GB2220732A - Sighting aperture clearance means for flame monitoring apparatus - Google Patents

Abstract

To reliably remove any encrusting slag (A) which obscures the sighting aperture (14) in a boiler wall (16), the sighting tube (12) is arranged within a sleeve (32) which is movable longitudinally of the tube to protrude through the aperture and break off debris and then to retract again. The sleeve (32) is preferably mounted on a piston (30) which slides within a housing (20) so that air expelled from the housing upon movement of the piston can be directed (via ducts 31, 33 and 39) into the sighting tube (12) to provide exhaust air blasts therealong. <IMAGE>

Description

SIGHTING APERTURE CLEARANCE MEANS FOR FLAME MONITORING APPARATUS This invention concerns flame monitoring apparatus, and more specifically means for clearing the sighting aperture of such apparatus.

The combustion of pulverised fossil fuel or oil in the furnace of -a boiler is conventionally monitored by continuous observation of the flame via a viewing or sighting aperture in the wall of the boiler. In this respect, complete failure of the flame can lead to build up of an explosive fuel-air mixture, so monitoring is necessary for safety reasons. It is also, of course, important to know when ignition has been achieved at start-up of boiler operation, and it is useful, for reasons of economy, to be aware of any flame instability so that the pulverised fuel can be supplemented by the more expensive, but more stably combustible, oil only when essential.

The present applicants have previously developed highly successful flame monitoring apparatus based on continuous detection of flame flicker frequency by photocells having spectral response similar to the human eye. These cells are mounted in detector heads fitted to sighting tubes which are arranged to project through respective sighting apertures in the boiler wall. The electrical signal from the cells is applied to a circuit which amplifies the modulation at flame frequency and rejects the steady state component, the degree of amplification being adjustable to discriminate over any interference from adjacent burners. The signal indicative of flame flicker frequency is transmitted to a central control logic unit where a micro-computor gives a flame fault warning when flicker frequencies drop below predetermined danger thresholds for longer than pre-set delay periods.

Other monitoring systems are known which also involve use of optical detector means for continuous flame observation via a sighting aperture.

The sighting aperture, or rather the outer end of the sighting tube which lies in or projects slightly beyond the sighting aperture in the boiler wall rapidly becomes obscured during boiler operation by coal dust, slag and clinker which occumulates and/or encrust. Obviously, this prevents effective operation of the relevant flame monitoring system. Hitherto, in an attempt to prevent slagging or fowling of the tube end, purging air has been supplied through the tube itself from a fan or pneumatic air blaster connected to a tee-piece on the tube. The force of air from a fan, whether supplied continuously or only intermittently was often insufficient to remove encrusting clinker. Therefore, more recently, intermittent blasts of compressed air (e.g. at 20 minute intervals) from an air blaster has been the most favoured method of removing debritus.

Even with this measure, there are still occasionally stubborn encrustations of slag which are not prevented from forming and which can only be removed by disconnecting the detector head and manually clearing the tube and aperture using a metal rod. This is clearly inconvenient, time consuming and costly.

It is an object of the present invention to provide more effective means than hitherto for cleaning and keeping clear the sighting aperture and/or the end of the sighting tube of flame monitoring apparatus.

Pursuant hereto, the invention proposes flame monitoring apparatus comprising a sighting tube for observation of a flame via a sighting aperture in a boiler wall, characterised in that the tube is arranged within a sleeve which is mounted so as to be movable longitudinally of the tube to the extent of being protrudable through the aperture.

Preferably the sleeve is mounted on a piston which is slidingly movable within a cylindrical housing which surrounds the sighting tube.

In this respect, the piston is advantageously sealed with respect to both the sighting tube and the housing and the housing provided with inlet and/or outlet ports adjacent its respective ends. Air expelled from the housing as the piston advances to protrude through the aperture can then be fed into the rear of the sighting tube to provide a blast of exhaust air along said tube and out of said aperture. Similarly, upon retraction of the piston, air expelled from the housing can again be fed into the rear of the sighting tube to provide another blast of exhaust air via the tube and the sighting aperture.

