Know Brazed Joints More on Piping & Fabrication

Know Brazed Joints More on Piping & Fabrication is one of the post that I will gave to all of you now and hope that would be useful.

Brazed Joints
Lap or shear-type joints generally are necessary to provide capillary attraction for brazing of connecting pipe. Square groove butt joints may be brazed, but the results are unreliable unless the ends of the pipe or tube are accurately prepared, plane and square, and the joint is aligned carefully, as in a jig. High strengths may be obtained with butt joints if they are properly prepared and brazed.

However, owing to the brittleness of the brazing alloy, they are not normally applicable. The alloys generally used in brazing exhibit their greatest strength when the thickness of the alloy in the lap area is
minimal. Thin alloy sections also develop the highest ductility. For brazing ferrous and nonferrous piping with silver and copper-base brazing alloys, the thickness of the brazing alloy in the joint generally should not be more than 0.006 in (0.15 mm) and preferably not more than 0.004 in (0.1 mm). Thicknesses less than 0.003 in (0.07 mm) may make assembly difficult, while those greater than 0.006 in (0.15 mm) tend to produce joints having lowered strength. The brazing of certain aluminum alloys is similar in most respects to the brazing of other materials. However, joint clearances should be greater because of a somewhat more sluggish flow of the brazing alloys. 

FIGURE A2.28 Eccentric insertion of consumable insert ring in pipe welded in the fixed horizontal pipe position.

FIGURE A2.29 Examples of typical fillet welded joints

For aluminum, a clearance of 0.005 to 0.010 in (0.12 to 0.25 mm) will be found satisfactory. Care must be exercised in fitting dissimilar metals, since the joint clearance at brazing temperature is the controlling factor. In these cases, consideration must be given to the relative expansion rates of the materials being joined.
The length of lap in a joint, the shear strength of the brazing alloy, and the average percentage of the brazing surface area that normally bonds are the principal factors determining the strength of brazed joints. The shear strength may be calculated by multiplying the width by the length of lap by the percentages of bond area and by taking into consideration the shear strength of the alloy used. 

TABLE A2.22 Standard Dimensions of Bell-and-Spigot Joints for Pipe Centrifugally Cast in
Metal Molds

TABLE A2.23 Standard Dimensions of Mechanical (Gland-Type) Joints

An empirical method of determining the lap distance is to take it as twice the thickness of the thinner or weaker member joined. Normally this will provide adequate strength, but in cases of doubt, the fundamental calculations should be employed. Such detailed determinations are generally unnecessary for brazed piping, since commercial brazing fittings are available in which the length of lap is predetermined at a safe value. For brass and copper pipe, cast or wrought bronze and wrought copper fittings are available. A bore of correct depth to accept the pipe is provided and midway down this bore may be a groove into which, at the time of manufacture, a ring of brazing alloy is inserted. Since the alloy is preplaced in fittings with such a groove, separate feeding of brazing alloy by hand is generally unnecessary.

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