Siphon effect in pumping systems

[Achman]

I have a scenario where a sump pump is used to pump liquid 300m away into a tank. Due to presence of solids I don't want to install a check valve on the pump discharge line and have designed a high point in the system to minimize the amount of liquid draining back into the sump when the pump turns off, however, we have concerns that the whole content of the line might drain back into the sump due to syphon effect as the pump elevation is 5 meters lower than the tank. We are not allowed to put a vacuum breaker in the high point due to potential leakage and environmental issues.
I have attached a sketch that shows the arrangement, does anyone know if a calculation can be done to determine whether the syphon effect will actually happen in this scenario ?
Regards,

 

[LittleInch]

You don't need a calculation here - this will syphon back as the up lift side is only a total of 4m. You would need a total of ~10m before you pull a vacuum on the top of the syphon and you can't "lift" the pipe contents back over the syphon.

Vacuum breakers are common in this circumstance and if necessary install two in series.

Or make your top loop 6m higher...

Or fit an actuated valve which closes on pump stop.

Reverse flow won't do your pump a lot of good either.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[JJPellin]

Can you install a check valve at the far end of the line just before the tank? Since the check valve would be in (I assume) a vertical line with downward flow, would that resolve your concerns about solids? In this arrangement, the check valve would need to be spring loaded closed depending on the type of check valve selected.

We use this arrangement in our oily water sewer transfer pumps that have a similar problem. Placing the check valve at the far end of the line works well for us in that service.

Johnny Pellin

 

[EdStainless]

We have some lines with high point vents in order to prevent siphon.
When the motor shuts off the vent opens, when the motor is energized the vent closes.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[katmar]

One way to do it without any check valves, vacuum breakers or vents would be to ensure that the section from the high point down to the delivery point does not run full. You need to get the diameter and slope of the 300 m section correct to guarantee this. It would be a tricky installation because the slope must be constant over the full length to ensure there are no low points where the pipe would run full. The discharge into the final tank must also be above the top surface level so that there is always an air gap. Maybe this was what you had in mind when you made the initial section 2" and the rest 3". Without knowing your flow rate I can't really be sure.

Another option might be to make the vertical downflow section between the high point and the reducer with a very large diameter, and then ensure that the pressure drop along the horizontal section would not require the fat vertical section to run full. This fat section would also have to be sized to make sure that when the vertical upflow section at the start drains back only air is drawn back from the fat section. This doesn't seem to be an elegant solution!

Katmar Software - AioFlo Pipe Hydraulics

http://katmarsoftware.com

"An undefined problem has an infinite number of solutions"

 

[georgeverghese]

How much of this discharge line will dribble back through the pump casing will depend on the low level elevation of the source tank that this pump takes its suction from - you havent said much about this source tank's liquid elevation range. Reverse flow will stop when level in the discharge line becomes the same as that as in the source tank when the pump stops.

 

[JStephen]

A variation of katmar's idea would be to run a separate 1" or so line from the high point over to the tank, but keep it elevated so it doesn't fill with fluid- then it serves as a vent line to avoid a vacuum condition at the high point.