US3276437A

US3276437A - Soot blower operation for vapor generator furnaces

Info

Publication number: US3276437A
Authority: US
Publication date: 1966-10-04
Anticipated expiration: 1983-10-04

Description

Oct. 4, 1966 J. JONAKIN ET AL SOOT BLOWER OPERATION FOR VAPOR GENERATOR FURNACES 5 Sheets-Sheet 1 Filed Sept. 14, 1964 Oct. 4, 1966 JONAK(N ET AL 3,276,437

SOOT BLOWER OPERATION FOR VAPOR GENERATOR FURNACES Filed Sept. 14, 1964 s Sheets-Sheet 2 F'IC5-2 PIC-L4 Oct. 4, 1966 JONAKIN ET AL 3,276,437

SOOT BLOWER OPERATION FOR VAPOR GENERATOR FURNACES Filed Sept. 14, 1964 5 Sheets-Sheet 3 w F 7 WJRM/ZZL V D/FTV 1 United States Patent 3,276,437 500T BLOWER OPERATION FOR VAPOR GENERATQR FURNACES James .lonalrin, Simsbury, and Donald J. Frey, Hazardville, Conn., assignors to Combustion Engineering, Inc., Windsor, Conn, a corporation of Delaware Filed Sept. 14, 1964, Ser. No. 396,277 13 Claims. (Cl. 122392) This invention relates to furnace wall soot blowers in vapor generating units and in particular to a method and apparatus for selectively operating the various wall blowers.

In steam generating units burning coal or other ash bearing fuels there has always been a problem with ash deposits on the walls of furnaces. These deposits effectively insulate the walls, decreasing the furnace heat absorption, disturbing the balance of heat absorption throughout the unit, and in some cases, decreasing the efficiency of the unit. Furthermore, excessive buildup of this ash can lead to conditions where the slag bridges over the burners requiring a shutdown for cleaning and, in some cases, a heavy accumulation of slag suddenly falls off the walls damaging the floor of the furnace. This problem is becoming more critical since units are being built with high furnace heat relase rates and poorer quality coals are being burned.

Generally the furnace wall soot blowers are set for sequential operation and their application is dictated by general furnace performance conditions. A typical furnace wall soot blower is described in US. Patent 2,662,241 and typical methods of operating these blowers are shown in US. Patents 2,811,954 and 3,137,278.

As the furnace walls become dirty the gas temperature leaving the furnace tends to increase and superheaters located in the gas stream produce steam of an increased temperature. Various means are used to control this steam temperature in order to compensate for the dirtiness of the furnace. This includes methods such as tilting burners, spray desuperheating, gas recirculation and superheater gas bypasses. As the furnace dirties up, the control means operates to maintain a preselected steam temperature. The position of the steam temperature controller is sensed and when it reaches a predetermined position, it is known that the furnace walls have reached a particular overall dirtiness condition. At this time all of the furnace Wall blowers are operated to clean the walls with the steam temperature controller moving to an earlier position to maintain steam temperature. As the furnace dirties up again, this cycle is repeated.

The deposition of ash on the furnace walls is not uniform with there being certain areas that tend to dirty rapidly and others which remain relatively clean. General soot blowing of the entire wall accomplishes little in the relatively clean areas. The operation of the soot blowers in these clean areas uses expensive soot blowing media and prolongs the time of the actual soot blowing cycle. Since there is always the possibility of tube erosion due to the soot blower action, operation of soot blowers in a clean furnace zone creates an unnecessary risk with no compensating advantage.

In our invention the need for cleaning in a particular zone within the furnace is sensed by measuring a temperature or temperature difference on the furnace wall pressure parts. These temperatures are taken at positions which indicate a measure of the local heat absorption rate in the pressure parts. The temperatures are compared to temperatures known to exist for a normally dirty furnace, and the necessary soot blower action is taken only in those zones which indicate a need for soot blowing. The corresponding soot blowers may be operated immediately or activated at the time the heavy ash condition exists but not operated until the over-all furnace dirtiness condition indicates a need for general cleaning.

It is an object of our invention to provide a control action for soot blowers indicating a need for soot blowing in the general area of the particular soot blower.

It is a further object to provide means for sensing the amount of ash deposit in a particular zone of a furnace which is simple to install and is protected from damage due to hot furnace gases.

It is a further object to provide a method and apparatus which yields superior furnace wall cleaning by activating soot blowers selectively in those zones requiring cleaning.

It is a further object to provide an improved soot blower operation avoiding unnecessary blowing, thereby avoiding excessive use of the soot blowing media and tube erosion.

