EP0023937B1

EP0023937B1 - Soot blowing system

Info

Publication number: EP0023937B1
Application number: EP79301627A
Authority: EP
Inventors: Herb Michelson
Original assignee: Individual
Current assignee: Individual
Priority date: 1979-08-10
Filing date: 1979-08-10
Publication date: 1984-07-04
Also published as: DE2967085D1; EP0023937A1

Description

This invention relates to cleaning systems for use in removing deposits of soot, dirt and debris from the surfaces of heat exchange tubes used in connection with furnaces or boilers.
The heat exchange tubes are customarily provided in banks or clusters. The hot combustion gases in the boiler or furnace pass over these banks of tubes which absorb the heat therefrom and thus heat the contents of each tube in the usual heat exchange function. In so doing, the exterior of the tubes collect dust or soot emitting from the combustion zone of the furnace or boiler which makes the tubes less effective as heat exchange units unless thoroughly cleansed from time to time.
To remove such deposits, it has long been customary to use air or steam under high pressure to literally blow the deposits off the tubes and thus prevent the deposits from becoming a layer of insulation which would reduce the exchange or transfer of heat by the tubes and adversely affect their function. Devices for doing this are usually referred to in the trade as soot blowers.
There are in the main two known types of soot blower in commercial use.
One of these commercially used soot blowers utilises a tube, called a lance, which is rigidly secured to the housing containing the tube banks to extend permanently across the full width of the tube banks. The lance is provided with fluid emission openings or nozzles arranged thereover at pre-determined spaced distances. High pressure steam is introduced into the lance and this steam emits through the various nozzles and jets of steam impinge on the tubes to blow away the soot, dust and other debris. Drive means disposed exteriorly of the housing rotate the lance continuously through successive 360° cycles so as to blow the soot and debris away in a complete circle around the lance.
This type of soot blower has various disadvantages: It has been found that the soot blowing range of any one lance is very limited because the steam velocity decreases rapidly as it leaves each nozzle. Thus a pluraliity of the lances must be mounted in proximity to each other at respective soot blowing stations along the length of the banks of tubes which is a relatively costly arrangement. In addition, the lances remain interiorly of the housing carrying the banks of tubes when not in use. Thus the lances are exposed to the condition or atmosphere of the hot corrosive flue gases at all times, which causes the lances to deteriorate and necessitates eventual replacement of the lances or use of an expensive corrosion resistant alloy.
Furthermore, the lances, as aforesaid, are multi-nozzled. In fact, customarily, nozzles are provided every 200 to 300 mm. along the length of each lance in order to assure its effectiveness. When this system is used in extremely wide furnaces, many nozzles are required. It has been found that in order to be effective, each nozzle must pass steam at a rate of thousands of pounds an hour to provide a high momentum needed to effectively clean the banks of tubes. Given the number of steam nozzles required per lance, it is necessary to install an expensive steam generator because of the high capacity required simultaneously for all the nozzles. Since soot blowing is carried out for a small fraction of the time that the boiler or furnace operates, such expensive steam generator is idle most of the time.
Another soot blower in current commercial use is one which also employs a lance mounted at a fixed location on the housing for the tube banks but in which the lance is slidably supported in a stationary casing so as to be insertable and retractable, within an aperture in the housing by rotation of a threaded guide engaging a nut attached to a rear portion of the lance. Such a soot blower is described in U.S. Patent No. 2,696,016. In this soot blower, the lance, which has two opposed nozzles at its terminus, rotates continuously through successive 360° cycles while it is being advanced across the entire width of the tube banks. A reversing mechanism is provided to retract the lance after it reaches the end of its forward travel and it is brought back to a position outside the housing containing the tube banks.
Because there are only two nozzles in the lance of this soot blower, the steam generation capacity required is much smaller and there is a consequent saving in capital investment. In addition, the stored position of the lance outside of the housing when not in use prevents it from becoming subject to the destructive effects of the hot corrosive flue gases.
However, the aforesaid soot blower still has certain disadvantages. In particular, a plurality of lances insertable into the housing through apertures at respective soot blowing stations at pre-determined spaced intervals along the length of the tube banks are required because the effective area which may be cleaned by each lance is quite limited extending only to about a 3 or 4 foot radius (0.9 or 1.2 m).
Variations on the type of soot blower described in U.S. Patent No. 2,676,016 are disclosed in French Patent No. 1,421,870 and U.K. Patent No. 1,160,439. These variations are similar to the arrangement of U.S. Patent No. 2,696,016 in that the site of the lance is fixed and the lance is both displaceable and rotatable within an aperture in the housing for the tube banks. However, the mounting for the lance is different. That is, it is supported on a carriage which is movable towards and away from the housing along a fixed beam secured outside the housing and which displaces the lance within the housing aperture as a result of such movement. A plurality of these lances are supported from respective beams at stations at intervals along the housing for cleaning the full length of the banks of tubes therein.
A significant disadvantage of all these known soot blowers is the number of lances which are required in order to achieve effective cleaning.
The soot blower of the present invention overcomes this problem and presents a system which requires only a minimum amount of parts while nonetheless effectively cleaning the surfaces of the tube banks in an extremely efficient manner.
