TECHNICAL FIELD

The present invention relates to a device according to the preamble of

Claim 1, for protecting condenser tubes against erosion by the water- droplet-containing steam that flows rapidly into a condenser.

TECHNICAL FIELD

In order to utilize the energy contained in the steam used in a steam power plant to the greatest extent possible, the steam is normally conducted from the low pressure turbine of the plant to a condenser. . The steam is caused to condense in the condenser under a pronounced subpressure, by cooling the condenser with cold water. This cooling water is conducted through a large number of cooling tubes which are mounted in groups or bundles and which extend horizontally in the condenser housing. The cooling tubes of each group are placed in mutually parallel and closely adjacent relationship. Since the steam is condensed primarily within the groups while the majority of the heat is removed from the narrow spaces defined between the closely adjacent cooling tubes, the largest subpressure occurs in this region. In the case of large power plants, the velocity of the steam at the condenser inlet may be in the order of 200 m/s, and the steam flowing into said condenser seeks a path to these regions of highest subpressure. In doing so, the steam is forced to pass between the outermost cooling tubes in the groups of cooling tubes, the so-called rim tubes. Because the steam contains water droplets which cannot be guided in between the rim tubes, but which strike against said tubes at the same velocity as that of the steam arriving at the tubes, the water droplets cause erosion on the outwardly-facing sides of the rim tubes. After a time, the erosion, which is due to fatigue of the grain

» boundaries, will be so extensive as to form penetrating holes in the tube walls. When salt water is used as cooling water, these holes _{ will permit salt to enter into the condensate which is reused as supply water in a closed water circuit and form harmful deposits on the heating surfaces of the plant. Consequently, it is necessary to

inspect this damage constantly, in order to rectify the damage before erosion of the cooling tubes has gone too far.

Large plants are normally shut-down once a year, for inspection purposes. Rim tubes that have suffered erosion damage are then plugged, or repaired in some other way. If damage is discovered between these shut- down periods, the damage is often rectified during the operating or running season, by plugging the eroded rim tubes.

For this operation, however, the plant needs to be shutdown or temporarily slowed down.

Since this period in which it is necessary to stop or slow down the running of the plant often falls within a time of the year in which the energy produced by the plant is most desired, this loss in production, together with the reduced efficiency caused by plugging of the rim tubes, means that erosion damage results in high economic losses.

U.S. 3,568,763 teaches a device which partially solves the erosion problem. This device comprises a structure in the form of a ridge roof which is mounted immediately above each group of cooling tubes. The device protects the uppermost rim tubes of a respective group against the steam that enters directly from the condenser inlet, but affords small protection to the rim tubes located on a lower level. A further drawback is that the device cannot be used with existing steam power plants without reconstructing the plants at high costs.

Another known protective device has the form of a grid which is mounted immediately outside the rim tubes of a group of cooling tubes, so as to take the force of the erosion attack. This solution, however, results in high losses due to the resistance presented to the flow of steam.

DISCLOSURE OF THE INVENTION

Technical Problems

There has long been the need for a device which will protect particularly the rim tubes of condensers against erosion and which can be fitted to existing structures in steam power plants without incurring heavy costs. The device shall be capable of protecting all types of cooling tubes in a tube group, shall result in the smallest possible power loss and shall have a long useful life. The device shall also be suitable for use in both new plants and existing plants, and in the latter case shall be capable of being fitted during the operational season of the plant without requiring the plant to be shut down for long periods of time.

Solution

The object of the present invention is to provide a device of the kind mentioned in the introduction which will solve the aforesaid technical problems. This object is achieved in accordance with the invention with a device having the characteristic features set forth in the following Claims.

In accordance with the invention, there is proposed a device which is comprised of a casing in the form of a shell which is intended to cover that part of a condenser tube which is subjected to erosion. For the sake of illustration, the shell can be considered to have the form of a so-called half-tube which is secured to each cooling tube to be protected, so as to be located on the side of said tube which is subjected to erosion or exposed to the risk of erosion. The casing has a length which is appropriate with respect to its position and also with respect to the length of the free cooling-tube section to be protected, etc. The half-tube includes that part of a circular "base tube" of preferably uniform thickness which is defined by two generatrices on the "base tube" with a centre angle of about 180°. To enable the half-tube, or shell, to be attached to the cooling tube, said shell is provided with mutually converging devices which project from the long sides of the shell and which are preferably made from the same material and have the same thickness as said shell. These attachment devices first extend generally parallel with one another and are then bent in towards one another into engagement with the rear side of the cooling tube, thereby providing means for holding the

