Socket-welded pipe joint

A welded pipe joint having reduced internal stresses comprises a pipe, a socket fitting, and an annular weld. The end surface of the fitting is chamfered in a concave profile, so that the weld profile subtends an angle of from about 100 degrees to about 120 degrees, rather than the customary 90 degrees. The fitting and the pipe are welded together by applying, from outside the pipe, an annulus of added bead metal to the chamfered end surface of the fitting so as to surround the chamfered end surface, and fusing the base metal of the annular wall and pipe. The pipe has no dam or other protuberance to confine the bead of added metal. Prior to welding, the fitting may have an annular lip immediately surrounding the surface of the pipe and may also have a ring spaced from the lip. The ring may be integral with the fitting and spaced from the lip by thin, radially spaced metal bars or may be separate from the fitting.

RELATED APPLICATIONS

TECHNICAL FIELD

This invention pertains to the field of welding metal pipe segments and metal pipe fittings to each other and to equipment.

BACKGROUND

In plants such as power plants, process plants, and the like, it is necessary to transfer fluids, including liquids, gases, and fluidized solids, from one location to another, and between different equipment. This is accomplished by piping layouts. When a piping layout is created, it must provide for changing directions of the piping, connecting two or more pipe segments, and/or connecting pipe segments to equipment components or fittings.

Based on issued patents relating to welding and pipe connections, as well as on a survey of the industry, it can be said that there are presently two approaches for making such welded connections. One is known as a butt-welded connection, in which both the connecting elements are butted against each other and welded together. The other is known as a socket-welded connection, in which one of elements is a socket and the other is a pipe segment. An end of the pipe segment is inserted into the socket, and the two are welded together. Socket-welded connections are commonly used in small diameter piping. In nuclear power plants, for example, socket-welded connections are normally used only in piping with a nominal diameter of two inches and smaller. Normally, butt-welded joints are stronger than socket-welded joints. However, the construction tolerances for butt-welded joints are more stringent than those for socket-welded joints.

In many butt-welded joints, the end surfaces of the pipes being joined are beveled or chamfered. As disclosed in U.S. Pat. No. 1,842,298 and No. 1,903,315, some socket-welded joints may simulate a butt-welded joint, as far as the shape of the weld is concerned. Such simulations required a non-standard pipe having on its outer surface a dam for confining the added weld metal, which must be configured and positioned to specifically complement a particular fitting.

Failures of socket-welded connections have been identified in the past, resulting in a loss of production and revenues due to shutdown of plants. These pipe segments may contain high-pressure fluid. Bursting of such a pipe segment can result in personal injury, even death. Significant research has been performed to reduce these failures. The following are examples of such research papers:J. K. Smith, “Vibrational Fatigue Failures in Short Cantilevered Piping with Socket-Welded Fittings,” ASME PVP-Vol. 338-1, 1996.P. C. Riccardella et al, “Fracture Mechanics Analysis of Socket-Welds under High Cycle Vibrational Loading,” ASME PVP-Vol. 353, 1997.M. Higuchi et al, “Fatigue Strength of Socket-Welded Pipe Joints,” ASME PVP-Vol. 313-1, 1995.M. Higuchi et al, “A Study on Fatigue Strength Reduction Factor for Small Diameter Socket-Welded Pipe Joints,” ASME PVP-Vol. 338-1, 1996.

It is well known in the industry that the process of welding creates significant residual stresses in the components welded together. These stresses are so strong that they are capable of warping very thick steel plates. Research directed to failures of socket-welded fittings attributes the failures to such residual stresses created by the welding process. To reduce the residual stresses created by the welding process, the welding process needs to be very, very slow. The research has also led to recommendations that welding beads be applied in a very specific sequence. These recommendations are difficult to follow in practice, and they tend to be costly and time-consuming.

