MAWP of a Thread-o-let

[zakk]

Is there anything in ASME Section VIII Div 1 that covers Thread-o-let fittings? Can they be used at the MSS SP-97 rating of CL3000 and CL6000 without analysis (MSS SP-97 is the design Standard for Thread-o-let fittings, but isn't listed in UG-44)?

The various Standards associated with weld fittings state that the MAWP for these fittings shall be calculated similarly as for straight pipe (ASME Sec VIII Div 1 UG-44, ASME B16.9 2.1, MSS SP-97 2.3, etc.), taking into account pipe undertolerance, corrosion, threading allowance, etc.

Using this criteria, I tried to calculate the MAWP of a NPS1/2 CL3000 Thread-o-let using NPS1/2 XH pipe as the basis for the CL3000 rating per MSS SP-97 (my vessel has a 1/16" C.A.):

Minimium Pipe Wall = (Nominal Wall-12.5%) - C.A. - Thread Depth

Minimium Pipe Wall = (0.147" * 0.875) - .0625" - 0.571"

Minimum Pipe Wall = 0.009"

A 0.009" wall will handle a full CL3000 rating??? I calculate around 500 psig using Appendix 1. Am I overlooking something? Is there something in the ASME Code that will exempt MAWP calculations for these fittings?

 

[GenB]

you did not have to deduct depth of thread since you are calc the fem part and not the pipe itself
ussually the weld'olet will have a little more wall than the pipe. and yes the final wall is the small # that is required to hold the pressure
it is a lot less than the safety required corrossion
(note, the forgind) does not have to suffer the 12.5% when calculated so it can be lot thinner than pipe.
You need the ASME/ANSI B16 fittings book, it pays to have it.
also the bonney forge specs.
genb

 

[zakk]

GenB, thanks.

There is no ASME B16 spec for thread-o-lets, only MSS SP-97.

I talked to my AIA and they said to treat the thread-o-let as a nozzle and go from there. This was my first take also, but I was thrown by the Standards which state to treat the fitting as straight seamless pipe. I must be misinterpretating this. If the vessel had a 1/8" CA and I used this in the NPS1/2 pipe wall calc, the thickness would be less than zero and I know these fittings are used on vessels with 1/8" CA. Again, maybe I'm misreading something.

 

[GenB]

if the wall t on 1/2in is 1/4" (i do not have specs on hand)
less 1/8 = c1/8" what are you taling about.

I use thread'olets a lot and they are B16 .- Am I missing something.- I will look at it further on Friday.
If not you can calc them as pipe and ..comply with the Code..
genblr

 

[zakk]

NPS1/2 XH, 1/8" CA, 14tpi

tmin = 0.875*tnom - CA - Thread Depth (0.8/n)

tmin = 0.875(0.147") - 0.125" - 0.057" = -0.053"

Thread-o-lets are not covered under B16.9 or B16.11. Which B16 Code can you find them? Bonney Forge references only MSS SP-97.

 

[GenB]

1st check your calcs: YOU do not needf to take 12.5 pc off
on forgings.- thats why.
I will look closer to the ansi spec.
genblr

 

[GenB]

I was going to ask you of how you've got the .147" t.
but I found that you took the pipe t. of 1/2 S80
The Code says: similar to the pipe run, it does not mean the same as the pipe run.
you are mistaken: take the t of the thread'olet less corrossion then run the calc as or using the formula for t. (or pressure) or both.
if that does not give you the require output... guess what:
use 6000# thread'olet.
It will end it here...
...THANK YOU...GenBlr

 

[zakk]

I just can't let it go...

MSS SP-97 para 2.2: ...Within these limits the maximum allowable pressure of a fitting shall be that computed for straight seamless run pipe of equivalent material...The wall thickness used in such computation shall be tabulated in B36.10M for the size and applicable schedule of pipe reduced by applicable manufacturing tolerances and other allowances (eg., threaded allowances).

MSS SP-97 para 2.3: Any corrosion allowance and any variation in allowable stress due to temperature or other design shall be applied to the pipe and fitting alike.

Therefore:

tmin = 0.875*tnom - CA - Thread Depth (0.8/n)
tmin = 0.875(0.147") - 0.125" - 0.057" = -0.053"

Too much time spent on this... THANKS anyway.