CA1060280A - Method of and apparatus for electrostatically spray coating powder material onto metal pipes - Google Patents
Method of and apparatus for electrostatically spray coating powder material onto metal pipesInfo
- Publication number
- CA1060280A CA1060280A CA238,732A CA238732A CA1060280A CA 1060280 A CA1060280 A CA 1060280A CA 238732 A CA238732 A CA 238732A CA 1060280 A CA1060280 A CA 1060280A
- Authority
- CA
- Canada
- Prior art keywords
- pipe
- spray
- powder
- powder material
- housing
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05D7/00—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials
- B05D7/14—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials to metal, e.g. car bodies
- B05D7/146—Processes, other than flocking, specially adapted for applying liquids or other fluent materials to particular surfaces or for applying particular liquids or other fluent materials to metal, e.g. car bodies to metallic pipes or tubes
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05B—SPRAYING APPARATUS; ATOMISING APPARATUS; NOZZLES
- B05B13/00—Machines or plants for applying liquids or other fluent materials to surfaces of objects or other work by spraying, not covered by groups B05B1/00 - B05B11/00
- B05B13/02—Means for supporting work; Arrangement or mounting of spray heads; Adaptation or arrangement of means for feeding work
- B05B13/0221—Means for supporting work; Arrangement or mounting of spray heads; Adaptation or arrangement of means for feeding work characterised by the means for moving or conveying the objects or other work, e.g. conveyor belts
- B05B13/0235—Means for supporting work; Arrangement or mounting of spray heads; Adaptation or arrangement of means for feeding work characterised by the means for moving or conveying the objects or other work, e.g. conveyor belts the movement of the objects being a combination of rotation and linear displacement
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05B—SPRAYING APPARATUS; ATOMISING APPARATUS; NOZZLES
- B05B5/00—Electrostatic spraying apparatus; Spraying apparatus with means for charging the spray electrically; Apparatus for spraying liquids or other fluent materials by other electric means
- B05B5/08—Plant for applying liquids or other fluent materials to objects
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B05—SPRAYING OR ATOMISING IN GENERAL; APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05D—PROCESSES FOR APPLYING FLUENT MATERIALS TO SURFACES, IN GENERAL
- B05D1/00—Processes for applying liquids or other fluent materials
- B05D1/002—Processes for applying liquids or other fluent materials the substrate being rotated
Landscapes
- Life Sciences & Earth Sciences (AREA)
- Engineering & Computer Science (AREA)
- Wood Science & Technology (AREA)
- Electrostatic Spraying Apparatus (AREA)
- Application Of Or Painting With Fluid Materials (AREA)
Abstract
A METHOD OF AND APPARATUS FOR ELECTROSTATICALLY
SPRAY COATING POWDER MATERIAL ONTO METAL PIPES
Abstract of the Disclosure A method of and apparatus for electrostatically spray coating powder material onto a metal pipe enable a homogenous and well-adhering coating of uniform thickness to be provided. The pipe is preheated and rotated about its longitudinal axis at constant speed, and is displaced relative to a powder spray coating device longitudinally of the pipe. Powder material is discharged onto the pipe from the spray coating device, and excess powder material is withdrawn from the coating device. For all pipe diameters, the speed of rotation of the pipe, the spacing of the coating device from the pipe and the amount of air sucked in per unit of time from the atmosphere are maintained constant, while the speed of the the relative displacement of the spray coating device and the pipe, longi-tudinally of the pipe, is varied in accordance with the pipe diameter. The powder material is sprayed with an inert gas. The coating device comprises a spray chamber in the form of a downwardly open housing adjustable in position relative to the pipe, and adapted to extend through, at most, 120°
around the pipe, with the edge of the housing spaced from the pipe to define an air inlet gap. A spray gun or the like is fixed relative to the housing for discharging the powder material within the housing.
SPRAY COATING POWDER MATERIAL ONTO METAL PIPES
Abstract of the Disclosure A method of and apparatus for electrostatically spray coating powder material onto a metal pipe enable a homogenous and well-adhering coating of uniform thickness to be provided. The pipe is preheated and rotated about its longitudinal axis at constant speed, and is displaced relative to a powder spray coating device longitudinally of the pipe. Powder material is discharged onto the pipe from the spray coating device, and excess powder material is withdrawn from the coating device. For all pipe diameters, the speed of rotation of the pipe, the spacing of the coating device from the pipe and the amount of air sucked in per unit of time from the atmosphere are maintained constant, while the speed of the the relative displacement of the spray coating device and the pipe, longi-tudinally of the pipe, is varied in accordance with the pipe diameter. The powder material is sprayed with an inert gas. The coating device comprises a spray chamber in the form of a downwardly open housing adjustable in position relative to the pipe, and adapted to extend through, at most, 120°
around the pipe, with the edge of the housing spaced from the pipe to define an air inlet gap. A spray gun or the like is fixed relative to the housing for discharging the powder material within the housing.
