Sanitary Sewer Air relief Valve

[SamL]

I have a 2" force main that I have to relocate so it is going up and over a new culvert. I am showing an air relief valve to be installed in a sidewalk adjacent to the rise in the pipe, however when I went to look for details the closest I could find is an air relief valve that comes off of a 6" main with a 2" line. To me it seems to use a 2" line off of a 2" line would be overkill, any suggestions as to the line size?

thanks

 

[EricC]

What type of forcemain is this? 2" seems like it would serve a single home or building. I'll defer to the others in this forum, but I would assume that an air relief valve on a small system like this one is not required. The safety factor provided on small piping systems is so large compared to larger systems (4" or greater), I'd be surprised if there'd be a problem with water hammer. It's the same reason there aren't air relief valves commonly installed in a water service to a single home...it's overkill usually, and if there is a problem, it's easier to correct later. Anyone else want to chime in?

 

[chip1998]

I agree with Eric C. In small distribution sysytems air release valves are not normally reqired as you have lots of air bleeding pints such as hydrants ,taps etc.

 

[saxon]

SamL, The size of the Air relief valve req'd. is dependant on the amount of air that needs to be relieved at the hi-point in the piping. Clow Corp. manu'f. them at 1/2" @ 12 scfm release up to 2" @ 240 scfm release all at various op. press. it sounds like a case of, "We don't know what the air load will be, so let's put in the biggest air vent, that way it will never be undersized." At start up there's an awful lot of air that needs to be purged.

saxon

 

[semo]

Saxon is correct. The valve is dependent on the air being released. An oversized valve will not hurt anything.

Just because the line size is small does not mean there can't be problems. You can get airlock with a 2" or 200" main if the conditions are right. The water hammer only occurs when these valves slam shut forcing the flow of liquid through the forcemain instead of air out of the valve. Some valves are designed to reduce this.

There are numerous manufacturers of air release valve and some have more benefits than others. Some of them also have backflow options available that help with the O&M.

These valves will spit some wastewater so you must put them in a location that can drain. You also do not want to put them in sealed environment. I had one in a manhole with a cast-iron lid that sealed itself just enough that the valve did not function properly. Once a small hole was drilled in the lid, the valve operated fine.