Sustained Allowable- ASME B31.3 & ASME SECTION VIII-2

Dear All,

I am working on calculation of Rpad thickness using Roarks equation.

These are the terms in that equations.

1. Hoop stress due to pressure, σhp - General Primary membrane stress, Pm

2. Circumferential membrane stress, σ2 - Local Primary membrane stress, PL

3. Circumferential Bending stress, σ ́2 - Secondary stress, Q

According to ASME Section VIII Div-2. Pm + PL + Q = 3 Smavg

Based on ASME B 31.3 for sustained allowable is only Sh. In sustained we are considering Due to pressure, Due to axial and Due to Bending then Why in ASME 31.3 has less allowable compared with SECTION VIII –DIV 2??

Please correct if I am wrong.

Durga,

I think you are mistaken something. The allowable loads (at room temperature) as per VIII div-2 will be on lesser side than in B 31.3. I rem for A 106 Gr B material the allowable starts from 20 ksi and in section VIII it starts from 17 ksi.

Regards,
R.K.

RK,

Thanks for your response,

1. My question is Pb primary bending stress. As per ASME SECTION VIII div2 , if we consider Pb the allowable is 1.5Smh

As per ASME B 31.3, sustained case(Primary stress) we are also considering bending loads due to W+P. why Sh allowable why not 1.5Smh as per Section VIII- Div 2.

Is the Pb Primary bending stress in Asme section - 8 div 2 is different with what we are considering bending stress in Sustained case in asme b 31.3?

If yes/ Please tell me what is that??

If my understading is wrong, please correct me.

hi Durga

stress classification defined by Asme section-8 div-2 is different from Piping code.

In piping code bending stress is calculated from bending moment while Asme section-8 div-2 bending stress is the changing stress through the wall.


Also note that as per Asme section - 8 div-2 the bending stress terms caused by external moment or internal pressure in the vessel wall should be classified as secondary stress(Q)regardless of whether they are caused by sustained or expansion.which cause Pb to disappear in the equation.


Regards
Jaypipingstress@gmail.com

Jay Stress,

Thanks for your reply.

Then How Pm Primary membrane stress(Pm<=kSmh)in ASME SECTION VIII taken as Sustained(SL<=Sh) in ASME B 31.3?

Pm also calculated along the thickness only(Across solid section)??

Please correct me, If I am wrong.