SUMMARY OF THE INVENTION

The object of this invention is to provide a socket-welded pipe joint which is practical, provides better fusion of metal, and reduces residual stresses. These will result in fewer failures of the joints. In the first two embodiments, the end of fitting is configured so that the fusion of the base metal of the end portion of the fitting will help to reduce the residual stresses at the root of the joint. These are the simpler of the embodiments presented herein. The remaining four embodiments will help to reduce the residual stresses at both the toe and the root of the joint by using the heat in better fusion of the base metal. Six embodiments of the invention achieve these objectives. This invention includes a welded joint and a method of making a welded joint, as well as a hollow metal fitting and an assembly ready to be welded, to which my application Ser. No. 10/646,583 is directed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1-3illustrate a typical socket-welded connection presently used in the industry. The socket-welded fitting shown inFIG. 1is available in the market. If the pipe segment is to be connected to equipment, the equipment may be configured like an end of a socket-welded fitting.FIG. 2shows an end of pipe segment2inserted in the end of a socket-welded fitting1. They are joined together by a weld3between the end surface4of the fitting and the exterior surface of the wall of pipe segment2.FIGS. 1-3, as well as the other figures of the drawings, are fragmentary, with the other, remote, left hand end along the longitudinal axis of fitting1not being shown. The view inFIG. 2is after the welding is performed with some of the pipe and fitting base metal fused. (The use of “fuse” and cognate terms herein is in accordance with a dictionary definition of that term, namely “To liquefy by heat: to render fluid; to dissolve; to melt. To unite or blend, as if melted together.”) It will be understood that the fused metal shown in this figure and other figures are for illustration purposes, as the precise locations of the boundaries between fused and unfused metal cannot be accurately predicted due to many variables that control the melting of the metal. The gap in the axial direction between pipe segment2and the edge inside the socket-welded fitting is a standard practice and it is believed that it reduces residual stresses. (As used herein, “axis” and cognate terms refer to the longitudinal axis, unless a different interpretation is required by the context.) The gap between socket-welded fitting1and piping segment2in the radial direction is a standard practice, makes the construction easier, and is due to the difference between the inner diameter of the socket-welded fitting1and the outer diameter of the pipe segment2. As shown inFIGS. 1-3and other figures of the drawings, these gaps are exaggerated for purposes of illustration. Failures of this type of joint may initiate at a “toe” or “root” of the weld. Location9inFIG. 3is called a “toe” and location10a “root”. The present invention is believed to be an improvement upon the prior art shown inFIGS. 1-3.

The first embodiment of the invention is shown inFIGS. 4,5and6. Similar to the prior art configuration, this embodiment has modified socket-welded fitting101, the end of the pipe segment102inserted inside the socket, and a weld103. The view inFIG. 5is after the welding is performed with some of the pipe and fitting base metal fused. The difference between the prior art configuration shown inFIGS. 1-3, and this embodiment is that the edge104is chamfered as shown inFIG. 5, while prior art edge4shown inFIG. 2is vertical and 90° to the wall of the pipe segment2. (As used herein, the term “chamfer” refers to a radially outward-facing annular surface which has been cut away, or is shaped as though it had been cut away, from surfaces which included a 90 degree angle; as viewed in cross section, the surface of the chamfer may include one or more straight lines and/or curves. Except for possible broadening to embrace a surface formed by a process that did not include physically cutting, and possible narrowing to specify that the surface is radially outward-facing, the use of “chamfer” and cognate terms herein is in accordance with a dictionary definition of that term, namely “The surface formed by cutting away the arris, or angle, formed by two faces of a piece of timber, stone, etc.” As used herein, “radially outward-facing” means that the entire chamfered surface would be visible to an imaginary observer viewing it from outside the fitting, from a direction generally perpendicular to the longitudinal axis of the fitting; that is, the innermost surface of the chamfer extends away from the shoulder and beyond the outermost surface of the chamfer, in the axial direction, and the chamfer would not appear to be “undercut” as viewed by such an observer. Additionally, the size and profile of the weld103inFIG. 5may be different compared to the profile of the weld3inFIG. 2. The weld size and profile have to be determined in all the embodiments based on the diameter and the thickness of pipe segment and the necessary strength of the weld joint. The slanting edge, in which the entire chamfer is a straight line, is shown for the whole thickness of the fitting inFIGS. 5 and 6. However, the slanting edge can terminate at a smaller height. This is applicable to all the embodiments identified here. The example of this type of edge is illustrated inFIG. 25.