Description
The present invention relates to a method of and an apparatus for the electrostatic spray coating of powder material onto a preheated metal pipe, which is rotated about its longitudinal axis, the pipe and a spray coating device being displaced relative to one another longitudinally of the pipe, and excess powder material being withdrawn from the spray coating device.
It is known, from German Offenlegungschrift -No. 2,256,865, to carry out this method with an apparatus 10 which comprises a powder spray chamber extending longitudinally of the pipe and connected with a suction device, spray guns or the like being adjustable in height within the spray chamber.
The stationary powder spray chamber is closed and comprises a substantially cylindrical housing, in which mutually aligned circular openings are provided~ through which pass the pipes which are to be coated. Within the housing, spray guns are arranged horizontally and vertically adjustable. The housing is connected by a suction pipe to a vacuum pump, by which excess powder material is withdrawn. Air is blown into the 20 powder spray chamber over an air deflector plate provided with air outlet openings and extending the length of the housing, so that, in conjunction with the suction pipe, which is arranged at the bottom of the housing, a circu]ar air movement is provided within the powder spray chamber. In this way, powder located within the powder spray chamber is fluidized.
To counteract accumulation of the powder on the inner wall of the housing, the housing wall is provided at its inner side, at a spacing therefrom, with a porous or perforated lining. Cold air is constantly drawn from the exterior through the housing 30 into the space between the wall and the porous covering. This air flows through the openings in the lining into the interior
It is known, from German Offenlegungschrift -No. 2,256,865, to carry out this method with an apparatus 10 which comprises a powder spray chamber extending longitudinally of the pipe and connected with a suction device, spray guns or the like being adjustable in height within the spray chamber.
The stationary powder spray chamber is closed and comprises a substantially cylindrical housing, in which mutually aligned circular openings are provided~ through which pass the pipes which are to be coated. Within the housing, spray guns are arranged horizontally and vertically adjustable. The housing is connected by a suction pipe to a vacuum pump, by which excess powder material is withdrawn. Air is blown into the 20 powder spray chamber over an air deflector plate provided with air outlet openings and extending the length of the housing, so that, in conjunction with the suction pipe, which is arranged at the bottom of the housing, a circu]ar air movement is provided within the powder spray chamber. In this way, powder located within the powder spray chamber is fluidized.
To counteract accumulation of the powder on the inner wall of the housing, the housing wall is provided at its inner side, at a spacing therefrom, with a porous or perforated lining. Cold air is constantly drawn from the exterior through the housing 30 into the space between the wall and the porous covering. This air flows through the openings in the lining into the interior
- 2 -of the powder spray chamber and thereby counteracts accumulation of excess powder material.
This prior method has numerous disadvantages. The main disadvantage is that the powder supply, which is required to produce a homogenous and well-adhering coating of very constant thickness, can be only insufficiently regulated in accordance with the heat of the heated pipe. Extensive observations of the coating process have shown that the best coating quality can be achieved when the powder supply for 10 achieving the required coating thickness is determined quantitatively with respect to the heat content of the heated pipe. The efficiency ofthe utilization of the powder is also determined by these factors.
When the above-described prior method is used for the electrostatic spray coating of synthetic resin powders, the observation of the powder supply during the coating process is limited by the closed chamber, and in fact can only be effected by observing the amount of powder which is withdrawn from the chamber. The adjustments of the powder supply which 20are required for improving the quality of the coating can only be effected at a relatively late point in time, due the -closed and fixedly positioned coating chamber. Also, it is necessary to individually adjust the supply and the spacing, relative to the pipe surface, of each single spray gun.
These adjustments are, however, subject to narrow limits due to the relatively high coating speed and the consequentially nonuniform overlappings ofthe spray cones, particularly when pipes of different diameters are coated in successive operations.
Since, for pipes having small diameters, a relatively large 30annular space remains open in the side walls of the powder spray chamber, the air flow conditions within the powder spray .
:
~ 106~2~30 chamber vary, as they al~o do for larger pipe diameters.
According to regulations in force in the Federal Republic of Germany, the concentration of powder within the spray chamber should not exceed 20 grams per cubic meter. The above-described prior apparatus therefore should not be operated in the Federal Republic of Germany. The arrangement of the spray guns and the air guiding and deflecting plates provided within the chamber is not only expensive, but also does not allow uniform flow conditions of the various air streams to be ensured 10 when pipes of different diameters are coated. This prior apparatus also requires expensive multiple air sup?lies. A particular disadvantage of this prior apparatus, finally, is that it can only be employed for a comparatively small range of pipe diameters since the diameters of the circular openings in the side walls of the chamber determine the maximum possible pipe diameter which can be treated. Consequently, if pipes of very different diameters are required to be coated, correspondingly different powder spray chambers must be employed.
It is an object of the present invention to provide 20 an improved electrostatic spray coating method which at least partly mitigates at least some of the above-described disadvantages of this prior method.
