Pressure class in valves and flanges

Understanding valve pressure classes requires knowledge of the factors which determine each class.

By Artur Mathias

The Pressure Class is a guideline and dimensionless number to define the pressure and temperature values at which flanges, and valves of any type (gate, globe, ball, safety, check valves, automatic control, etc.) with metal seats can operate according to their construction material. 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 pounds’ is not correct, but only ‘class 150, class 300, etc.’ The numbers determine the working range in which the valve can operate according to the temperature of the fluid and material of construction for the body and bonnet. These limits vary inversely, proportional to the fluid temperature. Thus, for a given pressure, the mechanical resistance of the material is reduced as the temperature increases.

Rating

The term rating describes the direct relationship between pressure, temperature and the construction material of the body of a valve. Rating indicates how much pressure the valve body can withstand according to the temperature of the fluid and the construction material of the body and bonnet, including the wall thickness of these parts.
The operating pressure must be reduced as the operating temperature increases. Likewise, the operating temperature must be reduced if the operating pressure is raised. 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 is reduced proportionally. Therefore, it is through the pressure and temperature rating that the pressure class of
a valve is defined. Valves that have seats and other components in non-metallic materials (for example, thermoplastics and elastomers) will have rating values much lower than those listed in ASME B 16.34 for carbon steel, alloy steel and stainless steel bodies and bonnets.
For valves built in accordance with ASME B16.34, the pressure class values found are 150, 300, 600, 900, 1500, 2500, and 4500 for temperatures between -29 °C to 270 °C in class 150, and up to 454 °C in classes 300 and above, for ASTM A carbon steel 216 Gr. WCB, for example.
Class 125 and class 250 are only for valves and flanges constructed of grey cast iron. For ductile cast iron, we can also find pressure classes 150 and 300, which are not included in the ASME B16.34 standard. The others are for carbon steel, alloys and stainless steel. Class 800 is only for forged steel (carbon, alloy and stainless) globe, gate and check valves. This pressure class is also not included in the ASME B 16.34 standard.

Classes and pressure

To determine the pressure and temperature curve In class 800 valves (intermediate class) according to ISO 15761 in paragraph 4.1.2 and API 602 in paragraph 4.1.3 the linear interpolation form must be used between classes 600 and 900, that is, 1/3 of class 600 (ASME B16.34) + 2/3 of class 900 (ASME B16.34). According to the ASME B16.34 standard, for example, a class 600 valve has a maximum working pressure, at a temperature between -29 °C to 38 °C, of 1480 psig (102.1 barg), and a class 900 has a maximum pressure of 2220 psig (153 barg) for WCB cast carbon steel. By the way of interpolation, a carbon steel class 800 forged in ASTM A 105 should be used with 493 psig (1/3 of the 600 class) + 1480 psig (2/3 of the 900 class), resulting in 1973 psig (pounds per square inch gauge) of 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 steel and stainless steel.
The values for both pressure and temperature in classes 600 and 900 must be in accordance with the temperature specified in the ASME B16.34 standard for that pressure class, which corresponds to that of the flowing fluid.
In valves that use stainless steel seats, regardless of the type or pressure class, the rating is limited by the construction of the body or by the material of the gaskets, while in those that have seats made of elastomers or thermoplastics, this rating is established by the limit of pressure and temperature supported by those materials. Those with metallic seats are suitable for use in a wide range of operating conditions of pressure, temperature, corrosivity and type of fluid, and each one must be carefully analyzed in relation to its constructive characteristics. Class 4500 is only for valves constructed of carbon steel, alloys and stainless steel, but with butt weld connections.
For those built according to the DIN standard, the values are given considering that the fluid temperature is between -10 °C to 120 °C and the values found are: PN 6, PN 10, PN16, PN 25, PN 40, PN 63, PN 100, PN 160, PN 250, PN 320, and PN 400. In this standard, the pressure values are in Bar and the temperature in °C. The initials PN stand for ‘nominal pressure.’

Maximum working pressures

Table 1 is taken from ASME B16.34 (2020 Edition) and shows maximum working pressure values according to temperature for materials such as ASTM A216 Gr. WCB (cast carbon steel) and ASTM A 105 Gr. II (forged carbon steel). All the dimensional characteristics of the valves, mainly those whose connections are flanged, are based on the value of their pressure class.

Safety and relief valves

For safety and/or relief valves, the pressure class of the inlet flange limits its set pressure. The outlet flange pressure class limits the back pressure if the valve is conventional. For bellows-balanced valves, the bellows material, in addition to the nozzle orifice area, limits the backpressure value, as determined by API Std. 526. In those that have a resilient seat (soft), it is its material (it may be some thermoplastic or elastomer), the area of the nozzle orifice and its set pressure, which limit the inlet pressure.
The pressure to be considered for selection of the pressure class of the valve must be limited to 75% of the value found in the tables of Standard B16.34 and according to the material of construction of the body and bonnet, and with the temperature. The temperature to be considered is always the operating temperature of the process fluid. If this percentage has to be exceeded for that pressure class and operating condition of pressure and temperature, and according to the construction material of the valve, the user must specify the next class above. This percentage must also limit the set pressure of the valve to be specified and sized.
Table 1 is based on Class 300, applied to valves with body and bonnet in WCB carbon steel or carbon steel forged in ASTM A 105 Gr. II. For other materials, consult ASME B 16.34. For example, in that table, at a temperature of 250 °C, the operating pressure must be limited to 31.4 kgf/cm2 (75% of the 41.9 kgf/cm2 in the table for 250 °C). If, for example, at this same temperature the operating pressure has to be 36 kgf/cm2, we must specify a Class 600 flange for the valve. And so on.
The temperature of a fluid, such as superheated steam, is what primarily raises the pressure rating value of a valve or flange for a given application. For example, for two different applications with equal pressures, one with saturated steam and the other with superheated steam, the one operating with saturated steam will have a lower pressure class than that for superheated steam, because for the same operating pressure, the superheated steam temperature will be higher.