Gther and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to attain the results desired, as hereinafter more particularly set forth in the following detail description of an illustrative embodiment, said embodiment being shown by the accompanying drawings wherein:

FIGURE 1 is a sectional side elevation of a steam generator illustrating the application of our invention to the furnace walls;

FEGURE 2 is a section through several tubes of the welded web furnace wall showing temperature conditions in a commercially clean furnace;

FIGURE 3 is a similar section showing temperature conditions in a normally dirty furnace;

FIGURE 4 is a similar section showing temperature conditions in a dirty furnace;

FIGURE 5 is a section through a welded web wall showing alternate thermocouple locations;

FiGURE 6 is a section through a tangent tube furnace wall showing temperature conditions in a commercially clean furnace;

FIGURE 7 is a similar section showing temperature conditions in a normally dirty furnace;

FIGURE 8 is a similar section showing temperature conditions in a dirty furnace; and

FIGURE 9 is a similar section through a tangent wall showing alternate thermocouple locations.

In the illustrative embodiment of FIGURE 1 coal is delivered through ti ting nozzles 2 into furnace 3 for combustion. The furnace walls 4 are comprised of a plurality of parallel tubes 5 wherein the adjacent tubes are joined by a welded web '7. Steam generated within the tubes is released from steam drum 8 and conveyed therefrom through superheating sections 9 and 10. The superheated steam is then conveyed at elevated temperature through steam outlet pipe 12 through a steam turbine (not shown).

Combustion gases leaving the furnace 3 pass through flue 13 wherein heat is transferred to the superheating surfaces 9 and 10, with the combustion gases then passing over an economizer and air heater exhausting to atmosphere through a stack. During operation of the furnace, ash and slag form on the furnace walls sticking to the tubes and reducing the heat absorption in the furnace. Furnace

wall soot blowers

14 and 15 are located throughout the furnace walls. Each of these soot blowers is operative to clean a section or

zone

17 and 18 of a furnace wall surface. These soot blowers are located on all walls of the furnace.

Steam temperature indicator 19 senses the temperature of the superheated steam passing through the steam line 12 and emits a control signal 20 to a tilting burner controller 21. As the furnace walls become dirty, the heat absorption in the furnace walls decreases and the temperature of gas leaving the furnace tends to increase with the temperature sensed by cont-roller 19 increasing. This operates through controller 21 to move tilting burners 2 in the downward position thereby decreasing the level of the zone of combustion within the furnace and controlling the steam temperature in line 12 at a preselected value. When the tilting burners reach a position of about 15 below horizontal, a control signal is emitted by controller 21 to activate all the furnace wall soot blowers. Only those of the furnace wall soot blowers which have been activated by the temperature sensing means, which will be subsequently described, are actually operated. When all of the activated wall blowers are operated, the furnace wall will be relatively clean and the burner tilts will resume a position above horizontal such as required to maintain the preselected steam temperature in the steam line 12.

FIGURES 2, 3 and 4 show the temperature pattern existing in the furnace wall tubes where the tubes 5 are 1% inches outside diameter with 0.250 inch minimum wall thickness. These tubes are spaced at 1% inches and are connected by welded web 7 which is inch thick and /2 inch long. The temperature profiles shown for the commercially clean furnace are those expected to exist for a heat absorption rate of 100,000 B.t.us per hour per square foot with boiling water within the tube at 2750 p.s.'i and 682 F. This absorption rate is that resulting due to the temperature difference between the furnace gases and the water wall surface. A commercially clean furnace is one which has been operating for some time and has just been cleaned by soot blowers. It is clean except for a thin tenacious slag layer which is essentially permanently bonded to the tubes.

FIGURE 3 illustrates similar temperature profiles which exist in a normally dirty furnace. Such a condition represents an ash lbuildup on the furnace wall which is such that we would be moved to operate a soot blower. These conditions are representative of the same furnace gas temperature but represent a lower heat absorption rate due to the insulating effect of the ash layer.

FIGURE 4 illustrates similar conditions with a very heavy ash layer. This again represents the same furnace gas temperature, but the heat absorption rate is again reduced due to the increased insulating effect of this heavy layer of ash.

