Method for prestressing tubes of a heat exchanger with precise tailoring of the prestress

Abstract

In order to prestress a tube ( 4 ) of a heat exchanger in tension in the axial direction, with precise tailoring of the prestress, at least one of the end parts ( 4 a, 4 b ) of the tube ( 4 ) is expanded diametrally using a hydraulic pressure while the tube ( 4 ) is fixed at its ends in the tube plates ( 3 a, 3 b ). The value of the prestress in each of the tubes ( 4 ) of the heat exchanger is tailored by altering the initial radial clearance between the external surface of the end part ( 4 a, 4 b ) of the tube ( 4 ) and the internal surface of the hole ( 5 ) passing through the tube plate ( 3 a, 3 b ) in which hole the end part ( 4 a, 4 b ) of the tube ( 4 ) is engaged before diametrally expanding the end part. The end part ( 4 a, 4 b ) of the tube which runs substantially over the entire length of the hole ( 5 ) passing through the tube plate ( 3 a, 3 b ) is hydraulically diametrally expanded. The initial radial clearance ( 6 ) between the tube ( 4 ) and the hole ( 5 ) is tailored to obtain longitudinal contraction of the tube ( 4 ) and therefore a prestress of a precisely determined value. The invention applies in particular to once-through heat exchangers.

1 . Method for prestressing, in longitudinal tension, tubes ( 4 ) of a heat exchanger ( 1 ) comprising at least one straight part and fixed at their ends ( 4 ′ a ,  4 ′ b ) in holes ( 5 ) passing through at least one tube plate ( 3   a ,  3   b ), in which the tensile prestress is obtained by hydraulically diametrally expanding at least one end part ( 4   a ,  4   b ) of each of the tubes ( 4 ) of the heat exchanger fixed at its two ends ( 4 ′ a ,  4 ′ b ) in the at least one tube plate ( 3   a ,  3   b ) in such a way as to induce deformation through longitudinal contraction of the tube ( 4 ) by subjecting the internal surface of the tube to a hydraulic pressure and in such a way that the prestress of the tube is tailored to a precise value, characterized in that the value of the prestress in each of the tubes ( 4 ) of the heat exchanger ( 1 ) is tailored by altering the initial radial clearance ( 6 ) between the external surface of the end part ( 4   a ,  4   b ) of the tube ( 4 ) and the internal surface of the hole ( 5 ) passing through the tube plate ( 3   a ,  3   b ) in which hole the end part ( 4   a ,  4   b ) of the tube ( 4 ) is engaged before diametrally expanding the end part ( 4   a ,  4   b ) and in that the tube ( 4 ) is hydraulically diametrally expanded along the entire end part ( 4   a ,  4   b ) of the tube which extends substantially over the entire length of the hole ( 5 ) passing through the tube plate ( 3   a ,  3   b ).  
     
     
         2 . Method according to  claim 1 , characterized in that hydraulic diametral expansion is performed on a first end part ( 4   a ) of the tube ( 4 ) which part is introduced into a through-hole ( 5 ) in a first tube plate ( 3   a ) to which one end ( 4 ′ a ) of the tube ( 4 ) is fixed, the second end ( 4 ′ b ) of the tube ( 4 ) being free inside a through-hole ( 5 ) of a second tube plate ( 3   b ), and in that a second diametral expansion is performed on the second part of the tube ( 4   b ) which part is introduced into the through-hole ( 5 ) in the second tube plate ( 3   b ), the tube ( 4 ) being fixed, at its two ends ( 4 ′ a ,  4 ′ b ) respectively, to the first tube plate ( 3   a ) and to the second tube plate ( 3   b ), respectively, the deformation through longitudinal contraction of the tube and the prestress being obtained solely during the second operation of diametrally expanding the second end part ( 4   b ) of the tube ( 4 ).  
     
