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  A flange is the name for the plate located at the end of a pipe, which connects to the flange of another similar pipe. This is accomplished by bolting the flanges together. Flanges usually have to be the same class to be bolted together. Flanges in the same class are made to the same pressure, temperature, size and tolerance specifications. Flange bolt specifications are also the same to allow a proper connection to be formed.
  
  

Class and Nominal Pipe Size

• The flange bolt specifications are determined based on the class and nominal pipe size specifications for a flange. Any two flanges that share these two specifications also have the same flange bolt specifications. The class is a series of specifications a flange must match. These specifications are created by standardization bodies, such as the American Society of Mechanical Engineers (ASME) or American National Standards Institute (ANSI). Nominal pipe size is the thickness of the pipe the flange is connected to. The class does not dictate the nominal pipe size, so flanges in the same class can be different thicknesses.
  
  

Number of Bolts

• The number of bolts indicates the number of flange bolts required to connect two flanges. There are always at least four bolts, no matter what size or class of flange is being used. The number of bolts increases when the nominal pipe size is increased. The number of bolts also increases as the class increases until Class 600, before it starts to decrease as the class is raised. This is because classes above 600 use thicker bolts. The thicker bolts reduce the number needed to seal the flanges.
  
  

Bolt Diameter

• The bolt diameter is the diameter of the flange bolts that should be used to connect the flange. The diameter of the bolt is the stem diameter, not the head diameter. The flange bolt diameter can range from 0.5 inches to 3.5 inches. Flanges in higher classes or with larger nominal pipe diameters use thicker bolts.
  
  

Bolt Circle

• The bolt circle is the diameter of an imaginary circle on the surface of the flange. This circle intersects the middle of all the bolt holes in the flange. The diameter of the actual flange is irrelevant for this measurement. The bolt holes in a flange are evenly spaced. Two flanges with the same number of bolt holes and the same bolt circle can be connected to one another. Flanges that might appear to be slightly different sizes may still fit together if they share the same bolt circle specification.