or breather valve at J6 to maintain the pressure at J6 at atmospheric. Installing a vent on a 6" line will result in air being sucked into the vent and entrained towards J3. This may or may not be a problem - depending on what is happening downstream of J3. To avoid air being entrained, pipe P4 should have Fr < 0.3 but this assumes that there will be some level maintained in P4.

If what happens downstream of J3 does not ensure that P4 remains full of water (for example a discharge into a dam) then it would be better to ensure that all the piping downstream of J6 is designed to guarantee part-full flow and there will be no pressure recovery when flowing down the P4 downleg.  In this case the pressure at J6 will be the same as at J3 and you can avoid the negative pressure problem.

Edited by katmar, 07 July 2025 - 10:03 AM.