Technical Field

The present invention relates generally to joining and sealing systems for pipe joints formed of moleculariy oriented plastic pipe, and to a simplified method for coupling pipe sections of moleculariy oriented plastic pipe using heat-shrinking technology to form such joints.

Description of the Prior Art

Pipes formed from thermoplastic materials including polyolefins such as polyethylene, polypropylene and PVC are used in a variety of industries. For example, such pipes are commonly used in municipal water and sewer applications, in forming a joint between sections of pipe, the spigot or mate pipe end is inserted within the female or socket pipe end. The actual manufacture of the mating sections of plastic pipe typically involves the reforming of the end of toe pipe by reheating and shaping to some desired profile to provide a means of mating with toe opposing end of the next pipe. The art of forming sockets (also called bells) on plastics pipes is well established, and there are numerous processes and methods in toe literature. An annular, elastomeric ring or gasket is typically seated within a grove or“raceway' formed in the socket end of the thermoplastic pipe to assist in forming a sealed pipe joint between adjoining sections of pipe. As the spigot is inserted within the socket, toe gasket provides the major seal capacity for the joint.

In recent yearn, pipe systems employing the so called ‘PVC-O* pipe have become increasingly popular. The designation“FVC-O’ stands for polyvinylchloride oriented, sometimes referred to simply as moteculariy oriented pipe, or 'MOP.' it is well established in the literature that molecular orientation of plastics can provide enhanced mechanical properties for plastic pipe of the type under consideration, and such materials are commonly used for plastics pipes. The moleculariy oriented thermoplastic materials enhance the strength of the artiste in certain directions by orienting the molecules in the plastic material in such directions.

Orientation Is achieved by drawing or stretching the material under appropriate conditions of temperature, such that a strain (i,e. deviation from the originally formed dimensions) is induced in tee plastics material to cause alignment of the molecules, and thereafter cooling the material while drawn to lock in teat strain. A number of methods have been proposed whereby this principle is applied to plastic pipes, in particular in order to enhance their strenoth under internal oressure bv circumferential and/or forces, or by external forces acting cm the pipeline.

For example, U.S. Pat No.4,428,900, shows a pipe of oriented thermoplastic polymeric material having an integral socket which Is manufactured by expanding a tubular blank. The tubular blank is heated by circulation of hot water to a temperature at which deformation will induce orientation of the polymer molecules. The blank is then expanded radially outward against a mold by application of internal pressure.

U.S. Pat No. 5,449,487. shows an apparatus and method for orienting plastic pipe. A heated pipe is oriented radially by means of a conically widening mandrel which is located downstream of the plastic extruder.

The above examples are intended merely to be illustrative of the general state of the art in the manufacture of mdeailarty oriented pipe. Whether the pipeline system in question is the more ordinary PVC pipeline, or the more exotic PVC-0 pipeline, it is often desirable to provide a“restrained joint* to insure that the spigot or mate pipe end and the female or socket end do not separate due to internal or external forces, such as hydraulic forces teat exist inside the pipeline, or external forces, such as bends in die direction of the pipeline, earthquakes or ground movement and the like. the problem is exacerbated In the case of PVC-0 pipe joints in that die prevalent socket end is Often provided with an "Anger Raceway" for receiving the seeding gasket Since an advantage of PVC-0 pipe is that it can be thinner with the same type strength as traditional PVC pipe, die Anger Raceway has a different geometry than the traditional socketed grooves provided in the more traditional‘Rieber* gasket sealing systems for traditional PVC pipe. This geometry has proved to be more difficult problem from the standpoint of providing a securely sealed joint than the traditional PVC pipe joint

Or® approach to joining moleculariy oriented pipe with a“restrained joint" was presented In U.S _> Patent Publication No. 2011/0062700 to Corbett, Jr„ assigned to the assignee of the present invention. That publication disclosed a method for joining moleculariy oriented pipe in which a coupling is provided which is formed of a material other than moleculariy oriented pipe, such as ordinary PVC pipe. The coupling is formed as a tubular body with a combination sealing and restraint mechanism located in each of two opposing end openings of the coupling that seal and restrain mating plain spigot ends of the moleculariy oriented pipe. Because the coupling is made of a material such as ordinary PVC _y the sealing arid restraint mechanisms can be installed In internal grooves provided in the coupling interior during normal pipe belting operations without introducing unacceptable levels of stress or strain into the product. While providing a workable solution, that approach had the disadvantage of requiring more coupling components, adding to the of each joint in the pipeline system.

