Method and system for monitoring the mass changes of heat exchangers of a steam boiler

Abstract

A method and a system for monitoring the mass changes of a heat exchanger bank of a steam boiler. The heat exchanger bank is supported by hanger rods and support beams to the frame beams of the steam boiler. In the method, the web plates of the support beams are provided with strain gages for measuring the changes in the strain state generated in the support beams by the mass of the heat exchanger bank. The system includes strain gages fixed to the web plates of the support beams for measuring the changes in the strain state generated in the support beam by the mass of the heat exchanger bank, data logging means for logging the measurement data and a computer or a corresponding calculation unit for processing the measurement data.

Claims

1. A method for monitoring mass changes of a heat exchanger bank of a steam boiler having an upper section including frame beams supporting support beams, wherein the support beams rest on the frame beams at support points, wherein the heat exchanger bank is supported by vertical hanger rods, and the vertical hanger rods are supported by the support beams, the method comprising: monitoring strain in vertical support elements of the support beams by strain gages mounted to the vertical support elements, wherein the strain gages are each located a horizontal distance from a nearest one of the support points no greater than a vertical width of the vertical support element, and using signals from the strain gages which indicate the strain in the vertical support elements to calculate a change in the mass of the heat exchanger bank.

2. The method according to claim 1, wherein the monitoring of strain includes monitoring changes of a strain state at each of the support points by four strain gages of the strain gages proximate each of the support points, wherein the four of the strain gauges form a Wheatstone full bridge connection.

3. The method according to claim 2, wherein two of the four strain gauges are provided in a direction of a principal strain and another two of the four strain gauges are provided perpendicularly to said direction of the principal strain.

4. The method according to claim 3, wherein the principal strain is determined by a finite-element method.

5. The method according to claim 1, wherein at least one of the support beams includes a parallel pair of the vertical support elements, wherein each of the parallel pair of the vertical support elements has mounted thereto a respective one of the strain gages.

6. The method according to claim 1, wherein the strain gauges are each located on a respective one the a vertical support elements at a point to which a principal strain provided by a support force (RFEM) of one of the frame beams is applied at an angle of 45 degrees.

7. The method according to claim 1, further comprising calibrating a mass measurement system by placing a known mass in a known position on one of the support beams.

8. The method according to claim 7, further comprising calibrating the mass measurement system by a finite-element method.

9. A system for monitoring mass changes of a heat exchanger bank of a steam boiler, wherein the steam boiler includes an upper network of horizontally oriented frame beams and support beams extending over and supported by the frame beams at support points where the support beams attach to the frame beams, wherein the heat exchanger bank is suspended below the network by vertical hanger rods connected to and hanging down from the support beams; strain gauges fixed to vertical support elements of the support beams and each of the strain gauges located a horizontal distance from a nearest one of the support points no greater than the vertical width of the vertical support element to which the strain gauge is fixed, wherein the strain gages are configured to measure changes of a strain state in one or more of the support beams due to the mass of the heat exchanger bank; a data logger configured to log the measurements of the strain gauges, and a computer configured to process the measurement data.

10. A method to detect a mass change of a heat exchanger bank suspended in a steam boiler, wherein the steam boiler includes an upper network of frame beams and support beams extending across the frame beams and resting on the frame beams at support points, wherein the support beams each include an upper flange, a lower flange and a vertical web plate joining the upper flange to the lower flange, and the lower flange rests on the frame beams; wherein the heat exchanger bank is suspended below the upper network of frame beams and support beams by vertical hanger rods, and the vertical hanger rods are attached to the upper flange of the support beams, the method comprising: monitoring strain in the vertical web plate of at least one of the support beams by strain gages mounted to the vertical web plates at locations proximate the support points, wherein each of the locations on the vertical web plates are a horizontal distance from a nearest one of the support points no greater than the vertical width of the web plate, and detecting a change in a mass of the heat exchanger bank using signals generated by the strain gages, wherein the signals indicate a strain on the vertical web plates to which is attached the strain gage generating the signal.

11. The method of claim 10 wherein the detection of the change in the mass includes comparing the signals from the strain gages on a first one of the vertical support beams to the signals from the strain gages on a second one of the vertical support beams, wherein the first one is upstream of the second one in a flow direction through the steam boiler.

