Robot for maintenance of a heat exchanger

Abstract

A robot 10, for traversing the exterior of tubes (22, Fig.2) of a furnace convection bank to transport a maintenance implement for inspection or cleaning, comprises a chassis 12 for supporting the mai

Description

FIELD OF THE INVENTION The present invention relates to a robot for maintenance of the tubes of a heat exchanger. BACKGROUND In many industrial plants, such as refineries, a fluid is heated by flowing through a heat exchanger, also referred to as a convection bank, comprising coiled tubes over which pass the flue gases of a furnace. In some cases, the tubes are bare radiant tubes having smooth outer surfaces, while in others each tube is a finned convection tube having closely spaced fins projecting from its outer surface to increase the surface area of the tube and thereby improve the heat transfer. Because of incomplete combustion of the fuel burned in the furnace, a deposit of soot and other combustion by-products can form on the tubes or between the fins, which, if allowed to build up, causes a serious deterioration in efficiency. To maintain good performance, it is therefore necessary to inspect and to clean the outer surfaces of the tubes in a furnace convection bank periodically. It is known to use a robot to clean such furnace convection bank tubes. GB2585500, for example, discloses a robot for cleaning the exterior of tubes of a furnace convection bank, and comprises a carriage supporting a cleaning lance and motorized traction assemblies for engaging the tubes and advancing the carriage along the furnace convection bank. Convection banks often additionally comprise baffle plates that support the tubes at intervals along their length and maintain the desired separation between them. Such baffle plates may protrude beyond the tubes of the bank, so as to present an obstruction to the movement of the robot and necessitate manual intervention to reposition the robot past the obstruction. OBJECT OF THE INVENTION The present invention seeks to reduce the need for human intervention and allow autonomous operation of the robot for more prolonged lengths of time. SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a robot for traversing the exterior of tubes of a furnace convection bank to transport a maintenance implement for inspection or cleaning, the robot comprising a chassis for supporting the maintenance implement, and a plurality of traction devices for frictionally engaging the exterior of the tubes to advance the robot in a direction parallel to the axes of the tubes of the bank, wherein the traction devices are movably mounted relative to the chassis in such a manner as to allow selected traction devices to be raised from contact with the tubes of the bank while maintaining clearance between the chassis and the bank, the robot being movable relative to the bank by the traction devices that remain in contact with the tubes of the bank while the selected traction devices are raised. To prevent the chassis from toppling when selected traction assemblies are raised, retractable supports may be provided on the chassis between the traction assemblies. In some embodiments, the supports may comprise omniwheels mounted on arms that are pivotable relative to the chassis to permit the support omniwheels to be moved between a tube engaging deployed position and a retracted position. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a view of the carriage and drive system of a robot capable of moving transversely to the length of a furnace convection bank, with the maintenance implements omitted in the interest of clarity, Figure 2 is a front view of the robot of Figure 1 positioned on tubes of a convection bank, Figure 3 is a view of a robot like that of Figure 1, but comprising additional support wheels to enable the robot to negotiate an obstacle preventing it from advancing along the length of the tubes of the convection bank, and Figure 4 is a view showing the robot of Figure 3 in an alternative configuration. DETAILED DESCRIPTION Figure 1 shows the carriage of a robot 10 designed to transport maintenance implements, i.e., equipment for cleaning or surveying the tubes of the coiled tubes of a convection bank. The cleaning or inspection equipment is not shown in the drawing as the present invention is only concerned with the carriage of the robot and its drive system. The carriage of the robot 10 comprises a chassis 12, and a plurality of individually controllable traction assemblies 14 mounted on the underside of the chassis 12. The chassis 12, which may be a thin and rigid sheet of any suitable type of metal, plastics, or composite, is appropriately sized to carry all the components of the robot, such as traction assemblies 14, circuitry, motors, sensors, and any required implements such as a high-pressure lance for cleaning tubes of a convection bank, or a camera for surveying the condition of said tubes. The maintenance implements are movably supported by the chassis so that they may be lowered to extend between the tubes of the bank. As the chassis 12 serves for mounting components, it may include such holes, slots, grooves