MXPA97001543A - Method for the restoration of the functionality of the equipment subject to heavy corrosion in a plant for the production of - Google Patents

Abstract

The present invention relates to a method for the repair and functional restoration of equipment of a section of high or medium pressure of an industrial plant, said equipment having at least one hole for recording and comprising, internally, an anticorrosive metallic coating which has at least one extended area subject to corrosion, the method comprises the following steps: (a) the cleaning of the corroded area, with the elimination of most of the waste produced from it, (b) the formation, in the cleaned area, of according to step (a), of appropriate support and / or fastening surfaces for the placement of a new metallic coating; (c) positioning, on these supporting and / or fastening surfaces and on the edges of the surface together with the corroded area, until it has been completely covered, with flat elements, suitably shaped to adapt to the internal profile of the equipment, and which consist of a metal resistant to the Orrosion under the operating conditions of the equipment and having dimensions such as to pass through the registration hole, these shaped elements being placed next to each other, (d) the welding of the adjacent edges of the elements attached to each other as in step (c) and, possibly, welding the same edges on the metal below them, so that this weld has at least one interrupted section for each simple shaped element, thereby forming new metallic coating resistant to the corrosion in which the interstices between each shaped element and the surface underneath, communicate with each other and with at least one of the drainage holes comprising the body of the equipment; (e) the coverage of the exposed surface to the process fluids, around the interrupted welding sections, with the strips shaped and properly adjusted to size, welded by the edges on the new or metallic coating, in order to obtain an internal surface of the equipment, which is completely sealed and resistant to corrosion under normal operating conditions, this method of repair is completely achieved by using the equipment's orifice as the only acce

