Compress Over Ride to Raise MDMT?

[weldtek]

Our engineer says that Compress does not allow the user to change the MDMT above -320F on an austenitic stainless design.
Does anyone know if there is a way to input a higher MDMT?

 

[SnTMan]

Edit the output

 

[TomBarsh]

COMPRESS provides an input for the designer to specify the "design MDMT". COMPRESS then determines the coldest possible MDMT rating based on the material, thickness, pressure, heat treatment, etc, and ASME Code rules. Both the design and the rated MDMT values are provided in the reports.

For example, for carbon steel the procedure follows the outline in Figure UCS-66.2. Similar procedures are followed for high alloy material and UHA-51. Because the "rated" MDMT is the coldest possible per Code rules without making any changes to the materials or design there is no benefit to making the "rated" MDMT a warmer value. However, there may be changes in the ratings if you adjust the design MDMT (eg: MAWP may vary).

Tom Barsh
Codeware (publisher of COMPRESS)

 

[weldtek]

Tom,
Thanks for the information. The reason I asked is that we often have confused customers. We submit calcs showing
-320F but our drawing shows something different based on the customer supplied requirement.
As an example, maybe our drawings show -50F. The customer then comes back and says he wants the vessel stamped for the lowest possible temp and he assumes, based on the calcs that it's -320 F. Of course it's not -320 F without involving additional costs for impact tests, so, an explanation is required.
When designing austenitic vessels, I'd like to have a standard procedure for those situations in which the customer hasn't specified a lower MDMT, where we default to -155 F.

 

[SnTMan]

That's exactly why I don't like all that output. I don't use Compress, but rather CodeCalc, and for submission to a customer, there is way too much information. It just confuses them and causes extra work for everyone. Aside from MDMT, CodeCalc prints % strain calculations for SEAMLESS PIPE.

On top of which I often don't trust the output for MDMT calculations, because it generally does not know what is connected to what. I also think the customer should SPECIFY the MDMT, same as the other design conditions (weldtek, I know you said yours do, but mine usually don't).

Regards,

Mike

 

[TomBarsh]

Weldtek, There should be no additional requirements imposed on the design or materials by using the "rated" MDMT over the "design" MDMT.

The principle is analogous to pressure rating: a given shell component might have a design pressure of 60 psi, but given the nominal thickness of the material as-built it may have excess thickness over that calculated by the ASME Code formula. This excess thickness allows the "maximum allowable working pressure", MAWP, of that shell to extend to a greater pressure. Similarly, the rated MDMT is determined based on the Code rules and using the material "as it is" without imposing additional impact testing, etc.

COMPRESS does provide some control for the MDMT rating. One can elect not to use the UG-20(f) exemptions, elect not to use UCS-68(c) exemption for optional PWHT, or not to use MDMT reduction permitted by use of the coincident ratio of Figure UCS-66.1. One can also control the pressure in order to control the MDMT (via the coincident ratio). Of course, not all of these are applicable to the high alloy material rules of UHA.