piping specified as Min Wall

[pipe15]

I have calculated the min wall required using B31.1 section 104 criteria and have accounted for all allowances for strength, erosion, etc. This pipe will not be bent, so the recommended bending allowances in 102.4.5 do not apply.

my question is:
Is there a rule of thumb that I should add some small amount to the calculated thickness in order to account for anything not addressed by the code requirements. (either an absolute value like maybe 1/64", or so many mills, or some small percentage increase)

thanks
Mark

 

[davefitz]

You can purchae the pipe as a schedule pipe, a forged pipe with a min wall specified, or as a tube with a min wall.

The schedule pipe is also ordered with an ASTM spec defined. There are mill tolerances that are applicable to the schedule piping, typically one uses an estimate that the min wall of a schedule pipe is 87.5% of the average wall that is listed on the schedule tables.

For a forged pipe as used for headers or thick wall piping, the pipe mill will determine the add'l tolerance needed between avg wall and req'd min wall , based on their experience with their forge equipment. You may need to know this tolerance for calcualtion of piping dead weights for suppors , shipping, crane loads, etc.

For a tube specified with min wall , you can use a historic estimate of variation of min wall vs avg wall, but the tubing will be orderd by min wall and ASTM mat'l spec. Each tube mill has different margins between min wall and avg wall, depending on finishing method ( hot findished, codl rolled finished, etc), and the technology the individual mill uses. You may need to know avg wall and statistical variation in wall thickness and ID in order to calculate some heat transfer , fluid thermohydraulic stability, and corrsoive life purposes.

 

[pipe15]

I think I have a handle on most of the issues in your reply.

for my application, a specific example is

30" OD with a min wall per B31.1 of 1.78"

is there a typical or general practice value that I should add to this calculated number in order to get me somewhat above that thickness required based just on temp, press, and all allowances for strength, erosion, etc. I do not like the idea of being exactly at the minimum calculated thickness.

thanks again
mark

 

[StressGuy]

I think your overthinking this one. The "Minimum" wall is was is calculated to wistand the design pressure of the pipe at the design temperature. There is already a safety factor in that calculation as the Allowable Stress is much reduced from the Yield Stress.

Further, the erosion/corrosion and mechanical allowances are already the "good practice" based on the experience of the industry, to get you to a point above the bare minimum.

In most cases, you are going to take the calculated wall and round up to the next highest commercially available pipe schedule. However, that isn't going to apply in your case as I don't see any standard schedules to cover pipe of that diameter and thickness.

In your case, it is going to come down to the tolerance level of the pipe fabrication technique. In your case, I would try to add as little as possible to your value for two reasons:

1. Thicker wall = stiffer pipe = higher anchor and equipment connection loads. With 30" pipe at 1.78", your stress guys are already going to have their work cut out for them.

2. From the size and wall and a B31.1 application, I'm going to guess that your looking at P11/P22/P91 for high pressure steam and those materials don't come cheap, especially in the current market.

Edward L. Klein
Pipe Stress Engineer
Houston, Texas

"All the world is a Spring"

All opinions expressed here are my own and not my company's.

 

[davefitz]

The reason for the use of erosion allowances and mill tolerances are to account for normal and expected wall thickness variations over the service life of the part. Added to this are all the implied safety factors associated with the determination of code allowable stresses, such as using the worst sampled material properties ( Yield , UTS, creep life, etc).

So , there are already mucho saftey factors added. If everyone who looks at the calculation adds another 5% to feel good, you will end up with a brick-!@#$house, or at least a non-competitive design.