Description

/ METHOD FOR THE RESTORATION OF THE FUNCTIONALITY OF THE EQUIPMENT SUBJECT TO HEAVY CORROSION IN A PLANT FOR THE UREA PRODUCTION The present invention relates to a method for restoring the functionality of equipment subject to heavy corrosion in a plant for the production of urea. More specifically, the present invention refers to a method for repairing and restoring functionality of metal parts or equipment subject to erosion and / or contact corrosion, under conditions of high temperature and pressure, with fluids comprising water mixed with Ammonia, urea and / or ammonium carbamate, typically present in a plant for the industrial production of urea. It is well known that urea is obtained with industrial processes that require conditions of operation of high temperature and pressure at least in some parts of the plant. In these processes, ammonia, which is generally in excess, and carbon dioxide are reacted in one or more reactors, at pressures usually between 100 and 250 bar and temperatures between 150 and 240 ° C, obtaining as final product a solution containing urea, ammonium carbamate not transformed into urea and the excess ammonia used in the synthesis. The above aqueous solution is purified from the ammonium carbamate contained therein by decomposing it into decomposers or disintegrants which operate, successively, at increasingly lower pressures. In most of the present processes, the first of these descomponedoreg operates at pressures that are basically the same as the synthesis pressure or slightly lower and generally uses purification agents to decompose the ammonium carbamate and at the same time eliminate the products of decomposition. The purifying agents can be inert gases, or ammonia or carbon dioxide, or mixtures of inert gases with ammonia and / or carbon dioxide, and the purification can possibly also be carried out by the use of excess ammonia dissolved in the mixture that comes from the reactor (self-cleaning), without requiring therefore any external agent. The decomposition products of ammonium carbamate (NH3 and C02), together with the possible purification agents, excluding inert gases, are normally condensed in appropriate condensers, obtaining a liquid mixture comprising water, ammonia and ammonium carbamate, which The synthesis reactor is recycled. In plants that are technologically more advanced, at least one step of condensation is carried out at pressures more or less equal to those of the reactor or slightly lower. As a reference, among the many in existence, US Pat. Nos. 3,886,210, US 4,314,077, US 4,137,262 and European Patent Application Publication No. 504,966, can be cited, the two describe processes for the production of urea with the above characteristics . A wide range of processes mainly used for the production of urea, is described in "Encyclopedia of Chemical Technology", 3a. edition (1983), Vol. 23, pages 548-574, John Wiley & Sons Ed. The most critical steps of the process are those in which the ammonium carbamate is at its highest concentration and temperature, and therefore, in the previous processes, these steps coincide with the reactor and the subsequent equipment for the decomposition (or purification) and the condensation of the ammonium carbamate operating in conditions similar or almost similar to those of the reactor. The problem to be solved in this equipment is that of corrosion and / or erosion caused by ammonium carbamate, ammonia and carbon dioxide, which behave as highly corrosive agents, especially in the presence of water, at high temperatures and pressures necessary for the synthesis of urea. This corrosion problem has. been faced with different solutions in existing industrial plants, and others have been proposed in the literature. There are, in fact, numerous metals and alloys capable of resisting for sufficiently long periods the potentially corrosive conditions that are created within a reactor for the synthesis of urea. Among these, lead, titanium, zirconium and various stainless steels such as, for example, AI $ I 316L steel (urea grade), INOX steel 25/22/2 Cr / Ni / Mo, special austenite-ferrite steels, etc., can be mentioned, however, for economic reasons, this type of equipment can not be completely made of these alloys or metals resistant to corrosion. In general, the containers or columns are used in normal carbon steel, possibly in multiple layers, having a thickness that varies from 40 to 350 mm, depending on the geometry and the pressure to be supported (pressure resistant body), which the surface in contact with corrosive or erosive fluids is uniformly coated with an anticorrosive coating from 2 to 30 m thick. In particular, the reactor basically consists of a vertical "container" with reagents entering from below and discharging the reaction mixture from above. The pressure resistant body normally consists of a cylinder that has a diameter of 0.5 to 4 m, made with the technique of solid walls or multiple layers, which has the two ends closed by covers properly welded to it. Within the reactor all the parts subject to corrosion are coated with an anticorrosive coating, which may be, for example, titanium, lead, zirconium, or preferably stainless steel (urea grade) of the aforementioned type. The subsequent carbamate decomposer, specifically if it operates at the same pressure as the reactor, consists of a shell and tube exchanger. In this case too, the "pressure-resistant body" is made of normal carbon steel, while titanium or urea-grade stainless steels are preferably used for the coating, gases that leave the decomposer are usually re-condensed in a Carbamate condenser, which is still in contact with a mixture similar to that of the decomposer (except for urea) and is therefore very corrosive.In this case, too, the internal coating is preferably constituted of stainless steels, urea grade, In the above equipment or in units in plant, the anticorrosive coating is made by assembling numerous elements that have an appropriate resistance to corrosion, to the formation, and at the end, a structure that is sealed at high pressure The different connections and welds carried out for this purpose often require technical special cases depending on the geometry and type of parts to be connected. While stainless steel is weldable to the "pressure resistant body" below, made of carbon steel, but has a higher coefficient of thermal expansion which, during operation, favors the formation of cracks along The welding line, the titanium can not be welded on the steel and in any case has similar problems of cracks in the welding since its coefficient of expansion is considerably less than that of the carbon steel. For this reason, techniques are used that often require complex equipment and complex operating procedures. In certain cases, the coating is carried out by depositing by welding instead of sheets welded to one another and on the body under pressure. In other cases, especially with materials that can not be welded to one another, it is necessary to "exploit" the coating on the pressure body to be sure of obtaining a satisfactory hold. In all the aforementioned equipment, however, there are a number of "drainage holes" to detect possible losses of the anticorrosive coating. A drainage hole usually consists of a small tube of 8 - 15 mm in diameter made of corrosion resistant material and is inserted into the body under pressure until it reaches the point of contact between the latter and the metal or plastic coating. corrosion resistant alloy. If there is a leak in the coating, due to the high pressure, the internal fluid, which is corrosive, diffuses immediately into the interstitial zone between the cladding and the body under pressure and, if not detected, this could cause rapid corrosion of the carbon steel, which is the latter. The presence of drainage holes makes it possible for these leaks to be detected. For this purpose all interstitial areas below the anticorrosive coating must communicate with at least one drainage hole. The number of drainage holes is usually 2 to 4 for each metal fitting and therefore, for example, in a reactor there are usually 30 to 60 drainage holes. The previous equipment also has at least one circular opening, generally in the upper part called "record well", which allows access to operators and equipment for controls and small internal repairs. These openings usually have diameters between 45 and 60 cm and at most allow the passage of objects having these sizes. Despite the numerous precautions and construction devices mentioned above, it frequently still happens that the large areas of the inner lining of the equipment operating at high or medium pressure in contact with aggressive process fluids, such as those, for example, used in a plant for the production of urea, suffer from strong and prolonged corrosion which quickly causes the risk of perforation of the lining, with the subsequent danger of catastrophic breaks, or at least makes it necessary to stop the plant for repairs, which sometimes take a considerable amount of time. The remedy for these corrosion phenomena involves problems that are not easy to solve. Very often it is necessary to replace the damaged equipment (reactor, exchanger or condenser) with a new one, which suffers from extremely high costs due to the plant's halting and the construction and installation of the new equipment. Attempts to repair the damaged part have always been considered impossible in practice when the damage is onsiderable, due to the conviction of not being able to offer a sufficient guarantee of safety for the operation, and also to the practical difficulty of carrying it out. Of course, every intervention on the equipment or units of the plant, to avoid its partial dismantling, must be carried out through the logging well mentioned above. It is therefore not possible to insert metallic laminates or other objects whose dimensions do not allow them to pass through the recording hole into the equipment. Considering the fact that corroded areas are frequently spread over surfaces of 20-30 m2 or more, it is easy to understand that it is absolutely impossible to re-coat the corroded area with new uniform and homogeneous coating. Another highly widespread damage is also related to the problem of fixing the repair on the previous coating. It was thought, of course, that due to the considerable deterioration of the metal in the corroded area, a repair involving the welding and support zones directly in the area of interest, as well as at the edges where the pre-coating still had characteristics, was not reliable. reliable. For this reason, it was the general opinion that it was not possible to restore the functionality of the coating that had extended areas of corrosion, with the sole use of log hole.

