FLANGE INTRODUCTION

Physical Specifications
First and foremost, a flange must fit the pipe or equipment for which it is designed. Physical specifications for pipe flanges include dimensions and design shapes.

Flange Dimensions
Physical dimensions should be specified in order to size flanges correctly.

Outside diameter (OD) is the distance between two opposing edges of a flange’s face.
Thickness refers to the thickness of the attaching outer rim, and does not include the part of the flange that holds the pipe.
Bolt circle diameter is the length from the center of a bolt hole to the center of the opposing hole.
Pipe size is a pipe flange’s corresponding pipe size, generally made according to accepted standards. It is usually specified by two non-dimensional numbers, nominal pipe size (NPS) and schedule (SCH).
Nominal bore size is the inner diameter of the flange connector. When manufacturing and ordering any type of pipe connector, it is important to match the bore size of the piece with the bore size of the mating pipe.
Flange Faces
Flange faces can be manufactured to a large number of custom shapes based design requirements. Some examples include:

Flat
Raised face (RF)
Ring type joint (RTJ)
O-ring groove
Types of Pipe Flanges
Pipe flanges can be divided into eight types based on design. These types are blind, lap joint, orifice, reducing, slip-on, socket-weld, threaded, and weld neck.

Blind flanges are round plates with no center hold used to close the ends of pipes, valves, or equipment. They assist in allowing easy access to a line once it has been sealed. They can also be used for flow pressure testing. Blind flanges are made to fit standard pipes in all sizes at higher pressure ratings than other flange types.

Lap joint flanges are used on piping fitted with lapped pipe or with lap joint stub ends. They can rotate around the pipe to allow for an easy alignment and assembly of bolt holes even after the welds have been completed. Because of this advantage, lap joint flanges are used in systems requiring frequent disassembly of the flanges and pipe. They are similar to slip-on flanges, but have a curved radius at the bore and face to accommodate a lap joint stub end. The pressure ratings for lap joint flanges are low, but are higher than for slip-on flanges.

Slip-on flanges are designed to slide over the end of piping and then be welded in place. They provide easy and low-cost installation and are ideal for lower pressure applications.

Socket weld flanges are ideal for small-sized, high-pressure piping. Their fabrication is similar to that of slip-on flanges, but the internal pocket design allows for a smooth bore and better fluid flow. When internally welded, these flanges also have fatigue strength 50% greater than double welded slip-on flanges.

Threaded flanges are special types of pipe flange that can be attached to the pipe without welding. They are threaded in the bore to match external threading on a pipe and are tapered to create a seal between the flange and the pipe. Seal welds can also be used along with threaded connections for added reinforcement and sealing. They are best used for small pipes and low pressures, and should be avoided in applications with large loads and high torques.

Welding neck flanges have a long tapered hub and are used for high pressure applications. The tapered hub transfers stress from the flange to the pipe itself and provides strength reinforcement that counteracts dishing.