GB2265853A

GB2265853A - Method for the protection of ferrous pipes with thermoplastics

Info

Publication number: GB2265853A
Application number: GB9206986A
Authority: GB
Inventors: John Brian Waddoups; David William Beale; Cyril Barrie Greatorex
Original assignee: Stanton PLC
Current assignee: Saint Gobain PAM UK Ltd
Priority date: 1992-03-31
Filing date: 1992-03-31
Publication date: 1993-10-13
Anticipated expiration: 2012-03-31
Also published as: GB2265853B; GB2271957B; GB2271957A; GB9324486D0; GB9206986D0

Abstract

Ferrous pipes are protected by extruding a thermoplastic tube, expanding said tube so as to incorporate potential stress shrinkage therein, sliding the extruded expanded tube over the pipe, and heating the thermoplastic tube to release the potential stress therein, causing the tube to shrink securely about the pipe. Expansion of the extruded thermoplastic tube may be carried out by heating the tube along at least one longitudinal continuous strip or area of its periphery, and stretching the heated strip transverse to its longitudinal direction whereby to expand the tube circumferentially in that strip or by passing the tube through a sizing sleeve with an intermittent movement such that an unexpanded portion of tube is heated within a heating zone of the sizing sleeve and subjected to zero or low internal pressure, then subjected to high pressure so that the heated unexpanded portion of the tube is expanded to fill the sizing sleeve in the heating zone, the pressure in the tube is then reduced causing the tube to shrink away from the internal surface of the sizing sleeve, whereafter the tube is moved forward such that the portion from the heating zone is disposed within a cooling zone of the sizing sleeve and a new portion is disposed in the heating zone and the sequence is repeated.

Description

Pipe Protection and Methods for its Attlication This invention relates to pipe protection and methods for its application. The invention relates more particularly to the external protection of ferrous pipes such as those formed of grey iron and those of ductile iron for carrying fluids of various kinds such as gas, water or sewage.

Such pipes are often used in circumstances, such as under the ground, where corrosion is a significant problem.

It has been proposed to overcome this problem by applying external protection to ferrous pipes by providing a thermoplastic tube having an internal diameter slightly greater than the external diameter of the pipe to be sheathed with inbuilt potential stress shrinkage, sliding the tube over the pipe and applying heat to the thermoplastic tube to cause it to shrink snugly onto the pipe.

It is possible to extrude the tube with the enlarged dimension and inbuilt potential stress shrinkage.

Alternatively the tube may be expanded after extrusion by heating and passing it over an expanding mandrel or (under pressure) into a sizing sleeve for example.

A problem encountered in expanding the thermoplastic tube after extrusion and locking in the stresses is that the heated tube becomes sticky and offers considerable resistance to actual movement by adhering to the surface of the expanding mandrel or to the sizing sleeve. The resultant high draw-off force can well be above the reduced strength capability of the heated sleeve leading, clearly, to significant production difficulties.

It is an object of the present invention to provide satisfactory techniques for enlarging such a thermoplastic tube after its extrusion to increase its diameter in such a way that adequate stress shrinkage potential is incorporated into the extruded and expanded tube to enable it to shrink snugly upon the relevant diameter ferrous pipe after it has been slid upon the pipe and heat treated.

In accordance with a first aspect of the present invention there is provided a method of applying external protection to a ferrous pipe comprising extruding a thermoplastic tube, expanding the diameter of the thermoplastic tube to an internal diameter slightly greater than the external diameter of the pipe to be sheathed such that a potential stress shrinkage is developed in the expanded tube, sliding the extruded tube over the pipe and applying heat to the thermoplastic tube whereby it is caused, by virtue of release of the potential stress therein, to shrink securely about the pipe; wherein the extruded tube is expanded by passing the tube through a sizing sleeve with an intermittent movement, the tube being heated during passage through a first heating zone the sizing sleeve and cooled during its passage through a second cooling zone of the sizing sleeve, the intermittent movement of the tube being such that an unexpanded portion of tube is held within the heating zone of the sizing sleeve with the tube subject to zero or low internal pressure and heated, the tube is then subjected to high pressure so that the heated unexpanded portion of the tube is expanded to fill the sizing sleeve at the heating zone thereof the tube is then subjected to zero or low pressure causing the tube to shrink away slightly from the internal surface of the sizing sleeve and the tube is then moved forward such that the portion from the heating zone is now disposed within the cooling zone of the sizing sleeve and a new expanded portion is disposed in the heating zone, whereafter the sequence is repeated.

A possible preheat zone is incorporated in which the tube is treated immediately prior to the heating zone of the sizing sleeve.

