The use of lipseals on FCC slide valves is common and is becoming the default standard for bonnet cover to bonnet flange joints. It is rare that I see gaskets being used any more. The chance of leakage at this joint is greatly reduced over that of a plain gasket as long as the weld is quality. I have seen the lipseals leak but it is very very rare. If you need additional information please feel free to contact me at the below listed email address.