Determination of pipeline pressure rating

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I. Pressure Class Apart from the design temperature and design pressure, which are the basic parameters for determining the pressure class of a pipeline, there are other factors as well that influence the determination of this pressure class. Among them, the application standard system, materials, media, and operating conditions are all important factors. 1 Selection of the application standard system: Since different standard systems have different nominal pressure grade series, the corresponding temperature-pressure charts also vary. Therefore, even under the same design conditions, if different application standards are chosen, their nominal pressure ratings may vary. Therefore, before determining the nominal pressure class of the pipeline, it is necessary to first clarify the applicable standard system. 2 Materials used in pipeline systems: The mechanical properties of different materials vary, and therefore their corresponding values at standard temperature and pressure conditions also differ. Before determining the nominal pressure of a pipeline, priority must be given to the selection of materials for the pipeline and its components, as the choice of material depends on the design temperature, design pressure, and the medium being handled.    Generally, different components of a pipeline use different material standards. For example, pipes are generally made of tubing, flanges are mostly made of forged materials, while valves are usually made of cast materials.    Regardless of the material standards used, they should possess the same material properties, that is, they should be suitable for the same operating conditions and have the same strength ； In addition, attention must also be paid to the compatibility between pipes, sheets, bars, and castings.    The flange pressure-temperature class refers to the highest impact-free operating pressure or maximum allowable operating pressure that a flange can withstand at different operating temperatures. It is closely related to factors such as the flange material and its high-temperature mechanical properties, as well as the calculation methods used for the flange. Based on the maximum impact-free operating pressure of the flange at different operating temperatures, the nominal pressure of the flange and its accessories should be selected appropriately to ensure compatibility among all these components, thereby guaranteeing the safe and reliable operation of the pipeline accessories within the piping system. If the bolts and gaskets of the flange joint meet the relevant requirements, and if the alignment and assembly of the flange joint are carried out in accordance with excellent practices, then such a flange joint can be used for the specified pressure-temperature ratings.    The rated temperature corresponding to the rated pressure refers to the shell temperature of the pressure vessel with flanges and flanged fittings. Under normal circumstances, this temperature is the same as the temperature of the fluid being stored. When the temperature is below -29°C (-20°F), the selected pressure rating shall not be higher than the rating at -29°C (-20°F).    The pressure-temperature ratings for the various materials listed in the pressure-temperature selection table refer to the maximum shock-free operating pressure at the specified operating temperature (expressed in gauge pressure) ； For pressures at intermediate temperatures, linear interpolation is allowed for calculation.    Operating temperature refers to the temperature of the flange metal under pressure. When the operating temperature is below 20°C, the maximum impact-free operating pressure value of the flange remains the same as that at 20°C. For ferritic steels, the maximum impact-free operating pressure at 100°C can be increased to 120℃ ； The maximum impact-free operating pressure value of austenitic stainless steel at 20°C can be used up to 50°C.    When pressure-temperature ratings are used for flanged connections, the risk of leakage caused by the forces and moments generated in the pipe connections must be taken into account. Therefore, in conditions of rapid temperature changes and thermal cycling, where the temperature is above 260°C, it is recommended not to use threaded flanges. 3 Characteristics of the operating medium: Under normal circumstances, the nominal pressure of the pipeline should fall within the allowable pressure range at the corresponding temperature, and must not exceed its design pressure.     For media that could cause serious harm or lead to major accidents due to the failure of pipes and their components, when determining their nominal pressure rating, it is not sufficient to rely solely on temperature-pressure charts; instead, the nominal pressure rating should be increased appropriately, that is, the safety and reliability factor should be raised. Standards such as SH3059 \"General Rules for the Selection of Materials for Petrochemical Piping\" provide detailed specifications; for example, pipelines transporting highly toxic substances, regardless of the operating pressure of these substances, must have a nominal pressure rating of at least PN5.0MPa (under the SH standard system) or PN4.0 (under the JB standard system) ；  For pipelines transporting media such as hydrogen, ammonia, and liquid hydrocarbons, the minimum nominal pressure rating shall be no less than PN2.0 MPa (SH standard system) or PN2.5 MPa (JB standard system) ；     For pipelines transporting general combustible media, the nominal pressure class should be no less than PN2.0MPa (SH standard system) or PN1.6MPa (JB standard system). 