For the many reasons mentioned above, it was generally believed that it was not advisable, or in any case economically inconvenient, to carry out operations for the repair and functional restoration of the corroded lining of the equipment in the high or medium pressure section of a plant for the production of urea. The solution usually recommended for these corrosion problems was consequently to replace the damaged equipment, although this involved high costs and the need to interrupt the production line for relatively long periods. The applicant has now found a satisfactory and advantageous solution to the above drawbacks, with a new procedure that allows the operative restoration of the plant equipment with greater reliability and without its removal, maintaining the repair cost at much lower levels than that one? necessary for the complete replacement of the equipment. The present invention therefore relates to a method for the repair and functional restoration of the equipment of the high or medium pressure section of an industrial plant, said equipment having at least one orifice of registration and comprising, internally, the metallic coating anticorrosive having at least one extended area subject to corrosion, said method comprising the following steps: (a) cleaning the area subject to corrosion, with the elimination of most of the waste produced from it; (b) forming, in the cleaned area according to step (a), support and / or maintenance surfaces appropriate for the placement of the new metallic coating; (c) the placement, on those support and / or maintenance surfaces, and on the edges of the surface next to the corroded area, until it has been completely covered, of properly shaped flat elements to adapt themselves to the internal profile of the equipment, and which consist of a metal resistant to corrosion, under the operating conditions of the equipment and having dimensions such as to pass through the hole of the record, these shaped elements being placed next to each other; (d) the welding of the adjacent edges of said elements joined to the other, as in step (c) and, possibly, the welding of the same edges on the metal below them, so that this welding has at least an interrupted section for each simple shaped element, thus forming a new corrosion-resistant metallic coating, in which the interstices between each shaped element and the surface below communicate with each other, and with at least one of the drain holes present in the body of the equipment; (e) the coverage of the surface exposed to the fluids of the process, around said sections of interrupted welding, with plates shaped and adjusted appropriately to size, welded by the edges on the new metallic coating, in order to obtain an internal surface of equipment which is totally sealed and resistant to corrosion under normal operating conditions; This method of repair is completely achieved by using the equipment's log hole, as the only access. The present invention also relates to the equipment obtained with the previous repair and the method of functional restoration. As specified above, the method of the present invention is particularly applicable to the equipment of the high or medium pressure section of a plant for Urea Synthesis. These can be substantially identified as urea synthesis reactors, equipment for the decomposition of untransformed carbamate, and containers for NH3 and C02 condensation with the formation of carbamate solutions. These equipment operate at pressures of between 15 and 250 atm and at temperatures between 70 and 300 ° C, in the presence of mixtures containing water, ammonia, carbon dioxide and ammonium carbamate, which is the condensation product of these compounds according to to the reaction: [2 NH3 + C02 + nH20 - NH4OCONH2 • nH20] The operating conditions are preferably a pressure of 100-250 atm and a temperature of 120-240 ° C. Under the above conditions the surfaces of the equipment in contact with the corrosive mixtures are preferably made of stainless steel, titanium, zirconium, lead, etc. The "urea grade" stainless steel is particularly preferable such as the AISI 316L steel (urea grade) ), INOX steel 25/22/2 Cr / Ni / Mo, special austenite-ferrite steels, etc. These metals or corrosion-resistant alloys do not normally form the complete body of the equipment but a coating having a thickness preferably of between 2 and 30 mm. In normal industrial plants for the production of urea, to which the present invention is particularly concerned, the above equipment belonging to the sections of high or medium pressure, usually contain volumes of between 2,000 and 400,000 liters and have a development of the surfaces internal in contact with corrosive liquids of between 8 and 400 m2, excluding the surface of the possible tubes (ihtercambiadores of heat, etc.). For purposes of the present invention, the equipment is considered to possess extended corroded areas, when its internal surface has at least one corroded zone whose area is greater than twice the section of the log hole. The terms "corrosion", "corrosive" and "corroded" as used in the present application refer indifferently to processes that comprise only corrosion of a chemical nature, or erosion of a physical nature or a combination of both. The method of the present invention can also be applied to equipment whose walls are not internally coated, but which are mainly made of a corrosion-resistant metal or metal alloy. In this case, the term "internal anticorrosive coating" as used in the present application also comprises the walls of the equipment having anticorrosive properties. According to the method of the present invention, in step (a) it is necessary to clean the area subject to corrosion. The corroded area normally appears as an irregular surface covered with residues caused by corrosion (oxides, carbides, etc.) or superficial layers of metal not completely corroded but no longer integral. The irregularities (or roughness) of the surface can also be very deep and extend through a thickness ranging from about 5% of the original thickness of the coating to 100%, in which case there is a perforation of the coating. In step (a) of the present invention, cleaning does not necessarily have to involve the total elimination of residues from corrosion, which could prove to be very laborious in terms of time (and without any advantage), but should ensure a surface without areas with dimensional instability and without residues able to act as initiators of corrosion during the subsequent operation of the equipment. In accordance with the present invention, the cleaning of the surface can be carried out using any of the methods appropriate for the purpose. These comprise abrasive methods by filing, abrasive treatment, sanding, knurling, etc., or cleaning methods are solvents or appropriate chemical agents capable of removing residues from corrosion, thereby producing a dimensionally stable layer. Cleaning methods that comprise a combination of the above are not excluded. Cleaning with abrasive methods is however preferred for purposes of the present invention. In step (b) of the method of the present invention, the appropriate support or maintenance areas are obtained on the suitably cleaned surface according to step (a), for the subsequent placement of the new coating. According to a preferred embodiment of the present invention, in step (b) a reasonably smooth surface is obtained, in the corroded area, which has a profile that is substantially in line with the profile of the previous coating in the areas adjacent to the corroded area, or where these are not present, which have a profile that is preferably in line with most of the the prominent surface irregularities. The entire corroded area is practically covered with a first metallic layer, usually of the same material as the previous coating, of a thickness such that the new surface of this first metallic layer is brought into line with that of the coating surrounding the corroded area. The covering is carried out by fixing on the cleaned surface, usually by welding, preformed metal elements, of relatively small dimension and with an irregular geometry, adapted to the irregularities caused by corrosion. Alternatively, it is also possible to carry out full coverage or part of it with solder deposits. In