and protrusions as necessary. Each traction assembly 14 comprises a traction device in the form of a wheel 16, described below in more detail, that is driven by a respective motor. The motor and its drive circuitry are protected by a cover 24 and cannot therefore be seen in the drawing. The entire traction assembly 14 is mounted on an arm 18 that is pivotable relative to the chassis 12 about an axis parallel to the axis of rotation of the wheel 16. Each of the traction devices 16 is a mecanum wheel, such wheels being known in the art. The outer surface of each such wheel, i.e., the surface that frictionally engages the tubes of the convection bank, is composed of a plurality of circumferentially spaced rollers each rotatable about an axis that lies in a tangential plane but is inclined at an angle of around 45° to the axis of rotation of the wheel. As a result, the frictional force between the wheel and the tubes of the bank has components both parallel to, and perpendicular to, the axis of rotation of the wheel 16. Mecanum wheels are handed as the inclination of the circumferential rollers in the tangential plane can be such as to cause the wheel to veer either to the right or to the left when rolling forward. The robot 10 in Figures 1 and 2 has four mecanum wheels, two at one end (the front end, seen on the right in Figure 1) and two at the other or rear end. The two front wheels are oppositely handed relative to one another, as are the two rear wheels. Furthermore, the left front and rear wheels are oppositely handed relative to one another, as are the right front and rear wheels. Transverse movement Hitherto, as previously mentioned, when cleaning a furnace convection bank, an operator of the robot needed to position a robot so that it would follow a path dictated by the tubes of the convection bank. As the robot in the prior art was only capable of advancing in the longitudinal direction of the tubes, once it reached the end of the convection bank, or met an obstruction, the operator needed to reposition it to follow the path of the next set of tubes. To avoid this requirement for manual operator intervention, the robot 10 shown in Figures 1 and 2 can move in a direction transverse to that of the tubes. This results in the robot being able to clean a full furnace convection bank without the need for manual intervention. While it would be possible to provide the robot 10 with separate traction devices dedicated to longitudinal movement and transverse movement, respectively, the use of four mecanum wheels 16, as shown in relation to the embodiment of Figures 1 and 2, enables both directions of movement to be achieved using only one set of wheels and motors. Figure 2 shows a front view of the robot 10 of Figure 1 situated on tubes 22 of a furnace convection bank. The wheel designated 16a in Figure 2 is therefore the left front wheel and the wheel 16b the right front wheel. If, as viewed in Figure 2, the wheel 16a is rotated clockwise and the wheel 16b is rotated counterclockwise, then both wheels will tend to move the robot backwards (out of the plane of the paper) and the transverse forces will cancel each other out. If the direction of rotation of the two wheels is reversed, then the robot will move forwards, parallel to the axes of the tubes, as the transverse forces will once again cancel each other out. During this time, the rear wheels are correspondingly rotated to advance the robot in the same direction as the front wheels 16a and 16b. If, however, in Figure 2, the two front wheels 16a and 16b are driven clockwise as viewed, the two transverse components of the frictional forces will reinforce one another and the front end of the robot will tend to move to the right, as viewed. At this time, one of the front wheels will be driving the robot forwards but the other will be driving it backwards and, were it not for the rear wheels, there would be a net torque tending to rotate the robot about a vertical yaw axis. However, if the rear wheels at the rear driven in a similar way at the same time, they too will tend to move the robot to the right as viewed in Figure 2, but the torque that they create tending to cause yaw of the robot will oppose the torque of the front wheels and there will be no net torque. In this way, by appropriately selecting the direction of rotation of each of the four mecanum wheels, it possible to drive the robot forwards and backwards, as well sideways in both directions. After traversing the length of the furnace convection bank, the robot of Figure 1 is therefore capable of moving sideways to the next pair of tubes and continuing with the operation that it is performing, without the need for human intervention. While the use of mecanum wheels provides an elegant solution to the problem of the enabling the robot to move between pairs of tubes, other mechanisms may be used. For example, regular wheels may be used as traction devices, if some are orientated to roll in the longitudinal direction and others in the transverse direction, the different wheels being movable relative to the chassis between deployed and retracted positions. To avoid the need