There are presently no other commercially available solutions to the above problem known to Applicant. The existing solutions have a limited pressure range and typically rely on indentation of the PVC-0 pipe which is typically hard and relatively brittle. A need continues to exist, therefore, for improved techniques for manufacturing and joining molecularly oriented pipe and specifically PVC-0 pipe, which techniques take into account toe unique properties of these types of molecularly oriented plastic materials. Disclosure of the invention

A method is shown for joining a first longitudinal section of mdiecuiariy oriented pipe to a second longitudinal section of moiecularty oriented pipe. Each of the longitudinal sections of moiecuiarty oriented pipe has at least one plain, spigot end having a given external diameter and a mating, oppositely arranged socket end having a given internal diameter, in the first step in the method, a fast section of pipe is provided having a straight, preformed socket with an internal cfiameter and with an end opening having enough clearance to allow a mating spigot section end having a given external diameter to be inserted into toe socket end opening anti be dosely received therein. After Inserting the spigot end to a preselected depth in the mating socket «id, the socket end is heated so that the internal diameterof the socket end comes into contact with the external diameter of the spigot end. The mdiecuiariy oriented pipe at this point Is in a rubbery state and exhibits a low elastic modulus due to the heating, which allows the socket end to conform tightly to the spigot end external diameter without deforming the spigot end. The socket end is then cooled sufficiently to cause the socket end to undergo thermal contraction to produce an interference fit with toe spigot end.

In one preferred: method of carrying out the process of the invention, a pipe joint of molecularly oriented pipe is formed by solely relying upon the pre-existing shrinking capabilities that are inherent in these particular types of plastic. In another aspect of the method of toe invention, a gasket fa provided between the internal diameter of the socket end and the external diameter of the spigot end to increase the sealing capacity of the joirif. In another aspect of the method ofthe invention, the spigot and socket tom a restrained joi nt by increasing the coefficient of friction cf the external diameter of the spigot end. The coefficient of friction of the spigot end can be increased, for example, by a technique selected from the group consisting of forming bumps in the external diameter of the spigot end and machining ridges, providing knurled surfaces and providing sanded surfaces on the spigot end.

The spigot end and socket end may also form a restrained joint by providing a mechanical gripping element between the spigot end outer diameter and the socket end internal dtemeter. The mechanical gripping element can be, for example, a metallic and rubber sleeve composite. The mechanical gripping element might also be a simple wire mesh sleeve.

The spigot end and socket end may also form a restrained joint by applying an adhesive between the intemal diameter of the socket ami the external diameter of ihespigot The adhesive might be, for example, an adhesive selected from the group consisting of epoxy adhesives, polyurethane adhesives and heat activated adhesives. The adhesive might also be provided in the form of a double sided stick tape, Preferably, in the inventive method for joining a first longitudinal section of molecuiariy oriented pipe to a second longitudinal section of molecuiariy oriented pipe, the heat shrinking step includes a heat shrinking technology isefeoted from the group consisting of an open flame, an oven, a pressure chamber over a selected area of the pipe joint being formed and a pit® run intide a selected area of the joint being formed.

An improved point joint is also show) which uses the previously described method steps to join two sections of molecuiariy oriented pipe and will be further described in the detailed description which follows. Additional objects, features and advantages wifl be apparent in die written description which follows.

Brief Description of the Drawings

Figure 1 is an end view of a socket section of molecuiariy oriented plastic pipe, partly broken away, and showing the socket end opening into which a mating spigot pipe end to about to be inserted. Figures 2 is a quarter sectional views of the previously described moieculariy oriented pipe socket end with the spigot end being inserted into the spigot end, but prior to any heating.

Figure 3 is a view similar to Figure 2, but showing the pipe socket end and spigot end after heating so that the socket end shrinks until it comes Into contact with the spigot end;

Figure 4 is a simplified view of a commercially available clam shell style oven of the type which could be used to heat shrink the moieculariy oriented pipe and form a pipe joint Figure 5 is a quarter sectional view of the pipe joint of Figure 2, just prior to beginning heating with the clam shell oven.

Figures 6-9 are simplified, partly schematic views of the pipe joint of Figure 3, but showing several possible failure mechanisms that must be taken into account when the pipe joint is subjected to pressure.