It is a further object of the present invention to provide an electrostatic spray coating method which enables the powder supply, required for producing a homogenous and well-adhering coating of uniform thickness, to be readily adjusted in dependence onthe heat content of the preheated pipe.
According to one aspect of the present invention, there 30 is provided a method of electrostatically spray coating a thermally fusible synthetic resin powder material onto a metal pipe, 1060Z8~
comprising the steps of preheating the pipe to a temperature above the fusion point of the powder material, rotating the pipe about its longitudinal axis at constant speed, effecting relative displacement of the pipe and a powder spray chamber longitudinally of the pipe, discharging the powder material in a spray from the spray chamber, and withdrawing excess powder material from the powder spray chamber by applying suction to the spray chamber, wherein, for all pipe diameters, the speed of rotation of the pipe, the spacing of the powder spray chamber 1~ from the periphery of the pipe and the amount of air sucked in per unit time from the atmosphere between the chamher and the metal pipe are maintained constant while the speed of the relative displacement of the powder spray chamber and the pipe, longitudinally of the pipe, is varied in accordance with the pipe diameter, and the powder material is sprayed with an inert gas.
According to another aspect of the invention there is provided electrostatic spray coating apparatus for coating a metal pipe, comprising: a powder spray chamber; a suction device connected to the spray chamber for removing excess powder there- :
20from; and at least one spray discharge device for discharging a spray of powder material within the spray chamber; the spray chamber comprising a downwardly open housing which is adjustable in position relative to the metal pipe for varying the spacing of the spray discharge device from the metal pipe; the spray chamber further being adapted te extend through, at most, 120 around the periphery of the metal pipe with the edge of the ~ -housing spaced from the periphery of the metal pipe to define therewith an air inlet gap; and the spray discharge device being fixed relative to the housing.
It is known to heat a pipe which is to be coated, after it has been cleaned, to a temperature which is sufficient ~ _ 5 _ 1060Z~30 to cause the subsequently sprayed material to melt and fuse.
When employing an epoxy resin coating material, this temperature, according to the prior art method disclosed in German Offenlegungschrift 2,256,865, is of the order of 230 to 260 centigrade. The thus heated pipe is displaced, while being continuously rotated about it longitudinal axis, through the powder spray coating chamber, which is comparatively short relative to the length of the pipe to be coated. The time required for the pipe to pass through the powder spray lOcoating chamber amounts, depending upon the length and the speed of advance, to several minutes. Consequently, the part of the pipe which is coated first has a higher local coating temperature than the end of the pipe which is coated last, and the temperature of which decreases, due to heat radiation, during the time required to spray coat the length of the pipe.
Extensive experiments have shown that the powder ~ 5a -, Z~30 material employed for spray coating develops its optimum properties only in a comparatively narrow temperature range.
Variations in the coating temperature, consequently, influence the quality of the coating.
In order to counteract this disadvantageous effect, when employing the method according to the present invention, the pipe is preferably preheated to different temperatures along the length of the pipe such that the pipe end at which the spray coating begins has the lowest preheated temperature, 10 and the pipe temperature increases substantially linearly along the length of the pipe to a maximum value such that the pipe temperature is at least approximately uniform at the powder spray coating device during the relative displacement of the powder spray coating device and the pipe.
The electrostatic powder spray coating method is a process for providing surface protection, in which synthetic resin powder, e.g. epoxy resin powder, is melted and thermally fused on the preheated pipe. The powder is sprayed onto the pipe, which is heated above the melting temperature of the 20 powder, the powder being electrostatically charged as it is ~
pneumatically conveyed in a spray unit. Between the electro- -statically negatively charged epoxy resin particles and the positively charged (grounded) pipe there is produced a strong magnetic field. The epoxy resin powder particles are distributed, corresponding to the disposition of the magnetic lines of force, in a uniform coating thickness on the surface of the pipe and, due to their different charge, adhere securely to the pipe.
For providing corrosion protection of a steel tube 30by a synthetic resin coating, it is not the thickness of the synthetic resin coating, but the quality of the adhesion of " 1060Z80 the synthetic resin to the steel pipe, which is important.
Extensive experiments have shown that the most important factors affecting the adhesion are the coating temperature of the workpiece, the melting temperature of the synthetic resin powder, the viscosity of the melted synthetic resin, and the temperature at which the hardner is effective.
It has previously been considered, by those skilled in the art, that the coating temperature, using epoxy resin, should lie within the range of 230 to 260 centigrade, and lOpreferably at approximately 245 centigrade (see e.g. German Offenlegungschrift 2,256,865). Also, in the "Specification for Planned Application of Scotchkote* no. 204 Prime System to Line Pipe" and "no. 206 Pipeline Coating to Line Pipe", coating temperatures of 218~centigrade to 244 centigrade are given. In these publications, furthermore, it is specifically stated that oxidation of the steel which is to be coated should be avoided under all circumstances (see. Section 3.1 thereof).
