Unequal Wall Thickness

Let's see, Unequal wall thickness, I think this is one of the important thing that we have to understand in the Piping System, and Piping & Fabrication will explain it.

In most piping systems there are components such as valves, castings, heavier header sections, and equipment nozzles which are welded to the pipe. In such instances the heavier sections are machined to match the lighter pipe wall and the excess thickness tapered both internally and externally to form a transition zone. Limits imposed by the various codes for this transition zone are fairly uniform. The external surface of the heavier component is tapered at an angle of 30° maximum for a minimum length equal to 11⁄2 times the pipe minimum wall thickness and then at 45° for a minimum of 1⁄2 times the pipe minimum wall.

Slip-on and socket welding flange welds

Internally, either a straight bore followed by a 30° slope or a taper bore at a maximum slope of 1 to 3 for a minimum distance of 2 times the pipe minimum wall are required. The surface of the weld can also be tapered to accommodate differing thickness. This taper should not exceed 30°, although some codes limit the taper to 1 to 4. It may be necessary to deposit weld metal to assure that these limits are not violated.

Fillet Welds. Circumferential fillet welds are used in piping systems to join slipon flanges and socket welding fittings and flanges to pipe. In welding slip-on flanges to pipe, the pipe is inserted into the flange and welded with two fillet welds, one between the outside surface of the pipe and the hub of the flange and the other between the inside surface of the flange and the thickness of the pipe.  Alignment is relatively simple since the pipe fits inside the flange. The B31.1 Code requires that the fillet between the hub and the pipe have a minimum weld leg of 1.09 times the pipe nominal wall or the thickness of the hub, whichever is smaller. The weld leg of the front weld must be equal to the pipe nominal wall or 1⁄4 in, whichever is smaller. The gap between the outside diameter of the pipe and flange inside diameter may increase with size, so the size of the fillet leg should be adjusted to compensate for this situation. Fillet welds are also used for circumferential welding of pipe to socket fittings.

Socket weld fittings and flanges are available in sizes up to NPS 4 (DN 100) but are most frequently used in sizes NPS 2 (DN 50) and smaller. Alignment is not a problem since the pipe fits into the fitting socket. Some codes require that the fillet have uniform leg sizes equal to 1.09 times the pipe nominal wall or be equal to the socket wall, whichever is smaller. In making up socket joints it is recommended that the pipe not be bottomed in the socket before welding. B31.1 and ASME Section III suggest a 1⁄16-in (2.0 mm) gap. In high-temperature service especially, the pipe inside the socket will expand to a greater degree than the socket itself, and the differential expansion may result in unwanted shear stress in the fillet and possible cracking during operation.

Intersection-Type Weld Joints. Intersection-type weld joints occur when the longitudinal axes of the two components meet at some angle. Such is the case where nozzle, lateral, and wye intersections are fabricated by welding. Weld joints in these cases may be butt, fillet, or a combination thereof. Nozzles are made either by seton or set-through construction. In set-on construction, the opening in the header pipe is made equal to the inside diameter of the branch pipe. The branch pipe is contoured to the outside diameter of the header and beveled so that the weld is made between the outside surface of the header and through the thickness of the branch. The through thickness weld is covered by a fillet weld to blend it into the header pipe surface. In set-through construction an opening is cut in the header pipe equal to the outside diameter of the branch pipe and beveled. The branch pipe is contoured to match the inside diameter of the header. See Fig. A6.19. The weld is between the outside surface of the branch and through the thickness of the header and is covered with a fillet weld to blend it into the outside surface of the branch. Either type of construction is acceptable; the usual practice is to use seton since the volume of required weld metal is less. However, when the header is is preferred.

Small nozzles are frequently made with socket welding or threaded couplings set on the header. In these cases it is difficult to assure complete root penetration, and specially designed couplings which permit drilling through the bore to remove the root of the weld are often used.  Welded-nozzle construction cannot be used at the full rating of the pipe involved, and suitability for particular pressure temperatures must be verified by component design methods found in Part B of this book. In all cases there must be a through thickness weld of the branch to the header. Where reinforcing pads are used, they should also be joined to the header by a weld through their thickness. See Fig. A6.19 for typical details. In designing headers with multiple outlet nozzles, sufficient clearance is needed between adjacent nozzles to provide accessibility for welding. Nozzles with reinforcing pads or flanges need greater clearance. PFI ES-731 gives suggested minimum spacings.