“Low temperature and low stress”
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There have always been many questions regarding low temperature and low stress. Appendix E of GB150 states that \"low temperature and low stress refers to the situation where the design temperature of the shell or its pressure-bearing components is below -20 degrees...\", which means that a temperature below -20 degrees is required in order to apply the low temperature and low stress condition? Then, when the temperature is between 0 degrees and -20 degrees, can’t low temperature and low stress be used? In that case, taking takeover as an example, if the design temperature is -30 degrees, 20# steel pipes can be used due to the low-temperature and low-stress conditions; but at -10 degrees, such conditions do not apply and therefore those pipes cannot be used. Isn’t that strange? Seeking advice from experts
This post was last edited by zhangjuhua on 2016-4-17 at 19:06. What you’re referring to is likely a situation of low temperature and low stress in a general sense. However, according to GB150 standards, ordinary carbon steel and low-carbon steel are considered low-temperature materials only when the temperature is below -20°C. The concept of low temperature and low stress mentioned in Appendix E applies specifically to low-temperature containers. As you said, “At -30°C, 20# steel pipe can be used due to low temperature and low stress conditions, but at -10°C it cannot be used because it’s not considered a low-temperature, low-stress condition.” Such a situation may indeed exist, but it’s quite special. For example, for designs with a temperature range of -20°C to 40°C, since the conditions meet those of low-temperature, low-stress operation, 20# steel can be used by selecting materials based on a temperature of +40°C. Otherwise, comparing conditions of low stress with those of high stress is inappropriate, as for the same material, the temperature at which brittle fracture occurs under low stress is lower than that under high stress. Only when stress levels and material are the same can a comparison be made. This is where the contradiction you mentioned arises. At its core, this issue stems from arbitrary definitions of what constitutes a low-temperature container; setting fixed temperature thresholds in such an arbitrary manner is not ideal and is an outdated approach. ASME handles this better – their UCS-66 standard uses curves, whereas our standard uses abrupt changes, which is rather rigid. In most cases, the stresses resulting from internal pressure along with other mechanical loads fall within the range of 50 MPa or 1/6 of the yield strength, as using higher values would be uneconomical. Unless the diameter and pressure are very low, even with ordinary pipes, the stress levels remain sufficiently low
Low-temperature and low-stress operating conditions must be used with caution; they cannot be employed casually. First, it is necessary to understand under what circumstances such conditions can be utilized Be sure to understand the concept first, then determine whether it falls under the conditions of low temperature and low stress; if so, then use this approach, otherwise it is not permissible.