shell firmly on the cooling tube. When the attachment devices are also bent inwards to an extent such that the intrinsic tension or spring forces will urge the shell against the cooling tube, the shell can be brought into intimate contact with said tube, when the shape of said shell conforms with the shape of the cooling tube, due to a wide contact surface and a satisfactory contact pressure over said surface. These spring or tension forces also contribute to the generation of frictional forces which ensure that the shell will be affixed against rotation on the cooling tube. The ability of the device to fasten to the cooling tube depends, among other things, on the extent to which the outermost parts of the device that are in contact with the cooling tube extend around said tube, i.e. on the centre angle encompassed by the device. This angle should be at least 180° in order for the device to fasten at all. Although a better attachment is obtained at greater angles, serious drawbacks occur when the angle exceeds 300°, for instance drawbacks in the form of long mechanical levers on the attachment devices and a large covered area on the cooling tube. Furthermore, difficulty may be experienced in opening out or widening the device sufficiently when fitting the device to a cooling tube, as described below.

According to one development of the invention, it is also proposed that the shell is made of an elastic material, of uniform thickness, and is configured so that, when fitted, it includes that part of the "base tube" which is defined by two generatrices having "a centre angle" slightly above 180°, preferably between 200° and 220°. In this instance, the cup-shaped bands which form continuations of the contemplated shell function as securing devices along its long sides.

In order to provide the intrinsic spring force in the fastener devices, the shell or casing is given a generally circular or possibly a slightly elliptical cross-sectional shape prior to being fitted, with an inner diameter which is slightly smaller than the outer diame¬ ter of the cooling tube. When the shell or casing is widened to a state in which it is fitted to the cooling tube, the whole of the inner surface of the shell or casing, including that part which is considered as the fastening means, will assume essentially the shape

of the cooling tube and will obtain the aforesaid retaining property and said intimate contact with said cooling tube.

When the shell has a relatively small wall thickness and is made of a material, such as a metal alloy, which has good spring properties, the shell can also be widened to an extent which will enable its mutually parallel long sides to be fitted over the cooling tube from one side thereof without plastically deforming the shell. When the shell strives to spring back to its original shape, the shell will partially embrace the cooling tube, as before mentioned.

The inventive fastening devices projecting out from the shell are not limited to parts of a "base tube". For instance, the fastener devices may have the form of strip-like extensions which first extend in mutually parallel relationship through a given distance and then folded in towards one another and into contact with the cooling tube. Although an advantage is afforded when the fastening devices directly abut the rear side of the cooling tube, it may be beneficial in some cases to place some form of insert between the fastener devices and the cooling tube, for instance in order to increase the pressure of the fastener devices against the cooling tube when fitting the shell. Neither need the fastener devices have the form of strips or bands which extend along the long sides of the shell. For example, the fastener devices may, instead, comprise a plurality of mutually spaced narrower tongues.

When, for some reason or other, greater tension is desired, it may be necessary to use a stronger material. In this case, it will not be possible to widen the shell to an extent which will enable it to be fitted over a cooling tube from one side thereof, without plastically deforming the shell. Closure of the casing or shell and the development of the intrinsic tension can then be achieved by pressing the casing or shell when fitting the same. This can be effected, for instance, by interrupting the shell with an Omiga-shaped fold which is located on the intersection of the shell with the symmetry plane and the flanks of which are pressed towards one another, such as to deform the shell plastically along the fold apex. Another method of closing

the shell around the tube, is to incorporate a memory metal in the device.

Advantages

The inventive device can be used both in planned plants and in existing plants for preventing erosion of cooling tubes and for protecting rim tubes which have already been eroded from further damage, therewith to avoid long plant shut-down periods during the plant operating season. If erosion has caused penetrating holes in the tubes, a good and effective seal can be obtained with the aid of glue or a locking liquid between the device shell and the cooling tube. This would also improve locking of the device against rotation. The device is inexpensive to produce and can be fitted quickly and simply. In the case of the preferred device, the device is pushed over the cooling tube from one side thereof by exerting pressure on the spine of the device, whereupon the device will open and then spring back around the tube.

