B31.3 Section 302.2.4 Verification

[guilermo]

Hi,
I am trying to verify the above mentioned clause of ASME B31.3 in order to determine if short pressure/temperature excursions above the design conditions of several piping systems are acceptable or not.
Besides other criteria also mentioned, I have the following question:
The paragraph (b) states:
#quote:
"Nominal pressure stresses shall not exceed the yield strength at temperature."
#unquote

How can I calculate the "Nominal pressure stresses"?
Can i rearrange the equation for required wall thickness in B31.3 sec. 304.1.2: S=((PD/t)/2)-PY
If so, what is the value of the thickness that I must consider in my calculations.

Perhaps I missunderstood the term "Nominal pressure stresses", but I could not find a definition for that.

 

[XL83NL]

The nominal pressure stress is the hoop, calculated using e.g. Lamé's eq., from which the equations in para 304 for wall thickness 'sizing' are derived.

 

[MITProfessor]

hi,

The formula S=((PD/t)/2)-PY can be rearranged within mathematical standards also can be rearranged to reverse calculate different values (incase thats what in needed).

 

[guilermo]

I did not get a (complete) answer yet to the above mentioned query:


Even if I make use of the rearranged formula: S=((PD/t)/2)-PY; what is the value of the thickness that I must consider in my calculations of nominal pressure stresses?

Anyway, it seems to me difficult (perhaps imposible) to exceed the yield strength if at the same time it fulfil paragraph f.1) and: occasional variations above design conditions exceed the pressure rating by no more than 33%. If so, what is the reason of paragraph b) then?

Is there anybody who has made use of Section 302.2.4 of B31.3 code before?

 

[weldstan]

If you have seamless pipe, .875 x the nominal thickness is used for t.

 

[TGS4]

To supplement what weldstan said, you also need to subtract the corrosion allowance to that calculation.

It may not be difficult to exceed Sy if you have a combination of higher temperature (lower yield strength) and higher pressure. Furthermore, there are some materials (think stainless steels) whose allowable stress may be as high as 90% of yield at temperature.

This section of B31.3 us used more frequently than one might think!

 

[jte]

The mill underthickness tolerance (0.875) is sensible for new construction.

If this issue is coming up in a post-construction environment, then ignore the mill underthickness tolerance as you are now using as-measured (typically UT) thicknesses.