EP0394084A1 - Electrostatic painting installation for a conductive liquid and isolating device for a supply system of a conductive liquid - Google Patents

Abstract

Installation for electrostatic spraying of a liquid product such as a relatively conductive coating product. According to the invention, the installation comprises at least one insulation device (11) comprising a section of insulating pipe (12) of sufficient length to withstand a predetermined tension and a scraping piston (14) capable of being moved in this section of duct to clean the internal wall of the latter.

Description

The invention relates to an installation for electrostatic projection of a conductive liquid product, in particular a coating product such as a water-based paint or a metallic paint; the invention relates more particularly to an improvement making it possible to quickly and effectively establish the necessary electrical insulation between the projection device brought to high voltage and other parts of the installation connected to the earth potential, such as for example distribution circuits and / or tanks for such liquids.

The invention also relates to an isolation device for a distribution circuit of a conductive liquid product.

In an installation for electrostatic projection of a relatively conductive coating product, such as for example a water paint mentioned above, all the elements of the supply circuit of the projection device must in principle be isolated from the potential of the earth. This is not possible when the installation is large. For example, when the paint spraying installation is that of an automobile manufacturing plant, it includes several very long closed-loop paint circulation circuits which can pass through a whole part of the plant. and which establish the connection between large paint tanks and the different projection booths. It is therefore necessary to provide at least one such circuit per color and another similar circuit for the solvent or the cleaning product. For obvious safety reasons, these circuits must be connected to earth. In addition, in each projection booth, the electrostatic projection devices are advantageously connected to a high voltage source. In the case of the use of a conductive paint, it is therefore imperative to electrically isolate the projection device and the high voltage source from structural elements necessarily connected to earth.

To solve this problem, it is known to use an intermediate tank electrically isolated from the ground, of relatively small capacity, and capable of being supplied from time to time with coating product. An adjustable or interruptible high voltage source is then provided and means for separating or at least electrically isolating the intermediate tank from the structural elements permanently connected to the ground, during the spraying phases of the coating product.

The invention relates to a new insulation device capable of being inserted between the structural elements connected to the ground and those which are capable of being brought to a high voltage.

With this in mind, the invention therefore relates to an installation for electrostatic projection of a conductive liquid product, in particular a coating product, of the type comprising a part connected to the ground potential, at least one intermediate reservoir isolated from the ground and at least one electrostatic projection device connected to an adjustable or interruptible voltage source, said intermediate tank being connected to supply said projection device, characterized in that it comprises at least one isolation device comprising a section of conduit made of material electrically insulating of a predetermined length and comprising an inlet and an outlet for liquid, respectively provided at the ends of this section of duct, a piston for scraping the internal wall of said section of duct, movable inside thereof and means for moving said piston in said section of conduit.

The invention also relates to an electrical insulation device for a circuit for distributing a conductive liquid, characterized in that it comprises a section of conduit made of electrically insulating material of a predetermined length and comprising an inlet and an outlet for liquid respectively provided at the ends of this section of pipe, a scraper piston of the internal wall of said section of pipe, movable inside the latter and means for moving said piston in said section of pipe.

• - la figure 1 est un schéma d'un dispositif d'isolation conforme au principe de l'invention, destiné à être inséré dans une installation de projection par voie électrostatique d'un produit liquide relativement conducteur;
• - la figure 2 est un schéma de principe d'une première installation de projection de produit de revêtement liquide incorporant un tel dispositif d'isolation;
• - la figure 3 est un schéma d'une seconde installation comprenant de tels dispositifs d'isolation;
• - la figure 4 est un schéma montrant une variante de l'installation de la figure 3;
• - la figure 5 est un schéma montrant une autre variante de l'installation de la figure 4; et
• - la figure 6 est un schéma d'une troisième installation comportant des dispositifs d'isolation conformes à la figure 1.

The invention will be better understood and other advantages thereof will appear more clearly in the light of the description which follows, given solely by way of example and made with reference to the appended drawings in which:

• - Figure 1 is a diagram of an isolation device according to the principle of the invention, intended to be inserted in an installation for electrostatic spraying of a relatively conductive liquid product;
• - Figure 2 is a block diagram of a first installation for spraying liquid coating product incorporating such an insulation device;
• - Figure 3 is a diagram of a second installation comprising such insulation devices;
• - Figure 4 is a diagram showing a variant of the installation of Figure 3;
• - Figure 5 is a diagram showing another variant of the installation of Figure 4; and
• - Figure 6 is a diagram of a third installation comprising insulation devices in accordance with Figure 1.