Figures 10-12 am simplified, partly schematic quarter sectional views of the pipe joint of moieculariy oriented pipe illustrating the use of an annular sealing gasket to prevent separation of the pipe sections and loss of seating capacity. Figures 13-15 are views similar to Figures 10-12 but showing the use of an external reinforcement element as a method for preventing separation of the pipe joint

Figures 16-18 are similar to Figures 13-15, but show toe use of a thickened socket end as a method of preventing pressure penetration between the spigot end and socket end of the pipe joint

Figures 18-20 are quarter sectional views of the pipe joint of the invention showing the use of a mechanical gripping element, along with a rubber gasket to insure the integrity of the pipe joint

Description of toe Preferred Embodiment

As briefly discussed in the Background portion of the present application, plastic pressure pipe systems are used for the conveyance of drinking water, waste water, chemicals, heating and coding Adds, foodstuffs, ultrapure liquids, slurries, gases, compressed air and vacuum system applications, both for above and below ground applications. Plastic pressure pipe systems have been in use in the United States for potable (drinking) water systems since at teast about the 19S0s. The types of plastic pipe in commercial use in toe world today indude, for example, unpiestidzed polyvinyl chloride (referred to as PVC or PVC-U), acrylonitrile butadiene styrene (ABS), post chlorinated polyvinyl chloride, (CPVC), polypropylene (PP), polyethylene (PE), pdyviny!idene fluoride (PVDF) and poiybiitylene (PB) and more recently the molecularly oriented plastics. As discussed in the Background section above, the newer ferroof plastic material used in plastic pipe manufacture is the so called“PVC Molecuiariy Oriented Pipe", sometimes tailed TVC-0 pipe" or simply "MOP* for simplicity. As has been briefly explained, these mdecuiarty oriented thermoplastic materials often exhibit enhanced strength ofthe article in certain directions by orienting the molecules in the plastic material in such direction, whereby the tensile strength of the plastic increases and the stretch decreases in such direction. Oils can provide advantages, for example when applied to tubular articles, where orienting is effected in the radial direction, for instance to increase the pressure resistance of the pipe, or In the longitudinal direction of the pipe, for instance to increase the tensile strength of the pipe, or in both directions (biaxial orientation). In the case of PVC-O pipe systems for municipal water and sewer pipe, toe molecular (mentation approximately doubles toe material strength, so that only about half the waB thickness for the same pipe class is required to be used to meet the applicable specifications.

A disadvantage of the mo!ecutariy oriented pipe (MOP), however, when used in such processes as the well known“Rieber* belling process is that the MOP is more difficult to bell. During the Rleber belling operation, a heated pipe end is forced over a forming mandrel which typically has a sealing ring, and perhaps other components, mounted about the mandrel. It is necessary to deform the heated pipe end as it passes over the forming mandrel and accommodates the sealing ring or other components, in some cases, the material of the MOP is already stretched to near its limit during pipe manufacture. The belting operation may fa# when such MOP feedstock is used in a Rleber belting process, or at the very least, the otherwise desired properties of the MOP may be altered.

S&B Technical Products, IncVHultec, the assignee of the present Invention, has previously deveioped specialized sealing gasket designs for PVC-O pipe. These designs are generally referred to as the PRESSURE FIX™, In Europe, and as the MAMBO™ in North America. Although these gaskets have been shown to be effective sealing solutions for PVC-O in many instances, there continue to he instances where MOP and particularly PVC-O pipe is not abie to adequately withstand the stresses encountered during pipe befling operations, or in maintaining seeling integrity at pipe joints in field app toaaons. The present invention offers a solution to the previously described problem with MOP by utilizing a heat shrink joining method for forming joints of PVC-0 pipe. While heat shrink techniques exist in the literature and have been used with other plastic pipe systems, particularly corrugated pipe systems, they have typically employed a shrinking element which was a separate, distinct part, like a sleeve. The use of a sleeve, which by its nature did not become a structural integral part of the pipe, presented a weak point in the pipeline system. Applicant is also unaware of any previous work in joining PVC-0 pipe which specifically took advantage of the pre*e>dsting shrinking capability of PVC-O pipe which is an inherent property of the material.

