The invention relates to a method in a bag filter comprising a hole plate as well as elongated filter elements installed in the apertures of the hole plate. The invention also relates to a bag filter.

Such a filter element installed in an aperture of a hole plate normally comprises a frame that supports the filter hose or bag forming the filtering structure.

Bag filters of the above-mentioned kind are used to separate solids from flue gases. The general principle of operation of a bag filter is disclosed, for example, in the international publication WO 88/07404. The bag filter is placed in a filter chamber in a flue gas duct in such a way that the hole plate of the filter delimits two spaces. The solids filtered by the filter material are left in the space in the direction of entry of the flue gases, whereas the flue gases filtered by the filter material flow to the clean space on the other side and from there further to a flue gas duct. In the general structure of a bag filter, the hose-like filter material is supported from the inside by a supporting frame suspended in an aperture of the hole plate in such a way that the elongated filter element extends in the direction of entry of the flue gases. In practice, the hole plate is supported in a horizontal position in the filter chamber in such a way that supporting frames suspended in the apertures, and the filter bags around them hang downwards. In this way, a large filtering surface is obtained on the inlet side. The hole plate may also be in a vertical position.

The filter element has to be connected to the hole plate in such a way that the joint is gas-tight as well, to prevent leaks from between the filter element and the aperture. Also, the connection has to be sufficiently sturdy. One example of the attachment of a filter element to an aperture of a hole plate is disclosed in German Utility Model DE 20305310 U1. Power boilers normally comprise 3 to 4 bag filter modules of the above- described kind placed in parallel, in which modules a hole plate with a filter element is placed in the space through which the flue gases flow. A problem with the bag filters is that the filter material is sometimes broken so that impurities can leak through the bag filter to the clean side through the filter element. The filtering capacity of the filter is thus impaired, and the filter element has to be replaced. The module that contains the broken filter element has to be removed from the flue gas filtering process so that replacement or plugging can be performed during the running of the power boiler. The output of the boiler may have to be restricted so that there would be enough space for the flue gases to pass through the remaining modules. If it is undesirable to limit the boiler output, then the modules have to be oversized to maintain the boiler output also during the replacement. Furthermore, the bag filter module has to be cooled down before the replacement so that a maintenance person can enter the module to replace or plug the filter element.

To solve this problem, flaps or corresponding blocking devices have been previously developed to block the element automatically at its upper end upon an increase in the gas flow through the element, indicating a failure, or to act as "fuses", being blocked when the solids content in the gas flow increases. Examples of the first and last mentioned cases include publications US 4,297,1 13 and GB 2 293 777, respectively. In this way, the filter elements of the bag filter can be automatically shut off from the process, one by one, and the bag filter is still in use at a capacity that has only been reduced by the filter elements shut off. Broken filter elements can be replaced with new ones during normal downtimes when the bag filter is maintained. A problem in the structure based on such a concept is that each element has to be equipped with a separate automatic blocking device. Furthermore, such extra structures may cause further pressure losses in the gas flow.

The aim of the invention is to eliminate the drawbacks of prior art and to pre- sent a method in a bag filter, whereby the filter elements can be shut off selectively, one by one, without substantially changing the structure of the filter elements or the hole plate supporting them and without causing further pressure losses. To achieve this aim, the method according to the invention is primarily characterized in that in the first step of the invention, a damaged filter element is detected, and in the second step, a separate plugging piece is inserted in an aperture of the hole plate corresponding to the damaged filter element in such a way that the plugging piece shuts off the flow from the filter element out of the hole (to the clean side, that is, above the hole plate).

The feeding of plugging pieces is provided from the clean side of the hole plate in the bag filter module, where it is easy to provide various devices for implementing the insertion and where various cleaning devices are already used for maintaining the filter elements.

According to a particular embodiment of the invention, a means for feeding plugging pieces is guided above said aperture of the hole plate, and the plugging piece is dropped into the aperture to plug the filter element underneath it.

