Method of repairing by lining heat exchanger tubes

A method of repairing a defective tube in a heat exchanger of the tube-in-shell type comprises inserting a tubular sleeve into the defective tube and extending through the tube from one tubeplate to the other, and bonding the sleeve to each tubeplate. The extremities of the sleeve are preferably welded to the outer surface of the respective tubeplate. In repairing a defective tube which has one end which is bonded to a tubular tubeplate boss having a bore which is of larger diameter than the external diameter of the tube, the diameter of a poriton of the sleeve is preferably expanded to fit the bore of the tubeplate boss after insertion of the sleeve into the tube.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to heat exchangers, and particularly to a method of 
repairing a defective tube in a tube-in-shell heat exchanger, such as a 
steam generator unit (SGU) of a nuclear reactor. 
2. Description of Related Art 
A heat exchanger of the tube-in-shell type comprises a shell in which are 
mounted spaced-apart substantially parallel tubeplates having apertures 
into which open-ended tubes are welded, so that a bundle of substantially 
parallel tubes extends between the tubeplates. A first fluid, such as 
liquid sodium, passes through the shell, in contact with the outside 
surfaces of the tubes, and a second fluid, such as water, flows through 
the tubes, so that heat is transferred from one fluid to the other. 
In operation of such a heat-exchanger, a leak may develop in a tube. In the 
case of a liquid sodium cooled reactor, this will allow sodium and steam 
to mix and to produce a chemical reaction. Neighbouring tubes may be 
weakened or ruptured by this reaction. 
The conventional method of bringing an SGU back into operation following a 
tube leak is to plug that tube and, for example, at least the neighbouring 
six tubes, so that all of those tubes become inoperative. This obviously 
results in a decrease in efficiency of the heat exchanger. Furthermore, 
because heat is not being extracted from the coolant in the region of the 
plugged tubes, a hot spot is created in the tube bundle, which can cause 
stressing of further tubes around the plugged tubes. It will be apparent 
that only a limited number of leaking tubes and their neighbouring tubes 
can be plugged before the number of inoperative tubes becomes too large 
for the SGU to continue to operate. 
An alternative method of dealing with leaking tubes, which does not involve 
plugging, is disclosed in our European Patent No: 0132950. In that method, 
a short sleeve is inserted into a defective tube through one tubeplate and 
is explosively welded to that tubeplate and to the inner surface of the 
tube so that the defect is bridged by the sleeve. This reduces the flow 
area of the tube, but allows it to remain operative and still contributing 
to the heat exchange function. Since any hot spots created around these 
tubes due to the reduced flow will be substantially cooler than if the 
tubes were plugged, more leaking tubes can be repaired before the SGU has 
to be finally taken out of service. 
However, that method has certain disadvantages. Firstly, the method 
involves welding on to the tube surface, which might impair the strength 
of the tube. Secondly, the method is primarily intended for bridging a 
leaking wed between the tube and the tubeplate, and is less applicable to 
the bridging of a leak in the wall of the tube itself. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved method of 
repairing a defective heat exchanger tube. 
According to the invention there is provided a method of repairing a 
defective tube in a heat exchanger of the tube-in-shell type, comprising 
inserting a tubular sleeve into the defective tube and extending through 
the tube from one tubeplate to the other; and bonding the sleeve to each 
tubeplate. 
The sleeve is preferably formed of the same material as the tube. 
The sleeve is preferably bonded to each tubeplate by welding, and 
preferably the extremities of the sleeve are welded to the outer surface 
of the respective tubeplate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawing, a tube bundle 1 comprises a number of vertical 
tubes, such as the tubes 3, 5, extending between horizontal tubeplates 
7,9. The tubeplate 7 has integral tubular bosses 11,13 to which the tubes 
3,5 are welded at weld areas 15, 17. The tubeplate 9 has integral tubular 
bosses 19,21 the bore of which is slightly larger than the outer diameter 
of the tubes 3,5, so that the tubes can be readily inserted through the 
bosses 19,21 during assembly. The lower end of each tube is welded to its 
respective boss 19,21 at weld areas 23,25. Water enters the tubes 3,5 via 
tubeplate apertures 27,29 and passes upwards through the tubes where it is 
heated by heat transferred from liquid sodium which fills the space around 
the tubes and between the tubeplates. Steam is thereby generated, and 
leaves the tubes at their upper ends 31,33. 
Let us assume that a split 35 has occurred in the wall of the tube 5. In 
order to bridge the split, a sleeve 37 is inserted into the bore of the 
faulty tube 5, the sleeve extending over the full length of the tube, so 
that its ends 39,41 are level with, or stand just proud of, the outer 
surface 43,45 of the respective tubeplate. The lower end of the sleeve is 
expanded to fit the bore of the tubeplate aperture 29. The ends 39,41 of 
the sleeve 37 are welded to the respective tubeplates 7,9 at the outer 
surfaces 43,45 thereof. The sleeve is thereby sealed to each tubeplate. 
The sleeve 37 is preferably formed of the same material as the tube 5. The 
wall thickness of the sleeve is determined by the creep strength required 
during the remainder of the expected life of the SGU. Hence, a repair 
effected late in the life of the SGU can use a slightly thinner-walled 
sleeve than one effected earlier, and can therefore provide more efficient 
heat transfer. The outer diameter of the sleeve 37 is chosen to give a 
minimal gap between the outer surface of the sleeve and the inner surface 
of the faulty tube 5, but sufficient to allow smooth insertion of the 
sleeve. The inner surface of the tube 5 is preferably cleaned with acid, 
to remove some corrosion, before the sleeve is inserted. 
The sleeve can be inserted while the SGU is still in situ. 
The present invention has advantages over the method described in our 
above-mentioned European patent. Due to the use of a full-length repair 
sleeve in the faulty tube, it is totally immaterial where the fault has 
occurred; it may be at the tube/tubeplate boss weld area or it may be 
anywhere along the length of the tube. Furthermore, more than one fault in 
a tube can be bridged by a single sleeve. Since the welding of the sleeve 
is effected at the tubeplates, no welding to the tube wall is required. 
Furthermore, the welding positions are both very accessible. The welding 
may be effected by any suitable method, or other bonding methods might be 
acceptable. 
The invention therefore provides a repair method which is simple, is 
applicable to all kinds and positions of tube leaks and is of high 
integrity. The tube after repair should be still capable of operating 
under sustained full load conditions. 
The method is suitable for use in repairing other heat exchangers of the 
tube-in-shell type, besides those used in liquid sodium cooled nuclear 
reactors.