The piston may conveniently be pneumatically driven, compressed air to cause it to advance or retract entering the housing via the same inlet/outlet ports at the respective ends thereof as are used for exhaustion of air.

The invention will be described further, by way of example with reference to the accompanying drawings, in which: Figures 1 to 4 are respective diagramatic crosssections of a prefered practical embodiment of flame monitoring apparatus in accordance with the invention showing successive stages of operation, namely: Figure 1. Normal or rest condition; Figure 2. Advancement of pistion causing protrusion of sleeve through sighting aperture; Figure 3. Fully advanced position of piston; and Figure 4. Retraction of piston causing sleeve to be drawn back through sighting aperture.

As illustrated, the prefered apparatus comprises a flame viewing device 10 mounted at one end of a sighting tube 12, the other end of which is aligned with and adjacent to a sighting aperture 14 in the wall 16 of a boiler. The viewing device may, for example, be a photocell for detection of flame flicker frequency, but the nature thereof is not pertinent to the present invention. The tube 12 is mounted concentrically within a cylindrical housing 20 which has respective end closures 18,22. In the illustrated embodiment the tube is in fact shown as being integral with the inner end closure 18. The outer end closure 22 is provided integrally with a bush 24 of wider diameter than the tube 12 which bush 24 is a tight fit in the aperture 14.

Terminal inlet/outlet ports 26, 28 for the housing 20 are provided in the respective end closures 18,22.

A piston or ram 30 is a sliding fit in the annular chamber between the tube 12 and the housing 20. A sleeve 32 is carried by the aforesaid piston or ram 30.

This sleeve 32 surrounds the tube 12 with only a slight spacing therebetween and extends into the bush 24. The piston 30 and the sleeve 32 may be integrally formed of stainless steel. As indicted, they are sealed relative to the housing 20 and the bush 24 at one side and relative to the tube 12 at the other side by respective sealing rings 34, 35, 36, 37.

Respective air ducts 31, 33 lead from the ports 28 and 26 at the forward and rear ends of the housing 20 to a directional control valve 40, which is also connected, via a pipe 38, to a supply of compressed air and, via a further duct 39 to an opening 21 at the rear end of the sighting tube 12, outside the housing 20 and adjacent to the flame viewing device 10.

While the boiler is in operation, the inner surface of the wall 16 tends to become encrusted with slag (indicated by reference letter A), which if unchecked would soon obscure the sighting aperture 14 through which the flame (indicated by reference letter B) is monitored by the viewing device 10. This is the situation illustrated in Figure 1. in which the clearance mechanism, consisting of the above-described piston 30 and sleeve 32 as well as the air ducts 31, 33 39 and the control valve 40, is in its rest position.

Periodically, for example at 20 minute intervals, the piston 30 is slid forwards along the sighting tube 12 so that the sleeve 32 protrudes through the aperture 14 into the boiler interior and in doing so physically breaks off any overhanging encrusting slag. This is illustrated in the successive piston advancing stages shown in Figures 2 and 3. The piston 30 may additionally be slid forward to cause physical removal of debris at any time when the viewing device 10 registers the aperture 14 as being obscured.

The piston and sleeve are moved forward relatively slowly, but with a steady force, preferably in the region of 800 lbf, which is more than adequate to remove any encrusting slag likely to be encountered. The overall sequence of advancement and retraction of the piston and sleeve 30, 32 may, for example, take only 5 seconds or thereabouts, made up of 2 seconds for forward movement to the fully advanced position shown in Figure 3, one second at that position and 2 seconds for retraction to the next position shown in Figure 1.

The clearance mechanism is also designed to provide an automatic exhaust air blast via the sighting tube 12 and the sleeve 32 upon forward movement of the piston 30 to project the'sleeve 32 into the boiler and again upon rearward movement thereof. These air blasts serve to remove any dust and/or any loose slag/clinker which may have accumulated in the region of the sighting aperture 14, or indeed in the end of the sleeve 32 itself, as indicated in Figure 3.

The entire sequence of operation of the clearnace mechanism, referring in turn to Figures 1 to 4 is, in detail, as follows.