According to the present invention there is provided a soot blowing system for cleaning banks of tubes or the like disposed in a housing having apertures, the system comprising a movable carriage located externally of the housing for moving a lance mounted on said carriage, drive means operable for displacing the lance axially within a respective aperture in the housing and for rotating the lance within the respective aperture, and means for connecting the lance to a fluid supply whereby fluid under pressure may be passed through at least one fluid emission opening in the lance, characterised by said carriage being movable between a selected plurality of the apertures in the housing, by additional drive means operable for moving the carriage between said selected plurality of the apertures, and by control means arranged to operate sequentially the additional drive means to move the carriage automatically from one selected aperture and to stop the carriage automatically at the next,selected aperture and the first drive means to automatically insert the lance into and move the lance within each selected aperture and to automatically withdraw the lance from each selected aperture.
Thus, the present invention provides an automatic soot blowing system in which only a single lance is employed, this lance being controlled automatically for movement to each of a plurality of soot blowing stations and movement within apertures in the heat exchange tube housing at each of these stations.
In U.S. Patent No. 3,184,774, which relates to a checker cleaning operation rather than a soot blowing operation, there is disclosed a cleaning system having a single lance movable on a carriage from one aperture to another in a checker brick housing and insertable successively into each aperture. However, in this case, movement of the carriage and of the lance is manually controlled.
The embodiments of the present invention described below feature a step by step sequential cleansing operation in which a flexible hose is employed for the supply of fluid. In addition, the lance of these embodiments may have only two opposed fluid emission openings or nozzles and thus requires only a low steam generating capacity for cleansing purposes with the consequent large saving in steam generation capacity.
In one such embodiment, the lance does not rotate or turn in any way while travelling into and advancing within a particular aperture and steam cleaning an area of each tube bank above and below the lance. In this version at the end of its initial travel in the aperture, the lance is rotated between approximately 5° and approximately 20° and is then gradually retracted while continuing to emit steam for cleansing purposes during its retraction. The flexible hose to which the lance is attached is also rotated between 5° and 20° in this step. At the end of the retraction step, which stops short of complete withdrawal from the housing, areas covered by arcs between 5° and 20° on the tube banks above and below the lance have been cleansed. At this point, the lance is rotated or twisted an additional 5° to 20° in which position it advances again into the housing and steam continues to be emitted through the nozzles for cleansing an additional area. At the end of this advancing step, the lance makes an additional 5° to 20° rotation or twist and is retracted with steam emitting therefrom for cleansing thoroughly further arcuate areas. These sequential cleansing operations at differing angles of the lance ultimately provide a total rotation of about 180° by the lance which results in cleansing of the entire area of the tube banks above and below the lance.
After the final pass of the lance to clean the tubes has been accomplished at a particular station, the lance is completely removed from the housing and the steam is shut off. The carriage utilised in the present invention then moves the lance to the next station. While the lance and attached hose may be returned to the original "untwisted" condition before entering the next aperture, it is preferable that the lance and the hose remain in the "twisted" condition upon entering the next aperture and then be rotated in a reverse direction in sequential increments of about 5° to 20° as aforesaid so that the flexible tube to which the lance is attached untwists and finishes the second cycle in the straightened condition that it was in at the outset of the previous cycle.
In another version of this invention, the lance enters the housing containing the tube banks at a first station but does not travel immediately to the far side of the tube banks. Instead, the lance enters the housing a relatively short pre-determined distance equivalent to the length of the arc of 5° to 20° set forth above with its nozzles facing substantially horizontally. The travel of the lance is stopped and the lance is turned or twisted over an arc up to 180°. The nozzles thus emit steam over a segment of the tube banks around the lance. The lance is then moved forward another pre-determined increment also equivalent to the length of the aforesaid arc of from 5° to 20° and turned or twisted in a direction reverse to that of the previous turn or twist above described. As the steam is emitted through the apertures or nozzles, the next segment of the tube banks around the lance is also cleaned. This step by step sequential procedure continues over the entire width of the bank of tubes whereupon the lance is withdrawn and moved with the carriage to the next cleaning station. The lance is then reinserted into the housing and the above described sequential operation continues.
These embodiments of the present invention utilise sequential steps of slightly different form to accomplish cleaning. The relatively rigid lance utilises a flexible hose attached thereto, as by a clamp or other attaching means, for the supply of steam and the overall 180° twist required for each cycle in the cleaning operation can be accommodated by the flexible hose without the requirement of any seal. This twist in no way blocks the passage in the hose. And the aforedescribed cycles can be continued in each embodiment throughout the entire length of tube bank until cleaning of the entire tube bank is accomplished.
While two nozzles are employed in connection with these embodiments of the invention, a greater number may be utilised, or only one may be used with the lance rotating through 360° in total.
The soot blower of the present invention is described further, by way of example, with reference to drawings, in which:

