The pressure class is a guideline and dimensionless number to define the pressure and temperature values at which valves of any type of valve can operate.
The number that follows the word "class" has nothing to do with the pressure at which the valve can operate. Thus, we can say that the expression "150 pound" is not correct, but only class 150, class 300, etc.
The numbers define the operating range within which the valve can operate, depending on the temperature of the fluid and the construction material of the housing and upper section. These limits vary inversely with fluid temperature. Thus, for a given pressure, the mechanical resistance of the material decreases as the temperature increases.
Rating.. the term rating describes the direct relationship between pressure, temperature and the structural material of a valve body. Rating indicates how much pressure the valve body can withstand, depending on the temperature of the fluid and the structural material of the body and upper part, including the wall thickness of these parts.
The operating pressure should be reduced as the operating temperature increases. Similarly, the operating temperature should be reduced as the operating pressure increases. This means that when pressure and temperature increase proportionally, for example, for a fluid such as saturated steam, the mechanical resistance of the valve materials decreases proportionally.
Therefore, the pressure class of a valve is defined by its pressure and temperature class.
Valves with seats and other parts made of non-metallic materials have much lower ratings than those in ASME B 16.34 for carbon steel, alloy steel and stainless steel bodies.
For valves built in accordance with ASME B16.34, the pressure class values found are 150, 300, 600, 900, 1500 and 2500 for temperatures between -29°C and 270°C in class 150 and up to 454°C in classes 300 and above, e.g., ASTM A216 Gr.WCB carbon steel.
Class 125 and Class 250 are only for gray cast iron valves and flanges. For ductile iron, we also find pressure classes 150 and 300, which are not included in the ASME B16.34 standard. The others are for carbon steels, alloys and stainless steels. Class 800 is only for wrought steel (carbon, alloy and stainless) globe, slide and check valves. This pressure class is also not included in the ASME B16.34 standard.
Maximum working pressures
The table below (Table 2-1.1) is from ASME B16.34 (2017 edition) and shows the maximum working pressure values by temperature for materials such as ASTM A216 Gr.WCB (cast carbon steel) and ASTM A105 (forged carbon steel) etc. All dimensional characteristics of valves, especially those with flange connections, are based on the value of their pressure class.
Table 2-1.1 Ratings for Group 1.1 Materials
| A105 (1) (2)
A216 Gr.WCB (1)
A350 Gr.LF2 (1)
| A350 Gr.LF3 (6)
A350 Gr.LF6 Cl.1 (5)
A516 Gr.70 (1)
| A516 Gr.70 (1) (4)
A537 Cl.1 (3)
A696 Gr.C (3)
| A672 Gr.B70 (1)
A672 Gr.C70 (1)
(1) Upon prolonged exposure to temperatures above 425°C, the carbide phase of steel may be converted to graphite. Permissible, but not recommended for prolonged usage above 425°C | (2) Only killed steel shall be used above 455°C | (3) Not to be used over 370°C | (4) Not to be used over 455°C | (5) Not to be used over 260°C | (6) Not to be used over 345°C
|−29 to 38||19.6||51.1||102.1||153.2||255.3||425.5||765.9|
About 50 tables are shown in ASME B16.34. This table is just an example of the Table 2-1.1 Ratings for Group 1.1 Materials.
Classes and pressure
To determine the pressure and temperature curve in Class 800 valves (intermediate class) according to ISO 15761 in Section 4.1.2 and API 602 in Section 4.1.3, use the linear interpolation form between Classes 600 and 900, that is, 1/3 of Class 600 (ASME B16.34) + 2/3 of Class 900 (ASME B16.34).
For example, according to the ASME B16.34 standard, a class 600 valve has a maximum working pressure, at a temperature between -29°C and 38°C, of 1480 psig (102.1 barg) and a class 900 valve has a maximum pressure of 2220 psig (153 barg) for WCB cast carbon steel.
By way of interpolation, a carbon steel class 800 forged in ASTM A105 should be used with 493 psig (1/3 of 600 class) + 1480 psig (2/3 of 900 class), resulting in 1973 psig (pounds per square inch gauge) maximum working pressure at temperatures between -29°C and 38°C.
The same method should be used for temperatures above 38°C and/or on materials such as alloy steels and stainless steels.
The values for both pressure and temperature in classes 600 and 900 shall be in accordance with the temperature specified in the ASME B16.34 standard for that pressure class corresponding to that of the flowing fluid.
For valves with stainless steel seats, regardless of type or pressure class, the rated value is limited by the construction of the body or by the material of the gaskets, while for valves with seats made of elastomers or thermoplastics, the rated value is determined by the pressure and temperature limit supported by these materials.
Valves with metal seats are suitable for use in a wide range of operating conditions of pressure, temperature, corrosivity and fluid type, and each valve must be carefully analyzed in relation to its structural properties.
For the models built according to the DIN standard, the values are given assuming a fluid temperature between -10°C and 120°C and the values found are: PN6, PN10, PN16, PN25, PN40, PN63, PN100, PN160, PN250, PN320 and PN400. In this standard, pressure values are in Bar and temperature are in °C. The initials PN stand for "nominal pressure".
Safety and relief valves
For safety and/or relief valves, the pressure class of the inlet flange limits the setting pressure. The pressure class of the outlet flange limits the back pressure if the valve is conventional. For bellows-balanced valves, the bellows material, in addition to the nozzle area, limits the back pressure value, as determined by API Std. 526. For valves with a resilient seat (soft), the inlet pressure is limited by the material (it can be a thermoplastic or elastomer), the nozzle orifice surface, and the set pressure.
The pressure to be considered for selecting the pressure class of the valve must be limited to 75% of the value found in the tables of standard B16.34 and in accordance with the construction material of the housing and upper part and the temperature. The temperature to be taken into account is always the operating temperature of the process fluid. If this percentage must be exceeded for that pressure class and operating condition of pressure and temperature and according to the valve's construction material, the user must specify the next higher class. This percentage should also limit the setting pressure of the valve to be specified and sized.
The above table (Table 2-1.1) is based on Class 300 applied to valves with body and upper part in WCB carbon steel or carbon steel forged according to ASTM A105. Refer to ASME B16.34 for other materials. In that table, for example, at a temperature of 250°C, the operating pressure should be limited to 31.4kgf/cm2 (75% of the 41.9kgf/cm2 in the table for 250°C). For example, if the operating pressure at that same temperature should be 36kgf/cm2, we should specify a Class 600 flange for the valve.
The temperature of a fluid, such as superheated steam, determines the pressure rating value of a valve or flange for a given application. For example, for two different applications with equal pressure, one with saturated steam and the other with superheated steam, the application with saturated steam will have a lower pressure rating than that for superheated steam, because for the same working pressure, the temperature of the superheated steam will be higher.