Q. Air release valves (ARV) are conventionally installed on long distance water pipelines. Why are ARVs not required on slurry pipelines?
A. With regards to any piping system that has the potential to become pressurized (i.e. not open channel flow); it is a good idea to have some sort of air control strategy. This applies whether the medium fluid is clean supply water, wastewater, or sewage (slurries). The reason that air valves might not be used on slurry systems is often because of the potential for the solids to foul the valve seals and seats. However, there are air valve designs now, which can overcome the fouling problems traditionally associated with waste water systems.
The presence or absence of air in a piping system can cause severe problems and even damage. The presence of air in a pressurized system can cause obstructions to fluid flow up to and including complete obstruction. Other problems include increased head losses, water hammer, and corrosion. The absence of air in certain vacuum conditions can be equally if not more destructive. You can have pipeline collapses, and increased surge and slam damages.
A properly air-controlled system should have large orifice valves for releasing air during the filling of the pipes and admitting air during the drainage of the pipes. It should also have small orifice valves that allow for the continuous release of entrained air bubbles while the system is pressurized. Air pockets tend to accumulate at the high points in a piping system, so these valves are often installed at the high points. There are combination valves manufactured which combine both large-orifice and small-orifice functions into one valve. These combination valves can take care of all air control requirements and can be used in clean water and slurry operations.