Preventing back siphoning

[mae1133]

We have a client with an aeration system that consists of a 32' tall aeration tank with approx. 28' of liquid. The PD blowers are in a nearby building at ground level. The piping goes up and over the wall of the aeration tank to the bottom of the tank where the aeration diffusers are. When the blowers stop, the cooling effect of the piping is causing the liquid to rise in the pipe, go up and over the hump at the top of the tank and then down to the blowers. I have requested a siphon break be installed at the top of the pipe to prevent this from happening consisting of a check valve positioned to allow air into the pipe. As the cooling occurs, a vacuum is formed within the pipe causing the check valve to open allowing air into the pipe and breaking the siphon. The problem is there is virtually no way to access this location and the owner asked why it can't be located inside the building near the blower as all they need to do is allow air into the line to prevent the siphon from forming. I want to explain this properly to them so I'm looking for a simple explanation from you guys. I believe if the siphon break is located at any point below the liquid surface, there will be positive pressure from the tank which would still cause the liquid to rise, and not providing a siphon break at all, but is this still the case in this type of application? Thanks

 

[LittleInch]

A section drawing would be nice, but my interpretation is that based on bottom of the tank at 0 feet, you have liquid at 28 feet.

From your description though I'm not sure where the high point of the air pipe is.

Is is 32 feet or higher?

Assuming it's 32 feet, then the owner I believe is correct. The inlet air vale to prevent a syphon in this instance ( not if liquid filled) can be anywhere on the air side up to the high point. Basically all you need is for the pipe pressure to be the same as the pressure in the tank above the liquid

However you then say the liquid is being pushed up the extra 4 feet because the pressure above the liquid is higher than atmospheric pressure? In that case you really need a check valve on the inlet into the thank or some sort of isolation valve.

All a valve at the high point will do will stop a syphon developing, but won't stop high tank pressure pushing the liquid backwards and up the pipe and then down the pipe into your blowers.

Really needs a good diagram!

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

 

[georgeverghese]

Agreed, the high point on this aeration air line has to be at 32 ft or more to prevent reverse flow of water back in to the blowers. The proposed syphon break doesnt do much in this case. Else, with some risk, install a check valve and a PLC operated SDV at the aeration tank air inlet nozzle to auto close whenever the PD blowers shutdown or when there is loss of pressure in the aeration air line ( there is still the risk of a reverse leak through the closed SDV and check valve).

 

[mae1133]

Attached is a diagram of the existing system. I'm recommending a siphon break at the high point of the piping at the top of the tank, but this is an inaccessible location, so the client has suggested locating it inside the building right after the blower discharge. I certainly prefer it being in the building but concerned it will not perform as intended. As indicated, the siphoning is occurring after the blower turns off as the pipe cools, causing a liquid column to rise enough to make it over the high spot of the pipe and then back down the pipeline to the blower. As a siphon break, I'm suggesting a branch off the main line with a check valve to allow air flow into the aeration line as the air cools and contracts, but when the blower is on, the aeration air closes the check valve. So, we just have to determine whether this can be placed within the building as opposed to the top of the pipeline, above the liquid surface. When we have designed a system such as this in the past, we have always placed this type of siphon break at the high point and it has always worked, but it has always been accessible or done during construction. In this case, it would be very difficult to access the high point of the tank where the piping is, which is why we are considering placing it inside the building.

 

[LittleInch]

OK, that's great.

As said above as this is an air filled pipe then anywhere on the pipe from the blower to the top of the pipe will allow air in to equalise the pressure and prevent back flow.

If this was a liquid filled pipe then yes, you need it at the top, but it isn't so you don't.

Somewhere though there must be a valve or other restriction which stops the pipe from equalising and creating a lower than atmospheric pressure??

This air inlet valve will need to be downstream of any such device.

You should also consider a non return valve somewhere in the pipe going down into the tank. An issue might just be that as your air flow stops, the water rushing into the pipe to fill it has enough velocity to climb up the 4 feet of pipe and create a syphon.

If a syphon develops then you will need the valve and air inlet at the top.

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

 

[HTURKAK]

....... When the blowers stop, the cooling effect of the piping is causing the liquid to rise in the pipe, go up and over the hump at the top of the tank and then down to the blowers. I have requested a siphon break be installed at the top of the pipe to prevent this from happening consisting of a check valve positioned to allow air into the pipe....

The raise (or jump ) of water in the pipe over the hump is , due to sudden pressure drop in the riser when the PD blower stops rather than siphon effect. The static water level in the riser (water side ) is 28 ft . During operation of PD, water level in the tube is 28ft below around diffuser level and when the PD blower stops, causing damping oscillation of water in the riser.My suggestion will be, provide CV at hump or prevent rapid pressure drop in the pipe with increasing the stopping time or with other means.

 

[georgeverghese]

Agreed there is no need for a syphon break here since this is in gas service. This 32ft riser should do the job. Also include a 1inch valved connection a the bottom of the gas riser ( on the blower side ) with a bottom autodrainer to drain off any water vapor that may have condensed during cooler weather - we dont want this water pooling up here. Else, if you dont want the autodrainer, just leave the bottom bleed valve here slightly open all the time.

 

[mae1133]

Good point guys! The water rise is likely more due to oscillation during shut down as opposed to the vacuum created from the pipe cooling. I'm having them adjust the ramp down time on the blower to try to minimize this effect. They do have manual drains at the low point of the piping.

 

[LittleInch]

A simple check valve somewhere on that system from the blower to the aerator should prevent this, but as close to the end point as possible would be best to reduce back flow from the tank

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

 

[bimr]

Not sure what type of aerators that you are using, but some types of aerators will be destroyed if you allow back flow through the aerators.

 

[itsmoked]

A no plumbing solution: If the blower has a VFD on it you can solve this by simply programming the deceleration speed to something like 20 seconds. As the blower slows down the pressure will drop more slowly allowing the water to slowly rise in the standpipe instead reaching a too-high carry-it-over-the-top velocity.

Keith Cress
kcress -

http://www.flaminsystems.com