Gasket of a valve or in-line meter with lining

[Instr13]

Hi, The pipe specification is asking for a spiral wound gasket but the flow meter has a PTFE lining. The spiral gasket has already damaged the face of the lining. Could you please give me an idea that if only replacing the gasket with a non-metallic one is sufficient.
Another question that I have is the fact that with lining overlapping the flange face do we still need any gasket between flanges? mating flage is a standard metallic (not lined) RF flange. Thanks.

 

[Scipio]

Depends I don't believe any of the PTFE lined flow meters I've seen have required gaskets, the PTFE lining serves as a gasket material. Normally the PTFE overlay essentially forms the raised-face portion of a RF flange. I have seen other instruments and valves with a complete full-face overlay of PTFE, however, which do require gaskets (always composite or elastomer, never spiral wound). Also the meter usually has specific bolt torque requirements to avoid/minimize cold extrusion of the PTFE. The damage from the spiral wound gasket windings may have destroyed the sealability of the overlay, you won't know until you bolt it up and try bringing the system up to pressure. Best thing is to double check with the flow meter installation manual - most manufacturer's of instrumentation even have 'em online these days, so they're pretty easy to come by.

 

[Instr13]

Thanks Scipio. The lining is already damaged but I am not sure if we need to replace the meter or just the gasket, for sure with a non-metallic one. Are you aware of any standard that might help us determining if we can still use this meter considering scratches on the surface of the lining due to spiral gasket.