Fig. 1 is a perspective view of the system of the present invention showing a furnace or flue housing, a carriage for conveying a soot blower lance and an illustrative steam supply area;
Fig. 2 is a sectional view taken along the lines 2-2 of Fig. 1 and showing in dotted lines a direct connection between the lance and a steam supply hose;
Fig. 3 is a perspective view showing an initial position of the lance in one aperture in the housing and, in dotted lines, a subsequent position of the lance and also showing in dotted lines the rotation of the lance as it reaches the end of its travel between banks of the tubes;
Fig. 4 is an end view of the lance showing the area covered by a fluid emission opening in the lance at the end of its insertion between the tube banks;
Fig. 5 is a diagram showing the path of the lance in the sequence of steps illustrated in Figs. 2 and 3 and showing the incremental rotation of the lance during such sequence;
Fig. 6 is a perspective view illustrating a modified form of cleansing operation accomplished by the lance showing in dotted lines the rotation of the lance at different locations of the tube banks;
Fig. 7 is a diagram showing the path of the lance in the sequence of steps illustrated in Fig. 6;
Fig. 8 is a schematic view illustrating one form of control and drive arrangement to accomplish the various sequential steps of the cleansing operation of the system as shown in Figs. 1-5.

As illustrated in the drawings, there is shown a housing 10 through which hot flue or exhaust gases flow. This housing may be the furnace housing or it can be located in an area adjacent thereto for heat exchange purposes as is well-known in the art. Tubes 11 are disposed within the housing 10 and are distributed in banks 12. The tubes 11 become covered with soot and other debris as a result of combustion in the furnace when in use. This soot and debris provides undesirable insulating cover and detracts from the conventional heat exchange function of the tubes. The system of the invention provides relatively inexpensive means for cleaning the tubes and removing the deposited soot and other debris to permit the tubes to function effectively.
The system in accordance with the invention, designated 13, comprises a movable carriage 17 disposed outside the housing 10 adjacent a wall 14 thereof separating a compartment 15 containing the system from the interior of the housing 10. The system 13 comprises a track 16 which carries the movable carriage 17 having wheels 18 riding on the track 16. Secured to the upper part of the carriage is a lance 19 which travels with the carriage along the track 16 to pre-determined desired positions of respective soot blowing stations. These positions in each instance are aligned with one of a plurality of apertures 20, 20a, etc. in the wall 14 of the furnace or flue housing 10. The apertures make accessible the banks of tubes 11 disposed within the housing.
When the movable carriage is located adjacent to a respective aperture 20 at the first soot blowing station, the carriage 17 comes to a stop and the lance 19 is automatically thrust through the aperture 20 into the interior of the furnace or flue housing 10.
In the method of operation of the cleaning system of the present invention shown in Figs. 3-6, the lance 19 is inserted into the housing through the aperture in the direction of the arrow shown in Fig. 3. The lance is substantially hollow and, as hereinafter set forth, is at pre-determined times filled with steam under pressure through a flexible steam hose 21.
The lance 19 is directly connected to the flexible steam hose 21 by means of a clamp 22. While a clamp has been illustrated, it is to be understood that any suitable means for affixing the steam hose to the lance is within the purview of this disclosure. There is no extraneous or separate seal required between the lance 19 and the flexible hose 21. The lance 19 is preferably provided near its terminal end with two diametrically opposed fluid emission openings 23 and 24. The steam hose 21 is connected to a condensation hose ending in steam trap 25 to prevent condensation from entering the furnace which could cause damage to the housing 10.
As the lance is inserted into the housing 10, steam under pressure is admitted to the lance and is emitted from the lance through the openings 23 and 24. This steam cleanses arcuate areas of the tubes in the path of its emission. The arcuate area of each tube bank 12 which is cleaned amounts to an area which is covered by an arc of from almost 5° to 20° from the respective opening 23 and 24.
