An isolation valve for a waste water treatment plant. Our inlet pipe is above the level of the treatment plant, so on power failure of the plant, I would like to isolate the inlet flow so:
1. The pipe does not drain and draw air through vacuum valves.
2. The inlet tank does not overflow as the inlet pipe drains to it.
My thought is to use a straight through diaphragm plug valve, with a pneumatic fail closed actuator and a energised closed solenoid on the outlet of the actuator. This way, if we have a power failure, the solenoid with de-energise, release actuator air pressure and close the valve.
This is on a DN250 pipe line, sub 5 bar operating pressure. Has anyone got a better solution or other ideas? I think this solution may be expensive I'm not sure diaphragm valve is best for sewage (ideally knife gate).
For a DN250 mm the post from stanier above is also in my opinion correct, and this will normally be the cheapest choice, and also technically satisfying.
Maintenance: piston rod and knife to be checked and kept lightly greased with waterproof grease.

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