It has now been found that, in the method of the 20present invention, the surface of the pipe which is to be coated may preferably be temporarily preheated to a surface temperature of 280 to 380 centigrade to form an oxide coating of approximately molecular thickness.
The coating temperature is the temperature which is uniform over the entire wall thickness of the pipe and which is produced, after the preheating, by a temperature equalization period. By this equalization period, it is possible to temporarily reach a surface temperature of 380 centigrade during the heating. If the heating is carried out 30by using gas burners, then these are preferably operated with excess air and hot flames. When the above-mentioned temperature * Trade Mark : ~
: , :
is reached, a FeO coating of approximately molecular thickness is produced, which is recognizable by a corresponding oxidation tint.
Extensive investigations have shown that a-ferrous oxide coating formed in this way produces a very good bonding of the subsequently spray coated syntheticresin. These findings are in contrast to previous opinions of those skilled in the art, according to which the preheating temperatures should not exceed about 260 centigrade. The positive results obtained by the present process are therefore surprising for those skilled in the art. The present process has an important 10 economic significance. Tests have shown that the requirements for the protective coating of steel pipes when they are buried in the ground are fulfilled. The same holds true with respect to the mechanical resistance to wear and tear which occur during transport and on site and also with respect to corrosion resistance. By employing the present process, it should now be possible, for the first time, to employ co~ted pipes buried in the ground for conveying substances at temperatures of up to 130 centigrade over long distances.
Also according to the present invention, there is provided an electrostatic spray coating apparatus for coating a pipe, comprising a powder spray chamber, a suction device connected to the chamber for removing excess powder therefrom, and at least one spray discharge device for discharging a spray of powder material within the spray chamber, the spray chamber comprising a downwardly open housing which is adjustable in position relative to the pipe for varying the spacing of the spray discharge device from the pipe, the spray chamber further being adapted to extend through, at most, 120 around 30 the periphery of the pipe with the edge of the housing spaced from the periphery of the pipe to define therewith an air inlet .
- ~6~280 gap, and the spray discharge device being fixed relative to the housing.
Preferably, at least the inner surface of the housing is coated with an electrostatic charge resistant material.
The adJustable housing may be telescopically movable in a fixed housing.
The invention will be more readily understood from the following description of a preferred embodiment thereof given by way of example with reference to the accompanying 10 diagramatic drawings, in which:-Figure 1 shows a view in perspective of a spraycoating apparatus;
Figure 2 shows a partially broken-away view in perspective of parts of the apparatus of Figure 1 on a larger scale; and Figure 3 is a diagram illustrating the preferred preheating temperature variation along a pipe having a length of 15 meters.
Referring firstly to Figure 1, there is illustrated 20a metal pipe 1 which is preheated and which is rotated about its longitudinal axis at constant speed.
A spray coating device 3 is displaceable longitudinally of the pipe 1, and includes a powder spray chamber or housing 4, which is fixed in position. A second housing 5 is telescopically received in the housing 4 and is spaced from the periphery of the pipe. The housing 5 is adjustable in height and downwardly -open, and extends around the periphery of the pipe through, at most, 120.
Substantially linearly arranged spray guns 6 are 30fixedly connected to the adjustably displaceable housing 5.
The edge of the housing 5 is spaced from the periphery of the 4 .
1~6~Z810 pipe to define therewith an air inlet gap 7.
The spraying distance between the spray guns 6 and the periphery of the pipe is indicated by reference numeral 2.
By means of this apparatus it is possible .o adapt the powder supply, at the earliest possible time, to the amount of powder which can be taken up by the heated pipe, e.g. by simple visual observation of the air inlet gap 7 and the coated pipe. In this way, the amount of powder required for providing a homogenous and adhesive coating of constant thickness can be 10 regulated during the coating process, and approximately 100%
utili7ation of the powder can be achieved. Consequently, the provision of an expensive powder reclaiming installation is not necessary.
By the uniform sucking in or out of air, a constant air movement is obtained for all pipe diameters. Since the peripheral speed of the pipe surface which is to be coated increases as the pipe diameter increases, a corresponding adjustment for providing a uniform coating is effected by reducing the relative speed of movement of the spray coating 20 device~3 and the pipe 1 in the longitudinal direction of the pipe 1.
Whlle it has been stated hereinabove that the spray coating device 3 is moved along the pipe, it is to be understood that alternatively the spray coating device 3 could remain stationary, while the pipe 1 is moved longitudinally relative thereto, or the spray coating device 3 and the pipe 1 could be moved in opposite directions.
Since the housings 4 and 5 do not extend entirely around the pipe, the apparatus is readily adaptable to a 30 wide range of pipe diameters. Moreover, it is advantageous that not only the spray guns 6 but also the housing 5 is - 10 ~ ,~
, -:~0602~310 adjustable. In this way, the longitudinal walls of the housing 5 serve as air deflector plates which define, at their lower edges, the air inlet space 7 which is relatively adjustable in a simple manner to the same ~alue for different pipe diameters.