As a result of the resultant contact pressure and the possibility of using thin shells which cover only half the surface of the exposed rim tubes, the cooling ability of the tubes is not impaired to any appreciable extent. Neither do the thin shells increase the flow losses of the steam between the rim tubes to any appreciable extent. Tests and calculations have shown that when the casings or shells are used over the full length of the rim tubes, between their points of attachment in support plates and distribution boxes, no flow-induced vibrations will occur which are liable to give rise to harmful oscillations or wear between the shells and respective cooling tubes.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive will now be described in more detail with reference to a preferred exemplifying embodiment thereof and also with reference to the accompanying drawings.

Figure 1 is a vertical, sectional view of a condenser.

Figure 2 illustrates a cooling tube of the condenser shown in Figure 1, fitted with a protective device constructed in accordance with the invention.

Figure 3 illustrates the protective device shown in Figure 2 t

Figure 4 and 5 illustrate alternative protective devices.

BEST MODE OF CARRYING OUT THE INVENTION

Figure 1 is a cross-sectional view of the upper part of a condenser section 1 which includes a cooling tube group 2 comprised of a number of cooling tubes 3 from which heat is removed by means of internally- flowing cooling water. For the sake of clarity, the illustrated group is shown to contain fewer cooling tubes than it would in practice. The cross-sectional view is taken on a plane which extends between two support plates which function to support the cooling tubes 3. The distal plate of these support plates is referenced 4 in the Figure.

The used steam exiting from the low pressure turbine 5 of the plant flows through the condenser inlet 6 and into the condenser section 1. The steam flow at the inlet 6 has a significant velocity which, in the case of large plants, can be in excess of 200 m/s. The steam flow divides and seeks entry into the group of cooling tubes 2 from different directions, between the peripherally located rim tubes 7 (as indicated by the arrows in Figure 1). The used steam contains small water droplets which are not guided in between the rim tubes 7, but which strike the rim tubes 7 at a velocity corresponding to the inlet velocity of the steam, and gradually result in needle-like pits or holes and other erosion damage to an extent such as to form penetrating holes through the tube walls.

In order to counteract this erosion, the present invention proposes , the use of protective devices 8 which in the case of the preferred embodiment (see Figure 2) are configured as tubular shells having a £ centre angle v which, when the shell is fitted, is about 210° and a length which is somewhat shorter than the distance between the support plates 4. The protective device is made of a thin, elastic material,

such as acid-proof stainless steel, which has good spring properties and is highly resistant against this type of corrosion. In the electrogalvanic stress chain, the material also lies close to the material from which the cooling tubes are made, in the present case titanium, and hence crevice or contact erosion between the protective device and its associated cooling tube need not be feared. The protective device can be manufactured directly, by rounding strip material in a known manner, or indirectly by dividing standard tubes along a generatrice and thereafter pressing the tube into the shape intended. The protective device is given a generally circular-arcuate cross-sectional shape with an inner diameter which is slightly smaller than the outer diameter of the cooling tube to be protected, and consequently with a larger centre angle than in its fitted state (see Figure 3).

When the protective device is to be fitted around a cooling tube, the device is simply snapped over the cooling tube from one side thereof, so as to be positioned over those parts of the tube which are exposed or subjected to erosion damage. In order to ensure that the gap between two adjacent rim tubes will not be decreased more than is necessary, the protective devices are suitably placed in positions in which only one of the devices will encroach on this gap (see Figure 1).

When the dimensions of the protective device are correctly chosen, it is possible, when fitting the device, to first open out said device against its intrinsic spring restoring force and then allow the device to spring back over and around the cooling tube so as to obtain a high contact pressure against said tube and to ensure good heat transfer and and an effective locking action.

Erosion on the lower cooling tubes in the group is restricted to a narrower region than on the upper cooling tubes. Consequently, the width of the shell can be made smaller. In this case, in order to ensure that the protective device, or shell, will sufficiently embrace the cooling tube, the device may be provided with separate fastener devices which extend out from the shell. Figure 4 illustrates a protective device 9 provided with tongues 10 which are attached to

both long sides of the shell. Figure 5 illustrates a protective device 11 in which fastener devices on one edge are included by the shell and on the other edge have the form of tongues 12. As illustrated in the drawing, the tongues may be formed from parts of the same tube as that from which the shell was formed.