Storm water ventilation

[arj00]

Recently a new storm water system has been constructed in a flat city with big diameters(Dia. range 1meter-2.5 meters),
last week there was a big storm and we noticed there are a lot of air coming from the Manholes ,so we opened several manholes to relief all the air from the system.
Some engineers in the municipality thinks we should add a ventilation to the system to improve storm water flows, however in my long experience I have not seen any ventilation to storm water system,in case some one have
an experience in such a problem please let us share his experience.
Best Regards

 

[Bbird]

arj00

Air is always trapped if the downstream end of a gravity drain is drowned or running full. The full bore flow if travelling upstream can cause a large volume of air to be displaced.

Storm water system rarely run full and open channel flow should operate most of the time.

In a power plant trapped air is an important issue for the gravity portion of a cooling system as the downstream exit can be drowned by tides on a daily basis. In such cases air valves are often necessary to vent the air out.

A cooling water system does not have frequent manholes. Both running full and partially full are normal operating conditions. Flow velocities are higher than a storm water system and the pipe diameter can go up to above 4m.

If the storm water system is flooded frequently my first approach in your case would be altering the affected manhole covers to open grid type where air can be easily expelled. If air is able to dissipate continuously and not compressed then its effect should be less alarming and may be acceptable.

 

[RWF7437]

Why not install open grate manhole covers ? These should allow lots of air to escape as the storm drain fills. Solid covers make sense on a sanitary sewer where one wishes to minimize infiltration/inflow but on a storm drain we'd like to maximize I/I.

If the open grate cover don't provide enough ventilation, you could then consider installing vent at high points in the system or at regular intervals. Experimant until things seem to work. Shouldn't be an expensive fix.

good luck

 

[PECPESC]

Inlets on the system should be upgrade from the storm main and should let most of the air escape. Anything remaining should escape through the lift holes in the manhole covers.