The housing is preferably coated, on at least its inner surfaces, with an electrostatic charge resistant material, e.g. polytetrafluoroethylene or polyvinylchloride. This coating prevents powder buildup on these surfaces.
Blowing of the powder from the apparatus is safely 10 prevented. The present spray chamber can therefore be very simply constructed, and only a simple housing structure is required. There is no need for any additional air supply.
In order to avoid risk of explosions, the powder is sprayed in an inert gas, and it is not necessary to effect air cleaning and drying.
Example.
A steel pipe of 811 mm diameter, 9.3 mm wall thickness and 15 mm length was coated. The relative speed of the pipe and the spray coating device 3 was 3 meters per 20minute. A constant temperature, at the spray coating device
This prior method has numerous disadvantages. The main disadvantage is that the powder supply, which is required to produce a homogenous and well-adhering coating of very constant thickness, can be only insufficiently regulated in accordance with the heat of the heated pipe. Extensive observations of the coating process have shown that the best coating quality can be achieved when the powder supply for 10 achieving the required coating thickness is determined quantitatively with respect to the heat content of the heated pipe. The efficiency ofthe utilization of the powder is also determined by these factors.
When the above-described prior method is used for the electrostatic spray coating of synthetic resin powders, the observation of the powder supply during the coating process is limited by the closed chamber, and in fact can only be effected by observing the amount of powder which is withdrawn from the chamber. The adjustments of the powder supply which 20are required for improving the quality of the coating can only be effected at a relatively late point in time, due the -closed and fixedly positioned coating chamber. Also, it is necessary to individually adjust the supply and the spacing, relative to the pipe surface, of each single spray gun.
These adjustments are, however, subject to narrow limits due to the relatively high coating speed and the consequentially nonuniform overlappings ofthe spray cones, particularly when pipes of different diameters are coated in successive operations.
Since, for pipes having small diameters, a relatively large 30annular space remains open in the side walls of the powder spray chamber, the air flow conditions within the powder spray .
:
~ 106~2~30 chamber vary, as they al~o do for larger pipe diameters.
According to regulations in force in the Federal Republic of Germany, the concentration of powder within the spray chamber should not exceed 20 grams per cubic meter. The above-described prior apparatus therefore should not be operated in the Federal Republic of Germany. The arrangement of the spray guns and the air guiding and deflecting plates provided within the chamber is not only expensive, but also does not allow uniform flow conditions of the various air streams to be ensured 10 when pipes of different diameters are coated. This prior apparatus also requires expensive multiple air sup?lies. A particular disadvantage of this prior apparatus, finally, is that it can only be employed for a comparatively small range of pipe diameters since the diameters of the circular openings in the side walls of the chamber determine the maximum possible pipe diameter which can be treated. Consequently, if pipes of very different diameters are required to be coated, correspondingly different powder spray chambers must be employed.
It is an object of the present invention to provide 20 an improved electrostatic spray coating method which at least partly mitigates at least some of the above-described disadvantages of this prior method.
It is a further object of the present invention to provide an electrostatic spray coating method which enables the powder supply, required for producing a homogenous and well-adhering coating of uniform thickness, to be readily adjusted in dependence onthe heat content of the preheated pipe.
According to one aspect of the present invention, there 30 is provided a method of electrostatically spray coating a thermally fusible synthetic resin powder material onto a metal pipe, 1060Z8~
comprising the steps of preheating the pipe to a temperature above the fusion point of the powder material, rotating the pipe about its longitudinal axis at constant speed, effecting relative displacement of the pipe and a powder spray chamber longitudinally of the pipe, discharging the powder material in a spray from the spray chamber, and withdrawing excess powder material from the powder spray chamber by applying suction to the spray chamber, wherein, for all pipe diameters, the speed of rotation of the pipe, the spacing of the powder spray chamber 1~ from the periphery of the pipe and the amount of air sucked in per unit time from the atmosphere between the chamher and the metal pipe are maintained constant while the speed of the relative displacement of the powder spray chamber and the pipe, longitudinally of the pipe, is varied in accordance with the pipe diameter, and the powder material is sprayed with an inert gas.
According to another aspect of the invention there is provided electrostatic spray coating apparatus for coating a metal pipe, comprising: a powder spray chamber; a suction device connected to the spray chamber for removing excess powder there- :
20from; and at least one spray discharge device for discharging a spray of powder material within the spray chamber; the spray chamber comprising a downwardly open housing which is adjustable in position relative to the metal pipe for varying the spacing of the spray discharge device from the metal pipe; the spray chamber further being adapted te extend through, at most, 120 around the periphery of the metal pipe with the edge of the ~ -housing spaced from the periphery of the metal pipe to define therewith an air inlet gap; and the spray discharge device being fixed relative to the housing.
It is known to heat a pipe which is to be coated, after it has been cleaned, to a temperature which is sufficient ~ _ 5 _ 1060Z~30 to cause the subsequently sprayed material to melt and fuse.
