Leak Testing (hydro) w/o Considering Stress Ratios 3

[eduardoeh]

Hello,

I've been inspecting local fabrication shops and i've noticed that at each one they do not factor the stress ratio [Stress Value of the metal at test temperature / Stress Value of the metal at design temperature].

E.g.:
Contact Tower
MAWP: 1360 psi @ 195 deg F
Leak test was done at 1768 psi @ ~85 deg F (cant remember the exact temperature).
So, they simply multiplied the MAWP by 1.3. I do not have any books with me to find the stress values but I am sure that for that ambient temperature, the test pressure should be much higher. Note, it has 600# flanges.

I did mention it to my superiors and their response was that the MAWP is not "high enough" to consider the stress ratios. The same thing happened with piping.

What are your opinions?

 

[TGS4]

I am going to assume that you are referring to pressure vessel fabricated according to ASME Section VIII, Division 1, post 1999 Editions.

The calculation in UG-99(b) states that the minimum hydrostatic test pressure, at the top of the vessel, must equal 1.3*MAWP* the lowest stress ratio (LSR) for the materials of which the vessel is constructed. The stress ratio is the allowable stress value at the test temperature - St, divided by the allowable stress value at the design temperature - S. Hence, St/S.

For typical carbon steel vessels (e.g. SA-516-70), S=St up until (and including) 500°F. So, in your example temperatures of 195°F DT and ambient of 85°F, St/S=1.

It's certainly not an issue of the MAWP being "high enough". That's just wrong.

 

[DSB123]

If the fabricators do what you say all of the time (even when the design temp is above 500 Deg F when S<St) then they are wrong!!!!

 

[r6155]

eduardoeh
You are wrong with stress ratio
You are wrong with leak test. This is Hydrostatic test. Please see ASME V for leak test

Regards
r6155

 

[eduardoeh]

TGS4 thank you! It makes sense what my superiors meant to say. They meant that the allowable stress doesn't change up to and including 500 deg F. I wish I had the code book to view these tables and not to search the internet.
By the way, the material is SA-516-70 and Sec VIII Div 1.

 

[Duwe6]

You will find the tables for c/s in Section IIA. You need to look the common materials up ASAP. The shops all have Sect. II books, and the tables don't change [since 1999]. Just ask to look at an old one.