The method of the invention takes advantage of the reversion and contraction properties of PVC-0 pipe in a visco-elastic process. When foe PVC-0 material is heated to about 8Q ^* C, it readies a "robbery" state where it remains very resilient but with a low viscosity. A low "long-term" elastic modulus is reached very quickly due to the material’s low viscosity. The process involves very little permanent (plastic) deformation.

When the PVC-O material is cooled in foe expanded shape to, for example, about 55*C, it returns to a very high viscosity before it can shrink back to its original geometry. The material, to effect, remains virtually "frozen" in foe deformed state due to the very high viscosity, The material then undergoes thermal contraction as it further cools to ambient temoerature.

The method of the invention takes advantage of a further processing step, not normally used in the manufacture of PVC-0 pipe. If, after the normal manufacturing operation, the PVC-0 pipe is heated again to near gassy transition, its viscosity will drop again and it will recover much of its original shape. Another way of saying this is that when PVC-0 pipe is heated above its glass transition temperature it reverts; the CD shrinks, walls thicken, and some of foe orientation of foe molecules is lost. It will harden when coded again, although, as explained, it may have lost some of its molecular orientation. This is why normal belling of PVC-O pipe must be done at cold temperatures yet above the glass transition. The most important point for purposes of the present invention is that the PVO O pipe will shrink, rather than expand with the additional heating step, including the formed socket.

The two commonly used methods for manufacturing PVC-0 pipe wilt now be briefly discussed. Both processes start by extruding a length of PVC-U pipe at a reduced diameter and increased wall thickness, followed by heating the pipe to the glassy transition tern Denature and exoandino it to foe desired diameter and wall thickness.

The following is an example of a "batch process' for producing PVC-0 pipe:

Extrude a pipe at 50% OD «id 200% thickness of desired product Cut into 7,1m sections.

Insert length of starting stock into a closed mold. This mold is a jacketed cylinder approximately 7.6 meters (25 ft) long including bell-forming segment that is bolted on its «id. Each end of file starting stock Is "pinched down" and held In place.

Heat the pipe near its glassy transition temperature. Use internal pressure to expand the pipe until it comes to contact with foe mold.

Cool the mold. After the pipe hardens again it will undergo thermal contraction so it detaches from the mold surface.

Remove expanded ole from the mold.

The pipe is transported to a cutting station where each of file "pinched down" ends is cut off to form the final 6.1 meter (20 ft) laying length.

The following is an example of a 'continuous process' for producing PVC-0 pipe:

Extrude a pipe, or starting stock, 50% of the desired OD and having twice foe wail thickness of the desired finished product The starting stock goes through a "cohctitibhing tank* where it is uniformly heated to a desired temperature.

The starting stock is pulled through an ''expansion zone” by a second haul-off where further heat brings the stock above the Tg of PVC. Desired dimensions are attained in the expansion zone.

The oriented pipe is coded in a spray tank.

The oriented pipe Is cut to length using a special rotary saw.

The oriented pipe is transported to a belling machine and the beli socket is formed.

Belling and gasket options for commercially avaiiabie PVC-O pipe may vary, but a common approach is tp farm a bell or socket end with an internal circumferential groove for receiving an annular sealing gasket FVCO pipe, uniike ordinary PVC-U pipe, is typically provided with what Is called a "30/60" Internal circumferential groove or "Anger Groove" for forming a non-restrained joint. The Anger Groove provides a very limited space for any type of joint restraint and to Applicant's knowledge, there are presently no successful joint restraints in tire marketplace for these types of pipe systems. Holding gasket raceway dimensions is difficult in FVC-0 pipe, with the belling process being the greatest contributor to scrap. As the PVC-O pipe wall becomes relatively thicker, it becomes more difficult to meet specifications.

Turning now to Figure 1, there is shown a mate or spigot pipe end 11 of one section of PVC-0 pipe about to be inserted into the mouth or end opening 13 of a socket or bei! pipe end 15 of a second, mating section of FVC-0 pipe of the type under consideration, the two sections being shown hi somewhat exaggerated fashion tor ease of illustration.