When employing an epoxy resin coating material, this temperature, according to the prior art method disclosed in German Offenlegungschrift 2,256,865, is of the order of 230 to 260 centigrade. The thus heated pipe is displaced, while being continuously rotated about it longitudinal axis, through the powder spray coating chamber, which is comparatively short relative to the length of the pipe to be coated. The time required for the pipe to pass through the powder spray lOcoating chamber amounts, depending upon the length and the speed of advance, to several minutes. Consequently, the part of the pipe which is coated first has a higher local coating temperature than the end of the pipe which is coated last, and the temperature of which decreases, due to heat radiation, during the time required to spray coat the length of the pipe.
Extensive experiments have shown that the powder ~ 5a -, Z~30 material employed for spray coating develops its optimum properties only in a comparatively narrow temperature range.
Variations in the coating temperature, consequently, influence the quality of the coating.
In order to counteract this disadvantageous effect, when employing the method according to the present invention, the pipe is preferably preheated to different temperatures along the length of the pipe such that the pipe end at which the spray coating begins has the lowest preheated temperature, 10 and the pipe temperature increases substantially linearly along the length of the pipe to a maximum value such that the pipe temperature is at least approximately uniform at the powder spray coating device during the relative displacement of the powder spray coating device and the pipe.
The electrostatic powder spray coating method is a process for providing surface protection, in which synthetic resin powder, e.g. epoxy resin powder, is melted and thermally fused on the preheated pipe. The powder is sprayed onto the pipe, which is heated above the melting temperature of the 20 powder, the powder being electrostatically charged as it is ~
pneumatically conveyed in a spray unit. Between the electro- -statically negatively charged epoxy resin particles and the positively charged (grounded) pipe there is produced a strong magnetic field. The epoxy resin powder particles are distributed, corresponding to the disposition of the magnetic lines of force, in a uniform coating thickness on the surface of the pipe and, due to their different charge, adhere securely to the pipe.
For providing corrosion protection of a steel tube 30by a synthetic resin coating, it is not the thickness of the synthetic resin coating, but the quality of the adhesion of " 1060Z80 the synthetic resin to the steel pipe, which is important.
Extensive experiments have shown that the most important factors affecting the adhesion are the coating temperature of the workpiece, the melting temperature of the synthetic resin powder, the viscosity of the melted synthetic resin, and the temperature at which the hardner is effective.
It has previously been considered, by those skilled in the art, that the coating temperature, using epoxy resin, should lie within the range of 230 to 260 centigrade, and lOpreferably at approximately 245 centigrade (see e.g. German Offenlegungschrift 2,256,865). Also, in the "Specification for Planned Application of Scotchkote* no. 204 Prime System to Line Pipe" and "no. 206 Pipeline Coating to Line Pipe", coating temperatures of 218~centigrade to 244 centigrade are given. In these publications, furthermore, it is specifically stated that oxidation of the steel which is to be coated should be avoided under all circumstances (see. Section 3.1 thereof).
It has now been found that, in the method of the 20present invention, the surface of the pipe which is to be coated may preferably be temporarily preheated to a surface temperature of 280 to 380 centigrade to form an oxide coating of approximately molecular thickness.
The coating temperature is the temperature which is uniform over the entire wall thickness of the pipe and which is produced, after the preheating, by a temperature equalization period. By this equalization period, it is possible to temporarily reach a surface temperature of 380 centigrade during the heating. If the heating is carried out 30by using gas burners, then these are preferably operated with excess air and hot flames. When the above-mentioned temperature * Trade Mark : ~
: , :
is reached, a FeO coating of approximately molecular thickness is produced, which is recognizable by a corresponding oxidation tint.
Extensive investigations have shown that a-ferrous oxide coating formed in this way produces a very good bonding of the subsequently spray coated syntheticresin. These findings are in contrast to previous opinions of those skilled in the art, according to which the preheating temperatures should not exceed about 260 centigrade. The positive results obtained by the present process are therefore surprising for those skilled in the art. The present process has an important 10 economic significance. Tests have shown that the requirements for the protective coating of steel pipes when they are buried in the ground are fulfilled. The same holds true with respect to the mechanical resistance to wear and tear which occur during transport and on site and also with respect to corrosion resistance. By employing the present process, it should now be possible, for the first time, to employ co~ted pipes buried in the ground for conveying substances at temperatures of up to 130 centigrade over long distances.
Also according to the present invention, there is provided an electrostatic spray coating apparatus for coating a pipe, comprising a powder spray chamber, a suction device connected to the chamber for removing excess powder therefrom, and at least one spray discharge device for discharging a spray of powder material within the spray chamber, the spray chamber comprising a downwardly open housing which is adjustable in position relative to the pipe for varying the spacing of the spray discharge device from the pipe, the spray chamber further being adapted to extend through, at most, 120 around 30 the periphery of the pipe with the edge of the housing spaced from the periphery of the pipe to define therewith an air inlet .