this way a first irregular filling is obtained which is then smoothed, with appropriate techniques normally available, until a maximum depth of the residual irregularities is reached, which is preferably less than 1 mm. For purposes of the present invention, it is not necessary that this covering with a first metallic layer be hermetically sealed with respect to the interstices below. According to another preferred embodiment of the present invention, step (b) consists of placing small strips or tapes having an appropriate width and thickness along the support lines of the metal shaped elements, subsequently placed in accordance to step (c). These small strips, suitably curved according to the curvature of the surface, are made of a metallic material which can be welded to the metal of said shaped elements and are preferably of the same metal. These preferably have a thickness approximately equal to that of the original undamaged coating. The width of the strips is not critical, although this makes possible the welding of the corrosion resistant coating in the subsequent step (d), so that it is carried out more easily and with a good seal. The width of these small strips is preferably between 20 and 200 mm. According to the present invention, the plates are placed on the underlying support plane without necessarily effecting any welding. Small welds can be carried out when allowed by the project and by the construction codes of the pressure body, to fix the strip until the placement of the subsequent layer. The plate is preferably inserted into an appropriate notch formed by the corroded coating, possibly reaching the underlying metal of the pressure-resistant body. The final seal of the cover is subsequently ensured by the operations carried out in steps (d) and (e), where the welds exposed to the corrosive fluid are in a sealed form. The surface between the different strips can be filled as described in the previous form of the mode of step (b), or left as such, especially if the difference in level between the surface of the plate and the neighboring area is reasonably small ( so as not to endanger the weld seal of the successive layer with excessive deformations). Step (c) of the process of the present invention comprises placing, on the prepared surface according to step (b), flat metallic elements, suitably shaped, and resistant to corrosion under the operating conditions of the equipment. In most cases and mainly in plants for the production of urea, the chemical equipment has cylindrical or at least curved sections. The above planar elements must therefore be properly curved or shaped to adapt themselves to the surface to be coated. Since these are not easily deformed, the curvature can be obtained with normal instruments available to an expert in the field. The shaped elements must be arranged next to one another to facilitate subsequent welding in step (d). The shape of the metallic elements must therefore be properly selected, so that they form a regular plane without empty spaces, after placement. A rectangular shape is preferred with most elements having the same dimensions. These should be such as to allow insertion into the internal part of the equipment through the registration hole and, at the same time, allow easy coverage of the corroded area, preferably with as few elements as possible. The present invention does not however exclude the different shapes, which can be better adapted, in each occasion, to the geometries of the corroded area. The thickness of the above metallic elements varies according to the metal from which the equipment is made and the operating conditions thereof. Thicknesses between 2 and 30 mm are preferably used. According to a preferred aspect of the present invention, the shaped pieces are made of the same metal or alloy as the original coating, or a metal or alloy that can be welded thereto. According to another preferred aspect of the invention, the metallic material of the new coating can also be non-weldable to the original coating. In this last case, in step (b) the supporting and / or maintenance strips made of a metal that can be welded to the new coating will be suitably accommodated on the corroded surface. The metal or metal alloy forming the shaped elements (and the new coating) of the present invention can each be selected from known materials resistant to corrosion under the operating conditions of the equipment. This metal or metal alloy is preferably selected from titanium, zirconium or its alloys or, particularly, from urea grade stainless steel such as, for example, AISI 316L steel (urea grade), INOX steel 25/22/2 Cr / Ni / Mo, special steels of austenite-frrita, etc. The selection of a material having a higher corrosion resistance (however measured) than the original coating, is left to the person skilled in the art. The arrangement of metallic elements Forming can be carried out using all conventional methods, available to the person skilled in the art, with the proviso that these are compatible with the operating conditions of the equipment. The mechanical fastening or welding points can normally be used. Step (d) of the method of the present invention comprises the welding of the shaped metallic elements, according to what is removed for step (c). The welding method is not critical, and any of the methods available in the known art can be used, provided that the welds have corrosion resistance and mechanical properties that are adequate to the operating conditions of the equipment. The welding is preferably carried out with arc electrodes, or "T.I.G." with wire rods. The adjacent edges of the metal elements placed next to each other, are welded together and, preferably, the same edges are welded on the underlying support surface. At least one section on the edge of each metal element is systematically left unwelded. This non-welded part, which forms a connection point between the interstitial space below each element, and that of the adjacent element, preferably has a length between 5 and 30 mm. The number of non-welded sections varies according to the type and geometry of the equipment and the existing number of drainage holes and, on average, is preferably between 1.5 and 2.5 non-welded sections per shaped element. The distribution of these is not necessarily homogeneous. In summary, for purposes of the present invention, the non-welded portions and the consequent communication points must allow a fluid released into any interstitial space beneath the new coating to reach at least one of the drainage holes present in the body. resistant to pressure, the equipment. It is not necessary, however, for all interstitial spaces (or areas) to communicate with one another, with communication with the drainage hole being sufficient. A particular aspect of the present invention relates to the case where, in step (c) the formed elements of a metal or metal alloy are used, which can not be welded to the material from which the new coating is made. In fact, in this case, unless the entire equipment is covered with the new coating, these shaped elements must be welded to be sealed to the edges of the corroded area, in order to create a stable, waterproof and resistant connection to corrosion. Perhaps, in this case the new coating can not be directly welded on the pre-existing, nor is it enough to place a strip on the edge, which is weldable to the new coating since this does not guarantee sufficient sealing against fluids at high or medium pressure . With respect to this particular aspect of the present invention, the above drawback can easily be overcome with the aid of an appropriate bi-metal strip. This preferably consists of do? strips, one of which is welded to the metal of the new coating and the other that can be welded to the metal of the original coating, which are structurally joined to one another on one side, with physical methods, thus obtaining a connection Sealed for the full length of the strip. The two strips can also be connected only on the part of the joining face, to obtain an irregular structure as shown schematically in figure 5. The hermetically sealed connection on the bimetallic strip is preferably obtained by cutting the two strips at be united against each other, according to the techniques known to those skilled in the art.