12. The method of claim 11 further comprising controlling a soot blowing operation to remove deposits on the heat exchanger bank.

13. The method of claim 10 further comprising using the detected change in the mass to initiate at least one of a soot blowing operation, a temporary cooling cycle of the steam boiler and a shut-down of the steam boiler.

14. The method of claim 1 wherein each of the strain gages is adjacent a corresponding hole in the support element on which the strain gauge is attached.

15. The system of claim 9 wherein each of the strain gages is adjacent a corresponding hole in the support element on which the strain gauge is attached.

16. The method of claim 10 wherein each of the strain gages is adjacent a corresponding hole in the vertical web plate on which the strain gauge is attached.

Description

LIST OF DRAWINGS

(1) FIG. 1 illustrates as a side view the heat exchanger banks of a recovery boiler and their support on the frame beams of the boiler.

(2) FIG. 2 illustrates the support structures of two heat exchanger banks as seen from above.

(3) FIG. 3 shows the support points of a support beam.

(4) FIG. 4 shows an example of the fixing of a hanger rod to a support beam.

(5) FIG. 5 illustrates a measurement point at the end of a support beam.

(6) FIG. 6 illustrates a measurement point in the middle of a support beam.

DETAILED DESCRIPTION OF THE INVENTION

(7) Suspending the structural parts of a steam boiler from the frame columns 16 and frame beams 15 facilitates their unobstructed thermal expansion. FIG. 1 illustrates the support of the heat exchangers 10, 11, 12 of a recovery boiler to the frame structures 15, 16 of the boiler. The number of heat exchanger banks 10, 11, 12 varies according to the type of boiler. In this case the recovery boiler comprises eight heat exchanger banks 10, 11, 12, of which in the flow direction of flue gas the five first ones are superheaters 10, which are followed by boiler tubes 11 and the last two heat exchanger banks are water preheaters 12. Each heat exchanger bank 10, 11, 12 comprises a number of plate elements consisting of heat transfer tubes, inside which elements the heat transfer medium (water or steam) flows. The heat releasing hot flue gas flows between the heat exchanger plates, which form the heat exchanger bank 10, 11, 12. Each heat transfer plate is suspended by means of two hanger rods 13 from two support beams 14, which in turn are supported to the frame beams 15.

(8) FIG. 2 illustrates two heat exchanger banks 10, 11 as seen from above the boiler, each of which banks is suspended by means of hanger rods (not shown) from two support beams 14, which are supported by four frame beams 15. Each support beam 14 is supported by each frame beam 15 at four support points s1, s2, s3, s4. FIG. 3 shows the support beam 14 and its support points s1, s2, s3, s4 as a side view. FIG. 4 shows a way of fixing the hanger rod 13 to the support beam 14. As illustrated, the hanger rod 13 is arranged to pass through the support beam 14 and the upper end of the hanger rod 13 is secured to the upper surface of the support beam 14 by means of a nut 19.

(9) The support beam 14 illustrated in FIG. 4 comprises a lower flange 20, an upper flange 21 and two web plates 22 connecting the lower flange 20 to the upper flange 21. The upper flange 21 bears the weight of a heat exchanger bank 10, 11, 12 transmitted to the upper flange 21 by hanger rods 13. The lower flange 20 receives support forces R.sub.FEM exerted by frame beams 15 on the support beam 14 at support points s1, s2, s3 and s4. The web plates 22 do not substantially bear a load but are subject to the shear forces arising due to the support forces R.sub.FEM and the load transmitted by the hanger rods.

(10) The support beam 14 may be for example a single-web I-beam or a double-web box beam or rectangular beam. In all cases, the support beam 14 comprises an upper horizontal support element, a lower horizontal support element and at least one vertical support element that connects them. From the vertical support element, the stretching caused by the strains that are generated in the support beam 14 by the support reaction R.sub.FEM can be measured.

