Choosing the correct flange dimensions is often easier than you may expect, because most flange sizes are pre-determined and regulated by a national set of standards. In the United States, the American Society of Mechanical Engineers (ASME) establishes the regulations governing flanges. If the flange dimensions you need match an existing ASME flange, then the dimensions are already designated and available for you. The majority of flanges you may encounter have already been engineered, and proven to fit and work properly.
Flange sizes can vary widely. Fortunately, most countries maintain standards that typically provide the flange dimensions for us. For example, ASME oversees U.S. standards, while Germany’s governing body is known as DIN. This is the English translation for the German Institute for Standardization.
In the rare event that a flange does not need to meet any of the industry standards, you should still consider certain criteria when choosing the flange dimensions. For instance, the overall size of the flange assembly can help determine how many bolts you might need, as well as how thick the flange should be. In other words, a wide flange assembly may require more, or larger, bolts than a narrow flange does.
Another consideration when choosing the correct flange dimensions is the weight and/or pressure involved in the connection related to the flange joint. Flanges that are intended to hold large amounts of pressure, such as steam pipes at a factory, need to maximize the clamping pressure at the connection. Increasing the number of bolts around the pipe accomplishes this goal. If bolts are spaced too far apart, there is increased risk of leakage between them.
To help illustrate this idea, picture a picnic blanket on a windy day. Placing rocks at the four corners of the blanket will help hold it down. High winds might still manage to lift up gaps along the long sides. Splitting that dimension and placing four more rocks in between the rocks at the corners would strengthen the force holding the blanket down. This same idea applies when trying to maximize the clamping pressure of a flange assembly.
If you are still uncertain how to choose the best flange dimensions, contact a flange manufacturer. The majority of dimensions used in flanges and flange systems are already determined. Consulting an engineer may also help when the choice involves heavy loads, high pressures, or configurations that are not common, such as those covered by ASME.