1 reply to this topic

[hannah17]

Hello,


I am currently working on a project to determine the risk of brittle fracture on a number of old vessels at different pressure settings: Mawp, current operating pressure, and at psv set pressure. If there is a risk of brittle fracture (when MDMT> CET- critical exposure temperature) , i have to find the mdmt at each pressure settings above and figure out at which pressure setting brittle fracture will occur. When that figured out, i have to rerate that pressure setting to lower the MDMT to avoid brittle fracture.
Having said that, for example, here is some info of a vessel:
Mawp: 720psig
Operating pressure: 240psig
Psv set pressure: 560 psig
Governing thickness (uncorroded) : 2.125 inches
Obsolete a150
Calculated mdmt: 100.7F
CET: 73
-> brittle fracture will occur on this vessel..
So at those pressure settings above, i calculated the mdmt: 97F (at mawp), -25F (current opert -impossible to cause brittle fracture ), 72.5F (psv setting). Now we see a problem at psv setting ( 72.5 ~ 73F CET) . At what psv set pressure should i rerate to? And what is the mdmt at this pressure setting should it be to get a conservative value?

Thanks!

[fallah]

hannah17,

In general, if you want to lower the MDMT in order to avoid brittle fracture, you have to lower the MAWP because they are related to each other.

According the material and given information you can use the procedure and curves of UCS-66 in ASME Sec.VIII to lower the MAWP (as PSV set point) such that lowered correspondent MDMT will result in there would be no brittle fracture.

Fallah