Material of safety valve discharge pipe
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The safety valve on the spherical tank is used to release gaseous propylene; the release pressure is 1.9 MPa, and the release temperature is 45.67°C. The pressure behind the safety valve is 0.3 MPa, while the release temperature there is -20°C. Does this operating condition fall under the category of low-temperature and low-stress conditions? Experts with relevant design experience, please provide an answer. In such a situation, is it acceptable to use carbon steel for the pipes downstream of the safety valve?
The pressure in the pipeline behind the safety valve is generally not high, around the kilopascal range. Low-temperature stress conditions require consideration of factors such as temperature, pressure, and the medium. Therefore, it should not be in a low-temperature operating condition, and using carbon steel is no problem (which is indeed the case in practice).
When the ambient temperature is also -20°C and the discharge temperature reaches -20°C, as the gas moves backward, the temperature drops further due to the pressure drop (to -21°C). The minimum operating temperature for 20# steel is -20°C; then why is it okay to use carbon steel?
For example, in the northern region of Heilongjiang, the lowest winter temperatures can drop well below -20°! But the steel structure of the device (to take an extreme example) is made of ordinary carbon steel – why?
This post was last edited by xbl240 on 2016-8-24 at 11:13. The specifications for tank design stipulate that, when affected by atmospheric conditions, the lowest value of the monthly average minimum temperature should be used as the design temperature; if this temperature is below -20, low-temperature steel must be used for the materials of the equipment. Section 4.2.2.1 of GB50316-2000 (2008 edition) stipulates that: \"Except in low-temperature and low-stress conditions, the operating temperature of the material shall not exceed the upper and lower temperature limits specified in Appendix A of this code.\" ” ； The minimum operating temperature for 20# (GB8163, GB3087, GB9948) in Appendix A is -20°C. 4.3.4 Interpretation of the provision: \"For carbon steel materials under low-temperature and low-stress conditions, the minimum operating temperature can be reduced to -70°C without the need for impact testing. Depending on the extent to which the operating temperature drops under such conditions, 20# steel pipes, if made thicker, can be used in cold regions and frozen systems at temperatures below -20°C when the stress levels are very low.\" " If ordinary carbon steel is used for the structural steel of a device in low-temperature environments, there must be a reason for that; ordinary carbon steel cannot be used at temperatures below -20°C arbitrarily. Also, regarding what you said that \"it is not in a low-temperature operating condition, so using carbon steel is fine (and this is indeed the case)\\", has this safety valve actually been used for relief purposes?
Firstly, propylene emissions cannot be released into the atmosphere; they must be sent to the main flare stack of the plant (this is generally the case). The temperature of propylene is very low, making it unsuitable to use carbon steel pipes for such discharge. Although the back pressure after venting through safety valves is not high, the issue lies in the fact that carbon steel does not possess as good low-temperature impact toughness as A333 material; long-term use may lead to problems. I have worked on large-scale refrigeration plants, and from what I’ve found, the discharge pipes there are all made of 304 stainless steel, as designed by Hualu
The storage and transportation temperatures of propylene at your cryogenic station are relatively low; mine is stored at room temperature, with only low temperatures occurring when the safety valve releases pressure, and such release events do not happen very frequently