EP2236217A1 - Method and device for coating metallic tubes or other long components with limited cross-section - Google Patents

Abstract

The method involves feeding to-be coated pipes (1) in a processing line, in which a pipe is moved axially, and preheating the pipes or a pipe section by hot air by a pipe preheater (4), and applying a primer layer. The pipes are heated to reach a polarization between a primer and a pipe. The pipes are dried for complete expulsion of all soluble components. The pipes are fed into another processing line, and a coating is applied in an extruder with a cross head. The pipes are heated in an induction furnace, and the coated pipes are hardened. The coated pipes are cooled. An independent claim is also included for a device for executing a method for coating metallic pipes.

Description

The invention relates to a method for coating metallic pipes or other long components of limited cross-section, in particular of tubes for heat exchangers, and to an apparatus for carrying out this method.

For the power plant industry large heat exchangers are needed, which are installed in the flue gas ducts. The flue gas leaves the boiler and the downstream air preheater, called LUVO for short, with about 130 to 170 ° C. This temperature depends on the type of fuel and is in any case far above the acid dew point of the flue gas. If you were to cool the flue gases in the LUVO below the acid dew point, this and the downstream components would be destroyed by the acidity. For this reason, the flue gas temperature at the coldest point of the LUVO must be safely above the acid dew point.

However, depending on the driving style of the REA, only flue gas temperatures lower than 100 and 110 ° C are required in the flue gas desulphurisation plant downstream of the LUVO, known as REA for short. This temperature sink between LUVO and REA of> 30 K can be used to increase the power plant efficiency and thus to reduce the CO ₂ emissions or to reheat the flue gases behind the REA. To use this energy, but you need acid-resistant heat exchangers, which in the Flue gas stream can be switched and can dissipate the heat by means of a heat transfer medium.

It is also already known to build the acid-resistant heat exchangers described above either from fluoroplastics such as "PFA", "MFA" or "TFM" or from high-quality chromium-nickel material such as "A59". At times, enamelled pipes have also been used, but these have proven to be ineffective and can only be used under very specific conditions.

Another possibility is to use so-called "gelatinized pipes". In this process, a "PFA" layer, the so-called "liner" is applied without a solid composite on a pipe, in particular on a steel pipe and thus serves as corrosion protection. All other materials did not work and had to be discarded.

The advantages and disadvantages of the known materials used are briefly described below:

PFA (Perfluoropropyl vinyl ether)

PFA is absolutely acid resistant. Even after many years of operation no acidity is detectable. However, there are certain disadvantages. On the one hand, the material is very expensive, so that one must endeavor to keep the wall thickness of the tubes (one also speaks of tubes) as small as possible. In addition, there is the poor thermal conductivity, which also forces to low wall thicknesses. On the other hand, in comparison with steel, there is very little strength, which at high temperatures still decreases considerably, and thus either excessive wall thicknesses or leads to small hose diameters. High wall thicknesses, together with the poor conductivities at high costs and small diameters of the hoses, also lead to narrow passages for the flue gas passage, which in turn can lead to contamination of the heat exchangers. In the turbulence zones of the flue gas, the hoses can bang against each other causing mechanical damage.

Modified PTFE (polytetrafluoroethylene)

However, modified PTFE materials that have properties similar to PFA in the area of acid resistance do not have a melting point in contrast to PFA. For this reason, they can not be melt-extruded processed but only paste-extruded. That is, the material is not fused (as in PFA) but pressed together only in pasty form, which amounts to a sintering. In addition, since the molecules align only longitudinally to the extrusion direction, although a relatively high strength in the longitudinal direction, in the transverse direction, the creep rupture strength is relatively low. In addition, the material has a high cold flow property. That is, the material flows away under pressure over time. Unfortunately, this process can not be stopped so that leaks occur again and again at the sealing points of the hoses in the tubesheets. Otherwise, the same problems apply as with the PFA.

Chrome-nickel material

Chrome-nickel material "A59" is almost acid-resistant. However, even after a short period of operation, its smooth surface during production changes. The surface becomes rough and thus very susceptible to dirt, which can lead to the blocking of the flue gases. However, since "A59" has a high strength, pipes with a larger diameter can be used, which in turn partially counteracts the contamination. The biggest disadvantage of the "A59", however, is the high price. The costs for a heat exchanger of the same power are about twice as high for the "A59" as for the PFA.