The plugging pieces may be, for example, spherical in shape and dimen- sioned in such a way that they settle in the aperture of the hole plate to block the flow from the filter element via the aperture to the clean side. Spherical plugging pieces are preferable, because they always settle in the correct plugging position in the aperture, but alternatively, plugging pieces of other shapes can be used as well, in which case they must be guided to the correct position in the aperture.

According to a particular embodiment, a damaged filter element, that is, the damaged filter material of the filter element, is detected by a measuring sensor moving above the different apertures of the hole plate and capable of detecting a damage in the filter material below the aperture. The measuring sensor detects dust coming from the aperture, revealing the damaged filter material. Other measuring methods can be used as well, if their results correlate with the damage of the filter material. Before blocking the aperture with the plugging piece, a blowing nozzle of a cleaning device can be moved to the location of the aperture to clean the aperture from dust by blowing pressurized air.

A device arranged in a bag filter according to the invention, in turn, is characterized in that the device is a blocking device with a movable device for feeding plugging pieces, which device can be guided to a location above the different apertures of the hole plate for blocking the aperture and alternatively the filter element below the same. By means of the invention, maintenance operations can be performed at each filter element separately without a need to block the whole bag filter module. The blocking of the filter elements can be performed simultaneously when the bag filter module is in operation and the flue gases flow through it.

Thanks to the invention, the filtering apparatus of the boiler plant can be implemented with even a single bag filter module, because there is no need to have other modules in reserve. Similarly, there is no need to over-dimen- sion the filtering process. In general, the usability of the filtering apparatus is improved, because the apparatus or a single module does not need to be stopped because of a leak in a single filter element. Expensive filter socks can be worn out, because the filter element is not blocked until the filter material of the sock is clearly broken. Previously, filter material has been replaced with new material after a given service life, to be on the safe side. This extends the change interval of the filter socks.

In the following, the invention will be described in more detail with reference to the appended drawings, in which

Fig. 1 shows a side view of a cleaning device for filter elements belonging to a bag filter,

Fig. 2 shows a cleaning device seen from above,

Fig. 3 shows the principle of blocking a filter element according to the invention, in a cross-sectional view transverse to the plane of the hole plate,

Fig. 4 shows a blocking device of filter elements in connection with a cleaning device, seen from above,

Fig. 5 shows a perspective view of another blocking device for filter elements arranged in connection with a cleaning device, and

Fig. 6 shows the blocking device of Fig. 5 in a front view from the direction of a guide.

Figure 1 shows schematically the structure of a bag filter at a hole plate. The figure also shows a cleaning device, whose operation will be described further below. The bag filter is placed in a flue gas duct extending from a boiler for burning fuels. In addition to gaseous substances, the flue gas flow also comprises solid particles originating from the combustion process, which are separated by the bag filter. The bag filter comprises a so-called hole plate 4, with elongated filter elements 6, or filter bags, being suspended in its apertures 5. The filter bag comprises hose-like filter material 6a which is perme- able to gases and retains solid particles of a given size, as well as a supporting structure 6b to support the filter material from the inside, that is, a supporting frame or a kind of a "cage".

The hole plate 4 divides the space in the flue gas duct to an inlet side 1 , which is below the hole plate and in which the solid substances filtered out are retained, and a clean outlet side 2 which is above the hole plate and from where the gases purified from the solid substances move further on. Such a space, divided in two parts by the hole plate, constitutes one bag filter module for the filter apparatus in a boiler plant.

The filter material 6a may be any suitable filtering textile that resists well high temperatures. The filter material is pulled as a kind of a sock (filter sock) up onto a frame or cage suspended in the aperture. Because of the shape of the filter material, the filter is also called a bag filter. The filter material is also susceptible to blocking, due to solid material accumulated on its outer surface. Because of this, the filter material must be cleaned from time to time by a blow directed into the filter element, in a direction opposite to the direction of the gas flow to be filtered. Figure 1 shows nozzles 10c of a cleaning device 10 placed above the hole plate 4 for this purpose.