Referring more particularly to FIG. 1, there is shown an insulation device 11 comprising a section of conduit made of electrically insulating material 12 of a predetermined length, a scraping piston 14 of the internal wall of this section of conduit and means for moving this scraper piston in said section of conduit. The scraping piston is provided with an O-ring 15 made of elastomeric material which is applied against the internal wall of the section of conduit. In the example, the means for moving the piston consist of a jack 16, here a double-acting pneumatic cylinder, located in the extension of the section of conduit 12 and whose rod 18 of insulating material is fixed to said scraping piston 14. This the latter is either of conductive material or of insulating material and comprises, on the side of the rod 18, a conductive part in contact with the paint to avoid electrical leaks along the piston when the latter is in the vicinity of the conduit 36, which leaks could damage the seal 15.

The length of the insulating duct section 12 is predetermined so that the leakage current remains below a chosen value in the presence of a given high voltage, between its ends, as soon as the internal surface of this duct section is sufficiently rid of conductive product by said scraping.

In the example shown, the section of conduit made of electrically insulating material 12, straight, is defined in a cylindrical block 20 of rigid electrically insulating material and this block also forms the body of the jack 16 arranged in the axial extension of the section of conduit 12 This pneumatic jack 16 is axially delimited by two walls 22, 24 having the form of plugs screwed into tapped parts of a cylindrical recess 26 formed in the block 20. The wall 22 separates the jack from a cylindrical cavity 28, provided with an annular extension 29 surrounding the section of conduit 12; this part will be described in detail later. The wall 24 closes off an open end of the recess 26 and has an orifice 32 for connection to a source of pressurized air, not shown. Another orifice 33 for connection to a source of pressurized air is provided near the wall 22. The piston 34 of the jack 16 moves in the recess 26 between the two orifices 32 and 33. The rod 18 is fixed by one of its ends to the piston 34 and passes through the wall 22, which houses an O-ring 35 ensuring the tightness between the cylinder and the cavity 28.

The insulating duct section 12 is directly connected to an orifice 36, on the side of its end opposite to the jack and it communicates with an orifice 38 opening into said annular extension 29 via an isolation valve 40 which will be described later.

In the example described, it will be assumed that the orifice 38 constitutes a liquid inlet and that the orifice 36 constitutes the liquid outlet, but it should be noted that the device can be connected in the other direction, as will be seen further (depending on its location in the installation), the orifice 36 then serving as a liquid inlet and the orifice 38 serving as a liquid outlet.

The isolation valve 40 arranged in the vicinity of one end of said section of insulating duct 12 has the function of interrupting the circulation of liquid between the adjacent orifice 38 and said section of insulating duct 12. It is constantly urged towards its closed position and it is actuated on opening by the scraper piston itself, when the latter is in the vicinity of this end of said insulating duct section 12, that is to say when it is located in abutment on the side of the actuating cylinder 16, as shown in FIG. 1.

To do this, said isolation valve 40 comprises a tubular isolation valve 45 provided with a cylindrical sleeve 41 sliding on an internal surface 42 of said annular extension 29 of the cavity 28. The orifice 38 communicates with this annular extension 29 and the sleeve 41 has a passage 43 (here a single hole) allowing the liquid to flow.

The seal between the inlet 38 and the duct portion 12 is provided by the support of the end surface of the bearing surface 42 against the facing surface of the valve 45 which can be provided with an elastic seal.

Furthermore, the cavity 28 is coaxial with the section of conduit 12 and it communicates with it so that the tubular valve 45 is subject to movement in the axial extension of the section of conduit 12 of which it constitutes one of the ends. It is in fact provided with a bore 46 extending the section of conduit 12 and of the same diameter as the latter. The scraper piston 40 can engage at the end of its travel in this drilling, until it encounters a shoulder 47. A spring 49 is installed in the cavity 28 between the fixed wall 22 and a shoulder of said tubular valve 45. It is mounted with initial compression to urge said tubular valve towards its closed position. In addition, said cavity 28 is in communication, through an orifice 48 with a source of compressed air, not shown. The pressure which is thus established in the cavity 28 also urges the valve 45 towards its closed position. This pressure is exerted, thanks to the presence of a hole 50 in the valve, on the rear face of the scraper piston, that is to say that which is not in contact with the liquid present in the section of conduit 12. The piston 14 is thus subjected to an air back pressure opposing the pressure exerted by the liquid in the section of conduit 12. This arrangement allows a certain balancing of the pressures on either side of the piston doctor blade 14 and defines a kind of "air seal" preventing liquid infiltration along the side wall of the doctor piston and extending the life of the O-ring 15.

