Welded and brazed joints are the most commonly used methods for joining piping components because these joints are stronger and more leak-tight than threaded and flanged joints. Furthermore, they do not add weight to the piping system as flanges do, and they do not require an increase in pipe wall thickness to compensate for threading, as threaded joints do.
Pipe-Weld Joint Preparation and Design
Butt Welds. The most common type of joint employed in the fabrication of welded pipe systems is the circumferential butt joint. It is the most satisfactory joint from the standpoint of stress distribution. Its general field of application is pipe to pipe, pipe to flange, pipe to valve, and pipe to fitting joints. Butt joints may be used for all sizes, but fillet-welded joints can often be used to advantage for pipe NPS 2 (DN 50) and smaller.
The profile of the weld edge preparations for butt welds may be any configuration the welding organization deems suitable for making an acceptable weld. However, to standardize the weld edge preparation on butt-welded commercial piping components, standard weld edge preparation profiles have been established in ASME B16.25. These weld edge preparation requirements are also incorporated into the standards governing the specific components (e.g., B16.9, B16.5, B16.34). Figures A2.22, A2.23, and A2.24
illustrate the various standard weld edge profiles for different wall thickness.
On piping, the end preparation is normally done by machining or grinding. On pipe of heavier wall thicknesses, machining is generally done on post mills. On carbon and low-alloy steels, oxygen cutting and beveling are also used, particularly on pipe of wall thicknesses below ¹⁄₂ in (12 mm). However, the slag should be removed by grinding prior to welding.
Because of fairly broad permissible eccentricity and size tolerances of pipe and fittings, considerable mismatch may be encountered on the inside of the piping. Limitations on fit-up tolerances are included in several piping codes. For severe service applications, internal machining may be required to yield proper fit-up. When one is machining the inside diameter, care should be taken to ensure that minimum wall requirements are not violated. Table A2.21 lists the counterbore dimensions typically specified.
When piping components of unequal wall thickness are to be welded, care should be taken to provide a smooth taper toward the edge of the thicker member. The length of the taper desirable is normally 3 times the offset between the components, as outlined in ASME Boiler and Pressure Vessel Code Sections-I and III, and ASME B31.1, Power Piping Code. The two methods of alignment which are recommended are shown in Fig. A2.25.
The wall thickness of cast-steel fittings and valve bodies is normally greater than that of the pipe to which they are joined. To provide a gradual transition between piping and components, the ASME Boiler and Pressure Vessel Code and the ASME Code for Pressure Piping permit the machining of the cylindrical ends of fittings and valve bodies to the nominal wall thickness of the adjoining pipe. However, in no case is the thickness of a valve permitted to be less than 0.77tmin at a distance of 1.33tmin from the weld end, where tmin is the minimum valve thickness required by ASME B16.34. The machined ends may be extended back in any manner, provided that the longitudinal section comes within the maximum slope line indicated in Fig. A2.25. The transition from the pipe to the fitting or valve end at the joint must be such as to avoid sharp reentrant angles and abrupt changes in slope.
End Preparation for Inert-Gas Tungsten-Arc Root-Pass Welding. The pipe end bevel preparations shown in Fig. A2.24 are considered adequate for shielded metal- arc welding, but they pose some problems in inert-gas tungsten-arc welding. When this process is used, extended U or flat-land bevel preparations are considered more suitable since the extended land reduces the heat sink, thereby affording better weld penetration. The end preparations apply to inert-gas tungsten-arc welding of carbon- and low-alloy steel piping, stainless-steel piping, and most nonferrous piping materials. On aluminum piping, the flat-land bevel preparations are preferred by some fabricators.
Backing Rings. Backing rings are employed in some piping systems, particularly where pipe joints are welded primarily by the shielded metal-arc welding process with covered electrodes. For example, a significant number of pipe welds for steam power plants and several other applications are made with the use of backing rings. On the other hand, in many applications backing rings are not used, since they may restrict flow, provide crevices for the entrapment of corrosive substances, enhance susceptibility to stress corrosion cracking, or introduce still other objectionable features. Thus, there is little, if any, use made of backing rings in most refinery piping, radioactive service piping, or chemical process piping.
The use of backing rings is primarily confined to carbon- and low-alloy steel and aluminum piping. Carbon-steel backing rings are generally made of a mild carbon steel with a maximum carbon content of 0.20 percent and a maximum sulfur content of 0.05 percent. The latter requirement is especially important since high sulfur in deposited weld metal (which could be created by an excessive sulfur content in such rings) may cause weld cracks. Split backing rings are satisfactory for service piping systems. For the more critical service applications involving car- bon- and low-alloy steel piping, solid flat or taper-machined backing rings are preferred in accordance with the recommendations shown in Pipe Fabrication Institute Standard ES1 and illustrated in Fig. A2.24 and Table A2.21.
When a machined backing ring is desired, it is a general recommendation that welding ends be machined on the inside diameter in accordance with the Pipe Fabrication Institute standard for the most critical services—and then only when pierced seamless pipe that complies with the applicable specifications of the Ameri- can Society for Testing and Materials is used. Such critical services include high- pressure steam lines between boiler and turbines and high-pressure boiler feed discharge lines, as encountered in modern steam power plants. It is also recommended that the material of the backing ring be compatible with the chemical composition of the pipe, valve, fitting, or flange with which it is to be used. Where materials of dissimilar composition are being joined, the composition of the backing ring may be that of the lower alloy.
