Two styles of gasket are used for pipe flanges: full face gaskets and ring gaskets. Full face gaskets need holes for the flange bolts to pass through. When you order this type of gasket the manufacturer needs to know the diameter of the bolt circle. Here we’ll explain what that is and how to measure it. First though, an explanation of the two gasket styles.
A full face gasket has roughly the same outside diameter as the flanges being joined. The inside diameter is of course equal to the bore of the pipe. Flange bolt holes in the gasket simplify assembly as the bolts hold the gasket in place and stop it intruding into the flow. The downside is that all the bolts must come out when installing a new gasket.
In contrast, a ring face gasket sits inside the bolts and can be replaced without completely disassembling the joint. They also have a smaller outside diameter, which saves material. They are used principally on raised face flanges where the sealing surfaces are inside the bolt hole diameter.
A full face gasket can be used with raised face flange. Likewise, ring gaskets can be used on flat face flanges, providing the impact of the reduced gasket width is considered.
Bolt Circle Diameter Measurement
The bolt circle is defined as a circle running through the centers of all the bolt holes. As these points are in space they can’t be measured directly. However, there is way.
The bolt holes will all be the same diameter and arranged in diametrically opposite pairs. Looking at the flange end-on, measure from the left edge of the leftmost hole to the left edge of the hole diametrically opposite. (Or measure from right edge to right edge.)
To eliminate any measurement error, repeat on a number of other hole pairs and average the result. This is the number you’ll give your gasket manufacturer, along with inner and outer diameters, number of bolt holes, and the bolt hole diameter. With this information they’ll cut the gasket to the size you need.