Description clarification on Ball Valves Primary/Secondary seal??

[howearthmademe]

Hi all,

I am new to Eng-tips and also new to the Valve world...

Could anyone help me out?? I got a description for the seating of a 30inch ball valve as Primary metal and secondary soft. Does it mean that the ball is soft seated??

Thanks

 

[gerhardl]

The central question in this case is (as always) if a certain construction of valve is recommendable, adwiseable or allowed to be used for a certain application.

This would be ruled by the given general design data, certifications, restrictions and local laws and standards.

The second question is to check the construction against the given description. (Your present stage?). This should then again be backed by factory description, procedures, test, certificates and previous references.

Then back to your original question. There is a number of different constructions that could fit your description, where the seat and seat sealing are a combination of metal and soft material, and the answer could be yes, no or mixed (?)

If the valve application is for flammable liquids or applications that should resist a fire, the significant question could be if the construction is of firesafe design or firesafe tested or not.

If this is for other fluids, and additionally really for all applications, the seal and total construction details compared to normal standard use for the fluid will give the answer.

 

[hsbcn]

You're probably being offered a PTFE or RPTFE seat (soft seat) in one instance, probably the cheaper option and a metal seat in the other (more expensive).

Depending on the application, you may need to opt for the metal seat, particularly if the pressure-temperature combination puts the soft seat at risk of creeping and henec not offering a good seal.

Heather's focus: Alloy Valves Duplex Valves Super Duplex Valves Monel Valves Inconel Valves [link titanium-valves.com]Titanium Valves[/url] & 6 Moly Valves

 

[superweo]

Hi,
this is a typical "trick" that usually valve manufacturer play to follow the specification.
As general rule when we speak about this type of valve we intend that primary sealing (valve under pressure) is metal to metal, and secondary (valve not pressurized) is soft (basically with Viton delta seal).
To be practical this is a standard soft seated valve especially if the service is sweet natural gas.
There are variation to this design depending on the fluid.
In some case you can get also a real PMSS but in this case you will have ball and seats Coated with Tungsten Carbide or at least ENP plated with different hardness.
Ciao

 

[valvit]

The PMSS (primary metal-secondary soft) seat was originally developed by Grove Valve and Regulator back in the late 1950's. It is the most used and most copied ball valve seat design on the market today.

The PMSS is designed to engage the metal seat at all times, with the soft seat as a back-up to provide bubble-tight shut off for both gas and liquid service.

Factory testing have proven the primary metal seat alone, without the soft seat installed, will provide effective shut off for both gas and liquid. With leak rates for gas service in the range of ISO 5208 Rate D.

As stated by superweo, the ball and seats are commonly coated with either tungsten carbide, or most often an electroless nickel as it is important to maintain a differential hardness between the ball and seat to prevent galling of the substrate. When provided with ENP, the ball is heat treated following ENP for maximum hardness and the seat ring is not...thereby achieving the differential hardness.