Corrosion-resistant bimetallic tube and its use in tube bundle equipment

Abstract

A bimetallic tube consisting of at least one tubular element in a first metal resistant to the corrosive and/or erosive action of a process fluid with which it is put in contact, having at least one end, or an area close to an end, externally coated with a layer of a second metal, different from the first and more suitable, with respect to this, for being seal-welded to a support. Tube bundle equipment to be used for thermal exchange operations at high temperatures and pressures, under conditions of high aggressiveness of the process fluids, wherein the tube bundle comprises at least one tube having the above characteristics. Said equipment is particularly used as a heat exchanger and decomposer, for example as a stripper, in the cycle of urea synthesis processes where there are conditions of high pressure, high temperatures, high aggressiveness of the process fluids, and in which the tube bundle consists of at least one tube having the above characteristics.

1 : A bimetallic tube resistant to the corrosive action of a process fluid with which it is put in contact on its internal surface, comprising a first tubular element E 1  (1) facing said internal surface, consisting of a metal M 1  selected from Zr, Ta, Nb and Al or an alloy of said metals, homogeneously extending for the whole length of the tube, and at least a second tubular element E 2  (2), consisting of a second metal or alloy M 2  different from M 1 , arranged circularly outside said first tubular element, in a position close to one of its ends, for a section of less than a third of the length of the tube itself, and seal attached to said element E 1 . 
     
     
         2 : The bimetallic tube according to  claim 1 , comprising two of said tubular elements E 2 , of the same material or different materials, each situated close to one of the ends of the tube. 
     
     
         3 : The bimetallic tube according to  claim 1 , wherein said element E 1  consists of zirconium or an alloy with at least 60% by weight of zirconium. 
     
     
         4 : The bimetallic tube according to  claim 1 , wherein said metal M 2  of the element E 2  is selected from titanium, a titanium alloy and urea grade stainless steel. 
     
     
         5 : The bimetallic tube according to the previous  claim 4 , wherein said metal M 2  is selected from titanium or a titanium alloy. 
     
     
         6 : The bimetallic tube according to  claim 1 , comprising, in addition to said elements E 1  and E 2 , at least a further metallic tubular element positioned around E 1  and in contact with its outer surface. 
     
     
         7 : The bimetallic tube according to  claim 6 , wherein said further tubular element, consisting of a metal selected from titanium, a titanium alloy or a stainless steel, is contiguous to E 2  and extends in the central section of the tube itself. 
     
     
         8 : The bimetallic tube according to  claim 7 , wherein said further tubular element consists of a metal compatible with the welding with the metal M 2  of E 2 . 
     
     
         9 : The bimetallic tube according to  claim 1 , wherein the central section of said element E 1  has a thickness ranging from 1 to 15 mm. 
     
     
         10 : The bimetallic tube according to  claim 1 , wherein each element E 2  has a length ranging from 0.2 to 20% of the total length of the tube. 
     
     
         11 : The bimetallic tube according to  claim 1 , wherein said element E 2  has a uniform thickness ranging from 1 to 15 mm. 
     
     
         12 : A method for the production of the bimetallic tube according to  claim 1 , starting from a tube E 0  comprising for the whole of its length at least one tubular element E 0 , consisting of said metal M 1 , comprising the following steps:
 a) arrangement of the outer surface of at least one of the ends of E o , for a length sufficient for receiving a second tubular element E 2 , so as to produce a suitable insertion seat, preferably having an outer diameter of the tube smaller than the initial diameter;   b) positioning of a tubular element E 2 , having a length equal to or less than a third of the length of E o , consisting of a metal M 2  different from M 1 , arranged as a ring around at least a segment of said insertion seat;   c) joining of the surfaces of the metals M 1  and M 2 , for at least a part of the contact surface between them, so as to form a seal joint on the whole perimeter of the bimetallic tube thus obtained.   
     
     
         13 : The method according to  claim 12 , wherein, in step a), a metallic layer having a thickness ranging from 0.1 to 2 mm is removed from the insertion seat. 
     
     
         14 : The method according to  claim 12 , wherein, in said step c), the two metals M 1  and M 2  form a metallurgical bond with each other extending over all the contact area of the surfaces of the elements E 1  and E 2 . 
     
     
         15 : Use of the tube according to  claim 1 , in equipment for processing corrosive fluids in an industrial plant. 
     
     
         16 : Use according to  claim 15 , for the manufacturing of a tube bundle for heat exchange inserted in said equipment. 
     
     
         17 : Use according to  claim 15 , wherein said equipment forms an ammonium carbamate stripper in the high pressure cycle of plants for the production of urea. 
     
     
         18 : Industrial equipment for processing a highly corrosive fluid at high pressure, characterized in that it comprises at least one bimetallic tube, preferably a series of bimetallic tubes, according to  claim 1 , whose internal surface is put in contact with said fluid. 
     
     
         19 : The equipment according to  claim 18 , consisting of a tube-bundle heat exchanger. 
     
     
         20 : The equipment according to  claim 19 , wherein said tube bundle comprises from 100 to 6,000 bimetallic tubes having a diameter ranging from 10 to 100 mm. 
     
     
         21 : The equipment according to  claim 18 , comprising a tube plate on which said bimetallic tubes are welded, which is coated with at least one metallic layer resistant to the corrosion of said process fluid. 
     
     
         22 : The equipment according to  claim 21 , wherein said metallic coating layer consists of a metal M 3  compatible with the metal of the element E 2  of said bimetallic tube. 
     
     
         23 : The equipment according to  claim 22 , wherein said metallic coating layer is seal welded with said element E 2 . 
     
     
         24 : The equipment according to  claim 22 , wherein said coating and said element E 2  consist of a metal M 2  selected from titanium or a titanium alloy. 
     
     
         25 : A method for repairing or improving tube-bundle chemical equipment suitable for the treatment of a corrosive fluid, wherein said fluid is in contact with the internal wall of the tubes forming the tube bundle, comprising the substitution of at least one of said tubes with a bimetallic tube according to  claim 1 . 
     
     
         26 : The method according to  claim 25 , wherein said equipment comprises a tube plate coated with a metal selected from titanium, a titanium alloy or stainless steel. 
     
     
         27 : The method according to  claim 25 , wherein said equipment forms a stripper in the high pressure cycle in a urea synthesis process. 
     
     
         28 : The method according to  claim 25 , comprising the removal of at least one of the pre-existing tubes, cleaning of the cavity thus formed in the tube plate, insertion of a bimetallic tube according to the present invention, having a suitable length, in each cavity, positioning the mouth of each tube that it protrudes for a short distance, usually from 3 to 50 mm, and welding the coating of the tube plate with the outer surface of the element E 2  of each tube.