saw tooth profile for sewer pipelines 1

[ghkskn]

What is the reason creating "saw tooth profile" for buried water/wastewater pipelines while you can just lay it as per the ground profile which would give you peaks for trapped air anyway.

By the way, in our project, the difference of ground elevation between pipeline start point and end points is +2m. So we have the chance to make the pipeline (12km) all the way "uphill". Do you think we still need the "saw tooth profile".

 

[BRIS]

A Saw tooth profile is used on water trunk mains to aid removal of air. A minimum descending slope of say 3 in 1000 and ascending slope of 1 in 1000 is common.

Air valves are located at the peaks. You can just lay to the ground profile, but you will need an air valve at each peak. It is a balance of the cost and maintenance problems of air valves against the additional excavation and filling costs of a saw tooth profile.

http://www.hrwallingford.co.uk/projects/air_in_waterpipelines/index.html

 

[ghkskn]

Thanks BRIS.
Let me correct the link you sent.

http://ftp.hrwallingford.co.uk/projects/air_in_waterpipelines/index.html

They also have a book on the same subject

http://books.hrwallingford.co.uk/acatalog/ISBN 1898485119.html

I would like to add that there is also additional cost of HDPE elbows and welds (and of course time) at the "Saw tooth profile" side.

 

[BRIS]

The saw tooth gradients should not need elbows the change in gradient can be accommodated with pulling the joints on rigid pipe and pipe deflection with flexible (PVC, HDPE) pipe. The objective is to avoid flat pipe were setting out and laying tolerances will inevitably produce high spots and air traps.


Problems are most severe on trunk pipelines that operate continuously at near constant discharge. Air pockets can be held in equilibrium on descending gradients where the drag force is balanced by the buoyancy force. As a result the air pocket cannot work its way back upstream to the air valve at the peak. Some suggest that air valves are placed downstream of the peak.

On distribution systems with diurnal flow variation air pockets/bubbles will either get swept downstream under high flow or work there way upstream at low flow. I.e. they will find their way to an air valve.

How much of this research is relevant and how much is promoted by the air valve manufacturers to sell more air valves also needs to be balanced.