Basic Knowledge of Flange 02

C) Flange Types

Flanges have been designed and developed to be used in a myriad of applications. Each one has its special characteristics, and should be carefully selected to meet specific function requirements. Normally they can be classified as the following basic types,

Weld Neck Flange

Threaded Flange

Socket Weld Flange

Slip On Flange

Lap Joint Flange

Reducing Flange

Blind Flange

Orifice Flange

LWN Flange

Spectacle Blind Flange

a) The Weld Neck Flange (abbreviated as W/N Flange) is occasionally referred to as the "high hub" flange. It is designed to reduce high-stress concentrations at the base of the flange by transferring stress to adjoining pipe. Although expensive, the weld neck flange is the best-designed butt weld flange available because of its inherent structural value and ease of assembly.

Known for its strength and resistance to dishing, the weld neck flange is manufactured with a long tapered hub. The tapered hub is is created by gradual increase in metal thickness from the weld joint to the flange facing. The symmetrical taper transition is extremely beneficial under conditions of repeated bending caused by line expansion, contraction, or other external forces. Weld Neck Flanges  are normally used in  severe service applications involving high pressures, high temperatures, or sub-zero conditions(cryogenic).

Weld Neck Flanges are bored to match the ID of the adjoining pipe. In other words, the thinner the wall of the pipe, the large the bore through the flange. The thicker the wall of the pipe, the smaller the bore. Because of these matching IDs, there is no restriction to the flow. Turbulence and erosion are therefore eliminated.

b) The Slip On Flange(abbreviated as S/O Flange ) has a low hub that allows the pipe to be inserted into the flange prior to welding. Shorter in length than a weld neck flange, the slip on flange is used in areas where short tie-ins are necessary or space limitations necessitate its use.

Two significant disadvantages, however, are the requirements of two fillet welds, one internal and one external, to provide sufficient strength and prevent leakage, as well as a life span one third of that of the weld neck flange. They are preferred over weld neck flange by many users because of their lower initial cost. However the total cost after installation is not much less than the weld neck flange because of the additional welding involved.

 

c) The Lap Joint Flange is primarily used on carbon or low alloy steel piping systems. Attachment of a lap joint flange to the piping system requires a lap joint stub end. The lap joint flange and the stub end assembly are mainly used in piping systems that necessitate frequent dismantling for inspection or routine maintenance. It is also used in the erection of large diameter or hard-to-adjust piping configurations because of its quick bolt hole alignment.

Because of it's two piece configuration, the Lap Joint Flange offers a way to cut cost or simplify work. The cost saving comes when the piping system requires a high cost alloy for all "wetted" parts to reduce corrosion. The sleeve or Stub-end can be the required higher cost alloy but the flange can be the lower cost forged carbon steel.

 

d) The Threaded Flange similar to the slip on flange, but the bore is threaded. Its principal value is that it can be assembled without welding. This feature makes the threaded flange well suited to extreme pressure services that operate at normal atmospheric temperatures and in highly explosive areas where welding may create a hazard.

Threaded flanges are note suited, however, for conditions involving temperatures or bending stresses of any significance, particularly when cyclic conditions exist which may cause leakage through the threads. After just a relatively few cycles of expansion and contraction or movement caused by stress, the threaded flange no longer performs adequately. A seal weld is sometimes applied around the threaded joint to reduce the possibility of leakage. This technique, however, cannot be considered as entirely satisfactory nor is it always possible.

e) The Socket Weld Flange is also similar to the slip on flange. It is originally designed for use on small diameter(1/2" through 4") high pressure piping systems. Like socket welding fittings, pipe is inserted into the socket then welded. An internal weld is often employed for added strength. By grinding the internal weld smooth, turbulence and flow restriction are kept to a minimum.

The socket weld flange can be more expensive than a slip on flange due to more intricate machining, and they can be less readily available than slip on flanges due to the fact that the bottom half of the inside diameter must be made to the same schedule as the pipe that is being used.