GB2094435A - Repairing tubes - Google Patents

Abstract

To repair a defective heat exchange tube 2 in a tube-in-shell heat exchanger a sleeve 3 is inserted in the tube 2 to bridge the defect. The sleeve 3 is made of a material, such as nickel titanium alloy, which exhibits a shape memory when subjected to a restorative heat treatment. The sleeve 3 is preformed and heat treated to impart a memory configuration which provides a sealing interference fit with the tube 2. <IMAGE>

Description

SPECIFICATION Tube-in-shell heat exchangers This invention refers to tube-in-shell heat exchangers.

A heat exchanger of the tube-in-shell kind comprises a shell having at least one tube sheet into which open-ended tubes are welded. In use a first fluid is passed through the tubes whilst a second fluid is passed through the shell to effect heat transfer between the fluids. When a leak develops in a tube it is common practice to make the tube redundant by plugging but where numerous tubes are made redundant the heat exchanger loses efficiency. In British Publication No 2032559 there is disclosed a method of repairing heat exchanger tubes comprising inserting a tubular sleeve within a defective tube and sealably bonding the end regions of the sleeve to the tube and tube sheet to bridge the defect.

The preferred bonding method is to braze one end of the sleeve to the tube and explosively weld the other end of the sleeve to the tube plate.

Unfortunately, where a large number of defective tubes are required to be repaired full repair of the heat exchanger is a time consuming operation and it is an object of the present invention to reduce the time for effecting repair to a heat exchange tube by insertion of a tubular sleeve.

According to the present invention in a method of repairing a tube of a tube-in-shell heat exchanger wherein the defect is bridged by insertion of a tubular sleeve, the sleeve is made of an alloy which exhibits a shape memory and which on application of restorative heat treatment will tend to resume its memory configuration and thereby expand into sealing contact with the heat exchange tube. One kind of alloy which exhibits a shape memory is a nickel titanium alloy containing between 53 and 57 weight per cent nickel, generally known as 55-Nitinol the restorative heat treatment temperature depending on the composition of the alloy.

In the repair of a tube-in-shell heat exchanger used for the generation of steam by heat exchange with liquid alkali metal flowing through the shell of the heat exchanger, the preferred "memory" alloy composition is one for which the restorative heat treatment temperature is less than the normal operating temperature of the liquid alkaline metal, approximately 4000C.

A method according to the invention of repairing a tube of a tube-in-shell heat exchanger is described, by way of example, with reference to the sole accompanying drawing which is a fragmentary sectional view.

In the drawing there is shown a tube sheet 1 and a tube 2 of a tube-in-shell heat exchanger for effecting heat transfer between liquid alkali metal at temperature of about 4000 C flowing through the shell and water flowing through the tubes.

There is a multiplicity of tubes 2 each of which is sealed to the tube sheet by welding. In use, some of the tube to tube sheet welds become defective and in order to avoid making them redundant by plugging, a tubular sleeve 3 is inserted within each tube having a defective weld 4. The sleeve is made of an alloy which exhibits a shape memory, for example, a nickel titanium alloy in general having 53 to 57 weight per cent of nickel, the alloy and its properties being disclosed in USP 3174851. The "memory" configuration is such that the sleeve forms an interference fit with the tube and tube plate but before fitting is strained to a configuration that will render it a close sliding fit to the tube and tube plate.On putting the heat exchanger into normal service the operating temperature of the liquid alkali metal is such that the sleeve will restore to its "memory" configuration and thereby effect a sealing fit with the tube and tube plate. In one example the sleeve of nickel titanium is preformed to a stepped configuration such that it would form an interference fit with the complementary sealing surfaces and whilst constrained in that form the sleeve is heated to, and quenched at, a "memory" temperature of approximately 5000C thereby imparting a modified "memory" configuration.

After cold swaging the sleeve to render it a close sliding fit in the tube and tube plate it is fitted within the tube to bridge the defect and on reaching the normal operational temperature of the heat exchanger the sleeve restores to its modified "memory" configuration whereby it makes an effective seal.

By providing a supply of preformed sleeves which can be inserted readily in the heat exchanger tubes, the heat exchanger can be put back into service quickly without the need for individually bonding the sleeves to the tubes by welding and brazing thereby considerably reducing the time during which the heat exchanger is out of service.

1. A method of repairing a defective heat exchange tube of a tube-in-shell heat exchanger, the method comprising inserting a tubular sleeve in the tube to bridge the defect, the sleeve being of an alloy which exhibits a shape memory, and heat treating the sleeve to restore it to its memory configuration whereby it expands into sealing contact with the heat exchange tube.