Another maintenance operation of the filter is the elimination of the effect of broken filter material. To remove a filter element with broken filter material from the process, a separate plugging piece is used, which is inserted, from above the hole plate, in the aperture to which said filter element is fastened. The insertion takes place when the filtering process is running, that is, when the gas flow to be filtered passes via the filter element from the inlet side to the outlet side. The plugging piece is inserted in the aperture in such a way that it blocks the flowing path of gas and entrained solid particles via the defective filter element to the clean side.

Figure 3 shows part of a hole plate 4 at a single filter element 6 (the lower part of the filter element is not shown). The gas to be filtered flows upwards inside the filter element in the figure. The figure shows a supporting frame 6b for a filter element fixed in the aperture 5 of the hole plate, and filter material 6a placed on it. In Fig. 3, the filter material 6b has been broken at some location, for which reason a separate plugging piece 12, which is not a device or a part that is integral in the filter element 6 during its operation, has been dropped into the aperture 5. Said plugging piece 12 can be optionally dropped into any aperture 5 where a defect causing a leak has been detected in the filter element 6. The plugging piece 12 remains in the aperture because the lower part of the aperture 5 of the hole plate 4 comprises a stopper whose inner diameter is smaller than the inner diameter of the aperture 5, wherein the plugging piece 12 fits with a suitable free play into the aperture but blocks the upward flowing path at the stopper. Figure 3 shows how a horizontal part of the supporting frame 6b of the filter element 6 is used as such a stopper. The spherical plugging piece 12 is marked by bro- ken lines in Fig. 4.

Figure 3 also shows how a single filter element 6 can be placed in the aperture 5 of the hole plate. The hole plate 4 has a sandwich structure with an upper plate 8 and a lower plate 7, wherein the aperture 5 of the hole plate is formed by an aperture 5b in the upper plate 8 and an aperture 5a in the lower plate 7 facing each other. At the upper end of the supporting frame 6b of the filter element 6, a cylindrical tubular piece is provided, placed inside the aperture 5 between the upper plate 8 and the lower plate 9, the lower end of the tubular piece being provided with a latticed part of the supporting frame 6b of the filter material, to support the filter material. The stopper for the plugging piece 12 is formed on the inner surface of the cylindrical tubular piece, for example by providing an annular thread or the like.

The upper edge of the cylindrical tubular piece comprises a flange, by means of which the whole supporting structure of the filter bag can be suspended from the aperture 5, that is, the aperture 5b of the upper plate 8, in such a way that the flange comes on top of the edges of this aperture 5b. In a corresponding manner, the filter material 6a can be sealed at its upper edge to the edges of the aperture 5a of the lower plate 7, as shown in Fig. 3. Owing to this fixing arrangement, the aperture 5a in the lower plate 7 is slightly larger in diameter than the aperture 5b in the upper plate 8 but concentric with this. Figure 3 also shows reinforcements 9 between the upper plate and the lower plate, connecting the plates together.

The aperture 5 of the hole plate 4 with a sandwich structure has a dimension in the depth direction, wherein the plugging piece 12 can be easily fitted in it, above the filter material 6a. However, the invention is not limited to be used in connection with hole plates of sandwich structure only, and with a suitable structure of the filter element 6, a space for receiving the plugging piece can be provided in hole plates of other types as well.

The blocking device or "plugging device" for filter elements, arranged to dispense separate plugging pieces into selected apertures 5 of the hole plate 4, can be placed in connection with the cleaning device for the filter elements. Figures 1 and 2 show an example of such a cleaning device 10 which is placed above the hole plate 4 in such a way that its central housing 10a, comprising the rotation axis of the cleaning device, the related mechanics, and the channels for feeding and distributing cleaning air, is above the central area of the hole plate 4 with no apertures. The cleaning device comprises supporting arms 10b extending from the centre sideward, their downward directed ends being provided with nozzles 10c into which cleaning air is introduced via the arms 10b. The nozzles blow this air downwards into the apertures 5 and into the corresponding filter elements 6, to clean the filters by the principle of back flushing; that is, by a gas flow in a direction opposite to the flowing direction of the gas to be filtered. As shown in the figures, the nozzles may be suspended in the arms 10b one by one or in pairs. The nozzles 10c of the cleaning device are placed in such a way that at least one nozzle 10c is provided at each circle of apertures in the hole plate 4. The axis of rotation of the cleaning device 10 is concentric with these circles of apertures, wherein by rotating the cleaning device 10, each nozzle 10c can be guided to a location above any aperture 5 of the same circle of apertures, and the respective filter element 6.

