Stress Ratio as per B31.3 (MDMT calculations)

[octane]

I need some guidance for calculation of stress ratio as per ASME B31.3 for MDMT determination.
Material: A106 Gr. B
Design pressure: 150 bar
Size: 1/2" NPS Schedule XS (0.147 in. thick)
What would be MDMT for this pipe? As per curve A, it comes out to be -10 deg C.
Want to apply stress ratio to find any reduction, if possible.

[salzx64]

Dear Octane...can you specify the Design temperature of the said calculation?
For stress ratio you might need to calculate the tm = PD/2(SE+py), and for S you need design temperature.
Than for t (0.147 in.) you will again calculate the S. Ratio of both of them will give you a Stress ratio which you can use in the table 323.2.2B (31.3) for reduction in MDMT.

[octane]

Desired MDMT is -40 deg C. I do not know how can I get the value of S at this minimum design temperature so that I may divide the actual stress by this.
Appreciate your help.

[ben]

You are not looking at the right curve.
For A106 Gr. B, curve B applies as per Table A-1 of ASME B31.3 under Min. Temp. column. So the allowable MDMT without reduction as per curves is -29 deg C.

[mechcolor]

What's the corrosion allowance and the maximum design temperature?
What I have understood is that you want to avoid impact testing?

[octane]

Thanks everybody for the extended help.
The maximum design temperature is 150 C and corrosion allowance is 0.05".
Yes I need to evaluate that with this material, size is it possible to sustain -40 C without impact testing.

[ben]

When code says stress at minimum design temperature, you have to use the given value in Table A-1 that is for A-106 gr. B pipe, Sallowable would be 20ksi.
Now calculate the actual stress, Sactual, from the formula as provided by salzx64. And while calculating actual stress you must deduct the corrosion and other allowance from the nominal thickness.
I have done the calculations for you and the stress ratio comes out to be 0.64 which allows a reduction of almost 19 C.
See what you get.

[octane]

Have you any reference that we have to take the value stated under -20F to 100F, the maximum value of stress for minimum design temperature?

[ben]

Table A-1 of ASME B31.3 mentions the first stress value from Min. Temp. to 100F. It's very obvious to consider that stress value as the allowable stress at minimum design temperature.

[ashfaqanwer]

Octane, as Ben has mentioned you can get some reduction in MDMT while using a material not impact tested. Additionally, in such small thickness, likely hood of suspected brittle fracture is comparatively lower.
However as per my experience, A106 Gr. B piping must not be exposed to such lower temperature as -40 deg C. You may use that temporarily and I suggest you to move to A333 Gr. 6 piping and avoid any risk of brittle fracture.