Pressure level of flange 

 According to ASME B16.5, the steel flange has 7 pressure classes: Class150 - 300 - 400 - 600 - 900 - 1500 - 2500 

 The pressure level of the flange is very clear. The Class300 flange can withstand greater pressure than the Class150, because the Class300 flange needs to be made of more materials, so it can withstand more pressure. However, the compressure capacity of the flange is affected by many factors. The pressure level of the flange is expressed in pounds, and a pressure level is expressed in different ways. For example, 150Lb, 150Lbs, 150# and Class150 mean the same. 

 Examples of pressure levels 

 The pressure bearing capacity of the flange is also different. When the temperature rises, the pressure bearing capacity of the flange decreases. For example, the flange of Class150 is 270PSIG at room temperature, 180 PSIG at 400°F, 150 PSIG at 600°F, and 75 PSIG at 800°F. 

 In other words, when the pressure drops, the temperature is allowed to rise, and vice versa. Because flanges can be made of different materials, such as stainless steel, cast iron, ductile iron, carbon steel, etc., each material has different pressure levels. 

 The figure below is an example of the flange of NPS12. As you can see, the diameters of the inner holes and protruding planes of the flange are the same. However, the outer diameter of the flange, the diameter of the center of the bolt hole and the diameter of the bolt hole increase with the increase of the pressure level. 

 The number and diameter of bolt holes are: 

 CL 150 - 12 x 25.4 CL 300 - 16 x 28.6 CL 400 - 16 x 34.9 CL 600 - 20 x 34.9 CL 900 - 20 x 38.1 CL 1500 - 16 x 54 CL 2500 - 12 x 73 

 Pressure Temperature Rated - Example 

 Pressure-temperature rating refers to the maximum allowable worksheet pressure of the material and grade at the rated temperature (Celsius) (the unit of pressure is Bar). For intermediate temperature, linear insertion is allowed. However, insertion is not allowed between flange levels. 

 If the bolts and gaskets of flange joints meet the relevant qualifications, and the correction and assembly of flange joints meet good procedures, such flange joints are suitable for pressure-temperature rated values. If the latter is used for flange joints that do not meet these qualifications, it is the responsibility of the user. 

 The rated temperature corresponding to the rated pressure refers to the shell temperature of the pressure vessel of the flange and flange pipe fittings. Generally speaking, this temperature is the same as that of the stored fluid. The responsibility of selecting the pressure rating according to the storage fluid temperature is borne by the user. When the temperature is below -29°C (-20°F), the rated value should not be greater than -29°C (-20°F). 

 For example, the following are the two sets of materials divided into ASTM and their pressure-temperature rating according to ASME B16.5. 

 2. It shall not be used above 455°C. 

 3. It shall not be used above 370°C. 

 4. It shall not be used above 260°C. 

 5. It shall not be used above 425°C. 

 Rated value 

 The term rating describes the direct relationship between pressure, temperature and the structural material of the valve body. The rating indicates how much pressure the valve body can withstand according to the temperature of the fluid and the structural materials of the valve body and valve cover (including the wall thickness of these parts). 

 The working pressure must decrease with the increase of operating temperature. Similarly, if the working pressure increases, the operating temperature must be reduced. This means that when the pressure and temperature increase proportionately, for fluids such as saturated steam, the mechanical resistance of valve materials will be reduced proportionately. Therefore, it is through pressure and temperature levels that the pressure level of the valve is defined. 

 The seats and other components use valves of non-metallic materials (for example, thermoplastics and elastomers), which will be far lower than the values of carbon steel, alloy steel and stainless steel valve bodies and valve covers listed in ASME B 16.34. 

 For valves manufactured according to ASME B16.34, pressure grade values are found to be 150, 300, 600, 900, 1500, 2500 and 4500, with temperatures between -29C and 270C, above 300, and up to 454C, taking ASTM A carbon steel 216 Gr.WCB as an example. 