     
         3 . Method according to  claim 1 , characterized in that a first diametral expansion is performed on a first end part ( 4   a ) of the tube ( 4 ) which part is introduced into the opening ( 5 ) of a first tube plate ( 3   a ), and a second diametral expansion operation is performed on a second part ( 4   b ) of the tube ( 4 ) inside the opening ( 5 ) of a second tube plate ( 3   b ), the tube ( 4 ) being fixed at a first end ( 4 ′ a ) and at a second end ( 4 ′ b ), respectively, to the first tube plate ( 3   a ) and to the second tube plate ( 3   b ), the deformation through longitudinal contraction of the tube ( 4 ) and the prestress being obtained cumulatively by the first diametral expansion operation and the second diametral expansion operation.  
     
     
         4 . Method according to any one of claims  1 ,  2  and  3 , characterized in that the heat exchanger ( 1 ) is a once-through heat exchanger comprising a first tube plate ( 3   a ) and a second tube plate ( 3   b ) which plates are fixed in arrangements spaced apart in the longitudinal direction of the straight tubes ( 4 ).  
     
     
         5 . Method according to any one of  claims 1  to  4 , characterized in that the initial radial clearance ( 6 ) between the external surface of the end part ( 4   a ,  4   b ) of the tube ( 4 ) and the internal surface of the hole ( 5 ) passing through the tube plate ( 3   a ,  3   b ) is altered by tailoring the diameter with which the hole ( 5 ) through the tube plate ( 3   a ,  3   b ) is pierced, bearing in mind the tolerances on the outside diameter of the tube ( 4 ).  
     
     
         6 . Method according to any one of  claims 1  to  5 , characterized in that the at least one tube plate ( 3   a ,  3   b ) has a number of through-holes ( 5 ) for the mounting of tubes ( 4 ) of a heat exchanger bundle and in that the prestress in the various tubes ( 4 ) of the heat exchanger bundle is tailored by altering the diameters with which the holes ( 5 ) through the tube plate ( 3   a ,  3   b ) are pierced.  
     
     
         7 . Method according to  claim 6 , characterized in that the heat exchanger ( 1 ) is a once-through heat exchanger comprising two tube plates ( 3   a ,  3   b ) of circular overall shape placed parallel to one another at a certain distance apart and through which there pass holes ( 5 ) which are uniformly distributed between a central zone and a peripheral zone of the circular tube plate ( 3   a ,  3   b ), characterized in that the initial radial clearance ( 6 ) between the end parts of the tubes ( 4 ) and the surface of the openings ( 5 ) is altered by piercing openings ( 5 ) with diameters determined according to the clearance and the prestress required for the tubes fixed in the holes ( 5 ) distributed around the tube plate ( 3   a ,  3   b ).  
     
     
         8 . Method according to  claim 7 , characterized in that the holes ( 5 ) intended to accommodate the tubes with which the bundle of the heat exchanger ( 1 ) begins to be assembled are generally pierced with a smaller diameter than the holes ( 5 ) intended to accommodate the tubes ( 4 ) of the bundle of the heat exchanger ( 1 ) with which assembly of the bundle terminates.  
     
     
         9 . Method according to  claim 8 , characterized in that the holes passing through the tube plates ( 3   a ,  3   b ) have diameters which are generally different in the central part and in the peripheral part of the circularly-shaped tube plate.  
     
     
         10 . Method for prestressing, in longitudinal tension, tubes ( 4 ) of small diameter and long length comprising at least one straight part, so as to increase the buckling margin and reduce induced vibrations, and which are fixed at their ends ( 4 ′ a ,  4 ′ b ) in holes ( 5 ) passing through at least one tube plate ( 3   a ,  3   b ), in which method the tensile prestress is obtained by hydraulically diametrally expanding the ends ( 4   a ,  4   b ) of each of the tubes ( 4 ) so as to induce deformation through longitudinal contraction distributed uniformly along the length of the tube ( 4 ) by subjecting the internal surface of the tube to a hydraulic pressure and in such a way that the prestress of the tube is tailored to a precise value by predetermined diametral expansion of the end part ( 4   a ,  4   b ) and that the tube ( 4 ) is hydraulically diametrally expanded.