This first aspect of the invention includes apparatus for carrying out the methods as hereinabove described, and a ferrous pipe including plastic sheathing produced in accordance with the first aspect of the invention.

According to a second aspect of the present invention there is provided a method of applying external protection to a ferrous pipes comprising extruding a thermoplastic tube, expanding the thermoplastic tube such as to incorporate potential stress shrinkage therein, sliding the extruded expanded tube over the pipe, and applying heat to the thermoplastic tube whereby it is caused by virtue of release of the potential stress therein, to shrink securely about the pipe; wherein the expansion of the extruded thermoplastic tube is carried out by heating the tube along at least one longitudinal continuous strip or area of the periphery thereof, and stretching the heated strip transverse to its longitudinal direction whereby to expand the tube circumferentially in that strip with the inclusion of appropriate potential shrinkage stress therein.

The circumferential expansion of the longitudinal strip of heated thermoplastic tube may be carried out by drawing the tube after and/or during heating over an expansion mandrel In an alternative arrangement the circumferential stretching may be achieved by forming the extruded plastic tubing into a flat configuration, heating a longitudinal strip of the flattened tube, and subjecting the tube to stretching in the heated strip by means of gripping the flattened tube and drawing it at an inclined angle forward and outwardly from the strip so as to stretch, in a direction transverse to the direction of movement of the flattened tuber the heated portion whereby to expand the diameter of the tube. The tube will be returned to its normal open configuration after such expansion with its included potential shrinkage stress.

The second aspect of the invention includes apparatus for performing the method as hereinabove described, and a ferrous pipe including plastic sheathing in accordance with the second aspect of the invention.

It is to be understood that the thermoplastic tube used in the technique of the invention is usually as thin as is reasonable to provide protection against damage and corrosion, without being of such thickness as to be excessively costly. It may for example comprise a tube having wall thickness of the order 1 to 2mm.

It will be appreciated that in practice with pipes of the spigot and socket joint type, for example, separate protection will be needed for the joint, including the enlarge diameter socket, the barrel only of such pipes between the socket and spigot being provided with protection by means of the invention.

The extruded and expanded thermoplastic tubes may be produced in association with production of ferrous pipes or may be produced separately and brought for fitting to the tubes after production. Yet again and in another alternative, plastic tubing and ferrous pipes may be delivered separately to the relevant work site and the tubing slid over the pipes thereat and heat shrunk to fit upon the pipes.

The degree of expansion of the thermoplastic tubes mav be such that in its expanded and non-shrunk condition it fits as an easy sliding fit upon a pipe of maximum diameter within the tolerance range of the nominal outside diameter pipe size concerned.

In order that the invention may be more readily understood three embodiments thereof will now be described by way of example with reference with the accompanying drawings in which: Figure la to Id illustrates schematically an embodiment of the method and apparatus of the first aspect of the invention; Figure 2 illustrates schematically one embodiment of the method and apparatus of the second aspect of the invention; Figure 3 illustrates schematically a second embodiment of the method and apparatus of the second aspect of the invention; and Figure 4 is a scrap view on an enlarged scale of part of the arrangement of the embodiment of Figure 3.

Referring now to Figure la to ld of the drawings it is to be seen that the technique is an intermittent process where the tube is expanded by internal pressure.

It includes a preheat zone 1 where the temperature of plastic tube 2 is raised to just below the critical temperature for expansion This zone, which may not be essential in some instances enables the tube to be "soaked" in order to ensure that the temperature of the tube wall is essentially uniform. The next stage is to heat the tube to the critical expansion temperature and includes one zone 3 of a sizing sleeve surrounding the plastic tube 2. The third stage is to cool the tube and includes a second zone 4 of the sizing sleeve. The process consists of four stages as illustrated in Figure 1.

In stage 1 (Fig la) the plastic tube 2 is heated inside the first zone 3 of the sizing sleeve.

In stage 2 (Fig lb) the internal pressure is raised to expand the heated plastic tube 2 into contact with the first zone 3 of the sizing sleeve.

In stage 3 (Fig lc) the internal pressure is reduced allowing the expanded plastic sleeve to elastically recover sufficiently so that it shrinks away from the sizing sleeve.

In stage 4 (Fig ld) the plastic tube is indexed along by a draw-off mechanism (not shown) until the expanded section is inside the cooling zone 4 of the sizing sleeve.

On the next repeat of the cycle the still hot section is expanded into contact with the cool zone 4 of the sizing sleeve and its temperature is reduced to below the critical level.

