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The use of weld nipple and ferrule type fittings is widespread in Hydraulics, Pneumatics and Lubrication System for tube sizes upto 38mm OD and various thickness depending upon the pressure involved. For bigger sizes, ferrule and weld nipple type fittings are impracticable because of their heavy hexagonal sections which increase the pitch between pipes spreading the pipe installation over a large area. Also such fittings require very high torques for tightening which lead to loose joints.
In such a cases the use of flanges with Butt Weld or Socket Weld ends has been found to be more practical leading to compact piping installations.
CETOP PR 63H, AFNOR 48-0-54 and ISO 6164 specify flanges with Butt Weld ends and square clamping flanges for 250 bar and 400 bar static working pressure respectively. These correspond to 160 bar and 315 bar dynamic working pressures. The flange system consist of a sleeve to be butt or socket welded to the pipe and clamped together between two square flanges by a set of four hex bolts, nuts and spring washers. One of the sleeves carries an "O" ring groove fitted with an "O" ring for sealing the interface.
The use of these ISO 6164 flanges has one disadvantage. The size of the square flange require a higher pitch between pipes due to the flange sizes. In order to reduce the distance between pipes, SAE introduced their own standards which are now incorporated into ISO 6162 for the 3000 psi and 6000 psi static pressure ranges respectively. SAE System depends on the same set of Butt Welding sleeves held together by a set of split flange and solid flange secured by hex bolts and nut.

The design for the SAE system calls for forged flanges to ASTM A 105 with sleeves made from weldable material. In order to reduce costs, various manufacturers have introduced a two piece system shown on pages 24 & 26 where the sleeve and the flange were combined into one piece. Such a system is easily adaptable to both Butt Weld and Socket Weld systems. However, in practice, the two piece system is used mainly for Socket Weld flanges for cost considerations.

With the introduction of more corrosive fluids in piping systems, the total joint is not required to be of non-corrosive material, eg. SS. Only the sleeves which are wetted by the fluid are required to be of SS. This makes for an economical joint.

Further, the orientation of these flange joints can be so done to minimize the distance between pipes and provide for a compact installation.