Robster1us
Mechanical
I am attempting to perform a calculation for reduction of MDMT in non-impact-tested material in a piping system, and need to understand the stress ratio referred-to in Figure 523.2.2 of B31.5 (or the analogous section of B31.3). My main point of confusion is with the meaning of the provisions in the above-referenced code section (ASME B31.5, Par. 523.2.2(f)(4) and (5)). My question has three parts:
1. Can anyone say, definitively, what is meant by the words, "provided the maximum circumferential or longitudinal tensile stress resulting from coincident pressure, thermal contraction, or bending between supports does not exceed 35%" means? Is there an interpretation by the B31.5 committee that can be of some use? One way to read this is that you add the maximum circumferential stress from all three (coincident pressure, thermal, and weight) and the maximum longitudinal stress for all three, and compare each to the allowable stress for the material, and find the highest one to give you your stress ratio to use in Figure 523.2.2. However, I don't see how this is possible when understanding that allowable stress for sustained loads (pressure and weight, which would constitute "coincident pressure" and "bending between support") is different from allowable stress for thermal loads (thermal contraction). How can a stress ratio be calculated that adds all of these together? What is the code really asking be calculated?
2. Figure 523.2.2 has something strange going on with Celsius vs. Fahrenheit curves. The "reduction" starts at zero for the Fahrenheit scale, but some unknown nonzero number for the Celsius scale. What gives?
3. 523.2.2(f)(4) appears to put a limit of -150F on using non-impact-tested Ferrous materials. However, if you take a normal thin-walled pipe under 0.375" wall thickness, you can get substantially lower than that depending on what curve you're using (B or C) and what your stress ratio is. Is this meant to indicated that if you get a reduction of -140F, and your are on curve B, you can't go to -160F, you have to stop at -150F?
Please help me understand what this code language is attempting to ask for.
1. Can anyone say, definitively, what is meant by the words, "provided the maximum circumferential or longitudinal tensile stress resulting from coincident pressure, thermal contraction, or bending between supports does not exceed 35%" means? Is there an interpretation by the B31.5 committee that can be of some use? One way to read this is that you add the maximum circumferential stress from all three (coincident pressure, thermal, and weight) and the maximum longitudinal stress for all three, and compare each to the allowable stress for the material, and find the highest one to give you your stress ratio to use in Figure 523.2.2. However, I don't see how this is possible when understanding that allowable stress for sustained loads (pressure and weight, which would constitute "coincident pressure" and "bending between support") is different from allowable stress for thermal loads (thermal contraction). How can a stress ratio be calculated that adds all of these together? What is the code really asking be calculated?
2. Figure 523.2.2 has something strange going on with Celsius vs. Fahrenheit curves. The "reduction" starts at zero for the Fahrenheit scale, but some unknown nonzero number for the Celsius scale. What gives?
3. 523.2.2(f)(4) appears to put a limit of -150F on using non-impact-tested Ferrous materials. However, if you take a normal thin-walled pipe under 0.375" wall thickness, you can get substantially lower than that depending on what curve you're using (B or C) and what your stress ratio is. Is this meant to indicated that if you get a reduction of -140F, and your are on curve B, you can't go to -160F, you have to stop at -150F?
Please help me understand what this code language is attempting to ask for.