for the wheels to be retractable, they may be constructed as omniwheels, which provide traction in one direction but permit free movement in the transverse direction. Obstacle navigation As earlier mentioned, convection banks may have obstacles, such as baffle plates, that prevent a robot from traversing the entire length of a bank. Hitherto, the robot would stop on encountering such an obstacle and it would require human intervention before its operation could be continued. The embodiment shown in Figures 3 and 4 is adapted so that it may autonomously overcome an obstacle such as a baffle plate. This embodiment, in addition to the various components previously described by reference to Figures 1 and 2, comprises omniwheels 20 that serve to provide rolling support for the chassis 12 but are not driven by motors and do not therefore provide any traction. In the same way as the mecanum wheels 16, the omniwheels 20 are mounted on arms 18 that are pivotable relative to the chassis 12 about axes parallel to the axes of rotation all the various wheels 16 and 20. In this way, each of the mecanum wheels 16 and the support wheels 20 is retractable from a deployed position, as shown in Figure 3 to a raised position, as shown for the support wheels 20 in Figure 4. The length of the arms 18 and the degree of pivoting is sufficient to allow any mecanum wheel 16 or support wheel 20 to be raised above any obstacle, such as a baffle plate, lying in its path. Though the different wheels 16 and 20 are shown as mounted on pivotable arms, it will be clear to a person skilled in the art that any other mechanism capable of up and down movement may be used as an alternative. For example, the wheels may be mounted on a sub-frame capable of being raised and lowered vertically, such as by a hydraulic mechanism or a motor driven screw, or rack and pinion, mechanism. It is preferable for the arms that carry the different wheels 16, 20 to be pivotable about axes axis that lie substantially parallel to the axis of the tubes 22 on which the robot 10 is configured to be used, as this allows the wheels 16, 20 to be lifted higher and increase ground clearance, and therefore also allows the robot 10 to negotiate taller obstacles. The wheels 16, 20 may be retractable individually or they may be retractable in pairs. When retractable in pairs, each pair comprises the wheels 16 on the same longitudinal position on the robot, i.e., the wheels 16 on the front row are paired, the supporting wheels 20 on the second row are paired, and so on. To negotiate an obstacle when it is encountered, it is first necessary to raise any implement that is currently lowered into the convection bank. The robot is then advanced with its front traction devices 16 raised until they clear the obstacle and one set of the support wheels 20 encounters the obstacle. At this time, the traction devices 16 that have already cleared the obstacle are lowered and the first set of support wheels 20 is raised clear of the obstacle. The robot the advances and the same procedure is repeated until each pair of support wheels and the rear traction devices have cleared the obstacle. At all times, two pairs of traction devices are in contact with the convection bank to enable the robot to be advanced and the chassis needs to be sufficiently supported not to topple and to remain horizontal. The above description merely describes a small number of possible embodiments that fall within the scope of the invention. It will be appreciated that many of the embodiments can readily be combined, and this should be considered as disclosed, especially where only small modifications or adaptations are required.

Claims

1. A robot for traversing the exterior of tubes of a furnace convection bank to transport a maintenance implement for inspection or cleaning, the robot comprising:a chassis for supporting the maintenance implement, anda plurality of traction devices for frictionally engaging the exterior of the tubes to advance the robot in a direction parallel to the axes of the tubes of the bank,wherein the traction devices are movably mounted relative to the chassis in such a manner as to allow selected traction devices to be raised from contact with the tubes of the bank while maintaining clearance between the chassis and the bank, the robot being movable relative to the bank by the traction devices that remain in contact with the tubes of the bank while the selected traction devices are raised.

2. A robot as claimed in claim 1, wherein the traction devices are mounted on arms that are pivotable about an axis generally parallel to the axes of the tubes of the convection bank.

3. A robot as claimed in claim 1 or 2, wherein retractable supports are provided on the chassis between the traction devices to prevent the chassis from toppling when selected traction devices are raised.

4. A robot as claimed in claim 3, wherein the supports comprise omniwheels mounted on arms that are pivotable relative to the chassis to permit the support omniwheels to be moved between a tube engaging deployed position and a retracted position.

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