(11) Heat exchanger banks 10, 11, 12 suspended from hanger rods 13 exert a downwards drawing force on a support beam 14, which is compensated for by a support force, i.e. support reaction R.sub.FEM, applied from a frame beam 15 to the support beam 14 at support points s1, s2, s3, s4. These load forces and support forces cause local strain states in the support beam 14, which can be measured by means of strain gages. Therefore, in the method according to the invention, the mass changes of the heat exchanger bank 10, 11, 12 are measured by means of strain gages that are glued to the support beams 14 supporting the heat exchanger bank. Preferably these strain gages are located near to the support points s1, s2, s3, s4 of the support beams 14.

(12) FIG. 5 shows the preferred position of strain gages 17 on the web plate 22 of a support beam 14 near to a support point s1 or s4 at the end of the beam, at which support point a support force R.sub.FEM is applied to the support beam 14.

(13) FIG. 6 shows correspondingly the preferred position of strain gages 17 on the web plate 22 of a support beam 14 near to support points s2 (shown in the figure) and s3 (corresponds to s2) in the middle of the beam, at which support points a support force R.sub.FEM is applied to the support beam 14.

(14) FIG. 6 also shows that a spacer element 23 may be located at a support point s3 (and s2, respectively) between a support beam 14 and a frame beam 15 in order to limit the area subject to the support reaction, which facilitates the arrangement of the measurement point near to the lower edge of the web 22 in the position most preferred in terms of detecting the change in stretching caused by the principal strain. A vertical stiffener 24 may preferably be fixed to the web 22 of the support beam 14 at the support point s2, s3. A vertical stiffener 24 at the support point prevents buckling of the web 22 of the support beam.

(15) Preferably the strain gages used are two pairs of strain gages 17, the strain gages being disposed at an angle of 90? to one another and one strain gage in the pairs of strain gages 17 being positioned in the direction of the principal strain. Thus, in measuring a support point s, a total of four strain gages are used in order to simultaneously conduct measurements in two directions. Preferably the pairs of strain gages 17 are located on the web 22 of a support beam 14 at a point to which the principal strain caused by the support force R.sub.FEM of a frame beam 15 is applied approximately at an angle of 45?. Two pairs of strain gages 17 are provided in connection with each support point s to be measured, which gages are connected to each other so that they form a Wheatstone bridge. The pairs of strain gages 17 are preferably glued near to a hole or a notch 18 made near to the support point s1, s2, s3, s4 of the support beam 14. The hole or notch 18 amplifies the strain states of the support beam 14 and improves the sensitivity of the measurement. Measurements of the strain changes in the direction of the principal strains may also be measured by other known measurement methods, observing their known limitations and characteristics.

(16) When strain gages 17 are only arranged on one web plate 22 of a support beam 14, the possible twisting of the support beam in the direction of the longitudinal axis can cause inaccuracy in the measurement results. If so required, this can be eliminated by arranging the measurement points symmetrically on both web plates 22 of the support beam 14, whereby the measurement results from each support point s1, s2, s3, s4 are obtained by summing the measurement results from the measurement points of both web plates 22. In this case, the amount of the measurement channels needed is doubled.

(17) An error caused by temperature changes is automatically compensated for, when strain gages are connected in a Wheatstone full bridge connection and all sensors are at the same temperature. The measurement bridge is connected so that as the resistances of the sensors change in the same direction (temperature changes), the bridge remains is equilibrium, but as the resistances change in different directions, the bridge easily falls out of equilibrium. When a constant voltage source is fed into the bridge, the loss of equilibrium is seen as the output voltage of the bridge. Thus, the output voltage is directly proportional to the strain state of a support beam. The signal obtained from the bridge connection is amplified and entered via a data logging system into a calculation unit for saving and analyzing the data.

(18) The measurement system comprises at least a data logging system and a computer or another calculation unit having a computer program for processing the data recorded by means of strain gages. The computer program receives the measurement data coming from the data logging system, processes it by converting the measurement signal to data representing the mass or mass change of the heat exchanger, compiles statistical data and converts the data to other formats.

(19) The calibration of the measurement system can be performed as direct calibration by loading a support beam 14 with known masses or by means of the finite-element method. In direct calibration, the calibration coefficients are calculated using signals measured by means of known masses. In dynamic calibration, masses are moved along the support beam 14, while in static calibration, known static masses are suspended from several points of the support beam 14. In calibration based on the finite-element method, theoretical calibration coefficients are calculated by means of the setting values of the data logging system, characteristics of strain gages and theoretical stretch values in the optimal positions of the strain gages 17 obtained by finite-element modeling.

