Air Regulated Covered Siphons 2

[peterthompson]

The attached sketch refers to an air regulated siphon which consists of two concentric pipes with the ability to control the flow of water using air. Water flow is up the inner pipe, diameter (Da) and then down the second pipe, diameter (Db). When flowing down the outer pipe, the water is flowing down the annulus of the two concentric pipes. The outer pipe flows into a tank, which has an overflow level which ensures that the water level covers the lip of the pipe.

1. Is there a minimum flow/velocity required before a siphon will be initiated.
2. is there a relationship between the area of teh inner pipe and the area of annulus.
3. is there a minimum overflow height above the inner pipe/weir before a siphon will form

 

[katmar]

It seems to me that you are siphoning from and back to the same place? What is the purpose of this apparatus? The way it is drawn it will not achieve anything. You will have to give a bit more explanation if you want answers.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[peterthompson]

Please see an updated sketch of the siphon system. This is a gravity filter system and we use the siphone outlet to regulate the flow rate through teh filter. we are trying to downscale teh unit to build a small pilot plant

 

[zdas04]

Peter,
If you want us to see an updated sketch, you have to upload it again, engineering.com can't really see your hard disk.

David

 

[peterthompson]

Here is the updated sketch - forgot to upload

 

[Pumpsonly]

Peterthomsom,
Can't find any updated sketch. Any way your proposed design is called Airlift pump. Just Google Air lift pump and you will find more than you can handle. For your described usage, you will find it
being applied for the aquarium filtration system.
The top of the lift tube must be freely vented for it to work effectively. If you are in the water treatment industry you will find its application in lifting the silt from the bottom of the settling bay.

 

[zdas04]

Pumpsonly,
I think you should have waited for the updated picture. It isn't an airlift pump. He's using the air valve to maintain an artificial atmosphere to control the water level above the sand filter.

PeterThompson,
It looks like a pretty clever control scheme. To address your questions:
1. No, as long as the pneumatic head above the standpipe (above Da) is below the hydrostatic head of the water above the sand filter then water will flow. The farther below, the faster the water will flow.
2. Of course there is a relationship. Look in the FAQ's for this forum and you'll find an equation for the flow area (I would provide a link, but I can't go there without losing what I've typed so far). As long as the effective diameter of the annulus is bigger than Da it won't be a factor in the flow rate. The controlling factor will be air pressure until you reach a point where the pipe is running full (i.e., anytime flow rate in gpm is greater than 10.2*Da^2.5, with Da in inches). After the pipe is running full, then you will be friction limited.
3. Not really. I would expect that if the water level above the sand filter is more than 3-4 ft above the top of the stand pipe (hydrostatic pressure of 1.3-1.7 psig) then you would get strong flow. But remember that the apparent difference in those heights controls flow rate. With pressure above the standpipe, calculating the relative water level isn't straight forward. I would do it calculating a simulated elevation. For example, if the standpipe is 20 ft lower than the sand filter then the hydrostatic pressure is 8.6 psig. If you added 8 psig to the chamber then the apparent height difference becomes 1.4 ft and flow would be slow. The water seal might not hold with 8 psig, but that is controlled by how far the downcommer sits below the weir.

Hope this helps.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"It is always a poor idea to ask your Bridge Club for medical advice or a collection of geek engineers for legal advice"

 

[Pumpsonly]

The OP seems to have mooted the idea of this level / flow control based on siphon flow is established. With the air on top of the chamber siphon effect is not going to happen.

As explained by zdas04 The flow control from the sand filter will be regulated by the air pressure above the inner pipe only, is a matter of calculating the immersion depth of the outer pipe below the weir is needed to hold the pressure.

 

[BRIS]

An automatic air regulated siphon normally retains the upstream water level within a very narrow margin almost independent of flow rate. It gives automatic increase in flow rate with a nominal rise in upstream level. As the flow rate increases more air is entrained at the crest and the "Vacuum" increases. The air inlet is placed over the upstream water level. as the water level rises it restricts the inlet air flow, the vacuum at the siphon crest increase and the rate of flow increases. as the flow increases the rate of air entrainment increases and the whole system self regulates.
I do not see how this operates as an automatic air regulated siphon. I assume that the intention is to adjust the air valve to change the flow rate. The question is therefore
1)will the contraption entrain air and carry the air out of the siphon crest - it depends on the dimensions - you need a drop of feet rather than inches.
2) I would expect the area of the annulus would be the same as the area of the pipe, the air entrained flow in the annulus will be lees dense, but it is not significant.
3) The answer to 3) is the same as 1) there has to be sufficient turbulence to entrain air.

 

[peterthompson]

Thanks to everyone that has contributed to this discussion.

In response to Dave's questions, Yes, we do use an air partialisation valve to control the amount of air bled into the siphon neck and thus control the flow rate rate. The primary function is to control the water flow rate through the main filter by regulating the air bled into the neck of the siphon.

My primary problem is downscaling the full scale unit to suit the pilot plant.

I will only be running a 5m3/hr through the pilot plant siphon and want to ensure that the velocity in the siphon is high enough to start the siphon without introducing unneccessary pipe wall frictional head loss.

In the full scale plant, which runs at a flow rate of 210 m3/h, the inner pipe diameter is 235mm, outer pipe 335mm,
Inner area is 0.049 m2 and annualr area is 0.039 m2.
Inner water velocity is 1.18 m/s and the annular water velcoity is 1.49 m/s.

Should I size the pilot plant siphon such that I keep the velocities similar in each pipe area of the siphon?

This gives me a siphon of 39mm and 52 mm for the inner and outer pipes respectively.


Regards
Peter