Enamelled pipes

Enamelled pipes have not proven in practice to a large extent. Hydrogen forms between the steel tube and the enamel layer, which breaks up the enamel layer and thus leads to the destruction of the tubes. The cause of hydrogen formation is not yet clear. Corresponding investigations are currently being carried out by the email manufacturers. However, a result is not yet available.

Pipes with liner

"Gelinerte" tubes have the advantage that they use a steel tube as the pressure body. As a result, the PFA layer serves only as corrosion protection, and thus can be mounted in a small wall thickness on the pressure tube. This leads to a strong cost reduction. However, it is disadvantageous that the acid diffuses through the "liner" because of the differential pressure gradient and leads to corrosion products between the pressure pipe and the "liner". This bursts the "liner", which in turn leads to the destruction of the pipe by acid. In addition, the "liner" is mechanically very vulnerable. In the case of impacts against the pipes, whether intentionally during cleaning or unintentionally during other repair work, the "liner" often becomes inadvertently and unnoticed through the smallest holes damaged, which in the subsequent operation soon leads to corrosion damage.

The coating of machine components with organic fluoropolymers is known in many embodiments. It always occurs when non-stick coatings are required, which have become known under the brand name Teflon ^® or when it is important to protect the components from external influences. Corresponding linings or coatings are available, for example, in many areas of chemical plant construction. Generally speaking, a coating is necessary when there is a risk that the metallic components will be damaged by corrosion and thus the associated systems are impaired in terms of their service life. For example, pump impellers or stirrer components have been coated with organic fluoropolymers for years.

• Zunächst wird das Werkstück entfettet und gesandstrahlt und dann eine Haftgrundierung der sogenannte Haftprimer aufgebracht. Auf diesen Haftprimer wird anschließend die Fluorkunststoffschicht aufgebracht. Dazu werden die einzelnen Werkstücke bei vorgegebener Temperatur in einen Ofen gefahren und dort eine bestimmte Zeit belassen, bis sie ihre Beschichtungstemperatur erreicht haben. Dann werden sie aus dem Ofen entfernt und mit Haftprimer beschichtet. Anschließend werden sie erneut eine vorbestimmte Zeit in den Ofen gefahren, um die Haftprimerschicht mit dem metallischen Bauteil zu verbinden (sog. Polarisation). Anschließend werden sie wiederum aus dem Ofen geholt und mit der ersten Fluorkunststoffschicht beschichtet. Dieser Prozess wiederholt sich mehrere Male, da beim herkömmlichen Verfahren in jedem Beschichtungsvorgang nur etwa eine Schichtstärke von 500 µm aufgebracht werden kann und das Werkstück ca. fünf Mal in den Ofen hinein und wieder aus dem Ofen herausgefahren werden muss. Zudem unterliegt das bekannte Verfahren Einschränkungen hinsichtlich der Größe der Werkstücke, da die Werkstücke nicht größer als der Ofen sein dürfen. Der weltweit größte uns bekannte Ofen für diese Zwecke hat eine Länge von etwa 11 m. Dies bedeutet, dass mit dem bekannten Beschichtungsverfahren nur kürzere Bauteile beschichtet werden können.

The known procedure for coating larger components is as follows:

• First, the workpiece is degreased and sandblasted and then applied a primer so-called adhesive primer. The fluoroplastic layer is then applied to this adhesion primer. For this purpose, the individual workpieces are driven at a predetermined temperature in an oven and left there for a certain time until they have reached their coating temperature. Then they are removed from the oven and coated with adhesive primer. Then they are again driven a predetermined time in the oven to connect the adhesive primer layer with the metallic component (so-called polarization). Then they are taken out of the oven again and with the first one Fluoroplastic layer coated. This process is repeated several times, since the conventional method in each coating process only about a layer thickness of 500 microns can be applied and the workpiece about five times in the oven and out again has to be moved out of the oven. In addition, the known method is subject to restrictions on the size of the workpieces, since the workpieces must not be larger than the oven. The world's largest known oven for this purpose has a length of about 11 m. This means that only shorter components can be coated with the known coating method.