The operation of the device which has just been described follows clearly from the preceding description.

The device is connected so that the conduit 12 is inserted into a liquid distribution circuit driver. As long as the scraper piston 14 is in the position illustrated in FIG. 1 (air pressure maintained at the orifice 33 of the jack 16) it pushes the sliding valve to the right considering FIG. 1 and the passage 43 is free. The conductive liquid can therefore circulate between the orifices 38 and 36.

If it is desired to interrupt the circulation of this liquid, and to provide electrical isolation between the two parts of the liquid distribution circuit, it suffices to switch the pressure in the two chambers of the jack 16, which causes the displacement of the scraping piston 14 From the start of its stroke, said scraper piston releases the sliding valve 45 which interrupts the circulation of liquid. Then, the scraper piston 14 continues its stroke in the section of duct 12 by repelling the liquid and simultaneously cleaning the internal wall of said duct, so that, when it reaches the end of its travel, there is in the circuit, a portion of insulating conduit sufficiently free of conductive product to "hold" a predetermined high voltage.

FIG. 2 schematically represents an installation for electrostatic projection of an electrically conductive liquid coating product. This installation essentially comprises a coating product changing unit 60, known per se, at least one electrostatic projector 61 of said coating product, connected to an adjustable or interruptible high voltage source 62, an intermediate tank 64 capable of storing a certain amount of coating product and an insulation device 11 according to FIG. 1. The intermediate tank 64 is structurally isolated from the ground. The insulation device 11 is shown very schematically with its three essential elements: the insulating duct section 12, the isolation valve 40 and the jack 16. Conventionally, the coating product change unit 60 includes controlled valves 66, respectively connected to distribution circuits for different coating products A, B, C, a controlled valve 68 connected to a rinse aid distribution circuit R and a controlled valve 69 connected to a compressed air distribution circuit S, for blowing. All these valves open into the same manifold 70 connected to a volumetric counter 72, for example of the gear type. A controlled bypass valve 73 is connected in parallel to the counter 72. The output of this counter is connected to a flexible pipe 74, insulating, generally a few meters in length, connected to a projection unit 75. The unit of change of coating product 60 is structurally at ground potential. The pipe 74 is connected to a connection valve 76, the outlet of which is itself connected to the isolation valve 40. The "outlet" of the insulating duct section 12 is connected to the intermediate tank 64 as well as to the projection device 61, via a valve 78. Said insulating duct section 12 is therefore structurally inserted between the coating product changing unit 60 and the intermediate reservoir 64. This is here a sealed and pressurized container, the upper part of which receives compressed air, via a pressure regulator 80. The jack 16 is controlled by a source of compressed air 81, via a three-way valve 82. Finally, the rinse aid distribution circuit R is connected to the inlet of the isolation valve 40, via a rinse valve 84. A purge valve 86, for the evacuation of waste, has its inlet connected at the same point as the rinse valve. The three valves 76, 84 and 86 are arranged near the isolation device 11, as close as possible to the inlet of the isolation valve 40. The operation is as follows.

At the start of a cycle, one of the valves 66 corresponding to a chosen coating product is controlled and this coating product flows into the pipe 74, passes through the valve 76 and the isolation valve 40 (open), flows in the insulating duct section 12 and accumulates in the intermediate tank 64, the valve 78 being closed. During this phase, the voltage of the source 62 is brought back to zero. During the filling of the intermediate reservoir 64, the regulator 80 is in a position such that the pressure of the coating product can discharge the air contained in the reservoir through its venting hole.

As soon as a predetermined quantity of coating product has flowed through the counter 72, the valves 66 and 76 are closed and the valve 82 is controlled to cause actuation of the actuator 16 driving, as we have seen below above, the closure of the isolation valve 40 and the displacement of the scraper piston. The coating product contained in the insulating duct section is therefore pushed back towards the intermediate reservoir 64. When the scraper piston reaches the end of its travel, the intermediate reservoir and the electrostatic projection device 61 are already electrically isolated. of the coating product change unit 60 connected to earth.

