Polyurethane Valve Seats 1

[sewerfix]

What are the pros and cons in using knife gate valves with polyurethane seats for municipal sludge applications.
Obviously the suppliers talk about abrasion resistance, however I'm just curious if there are any negative aspects such as not being able to achieve a positive seal where foriegn matter (stones, fibre) causing the seat to deflect etc. Would a metal seat be a more superior unit.

Thanks
Andrew

 

[JimCasey]

PU seats are abrasion resistant and also resistant to most chemicals you would encounter in a waste stream. Another choice would be Ultra High Molecular Weight Polyurethane, which is abrasion and chemical resistant, but not as "springy" as PU. UHMWPE is Less able to conform around small particles, but more able to slice fibrous substances found in waste.

 

[BSQUAREDBUCKEYE]

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.

So whats my solution? An eccentric plug valve, a pinch valve, anything but a knife gate...