The cleaning device 10 is arranged to clean the filter elements 6 either in a predetermined order which can be programmed in the control unit of the device 10, or the control unit may control the device to clean single filter elements 6 according to the need. For this purpose, reduced filtering capacity caused by material accumulated onto the outer surface of the filter element 6, that is, the surface of the actual filter element, can be monitored by automatic control methods of prior art which will not be described in more detail, as they do not belong to the invention. Figure 4 shows a blocking device 11 or so-called plugging device for filter elements, connected to the cleaning device, by which a single filter element 6 can be blocked at its upper end in the way shown in Fig. 3. The blocking device 11 is suspended in an arm 10b of the cleaning device 10 in such a way that during the movement of the arm 10b, the feeding opening 11 of the cleaning device 11 moves along a given circle of apertures 5. The blocking device 11 comprises a storage (not shown) of separate plugging pieces 12, in this case balls, above the feeding opening 11a. At a broken filter element 6, the blocking device 11 dispenses one plugging piece 12 by opening the corresponding feeding opening 11a, wherein the plugging piece 12 can fall under gravity into the aperture 5 where it blocks the gas flowing path to the clean side. The feeding opening may be equipped with a shutter that opens for a moment to let the plugging piece 12 fall into the aperture 5 but prevents the next plugging piece 12, coming from above into its place, from falling. The feeding opening 11a of the blocking device 11 can be arranged to maintain the same circle of apertures 5 and corresponding filter elements 6 as the nozzle 10c fixed to the same arm 10b of the cleaning device 10, as shown in Fig. 7. Different arms 10b should be provided with a sufficient number of blocking devices 11 and respective storages so that all the possible apertures 5 can be covered by movements of the cleaning device 10. The feeding openings 11a can be arranged in pairs, as shown in the figure, in the same way as the nozzles 10c, which is also shown in the figure. The feeding openings 11a thus have a common storage of plugging pieces, from which the plugging pieces 12 drop down. The storage may be, for example, a verti- cal pipe whose lower end branches into two receptacles, each having a feeding opening 11a and a respective shutter at its lower end. The plugging piece 12 always drops into the receptacle from which a plugging piece has been dispensed to plug an aperture 5. The actuator that controls the shutter of the feeding opening 11a is indicated with the reference 11 b.

The blocking device 11 may be configured to function automatically in such a way that it is connected to a control unit, which also receives a message when a leak has been detected in the filter element 6 by a detection method known as such (for example, by a sensor movable above each aperture at a time). Then, the control unit guides the feeding opening 1 1 a of the blocking device 1 1 to a location above the corresponding aperture 5. The control unit may thus control both the cleaning device 10 and the blocking device 1 1 and automatically receive messages from the sensors, which messages are necessary for their operation. For example, the control unit may operate in a damage situation in the following way to carry out a so-called "plugging sequence":

- receive a damage message regarding a given filter element 6 from a sensor,

- guide the plugging piece feeding means of the blocking device 1 1 to a location above the corresponding aperture 5,

- give the feeding means a command to dispense a plugging piece 12 in such a way that it drops into the aperture 5.

The control unit may also be arranged to guide the nozzle 10c of the cleaning device 10 to a location above said aperture 5 before the means for feeding plugging pieces, and to give a cleaning command for directing a special cleaning blow into the aperture 5 from the nozzle 10c. However, this function is not necessary.