 Level and pressure 

 In order to determine the pressure and temperature curve, according to ISO15761 paragraph 4.1.2 and AP1602 paragraph 4.1.3, the 800-level valve (middle stage) must use linear interpolation between levels 600 and 900, that is, 1/3 of the 600 (ASME B16.34) + 2/3 of 900 (ASME B16.34). According to the ASME B16.34 standard, for example, the maximum operating pressure of the 600-level valve is 1480psig (102.1barg) between -29C and 38C, while the maximum pressure of 900-level WCB cast carbon steel is 2220psig (153 barg). By interpolation, the 800-grade carbon steel forged according to the ASTMA105 standard should use 493psig (1/3 of the 600) +1480psig (2/3 of the 900 class), so the maximum operating pressure between -29·C and 38·C is 1973psig (pound/square inch gauge pressure). For materials with temperatures above 38°C and/or alloy steel and stainless steel, the same method should be used. The pressure and temperature values of levels 600 and 900 must meet the temperature specified in the pressure level of ASMEB16.34, which corresponds to the temperature of the flowing fluid. 

 For valves using stainless steel seats, regardless of their type or pressure level, their grade is limited by valve body structure or washer material, while for those seats made of elastomers or thermoplastics, the grade is determined by the pressure and temperature limits supported by these materials. Valves with metal seats are suitable for working conditions of various pressure, temperature, corrosion and fluid types, and each valve must be carefully analyzed according to its structural characteristics. Grade 4500 is only suitable for valves made of carbon steel, alloy and stainless steel, but it has a butt welding connection. 

 For those valves built according to the DIN standard, considering that the fluid temperature is between -10°C and 120°C, the values given are: PN6, PN10, PN16, PN25, PN40, PN63, PN100, PN160, PN250, PN320 and PN400. In this standard, the unit of pressure is bar and the unit of temperature is °C. The initial PN stands for "nominal pressure". 

 Maximum working pressure 

 Table 1 is taken from ASME B16.34 (version 2020) and shows ASTM A216 Gr. WCB (cast carbon steel) and ASTM A 105 Gr. II (Forged carbon steel) and other materials have the maximum operating pressure value according to the temperature. The size characteristics of all valves, mainly those connecting flanges, are based on their pressure levels. 

 Safety valves and discharge valves 

 For relief valves and/or discharge valves, the pressure level of the imported flange limits its set pressure. If the valve is conventional, the pressure level of the outlet flange limits the back pressure. For bellows flat valves, in addition to the aperture area of the nozzle, the material of the bellows also limits the back pressure value, which is determined by API standard 526. In those valves with elastic seat (soft), the inlet pressure is limited by its material (possibly some thermoplastics or elastomers), the size of the nozzle and its setting pressure. 

 The pressure considered to select the pressure level of the valve must be limited to 75% of the value found in the table of standard B16.34, depending on the structural material and temperature of the valve body and valve cover. The temperature to be considered is always the operating temperature of the process fluid. If the pressure level and the working conditions of pressure and temperature must exceed this percentage, and according to the structural material of the valve, the user must specify the next level above. This percentage must also limit the set pressure of the valve to specify and determine the size. 

 Table 1 is based on grade 300 and is suitable for valve bodies and valves of carbon steel forged with WCB carbon steel or ASTM A105Gr.ll. For other materials please refer to ASME B16.34. For example, in this table, when the temperature is 250C, the working pressure must be limited to 31.4kgf/cm2 (75% of 41.9kgf/cm2 in the table at 250C). For example, if the pressure of 1 must be 36 kgf/cm2 at the same temperature, we must specify a 600-level flange for the valve. And so on. 

 Fluid temperature, such as hot steam, is the main reason for increasing the pressure level of valves or flanges in specific applications. For example, for two applications with the same pressure, one uses saturated steam and the other uses superheated steam, the pressure level of applications using saturated steam will be lower than that of applications that use superheated steam, because under the same working pressure, the temperature of superheated steam will be higher.