blow off for water service high point?

[TimMAz6]

A condo development has a 2" water service. Service needs to go over a 24" sewer forcemain (water company requirement); therefore, the service will have a high point where air can collect. The high point will be about 3' high. I don't want to install a blow off at the high point since our frost depth is 4' and the service cover is only 2'-1" (valve box will lead cold air directly to service). I'd rather blow out any air at the terminal end of the service with high velocity flow. What velocity is needed to blow out the air? Any experience with this? Or should I install a blow off in the frost zone?

PS, in low pressure sewer systems, air release valves are not needed unless the high point exceeds 30 vertical feet and system flows require 2 ft/sec to perge air (Environment One Design Manual).

 

[bimr]

Maybe this will help:

The Water Pollution Control Federation Manual of Practice No. FD-4 "Design of Wastewater and Stormwater Pumping Stations" states "A minimum velocity of 1.2 m/s (4 ft/sec) is required in the pipeline to shear the bubble and keep it moving downgrade."

 

[greggy]

Why not constuct a manual blow off in a valve box and use a foam frost lid with a handle to keep the warm air in the pit from rising and escaping?

 

[TimMAz6]

local water company won't allow manual valve box within 10' of sewer main; therefore, valve box will be 10'+ away from high point. My concern is that cold air will freeze valve and/or pipe leading to manual valve box since it's 10' away from high point.

 

[cvg]

install the service saddle and air line at the high point. run the line in the trench next to the mainline until you are well beyond the sewer line and below the frost depth and then install the air release valve. By the way, you need an air release valve, not a blow off. Typically blow offs are installed at low points to drain water, not air.

 

[TimMAz6]

cvg,

service saddle will be 5' below grade (existing main). I can't change this.

From the service saddle, the 2" service will go over an existing 24" sewer force main then down again 5' below grade. A 3' high point will be created.

When I clean my pool I create a 3.5' to 4' high point in the suction line and it takes a 1 minute to clean the line out of air.

Same is true with exterior hose. I can create a 3' high point and air will be expelled in a few seconds. Am I missing something here? Bimr's reference is about the only thing out there I can find which is a documented "engineering source". Any others out there?

 

[cvg]

when i said service saddle, I meant a service saddle to connect an air release line to your 2 inch pipe.

 

[CPENG78]

Tim,
It sounds like the service goes up over the force main and then comes down towards your property. From this I seem to read that the forcemain is shallow or the property been served by the water line is lower than road/street level or a maybe both. CVG is right, you will need an air release valve; not a blow off. By the way, if the agency allows it, can you provide a small concrete cap (I think this is the right term) over the forcemain and reduce the difference in clearance between the forcemain and your serivice.

It sounds like it may be a little bit of overkill for just one service but you have a special condition on your hands.

 

[bimr]

Think you are overly concerned about this. Hardly any water piping is installed with no pockets.

Unless your service connection is on the top of the water man, you are not likely to get large volumes of air in the line.

If you install a blow off valve in the building, it will enable you to flush out the line on start-up and you will not be likely to have problems.