Examination of FIGURES 2, 3 and 4 will show that the temperatures throughout the pressure parts vary in proportion to the heat absorption rate which is, in turn, effected by the layer of ash deposited on the furnace wall tubes. Consequently by sensing the temperature of the pressure parts at any selected location, we can obtain an indication of the thickness of ash on the furnace wall tubes. Thermocouple 22 is installed in a side of the web away from the furnace in approximately the center of this we'b. This location is preferred since it combines the maximum sensitivity to the absorption rate with ease of installation and freedom from operational difliculties in regard to the life of the thermocouple. Consequently this thermocouple 22 for the case shown will indicate a temperature of 750 F. in a commercially clean furnace, 710 F. in a normally dirty furnace and 690 F. in a dirty furnace.

Assuming for the moment that the particular furnace gas temperature condition illustrated exists, thermocouple 22 is installed at several locations in furnace wall zone 18. These temperatures are averaged in controller 23 which emits a control signal 24, representative of the temperature. Set point signal 25 is established, which in this case would represent normally dirty furnace temperature of 710 F. These two control signals are compared at summation point 27 with the resultant signal 28 indicating whether the sensed temperature is above or below the set point temperature. This control signal 28 passes to controller 29 which operates to activate soot blower 15 by closing switch 30 whenever the sensed temperature is lower than the set point temperature.

As previously described, cont-roller 21 activates the soot blowers sequentially when the over-all furnace dirtiness reaches a preselected condition. This is accomplished by closing switch '32 in control loop 33 for each soot blower in turn. When switch 32 is closed, a circuit is formed through controller 34 only if the circuit has been previously activated by the closure of switch 30. Controller 34 then acts to operate soot blower 15.

Since the furnace gas temperature in a particular location, however, varies "with both steam generator load and burner tilt, the set point temperature 25 is not a constant value. For accuracy of this system, set point temperature 25 should be varied in response to the position of the burners and the load on the steam generator. Although the proper temperature could be calculated, it is felt that the best method of obtaining a proper value is through actual test operation and observation of the unit when it is first put into operation. The controller may then be added to vary the set point in response to predictable furnace characteristic variations.

It is possible that during the operation of these soot blowers the furnace walls become clean to the extent that the tilting burners are in the fully upward position before the wall is completely clean. In such a situation a control signal may be added which blocks the soot blowing cycle until such a time as the tilting burners reach a preselected position at which time the cycle is permitted to continue. Furthermore, it is possible that at the completion of the cycle less cleaning than desired has been obtained 'in respect to the overall dirtiness of the furnace. In such a condition the signal may be produced operative to operate at the completion of the cycle in those soot blowers in zones where there had not been suflicient dirtiuess to indicate a need for soot blowing. Alternately on the repeat cycle all soot blowers could be operated including those which had just been blown.

Although with thermocouple 22 we are only measuring one temperature, we are actually dealing with the difference between that measured temperature and the temperature of the saturated fluid within the tubes. It is this temperature difference which is a function of the local heat absorption rate. Since steam generators are normally operated at constant pressure, the temperature of the fluid within the tube is not subject to much variation and may be assumed constant. Although this temperature will reduce at lower loads due to the lower pressures, the soot blowing would normally be cut out at these low loads. This is due to the general characteristic of a steam generator wherein the percentage of heat absorbed in a furnace tends to increase at reduced loads. Therefore during normal operation of a unit there is some advantage to permitting the furnace to become very dirty at low loads in order to more easily attain full superheated steam temperature.

For operation of units where the pressure varies significantly or for supercritical units where the temperature at a particular location may vary significantly, additional equipment will be required. Similar operation to that described may be obtained by either measuring the temperature of the pressure parts at two locations such as the location of thermocouple 22 and thermocouple 35. Alternately, a thermocouple 37 may be installed which actually senses the temperature of the fluid within the tube. On supercritical units where evaporation is occurring within the furnace tubes the drum pressure may be sensed, which is sufficient to determine the temperature of the saturated fluid.

FIGURE 5 shows a welded web wall indicating a variety of possible thermocouple locations. I hermocouple 22 is installed on the casing side of the fin near the center of the fin as previously described. This may he supplemented with thermocouple 37 sensing the temperature of the fluid within the tube. Alternately, thermocouple 22 may be combined with thermocouple 38 so that a temperature difference across the fin is obtained. Thermocouple 39 is located in the center of the fin, but is inserted in a small drilled hole so that the junction of the thermo couple is within the fin but near the furnace side. Thermocouple 40 senses the crown temperature of the tube and is installed by drilling a chordal hole and inserting the thermocouple so that the junction is near the crown of the tube. The lead portion of the thermocouple, which is inside the furnace, is then silver-soldered to the tube to protect it from the furnace gases. Either thermocouples 39 or 40 may be used as alternates for thermocouple 22.

