B31.3 nominal pressure stress from thickness calc

[JAYDEE23]

Can i rearrange the equation for required wall thickness in B31.3 sec. 304.1.2 eq.a to find nominal pressure stress refered to in sec 345.4.2 (test pressure)

i would leave out the E quality factor and the W weld joint strength reduction and substitute actual wall thickness for required wall thickness.

so
t=PD/2(SEW+PY)

would become

S=((PD/T)/2)-PY

 

[metengr]

No. P=design pressure, S=allowable stress. Why would you even consider this approach?

 

[JAYDEE23]

why? because i lack experience.. any suggestions to steer me in the right direction would be appreciated..

 

[metengr]

JAYDEE23;
Appreciate your candid response. I would suggest you look for one of the review classes offered by ASME.org on ASME B31 Piping. It is an excellent way to get introduced into the Code and applications.

 

[JAYDEE23]

It is on the list of to do's actually. I am familiar with small focused sections such as required wall thickness and was trying to extrapolate from there.

what i am looking to do is just follow code as to ensuring our hydro pressures are not violating individual component yield numbers. Just a side project right now.

Any insight is appreciated if you have the time...)

 

[JAYDEE23]

I used Circumferential hoop stress S = P D / 2 Tmin

and

Longitudinal Stress S = ( P D ) / ( 4 Tmin )

to check hyrdo test pressure does not exceen yield strengths of components...thoughts?

 

[ColonelSanders83]

You are still going about this incorrectly, rather than calculating the stress you should replace the allowable stress with the allowable stress under hydrostatic stress (90% of yield if my memory serves me correctly, check the section on hydrostatic testing), and then see if your thickness is adequete.

 

[JAYDEE23]

I believe you are refering to B31.1 section 102.3.3(B)

(summerized)
-Longitudinal and hoop stress shall not exceed 90% of yield strength during test at test temperature.

In B31.3 section 345.4.2(c)
(summerized)
-If test pressure would produce nominal or longitudinal stress in excess of the yield strengtrh at test temperature test pressure may be reduce to.............

Am i missing something here?

 

[JAYDEE23]

i spelled summarized wrong...

And i do appreciate any and all help ....thank you....

 

[rneill]

You might also want to look at the latest Edition of B31.3 (2008) as the language around test pressures for piping operating at elevated temperatures has changed somewhat.

I would use the basic formula but with 90% of the SMYS as the allowable and then see what the required pressure thickness was. I would compare this to my nominal thickness less tolerance but I would not deduct for corrosion allowance on new piping since it will be uncorroded.

 

[JAYDEE23]

i am using the latest code revisions and yes there is some slight wording difference. Why does 90% keep popping up. Can you refer me to this igure in B31.3

B31.3 2008

-345.4.2 Test Pressure. Except as provided in para.
345.4.3, the hydrostatic test pressure at any point in a
metallic piping system shall be as follows:
(a) not less than 1.5 times the design pressure.
(b) when the design temperature is greater than the
test temperature, the minimum test pressure, at the point
under consideration, shall be calculated by eq. (24).
When the piping system contains more than one material
or more than one design temperature, eq. (24) shall
be used for every combination, excluding pipe supporting
elements and bolting, and the MAXIMUM calculated
value of PT is the minimum test gage pressure.


This is of course limited by

(c) if the test pressure as defined above would produce
a nominal pressure stress or longitudinal stress in
excess of the yield strength at test temperature or a
pressure more than 1.5 times the component rating at
test temperature, the test pressure may be reduced to
the maximum pressure that will not exceed the lesser
of the yield strength or 1.5 times the component ratings
at test temperature.

 

[ColonelSanders83]

Just because the code allows me to go to yield on a hydrotest does not mean I would allow it on a system I designed. I do not have enough faith in the quality of the materieal, the quality of fabrication, and the quality of the test equipment to ever allow the piping to be tested up to its limit.

What happens if you yield the pipe?

How will you know if it yielded?

Where did it yield?

All question you won't be able to answer.

I prefer to play safe and stay just shy of the line rather than on it.


PS the 90% comes from the alternate allowable for occasional loads. (extreme earthquakes and such)

 

[rneill]

I'm pretty sure that many of us have a 90% figure in our heads since earlier versions of B31.3 limited hydrotest pressures to less than or equal to 90% of the SMYS. As per your note, B31.3 now references 100% of SMYS.

Note: if you work with CSA Z662, it permits testing to 100% of the allowable stress but requires a "yield plot hydrotest" if you exceed 90% of the allowable stress.

 

[JAYDEE23]

Thanks for all the advice. Makes perfect sense to limit to 90%. Thanks!!