The electrostatic spraying of the coating product can start as soon as the valve 78 is opened and the high voltage applied. The reservoir 64 is possibly filled, whenever it is necessary and as long as the coating product does not change, during a short interruption of the spraying, by bringing the high voltage to zero and by actuating the isolation device 11.

When a change of coating product is planned, the last filling of the reservoir 64 is extended after closing the valve 66 by opening the valve 73 and the valve 69. This has the effect of repelling with air practically all of the coating product contained in the conduit 74. At this time, the valve 76 is again closed and the last spraying phase before the change of the coating product takes place normally. During this last phase, during which the intermediate reservoir 64 gradually empties, the coating product change unit 60, the counter 72, the valve 73, the conduit 74 and the valve 76 can be cleaned by successive injections of product rinsing and compressed air by successively controlling the valves 68 and 69. During this phase, the purge valve 86 is open.

When the tank 64 is practically empty, the electrostatic spraying is interrupted and the valve 78 is closed. Rinsing product is then injected into the insulating duct section 12 and the intermediate tank 64 by controlling the valve 84 (the valve 86 being closed). The rinse aid is then discharged through the purge valve 86 (the valve 84 then being closed). These operations are possibly repeated several times until complete cleaning of the section of conduit 12 and of the intermediate tank 64. We end by cleaning the projection device 61 and the valve 78 by successively ejecting rinsing product and then air. , through said projection device. The installation is then ready to receive a new coating product by controlling one of the valves 66.

The installation of FIG. 3 comprises, like the previous one, a unit for changing the liquid coating product 60, a volumetric counter 72 and a bypass valve 73, a connection valve 76, a rinsing valve 84 and a purge 86. The arrangement of these various elements is the same as above and will not be described in more detail. The installation also includes at least one electrostatic projection device 61 and its adjustable or interruptible high voltage source 62. The installation differs from the previous one in that it comprises two branches 90A, 90B for circulation and intermediate storage of the coating product, arranged in parallel between the coating product changing unit 60 and the projection device electrostatic 61. Each branch 90A or 90B comprises an intermediate reservoir 64A, 64B, isolated from the ground, an upstream isolation device 11A₁, 11B₁ and a downstream isolation device 11A₂, 11B₂, respectively. An isolation device is said to be "upstream" if it is upstream of the intermediate tank or "downstream" if it is downstream of the intermediate tank by considering the direction of flow of the coating product. Each intermediate tank is here of the same type as above, that is to say a pressurized tank, supplied with compressed air, here by a source of compressed air 81, via a pressure regulator 80A, 80B, respectively. More precisely, each upstream isolation device 11A₁, 11B₁ is connected between the common point of the three valves 76, 84 and 86 (by its isolation valve 40) and the corresponding intermediate tank 64A, 64B. In other words, the three aforementioned valves are connected to the upstream junction point of the two parallel branches 90A, 90B and close to the isolation valves of the two corresponding upstream isolation devices. Each downstream isolation device 11A₂, 11B₂, is connected between the corresponding intermediate tank 64A, 64B and an input of a three-way valve 92 whose output is connected to the projector 61. Each downstream isolation device is connected to the valve 92 by its isolation valve 40. Consequently, in each branch, the isolation valve of the upstream isolation device is located on the side of its liquid inlet while the isolation valve of the downstream isolation device is located on the side of its liquid outlet, considering the normal direction of flow of the liquid during a spraying phase. In addition, in the example described, a three-way valve 82A, 82B (connected to the compressed air source 81) distributing in each branch the actuating air of the jacks 16, is connected to these jacks by connections suitable for actuating said cylinders simultaneously and in opposite directions.

Finally, it should be noted that in this installation, the intermediate tanks 64A, 64B can be of relatively small capacity (for example of the order of 50 cm³) so that they can be quickly cleaned with reduced consumption of rinse aid.

In normal operation, the two branches 90A, 90B are switched alternately by the appropriate control of the valves 82A, 82B and 92. The spraying interruption only lasts for the switching time of the three-way valve 92 and is therefore of no consequence on electrostatic projection. One of the intermediate reservoirs can be filled while the other supplies the projection device 61. In fact, the intermediate reservoir being filled is isolated from high voltage by the insulating duct section of its downstream isolation device .