(20) The digital signals measured by means of strain gages 17 can be converted to values representing the mass or mass change of a heat exchanger bank 10, 11, 12 by means of the following equation:
W.sub.i,s=CF.sub.?-W,s.Math.CF.sub.S-?.Math.(S.sub.i,s?S.sub.0,s)(1)
where W.sub.i,s is the mass measured at a support point s at a moment of time t.sub.i; CF.sub.?-W,s is the conversion factor between stretch and a mass applied to the support point s (equation 2). The theoretical value of the factor CF.sub.?-W,s can be calculated by the finite-element method from the strains of the beam; CF.sub.S-? is the conversion factor between the digital output signal of the data logging system and the stretch measured from the strain gages (equation 3). CF.sub.S-? depends on the settings of the data logging system and the configuration of the strain gages (in this case a Wheatstone full bridge having four active gages at each support point s); S.sub.i,s is the output signal produced by the data logging system from the strain gages at the support point s at a certain moment of time t.sub.i; S.sub.0,s is the output signal produced by the data logging system from the strain gages at the support point s, which corresponds to a 0-mass (i.e. the output signal at the beginning of the measurements while the boiler is clean).

(21) The conversion factor CF.sub.?-W,s can be calculated by means of the following equation:

(22) $\begin{array}{cc}{\mathrm{CF}}_{\mathrm{.Math.}-W,s}=\frac{R_{\mathrm{FEM}}}{{\mathrm{.Math.}}_{1,5}-{\mathrm{.Math.}}_{2,5}+{\mathrm{.Math.}}_{3.5}-{\mathrm{.Math.}}_{4.5}}& \left(2\right)\end{array}$
where R.sub.FEM is the support reaction at a support point s calculated by the finite-element method (in order to obtain the mass W.sub.i in kilograms, R.sub.FEM has to be converted to kilograms); ?.sub.1,s . . . ?.sub.4,s is the relative stretch of strain gages in a specific direction calculated by the finite-element method (FIGS. 4 and 5).

(23) The conversion factor CF.sub.S-? can be calculated by means of the following equation:

(24) $\begin{array}{cc}{\mathrm{CF}}_{S-\mathrm{.Math.}}=\frac{4.Math.U_{\mathrm{out}}}{\mathrm{GF}.Math.U_{\mathrm{exc}}}& \left(3\right)\end{array}$
where U.sub.out is the output voltage, i.e. measured voltage, of the data logging system (equation 4); GF is the gage constant given by the strain gage producer; U.sub.exc is the excitation voltage of the Wheatstone bridge.

(25) The output voltage U.sub.out can be calculated by means of the following equation:

(26) $\begin{array}{cc}{U}_{\mathrm{out}}=\frac{?U}{\mathrm{gain}.Math.2^m}& \left(4\right)\end{array}$
where ?U is the overall measurement zone of the data logging system; gain is the amplification coefficient of the data logging system; m is the resolution of the data logging system.

(27) By means of adding up the support reactions R.sub.FEM providing the equilibrium of the loads applied to one support beam 14 at a certain moment of time and comparing them to a situation where the heat exchanger bank 10, 11, 12 has just been cleaned, the change of the overall support reaction after cleaning of the heat exchanger bank and thus the mass increase of the heat exchanger bank are obtained.

(28) When comparing the strain state changes of support beams 14 supporting the front edge and the rear edge (as seen in the flow direction) of a heat exchanger bank 10, 11, 12 at individual measurement points, a view can be obtained on how the mass increases are accumulated in the horizontal plane, i.e. on the left or right side or on the side of the front edge or the rear edge. Since soot-blowing devices are also on different heights, the mass change exited during an individual soot-blowing operation allows one to estimate the mass accumulation of the heat exchanger bank 10, 11, 12 and the efficiency of the soot-blowing also in the vertical direction. Since the mass accumulation is often uneven and two- or even three-dimensional data on the accumulation and exit of mass is obtained, the soot-blowing devices can be controlled in the most optimal way by means of the measurement data obtained to remove the deposits at a proper time within the operational area of even a single soot-blowing device.