FIG. 6shows the angle108between the edge104and the longitudinal axes of the fitting and the pipe. For the best joint, angle108should be between 60° and 80° so that the supplemental angle110is between 100° and 120°.

The advantage of the first embodiment is that some of the heat of the welding will be used in melting the base metal at the bottom apex of the edge104, thereby reducing the residual stresses and providing better fusion. This is the simplest embodiment.

The second embodiment is shown inFIGS. 7,8,9and10. Similar to the first embodiment, the second embodiment has modified socket-welded fitting201, the end of the pipe segment202inserted inside the socket, and a weld203. In addition, chamfered edge204includes the surface of lip205, which is similar to lips in butt-welded joints. The weld size and profile in this embodiment will also have to be determined based on the diameter and the thickness of the pipe segment and the necessary strength of the weld joint. The detail of the end of the modified socket-welded fitting is shown inFIG. 10. Lip length211and lip thickness212are to be determined based on the diameter and the thickness of the pipe end and the necessary strength of the weld joint. For the best joint, the lip angle213should be between 0° and 30°. The view inFIG. 8is before the welding is performed. The view inFIG. 9is after the welding is performed with some of the pipe and fitting base metal fused.

FIG. 10shows the angle208between the edge204and the longitudinal axes of the fitting and the pipe. For the best joint, angle208should be between 60° and 80° so that the supplemental angle is between 100° and 120°.

The advantages of this embodiment are that more of the heat of the welding will be used in melting the base metal of the lip205and some of the base metal at the bottom apex of the edge204reduces the residual stresses and provides better fusion. However, this embodiment is slightly more complicated than the first embodiment.

FIG. 11shows a variation of the second embodiment in which the surface of lip205A merges gradually into the remainder of chamfered edge204A. That is, the profile of the major portion of the chamfered edge is a curve rather than two intersecting straight lines.

The third embodiment is shown in theFIGS. 12,13,14and15. Similar to the second embodiment, this embodiment has modified socket-welded fitting301, the end of the pipe segment302inserted inside the socket, and a weld303. Chamfered edge304includes inwardly flared surface305and thin metal bars306extending parallel to the longitudinal axis. The size and number of the bars will be based on the diameter and the thickness of the pipe segment302and the necessary strength of the weld joint. Chamfered edge304also includes a ring307, which provides better fusion and reduces residual stresses at the toe of the weld. The metal bars306are provided to connect the ring307to, and accurately spaces it from, the rest of the fitting. The size of the ring307will be based on the diameter and the thickness of the pipe segment302and the necessary strength of the weld joint. The detail of the end of the modified socket-welded fitting for the third embodiment is shown inFIG. 15. The dimensions311,312,320and323are to be determined based on the thickness of the pipe end and the necessary strength of the weld joint. The angle313should be between 0° and 30°. The angle313may be continued for metal bars306or a different angle may be used. The angle313may be continued for ring307or different angle may be used. The view inFIG. 13is before the welding is performed. The view inFIG. 14is after the welding is performed and shows some of the pipe and fitting base metal fused. The cross sections shown in theFIGS. 13 and 14, and the corresponding drawings for the fourth and fifth embodiments, are through the bars. However, as is best shown byFIG. 12, most of the welding will be between the bars306.

FIG. 15shows the angle308between the edge304and the longitudinal axes of the fitting and the pipe. For the best joint, angle308should be between 60° and 80° so that the supplemental angle is between 100° and 120°.

The advantage of this third embodiment is that more heat of the welding will be used in melting the base metal of the flared surface305, metal bars306, and the metal ring307, as well as some of the base metal of the remaining, larger diameter portion of edge304, thereby reducing the residual stresses and providing better fusion. This embodiment will also provide reduction in the failure initiated at the toe of the weld303in addition to reducing failure at the root. However, this embodiment is slightly more complicated than the first two embodiments.