When the lance reaches approximately the side of the housing 10 opposite the aperture 20, it is atuomatically twisted approximately 5° to 20° in the direction of the arrow shown in Fig. 4 while steam is still constantly being emited from the openings 23 and 24. The hose twists as the lance rotates. The lance is then in position for retraction and automatically begins the return while cleansing the ares of the tube banks 12 adjacent the heretofore serviced area described with relation to the first pass. This operation effects cleansing an additional area covered by an arc between 5° to 20°. The retraction of the lance 19 is automatically terminated prior to the time when the fluid emitting openings 23 and 24 reach aperture 20. At this position, the lance is again automatically stopped and rotated an additional 5° to 20° in the same direction as previously. Forward movement of the lance in the direction of the arrow shown in Fig. 3 is again resumed thereby cleaning an additional area of each tube bank covered by an arc of from 5° to 20° from each opening 23 and 24 by steam which is continuously emitting through these openings.
The sequential steps of forward movement, arcuate rotation, retracting movement, arcuate rotation and forward movement, etc. continues throughout the pattern diagramatically set forth in Fig. 5 until each of the fluid emitting openings 23 and 24 has rotated approximately 180° while the lance remains within the respective aperture 20. Thus, a complete 360° area is cleansed by the steam in this cycle of the lance 19. The steam hose supplying the steam to the lance will be twisted only 180° during this operation which, in view of the length of the hose, has no deleterious or adverse effect.
At the end of the aforedescribed sequence, the lance is totally withdrawn from the respective aperture 20 at which time a hinged door 26 closes the aperture to prevent the flow of gases in and out of the housing 10 and the supply of steam to the lance is automatically discontinued. The carriage 17 is then automatically moved along the track to the next adjacent aperture 20a where an automatic stop terminates movement of the carriage 17. In this second position, the lance is automatically reinserted into this next adjacent aperture 20a and the cleansing operation is continued. However, in accordance with the invention, the operative sequence does not follow the dia- gramatic arrangement set forth in Fig. 5 but instead reverses the turning direction. The 5° to 20° increments in rotation of the lance are in a direction opposite to the direction or rotation set forth with regard to the first cycle and thus opposite to the direction of the arrows in Fig. 5. As the advance, turning, retraction, turning, advance sequence develops, the steam hose 21 attached to the lance for its steam supply is untwisted and returns to its normal straight condition.
As a consequence, the hose need never be twisted through more than approximately 180° while nevertheless accomplishing all of the desired cleansing of the tube banks in sequential steps entering each housing aperture in turn. The aforesaid arrangement completely prevents any blockage of the passage of steam which might result from any over-tisting of the supply hose during rotation of the lance. In fact, the 180° twist can be easily accomplished with little stress or strain on the steam hose supply.
There is little, if any, need for replacement of parts and the system can operate efficiently over indefinite periods of time. Only one lance is required to service the entire length of even a huge tube cluster housing area. The steam generation capacity is small since only two fluid openings are used. In addition, in this version there are no extraneous rotary seals necessary because there is no 360° continuous rotation of the lance around the supply tube.
Thus, an efficient system is provided to overcome the problem of efficiently and economically cleansing heat exchange tubes in the path of the hot flue gases.
It is understood that, the steam emitting sequence of the lance can be so timed that steam is emitted only when heat exchange tubes are in the path of the fluid openings of the lance. Thus, in the illustration shown in Fig. 3, no steam need be emitted from the lance when the openings 23 and 24 are on a substantially horizontal plane as opposed to those vertical planes in which the tube banks 12 would be in the path of the emitted steam.
