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Metal to Metal Seal of Wedge Gate Valves

Metal to Metal Seal of Wedge Gate Valves

Metal to Metal Seal of Wedge Gate Valves

(OP)
Metal to Metal Seal of Wedge Gate Valves

How can I consider the additional preload needed in the seat-gate valve system to maintain the compressive load
on a STELLITE (Seat HF - Wedge HF / Roughness Ra=0.2 lapp) seal surface after internal pressure is applied.

Can I use for this problem the Gasket Factors "M" and "Y" (usually used for metallic gasket / flange design) and the related theory to estimate the required loads?

Thanks

RE: Metal to Metal Seal of Wedge Gate Valves

Please forgive me for being cynical and I certainly do not mean this to be insulting, but in the real world the seating load of a gate valve is governed by the length of the cheater bar the pipefitters use. (insert Tim Allen noise here)

If you feel a responsibility to analyze it mathematically, it's a function of the cosine of the wedge angle getting the specific loading up to the plastic yield stress of the surface, and throw in a correction for unlubricated Mu of Stellite on Stellite which is, by the way and luckily, non-galling.  

RE: Metal to Metal Seal of Wedge Gate Valves

JimCasey,
       is Stellite on Stellite actually non-galling?
       Mmmh... interesting!        
       Are you referring to a particular grade, or in general terms?

Thanks and Regards,      'NGL

RE: Metal to Metal Seal of Wedge Gate Valves

>>is Stellite on Stellite actually non-galling?<<

Good question

There are a heckuva lot of control valves in the world with stellited parts that rub each other.  With loading as high as you'd need for a wedge gate, it might just be enough to pass a threshold and cause the metal to smear.  

Generally similar materials gall.  I  have always wondered why Stellite(6)  does not.  

RE: Metal to Metal Seal of Wedge Gate Valves

(OP)
Hi

For my experience as Valve Designer a gate valve stem has to support 3 major types of forces: Seat Friction ( pressure and wedge angle depending) – Packing Friction – Hydrostatic Force.

But for a Stem Thrust equal to the force sum I don’t have any well seal, because I need an additional thrust to press the Metal to Metal HF Seal Area ( LIKE A GASKET).

Usually I use a safety factor to obtain the additional force, but now a customer ask my a correct analytical evaluation.

Can anyone help me???
Thanks a lot!!!!

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