Most water and wastewater treatment plant personnel understand check valves are a vital component in their water pump systems. Without a check valve, they could not safely operate their pumps. Check valves in their WWTP air supply systems are not as well understood.
In aeration blower systems, check valves are installed in the outlet piping to prevent reverse air flow through the blower. If there is only one blower on the system, the check valve is not needed. But since 99% of blower aeration systems have at least 1 backup blower, each blower will have a check valve. There are many types, but the wafer type check is most common with centrifugal blowers. It consists of 2 half moon wafers with a center hinge pin. Simple, effective, maintenance free, and expendable--it does not last forever.
The +200 degrees F temperatures of the outlet piping control the life of the silicone or buna flex sealing material in the check. Old age also shortens the flexibility of the sealing material to the point where it dries & cracks and no longer allows the half moon flapper to move to the sealing position. Oftentimes one of the half moons would stick in the closed position, which reduces the air flow out of the blower.
Without a check valve, another blower running on the same system would push air into the outlet of the blower, causing the blower to run backwards. The blower running backwards isn't necessarily bad for the blower unless you try to start it while the rotor is turning. Energizing the motor at this point "brakes" the rotor putting excessive stress on the shaft, coupling, and impellers. Coupling bolts can shear, keyways crack, or aluminum impellers shatter!
At some plants, closing the inlet and outlet butterfly valves can prevent this reverse flow.
The safest bet is still to replace the bad check. The check valves may be expensive, but they are never as expensive as the blower repair or replacement. Manufacturers have also increased the cost of their rebuild kits consisting if the internals only, to about 75% of the cost of new!
So it comes down to the age old adage: preventative maintenance is expensive. But still, invest in PM, it will save you money!
