How does this impact ASME Pressure Vessel Design?
The SA-105 MDMT curve change is probably the most impactful change to code users in the 2019 coded that will take effect in 2020. It’s a simple issue, but with complex consequences.
In 2017 and earlier editions of the code, SA-105 was assigned a curve B material. Now in 2019, the language is different, and “as forged” SA-105 is curve A. This means for the same governing thickness, there is nearly a 40 °F (21 °C) jump in impact exempt MDMT.
These MDMT changes mean that designs that passed in 2017 may be failing in 2019. Luckily, there are several options available to designers in this situation.
What are Options for Pressure Vessel Designers?
The cheapest option is to use the UCS 66(b) reduction.
As the material becomes thicker, the MDMT rating gets warmer. The UCS 66(b) reduction works to mitigate that. Some of the thickness is excess material not needed for the loading requirements. The UCS 66(b) reduction gives you credit for that and helps reduce the impact-exempt MDMT.
Next, consider if there’s any flexibility in the original MDMT rated for your design. The odds are against you in this pursuit, but it’s worth investigating since this option won’t cost you anything.
Another option is heat treatment after forging. In the 2019 version of UCS-66, you’ll find information about receiving a curve B assignment. There is information about heat treatment, which can allow you to use curve B still. It’s possible that much of the information for the forgings won’t be available in your shop. If that’s the case, you can contact your forging supplier and see if they can provide documentation that enables you to keep curve B in your design.
If none of these options work for you, the last option is the UCS-68(c) reduction, which unfortunately isn’t a free reduction. It will provide you with a 30 degree °F reduction; but it requires a post weld heat treatment for no other reason than to get the reduction.
To learn more about how CEI’s Finglow software can help you take advantage of these changes, CLICK HERE.