4 Medium operating temperature and additional forces on the piping system: Many flange standards specify that the temperature-pressure values indicated refer to those under conditions where the flange is not subjected to impact loads. In fact, loads such as bending, vibration, and temperature cycling exerted by external pipes on the flanges all affect their sealing performance and reliability. Therefore, when determining the nominal pressure required for the pipeline, these external loads should be converted into an equivalent medium pressure.    Thermal expansion and contraction of piping systems often cause bending loads on flanges. For PN2.0 class flanges, when the operating temperature is greater than 200°C, or for flanges of PN5.0 class and above when the operating temperature is greater than 400°C, the effect of the additional loads exerted on the flanges by the piping system must be taken into account. Otherwise, the nominal pressure rating of the piping system should be increased.    When the operating temperature approaches the material’s creep temperature, the flange connection gradually loosens, which continuously reduces the bolt load and thus affects the tightness of the connection. Therefore, it is necessary to tighten the bolts regularly to prevent leaks.   When the operating temperature is higher than those listed in the table and no exact (rated) value is available, it can be determined by the designer based on practical experience or calculations.    When materials other than those listed in the table are used, the maximum impact-free operating pressure of the flange can be determined by referring to the materials listed in the table, on the principle that the allowable stress of the material is equivalent, provided that it does not exceed the nominal pressure. II. Allowable stress of material with wall thickness class 1: The allowable stress of a material is the value obtained by dividing the material’s strength index by the corresponding safety factor. The mechanical property indicators of materials include yield limit, strength limit, creep limit, fatigue limit, etc.; these indicators reflect the ultimate values of failure under different conditions.    To ensure reliable strength during pipeline operation, the stress in pipeline components is often restricted to a certain value within the limits of various strength criteria, and this value is known as the allowable stress. When the stress in a pipe component exceeds its allowable stress value, its strength is considered no longer guaranteed.    Therefore, the allowable stress of the material is a fundamental parameter for determining the pipe wall thickness class.    Under different design standards, the allowable stress values for selected materials vary. For pressure pipelines, the domestic design standards use the allowable stress values specified in GB 150 for steel pressure vessels, while for ASTM materials, the allowable stress values determined according to the ANSI B31.3 Process Piping standard are applied. 2 Determination of the corrosion allowance   The corrosion allowance is the value added after the pipe wall thickness has been reduced; it is necessary to account for the corrosion of the pipe caused by the medium. This value directly affects the selection of the pipe wall thickness, or rather, the determination of the wall thickness class. At present, China has not established a system that includes data on the corrosion rates of various materials under different conditions caused by various corrosive agents.    Therefore, in engineering, the corrosion allowance is usually still determined based on experience. Many engineering companies or design institutes both at home and abroad typically classify the corrosion allowance into four levels: a) No corrosion allowance, meaning the corrosion allowance is 0. Generally suitable for stainless steel pipes ；    b) 1.6mm corrosion allowance. This value is usually chosen for carbon steel and chromoly steel pipes with minor corrosion ；    c) 3.2mm corrosion allowance. This value is generally used for carbon steel and chromium-molybdenum steel pipes that are severely corroded ；    d) Enhanced grade (greater than 3.2 mm) corrosion allowance. For pipelines under special conditions such as solid particle erosion, the specific corrosion allowance must be determined based on the actual situation. 3 Wall thickness tolerances in the manufacturing of pipes and their components: During the production of pipes and their components, there are usually positive and negative deviations relative to the nominal wall thickness (or theoretical wall thickness). Therefore, when determining the nominal wall thickness of pipes and their components, the possibility of a negative deviation value must be taken into account.   The negative deviation values specified in various steel pipe standards are not exactly the same. The wall thickness deviation values specified for seamless steel pipes used for fluid transport in GB/T8163 and for stainless steel seamless steel pipes used for fluid transport in GB/T 14976 are as follows: Wall thickness deviation values of commonly used standards