The bade heat shrink pipe joining process of the invention will now be described with respect to the quarter sectional, axis-symme¾ie views of Figures 2 and 3. in each case, the upper section 15 represents a section of the socket, white the lower section 11 represents a section of the spigot of the pipe system of Figure 1. The process starts from a length of PVC-0 pipe with a straight, pre-formed socket 15. Note that there is enough clearance (indicated generally at 17) between the male, spigot end 11 and the socket end 15 to allow easy insertion of the spigot 11. After inserting the spigot, the socket is heated to about 80 ^e C, causing the socket to shrink until it comes into contact with the spigot Note that the socket is shrinking with temperature, rather than expanding. The PVC-0 material is now in a rubbery state and toe low elastic modulus will help toe socket conform tightly to the spigot surface without deforming toe spigot The heating process can be accomplished in various ways, including the use of open flame (propane), various types of ovens (hot air, steam, resistance heaters) or by using an industrial heat blanket

Figures 4 and 5 illustrate a dam shell type heater 18, of file type commerdatly available in the marketplace today, having internal resistance heating elements 21 which could be used for the heating step, in some cases, a pressure chamber might be placed over the overlapping socket area which would be heated inside and later water coded. Optional cooling may be provided inside the pipe with a mandrel or air. A pity might also be run inside the pipes beneath tire socket area. This could be. for example, a metallic cylinder which could be hdiow and also provide for cold water delivery. The plug could also be of the inflatable type, insulating and coding elements may be provided to prevent the spigot from heating and softening at the same time as tire socket Moderate pressure, for example, 1 bar, might also be applied to accelerate shrinkage.

In the next step in the process, the socket 15 is coded to about, for example, 55 ^e C, causing the socket to harden. The socket will undergo thermal contraction to produce about an 0.5% interference fit with the spigot 11. This will insure that the spigot and socket remain firmly attached at the joint The Joint lnterference Contact Pressure Can Be Estimated;

Uting thin wall theory for simplicity and assuming foe same elastic modulus for spigot and bell, the contact pressure between them is

Pit ; nominal pressure using the same units as pi

<x _e : design stress (12.5 MPa for pipes at and above DN 110 by ISO 4422)

From here the interface pressure is roughly the same as the nominal pressure.

The Effect of lntemal Fluid Pressure Must Be Taken Into Account

When and internal flute pressure is applied to the pipe, it pushes the spigot against the

socket so the interface pressure increases roughly to:

Then for example at nominal pressure the interface pressure between spigot and socket is roughly 50% greater, which would ensure sealing. Under pressure tire pipe also expands, expressed in terms of strain:

At the joint, tee effective SDR is about twice. So, expansion under fluid pressure occurs at naiT me rate .

Considering this, expansion of the socket next to the overlap will exceed the thermal interference at roughly 2 P _s . At this point the interface pressure at the edge of tee overlap becomes 0, fluid penetrates it expands the socket and the joint fails. Pressure penetration can occur an interface pressure greater than 0. Sometimes this is referred to as the‘critical pressure ^* and it depends on smoothness of the contact surfaces to a large extent Failure can occur anywhere between P _N and 2P _W .

Figures 6-9 are intended to illustrate, in simplified fashion, the possibility of a failure mechanism under pressure. Any deformation due to heat shrink is not shown. The deformation due to pressure is shown in exaggerated fashion for illustration purposes.

Figure 6 shows the initial pipe condition without any internal pressure. In Figure 7, as pressure is applied, tee pipe length spaced away from tee overlap undergoes more expansion. This is illustrated generally at 23 in figure 7. When the difference in expansion overcomes tee shrink fit interference, a gap starts to open and fluid pressure penetrates the interface. This Is illustrated In exaggerated fashion generally at 24 In figure 8, As pressure is exerted on bote sides of the spigot (as at 25, 27 in Figure 9), it returns to its original diameter, white the socket continues expanding. Resistance To Axial thrust Can Also Be Estimated:

Equating friction force that the joint can generate and axial thrust that fluid applies on the spigot

The length of socket required to resist axial thrust would be:

Considering

This would be a reasonable socket length for most applications. Also, a practical socket length for joint restraint is achievable as long a® the joint doesn't fell due to socket expansion under internal pressure. Still, a more secure restraint mechanism would be desirable in some circumstances. Nevertheless, a very long overlap would ensure axial restraint where other types of restraint mechanisms are not practical or desirable. Enhancements To Sealing Performance:

In some instances, as where a very long pipe overlap is not practical, other enhancements can be made to the heat shrinksysfem to fcnprove sealing performance. The object is to prevent water from penetrating the spigot-socket interface, thereby preventing leakage and preventing separation due to pressure penetration. There are many enhancements which might be employed, including: Gaskets between layers; the simplest approach, probably most cost effective and probably the best way to prevent leakage.

