Isolation valve for sewage

[rgvrider]

Hi,

I'm after 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 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).

Thanks

 

[stanier]

Single acting (spring return) pneumatically operated knife gate valve using solenoid valve as you have suggested.

Knife gate valve will cut through any debris whereas a diaphragm valve may only partically close .

 

[gerhardl]

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.

Others to be considered:

Gate-valve (more suspectible for debris)

Double eccentric plug valve (cost much more)

As a failure safe solution electrical with UPS (battery backup) is possible but will need a longer closing time, and for this size cost considerably more than pneumatic with spring or pneumatic with air vessel power-source backup.