When it is necessary to change the coating product (color change), proceed as follows. At the time of the last filling of one of the intermediate reservoirs, for example the reservoir 64A, air is injected into the conduit 74 to repel most of the coating product in the intermediate reservoir during filling. Then, when the tank 64B is empty, the branches 90A and 90B are switched one last time to complete the projection by feeding the projection device 61 from the intermediate tank 64A. During this time, it is possible to clean the color changing unit 60, the volumetric counter 72 and the pipe 74 and then introduce the new coating product until the valve 76 closed, as indicated for the installation of Figure 2. Then, the branch 90B is cleaned up to the downstream isolation device 11B₂, in the same way as for the installation of Figure 2. The isolation device 11B₂ is, meanwhile, already cleaned by scraping up to its isolation valve 40.

At the end of the spraying of the coating product coming from the intermediate tank 64A, the spraying is interrupted, which corresponds to the time necessary to pass from one object to be covered to another. During this time, the high voltage is reduced to zero. The branch 90A is then cleaned in the same way as the branch 90B and the parts extending between the isolation valves of the downstream isolation devices and the projection device 61 are then cleaned. To do this, liquid rinsing is alternately pushed through the downstream isolation devices and evacuated by the projection device 61. After purging of the entire installation by injection of compressed air, the new coating product can be admitted into the one of the intermediate tanks and the spraying of this new product can start while the second intermediate tank is filling.

The installation which has just been described allows extremely rapid changes of coating product, in particular because of the low volumes to be cleaned, as indicated above, and also because the duration of spraying interruption is reduced to the time which is necessary to clean only one of the two branches and the projection device itself.

To facilitate cleaning and above all avoid passing too much rinse aid through the projection device 61, the installation in FIG. 3 can be completed by the assembly in FIG. 4. According to this variant, each isolation valve 40 of the downstream isolation device 11A₂, 11B₂, is connected to an inlet of a three-way valve 92A or 92B, respectively. Each of these valves has a single inlet and two outlets. The outputs are connected two by two. Two such outlets are connected to the projection device 61 while the other two are connected to a waste recovery tank 95, electrically isolated from the ground. Valves 92A, 92B can be disposed near the projector 61. With this arrangement, it is possible to pass through the projection device 61 an amount of rinse aid strictly necessary for its own cleaning. It saves time because it becomes possible to completely clean one of the branches (up to the corresponding valve 92A or 92B) during the last projection phase provided by the other branch and even to fill it with the new coating product. .

In the case where it is not possible to use an isolated waste recovery tank, the installation of FIG. 3 can nevertheless be improved by completing it as shown in FIG. 5. According to this variant, a device is inserted additional insulation 11A₃, 11B₃, in each branch, between the corresponding intermediate tank 64A, 64B and a waste recovery tank 98 at earth potential. Each additional isolation device is mounted so that its isolation valve 40 is on the side of said intermediate tank. This arrangement simplifies the rinsing cycles of the intermediate tanks by avoiding the return of rinsing liquid to the purge valve 86 (not shown in FIG. 4, see FIG. 3), the rinsing liquid always flowing in the same direction, from the valve 84 to the recovery tank 98. The two isolation devices 11A₃ and 11B₃ are controlled "in phase opposition".

In the previous embodiments, the pressurized intermediate tanks can be replaced by membrane or rigid piston tanks. In such a reservoir, known per se, the membrane or the piston separates the reservoir into two chambers with variable volume, one of the chambers receiving the coating product and the other an actuating fluid (air or liquid) ensuring the pressurizing said coating product.

The installation of figure 6 is more particularly indicated whenever it is necessary to power an electrostatic projection device without interruption for long periods of time. In this case, it is known to use two intermediate tanks in cascade.

The installation comprises, as before, a coating product changing unit 60 connected by a pipe 74 to a downstream isolation device 111, then to a first intermediate tank 100, then to a downstream isolation device 112. The two isolation devices are called "upstream" and "downstream" with respect to the intermediate tank 100. They are mounted in the same way as in one of the branches 90 of FIG. 3 and controlled simultaneously in opposite directions by a gate valve. three ways 115 supplied with compressed air and connected in the same way as a valve 82 in FIG. 3. The connection point of the two isolation devices and of the intermediate tank 100 is connected to a waste recovery tank 118, isolated of the earth, via a controlled purge valve 120.

