4 replies to this topic

[MikeO2]

Hi

Just wondering if someone can help me out with a little fluid flow problem. I'm looking to assess the air entrained in the fluid stream (purge stream) is flowing at 50m3/hr. The main pipe is 150NB and the Air, anti-siphon pipe is 25NB. The schematic diagram is on the file attached. My idea was to used Bernoulli to calculate the pressure P2 and the use Bernoulli again to calculate the velocity of air drawn into the main fluid pipe (as P1 is known, i.e. atmospheric pressure). Do you think that this is the right approach, or do you think that a gravity flow apprach should be adopted.

Thanks in advance for any comments

Regards

Mike
michael.orley@akersolutions.com

Attached Files

•  Fluid Flow.jpg   31.39KB   55 downloads

[Zauberberg]

What makes you think that there will be an ingress of air at all?
I'm not sure if I missed something from your sketch and description.

[Art Montemayor]

Mike:

Zauber is right. Why do you complicate the query by not telling us what exactly it is that you are trying to do? Are you trying to drain off (or dispose of) waste or unwanted liquids (water?) to a pond? Or are you trying to aerate a drain liquid with air (or its oxygen content)? Do you believe that your sketch is not one of a typical gravity flow? It certainly looks like one.

Why do you propose to introduce air through an inverted nozzle and through the vertical last leg of the drain? Without a detailed and accurate explanation of what the scope of this proposal is, it is virtually impossible to put together any constructive comment to resolve the task at hand. Your sketch is useful in describing what you think you need; but can you furnish us with a detailed explanation of what you propose?

[MikeO2]

Thanks for the replies

Should have put a little more information on there. It should be noted that this is an existing design.

The purpose of this existing system is to top up a pond (open to the atmosphere). The purge inlet also comes from another pond which is at atmospheric pressure. The intention of the air inlet line (again at atm pressure) was not as a purge, but to act as an anti-siphon device, thus preventing backflow of liquor back up the purge line (the 150NB line).

As the anti-siphon line is located on the vertical section of pipe, it is envisaged that the pipe may not be fully flooded, having the potential to draw air into the main purge line.

This is where the problem may lie. It has been pointed out that the entrained air may be causing enhanced turbulence at the exit of the purge pipe, thus increasing aerial releases local to the purge exit from the surface of the pond.

I have used the following equation from the htfs handbook (for self venting flow in vertical pipelines):

d > (4M/(0.3*PI*D*(g^0.5)))^0.4, where M is the mass flowrate of water (purge), d is the purge pipe diameter, D is the density of water and g is the gravitational constant. If d is greater than the right hand side of the equation, this minimises the potential for gas entrainment.

In this case, d is not greater than the right hand side of the equation and therefore there is a much greater potential for gas entrainment.

What do you thinkwould be the correct approach for estimating the flow rate of gas/air into the pipe (solving Bernoulli??)?

Regards

Mike

Edited by MikeO2, 15 December 2009 - 05:59 AM.

[Zauberberg]

I still can't see how there can be an ingress of air unless the piping itself is under slight sub-atmospheric pressure, which cannot happen. The formula that you have used is related to self-venting gravity flow.

Attached Files

•  A. Sloley - Gravity Flow.pdf   80.92KB   89 downloads