Our invention may also be applied to units wherein the furnace walls are formed of tangent tubes as can be seen by an examination of FIGURES 6, 7 and 8. These figures show the temperature profiles for a commercially clean, normally dirty and dirty furnace.

Typical thermocouple locations suitable for such a furnace wall are illustrated in FIGURE 9. Thermocouple 42 is installed on the side of the tube so that the maximum sensitivity to heat absorption changes may be obtained consistent with simple installation of the thermocouple. Thermocouple 43 may be used in conjunction with thermocouple 42 if operation on the basis of temperature difference is desired. Thermocouple 44, which senses the crown temperature, may be installed in a similar manner to thermocouple 40.

Also illustrated in FIGURE 9 is fin 45 which is welded to one of two adjacent parallel tubes. Although this fin is not a pressure part in the strict sense of the term, it functions as a pressure part in respect to the heat absorption of the furnace walls. This fin will have the same temperature characteristics as illustrated in the other embodiments and thermocouples such as thermocouple 47 may be located on this fin in accordance with our invention.

In the preferred embodiment soot blowers in each zone when they are activated are only put on the list to be blown when the over-all furnace dirtiness indicates a need for general soot blowing. Alternately our invention may be employed to operate a soot blower immediately when the sensing means in the particular zone indicates a need for cleaning in that zone. Obviously, conventional interlocks employed to avoid an excessive number of soot blowers operating at one time must be used to avoid overburdening the source of blowing media and to avoid upsetting furnace conditions excessively. When such an operation is employed, interlocks and overrides responsive to over-all furnace dirtiness may be employed to block soot blowing when the furnace is not sufficiently dirty for adequate operation of the unit, and to override the activation signals when soot blowing is required for the over-all dirtiness of the unit independent of a local heavy ash condition.

While we have illustrated and described a preferred embodiment of our invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. We therefore do not wish to be limited to the precise details set forth but desire to avail ourselves of such changes as fall within the purview of our invention. For instance, over-all furnace dirtiness may be sensed by other means such as measurement of the gas temperature leaving the furnace. Alternate temperature control schemes may be used such as spray desuperheating in which case either the injection quantity or the position of the spray valve may be used in lieu of burner tilt position.

What we claim is:

1. An apparatus for generating steam comprising a furnace; means for burning an ash bearing fuel within said furnace; pressure parts including tubular surface lining the walls of said furnace; a plurality of zones on said furnace walls; a soot blower located in each zone; means for sensing the temperature of the pressure parts at a preselected location within each zone where the temperature is a function of the local heat absorption rate, which is in turn a function of the thickness of an ash layer thereon, said temperature sensing means being in contact with the pressure parts; means responsive to said temperature sensing means for activating a soot blower in the zone in which said temperature sensing means is located when the sensed temperature drops to a preselected temperature level.

2. An apparatus for generating steam comprising a furnace; means for burning an ash bearing fuel within said furnace; pressure parts including tubular surface lining the Walls of said furnace, the pressure parts having one surface thereof exposed to the burning fuel and ash; a plurality of zones in said furnace walls; a soot blower located in each zone; means for sensing the temperature of the pressure parts at a preselected location within each zone, said location being other than on the surface exposed to the burning fuel and ash; means responsive to said temperature sensing means for activating a soot blower in the zone in which said temperature sensing means is located.

3. A vapor generator having an apparatus for cleaning the furnace walls thereof comprising: a plurality of parallel fluid heating tubes joined by welded webs forming the Walls of the vapor generator furnace; soot blowers located in said furnace walls; means for determining the temperature of the web on the side of said web remote from the furnace; means for comparing said temperature with a temperature known to exist for a preselected furnace dirtiness condition; means for activating soot blowers in the general area of the temperature sensing means when the temperature so obtained is less than the predetermined temperature.

4. A vapor generator having an apparatus for cleaning furnace walls thereof comprising: a plurality of parallel fluid heating tubes joined by welded webs forming the walls of the vapor generator furnace; soot blowers located in said furnace walls; means for determining the temperature of the web at a preselected location; means for determining the temperature of the fluid within the adjacent tubes; means for determining the difference between said temperatures; means for comparing said temperature difference with a predetermined temperature difference known to exist for a preselected furnace dirtiness condition; means for activating soot blow-ers in the general area of the temperature sensing means when the temperature difference so obtained is less than the predetermined temperature difference.