The intermediate tank 100 is, according to the example, a rigid piston tank separating the coating product from its pressurization fluid. This is admitted into the intermediate tank by a pressure regulator 122 or any other suitable control means. The output of the isolation device 112 is connected to a second intermediate reservoir 126 and to a positive displacement pump 130 supplying the projection device 61 brought to high voltage by the high voltage source 62. Between the output of the isolation device 112 and the pump 130 are also connected: a compressed air injection valve 132 supplied by a source S, a purge valve 134 and a third isolation device 138. The latter is connected, by its isolation valve 40 to the rinse aid distribution circuit R. The outlet of the purge valve 134 opens into a waste recovery tank 136, isolated from the ground. The intermediate tank 126 is here of the membrane type. Pressurizing fluid (compressed air, for example) is admitted into this intermediate tank using a pressure regulator 140 or any other suitable control means. The operation is as follows.

In continuous spraying, the reservoir 126 supplies the electrostatic spraying device 61 via the positive displacement pump 130. During this time, the intermediate reservoir 100 is sometimes connected to the coating product change unit 60 and sometimes connected to the intermediate tank 126 to fill it. The electrical isolation of the tank 100 is normally provided by the isolation device 112 when the intermediate tank 100 is actually connected to the coating product change unit and it is therefore at ground potential. When the tank 100 must fill the tank 126, it is the isolation device 111 which performs the electrical insulation between the tank 100 and the coating product change unit. The presence of the positive displacement pump 130 makes it possible to obtain a constant flow rate of coating product towards the spraying device.

When changing the coating product, it is possible, as in the previous cases, to clean and then fill with another coating product, the reservoir 100 while the other reservoir 126 empties the last coating product for the last time. . Rinse aid and air are supplied by the coating product change unit. The rinse aid is discharged to the insulated recovery tank 118, via the purge valve 120.

Then the projection is interrupted and the high voltage is reduced to zero. The tank 126, the pump 130 and the spraying device 61 are then cleaned and dried. The rinsing product is introduced from the isolation device 138 and the air is introduced by controlling the valve 132. . We can then fill the intermediate reservoir 126 with coating product taken from the reservoir 100.

It should be noted that, in the example which has just been described, as in the examples in FIGS. 3 to 5, the simultaneous movement of the two upstream and downstream isolation devices (here the devices 111 and 112) takes place without significant variation in the volume of coating product stored in the intermediate tank located between these two devices. This can make it possible to control the position of the rigid piston or of the membrane, for example by replacing the compressed control air with an incompressible fluid, such as insulating oil, the flow rate of which is controlled. It should also be noted that the intermediate tanks described with reference to FIG. 6 can be replaced by intermediate tanks pressurized by air, like those of FIGS. 2 to 5.

In the embodiments of FIGS. 3 to 6 comprising “upstream” and “downstream” isolation devices, there have been shown and described pairs of such devices supplied by a single three-way valve so that their actuation is reciprocal, simultaneous and automatic. This avoids any risk of short circuit which could result from the actuation of a single device. However, if it is desired, for example to perform cleaning, emptying, complete blowing, through and through, of an intermediate tank and of the isolation devices, it is possible to use two three-way valves controlled separately, the programming of the control ensuring either a reverse movement of the isolation devices, as in the previous case, or a joint movement, during the cleaning phase.

Claims (21)

