FR2558592A1 - Methods and devices for continuously checking the leaktightness of plastic extruded pipes. - Google Patents

Abstract

The invention consists of methods and devices for continuously checking the leaktightness of plastic extruded pipes. A device according to the invention is fitted to a machine for extruding pipes 10 comprising a die 1 and a sizing chamber 4 which is maintained at a reduced pressure by a vacuum pump 8 and a duct 11 which emerges in the centre of the die and which maintains atmospheric pressure in the pipe 10. A test gas, for example helium contained in a bottle 12 equipped with a pressure-reducing valve 13, is injected into the duct 11. A test gas sensor 14 analyses the gases pumped out by the vacuum pump. One application is the continuous checking of extruded pipes intended for transporting town gas.

Description

 Methods and devices for continuously checking the tightness of extruded plastic tubes.

 The subject of the invention is methods for continuously checking the seal of extruded plastic tubes and devices making it possible to carry out this check during the manufacture of the tubes.

 The technical sector of the invention is that of manufacturing plastic tubes by extrusion.

 More and more plastic extruded tubes are used, for example polyethylene tubes, for transporting gases, in particular combustible gases in urban distribution networks.

 In order to avoid accidents due to gas leaks, system operators require pipe manufacturers to check their tightness.

 To date, the leaktightness tests of the extruded tubes intended for the transport of gas are carried out by filling these tubes with a compressed gas for example with six bats, by closing the tube and by checking that the pressure is maintained. These tests require mount satm - 3 at the ends of the tubes and they involve manipulations. In addition, low flow leaks are difficult to detect by this method.

An object of the present invention is to provide
uo ^ yells for detecting the tightness of the continuously extruded tubes @@, at the exit of the extrusion die and means which are easy to adapt rdaptr to the arrangements which are used to extrude and to conform the tubes
When making a plastic tube, hot and pasty material is extruded through an annular die. At the kind of the die, the tube enters a shaping chamber through an orifice whose diameter corresponds to the outside diameter of the tube to be manufactured.

 The shaping chamber is several meters long and contains diar-osage ramps which project cooling water onto the external wall of the tube. During the passage of the shaping chamber, the inside of the tube is kept in slight overpressure compared to the outside, so that the tension which is exerted on the tube maintains its circular shape during the hardening of plastic.

 To keep the interior of the tube under overpressure, the center of the die is placed in communication with atmospheric pressure and a vacuum is created in the shaping chamber around the tube which passes through it. As a variant, the tube is closed at the outlet from the shaping chamber with a plug which is kept fixed and a gas is injected at the center of the die under a pressure higher than atmospheric pressure.

 The objective of the invention is achieved by means of a process by which an easy-to-detect test gas, preferably helium, is injected into the pipe which opens out at the center of the die and the presence is continuously detected. possible of said test gas in said shaping chamber by means of a sensor capable of detecting traces of said test gas.

 A device according to the invention is adapted to a manufacturing device of the type comprising an annular die, a shaping chamber placed at the outlet of said die and a conduit which opens out at the center of the die and which maintains inside the tube. a pressure higher than the pressure in the shaping chamber.

 A verification device according to the invention further comprises a reservoir containing an easy-to-detect test gas, preferably helium, which is connected to the duct which opens out at the center of the die and it further comprises , a sensor, capable of detecting traces of said test gas, contained in or leaving the shaping chamber.

 The invention results in the continuous verification of the tightness of the extruded plastic tubes.

 The essential advantage of the methods and devices according to the invention lies in the fact that they are very easy to adapt to conventional extrusion machines. They do not require costly transformations of these, nor use of expensive material to install or operate.

 The devices according to the invention make it possible to detect very small leaks from the outlet of the extrusion die and to immediately remove the sections of defective tubes.

 The methods: and devices according to the invention have been designed more particularly for checking the tightness of the extruded plastic tubes intended for the transport of combustible gas, in particular town gas. However, this application is not limiting, and given the low investment and low operating cost of these devices, they can be used to systematically check, continuously, the tightness of all the extruded tubes in material. plastic whatever the destination of these.

 It allows the tube manufacturer to fully guarantee the tightness of all tubes leaving production without having to carry out other leak tests which are costly in handling and in working time.

 The following description refers to the appended drawings which represent, without any limiting nature, exemplary embodiments of devices according to the invention.

 Figure 1 is a schematic overview of a first embodiment.

 Figure 2 is a schematic overview of a second embodiment.

 Figure 1 shows, in longitudinal section, a device for extruding plastic tubes, for example-polyethylene tubes.

