Reduction in MDMT Without Impact Testing

I have worked with this in the past, but not recently. Thus my information might be out of date. If no one else responds, I will try to reenter the discussion later, and to describe the testing and rationale involved. Please stay tuned.

If you can quote the code sections involved (I have no copies at hand), that would help. I realize full well that there are copyright issues about this, but for now I will refrain from rants and proposed solutions.

Hi Torando,

ASME sec viii, UCS-66.1 and UCS-66.2,

i could understand the spec but a detailed explanation about how to use them in practice would be much appreciated.

Regards

Manpreet Singh

The attached file is a full detailed design calculations for an actual pressure vessel certified U-stamp as per ASME BPVC, Section VIII, Div. 1. And that vessel was designed to resist a higher design temp of 65 ^oC and a lower design temp. MDMT of -12 ^oC.

So, we have to find the lower temp. for each component located at that vessel, and be sure that any of those temps. are still less or equal to (at least) the MDMT (-12 ^oC) of the vessel, otherwise we have to proceed an impact test for that components by implying that request through the specifications of the material requisition and implying that test through WPS's and PQR's .

Free down load this file and check pages 97 up to 107 for MDMT: Pressure_Vessel_Design_Calculation.

Thanks Mr. Abdel,

File is very usefull and require deep study to understand each factor associated.

Once again you proved to be an angel!

Regards

Manpreet Singh

Dear Dan W,

Thanks a lot, this is what i was looking for, now the concept is crystal clear except one thing:-

If I have nominal thickness in my hand, then is it necessary to use this formaula (reduction in MDMT W/O IMPACT TESTING) to check whether Impact is necessaary? since I can check the same with UCS-66 curve as well to check whether Impact testing is mandatory?

so basically i am asking why to use UCS-66.1 when I can check the impact testing exemption with UCS-66?

Please explain one again, looking to hear from you.

Regards

Manpreet Singh

Manpreet:

In the example that I gave you, the thickness you have in hand (as you phrase it – which in the explanation that I gave you is referred to as "nominal thickness", or it could also be the actual measured thickness via UT thickness testing) was 1.0", and as I said it was a curve B material, Fig UCS 66 gave a minimum temperature of + 30 deg F. If the application that the vessel is being designed for is not expected normally to see mean wall temperatures colder than + 30 deg F, than nothing further needs to be done.

If however the application is expected to cause mean metal temperatures colder than + 30 Deg F (or if the vessel will be used in a very frigid climate perhaps), you would have to explore the other options that I explained. If after having applied all of those additional options you still cannot arrive at an MDMT sufficiently cold, you would then be required to conduct Charpy V-Notch impact testing per UG-84 of ASME Section VIII Division 1, at a temperature at least as cold as the design or target MDMT.

Dan Willis

Thanks Dan W,

It was really helpful, now I got it clear. Hope to hear from u in future.

Regards

Manpreet Singh

Dear sir,

kindly share that file again to us...

Let me give an example. I work in refrigeration, which deals with pressure vessels that can cycle from warm to cold. A low pressure ammonia receiver might be at 90°F if the system is idle, but drop to -28°F when running. Because of the temperature/pressure relationship of ammonia, at 90°F the refrigerant pressure is about 180 psi (I'm too lazy to look up the precise value). Thus the receiver would need to be designed for a pressure of at least 180 psi. Here, pressure is the key criterion.

When carbon steel becomes quite cold, it also becomes brittle, and therefore can withstand less stress. However, in the case of refrigerants, their pressure decreases as the temperature decreases, and so the stresses also decrease. This is the concept of "coincident temperature and pressure." High pressure and low temperature would be a problem, but if pressure goes down along with temperature, the full strength of the steel is no longer needed.

So far this explanation is very incomplete, but I will try to continue later.

Thanks, consolidating all 3 responses from you, Dan W and Abdel, my concept is very clear now for MDMT.

Please try to help me if you can provide with some practical examples.

Regards

Manpreet Singh