Furthermore, the lance need not be removed from aperture 20 and inserted in the next aperture 20a before it is rotated to untwist the steam hose 21. In other words, either prior to or during its withdrawal from aperture 20, the lance may be automatically rotated in a direction opposite to that of the incremental rotation shown in Fig. 5 so that the hose is untwisted and straightened before it enters aperture 20a. Upon entry into aperture 20a, it will be understood that the lance can again rotate in the incremental steps shown in Fig. 5 or alternatively rotate incrementally in the opposite direction.
While a lance having two fluid emission openings has been shown, it is understood that the lance may have a single such opening and be rotated over 360° in the described 5° to 20° increments as such rotation will not twist the elongate steam hose 21 in such fashion as to prevent it from properly functioning. The sequential steps described above would be repeated. However, the lance would pass through a total of 360° with the steam being emitted when necessary to clean the tube banks in the path of the lance opening or whenever desired.
A modified form of the cleaning cycle is illustrated in Figs. 6-7. Fig. 6 shows the lance inserted into the aperture 20 in the wall 14 of the housing 10. The lance is carried by the same carriage 17 riding on tracks and has the same steam hose arrangement as in the embodiment illustrated in Figs. 1 and 2. In this embodiment, however, the lance 19 is inserted into the housing 10 to a lesser extent in the first portion of the cycle than is the case in the cycle of Figs. 3-5. As illustrated in Fig. 7 the initial advance is from the "start" position to position 30. The lance at this point is then rotated approximately 180° to position 31 shown in Fig. 7, which cleanses the tube banks in a 360° area in the vicinity of the lance itself because the two fluid emitting openings 23 and 24 each rotates 180°. The lance is then automatically inserted further into the housing to position 32 in Fig. 7 where it is rotated 180° in the direction of the next arrow which is reverse to that of the rotation between position 30 and 31. When the lance has made this rotation to position 33, it is again moved forward into the housing to position 34 and rotated in the same direction as the rotation accomplished between positions 30 and 31, as is indicated.
The aforesaid sequential steps continue through the entire movement of the lance into the housing as diagramatically illustrated in Fig. 7 until it reaches approximately the wall of the housing opposite the aperture 20. At this position, the lance makes its final 180° arcuate sweep and then is automatically withdrawn from the housing whereupon the steam feed is automatically stopped. Thus, cleansing takes place witin the housing through the complete 360° in proximity to the lance as it progresses inwardly.
The lance is then carried forward on the carriage 17 to aperture 20a where it is again inserted into the housing and the sequence described hereinbefore and shown in the dia- gramatic illustration in Fig. 7 is repeated. The operation continues until the entire length of the banks of tubes is cleansed from the debris and soot and maintained in functioning and operational condition. Again, in this operation the steam hose is not twisted by any increment exceeding approximately 180°. Further, there is little stress or strain and all of the benefits described with reference to the form of the invention shown in Figs. 3-5 are accomplished and obtained by this modified form.
It is to be noted that increments of travel of the lance into the housing 10 between points 31-32, 33-34, etc. are substantially equivalent of the length of each 5° to 20° arc traversed by the lance as shown in Figs. 4 and 5. This accomplishes a total cleansing of the tubes in a highly efficient manner. It is also to be understood that steam may be emitted from the lance only at such time when tube banks 12 are in the path of the fluid openings 23 and 24 in order that energy be conserved.
Fig. 8 is a simplified diagram of the electrical arrangement including the motors and switching structure utilised in connection with the embodiment of Figs. 1-5.

