How are the design pressure and design temperature of natural gas selected?

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A heat exchanger is to be designed, with natural gas flowing through the tube side. Its operating temperature ranges from minus 25 degrees to 10 degrees Celsius, and the operating pressure is 1.2 MPa. The main components of natural gas are methane, propane, and butane, with small amounts of nitrogen, carbon dioxide, etc. also present. May I ask, 1: What are the appropriate values for the design pressure and design temperature of this pipe train? 2. Should the design pressure be determined in accordance with the provisions for liquefied petroleum gas storage outlined in the Code of Practice, based on the saturated vapor pressure at 50 degrees Celsius for the corresponding natural gas medium? 3. Should we still choose the operating pressure at 1.05 to 1.1 times the normal value, and select the design temperature based on the low-temperature limit? 4. If this equipment is made entirely of stainless steel, must the tube sheet and tube box flanges be third-grade forgings? Is there any regulation regarding this? 5. Can asbestos rubber sheets or polytetrafluoroethylene be used as gasket materials for pipe flanges?