 A tube extrusion device comprises, in a known manner, an annular die 1 through which the resin passes in the pasty state in an annular space comprised between a cylindrical body 2 and an axial core 3. At the outlet of the die , the still hot tube 10 enters a shaping chamber 4 passing through a cylindrical calibration tube 5, the inside diameter of which corresponds to the outside diameter of the tube which is to be obtained.

 The shaping chamber, which has a length of several meters, comprises perforated ramps 6 for spraying the tube which project jets of cooling water against the external wall of the tube. The irrigation water is taken up at the bottom of the shaping chamber by a pump 7 and recycled. If necessary, the water is passed through atmospheric refrigerants to cool it.

 In order to obtain tubes having a perfectly circular section, a pressure slightly higher than the pressure prevailing in the chamber is maintained inside the tube, while passing through the shaping chamber, so that the material in progress of hardening which constitutes the tube is kept in tension during cooling and keeps a circular shape.

 Figure 1 shows a first manufacturing method, which is the most used, according to which, to maintain a pressure difference between the inside and the outside of the tube, a partial vacuum is made in the shaping chamber 4 by means of 'a vacuum pump 8, which sucks in this chamber. An air inlet 9, fitted with a valve, is located at the end opposite the vacuum pump and allows a small flow of sweeping air to be brought into the chamber.

 Is maintained inside the tube 10 a pressure equal to atmospheric pressure by means of a conduit 11 which opens at the center of the die and whose other end 11a is open to the atmosphere.

 The device for manufacturing extruded tubes of palstic material described above is known and it is not necessary to describe it in more detail.

 The problem to be solved is to verify that the tubes which exit from the shaping chamber are perfectly sealed, particularly in the case of tubes intended for the transport of gas. For this, a very small proportion of an easy to detect test gas is continuously injected into the tube 11.

 Since die 1 is at a temperature of around 200, a non-flammable and obviously non-toxic gas must be used. Preferably, helium is injected by means of a helium bottle 12 which is connected in bypass to the tube 11 through a regulator 13 which expands the helium to a pressure very slightly higher than atmospheric pressure.

 We know that helium is a very light gas which diffuses very easily through the slightest crack and we know how to detect traces of it, of the order of a part for a thousand (1 p.p.m.). In addition, helium is a non-toxic, non-flammable and inert gas.

 The tightness checking device also comprises a sensor 14 capable of detecting traces of the test gas, for example a helium sensor, which is connected for example in bypass to the discharge of the vacuum pump 8. In alternatively, the sensor 14 can be connected directly to the chamber 4.

 The sensor 14 continuously analyzes the gases contained in the chamber 4. If the tube has a leak, as soon as it enters the chamber 4 of the helium begins to diffuse in the chamber and the helium content of the gases contained in the chamber progressively increases as the leak progresses along the chamber and then decreases as soon as the leak leaves the chamber.

 The sensor 14 can be fitted with a graphical recorder of the helium content on which a very pronounced peak appears, as soon as the tube has the slightest crack. As a variant, the sensor 14 controls an alarm which is triggered as soon as the helium content exceeds a threshold which can be a very low threshold, of the order of a few ppm. Thanks to the detection device according to the invention, it is therefore possible to continuous sealing of the extruded tubes as soon as they leave the die. It is therefore also possible to immediately identify the position of a crack and immediately cut off the faulty tube section.

 Helium is used to check the tightness of a buried section of a gas transporting tea and to locate leaks.

 In this case, helium is injected into the tube and traces of helium are looked for on the surface by means of a helium sensor.

 The methods and devices according to the invention constitute a new application of this method which has been adapted to the extrusion machines so as to obtain a continuous verification, without having to make significant modifications to the known extrusion machines.

 The helium sensors used in the devices according to the present invention are of the same type as those which are used to detect leaks from a buried tube or of any other known type.

 FIG. 2 represents a device for continuously checking the tightness of the extruded tubes on a different lightweight extrusion machine. The homologous parts are represented by the ninth marks in FIGS. 1 and 2.

 This known extrusion machine differs from the previous one by 'es ncyen used to maintain a slight overpressure in the tube 10 which passes through the shaping chamber 4. On this machine, the interior of the chamber 4 is at atmospheric pressure.

 A piston 15 is placed inside the tube 10, which acts as a tight plug and which is retained in a fixed position, near the exit from the shaping chamber, by a cable 16 which is fixed by a ring 17 or by any other equivalent fixing means on the die 1.

 The conduit 11 is connected to a source of compressed gas at a pressure slightly higher than atmospheric pressure, for example on a bottle 18 of compressed air provided with a regulator i9.

 In this case, the device according to the invention comprises, on the one hand, a source of easily detectable test gas, for example a bottle of helium 12 which is connected in bypass to the tube 11, through a pressure reducer 13 which relaxes helium to a pressure slightly higher than atmospheric pressure.

