Air valves for Sewage and Slurry Pipelines and Pump Stations.

By Peter Telle

Air valves for the water industry are commonly the focal product for engineering, client and valve supply companies, and their marketing, R&D and sales reflect this. However, sewage air valves might just deserve the same attention.

Currently, high volume sales for air valve companies are in the water industry, and most of these companies merely supply sewage air valves as an extension of water air valves. This lack of development in the way sewage air valves operate has caused them a bad reputation: They generally always “foul up” causing sewage leakages, which in turn cause health hazards. This results in high maintenance costs and, overall, a totally unsatisfactory situation.

The poor reputation of sewage air valves also often results in design engineers going out of their way to eliminate their use – often at great cost.

To explain the problem consider the following:

The biggest problem with sewage air valves is that when the sewage comes into contact with the sealing mechanism, the particles get between seal and seat preventing a proper seal being formed, with resultant leakages. To overcome this, most sewage air valve manufacturers make the body longer than their standard water air valve to prevent sewage from coming into contact with the sealing mechanism.

The below picture is a cut-away of a well known sewage air valve from the USA, alongside a well known sewage air valve made in South Africa on the right.

Both these manufacturers install their standard mechanism in a longer body to overcome the “fouling” problem, but all they effectively achieve is to delay the time before the sewage contacts the sealing mechanism… maybe by a few hours or days at the most!

The reason long bodies don’t really work is that the level of sewage will rise in the air valve over time. The reason for this is as follows:

As sewage enters the sewage air valve, the float gets lifted at a certain point, which causes the float to rise and close the outlet seal. Due to the length of the body, a certain volume of gas is trapped in the top part of the valve, which prevents the sewage from reaching the seal.

However, because sewage is normally pumped by positive displacement pumps which have a continuously changing discharge pressure, as the pressure in the pipeline drops, the gas expands. This forces the sewage level downwards, opening the valve outlet, allowing a small volume of gas to escape. During the next cycle of increased pressure, the gas is now pressurized to a higher level with resultant smaller volume, and the level of sewage rises. So, after a short period of operation the gas volume decreases, and the sewage level rises until it comes in contact with the seal, causing it to leak.

AFC’s SKAR sewage air valve works as followed.

Thanks for the big body of the SKAR, the water is still far away from the sealing surface when valve is sealed. This allowed more tolerance of air compression. The tolerance between the float and middle tunnel is very small which prohibited the dirty stuff get into the tunnel to protect the sealing part.