The blocking device 1 1 may also be controlled purely manually. Figure 4 also shows sensors 1 1 c placed in the blocking device 1 1 and arrayed on the same circumference of apertures 5 as the feeding openings 1 1 a for the plugging pieces, thus coming above each aperture 5 in turn and thereby being able to monitor the condition of the corresponding filter element 6 underneath the aperture, to detect damages and to notify the control unit to start said plugging sequence.

The plugging of the apertures 5 does not disturb the operation of the bag filter as such, because it can be performed during the flow of the gas to be filtered and when the filter is in operation. Furthermore, plugging a few apertures 5 and blocking the corresponding filter element does not substantially reduce the filtering capacity, compared with the need to disconnect the whole bag filter module from the filter apparatus. In connection with the measures of normal maintenance of the bag filter, it is possible to replace the damaged filter elements 6 underneath the plugged apertures 5 with new ones, that is, to substantially extend the maintenance intervals, particularly when the service life of filter socks is extended. The blocking device 11 can also be arranged to be separate from the cleaning device 10, wherein it can operate in the same way as the cleaning device. In this case, a sufficient number of blocking devices and corresponding storages are connected to a structure that rotates in a controlled manner around an axis coinciding with the centre of the hole plate 4, or they can be moved to the correct locations in a controlled manner by a manipulator or the like, or in another way. In principle, it is thus possible to manage with only one blocking device 11 having a storage of plugging pieces 12 and a feeding opening 11a or a pair of feedings openings 11a underneath it, and moved by a manipulator to a location above such apertures 5 which need to be plugged. The con- trol unit may operate in the same way as described above, to implement the plugging sequence.

Figures 5 and 6 show a blocking device independent of the operation of the cleaning device, where the principle differs from the device of Fig. 4 in that the blocking device 11 is a guide 3 connected to a supporting base for the cleaning device, underneath the cleaning device 10, and arranged to be piv- otable. A carriage used as the actual means for feeding the plugging pieces 12 is arranged to run along the guide 3. The carriage comprises a vertical cage which is open on top and at the bottom, in which cage the plugging piece 12 can be placed before it is dropped into an aperture to be plugged. The guide pivots around a rotation axis running through the centre of the hole plate; in other words, the guide is placed in the radial direction. As the guide 3 can be turned into any angular position and the carriage can be moved in a corresponding manner to any location in the longitudinal direction of the guide, the location of the rotation axis of the guide is not, in principle, significant, because the feeding means and the respective opening 11a for feeding the plugging pieces can be guided, in principle, to a location above any aperture 5 in the x-y plane. However, if the rotation axis of the guide 3 is at the centre of the circumferences of concentric apertures 5, it is easy to scan through the apertures 5 of the same circumference when the guide is turned and the carriage has a constant location in the guide. In the device of Figs. 5 and 6, the storage of plugging pieces does not move with the means for feeding the plugging pieces, but it is stationary, and the plugging pieces are transferred into the means for feeding plugging pieces along a transfer line from the storage. The pipe or chute used as the transfer line for feeding plugging pieces extends from above and ends at a dispensing point where the plugging piece 12 passed by gravity along the pipe or chute from the storage above drops into the feeding means which is brought to the dispensing position along the guide 3, and remains above the feeding opening 11a (open lower end of the cage) in this means, held by the shutter. The terminal part of the feeding pipe or chute may be provided with a stopper, by which the plugging piece introduced along the feeding pipe or chute is stopped in a standby position and can be dispensed mechanically, for example by an empty means for feeding plugging pieces brought to the dispensing point, wherein the plugging piece can drop into the means for feeding plug- ging pieces, via the dispensing point. Figure 5 shows a situation in which the carriage is in a dispensing point, in which the open upper end of the cage is placed directly below the terminal end of the feeding pipe or chute.

