As a flange manufacturer for over 50 years, we discuss Navy and commercial ANSI flange drilling all the time at Piping Supplies. We know when it comes to pipe flanges, it is all about the drilling. Other physical aspects of a pipe flange need to match the application, sure, but the main purpose of a flange is to connect two parts of a piping system together. The bolting pattern at the gasket surface accomplishes that.
Elements of a flange bolt pattern
Navy and commercial ANSI flanges bolt patterns are made up of three elements: the bolt hole size, the number of bolt holes and the bolt circle.
Bolt hole size
The bolt hole size is larger than the bolt which connects the flange to another part of the piping system. For ANSI flanges, the bolt hole size is 1/8″ larger than the bolt or stud. So, a flange having a 5/8″ bolt will have a 3/4″ bolt hole. Navy flanges generally have bolt hole sizes 1/16″ larger than the bolts used. A Navy flange using 5/8″ bolts will have an 11/16″ bolt hole.
Number of bolt hole
The number of bolt holes and the size of the bolts determine how much pressure the flanged union can hold at a given temperature. Pressure and temperature ratings of flanges are found in flange specifications like ANSI B16.5 and others. Generally, higher pressure class flanges will have more bolt holes and/or larger bolt hole diameters. A 2″ Class 150# flange has 4 holes and a 2″ Class 300# flange has 8 bolt holes. Both will attach to a 2″ pipe but will not bolt to the same outlet.
Bolt circle
The bolt circle is the distance from the center of one bolt hole to the center of the bolt hole directly across from it. This also changes between each pressure class which is another reason the 2″ 150# and 2″ 300# flange from the example above will not bolt up to the same outlet. ANSI flanges all have even number of holes, so measurements are easier. It is more challenging to measure bolt circles on some Navy flanges with an odd number of bolt holes like three, five and seven.
As flange manufacturing experts, we at Piping Supplies Inc try to make getting the flange you need easier. We have created two documents you can download. One will help you identify a flange or flanged outlet based on the drilling. The other gives you all the Navy and commercial drill patterns. Both are good resources.