 A test gas sensor 14, for example a helium sensor, is connected directly to the chamber 4.

 Helium is the preferred test gas, since it easily diffuses through the slightest crack and exists in very small quantities in the atmosphere. However, it is a relatively expensive gas.

 The method according to FIG. 2 consumes a very small quantity of test gas because if there is no leakage either to the tube or to the piston 15, only the quantity necessary to pressurize the section of tube between the tube is consumed piston 15 and the die.

 Carbon dioxide, which is inert and inexpensive, can also be used as the test gas. In this case, since carbon dioxide is contained in the atmosphere, a differential gas sensor is used which compares the carbon dioxide content in the chamber to the content in the atmosphere.

 Figures 1 and 2 and the above description relate to devices in which the tightness check is carried out in the crossing of the shaping chamber, which is simpler.

 It is specified that these examples are not limiting. Test gas leaks can also be detected by placing a detection chamber traversed by the tube at any point along the path of the tube leaving the die. In this case, the gases contained in the detection chamber are sucked in by means of a pump, which has the effect of keeping the chamber in slight depression relative to the tube and the gases discharged by the pump are passed through a detector capable of detecting in them any traces of the test gas in the event of a leak in the tube. This variant can be installed on which extrusion line, whatever the shaping device used at the outlet of the die.

Claims (7)

 1. A method for continuously checking the tightness of the plastic tubes extruded through an annular die which comprises a conduit which opens into the center of the die, characterized in that a small amount of a test gas easy to detect, said tube is passed continuously through a detection chamber, the gases contained in said chamber are sucked up and these are continuously sent into an apparatus capable of detecting the possible presence of said test gas in said gases.  2. Method according to claim 1, characterized in that helium is injected into said conduit.  3. Method according to any one of claims 1 and 2 for checking the tightness of the extruded plastic tubes, which are shaped in a chamber (4), which is maintained in depression relative to the atmosphere by a pump vacuum (8) which sucks in said chamber and the interior of the tube is maintained at atmospheric pressure by a conduit (11) which opens at the center of the die and which places the interior of the tube in communication with the atmosphere, characterized in that a small quantity of an easy-to-detect test gas, expanded to atmospheric pressure, is continuously injected into said conduit and the gases passing through the vacuum pump are continuously analyzed by means of a sensor (14) capable of detecting possible traces of said test gas which indicate a leak in the tube.  4. Method according to claims let 2 for checking the tightness of extruded plastic tubes which are shaped in a shaping chamber (4) which is maintained at atmospheric pressure while placing inside the tube a plug (15) kept fixed and the portion of tube between said plug (15) and the die (1) is brought to a pressure higher than atmospheric pressure by means of a conduit (11) which opens in the center of the die and which is connected to a source of weakly compressed gas (18), characterized in that a small quantity of an easily detectable test gas is injected into said conduit (11) and the gases contained are analyzed in said conforming chamber by means of a sensor capable of detecting possible traces of said test gas which indicate a leak from the tube.  5. Device for continuously checking the tightness of extruded plastic tubes of the type comprising an annular die and a conduit which opens out at the center of said die, characterized in that it comprises means for injecting into said conduit a gas of easy to detect test, preferably helium, and it further comprises a detection chamber which is placed in the path of the tube continuously emerging from the die and which is equipped with a detector of said test gas which is provided with a pump which sucks the gases contained in said chamber and which discharges these into said detector which is capable of detecting in them traces of said test gas.  6. Device according to claim 5 for checking the tightness of extruded plastic tubes, of the type comprising an annular die (1), a conduit (11) which opens at the center of the die and which puts the inside of the tube (10) at atmospheric pressure, a tube conformation chamber (4) and a vacuum pump (8) which maintains said chamber (4) in depression relative to the atmosphere, characterized in that it comprises a easy-to-detect test gas reservoir (12), preferably helium, which is connected through a pressure regulator (13) to said conduit (11) and further comprises a sensor (14) capable of detecting traces of said test gas which analyzes the gases which pass through said vacuum pump.  7. Device according to claim 5 for checking the tightness of extruded plastic tubes of the type comprising an annular die (1), a shaping chamber (4) which is at atmospheric pressure, a piston (15) which is placed inside the tube and which is retained in a fixed position at the outlet of said shaping chamber and a conduit (1-1) which opens out at the center of the die and which is connected to a source (18) of gas compressed at a pressure higher than atmospheric pressure, characterized in that it further comprises a reservoir (12) containing an easy-to-detect test gas, preferably helium, which is connected, through a pressure reducer (13) on said conduit (11) and it further comprises a sensor (14) capable of detecting traces of said test gas which is connected to said chamber (4) and which analyzes the gases contained therein.