Dear all,

Anyone of you come across in-accurate stress ratio to compute further MDMT deduction per UCS-66.1?

The tr value used in below formula
Stress ratio = tr x E / (tg - c)
did not corresponse to correct tr @ MDMT temp.

The extracted report to show this doubt with opinion & simple PVElite input file are as attached for reference.

tr @ design temp. = 32.6 mm
tr used in above formula = 34 mm and not 32.6 mm.
This provides an in-accurate stress ratio 1.0 with zero further MDMT deduction.
Finally, PVElite concluded MDMT = -28°C and this is warmer than required -29°C. Impact test material required.

Your opinion to try and/or sharing of opinion and Code knowledge will be appreciated.

Rgds

S K Tan

I am using PVElite 2011. But my findings in PVElite 2011 are as follows.
1. No Staic Head due to Liquid
If, Tmin = min Specified thk in input menu, then
PVElite uses Tmin as tr and tg for MDMT calculations. As this would be the actual minimum thk of the material, the MDMT calculations should be based on actual minimum thickness of the material and not the calculated min thk.

2. With static Head due to Liquid
In this case, thicknesses considered are as follows.
If
A = Min. required thk for internal pressure + static head
B = Min. required thk for internal pressure alone
Then (A-B) = increase in thickness due to static head
If, Tmin = min Specified thk in input menu
Then, the values used in tr x E / (tg - c) are-
tr= Tmin -(A-B)
tg = Tmin

Even if tr-as calculated above, goes below the minimum required thk due to pressure (B), the same is used for MDMT calculations. I think, this is a bug and need to be fixed.

Dear Kunal,

By ASME defination, tr is the required corroded thickness under applicable loadings computed with material stress at MDMT temperature.

Example on L-9.2 illustrate this understanding.

Tr shall be calculated value using relevant formulas like UG-27. Not the specified thickness in input menu, in my view.

This come back to beginning of my query point since PVElite used tr as 34 mm and not calculated value .

Rgds!

S K Tan

Dear Mr.Tan

I have faced same problem while exploring MDMT calculation by program.

Please see Tools>Configuration>MDMT options

There is one option given for " USE MAWP TO CALCULATE MDMT"

By default this one is clicked.

If you have clicked this option program will use MAWP of element to compute tr.

hence your tr will be different than internal pressure calc.

Hence your ratio will be higher & reduction in temperature from graph is less.

This is conservative option.

Hope I have clarified your query.

One more option in MDMT regarding some interpretation reference which will also effect your MDMT for nozzle check out PV-elite documentation which you can access from the Help > View documentation.I always use this document to understand software in depth.

Thanks

I agree with Mitul. Try with unchecking the option as suggested by Mitul.

Thanks to both of you.

Tried check off and PVElite calculated a lower achievable MDMT.

Anyway, per ASME Code requirement, do not think we shall check it off. MDMT still need to be evaluated at MAWP unless design concept is using design pressure as MAWP (Rate MAWP = DP supported by UG 99 b with note 34).

Manual calculation for tr with MAWP(1536.05 KPa) + liquid head (41.85 KPa) get around 33.9 mm, close to PVElite reported 34 mm.

My doubt is clear now, it is not bug.

Regards!

S K Tan