UG-45(b) Corrosion Allowance 1

[mxms80809]

I have a 0.5" shell with an internal pressure of 5psi. A 2" sch 40 (strd pipe size) goes through the pressure boundary. The required thickness of the shell is 0.388". and the corrosion allowance is 0.04". When applying Section UG-45(b)[note that UG-45(a)<(b)] I get:
0.175 UG-45(b) [in]
0.428 UG-45(b1) [in]
0.175 UG-45(b4) [in]
0.100 min thickness provided w/ under tolerance [in]

which indicates the pipe thickness needs to be increased. If however, there was no corrosion allowance, the pipe would be sufficient with:
0.135 UG-45(b) [in]
0.388 UG-45(b1) [in]
0.135 UG-45(b4) [in]
0.135 min thickness provided w/ under tolerance [in]

So my question is, first is my interpretation correct, and if so, does this mean standard pipe size can only be used when corrosion allowance is 0?

 

[TomBarsh]

"So my question is, first is my interpretation correct,"

Seems to be. I'd rather have more detail in order to run all the numbers. But if your arithmetic is correct, then you can't use standard wall pipe for this particularapplication.


"and if so, does this mean standard pipe size can only be used when corrosion allowance is 0? "

It is not true that in general you cannot use standard wall pipe for ASME Section VIII Div 1 design, although I have heard this stated numerous times.

For example, for the situation you described you could use the standard wall pipe with 0.04" CA if the pressure (and shell required thickness) is reduced.


UG-45(b) states that the minimum wall thickness of
the nozzle neck "shall not be less than the smaller of... applicable rule in (b)(1), (b)(2), (b)(3), and the thickness determined by (b)(4).

In other words, determine the required thickness as per (b)(1), (b)(2), or (b)(3), whichever is applicable, then take the smaller of that value and that from (b)(4).

The current version of UG-45(b) is a mess to read and I have listened to people argue about the placement of the commas, etc.

I had one ol' feller tell me he'd been doing this for 40 y'ars and he'd always done it this a'way (nozzle neck no less than standard size pipe). Fortunately, I had a copy of the 1959 Code Edition and read it to him. That Edition was much more clear and it was explicit that the required value was the smaller value of the calculated shell thickness (plus CA) and min thk of standard wall pipe (plus CA).

 

[jamesl]

Tom is right. This question has been answered in Code Interpretation VIII-1-83-199. The minimum nozzle neck thickness shall be the least that is determined by UG-45(a), but not less than the smallest determined by UG-45(a)(1) through (4). Standard wall pipe can be used for nozzle neck with C.A. unless UG-45(b)(4) governs.

 

[mxms80809]

Thanks, You have all just confirmed what I thought was going on. Where can I find the Code Interpretation VIII-1-83-199 that was mentioned?

 

[jte]

You'll find interpretation VIII-1-83-199 in Interpretations Volume 14 published in 1984 on page 111. Available on IHS or (hopefully) in your company library. Otherwise, your AI might be able to provide you a copy or you might have to check with a technical library. If you have a company library with old codes, but not separate Interpretations volumes, look in the back of the old code books. Sometimes people put their Interpreations in the back of their current code books.

jt