I want to pump water up and over a hill. Somewhere on the downhill side, the velocity will increase such that the pipeline will run partially full (Manning open channel calc.). What should I be concerned about in this case? Do I need a vent at the high point? There will be too much head to install a stand pipe type vent.
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Pumping over a hill 1
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If the available head due to the height difference on the downhill side exceeds the friction loss over that section of pipe by more than the vapor pressure of the water you will get localized boiling in the pipe. These bubbles will be partly entrained and when they reach a higher pressure area they will collapse, causing cavitation. You could get vibration and gradual damage to your pipeline.
It seems that you have already done this calculation if you know that the pressure at the high point will be too high to use a stand pipe vent. If you have the reverse situation to that described above, i.e. If the friction loss over the downhill section of pipe exceeds the available head due to the height difference then you will not get a part full section of pipe and you will not have vibration.
The calculation probably needs to be done over a range of flow rates, unless you can be sure the pump will always run at the same rate.
Katmar Software
Engineering & Risk Analysis Software
It seems that you have already done this calculation if you know that the pressure at the high point will be too high to use a stand pipe vent. If you have the reverse situation to that described above, i.e. If the friction loss over the downhill section of pipe exceeds the available head due to the height difference then you will not get a part full section of pipe and you will not have vibration.
The calculation probably needs to be done over a range of flow rates, unless you can be sure the pump will always run at the same rate.
Katmar Software
Engineering & Risk Analysis Software
Draw a longitudinal profile of the pipeline.
Draw the hydraulic grade line (HGL) from the delivery pressure of the pump, down to the pipe outlet. The HGL must be a straight line to the outlet because there is no head at the end - it is at atmospheric pressure.
If the HGL is more than about 9 or 10 m below the pipe, column separation will occur. At the column separation, the pressure is about full vacuum (about -10 m of water) less the vapour pressure of the water (about 200 mm of water, absolute).
If the HGL is less than (say) 8 or 9 m of water, the system will operate like a syphon.
Positioning an air valve on top of the hill (so that you don't get an air lock on pump start up), will break the water column THEN the down stream pipe can become an open channel. For a quick check try Manning formula for Q = pump flow (based on the pump HQ curve & pipe characteristic curve) & see if the pipe can take the flow as an open channel. I usually check a full pipe first, then if the pump flow is less, the depth of flow can be calculated from the curves of flow vs depth (such as in AS 2200).
Summary
HGL above the hill - pipe flows full.
HGL down to 9 m below the pipe - pipe flows full (as a syphon).
HGL 10 m or more below the pipe - pipe flows full on the upstream side & can be full or as an open channel on the downstream side, depending on the flow, diameter & pipe grade.
Barry Eng
Draw the hydraulic grade line (HGL) from the delivery pressure of the pump, down to the pipe outlet. The HGL must be a straight line to the outlet because there is no head at the end - it is at atmospheric pressure.
If the HGL is more than about 9 or 10 m below the pipe, column separation will occur. At the column separation, the pressure is about full vacuum (about -10 m of water) less the vapour pressure of the water (about 200 mm of water, absolute).
If the HGL is less than (say) 8 or 9 m of water, the system will operate like a syphon.
Positioning an air valve on top of the hill (so that you don't get an air lock on pump start up), will break the water column THEN the down stream pipe can become an open channel. For a quick check try Manning formula for Q = pump flow (based on the pump HQ curve & pipe characteristic curve) & see if the pipe can take the flow as an open channel. I usually check a full pipe first, then if the pump flow is less, the depth of flow can be calculated from the curves of flow vs depth (such as in AS 2200).
Summary
HGL above the hill - pipe flows full.
HGL down to 9 m below the pipe - pipe flows full (as a syphon).
HGL 10 m or more below the pipe - pipe flows full on the upstream side & can be full or as an open channel on the downstream side, depending on the flow, diameter & pipe grade.
Barry Eng
At the risk of sounding lecturing: The velocity in the fluid wont change throughout the pipe.
Slag flow (as i also have heard it refernced as) is quite normal - and a special case of multiphase flow. I would consider using manning open channel flow as a practical and somewhat conservative solution. The section with the channel flow is going to be the limiting factor.
For your reference try to read this article regarding slack flow:
Best regards
Morten
Slag flow (as i also have heard it refernced as) is quite normal - and a special case of multiphase flow. I would consider using manning open channel flow as a practical and somewhat conservative solution. The section with the channel flow is going to be the limiting factor.
For your reference try to read this article regarding slack flow:
Best regards
Morten
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