On turned-and-bored and fusion-welded pipe, the design of the backing ring and internal machining, if any, should be a matter of agreement between the customer and the fabricator. Regardless of the type of backing rings used, it is recommended that the general contour of the welding bevel shown in Fig. A2.24 be maintained.
When machining piping for backing rings, the resulting wall thickness should be not less than that required for the service pressure. Wherever internal machining for machined backing rings is required on pipe and welding fittings in smaller sizes and lower schedule numbers than those listed in Table A2.21, weld metal may have to be deposited on the inside of the pipe in the area to be machined. This is to provide satisfactory contact between the machined surface on the pipe inside and the machined backing ring. For such cases, the machining dimension should be a matter of agreement between the fabricator and the purchaser.
Whenever pipe and welding fittings in the sizes and schedule numbers listed in Table A2.21 have plus tolerance on the outside diameter, it also may be necessary to deposit weld metal on the inside of the pipe or welding fitting in the area to be machined. In such cases, sufficient weld metal should be deposited to result in an ID not greater than the nominal ID given in Table A2.21 for the particular pipe size and wall thickness involved.
Experience indicates that machining to dimension C for the pipe size and schedule number listed in Table A2.21 generally will result in a satisfactory seat contact of 7/32 in (5.5 mm) minimum (approximately 75 percent minimum length of contact) between pipe and the 10° backing ring. Occasionally, however, it will be necessary to deposit weld metal on the inside diameter of the pipe or welding fitting in order to provide sufficient material for machining a satisfactory seat.
In welding butt joints with backing rings, care should be exercised to ensure good fusion of the first weld pass into the backing ring in order to avoid lack of weld penetration or other types of stress-raising notches.
Consumable Insert Rings. The chemical composition of a piping base metal is established primarily to provide it with certain mechanical, physical, or corrosion resisting properties. Weldability characteristics, if considered at all, are of secondary concern. On the other hand, the chemical composition of most welding filler metals is determined with primary emphasis on producing a sound, high-quality weld. The steel making process employed in the manufacture of welding filler metals permits closer control of the composition range, which is usually considerably narrower than would be practical for the piping base metal where much larger tonnages of steel are involved. On some base metals, the welding together by fusion of only the base-metal compositions may lead to such welding difficulties as cracking or porosity. The addition of filler metal tends to improve weld quality. However, in inert-gas tungsten-arc welding, the addition of welding filler metal from a separate wire, which the welder feeds with one hand while manipulating the tungsten-arc torch with the other, is a cumbersome process and interferes with welding ease. The welder may leave areas with lack of penetration, which generally are considered unacceptable as can be seen, e.g., in the rules of the ASME Boiler and Pressure Vessel Code. Since some types of serious weld defects are detected only with difficulty during inspection (if they are detected at all), it is extremely important to provide the easiest welding conditions for the welder to produce quality welds. One technique to produce high-quality welds is to employ consumable insert rings of proper composition and dimensions. Consumable insert rings which are available commercially are shown in Fig. A2.26.
The three primary functions of consumable insert rings are to (1) provide the easiest welding conditions and thereby minimize the effects of undesirable welding variables caused by the ‘‘human’’ element, (2) give the most favorable weld contour to resist cracking resulting from weld-metal shrinkage and hot shortness, or brittleness, in hot metal, and (3) produce metallurgically the soundest possible weld-metal composition of desirable strength, ductility, and toughness properties.
The best welding conditions are obtained where the flat-land and extended U-bevel preparations are used. These joint preparations are particularly helpful where welding is done in the horizontal fixed pipe position (5G), since they ensure a flat or slightly convex root contour and provide by far the greatest resistance to weld cracking in those alloys particularly susceptible to microfissuring.
The weld-root contour conditions to be expected from different bevel prepa- rations and consumable insert rings are illustrated in Fig. A2.27.
Where sink is not acceptable, it is considered obligatory to use consumable insert rings with the special flat-land or extended U-bevel preparation. In horizontal-rolled (IG) and vertical-position (2G) welding, the insert ring should be placed concentrically into the beveled pipe.
In horizontal fixed-position (5G) welding, the insert ring should be placed eccentric to the center line of the pipe (as shown in Fig. A2.28).
In this position, the insert ring compensates for the downward sag of the molten weld metal and aids in obtaining smooth, uniform root contour along the inner diameter and the joint.
Circumferential fillet-welded joints are generally used for joining pipe to socket joints in sizes NPS 2 (DN 50) and smaller. Figure A2.29
illustrates three typical fillet-welded joints. These types of welds are subjected to shearing and bending stresses, and adequate penetration of the pieces being joined is essential. This is particularly important with the socket joint, since the danger of washing down the end of the hub may obscure, by reason of fair appearance, the lack of a full and sound fillet weld. This condition is one which cannot be detected in the finished weld by the usual visual inspection. Additionally, a 1/16-in (1.5-mm) gap (before welding) must be maintained between the pipe end and the base of the fitting to allow for differential expansion of the mating elements.
There are service applications in which socket welds are not acceptable. Piping systems involving nuclear or radioactive service or corrosive service with solutions which promote stress corrosion cracking or concentration cell action generally require butt welds in all pipe sizes with complete weld penetration to the inside of the piping.
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. 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. 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. #Little_PEng