Principles for determining the design pressure and design temperature of pipelines

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Principles for determining the design pressure and design temperature of pipelines 2.1 Scope of responsibilities 2.1.1 The process system discipline determines the maximum operating pressure of the pipeline (i.e., the design pressure of the pipeline as referred to in this chapter), based on the operating pressure data for various conditions during normal operation provided by the chemical process discipline, together with any additional factors specific to that pipeline system (such as pressure variations in the pipeline system and the properties of the fluid). This value serves as the reference pressure for the pipeline materials discipline to determine the appropriate pressure class for the pipeline and to calculate the wall thickness. 2.1.2 The process engineering team does not determine the design temperature of pipes; however, based on the data provided by the chemical process engineering team, it is necessary to provide the pipe engineering and pipe material teams with the normal operating temperature and/or the maximum (or minimum) operating temperature at the corresponding design pressure. If a professional in process systems is required to specify the pipeline design temperature (as a design condition), it can be determined with reference to the provisions in 2.6. 2.2 Glossary of Terms 2.2.1 Design Pressure of Pipes The design pressure of pipes refers to the pressure under the most severe conditions among all possible combinations of operating pressure and temperature that may occur in the piping system under normal operating conditions. 2.2.2 Maximum operating pressure and highest operating temperature: The pressure and temperature under the most severe conditions among all the combinations of operating pressures and corresponding operating temperatures that may occur during normal operation. 2.3 Scope of Application 2.3.1 Applicable to pipelines within the following range: a) Pressure pipelines: 0 MPa(gauge) ≤ design pressure ≤ 35 MPa(gauge) ； b) Vacuum pipeline: Design pressure < 0 MPa (gauge). 2.3.2 Applicable to pipelines transporting all fluids, including fluidized solids. 2.4 Principles for determining the design pressure of pipelines 2.4.1 The design pressure of pipelines shall not be lower than the maximum operating pressure. 2.4.2 For pipes equipped with safety relief devices, their design pressure shall not be lower than the opening pressure (or burst pressure) of the safety relief devices. 2.4.3 The design pressure of all pipes connected to the equipment shall be no less than the design pressure of the equipment to which they are connected. 2.4.4 For pipes transporting low-boiling-point media such as refrigerants and liquefied gases, the design pressure is determined based on the maximum saturated vapor pressure that the medium may reach when the valve is closed or when no flow of the medium is occurring. 2.4.5 When the path between the pipeline or pipeline components and the overpressure relief device may be blocked or severed, the design pressure shall be determined to be not lower than the maximum working pressure that may occur. 2.4.6 For pipes for which the engineering design requirements dictate that the wall thickness be calculated, the calculation pressure listed in the “Pipe Wall Thickness Table” represents the design pressure of such pipes, and the operating temperature corresponding to that calculation pressure is the design temperature of the pipes. 2.5 Selection of the design pressure for pipelines 2.5.1 Pressure pipelines a) Pressure pipelines equipped with safety valves: The design pressure of the pipeline shall be ≥ the opening pressure of the safety valve. b) Pressure pipelines connected to equipment without safety valves: The design pressure of the pipeline shall be ≥ the design pressure of the equipment. c) Pipelines at the outlet of centrifugal pumps: The design pressure of the pipeline shall be ≥ the shut-off pressure of the pump. d) Pipelines at the outlet of reciprocating pumps: The design pressure of the pipeline shall be ≥ the opening pressure of the safety valve at the pump outlet. e) Pipelines leading from compressors: The design pressure of the pipeline shall be ≥ the opening pressure of the safety valve + the pressure drop along the pipeline from the compressor outlet to the safety valve under maximum normal flow conditions. f) Vacuum pipelines: The design pressure of the pipeline shall be equal to full vacuum. g) Pipelines that do not fall into the above categories: The design pressure of the pipeline shall be ≥ the maximum value among the variations in operating pressure. 2.6 Principles for determining the design temperature of pipelines The design temperature of a pipeline refers to the temperature of the pipeline material that it may reach under the corresponding design pressure during normal operation. The professionals in the process system select the pipeline design temperature based on the operating temperatures under various conditions during normal operation, as provided by the chemical process specialists, following the \"pressure-temperature combination under the most severe conditions\". The pipe design temperature proposed by the process systems specialty (which in this section refers to the highest operating temperature of the medium in the pipe) can be determined using the following method: 2.6.1 The pipe wall temperature under the highest operating temperature of the medium during normal operation, as obtained through heat transfer calculations or actual measurements, is used as the design temperature. 2.6.2 In cases where it is not convenient to perform heat transfer calculations or to measure the wall temperature of the pipe, the highest (or lowest) operating temperature of the medium during normal operation shall be used as the design temperature for the pipe. 2.6.2.1 Metal pipes a) Uninsulated pipes 1) Pipes with a medium temperature < 38°C: Design temperature of the pipe = highest temperature of the medium. 2) Pipes with a medium temperature ≥ 38°C: Design temperature of the pipe = 95% of the highest temperature of the medium. b) Pipes with external insulation: Design temperature of the pipe = highest temperature of the medium. c) Pipes with internal insulation (lined with insulating material): Design temperature of the pipe = wall temperature determined through heat transfer calculations or the wall temperature measured experimentally. d) When the medium temperature ≤ 0°C, the design temperature = lowest temperature of the medium. 2.6.2.2 Non-metallic pipes and metal pipes with non-metallic linings a) Pipes not affected by ambient temperature: Design temperature of the pipe = highest temperature of the medium. b) Pipes installed in an environment where the ambient temperature is higher than the highest temperature of the medium (except those for which protective measures have been taken): Design temperature of the pipe = ambient temperature. 2.6.3 The design temperature is determined by taking the normal operating temperature of the medium during normal operation as specified by the chemical engineering department and adding (or subtracting) a certain margin, as follows: If the normal operating temperature of the medium is between 0 and 300°C, the design temperature shall be ≥ the normal operating temperature + 30°C. If the normal operating temperature of the medium is above 300°C, the design temperature shall be ≥ the normal operating temperature + 15°C. 2.6.4 When the temperature of the fluid medium is close to the upper limit of the temperature range allowed for the selected material, the design temperature should be chosen carefully based on specific circumstances, in order to avoid increased costs or reduced safety levels. For example, if the calculations according to 2.6.3 require the use of a higher-grade material, from an economic perspective it is permissible to reduce the 15°C allowance as specified in the engineering design requirements; however, measures must be put in place to ensure that the temperature does not exceed the allowed level during operation. 2.7 Other notes: When there are various operating conditions or periodic changes in working pressure and corresponding operating temperature, the process system designer should list all the operating condition data provided by the chemical process specialists and provide explanations to the pipeline material specialists. The foundation of design lies in the fact that pipes are necessary; it is important to choose the appropriate material for them. What matters are the details: various fittings such as tees, crosses, elbows, valves, thermometers, expansion joints, filters, pressure gauges, flow meters, flame arrestors, and so on. There are also safety valves, vent valves, pressure reducing valves, steam traps, and many more. For professional installation and equipment operation, what is needed are pipes and the medium flowing through them, along with additional equipment, as well as the source of the medium (i.e., the quantity of it). What needs to be done is to select the appropriate fittings and models. For example, in the case of electricity, cables, air switches, switches, fuses, and leakage protection devices are used