Air valves versus gooseneck? Secondary effluent... 1

[RRichardson]

I'd like to know what the advantage is of a combination air valve versus a goose neck.

The scenario is a 42 inch diameter concrete lined steel pipe that is fed by two secondary clarifiers (three at peak flows). The clarifiers drain into two basins where four vertical turbine pumps (two for each basin) lift the effluent to the tertiary filters. The difference in invert elevation from secondary clarifiers and basins is approximately 1.5 feet in the course of about 700 feet. The difference in water level is about 1 foot. There is one known high point in the pipe with an air release, but it is currently low pressure and taking air into the pipe (we're figuring that we have to raise the water level in the basins to correct this issue). There is another suspected high point in the pipe without an air release which we are exploring now. Is there any reason to use an expensive combination air valve versus a goose neck?

 

[bimr]

You do not want a combination valve since a combination valve will allow air to enter the pipe. The application if for an air release valve.

You would be better off with a standpipe or gooseneck rather than an air release valve as there is no maintenance required. There is a little maintenance required with an air release valve.

 

[RRichardson]

bimr,

Thank you for the response. Now that you mention that we don't need a combination valve, that makes sense. There is no reason I can think of to ever allow air back into the pipe. However, doesn't a typical air release valve trigger by water reaching some sort of float? The air release valve would have to be installed on top of the pipe because the EGL is well below, and will always be below, the ground surface. I think operations would prefer things to be above ground when possible.

I think a standpipe or gooseneck has the same issue as the combination air valve unless some sort of check valve could be installed.

 

[stanier]

No maintenance required for a goose neck. No odour problems with a goose neck.

Air release valves are generally designed for line filling and emptying. Their use to solve surge related events is over stated and unreliable in effluent systems. If you rely upon air release valves for surge mitigation then you need to treat them with the same respect as a relief vvalve. ie surge analysis of all possible operating conditions, full documentation, routine maintenance and testing and a formal risk analysis.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[RRichardson]

Stanier,

Thanks for the reply. Secondary effluent doesn't have much odor relative to primary effluent. Also, we're in a canyon so odor isn't much of a concern. I don't think there are any surge issues either. The primary reason for having an air release in this particular situation is to maximize the flow-able cross section of the pipe.

 

[stanier]

I would have thought a 42" gooseneck would cost a lot more than an air release valve.

Go onto the

http://www.ventomat.com

website and download technical articles and modelling software. You could model the system both ways.

Free surface flow is maximum when pipe is about 87% full so air ingress may help the hydraulics.

“The beautiful thing about learning is that no one can take it away from you.”
---B.B. King

http://waterhammer.hopout.com.au/

 

[bimr]

The combination valve is slightly more complicated than an air release. That is why you should use the air release, if you decide to use an air release. However, unless the water level is higher than the top of the pipe (to pressurize the pipe), the air release valve will not solve the problem.

I agree that you need to operate with a higher water level in the basin. The higher water level will cause the pipe to remain full at all times. If you operate at a low basin level, you will get air into the pipe and the air may limit the flow through the pipe.

I had a similar situation at a treatment plant. The plans showed an inverted siphon between the clarifiers and a step aeration device. The Contractor put a manhole in between. Fortunately the water level in the manhole ended up being just below grade.

You probably can get by with a small vent, maybe 4-Inch size. The important thing is to raise the level in the basin to keep the water level above the top of the pipe.

 

[RRichardson]

The main issue is that the water level would never reach the float in an above ground air release valve due to a lower HGL. There are commercially available underground air release valves, but I have a feeling these are quite expensive. I think it would be best to install the vent instead, with a manual close valve. Thank you all for the input.

 

[cvg]

the only reason for the float is to shut the valve and prevent water from shooting out through the air/vac valve. If you are sure that there will never be surges causing the float to be functional, then there is no reason for a float. However, since you plan to raise the operating level in the basin to make the pipe flow full, I can see the pipe sealing and potential surges causing water to discharge. Plan on dealing with this water unless using a valve with a float. A common way to install an air release valve would be directly on top of the pipe, located in a manhole. Without knowing the flow velocity or the profile of your pipe, hard to know if air accumulation at a high point will even be a problem.

 

[RRichardson]

We'll be running an experiment today to measure basin levels and try to determine what our actual head loss is. It is unlikely there will ever be any significant surges... the flows appear to be subcritical.

 

[bimr]

Since this is the effluent from a clarifier that has just overflowed a weir, there is no way that there will be surges.

 

[hsbcn]

I'd ensure that there's a float available, just in case. Although you're counting on not having surges, a single surge is sufficient to put your system out of order accompanies by a costly maintenance and pre-relaunch testing, health and safety issues and, eventually, yet another valve replacement.

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