- ~6~280 gap, and the spray discharge device being fixed relative to the housing.
Preferably, at least the inner surface of the housing is coated with an electrostatic charge resistant material.
The adJustable housing may be telescopically movable in a fixed housing.
The invention will be more readily understood from the following description of a preferred embodiment thereof given by way of example with reference to the accompanying 10 diagramatic drawings, in which:-Figure 1 shows a view in perspective of a spraycoating apparatus;
Figure 2 shows a partially broken-away view in perspective of parts of the apparatus of Figure 1 on a larger scale; and Figure 3 is a diagram illustrating the preferred preheating temperature variation along a pipe having a length of 15 meters.
Referring firstly to Figure 1, there is illustrated 20a metal pipe 1 which is preheated and which is rotated about its longitudinal axis at constant speed.
A spray coating device 3 is displaceable longitudinally of the pipe 1, and includes a powder spray chamber or housing 4, which is fixed in position. A second housing 5 is telescopically received in the housing 4 and is spaced from the periphery of the pipe. The housing 5 is adjustable in height and downwardly -open, and extends around the periphery of the pipe through, at most, 120.
Substantially linearly arranged spray guns 6 are 30fixedly connected to the adjustably displaceable housing 5.
The edge of the housing 5 is spaced from the periphery of the 4 .
1~6~Z810 pipe to define therewith an air inlet gap 7.
The spraying distance between the spray guns 6 and the periphery of the pipe is indicated by reference numeral 2.
By means of this apparatus it is possible .o adapt the powder supply, at the earliest possible time, to the amount of powder which can be taken up by the heated pipe, e.g. by simple visual observation of the air inlet gap 7 and the coated pipe. In this way, the amount of powder required for providing a homogenous and adhesive coating of constant thickness can be 10 regulated during the coating process, and approximately 100%
utili7ation of the powder can be achieved. Consequently, the provision of an expensive powder reclaiming installation is not necessary.
By the uniform sucking in or out of air, a constant air movement is obtained for all pipe diameters. Since the peripheral speed of the pipe surface which is to be coated increases as the pipe diameter increases, a corresponding adjustment for providing a uniform coating is effected by reducing the relative speed of movement of the spray coating 20 device~3 and the pipe 1 in the longitudinal direction of the pipe 1.
Whlle it has been stated hereinabove that the spray coating device 3 is moved along the pipe, it is to be understood that alternatively the spray coating device 3 could remain stationary, while the pipe 1 is moved longitudinally relative thereto, or the spray coating device 3 and the pipe 1 could be moved in opposite directions.
Since the housings 4 and 5 do not extend entirely around the pipe, the apparatus is readily adaptable to a 30 wide range of pipe diameters. Moreover, it is advantageous that not only the spray guns 6 but also the housing 5 is - 10 ~ ,~
, -:~0602~310 adjustable. In this way, the longitudinal walls of the housing 5 serve as air deflector plates which define, at their lower edges, the air inlet space 7 which is relatively adjustable in a simple manner to the same ~alue for different pipe diameters.
The housing is preferably coated, on at least its inner surfaces, with an electrostatic charge resistant material, e.g. polytetrafluoroethylene or polyvinylchloride. This coating prevents powder buildup on these surfaces.
Blowing of the powder from the apparatus is safely 10 prevented. The present spray chamber can therefore be very simply constructed, and only a simple housing structure is required. There is no need for any additional air supply.
In order to avoid risk of explosions, the powder is sprayed in an inert gas, and it is not necessary to effect air cleaning and drying.
Example.
A steel pipe of 811 mm diameter, 9.3 mm wall thickness and 15 mm length was coated. The relative speed of the pipe and the spray coating device 3 was 3 meters per 20minute. A constant temperature, at the spray coating device
3, of 290 centigrade was provided by non-uniformly preheating the pipe so that a temperature of 300 centigrade was measured at the end of the pipe which was coated first, the pipe temperature increasing substantially linearly to a temperature of 380 centigrade at the opposite end of the pipe, as illustrated in Figure 3.
~ ' :
,
~ ' :
,
Claims (6)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of electrostatically spray coating a thermally fusible synthetic resin powder material onto a metal pipe, comprising the steps of preheating the pipe to a temperature above the fusion point of the powder material, rotating the pipe about is longitudinal axis at constant speed, effecting relative displacement of the pipe and a powder spray chamber longitudinally of the pipe, discharging the powder material in a spray from the spray chamber, and withdrawing excess powder material from the powder spray chamber by applying suction to the spray chamber, wherein, for all pipe diameters, the speed of rotation of the pipe, the spacing of the powder spray chamber from the periphery of the pipe and the amount of air sucked in per unit time from the atmosphere between the chamber and the metal pipe are maintained constant while the speed of the relative displacement of the powder spray chamber and the pipe, longitudinally of the pipe, is varied in accordance with the pipe diameter, and the powder material is sprayed with an inert gas.
