Minimum Wall Thickness

[sowhatso]

I did a calulation to know the minimum wall thickness required for carbon steel pipe (ASTM A53 Gr.B , type E) that will be used for potable water pipes.

I did the calculation according to two ASME codes :

ASME B31.1 : Power Piping
ASME B31.4 : Pipe line transportation system

The calculated minimum wall thickness according to the first code (ASME B31.1) was much greater than the MWT calculated according to the second code , is that because the power piping code assumes that number of joints , valves ,and fittings existing much more than that at the pipeline ? Inaddition , the allowable stress value (S=25,200 psi ) at the first code was higher than the maximum allowable stress at the second code (S=12800 also) , Moreover , the weld joint efficiency factor at the first code is 1 , while at the second code 0.85 ? why we have this difference if we are dealing with the same material ?

 

[stanweld]

Per Table A-1 of B31.1, the design allowable stress for A-53 B Class E is 14.6 ksi at T < or = 650 F. The .85 E factor has been taken into account in the design allowable stress. Design allowable is based on a Safety Factor of 3.5 on the minimum specified tensile strength in that temperature range.

Other Codes use different design allowables based on different Saftey Factors (3:1 for B31.3) or are based on a percentage of specified minimum yield strength (B31.4 & B31.8).

Codes are written by people who represent companies and Jurisdictions with materials in differing services and margins of safety or additional inspection/testing criteria.

 

[sowhatso]

Dear stanweld ,

According to the ASME B31.1 (Version 2001) , the maximum allowable stress for ASTB A53 Gr.B , Type S (Seamless ) , is 15 ksi , and for Type E ( Electric resistance welded pipe) is 12.8 ksi, maybe for new update version it is different.

 

[stanweld]

The design allowable stresses quoted are for the current edition. And yes, the Safety Factor changed from 4:1 to 3.5:1.