2. A method according to claim 1 comprising the steps of preforming the sleeve to a configuration which will form an interference fit with the complementary sealing surfaces of the tube, imparting a modified memory configuration by constraining the sleeve in said configuration and heating to its memory temperature followed by quenching, cold forming the sleeve to a configuration which renders it a close sliding fit in the tube, inserting the sleeve in the tube and heating it to its memory temperature whereby the sleeve restores to its modified memory configuration.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Tube-in-shell heat exchangers This invention refers to tube-in-shell heat exchangers. A heat exchanger of the tube-in-shell kind comprises a shell having at least one tube sheet into which open-ended tubes are welded. In use a first fluid is passed through the tubes whilst a second fluid is passed through the shell to effect heat transfer between the fluids. When a leak develops in a tube it is common practice to make the tube redundant by plugging but where numerous tubes are made redundant the heat exchanger loses efficiency. In British Publication No 2032559 there is disclosed a method of repairing heat exchanger tubes comprising inserting a tubular sleeve within a defective tube and sealably bonding the end regions of the sleeve to the tube and tube sheet to bridge the defect. The preferred bonding method is to braze one end of the sleeve to the tube and explosively weld the other end of the sleeve to the tube plate. Unfortunately, where a large number of defective tubes are required to be repaired full repair of the heat exchanger is a time consuming operation and it is an object of the present invention to reduce the time for effecting repair to a heat exchange tube by insertion of a tubular sleeve. According to the present invention in a method of repairing a tube of a tube-in-shell heat exchanger wherein the defect is bridged by insertion of a tubular sleeve, the sleeve is made of an alloy which exhibits a shape memory and which on application of restorative heat treatment will tend to resume its memory configuration and thereby expand into sealing contact with the heat exchange tube. One kind of alloy which exhibits a shape memory is a nickel titanium alloy containing between 53 and 57 weight per cent nickel, generally known as 55-Nitinol the restorative heat treatment temperature depending on the composition of the alloy. In the repair of a tube-in-shell heat exchanger used for the generation of steam by heat exchange with liquid alkali metal flowing through the shell of the heat exchanger, the preferred "memory" alloy composition is one for which the restorative heat treatment temperature is less than the normal operating temperature of the liquid alkaline metal, approximately 4000C. A method according to the invention of repairing a tube of a tube-in-shell heat exchanger is described, by way of example, with reference to the sole accompanying drawing which is a fragmentary sectional view. In the drawing there is shown a tube sheet 1 and a tube 2 of a tube-in-shell heat exchanger for effecting heat transfer between liquid alkali metal at temperature of about 4000 C flowing through the shell and water flowing through the tubes. There is a multiplicity of tubes 2 each of which is sealed to the tube sheet by welding. In use, some of the tube to tube sheet welds become defective and in order to avoid making them redundant by plugging, a tubular sleeve 3 is inserted within each tube having a defective weld 4. The sleeve is made of an alloy which exhibits a shape memory, for example, a nickel titanium alloy in general having 53 to 57 weight per cent of nickel, the alloy and its properties being disclosed in USP 3174851. The "memory" configuration is such that the sleeve forms an interference fit with the tube and tube plate but before fitting is strained to a configuration that will render it a close sliding fit to the tube and tube plate.On putting the heat exchanger into normal service the operating temperature of the liquid alkali metal is such that the sleeve will restore to its "memory" configuration and thereby effect a sealing fit with the tube and tube plate. In one example the sleeve of nickel titanium is preformed to a stepped configuration such that it would form an interference fit with the complementary sealing surfaces and whilst constrained in that form the sleeve is heated to, and quenched at, a "memory" temperature of approximately 5000C thereby imparting a modified "memory" configuration. After cold swaging the sleeve to render it a close sliding fit in the tube and tube plate it is fitted within the tube to bridge the defect and on reaching the normal operational temperature of the heat exchanger the sleeve restores to its modified "memory" configuration whereby it makes an effective seal. By providing a supply of preformed sleeves which can be inserted readily in the heat exchanger tubes, the heat exchanger can be put back into service quickly without the need for individually bonding the sleeves to the tubes by welding and brazing thereby considerably reducing the time during which the heat exchanger is out of service. CLAIMS

1. A method of repairing a defective heat exchange tube of a tube-in-shell heat exchanger, the method comprising inserting a tubular sleeve in the tube to bridge the defect, the sleeve being of an alloy which exhibits a shape memory, and heat treating the sleeve to restore it to its memory configuration whereby it expands into sealing contact with the heat exchange tube.

2. A method according to claim 1 comprising the steps of preforming the sleeve to a configuration which will form an interference fit with the complementary sealing surfaces of the tube, imparting a modified memory configuration by constraining the sleeve in said configuration and heating to its memory temperature followed by quenching, cold forming the sleeve to a configuration which renders it a close sliding fit in the tube, inserting the sleeve in the tube and heating it to its memory temperature whereby the sleeve restores to its modified memory configuration.

3. A method according to claim 2 wherein the tubular sleeve is of a nickel titanium alloy containing between 53 and 59 percent nickel.

4. For repairing a defective heat exchange tube in a tube-in-shell heat exchanger, a method substantially as hereinbefore described.