Reduction in MDMT

http://www.acronymfinder.com/Minimum-Design-Metal-Temperature-(MDMT).html

Increasing the thickness of any pressure part from its min. required t_r to the adopted/nominal t_n (due to corrosion/erosion allowance, threaded, mill tolerances, … etc.) will lead to an increasing of MDMT (disadvantage). And the ASME try to compensate that disadvantage by adding a correlation factor represents the Reduction in MDMT which depends on the ratio (t_r x E)/ (t_n-c).

See CR Thread Re: Impact Tests for Pressure Vessels.

Say the reduced MDMT came 64 degree F.. and the MDMT came to be -20 degree F..

so for adjusted MDMT we have 44 degree F?? :s

Based on your assumption (calculated reduction is 64 ^oF and MDMT is - 20 ^oF), the adjusted MDMT shall be equal to - 20 - (64) = - 84 ^oF

my interpretation is that you can use a material at a lower temperature if the stress ratio is below a certain value.

have a look at this page:

http://www.gowelding.com/weld/fracture/impact.html

hope this help

S

corrosion prevention & control