The invention is therefore based on the object mentioned above and described in more detail above method and a corresponding device for coating of metallic components and further develop that even relatively long components can be mechanically coated under temporally and economically optimal conditions. In particular, it should be possible to continuously coat the components.

• Zuführen des zu beschichtenden Rohres in eine erste Bearbeitungslinie, in der das Rohr axial transportiert wird,
• Vorwärmen des Rohres oder eines Rohrabschnittes,
• Aufbringen einer Haftgrundschicht,
• Erwärmen des Rohres zum Erreichen einer Polarisation zwischen Haftgrund und Rohr,
• Trocknen des Rohres zum vollständigen Austreiben aller löslicher Bestandteile,
• Zuführen des Rohres in eine zweite Bearbeitungslinie, in der das Rohr axial transportiert wird,
• Vorwärmen des Rohres,
• Aufbringen der Beschichtung in einem Extruder mit Querkopf,
• Aufheizen des Rohres in einem Induktionsofen,
• Aushärten des beschichteten Rohres und
• Abkühlen des beschichteten Rohres.

The solution to this problem is next in a process for coating metal pipes or other long components of limited cross-section with acid-resistant corrosion protection layers, which is characterized by the following steps:

• Feeding the pipe to be coated into a first processing line, in which the pipe is transported axially,
• Preheating the pipe or a pipe section,
• Applying a primer layer,
• Heating the tube to achieve a polarization between the primer and the tube,
• Drying the tube to completely expel all soluble constituents,
• Feeding the tube into a second processing line in which the tube is transported axially,
• Preheating the pipe,
• Applying the coating in an extruder with a crosshead,
• Heating the pipe in an induction furnace,
• Curing the coated tube and
• Cooling the coated tube.

A corresponding device for carrying out the method achieves the object according to the invention by a first processing line with a first drive, a first preheater, a device for applying the primer layer and at least one oven for curing and drying and a second processing line with a second drive, a second preheating , an extruder with a crosshead for applying the coating, an induction furnace and a curing oven.

When the term "tube" is referred to in the following, it is intended to encompass all possible components to be coated whose length exceeds the extent and the cross section by a multiple. Since the coating takes place from the outside, the tubes or other hollow components can also be closed on one side.

According to another teaching of the invention, it is possible that the first and second processing line are operated separately. Alternatively, it is also conceivable to operate them together in succession. The separate embodiment, however, will generally be the preferred one, since on the one hand the length of the processing lines otherwise becomes relatively large and it is possible by the division to perform the coating with adhesive primer and the later actual fluoroplastic coating at different times and at different locations. In particular, in the case of separate lines, the standstill of a line does not immediately stop the entire system.

A further embodiment of the invention provides that the pipes to be coated are connected before each processing line with suitable pipe connection elements and the treatment process proceeds continuously. In this way, so to speak, it is possible to coat "endless" components, which is particularly expedient from an economic point of view.

According to further teachings of the invention, the components to be treated are to be degreased before being fed into the first processing line and / or sandblasted in order to ensure a secure connection between primer layer and component. If "sandblasting" is mentioned here, this also means the irradiation with corundum, glass bodies or the like.

According to another preferred embodiment of the invention, the tube is transported not only axially in the first processing line, but at the same time about its longitudinal axis rotated so that its surface undergoes a sense of a helical movement. In this way, it is possible that the application of the primer layer can be carried out by spraying, wherein the range of spraying in the axial direction can be relatively short, when the rotational speed is adjusted in relation to the feed speed accordingly. The adhesive primer is so to speak "helical" applied to the component to be coated.

In a further embodiment of the invention, the preheating of the component in the first processing line is effected by means of hot air. A preheated tube allows better primer-to-tube contact when applying the primer layer, which is essential for even coating.

Another teaching of the invention provides that the preheating of the component takes place in the second processing line by means of hot air. Preheating is of particular importance here since the fluoroplastic applied in liquid form in the extruder would otherwise crosslink prematurely on the cold metal surface of the component and, in extreme cases, even lead to dripping of the fluoroplastic after leaving the extruder crosshead. Of course, the preheating can also be done by other suitable measures.