Applying an adhesive between layers; tills would Improve both sealing pressure and axial thrust performance. Adhesives could include such things as epoxy, polyurethane heat activated and such foincss as double sided stick! no tone.

Another approach Is socket thickening; this would increase the pressure at which heat shrink Interference Is overcome by expansion.

A further approach would be toe use of an external socket reinforcement this would increase the pressure at which heat shrink interference is overcome by expansion.

Figures 10-12 illustrate the use of a gasket to prevent separation. Figure 10 illustrates, in simplified fashion, the condition of the joint prior to heating. The preferred seal ring or gasket 20 would be fiat so that it wouldn't tend to roll. Figure 11 shows toe condition of the joint after heating with toe seal ring 29 being confined and compressed between the spigot 11 and socket 15. Figure 12 iilustrates the joint condition at the pressure at which the difference in expansion between the overlapped and non-overlapped pipe is greater than the interference fit The socket and spigot start separating. However, toe seal ring 29 biocks pressure penetration and stops the separation process. The integrity of the overlap on toe dry side of toe joint prevents seal extrusion and preserves joint restraint

Figures 13-14 illustrate, in simplified fashion, the use of an external reinforcement on the socket 15. The reinforcement 31 is, in tois case, a piece of the same pipe material as the remainder of toe pipe joint at toe socket O.D; It can be shrunk at the plant or in the field. An optional chamfer 33 can be provided over toe socket transition to improve fit It could also be another type of reinforcement such as a strap. figure 14 shows the condition of the joint after shrinking. Figure 15 shows what happens when pressure is applied. The reinforcement 31 prevents the socket 15 from expanding at the point of pressure penetration. However, them is still some risk of pressure penetration If the mating surfaces are not smooth enough.

Figures 15-18 are intended to illustrate the use of a thickened socket (bell) to enhance sealing performance. Figure 18 shows the condition of the joint before shrinking. However, a thicker socket will be more difficult to produce and shrink. Figure 17 shows the condition of the joint after shrinking. When pressure is applied; the thick socket expands less to the pressure at which pressure penetration occurs will increase. There is still a risk of pressure penetration if the mating surfaces «re not smooth enough.

Enhancements to Axial Restraint:

As mentioned briefly, there are also various enhancements to axial restraint that can be utilized. One is to Increase the coefficient of friction of the spigot. This would improve the resistance to axial thrust. The coefficient of friction could be increased, for example, by forming "bumps" in spigot, or by machining ridges, providing knurled surfaces, or sanded surfaces on the spigot,

Another technique would be to provide a mechanical grip between spigot OD & socket ID layers. This would improve both the sealing pressure and axial thrust properties of the joint The mechanical grip might comprise, for example, a metallic and rubber sleeve composite, or a simple wire mesh sleeve.

Figure 19 shows the pipe joint equipped with a sealing ring 35 and a mechanical gripping element 37 before shrinking. It is possible that the seal element and the gripping element might be integrated into a single part for easier handling. Possible variations in seal shape can lead to an optimized socket profile and improved sealing performance. The seating element shown in Figures 19 ami 20 Is a“barbell” shaped profile. Figure 26 shows tie joint after shrinking. The particular type of gripping element shown in Figures 19 and 20 is a portion of am expanded wire mesh sleeve. Thewire mesh sleeve has sharp edges for Indentation of the spigot end can be supplied in a variety of shapes and thicknesses.

An invention has been shown with several advantages. The heat shrinking method of tie Invention provides a simple and economical way to join PVC-O pipe, as well as providing a restrained joint for PVC-0 pipe. The approach avoids many of the problems involved in belling PVC-O pipe in order to provide an internal groove or raceway for a sealing gasket, or in attempting to use a Rieber type belling process. The method is simpler and more economic^ than using an external coupling or external restraints. The technique does not necessarily rely upon the indentation of PVC-O which is hard and brittle by nature. The present method differs from prior heat shrinking techniques for plastic pipe which involved the use of a separate sleeve where the sleeve constituted a separate, distinct part, which never became a structurally integral part of the pipe, and thus became a poirrt of weakness.

While the invention has been shewn in several of Its forms, it is not; tot» limited but is susceptible to various changes and modifications without departing from the spirit thereof.