This procedure avoids the high draw-off forces which would otherwise break the weakened hot tube. It is possible that the actual stages may be modified eg stages 1 and 2 may be combined.

The length of each stage is not limited; it is possible to have full ductile iron pipe length stages.

Conversely if the stages are relatively short, eg Tm, it may be possible to couple the intermittent stretching process to a continuous tube extrusion process. This can be done by providing lateral displacement of the tube to provide "slack" between the two stations.

The expanded tube is fitted and shrunk onto a pipe.

Figure 2 illustrates one embodiment of the second aspect of the present invention. This is a continuous or semi-continuous process where a tube 5 is heated over one or more longitudinal strips 6 of controlled width(s) in a heating zone 7 to a pre-determined temperature. The very low thermal conductivity of the polymer plastic of the tube ensures that the unheated segment(s) of the tube remain near to ambient temperature and retain virtually their full tensile strength. This high strength enables the tube to be pulled over an expansion mandrel (not shown) in a stretching zone 8 which stretches the tube in a circumferential direction 9 over the locally heated segment(s). The strong unheated segment(s) prevent significant longitudinal stretching occurring. The tube is cooled in zone 10 on the mandrel and then drawn off.

Where more than one longitudinal strip is heated and stretched, the heating and expansion may be done on both strips at the same time or the strips may be heated and stretched one after the other.

The resultant tube has one or more longitudinal strips running along its length which have locked in circumferential strain of up to 50%.

The local expansion process produces a slightly distorted product tube not fully circular and should it be considered necessary a more nearly circular tube can be produced by using an extrusion die so shaped as to produce a suitably non circular tube for subsequent heating and expansion to a circular configuration.

The local expansion produces a tube having a locally thinned strip(s) and, even after subsequent shrinking onto the pipe, this strip(s) will tend to remain thinner than the rest of the tube. If desired this can be compensated for by extruding a tube having a local thickened strip(s) where the subsequent heating and stretching occurs.

Figures 3 and 4 illustrate a second embodiment of the arrangement of Figure 3. In this instance a tube 11 after extrusion is rolled or laid into a flat configuration and passed through similar heating and stretching and cooling zones 12, 13, 14 of the arrangement of Figure 2. However in this instance it will be seen that stretching of strip 15 instead of being by means of an internal mandrel, is by means of gripping means 16 located on either side of the flattened tube and transversely on either side of the heated strip of the plastic tube.In the arrangement illustrated in Figures 3 and 4, the essential requirement to provide the stretching in a peripheral or circumferential direction to enlarge the diameter of the tube once reformed into its open configuration, is met by the means 16 for gripping the tube being inclined to the forward direction of travel of the tube.

In the arrangement illustrated in Figure 4 the gripping means 16 comprises driven belt traction units 17 bearing upon the tube and moving the tube in the direction of travel in the heating stretching and cooling zones 12, 13, 14 and inclined to that direction of travel so that the heated strip of the plastics tube is stretched circumferentially thereof. Alternatively other forms of diverging grippers mounted for example on inclined tracks could be used for the circumferential stretching in this embodiment.

By means of the invention simple effective and relatively inexpensive corrosion protected sheathing for ferrous pipes can be provided, with effective and simple expansion of the tube to enable the tight fitting of a sheath satisfactorily and simply upon a pipe to be protected.

It is to be understood that the foregoing is merely exemplary of the sheathing of ferrous pipes accordance with the first aspect and the second aspect of the invention and the modifications can readily be made thereto without departing from the true scope of the invention.

Claims

1. A method of applying external protection to a ferrous pipe comprising extruding a thermoplastic tube, expanding the diameter of the thermoplastic tube to an internal diameter slightly greater than the external diameter of the pipe to be sheathed such that a potential stress shrinkage is developed in the expanded tube, sliding the extruded tube over the pipe and applying heat to the thermoplastic tube whereby it is caused, by virtue of reiease of the potential stress therein, to shrink securely about the pipe; wherein the extruded tube is expanded by passing the tube through a sizing sleeve with an intermittent movement, the tube being heated during passage through a first heating zone of the sizing sleeve and cooled during its passage through a second cooling zone of the sizing sleeve, the intermittent movement of the tube being such that an unexpanded portion of tube is held within the heating zone of the sizing sleeve with the tube subject to zero or low internal pressure and heated, the tube then being subjected to high pressure so that the heated unexpanded portion of the tube is expanded to fill the sizing sleeve at the heating zone thereof, the tube then being subjected to zero or low pressure causing the tube to shrink away slightly from the internal surface of the sizing sleeve, the tube then being moved forward such that the portion from the heating zone is now disposed within the cooling zone of the sizing sleeve and a new expanded portion is disposed in the heating zone, whereafter the sequence is repeated.