Three motors may be involved as follows: Motor I - for effecting the travel of the lance in and out of the housing; and
Motor II - for effecting the rotation of the lance; and
Motor III - for driving the carriage along the tracks outside the housing.

All three motors are essentially standard Direct Current motors so that when the current to the motor is reversed, the motor turns in the opposite direction. Also, the motors each have a gear drive and require only a fraction of a horsepower for the necessary functions.
Each motor is supplied with current through a number of switches. These switches are multiple pole, double throw. They are capable of performing one or both of the following two functions, depending on the way they are wired:

1. Reversing the current to a motor.
2. Interrupting the supply of current to a motor until another switch in the circuit is actuated. This is similar to the multiswitch home light circuit where any one switch when actuated determines whether current is supplied to a light bulk or not.

The cycle set forth with relation to the embodiment of Figs. 1-5 is started by actuating switch A (interrupt) which is mounted anywhere convenient to the operating personnel. This supplies current to the carriage travel Motor III. The carriage travels until it reaches the first soot blowing station, at which position switch B (interrupt) engages a strikerplate. Switch B interrupts the current to Motor III and passes the current to Motor I propelling the lance into the housing. After the head of the lance is in the housing, switch C (interrupt) is actuated which opens the solenoid valve admitting the steam to the lance. Just immediately before the end of the inward stroke, switch D (interrupt) is actuated which will start the lance rotation Motor II. Just after that, switch E (reverse) is actuated which causes Motor I to retract the lance. Then switch F (interrupt) is actuated which stops the rotation caused by Motor II. The lance then is withdrawn from the housing and properly positioned stops rapidly actuate switches D, E and F so that the lance travels inward again after a slight rotation. After completing an in and out cycle covering almost 180° the lance travels withdraws from the housing. There are no stops located in the path of the outgoing lance to activate any of the switches which previously caused the lance to reverse and rotate. As the lance is withdrawn, switch C is activated, stopping steam flow. Once the lance is withdrawn from the housing, a striker plate on the lance actuates switch G thereby causing Motor I, which was operating to move the lance 19 outwardly from the housing 10, to stop and starting Motor III. The carriage 17 is thereupon moved until it reaches the next soot blowing station where a striker plate has been positioned to reactuate switch B which stops Motor III and starts Motor I. The lance continues its outward movement and immediately thereafter engages a striker plate to actuate switch E and reverse the lance travel so that it begins to move through the aperture 20a and into the housing 10. That striker plate also actuates switch H which reverses the current to Motor II. In this manner, when the lance reaches the end of its inward movement it will rotate in a direction opposite to that of the previous rotational steps. The aforedescribed series of steps continues until all the soot blower stations at each aperture have been serviced. When this is accomplished, the carriage travel Motor III will begin to drive the carriage past the last soot blower station whereupon it contacts a striker plate which engages switch I which serves to simultaneously interrupt the current and reverse the supply thereof in preparation for the next cleaning operation wherein the carriage will pass the housing in the direction opposite to the aforedescribed direction of travel.
It is to be understood that the wiring schematics are merely illustrative of one means for carrying out the present invention, which provides a novel and unique soot blowing system.

Claims

1. A soot blowing system for cleaning banks of tubes (11) or the like disposed in a housing (10) having apertures (20), the system comprising a movable carriage (17) located externally of the housing, for moving a lance (19) mounted on said carriage (17), drive means (I, II) operable for displacing the lance (19) axially within a respective aperture (20) in the housing (10) and for rotating the lance (19) within the respective aperture (20), and means (21) for connecting the lance (19) to a fluid supply whereby fluid under pressure may be passed through at least one fluid emission opening (23, 24) in the lance (19), characterised by said carriage (17) being movable between a selected plurality of the apertures (20) in the housing (10), by additional drive means (III) operable for moving the carriage (17) between said selected plurality of the apertures (20), and by control means arranged to operate sequentially the additional drive means (III) to move the carriage (17) automatically from one selected aperture (20) and to stop the carriage (17) automatically at the next selected aperture (20) and the first drive means (I, II) to automatically insert the lance (19) into and move the lance (19) within each selected aperture (20) and to automatically withdraw the lance from each selected aperture (20).

2. A system as claimed in claim 1, characterised in that the control means are arranged to operate the first drive means (I, II) selectively to rotate the lance (19) through a predetermined angle within each selected aperture (20).

3. A system as claimed in claim 2, characterised in that said angle is between approximately 5° and approximately 20°.

4. A system as claimed in any of claims 1 to 3 characterised in that the control means are arranged to operate the first drive means (I, II) selectively to displace the lance (19) a pre- determined distance within each selected aperture (20).

5. A system as claimed in claim 1, characterised in that the control means are arranged to operate the first drive means (I, II) to advance the lance (19), then rotate the lance (19), then retract the lance (19), then further rotate the lance (19), and then further advance, rotate, and retract the lance (19) within each selected aperture.

6. A system as claimed in claim 1, characterised in that the control means are arranged to operate the first drive means (I, II) to rotate the lance (19) in one direction, then advance the lance (19), and then rotate the lance (19) in the other direction within each selected aperture (20).

7. A soot blowing system as claimed in any preceding claim, characterised in that the lance (19) has two said fluid emission openings (23, 24) which are substantially diametrically opposed in the lance (19).

8. A system as claimed in any preceding claim, characterised in that the connecting means comprise a flexible hose (21). ).

9. A system as claimed in any preceding claim, characterised in that the apertures (20) in the housing (10) are provided with covers (26) arranged to open when the lance (19) is inserted.