Differences in Minimum Allowable Stresses in ASME B31.3 for Low and High Pressure

[Drilling Integrity Super.]

While working on an inspection procedure for process piping in drilling units, I noticed an interesting detail in the ASME B31.3 code. This standard separates low-pressure and high-pressure systems into different chapters.

A key point in this process is the calculation of minimum required thickness, which depends on the minimum allowable stress of the material. These values are listed in the code:

• Table A-1: For low-pressure systems.
• Table K-1: For high-pressure systems.

However, when checking Table K-1, I saw that for the same material and temperature, the minimum allowable stress values are different from those in Table A-1. This raises questions about the reasons for these differences and how they affect material selection and calculations.

Have you come across this in your work? Share your thoughts in the comments!

 

[GBTorpenhow]

Its not a low pressure vs high pressure thing, "High Pressure Fluid Service" is a special service designation (vs "Normal Fluid Service", "Category D Fluid Service", etc.) that triggers additional requirements and different allowables. There is no specific pressure that triggers this, it is an owner driven service designation.

It has to do with the thin wall behavior assumption that drives standard code stress calcs starting to not be as applicable at certain pressure levels and wall thicknesses. It is explained pretty well in the text of the code IIRC.

 

[Drilling Integrity Super.]

Its not a low pressure vs high pressure thing, "High Pressure Fluid Service" is a special service designation (vs "Normal Fluid Service", "Category D Fluid Service", etc.) that triggers additional requirements and different allowables. There is no specific pressure that triggers this, it is an owner driven service designation.

It has to do with the thin wall behavior assumption that drives standard code stress calcs starting to not be as applicable at certain pressure levels and wall thicknesses. It is explained pretty well in the text of the code IIRC.

Thanks

 

[GD2]

While working on an inspection procedure for process piping in drilling units, I noticed an interesting detail in the ASME B31.3 code. This standard separates low-pressure and high-pressure systems into different chapters.

A key point in this process is the calculation of minimum required thickness, which depends on the minimum allowable stress of the material. These values are listed in the code:

• Table A-1: For low-pressure systems.
• Table K-1: For high-pressure systems.

However, when checking Table K-1, I saw that for the same material and temperature, the minimum allowable stress values are different from those in Table A-1. This raises questions about the reasons for these differences and how they affect material selection and calculations.

Have you come across this in your work? Share your thoughts in the comments!

You are right. For t<D/6, the Lame's equation for thickness calculation becomes overly conservative. The stress across the wall thickness is no more assumed as membrane (equal) and varies across the wall.
That's where the base code and high pressure fluid system Chapter IX kicks in.
If you compare the Para 304.1 and K304.1 the thickness equations are different. So are the Bases of Allowable stresses explained in Para 302.3.2 and K302.3.2 (it is something similar to BPVC Sec VIII Div 1 and Div 2 allowable stresses). It's all about defining the stresses more accurately for thicker vessels or piping and the risk that comes with higher pressure.
Note that the Owner will require to designate the piping system as being in 'High Pressure Fluid Service'. There is no limitation on the pressure but the entire Chapter IX must be applied in entirety.