Pressure parts?

[carletes]

Hello all,

As you know, any pressure containing element within the scope of ASME Pressure Vessel Code must be fabricated of materials according to ASME (materials SA). My question refers to valves and the materials to be used: which parts of valves are considered to be under pressure? I suppose that body or bonnet are evident, but stem or wedges for example? Must they be manufactured of ASME materials as well? Is it stated in any code?

Thank you a lot for any help.

 

[nealc]

In my understanding of the code, this is somewhat left up to the engineer. I do alot of design work on non-standard valves or "valve-like" components and systems, so make these decisions regularly. At a mionimum, ask yourself "If this material fails, what will happen". If the answer is a potential safety issue, then assume that this material should be per the code.

Second issue is that ASME allows for materials not explicitly listed in the code. The material must be purchased with appropriate material specifications to a recognized material standard. And this puts the burden on the engineer to prove the suitability of a material for the application. But if you are working on a product that has a long history of successful application using a material that is not listed in the code, this may be a worthwhile path. Just because a material is listed in the code does not make it appropriate for an application, and vice-versa

 

[bcd]

This is a question to which you will receive many different replies and theories. The simplest explanation I have ever heard from an Engineer is "if you can drill a hole thorugh it and get wet, then it is pressure retaining." Horribly misleading.

The pressure vessel codes refer to ASME B16.34. This specification defines allowable materials of construction for valve bodies and trim. If the valve is built in conformance with this specification, then it meets code.

The term "pressure retaining" in relation to valve parts is what creates much confusion and excessive documentation. The main issue is if pressure is a significant design factor. The stress in a stem from turning or linearly moving the control element is many times greater than the stress applied to hold the stem against internal pressure. Same holds true for bonnet plates. Usually, the stress applied to compress the packing is many times greater than the stress from internal pressure. Point is that if such parts do not break at assembly or when the valve is operated, they will not break when internal pressure is applied, so added documentation or testing solely to give assurance that the part will safely hold pressure is just a waste of time and money.

bcd