In step (e) of the method of the present invention, the weld sections interrupted between the shaped metal elements, they are covered, by placing metal strips suitably shaped on the upper part of these, and subsequently sealingly welding the edges on the underlying metal. This operation must be carried out in such a way as to ensure that the total surface exposed to the process fluids in the equipment is sealed, but that communication between the interstitial spaces is maintained under the new coating. The welding on the edges of each metal strip is carried out using one of the normally available methods and, preferably, with one of the previously mentioned methods in the description of step (d). The plates suitable for the purposes of the present invention have appropriate dimensions to cover the entire length of the interruption sections, and are preferably square or rectangular in shape.The dimensions are preferably between 20 and 200 mm. it is preferably between 4 and 25 mm These plates are preferably made of the same metal or alloy as the shaped elements which form the new coating, but may also be made of a different material, provided that it can be welded thereto. According to the method of the present invention, and with reference to what is specified above, the coverage of the welding strips according to step (e) allows the maintenance of communication of each interstitial area under the coating with at least a drain hole and, at the same time, ensures that the surface in contact with the process fluids is sealed. , the functionality of the equipment is fully restored, with the guarantee that the control and safety standards are maintained. The drainage holes originally present in the equipment are preferably extended beyond the limit of the original coating, to reach the limit of the new coating, to provide or facilitate communication with the interstitial areas. It may be occasionally necessary to produce a new drainage hole where the corroded area does not have a sufficient number of them.