Another preferred embodiment of the invention provides that the application of the primer layer and the coating in the extruder are each carried out in a single operation. This is for an economic continuous manufacturing operation of particular interest. Attention must be paid to the application and drying process, so that the layer thickness on the component to be coated is the same everywhere. For example, when the primer is applied to a pipe, no "seams" may occur due to too thick or too thin "coating boundary lines".

According to another teaching of the invention, the curing of the coated component takes place in an oven. In this way, it is ensured that a uniform curing of the fluoroplastic layer is made possible.

Preference is given to using tubes as components and there especially steel tubes. The organic fluoropolymers PFA (perfluoropropyl vinyl ether) or MFA (perfluoromethyl vinyl ether) are preferably used for the coating.

According to a further embodiment of the invention, in a corresponding device for coating pipes, the first drive of the first processing line is provided with a rotary feed device. This rotary feed device can be controlled so that axial and rotational movements can be influenced individually in order to allow optimum adaptation to the width of the application zone of the adhesion prime. Preferably, a spray nozzle is used to apply the adhesive primer layer, which is formed in an advantageous embodiment of the invention as a broad jet nozzle, which extends substantially parallel to the tube longitudinal axis.

• Fig. 1 schematisch, die Komponenten einer ersten erfindungsgemäßen Bearbeitungslinie und
• Fig. 2 schematisch, die Komponenten einer zweiten erfindungsgemäßen Bearbeitungslinie.

The invention will be explained in more detail below with reference to a drawing illustrating only a preferred introductory example. In the drawing show

• Fig. 1 schematically, the components of a first inventive processing line and
• Fig. 2 schematically, the components of a second inventive processing line.

For the coating a kind "on line" procedure was developed. An arbitrarily long tube 1, preferably made of steel, lies on a roller conveyor 2 and is advanced in rotation by a special rotary feed 3. An endlessly long tube 1 is formed by fastening a tube to the next one by means of connecting elements in order to achieve a continuous process. Here, the rotation of the tube 1 is in a certain ratio to the feed. This ratio is diameter dependent.

Behind the rotary feed 3, a tube preheating 4 is switched to preheat the tube, preferably with a hot air blower 5 to a primer specific temperature.

After preheating a developed spray device 6 is installed. In cooperation of the tube preheating 4 and spray device 6, it is possible to apply a uniform primer layer with the predetermined layer thickness on the rotating tube 1 in a single operation.

Subsequently, the tube 1 is driven by the rotary feed 3 through a drying oven 7, whose length is sized is that it ensures the predetermined residence time in the drying oven 7 in connection with the feed. This residence time must be strictly adhered to in order to complete the polarization, that is to say the connection of the primer to the steel tube 1. In addition, during this time, the volatiles must be completely expelled from the primer to prevent subsequent blistering in the PFA coating. It must be ensured that the drying takes place uniformly over the layer thickness. If, for example, the outer layer is dried first, microcracks occur in the layer since the residual moisture from the inner layers can no longer escape and therefore ruptures the already dried outer layer.

The drying oven 7 is then followed by a polarizing oven 8, in which the polarization, ie the combination of adhesive primer and steel tube 1 takes place. The drying and polarization temperature in the oven 7 and 8 must be chosen and ensured so that, although the polarization temperature is reached and maintained over the full length of the furnaces, the polymerization temperature through which the subsequent connection between primer and PFA or MFA , but not reached. After passing through the drying and polarizing furnace 7 and 8, the primer coating is completed. Behind the furnaces 7, 8 then provided with the adhesive primer tube 1 'can be removed and optionally separated. It goes without saying that the ovens 7 and 8, which are shown individually in the exemplary embodiment shown and preferred so far, can also be realized as a common structural unit.

In addition to the coating line for the primer, a similar line is installed for the second coating with PFA or MFA. In this line, the previously coated with primer tube 1 'is again placed on a roller conveyor 10 and advanced by means of a belt withdrawal 11 at a constant speed. According to the invention, the individual fluoropolymer layers are not applied successively as usual, but are melted by means of an extruder 14 with a crosshead (not shown) in a single operation. In order not to drive the cold pipe 1 'into the multi-degree crosshead, the pipe 1' is preheated by means of a special device 12, whereby it must be ensured that the polymerization temperature is not reached.

