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Governing Temperature (UCS-66 ASME SecVIII Div 1)

Governing Temperature (UCS-66 ASME SecVIII Div 1)

Governing Temperature (UCS-66 ASME SecVIII Div 1)

Refer case as below for pressure vessel (design code is Sec VIII Div I - Section UCS-66):
- Required thickness of vessel - 9.5 mm
- Vendor offered A106 GrB SCH80 for shell so nominal thickness is 10.97mm
-Minimum Design Temperature - minus 29degC (-20degF)

As per the curve, A106 should fall in curve B and hence the max. thickness up to which impact testing is not required is 10mm(0.394inch).

Do we need impact tested material since the required thickness is less and the actual and nominal thickness would be more than the allowable non-impact tested material thickness of 10mm.
Which thickness should be the used for governing thciknes? Required Thickness, Nominal Thickness, Actual Thickness?


Materials & Corrosion Engineer

RE: Governing Temperature (UCS-66 ASME SecVIII Div 1)

No impacts required.....UG-20(f) exempts P1 material to -20F....some restrictions, check it out. You don't have much excess thickness above that required, when you take into account the 12.5% undertolerance of SA-106B pipe. 10.97 x .875 = 9.6mm.

RE: Governing Temperature (UCS-66 ASME SecVIII Div 1)

Which is why the MDT was set at -29 deg C in the first place. Nothing to do with process and exposure conditions, everything to do with the minimum temperature where Charpy testing can be avoided.

Steve Jones
Corrosion Management Consultant


All answers are personal opinions only and are in no way connected with any employer.

RE: Governing Temperature (UCS-66 ASME SecVIII Div 1)

The thickness for impact exemption curves in UG-66 is the governing thickness as defined UCS-66 and Figure UCS-66.3. The examples are obvious.

RE: Governing Temperature (UCS-66 ASME SecVIII Div 1)

Thanks for your expert opinions.

Can you also confirm as which thickness do we generally look at while deciding for impact test?

Although I have gone through the UCS66 and Fig UCS66.3 earlier also but am not clear as which thickness are they referring to?

Is governing thickness based on the
-Required Thickness,
-Nominal Thickness or
-Actual Thickness?


Materials & Corrosion Engineer

RE: Governing Temperature (UCS-66 ASME SecVIII Div 1)

Nominal thickness would be tg (governing thickness).

RE: Governing Temperature (UCS-66 ASME SecVIII Div 1)

You need to read paragraph UCS-66 as well as the table. Paragraph UCS-66 clearly defines what the governing thickness is.

RE: Governing Temperature (UCS-66 ASME SecVIII Div 1)

you can find the exact definition of governing thickness on ucs-66 (a) 1&2. although by role of thumb it could be nominal thickness.

RE: Governing Temperature (UCS-66 ASME SecVIII Div 1)

Starting with nominal, uncorroded thickness, follow Part UCS-66(a) rules to determine governing thickness, and in turn, MDMT.



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