According to a particular form of embodiment of the present invention, steps (c) and (d), or also steps (c), (d) and (e) can be carried out at the same time, in the sense of that each of the steps above, can be carried out independently at different points in the area subject to corrosion. For example, the positioning of the shaped elements of step (c) can be carried out in a certain area of the operation zone, and the welding of these elements (step (d)) started at the time of the placement of the others shaped elements, can be continued in a different, preferably adjacent, area. Of course, in each simple point of the corroded area, the operation according to the present invention must be carried out following the different steps in the order previously specified. The method of the present invention is suitable for the functional restoration of areas subject to corrosion, which can vary greatly in extent and depth. When the corroded areas are very widespread or even when corrosion involves the total internal surface of the equipment, the present method gives particularly advantageous results.

In particular, the method of the present invention allows the original functionality of the equipment to be completely restored, completely in accordance with safety regulations. It has also surprisingly been found that, although the new coating has a considerable number of welding lines, this does not impair its duration, thus ensuring the proper functioning of the equipment for the normal times estimated for the corresponding new equipment. The repair operation of the present method also allows excellent safety control of the complete equipment, particularly with respect to the identification of possible leaks of the new coating, without having to produce additional drainage holes in the pressure body or, possibly, produce a number very limited thereof (which does not exceed 20% of the original density of the drainage holes in the local area subject to corrosion). There is therefore the additional advantage of having a less complex repair operation. Although the present method is particularly suitable for the functional repair of the equipment operating in a plant for the production of urea, the scope of the present invention does not exclude its application, or a method equivalent thereto, to the equipment operating in different plants a pressures and / or medium or high temperatures, but which have similar problems of widespread corrosion and repair difficulties. The application characteristics of the method of the present invention can be better understood if reference is made to the drawings, diagrams and photographs shown in the appended figures, wherein: FIGURE 1 schematically represents a view of the longitudinal section of a reactor for the synthesis of urea; FIGURE 2 schematically depicts an enlarged detail of the section of Figure 1, related to the area particularly subject to corrosion, after the functional restoration of the present invention; FIGURE 3 schematically represents a detail of the longitudinal section of a reactor subject to corrosion, after the functional restoration of the corroded area using the placement of the strips in step (b) of the method of the present invention; FIGURE 4A reproduces a photograph of a welded strip on the metal sheath at the point of communication between the interstitial areas under two pieces of the sheath, whose edges are welded together and to the underlying surface; FIGURE 4B reproduces an enlargement of the detail shown in Figure 4A; FIGURE 5 schematically represents the section of a bimetallic strip, where two strips of two metals Mi and M, not weldable one to the other, are partially joined by a face to form an alternating structure.

In the figures, the corresponding parts have identical reference numbers for purposes of simplicity. In figures 1, 2 and 3 the different elements are not drawn to scale to provide a better illustration of the distinctive features of the present invention. The various appended figures illustrate the present invention, but do not limit it in any way. In Figure 1, the reactor basically comprises the pressure-resistant body in which there are the forged lids 1 and the cylindrical body 2 of multiple layers, which is internally coated with a metallic coating 4 normally resistant to corrosion. The reactor ends at the top with a registration hole 9, with a relative blind flange. On the lower side there are the input lines for ammonia 5, carbon dioxide 6, and aqueous carbamate solution 7 that come from the recycling of reactants not transformed into urea, as well as ring 3 which acts as a distributor of currents . On the intermediate lower side of the reactor, circularly accommodated around the inner liner, is the area subject to corrosion 8, whose functionality has been restored according to the method of the present invention. Any possible support and control equipment is not shown in Figure 1, nor are the drain holes or safety valves shown. In the section of the reactor represented by FIG. 2, the pressure-resistant body 2 can be observed again, and also the original coating of the reactor 4, which has undergone corrosion to an approximate depth schematically represented by section 8. on top of the corrosion area 8 is the new coating consisting of the formed elements 11, welded together on the lines 14. At the numerous points of the welding lines 14, there are the metal strips 14, welded by the edges to the elements 11. Below the strips 12, and not shown in figure 2, are the interrupted weld sections, to allow communication between the interstitial areas 15, located between the elements 11, and the support areas below these. On the sides of the reactor, four drainage holes 13 are schematically shown. Figure 3 again shows most of the previously mentioned components with reference to Figure 2, for example: the pressure-resistant body 2, the original coating of the reactor 4 , the area subject to corrosion 8, the shaped elements 11, welded to one another on the lines 14, the metal strip 12, the interstitial areas 15 and the drain hole 13. Figure 3 also schematically provides the strip 16 in the section which forms the support of the welding line 14 between two shaped elements 11, the latter not being weldable to the metal of the pre-coating 4. After this general and detailed description of the present invention, a practical example follows for its embodiment, without limiting notwithstanding the scope of the invention in any way.