The fourth embodiment is shown inFIGS. 16,17,18and19. Similar to the third embodiment, this embodiment has modified socket-welded fitting401, the end of the pipe segment402inserted inside the socket, and a weld403. Chamfered edge404includes inwardly flared surface405and thin metal bars406extending parallel to the longitudinal axis. The size and number of the bars will be based on the diameter and the thickness of the pipe segment402and the necessary strength of the weld joint. Chamfered edge404also includes outwardly flared surface415at the end of the metal bars406, providing better fusion, and reducing residual stresses at the toe of the weld. A thicker ring417is provided at the terminus of chamfered edge404. The detail of the end of the modified socket-welded fitting for the fourth embodiment is shown inFIG. 19. The dimensions411,412,414,420,421and424are to be determined based on the thickness of the pipe end and the necessary strength of the weld joint. The angle413should be between 0° and 30°. The angle413may be continued for metal bar406or a different angle may be used. The angle of the flared surface415is identified as416, and thickness of ring417is identified as414. The angle416should be between 0° and 30°. As mentioned previously, the weld size and profile will have to be determined based on the diameter and the thickness of the pipe segment and the necessary strength of the weld joint. The weld directly deposited on the pipe induces residual stresses. Additional welding metal deposit on the weld or fitting in the vicinity of the pipe will reduce the induced residual stresses. Hence, the welding should not go beyond the edge of the socket-welded fitting in order to minimize the residual stresses in the pipe adjacent to the weld. This can also be done in the third, fifth, and sixth embodiments. The view inFIG. 17is before the welding is performed. The view inFIG. 18is after the welding is performed and shows some of the pipe and fitting base metal fused.

FIG. 19shows the angle408between the edge404and the longitudinal axes of the fitting and the pipe. For the best joint, angle408should be between 60° and 80° so that the supplemental angle is between 100° and 120°.

The advantage of this fourth embodiment is that more heat of the welding will be used in melting the base metal having the inwardly flared surface405, metal bars406and metal ring417, as well as some of the base metal of the remaining, larger diameter portion of edge404, thereby reducing the residual stresses and providing better fusion. Fusion of the base metal at the flared surface415and the additional thickness of the ring417will reduce the residual stresses in the pipe adjacent to the weld, reducing the possibility of a failure initiated at the toe of weld403. Ring417also serves as a dam for minimizing the chances of the weld material going beyond it. However, this embodiment is more complicated compared to the first three embodiments.

The fifth embodiment is shown inFIGS. 20,21,22and23. Similar to the fourth embodiment, this embodiment has modified socket-welded fitting501, the end of the pipe segment502inserted inside the socket, and a weld503. Chamfered edge504includes inwardly flared surface505and thin metal bars506extending parallel to the longitudinal axis. The size and number of the bars will be based on the diameter and the thickness of the pipe segment502and the necessary strength of the weld joint. Chamfered edge504also includes outwardly flared surface515at the end of the metal bars506for stability, providing better fusion, and reducing residual stresses at the toe of weld503. The detail of the end of the modified socket-welded fitting for the fifth embodiment is shown inFIG. 23. The dimensions511,512,514,520,521,522and524are to be determined based on the thickness of the pipe end and the necessary strength of the weld joint. The dimensions511and512are to be determined based on the thickness of the pipe end. The angle513should be between 0° and 30°. The angle513may be continued for metal bar506or different angle may be used. In addition to outwardly flared surface of515, which corresponds to outwardly flared surface of415in the fourth embodiment, another outwardly flared surface518and thicker ring517are included in this embodiment. The angle of the flared surface515is identified as516, and the length of flared surface515as521. The angle516should be between 0° and 30°. The angle519for the flared surface518should be between 60° and 80°. The length of flared surface518is identified as522. The thickness of the ring517is identified as514. This edge shape creates an annular well for deposit of the weld material, will increase the strength of the joint and minimize chances of weld material going beyond it. In this sense, ring517serves as a dam. This edge shape will also minimize the residual stresses in the pipe adjacent to the weld. The view inFIG. 21is before the welding is performed. The view inFIG. 22is after the welding is performed and shows some of the pipe and fitting base metal fused.