1- Installation for electrostatic projection of a conductive liquid product, in particular a coating product, of the type comprising a part connected to the earth potential, at least one intermediate reservoir (64-100, 126) isolated from the earth and at least one electrostatic projection device (61) connected to an adjustable or interruptible voltage source (62), said intermediate tank being connected to supply said projection device, characterized in that it comprises at least one isolation device ( 11-111, 112) comprising a section of pipe made of electrically insulating material of a predetermined length and comprising a liquid inlet and outlet, respectively provided at the ends of this section of pipe (12), a scraper piston (14) of the internal wall of said section of conduit, movable inside thereof and means for moving said piston in said section of conduit. 2- Installation according to claim 1, characterized in that said insulating duct section (12) is extended axially by a jack (16) whose rod is connected to said scraper piston. 3- Installation according to claim 2, characterized in that an isolation valve (40) is arranged in the vicinity of one of the ends of said insulating duct section and in that means are provided to open this valve isolation when said scraper piston (14) is in the vicinity of this end. 4- Installation according to claim 3, characterized in that said isolation valve (40) comprises a tubular isolation valve (45) mounted inside a cavity of said section of rigid material, in that this valve is subject to move coaxially to said section of conduit of which it constitutes a movable end, between a closed position and an open position and in that said isolation valve comprises a passage (43) capable of allowing the flow of said liquid from or towards the interior of said section when it is in said open position. 5- Installation according to claim 4, characterized in that a spring (49) is mounted in said cavity between a fixed wall and a shoulder of said tubular isolation valve (45) to urge the latter towards its closed position. 6- Installation according to one of the preceding claims, characterized in that the face of said scraper piston (14) which is not in contact with said liquid, is subjected to an air back pressure defining a gasket air. 7- Installation according to one of claims 1 to 6, characterized in that the part connected to the earth is a liquid product change unit (60), known per se. 8- Installation according to claim 7, characterized in that it comprises a connecting valve (76) connected to said liquid product change unit, a rinsing valve (84) connected to a source of rinsing product and a valve purge (86), these valves being connected upstream of said isolation device (11), near the latter. 9- Installation according to claim 7, characterized in that it comprises two branches (90A, 90B) for circulating the liquid, arranged in parallel between said liquid product change unit (60) and said electrostatic projection device (61) and in that each branch comprises an aforementioned intermediate tank (64A, 64B), an upstream isolation device (11A₁, 11B₁) interposed between said liquid product changing unit and said intermediate tank and a downstream isolation device (11A₂ , 11B₂) interposed between said intermediate reservoir and said electrostatic projection device. 10- Installation according to all of claims 3 and 9, characterized in that said isolation valve (40) of said upstream isolation device is located on the side of its liquid inlet while said isolation valve of said device he downstream insulation is located on the side of its liquid outlet. 11- Installation according to claim 10, characterized in that it comprises a connecting valve (76) connected to said liquid product change unit, a rinsing valve (84) connected to a source of rinsing product and a valve purge (86), these valves being connected to the junction point of the two parallel branches and close to the isolation valves of the two corresponding upstream isolation devices. 12- Installation according to one of claims 9 to 11, characterized in that each downstream isolation device (11A₂, 11B₂) is connected to an input of a three-way valve (92A, 92B) whose outputs are respectively connected to said electrostatic projection device (61) and to a waste recovery tank (95) isolated from the ground. 13- Installation according to one of the preceding claims, characterized in that the or each intermediate tank (64A, 64B) is connected to a waste recovery means (98) at the earth potential by an isolation device (11A₃ , 11B₃) additional. 14- Installation according to one of claims 1 to 7, comprising two intermediate tanks (100, 126) in cascade, characterized in that a first intermediate tank (100) is connected to a part of the earth potential by a device upstream insulation (111) and to the second intermediate tank by a downstream isolation device (112). 15- Installation according to one of the preceding claims, characterized in that an intermediate tank (126) is connected to a distribution circuit rinse aid (R) by an isolation device (138). 16- Electrical insulation device for a circuit for distributing a conductive liquid, characterized in that it comprises a section of conduit made of electrically insulating material (12) of a predetermined length and comprising a liquid inlet and outlet respectively provided at the ends of this section of pipe, a scraping piston (14) of the internal wall of said section of pipe, movable inside thereof and means for moving said piston in said section of pipe. 17- Isolation device according to claim 16, characterized in that said insulating duct section (12) is extended axially by a jack (16), the rod of which is connected to said scraping piston. 18- Isolation device according to claim 17, characterized in that an isolation valve (40) is arranged in the vicinity of one of the ends of said insulating duct section and in that means are provided to open this isolation valve when said scraper piston (14) is in the vicinity of this end. 19- Isolation device according to claim 18, characterized in that said isolation valve (40) comprises a tubular isolation valve (45) mounted inside a cavity of said section of rigid material, in that that this valve is subject to move coaxially to said section of conduit of which it constitutes a movable end, between a closed position and an open position and in that said isolation valve has a passage (43) capable of allowing the flow of said liquid from or towards the interior of said section when it is in said open position. 20- Isolation device according to one of claims 16 to 19, characterized in that the face of said scraper piston (14) which is not in contact with said liquid is subjected to an air back pressure defining an air seal. 21- Isolation device according to one of claims 16 to 20, characterized in that the scraper piston (14) is conductive or has a conductive part.

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