2. A method as claimed in claim 1, which includes preheating the pipe to different temperatures along the length of the pipe such that the pipe end at which the spray coating begins has the lowest preheated temperature, and the pipe temperature increases substantially linearly along the length of the pipe to a maximum value such that the pipe temperature is at least approximately uniform at the powder spray coating device during the relative displacement of the pipe and the powder spray coating device.
3. A method as claimed in claim 1 or 2, which includes temporarily preheating the surface of the pipe which is to be coated to a surface temperature of 280 to 380° centigrade to form an oxide coating of approximately molecular thickness on the pipe.
4. Electrostatic spray coating apparatus for coating a metal pipe, comprising:
a powder spray chamber;
a suction device connected to the spray chamber for removing excess powder therefrom; and at least one spray discharge device for discharging a spray of powder material within the spray chamber;
the spray chamber comprising a downwardly open housing which is adjustable in position relative to the metal pipe for varying the spacing of the spray discharge device from the metal pipe;
the spray chamber further being adapted to extend through, at most, 120° around the periphery of the metal pipe with the edge of the housing spaced from the periphery of the metal pipe to define therewith an air inlet gap; and the spray discharge device being fixed relative to the housing.
a powder spray chamber;
a suction device connected to the spray chamber for removing excess powder therefrom; and at least one spray discharge device for discharging a spray of powder material within the spray chamber;
the spray chamber comprising a downwardly open housing which is adjustable in position relative to the metal pipe for varying the spacing of the spray discharge device from the metal pipe;
the spray chamber further being adapted to extend through, at most, 120° around the periphery of the metal pipe with the edge of the housing spaced from the periphery of the metal pipe to define therewith an air inlet gap; and the spray discharge device being fixed relative to the housing.
5. Electrostatic spray coating apparatus as claimed in claim 4, wherein at least the inner surface of the housing is coated with an electrostatic charge resistant material.
6. Electrostatic spray coating apparatus as claimed in claim 4 or 5, wherein the adjustable housing is telescopically movable in a fixed housing.
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19742451672 DE2451672C3 (en) | 1974-10-31 | Method and device for the electrostatic spraying of powdery substances onto preheated metal pipes |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1060280A true CA1060280A (en) | 1979-08-14 |
Family
ID=5929646
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA238,732A Expired CA1060280A (en) | 1974-10-31 | 1975-10-31 | Method of and apparatus for electrostatically spray coating powder material onto metal pipes |
Country Status (4)
| Country | Link |
|---|---|
| CA (1) | CA1060280A (en) |
| FR (1) | FR2289297A1 (en) |
| GB (1) | GB1496833A (en) |
| NL (3) | NL167866C (en) |
Families Citing this family (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB9202088D0 (en) * | 1992-01-31 | 1992-03-18 | Thomas Robert E | The manufacture of cylindrical components by centrifugal force |
| CN108580106B (en) * | 2018-06-26 | 2020-09-01 | 马鞍山楚锐科技信息咨询有限公司 | Automatic paint spraying instrument for entertainment type pipe fitting |
| CN108580108B (en) * | 2018-06-26 | 2020-09-01 | 马鞍山楚锐科技信息咨询有限公司 | Automatic paint spraying instrument for flexible pipe fittings |
| CN108580107B (en) * | 2018-06-26 | 2020-09-01 | 马鞍山楚锐科技信息咨询有限公司 | Automatic paint spraying instrument for multiple adjustable pipe fittings |
| CN109550616B (en) * | 2018-12-05 | 2020-07-17 | 安徽精工钢结构有限公司 | Steel construction paint spraying apparatus |
| CN111905958A (en) * | 2020-08-01 | 2020-11-10 | 安徽家园铝业有限公司 | Powder spraying aluminum alloy ex-trusions drying device |
-
1975
- 1975-10-30 GB GB44825/75A patent/GB1496833A/en not_active Expired
- 1975-10-30 NL NL7512744A patent/NL167866C/en not_active IP Right Cessation
- 1975-10-31 FR FR7533437A patent/FR2289297A1/en active Granted
- 1975-10-31 CA CA238,732A patent/CA1060280A/en not_active Expired
-
1978
- 1978-09-27 NL NL7809785A patent/NL7809785A/en not_active Application Discontinuation
- 1978-09-27 NL NL7809786A patent/NL7809786A/en not_active Application Discontinuation
Also Published As
| Publication number | Publication date |
|---|---|
| NL7512744A (en) | 1976-05-04 |
| NL7809786A (en) | 1978-12-29 |
| NL7809785A (en) | 1978-12-29 |
| GB1496833A (en) | 1978-01-05 |
| DE2451672A1 (en) | 1976-05-06 |
| DE2451672B2 (en) | 1976-12-02 |
| NL167866B (en) | 1981-09-16 |
| FR2289297A1 (en) | 1976-05-28 |
| NL167866C (en) | 1982-02-16 |
| FR2289297B1 (en) | 1978-05-12 |
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