FIG. 23shows the angle508between the edge504and the longitudinal axes of the fitting and the pipe. For the best joint, angle508should be between 60° and 80° so that the supplemental angle is between 100° and 120°.

The advantage of this fifth embodiment is that more heat of the welding will be used in melting the base metal of the lip505, metal bars506, and metal ring517, as well as some of the base metal of the remaining, larger diameter portion of edge504, thereby reducing the residual stresses and providing better fusion. Fusion of the base metal at the outwardly flared surfaces515and518and the additional thickness of the ring517will reduce the residual stresses in the pipe adjacent to the weld, thereby reducing the possibility of a failure initiated at the toe9(as identified inFIG. 3) in addition to reducing possibility of a failure initiated at the root10(as identified inFIG. 3). However, this embodiment is the most complicated of the embodiments proposed here.

The advantages of the bars in the third, fourth and fifth embodiments are that the bar allows a fixed distance between the main fitting and the ring and connect them together for ease of handling and manufacturing efficiency.

The sixth embodiment is shown inFIG. 25. If flexibility in width of the weld is needed, the bars used in the third, fourth and fifth embodiments can be eliminated. The fitting will be made into two pieces. The end of pipe segment602is inserted into ring617and then into fitting601. Chamfered edge604of the fitting and outwardly flared surface618of the ring correspond to chamfered edge504and outwardly flared surface518in the fifth embodiment, respectively. The distance between the two pieces can be increased or decreased according to the size of the weld required. The residual stresses will be reduced at both the toe and the root locations due to melting of the end portions of the fitting601and the ring617. The chamfered edges604and608are similar to the chamfered edges described previously.

The ring617may be made in more that one piece, put in place and welded together for ease of assembly, if necessary or desired. A ring made in two parts617A and617B is shown inFIG. 26. The two-piece ring alleviates the need for inserting an end of the pipe through a one-piece ring.

In all of the embodiments described, the pipe, at least at its end portion being welded to the fitting, has a cylindrical outer surface with a profile which is uniform as viewed in a cross-section taken along the longitudinal axis (i.e., a radial profile). That is, the portion does not have any dam, pocket, projection, recess, or similar functional feature that could require the pipe to be configured, dimensioned, selected, or positioned to work with or otherwise correspond to a particular fitting. Typical applications of the invention will utilize standard pipe in which the entire length of the outer surface will be free of any such feature. In all of the embodiments except the first embodiment, the radial profile of the chamfer is concave, which is preferred. In these preferred profiles, the chamfer is also radially outward-facing, as previously described. Thus, on the radial profile of the annular wall of the fitting, the intersection of the chamfered end surface with the outer cylindrical surface is closer to the remote end of the fitting than any other point on the chamfered end surface is to that remote end. Also, on the radial profile of the annular wall of the fitting, that intersection is radially farther from the annular wall's cylindrical inner surface than any other point on the chamfered end surface is from the annular wall's cylindrical inner surface. In addition, with the exception of the embodiments shown inFIGS. 12-24, on the radial profile of the annular wall of the fitting the radial distance between any given point on the chamfered end surface never exceeds the corresponding distance between any other point on the chamfered surface which is closer to the remote end than the given point. There is no concavity or chamfer on the inner surface of the fitting's annular wall.

In addition, in all of the embodiments described the supplemental angle corresponding to angle110inFIG. 6(i.e., the angle facing away from the remote end of the fitting) is desirably in the same range of 100° to 120°, as measured between the major portion of the end surface and the longitudinal axis of the fitting.

Instead of a circular cross section, the fitting, pipe and weld may have an elliptical or other non-circular cross section.

It will be understood that, while presently preferred embodiments of the invention have been illustrated and described, the invention is not limited thereto, but may be otherwise variously embodied within the scope of the following claims. It will also be understood that the method claims are not intended to be limited to the particular sequence in which the method steps are listed therein, unless specifically stated therein or required by description set forth in the steps.