Materials of discs and seats

[carletes]

Dear all,

As far as I know ANSI B16.34 defines the allowable materials for body and cover but not for other like seats or discs. Are there alternative rules for those parts? What I usually do when specifying valves is to select those materials according to the type of body material (carbon steel, stainless etc) and design temperature, using ASME Sec. II Part D to get the allowable temperatures of each material.

Thankis

 

[unclesyd]

The seats and disc are usually selected according to the compatibility with the process. This is done within each manufacturer's available materials for a specific figure valve. The disc and seat materials offered for each figure valve will usually operate within the pressure/temperature limits of the valve body unless otherwise noted. Another area you have to watch is the type of packing supplied. Is the supplied packing compatible with your process.

 

[carletes]

Thank you for your help unclesyd!!

But, is there any code to say if a materail is allowable or not depending on the temperature of the service? (apart from other issues like corrosion, for example)

ANSI b16.34 as far as I know says nothing about those parts of the valves.

 

[unclesyd]

I don't know of any though there are specific recommendations from agencies like NACE and API Etc. for certain conditions that are in their fields of expertise. There is a lot more to selecting a seat material than pressure boundary considerations. The selection of seat material is usually covered in company standards based on experience, all of which are based on the availability of a particular seat materials for a certain figure of a valve. You also have to depend on the valve manufacturer's recommendation, which is also based on experience and feedback from the customer.
Most valve companies have a table of compatibility of their valve seat materials with common environments and if you don’t see it ask. The rub come when you have a new process or process that is not covered in the tables. This is the time that one my require the assistance of a materials engineer to get you in the ballpark on the material selection or better yet recommendations for a specific combination of materials.
If you can get a standard seat material from a particular valve manufacturer that will work use it even though may be a little more costly than another valve with an adder. One always wants to stay away from specials unless it is absolutely necessary.

 

[RXH]

Carletes

You would use the relevant Code and other references to determine the suitability of the trim materials just as you would for the body material keeping one thing in mind: the trim materials have to be equal or better than the body material because they are "working" parts.

For example, let's say you are selecting the materials for a gate valve in a hydrocarbon process in a refinery. The governing code is ASME B31.3 and you would look at the Appendix to determine temperature limitations. If your piping is carbon steel then you would probably select the same valve body material knowing that the limits are -20 to 800 F. Next if you look at manufacturers catalogs (and API 600) you would see that SS 410 and SS 316 are the most common trims available. In this case we know that temperature limitations for these materials are higher so you are OK. By reviewing corrosion data you can verify whether these materials will be acceptable for your service (they probably are). Lastly, check out packing material. Graphite packing is standard and will work for a large range of services.