The means for feeding plugging pieces can thus be used to fetch one plug- ging piece 12 at a time from the dispensing point, which plugging piece 12 is then, according to the need, dropped into an aperture 5, when the means for feeding plugging pieces is guided, according to the plugging sequence, to an aperture 5 that needs to be plugged. Alternatively, the plugging sequence can be implemented in such a way that when the sensor 11c of the blocking device has detected a damaged filter element 6, the control unit first gives a command to the means for feeding plugging pieces 12 to move to a position of delivering a plugging piece, and if there is no plugging piece ready in the standby position, the control unit gives the storage a command to dispense a plugging piece 12 to the transfer line, wherein the plugging piece is moved from the storage via the dispensing point to the means for feeding plugging pieces. After this, a cleaning blow is possibly performed into the aperture 5 by the nozzle 10c of the cleaning device, and at the end of the plugging sequence, the means for feeding plugging pieces, having a plugging piece 12 fetched from the dispensing point, is moved to a location above the same aperture 5, and the plugging piece 12 is dispensed by opening the shutter of the feeding opening 11a. Figures 5 and 6 show how the movable means for feeding plugging pieces also comprises a sensor 1 1 c for detecting a damaged filter element. The sensor 1 1 c is placed in the same carriage, movable along the guide, as the means for feeding plugging pieces, on the other side of the guide 3. The sen- sor 1 1 c is fastened inside the vertical tube to be guided to a location above the aperture 5, so that it would only measure the gas flow from said aperture. Said pipe and the respective sensor 1 1 c can be guided to a location on top of different apertures 5 of the hole plate by moving the means for feeding plugging pieces.

The movable and plugging blocking device 1 1 of Figs. 4 and 5 presented above can also be used in connection with bag hoses with a structure different from that shown in Figs. 1 to 3. The blocking device can also be implemented with other mechanical solutions by which the sensor and the feeding opening of the means for feeding plugging pieces can be brought above any aperture of the hole plate. The blocking device may comprise several sensors and openings for feeding a plugging piece, wherein it is sufficient that they jointly cover all the apertures of the hole plate, or it may only comprise one of each, as in Fig. 5, in which case it should be possible to move both the sensor and the feeding opening to all the apertures.

Preferably, the plugging piece 12 is a sphere, because it will always block a circular opening with suitable dimensions, irrespective of the position. It is also possible to feed spheres from one place to another by rolling, for example along the transfer line of Fig. 5. However, it is also possible to use pieces of other shapes, such as e.g. conical pieces, wherein the device for feeding the plugging piece must be capable of placing the plugging piece in a correct position in the aperture. The plugging piece should have a sufficient mass so that it can be used by its weight as a plug to the gas flow tending to come out of the aperture 5. In bag filters of normal size, it is possible to use a sphere with a weight of a few kilograms and having a metal shell, for example an aluminium shell, filled with sand. Naturally, the dimensions and the weight of the plugging piece are determined according to the dimensions of the aperture in each bag filter and, in a corresponding manner, according to the pressure of the gas effective underneath the aperture. The sensors 11c used for detecting a damaged filter element are placed in such a way that they can be moved in the direction of the plane of the hole plate 4 to a location above different apertures 5. However, they can be mov- able up and down in the vertical direction, if it is necessary to move the detecting end of the sensor into the aperture 5 or the filter element 6, to detect the condition of the filter material in a more reliable way. The sensors used can be dust sensors known from condition monitoring of filters and capable of detecting the presence of particles in a gas flow. The sensor of the blocking device may be arranged to be movable to different apertures of the hole plate under the control by the control unit according to a predetermined program; in other words, it can scan through all the apertures 5 and respective filter elements 5 at certain intervals in a predetermined order. The data transmission by means of the control unit, the sensors and the different actuators can be arranged in a wireless manner and/or by cabling. The automatic control unit, with the electronics contained in it, is preferably outside the bag filter module, and it may comprise the normal peripheral devices (display, data input device) required for data processing.

The blocking device according to the invention can be installed in all bag filter modules of an apparatus for filtering flue gases in a boiler plant, or only one large bag filter module in which this device is installed is needed for the boiler plant, and no parallel modules will be necessary. This is an advantage, for example, in the design of new boiler plants.