ASME B31.4 Hoop Stress

[MechEngFiji]

Hi All,

What wall thickness should be used to calculate the operating hoop stress as per ASME B31.4 Clause 402.3? Should it be the corroded thickness? Is there an ASME B31.4 interpretation for this?

Thanks.

 

[LittleInch]

There is no interpretation because it is quite clear if you read the code and look up the nomenclature.

In 402.3 t is the "design" thickness, without any other allowances.

403.2.1 makes it very clear than the t nom >= t + A, where A is the sum of allowances for corrosion, threading etc

Note B 31.4 makes NO allowance for manufacturing tolerance unlike many other codes as this is taken into consideration in the Design factor used.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MechEngFiji]

Thanks LittleInch for your usual prompt reply.

My issue is that I need to work out the hoop stress to find out if impact testing (423.2.3) requirements apply.

The pipe wall thickness is 7.11mm and the calculated design wall thickness (based on 403.2.1) computes to 2.8mm (which includes a 12.5% mill tolerance). We have specified a corrosion allownace of 2mm.

Do I use 7.11mm to calcuate the hoop stress? The Autopipe model for the system that my colleague has done computes hoop stress based on the corroded wall thickness.

 

[MechEngFiji]

I have had look at the equations used by AutoPipe to calculate hoop stress and have discovered that for ASME B31.4 it uses the nominal (actual) wall thickness as a default.
Therefore, I would assume that the use of corroded wall thickness can be deemed as conservative, exceeding the requirement of the Code.

 

[LittleInch]

Well that's a different question.

For 423.2.3 I would use t nom - your CA.

Whilst the t for your design was only 2.8mm, you have quite rightly chosen a pipe much thicker than that for practical reasons and IMHO, you can use that additional metal in the relevant calculations. this is because it specifically states ..to be operated...", hence the actual reality of life in that you have a much thicker pipe than you need comes into play.

How autopipe works isn't that clear to me, but maybe if you actually insert a CA it will (or should) use t nom - the CA. Therefore it is not conservative, but actually following the general principles of the code to remove the CA from the nominal.

I don't understand your comment "...2.8mm (which includes a 12.5% mill tolerance)." B 31.4 or 31.8 do not include manufacturing tolerance in their calculations of t. This is stated as being "allowed for" within the design factor used in the calculation.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.