EXAMPLE The repair and functional restoration was carried out with the internal anticorrosive coating of a reactor of a plant for the production of 600 tons / day of urea. This reactor operated at 230 bar and 190 ° C / with a reaction mixture comprising, under conditions at rest, NH3, CO2 and urea, water and air as a passivation agent. The reactor, according to the scheme of figure 1, is basically comprised of a vertical container consisting of a cylindrical, multilayer, pressure resistant body 2 (thickness 3 x 47 cm), having an internal diameter of 1.5 m and a length d 17.2 m, and two hemispherical forged lids at the lower and upper ends. On the upper end was the registration hole 9, circular and with a diameter of approximately 550 mm. The internal anticorrosive coating 4 was made of AISI 316L steel, urea grade and had a thickness of approximately 17 mm. The internal volume of the reactor was approximately 32,000 liters, with an internal surface area of the pressure resistant body of approximately 85 m2. In the pressure vessel there were a total of 26 drainage holes at a suitable distance from each other. A wide cylindrical segment 8, in the lower intermediate part of the reactor had suffered corrosion by a height of approximately 4 meters and a total surface area of approximately 18 m2. The corrosion involved an average depth of 11 mm in the coating. After testing the integrity of the pressure-resistant body, and ensuring that no infiltration of the process fluids had attacked the carbon steel, the surface of the coating in the corroded area was brushed to remove most of the residue from corrosion and the unstable metallic layers, possible. The most obvious irregularities of the surface, and particularly the points where the cavities due to erosion were in their deepest state, were filled with steel strips 25/22/2 Cr / Ni / Mo that had a thickness from 3 to 5 mm, fastened to the previous coating by spot welding. The less deeply corroded remnant areas were filled with AISI 316L grade urea steel weld deposit. The entire corroded surface was then reasonably smoothed by filing and knurling. A satisfactory support surface was thus obtained by practically filling the cavity due to erosion and schematized by section 8 in figure 2. The drainage holes 13 present in the corroded area were extended through the pre-existing partially cladding. corroded, and the metallic filler form fixed as described above. A new coating was fixed on the surface prepared in this way, obtained by joining the rectangular metal sheets 11 (metal elements) made of stainless steel 25/22/2 Cr / Ni / Mo having a thickness of 5 mm, appropriately curved (shaped) to favor a homogeneous and well distributed support on the surface of the reactor. Each metal sheet was secured by spot welding. The dimension of each sheet was approximately 400 x 1500 mm, so that it was easily inserted through the hole in the record. The corrosion resistance of the metal sheets was verified on the samples subjected to the anticorrosion test according to the regulation "ASTM A.262, Practice C", in which there was no corrosion phenomena. The leaves were arranged to completely cover the corroded area, until they partially overlapped, over the edge of this area / the previous coating not damaged by corrosion. The metal sheets were then welded by arc to each other, along the adjacent edges, using an alloy that has the same composition as the sheets for welding. The interrupted weld sections of 10 to 20 mm in length were left along the weld line, so that all areas below the metal sheets communicated with at least one drain hole. For this purpose, no particular geometrical regularity was followed, solely taking into account the fact that a series of communications branched around each drainage hole from one metal sheet to the other, thereby connecting all the adjacent interstices to the vicinity of each other. the successive drainage holes. The outer part of each interrupted weld section was subsequently polished with a plate, of the same nature as the reactor material, which was square in shape with a side of approximately 40-50 mm. The thickness was again 5 mm. The edges of each strip were welded in the same way as the facing sheets. Welding of metal sheets to pre-coating (made of AISI 316L grade urea, weldable to steel 25/22/2 Cr / Ni / Mo) was carried out along the entire edge of the new coating thus obtained (covering the entire corroded area), thus ensuring that the complete coating of the reactor it was hermetically sealed. At the end of the operation, each of the interstitial spaces 15 below the new coating, communicated with one or even two drainage holes without it being necessary to make any additional drainage holes, with respect to those originally existing in the body. resistant to pressure. Figure 4 shows a significant detail of the appearance of the new coating at the points where a plate has been welded covering a section of the weld interrupted between two metal sheets. At the end of the operation, the reactor was subjected to conventional control tests for satisfactory operation. In particular, the following tests were carried out: Verification of welding with penetration liquids according to the regulation "ASME VIII, div. 1, appendix 8"; Gas seal test, according to" ASME V; Article 10", carried out with helium, Pressure test, carried out by putting the internal pressure of the reactor to the value specified by the regulation of the project (320 barias).

All the previous tests gave satisfactory results.

The reactor repaired in this way was subsequently operated under standard plant conditions and continued to operate for at least two years, excluding periodic interruptions for routine maintenance. After careful examination, it turned out that there were no additional phenomena of widespread corrosion.