In the rest or normal condition (Figure 1) the control valve 40 is in a first position, whereby compressed air is supplied from the pipe 38 to the interior of the housing 20 via the duct 31 and the port 28 (acting as inlet) so that the piston 30 is held in its fully retracted position at the rear of the housing 20, adjacent the end closure 18. In this valve position the other housing port 26 is in communication with the tube aperture 21 via the ducts 33 and 39.

When the aperture clearance mechanism is initiated the valve 40 is switched to a second position (Figure 2) whereby the supply of compressed air from the pipe 38 is switched via the duct 33 to the other port 26 at the rear of the housing 20. This forces the piston 30 forwards (to the left in the drawings as indicated by the arrows in Figure 2) so that the sleeve 32 moves through the aperture 14 and projects into the interior of the boiler, thus removing encrusting debris as already mentioned. In this second valve position the housing port 28 is in communication with the tube aperture via the ducts 31 and 39, so as the piston 30 moves the air in front of it is expelled via this route, providing an exhaust air blast down the tube 12, as previously referred to.

The fully advanced position of the piston 30, lying adjacent end closure 22, is shown in Figure 3. The valve 40 is then returned to its first position (Figure 4) so that compressed air is again supplied to the port 28 to push the piston 30 rearward, as indicated by the arrows in Figure 4. As it moves, air to the rear of the piston 30, is expelled via the port 26 and travels via the ducts 33 and 39 to provide a second exhaust air blast down the tube 12. Thus there are two air blasts in quick succession during advancement the retraction of the piston 30 with a pause therebetween while the piston dwells at its fully advanced position. When the piston 30 reaches the rear of the housing 20 again it has come a full cycle back to the rest position (Figure 1).

The foregoing is illustrated and not limitative of the scope of the invention and many variations in construction detail are possible.

Claims (6)

1. Flame monitoring apparatus comprising a sighting tube for observation of a flame via a sighting aperture in a boiler wall, characterised in that the tube is arranged within a sleeve which is mounted so as to be movable longitudinally of the tube to the extent of being protrudable through the aperture.

2. Apparatus as claimed in claim 1 wherein the sleeve is mounted on a piston which is slidingly movable within a cylindrical housing which surrounds the sighting tube.

3. Apparatus as claimed in claim 2 wherein the piston is sealed with respect to both the sighting tube and the housing, and the housing is provided with inlet/outlet ports adjacent its respective ends.

4. Apparatus as claimed in claim 3 further including ducting whereby air expelled from the cylindrical housing as the piston advances to protrude through the aperture is fed into the rear of the sighting tube for exhaustion along said tube and out of said aperture.

5. Apparatus as claimed in claim 3 or 4 further including ducting whereby air expelled from the cylindrical housing as the piston retracts is fed into the rear of the sighting tube for exhaustion along said tube and out of the sighting aperture.

6. Flame monitoring apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

6. Apparatus as claimed in claim 3, 4 or 5 wherein the piston is pneumatically advanced at regular intervals.

7. Flame monitoring apparatus substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

n n Amendments to the claims have been filed as follows

1. Flame monitoring apparatus comprising a sighting tube for observation of a flame via a sighting aperture in a boiler wall, the tube being arranged within a sleeve which is mounted so as to be movable longitudinally of the tube to the extent of being protrudable through the aperture, and the sleeve being mounted on a piston which is sealingly slidable within a cylindrical housing which surrounds the sighting tube.

2. Apparatus as claimed in claim 1 wherein the housing is provided with inlet/outlet ports adjacent its respective ends.

3. Apparatus as claimed in claim 2 further including ducting whereby air expelled from the cylindrical housing as the piston advances to protrude through the aperture is fed into the rear of the sighting tube for exhaustion along said tube and out of said aperture.

4. Apparatus as claimed in claim 2 or 3 further including ducting whereby air expelled from the cylindrical housing as the piston retracts is fed into the rear of the sighting tube for exhaustion along said tube and out of the sighting aperture.

5. Apparatus as claimed in claim 2, 3 or 4 wherein the piston is pneumatically advanced at regular intervals.