Metal to Metal Seal of Wedge Gate Valves

[benzy4]

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

 

[JimCasey]

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.

 

[NGiLuzzu]

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

 

[JimCasey]

>>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.

 

[benzy4]

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!!!!