2. A method as claimed in claim 1 including a pre-heated zone wherein the tube is treated immediately prior to the heating zone of the sizing sleeve.

3. A method as claimed in claim 1 or 2 wherein the thermoplastic tube is of sufficient thickness to be capable of providing protection against damage and corrosion to the pipes in which it is to be fitted.

4. A method as claimed in claim 3 wherein the tube has a wall thickness of the order of 1 to 2 mm.

5. A method as claimed in any one of the preceding claims for use with pipes of a spigot and socket joint type, wherein the barrel only of such pipes between the socket and spigot is arranged to be provided with protection by means of the invention.

6. A method as claimed in any one of the preceding claims operated physically adjacent the production of ferrous pipes. wherein the resultant plastic tube is applied to the pipes as they proceed from their production line.

7. A method as claimed in any one of claims 1 to 5 wherein the resultant plastic tube is applied to ferrous pipes previously produced at a separate mill.

S. A method as claimed in any one of the preceding claims wherein the degree of expansion of the thermoplastic tube is such that in its expanded and nonshrunk condition it fits as an easy sliding fit upon a pipe of maximum diameter within the tolerance range of the nominal outside diameter pipe size concerned.

9. A method of applying external protection to a ferrous pipe substantially as shown in and as herein before described with reference to Figures 1A to 1D of the accompanying drawings.

10. Apparatus for performing the method of applying external protection to a ferrous pipe as claimed in any one of the preceding claims.

11. Ferrous pipe including plastic sheathing applied in accordance with the method as claimed in any one of claims 1 to 9, or by the apparatus as claimed in claim 10.

12. A method of applying external protection to a ferrous pipes comprising extruding a thermoplastic tube, expanding the thermoplastic tube such as to incorporate potential stress shrinkage therein, sliding the extruded expanded tube over the pipe, and applying heat to the thermoplastic tube whereby it is caused by virtue of release of the potential stress therein, to shrink securely about the pipe; wherein the expansion of the extruded thermoplastic tube is carried out by heating the tube along at least one longitudinal continuous strip or area of the periphery thereof, and stretching the heated strip transverse to its longitudinal direction whereby to expand the tube circumferentially in that strip with the inclusion of appropriate potential shrinkage stress therein.

13. A method as claimed in claim 12 wherein the circumferential expansion of the longitudinal strip of heated thermoplastic tube is carried out by drawing the tube after and/or during heating over an expansion mandrel.

14. A method as claimed in claim 12 wherein the circumferential stretching is achieved by forming the extruded plastic tubing into a flat configuration, heating a longitudinal strip of the flattened tube, and subjecting the tube to stretching in the heated strip by means of gripping the flattened tube and drawing it at an inclined angle forward and outwardly from the strip so as to stretch, in a direction transverse to the direction of movement of the flattened tuber the heated portion whereby to expand the diameter of the tube, the tube returning to its normal open configuration after such expansion with its potential shrinkage stress.

15. A method as claimed in any one of claims 12 to 14 wherein the thermoplastic tube is of sufficient thickness to be capable of providing protection against damage and corrosion to the pipes in which it is to be fitted.

16. A method as claimed in claim 15 wherein the tube has a wall thickness of the order of 1 to 2 mm.

17. A method as claimed in any one of claims 12 to 16 for use with pipes of a spigot and socket joint type, wherein the barrel only of such pipes between the socket and spigot is arranged to be provided with protection by means of the invention.

18. A method as claimed in any one of claims 12 to 16 operated physically adjacent the production of ferrous pipes, wherein the plastic resultant tube is applied to the pipes as they proceed from their production line.

19. A method as claimed in any one of claims 12 to 16 wherein the resultant plastic pipe is applied to ferrous pipes previously produced at a separate mill.

20. A method as claimed in any one of claims 12 to 19 wherein the degree of expansion of the thermoplastic tube is such that in its expanded and non-shrunk condition it fits as an easy sliding fit upon a pipe of maximum diameter within the tolerance range of the nominal outside diameter pipe size concerned.

21. A method of applying external protection to ferrous pipe substantially as shown in and as hereinbefore described with reference to Figures 2 and 3 of the accompanying drawings.

22. Apparatus for performing the method of applying external protection to a ferrous pipe as claimed in any one of claims 12 to 21.

23. Ferrous pipe including plastic sheathing applied in accordance with the method as claimed in any one of claims 12 to 21, or by the apparatus as claimed in claim 22.