Claims (17)

1. A method for the repair and functional restoration of equipment of a high or medium pressure section of an industrial plant, said equipment having at least one registration orifice and comprising, internally, an anticorrosive metallic coating which has at least one extended area subject to corrosion, the method comprises the following steps: (a) the cleaning of the corroded area, with the elimination of most of the waste produced from it; (b) forming, in the cleaned area, according to step (a), appropriate support and / or fastening surfaces for the placement of a new metallic coating; (c) the placement, on these support and / or support surfaces and on the edges of the surface next to the corroded area, until it has been completely covered, with flat elements, suitably shaped to adapt to the internal profile of the equipment, and which consist of a metal resistant to corrosion under the operating conditions of the equipment and which have dimensions such as to pass through the registration hole, these shaped elements being placed next to each other; (d) Welding the adjacent edges of the elements attached to each other as in step (c) and, possibly, the welding of the same edges on the metal below them, so that the weld has at least one interrupted section for each simple shaped element, thus forming new corrosion-resistant metallic coating in wherein the interstices between each shaped element and the surface below communicate with each other and with at least one of the drainage holes comprising the body of the equipment; (e) the coverage of the surface exposed to the fluids of the process, around the interrupted welding sections, with the strips formed and suitably adjusted to size, welded by the edges on the new metallic coating, in order to obtain a surface internal of the equipment, which is totally sealed and resistant to corrosion under normal operating conditions; This repair method is completely achieved using the equipment's log hole as the only access.

2. The method according to claim 1, wherein the equipment is part of a plant for the production of urea.

3. The method according to claim 2, characterized in that the equipment is a reactor for the synthesis of urea, or a carbamate condenser, or a carbamate decomposer.

4. The method according to any of the previous claims, wherein the operating pressure of the equipment is between 10.13 MPa (100) and 25.33 MPa (250 atm).

5. The method according to any of the previous claims, wherein the equipment has a volume between 2,000 and 400,000 liters and an internal surface area in contact with the process fluids (excluding pipes) of between 8 and 450 m.

6. The method according to claim 2, wherein the anticorrosive metallic coating has a thickness between 2 and 30 mm and is basically made of a metal, or metallic alloy, selected from titanium, zirconium, lead, AISI 316L steel (urea grade) ), INQX steel 25/22/2 Cr / Ni / Mo or special austenite-ferrite steels.

7. The method according to any of the previous claims, wherein in step (c) the shaped metallic elements are rectangular and have a thickness between 2 and 30 mm.

8. The method according to any of the previous claims, wherein, in step c), the shaped metal elements are basically made of a metal or metal alloy selected from titanium, zirconium, titanium-zirconium alloys, INOX steel 25/22 / 2 Cr / Ni / Mo or special austenite-ferrite steels.

9. The method according to any of the previous claims, wherein in step (d), the sections of interrupted welding are on average between 1.5 and 2.5 in number, for each shaped metal element and have a length of between 5 and 30 mm.

10. The method according to any of the previous claims, wherein in step (e) the strips are square or rectangular, having dimensions of between 20 and 200 mm and a thickness of between 4 and 25 mm.

11. The method according to claim 10, wherein said strips are basically made of the same metal or metal alloys as the new coating.

12. The method according to any of the previous claims, wherein at the end of the repair, the same drain holes originally present in said equipment are maintained.

13. The method according to any of the previous claims, comprising in step (d) the formation of a new metallic coating consisting basically of a metal that can not be welded to the original anticorrosive coating of said equipment, wherein in step ( b) the metal strips not welded to the surface of the area subject to corrosion are placed, said strips being weldable to the metal elements forming the new coating, and their placement corresponding to the support lines thereof.

14. The method according to claim 13, wherein, in step (d), the sealed weld of the new coating on the edge of the area subject to corrosion is carried out using a bimetallic strip whose upper part is welded to the new coating and the lower part to the original anticorrosive coating of the equipment.

15. The chemical equipment comprising a recording orifice, a pressure-resistant body internally coated with an anticorrosive metallic coating having at least one drainage hole in said body, wherein, over the corroded area, a new coating is placed and seal, consisting of shaped elements, joined and welded to one another, which consist of a metal resistant to corrosion under the operating conditions of the equipment, and have dimensions such as to pass through the registration orifice, characterized in that at least a section of the interrupted weld exists on the edge of each shaped element, to allow the communication of the interstices located below the elements with at least one drainage hole, and a properly shaped metal strip is placed on each section and welded by its edges on the new metallic coating and also characterized in that said equipment is obtained average A method according to any of the preceding claims.

16. The chemical equipment according to claim 15, comprised in the high or medium pressure section of a plant for the production of urea.

17. The chemical equipment according to claim 16, which consists of a reactor for the synthesis of urea. ABSTRACT The present invention relates to a method for the repair and restoration of the functionality of the equipment subject to internal corrosion during operation at high or medium pressure in a plant for the synthesis of urea. This method includes: 1) cleaning the corroded area; 2) the formation, in the cleaned area, of appropriate support surfaces and / or glue for the placement of new metal coating; 3) the formation of new coating, sealing, anticorrosive, obtained by placing and welding flat elements and metal plates properly shaped and placed next to each other to get to adapt to the internal profile of the equipment, so that the spaces and interstices underneath this new coating, they all communicate with at least one drain hole present in the pressure resistant body. The complete repair is carried out through the equipment's record hole and makes possible the restoration of its functionality for times similar to the normal duration of newly constructed corresponding equipment.