5. A method of cleaning ash off the furnace Walls of a steam generator having the furnace walls made up of generally tubular pressure parts comprising: measuring the temperature of a portion of the pressure parts, with temperature sensing means being in contact with the pres sure parts; comparing the temperature so obtained to the temperature known to exist at the same location if the unit were operating with a normally dirty ashing condition; activating soot blowers to clean the general area in which the temperature was measured when the sensed temperature is lower than that known to exist for a normally dirty furnace.

6. A method of cleaning slag off the furnace walls of a steam generator having the furnace walls made up of generally tubular pressure parts, with the furnace walls having a plurality of areas each containing a soot blower, comprising: measuring the temperature of a portion of the pressure parts within an area of the furnace wall, with temperature sensing means being in cont-act with the pressure parts; comparing the temperature so obtained to the temperature known to exist at the same location if the unit were operating with a normally dirty slagging condition; activating soot blowers to clean the general area in which the temperature was measured when the sensed temperature is lower than that known to exist for a normally dirty furnace.

7. In a vapor generator having a furnace lined with tubes having a welded web connecting adjacent tubes, a method of removing ash from said walls comprising: sensing a temperature near the center of said Web intermediate the tubes in a selected location; sensing a temperature near the junction of said web and one of said tubes in said selected location; determining the difference in the two sensed temperatures; comparing said difference with a preselected value representing the temperature differonce known to exist at the selected furnace location for a normally dirty furnace; activating a soot blower in the zone of said temperature measurements when the temperature difference drops below said preselected value.

8. An apparatus for generating steam comprising a furnace; means for burning an ash bearing fuel within said furnace; pressure parts including tubular surface lining the walls of said furnace; a plurality of zones on said furnace Walls; a soot blower located in each zone; means for sensing an excess amount of ash in each Zone including temperature sensing means sensing at least one temperature of the pressure parts within said zone for determining a value indicative of the local heat transfer rate through the pressure parts, said temperature sensing means being in contact with the pressure parts; means for comparing said measured value wit-h a preselected known value for a normal condition of furnace dirtiness within said zone; and means for activating said soot blower within said zone when the measured value indicates a lower heat transfer rate than indicated by said preselected value.

9. An apparatus as in claim 8 having also means for determining the over-all dirtiness of said furnace; means for operating said furnace soot blowers When the overall furnace dirtiness reaches a preselected value, this means operating only the soot blowers in those zones which have been activated as a result of said temperature measurements.

10. An apparatus as in claim 9 including also means sensing the over-all dirtiness of said furnace after the completion of the soot blowing cycle; means for operating those soot blowers which have not been activated by the temperature sensing means and operating them sequential 1y until the over-all dirtiness of the furnace reaches a preselected value.

11. An apparatus as in claim 9 having also a flue for conveying combustion products from said furnace; steam heating surface located in said flue; and means for controlling the temperature of the steam leaving said steam heating surface; wherein the means for determining the over-all dirtiness of said furnace comprises means sensing the position of the steam temperature control means.

12. An apparatus for generating steam comprising a furnace; means for burning an ash bearing fuel within said furnace; pressure parts comprising tubular surface lining the walls of said furnace; a plurality of zones on said furnace walls; a soot blower located in each zone; means for sensing an excess amount of ash in each zone including means for sensing the temperature of the pressure parts within said zone, said temperature sensing means being in contact with the pressure parts, and means for comparing said measured temperature with a preselected known temperature for a normal condition of furnace dirtiness within said zone; and means for activating said soot blower within said zone when the measured temperature drops below said predetermined temperature.

13. An apparatus for generating steam comprising a furnace; means for burning an ash bearing fuel within said furnace; pressure parts comprising tubular surface lining the walls of said furnace; a plurality of zones on said furnace walls; a soot blower located in each zone; means for sensing an excess amount of ash in each zone including means for sensing a temperature difference across a portion of the pressure parts within said zone, means for comparing said measured temperature difference with a preselected known temperature difference for a normal condition of furnace dirtiness within said zone; and means for activating said soot blower within said zone when the measured temperature difference drops below said predetermined temperature difference.

References Cited by the Examiner UNITED STATES PATENTS 2,110,533 3/1938 Snow et al 122-392 2,811,954 11/1957 Hibner, et al 122-392 3,137,278 6/ 1964 Cantieri et a1. 122-392 References Cited by the Applicant Selection of Surface Thermometers for Measuring Heat Flux, by John W. Kurzrock, Cornell Aeronautical Laboratory, Inc. Report No. 124, February 1963.

CHARLES J. MYHRE, Primary Examiner.