With the help of the extruder 14, the melt PFA or MFA is then melted in full thickness in one operation on the tube 1 '. Only after the application of the melt is the tube 1 'heated by means of induction heat in an induction furnace 15 far above the polymerization point. This ensures the connection of primer PFA (or MFA). In a downstream oven 16, this temperature is maintained until the polymerization is complete. Again, as with the primer coating, the furnace length is directly related to the pipe feed. After leaving the furnace 16, the now completely coated tube 1 "is cooled and driven over a roller conveyor (not shown) for further use. As in the case of the primer line, the individual tubes are plugged together with connectors so that an infinitely long tube 1 'is formed and can be coated without interruption.

By means of the coating method according to the invention, it has been possible to produce an acid-resistant tube 1 "of any length, which is given both by the applied PFA (or MFA) layer and by the acid-resistant primer. Primer and PFA / MFA can not be infiltrated and detached by corrosion products, the pressure-resistant support tube can handle temperatures and pressures of any size, and the diameter of the pipes can be chosen as required by the overall process of the power plant it is possible to bend the tubes coated with the method according to the invention, for example coated U-tubes, such as those used in heat exchangers, can be produced.

Claims (22)

Method for coating metal pipes or other long components of limited cross-section, in particular pipes for heat exchangers, with an acid-resistant corrosion protection layer, characterized by the following steps: Feeding the pipe to be coated into a first processing line, in which the pipe is transported axially, Preheating the pipe or a pipe section, Application of a primer layer, Heating the tube to achieve a polarization between the primer and the tube, Drying the tube to completely expel all soluble constituents, Feeding the tube into a second processing line in which the tube is transported axially, - preheating the pipe, Application of the coating in an extruder with a crosshead, Heating the tube in an induction furnace, - curing the coated tube and - cooling the coated tube. Method according to claim 1,
characterized in that
the first and second processing lines are operated separately. Method according to claim 1,
characterized in that
that the first and second processing line are operated together. Method according to one of claims 1 to 3,
characterized in that
the pipes to be coated are connected before each processing line with suitable pipe connection elements and the treatment process proceeds continuously. Method according to one of claims 1 to 4,
characterized in that
the pipes to be coated are degreased before the first processing line. Method according to one of claims 1 to 4,
characterized in that
the pipes to be coated are sandblasted before the first processing line. Method according to one of claims 1 to 6,
characterized in that
the tube is transported axially and rotationally in the first processing line. Method according to claim 7,
characterized in that
the primer layer is applied by spraying. Method according to one of claims 1 to 8,
characterized in that
the preheating of the tube in the first processing line is done by means of hot air. Method according to one of claims 1 to 9,
characterized in that
Preheating the tube in the second processing line is done by means of hot air. Method according to one of claims 1 to 10,
characterized in that
the application of the primer layer and / or the coating in the extruder takes place in a single operation. Method according to one of claims 1 to 11,
characterized in that
Curing of the coated tube is done in an oven. Method according to one of claims 1 to 12,
characterized in that
be used as pipes steel pipes. Method according to one of claims 1 to 13,
characterized in that
for the coating, organic fluoropolymers PFA (perfluoroprapylvinylether) or MFA (perfluoromethylvinylether) can be used. Apparatus for carrying out the method according to one of claims 1 to 14, characterized by a first Processing line with a first drive (3), a first preheating device (4), a device (6) for applying the primer layer and at least one oven (7, 8) for curing and drying and a second processing line with a second drive (11), a second preheater (12), an extruder (14) with a transverse head for applying the coating, an induction furnace (15) and a furnace (16) for curing. Device according to claim 15,
characterized in that
the first drive (3) is a rotary feed device. Device according to claim 15 or 16,
characterized in that
the first preheating device (4) has a hot air blower (5). Device according to claim 15 or 16,
characterized in that
the second preheating device (12) has a hot air blower (13). Device according to one of claims 15 or 18,
characterized in that
as a means (6) for applying the primer layer, a spray nozzle is used. Device according to claim 19,
characterized in that
the spray nozzle is designed as a broad jet nozzle. Device according to one of claims 15 to 20,
characterized in that
the first processing line is preceded by a device (9) for degreasing and / or sand blasting. Device according to one of claims 15 to 21,
characterized in that
both processing lines are connected together to form a complete line.

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