MDMT for a fitting

[Jano6924]

Hi everyone:

This is the first time I face a situation like this: I'm working in an ASME VIII-1 horizontal vessel design. Vessel has a SA-106-B Shell and a SA-234-WPB Cap on each end, plus nozzles on Shell only. Both Caps are ASME B16.9 fittings, therefore the calculation of Caps is per UG-44 rules (no thickness calculation, just the pressure-temperature ratings). Design conditions, per my customer, are the following:

Design pressure = 200 psig (no static head)
Design temperatura = 375°F
MDMT required = -25°F
Corrosion allowance = 0.125"
Outside diameter of Shell = 18"
Thickness for Shell & Caps = SCH Std = 0.375" (nominal thickness)
Shell and Caps are curve B materials

Since required MDMT is -25°F paragraph UG-20(f) is not applicable, therefore I shall follow UCS-66 rules to calculate the rated MDMT for components. I have no problem with the shell and nozzles but ¿how can I calculate rated MDMT for the Caps when per UG-44 no thickness is calculated for ASME B16.9 fittings?

¿Shall I use a procedure similar to that outlined in UCS-66(b)(1)(-b)?

I cannot find any specifics on ASME VIII-1 on rated MDMT for fittings.

Any comments/guidance will be appreciated.

 

[SnTMan]

Jano6924, the cap required thickness is the same as a straight pipe, see UG-44.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Jano6924]

SnTMan:

Thanks for your reply. After reading UG-44 again and again, and per your comments I conclude that the rated MDMT for ASME B16.9 fittings shall be calculated as for straight seamless pipe but using the allowable stress of fitting material and considering the thickness tolerance for the fitting.

¿Correct?

 

[SnTMan]

Jano6924, correct, thickness tolerance for the fitting just means the 12 1/2% pipe wall tolerance rather than tolerances applying to the various fittings, which are not specified further. See B16.9.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[Jano6924]

Thanks again SnTMan!

 

[Mech2325]

can anyone please explain what is meant by the thickness as of a straight seamless pipe. From this i assume that it is the thickness of the pipe used as shell in this. If my understanding is correct, then, because the shell is of sa-106 gr B material, it will have a standard schedule (thickness). Now, the same will be the thickness of the pipe fitting. Now, in this case, the unadjusted MDMT will be the rated MDMT as we have no margin for additional thickness which will be used in MDMT reduction. Am i correct??

 

[SnTMan]

Mech2325, the difference (hopefully positive) between thickness used vs thickness required can provide a basis to reduce the MDMT. Again, see UG-44.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[r6155]

@ Mech2325 "can anyone please explain what is meant by the thickness as of a straight seamless pipe".
Instead of calculating the thickness of fittings,it is more easy to calculate the thickness (equivalent) as for straight seamless pipe. Please read ASME B16.9 2.1 and try to understand.

Regards
r6155

 

[Mech2325]

@sntman and r6156: My understanding from this is simple. Whatever Is the schedule of shell pipe will be the schedule for fitting?? Right??
Yes, i know that MDMT reduction is based on this but my question is, that how to calculate the "required thikness" of fitting as UG 44 states to use thickness of a straight seamless pipe?