My general belief is to try and use something other than a knife gate. These valves were originally designed for the Pulp&Paper Industry and would leak both through the seat and the bonnet. The nature of paper stock is that it will de-water and plug, stopping the leak. Most "metal seated" knife gates state their leakage to a MSS-SP-81 and TAPPI 405-8. This standard states that with a 40psi differential across the valve it will not leak more than the number stated in the standard. If you don't have 40psid, it will leak a lot more. Furthermore metal seated gate valves are typically one-way tight.
The next thing is to look for areas where the material will pack as the valve is closing. After a couple of cycles the gate can't get to the bottom of the body.
Do you need "chest buttons", "support strips", "backing rings"? What is the bonnet packing arrangement? These people have tried everything to get a bad idea to work.
How about the ones that have rubber liners and push the process fluid out through the rubber to atmosphere with every closure. They even sell boxes you can add on to catch the crap after your floor is so slippery it is dangerous to walk on.
High performance knife gates like WEY, or ITT Fabri's new XV150 have address many of these issues but they come with a price.

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