Factor of safety in valves designed as per ASME B16.34

[Jaivishnu]

Hi
I am a newbie in piping. I want to know if valves designed as per ASME B16.34 have any inherent factor of safety like say 3.5

 

[LittleInch]

FOS on what?

Pressure?

Shells get tested at 1.5 x. MAWP.

Burst pressure not specified. I wouldn't like to guess much more than 2 myself before something breaks.

Too many options in valve bodies.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jaivishnu]

Hi LittleInch
You are correct . I meant FOS on pressure. In ASME BPVC SecII ,part D materials , allowable stress for design is taken as Yield strength / 1.5 , which indicates that a safety factor of 1.5 is considered. Pardon me if I am wrong.
I was wondering if any similar consideration is taken for allowable stress when designing valves in ASME B16.34

 

[bcd]

The question is impossible to answer with a general single value. As previously stated, it is going to vary by size, pressure class, valve body configuration, material and design. Also note that B16.34 identifies the minimum wall thickness. In reality the actual wall thickness of the valve body is usually much more. Especially in cast bodies where the foundry requires extra wall thickness in order to pour a sound casting.

You can generalize by reworking equation B-1 in Appendix B for a given pressure class and compare the hoop stress of a pipe using yield as the criteria. Keep in mind that valves are not designed based on yield. There are factors of safety and allowable stress values based on time dependent properties to avoid creep. If the valve body slowly distorts over time, then it can fail to seal and operate.

Often, when burst testing valves per section UG-101 of the code, the burst pressure often exceeds 5 times the rated pressure. Sometimes it can be as low as 2.5 times the rated pressure before a body joint separates and leaks at the gasket. But the body is still intact.

 

[davefitz]

I don't have a final answer, but the B16.34 code only allows a ( 10%) overpressure above the P,T rating of the valve, while the connected piping allows a much higher temporary overpressure. Many of the older valve designs were based on room temperature pressure failure tests in the 1930's, and might not pass section VIII calculation procedures, just as many older flange designs would not. Also, the limiting design item might not be the valve body but could be the stem, bonnet, bolting,or plug.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick