A METHOD AND AN ARRANGEMENT FOR MEASURING MASS CHANGES OF HEAT EXCHANGERS OF A STEAM BOILER

Abstract

A method for measuring mass changes of a heat exchanger bank (1, 2, 3) or the heat exchangers thereof of a steam boiler, which heat exchanger (4) is supported by hanger rods (7) to support beams (5,8) above the steam boiler, wherein at least one hanger rod (7) of at least one heat exchanger (4) is connected a lower measuring element (9) and an upper measuring element (10), and the changes of the measuring length (X) between the measuring elements (9, 10) is measured by a measuring instrument (15) connected between the measuring elements (9, 10) for measuring the mass changes of the heat exchanger (4). The measuring instrument (15) may be attached in between the measuring elements (9, 10) and the change in the measuring length (X) is measured by the deformation of the measuring instrument (15). A connecting member (11) parallel to the hanger rod (7) may be located between the measuring elements (9, 10), which connecting member (11) relays the change in the length to the measuring instrument (15). An elastic member (16) may be attached between the measuring element (9) and the upper measuring element (10).

Claims

1. A method for measuring mass changes of a heat exchanger bank of a steam boiler or heat exchangers in the heat exchanger bank, wherein the heat exchangers are is supported by hanger rods attached to support beams above the steam boiler, wherein at least one of the hanger rods supporting at least one of the heat exchangers is connected a lower measuring element and to an upper measuring element, and the method comprises: measuring a change of a measuring length between the upper and lower measuring elements by a measuring instrument connected between the measuring elements, and calculating a mass changes of the heat exchanger based on the measured change in the measured length.

2. The method of claim 1, wherein the measuring instrument is between the measuring elements and the measurement of the change in the measuring length is a measurement of a deformation of the measuring instrument.

3. The method of claim 1, wherein a connecting member parallel to the hanger rod is between the upper and lower measuring elements, wherein the connecting member relays the change in the length to the measuring instrument.

4. The method of claim 1, wherein an elastic member is attached between the lower measuring element and the upper measuring element.

5. The method of claim 1, wherein the measuring length between the lower measuring element and the upper measuring element is in a range of 200 mm to 2000 mm.

6. The method of claim 1, wherein the measuring the at least one heat exchanger to which is connected the upper and lower measuring elements is a connected to a leading edge of the at least one heat exchanger.

7. A measuring arrangement for measuring mass changes of heat exchangers of a steam boiler, wherein the heat exchangers are in a heat exchanger bank and are supported by hanger rods attached to frame beams above the steam boiler, the measuring arrangement comprising: a lower measuring element connected to a first hanger rod of the hanger rods; an upper measuring element connected to the first hanger rod, wherein a measuring length is a vertical distance between the upper and lower measuring elements; a measuring instrument connected to the upper and lower measuring elements, wherein the measuring instrument is configured to measure a change in the measuring length between the lower measuring element and the upper measuring element, wherein the change in the measuring length is indicative of a change in a mass of at least one of the heat exchanger.

8. The measuring arrangement of claim 7, wherein the measuring instrument is between the measuring elements.

9. The measuring arrangement of claim 7, further comprising: a connecting member offset from and parallel to the hanger rod and between the measuring elements, wherein the measuring instrument is attached to the connecting member.

10. The measuring arrangement of claim 9, further comprising an elastic member attached to the connecting member between one of the upper and lower measuring elements and the measuring instrument.

11. The measuring arrangement of claim 10, wherein a coefficient of thermal expansion of the elastic member is similar to a coefficient of thermal expansion as the first hanger rod.

12. The measuring arrangement of claim 10, wherein the elastic member includes a frame to which is attached the measuring instrument, and the measuring instrument includes a strain gage configured to measure a deformation of the frame, wherein the measurement of the deformation is indicative of the change in the measuring length.

13. The measuring arrangement of claim 7, wherein the measuring instrument includes a metal wire and a device measuring a specific frequency of the metal wire.

14. The measuring arrangement of claim 7, wherein the upper measuring element is mounted to support beam connected to the first hanger rod.

15. The measuring arrangement of claim 9, wherein the measuring length is in a range of 200 mm to 2000 mm.

16. A method to measure a mass changes of a heat exchanger in a steam boiler, wherein the heat exchanger is suspended by a first hanger rod attached to a support beam above the heat exchanger and the first hanger rod, the method comprising: directing flue gases over the heat exchanger during operation of the steam boiler, wherein impurities from the flue gases accumulate on the heat exchanger and thereby increase a mass of the heat exchanger; measuring a change in a distance between an upper measuring element attached to the first hanger rod or the support beam, and a lower measuring element attached to the first hanger rod, wherein the distance is in a range of 20 centimeters to 2 meters; and cleaning the impurities from the heat exchanger in response to a value dependent on the measured change in the distance exceeding a predetermined value.

17. The method of claim 16, further comprising estimating a change in the mass of the heat exchanger due to accumulation of the impurities on the heat exchanger, wherein the estimated change is based the measured change in the distance, wherein the predetermined value in the step of cleaning the impurities is a predetermined value of a mass change of the heat exchanger, and wherein the estimated change in the mass is used to determine the value dependent on the measured change in the distance.

18. The method of claim 16, wherein a connecting element is between the upper measuring element and a lower measuring element, and the connecting element is offset from the first hanger rod, wherein the measurement in the change in the distance is based on an output signal from a load sensor measuring a load applied to the connecting element by the upper measuring element and the lower measuring element.

Description

LIST OF DRAWINGS

[0019] 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,

[0020] FIG. 2 illustrates a measuring arrangement for measuring the change of a distance,

[0021] FIG. 3 illustrates an advantageous measuring arrangement, in which the upper measuring element is the support beam and

[0022] FIG. 4 illustrates an advantageous measuring instrument.

DETAILED DESCRIPTION OF THE INVENTION

[0023] FIG. 1 illustrates that suspending the heat exchanger banks 1, 2, 3 by the frame columns 6 and the frame beams 5 facilitates their unobstructed thermal expansion. The number of heat exchanger banks 1, 2, 3 varies depending on the boiler type. In this case the boiler comprises eight consecutive heat exchanger banks, out of which the first four in the flow direction of flue gas are steam superheaters 1, followed by boiler tubes 2 for evaporating water and the last two heat exchanger banks are water preheaters 3. Each heat exchanger bank 1, 2, 3 comprises a number of heat exchangers 4 in which water of steam flows. There is typically 20-30 heat exchangers 4 situated side by side in each exchanger bank 1, 2, 3. Heat releasing hot flue gas flows between the heat exchangers 4. Each heat exchanger is suspended by two hanger rods 7 from two support beams 8, which in turn are supported to the frame beams 5. If, for example, the structure has a spacer beam supported by the support beams 8 by means of hanger rods 7, which supports the heat exchangers with the lower hanger rods 7, more accurate results are obtained from the lower hanger rods 7.

[0024] If there are spacer beams or collector pipes of inlet and outlet flows of a heat exchanger bank below hanger rods 7 supported from the support beam 8 spreading forces horizontally, measurement of the mass changes cannot directly be addresses to an individual heat exchanger 4. Still, by measuring from the hanger rods 7, it can be indicated in which section of the heat exchanger bank fouling has accumulated and soot-blowing can be targeted according to the indication. In that case a good resolution of measurement is especially important because the intermediate structures will dampen measurements of changes related to individual heat exchangers.

[0025] The change in mass of the heat exchangers 4 can be most accurately measured by combining the measurements of the hanger rods 7 of the leading edge and the trailing edge. Sufficient accuracy is already achieved with measurements of the leading edge hanger rods 7. The leading edge fouls faster than the trailing edge. Contamination also reveals itself as an increasing resistance of flow of flue gas, whereby the front edge hanger rods 7 are subjected to an additional load caused by the increasing flow resistance. Thus, in the front edge hanger rods 7, an amplification of the fouling indication always happens. In the case of the trailing edge hanger rods, the situation is the opposite.

[0026] FIG. 2 illustrates an arrangement, where the change in distance of measuring elements 9, 10 is measured indirectly with a load sensor 15. The load sensor 15 can be an arrangement based on one or plurality of strain gages connected to an item owing an applicable stiffness. Load sensors 15 are also widely available commercially. An elastic member 16 is fastened between the measuring element 9, 10 and the load sensor 15. The elastic member 16 may be, for example, an open, thin and short profile tube. If the elastic member 16 is not used, then the thickness of the rod acting as a connecting member 11 and the elasticity of the load sensor 15 as a measuring instrument must be carefully selected, because remarkable forces within the elasticities in the connecting member 11, the load sensor 15 and the fixtures of the measuring elements 9, 10 affect to the accuracy of the method and to the magnitude of the active forces.

[0027] The ratio of the data obtained from measurement means 15 and the mass changes of the hanger rod 7 must usually be calibrated during installation. The calibration can be performed for example by a known mass effecting loading of a hanger rod 7. The measuring arrangement can also be preloaded. Preferably an intermediate support 17 supports the thin and long connecting member 11 relaying the change of the distance. Especially when the measuring instrument extends from the lower measuring element to the upper measuring element, the connecting member 11 is not required.

[0028] Minimal movement and displacement of the measuring element 9, 10 on the surface of the hanger rod 7 may occur during the use of the boiler. Preferably, the bodies of the measuring elements 9, 10 are tightened around the suspension rods 7 asymmetrically in the vertical direction, e.g. with screws 14, preferably on the side closer to the second measuring element 9, 10. The tightest tightening point can further be designed to locate on the surface closest to the second measuring element. Due to the arrangement, elongation changes do not result in the movement of the measuring element 9, 10 on the surface of the suspension rods 7 and the distance between the fixing points of the measuring elements 9, 10 i.e. measuring length X remains unchanged.

[0029] FIG. 3 illustrates a very advantageous arrangement where the upper measuring element 10 is a support beam 8. The measuring length X extends to the upper surface of the support beam 8, on top of which the fastening element of the hanger rod 7 is located. In this solution, the elastic member 16 and the measuring instrument 15 are combined to the same component. The measuring instrument 15 is preferably attached to the lower flange plate of the support beam 8 and does not affect the measuring length X, because the support beam 8 does not substantially flex vertically due to mass changes or measuring forces and it has normally substantially the same thermal expansion coefficient as the hanger rod 7. The measuring instrument 15, the elastic member 16 or the connecting member 11 may also be attached to the upper flange plate or to the web either aside or through the lower flange plate, whereby the lower flange plate can act as an intermediate support for the connection member 11.

[0030] As illustrated in the embodiments of FIG. 2 or 3 the rod functioning as connecting member 11, an elastic member 16 and/or measuring instrument 15 can be replaced by a thin pre-stressed metal wire, and changes of the measuring length X can be measured by an instrument measuring the changes of the specific frequency of it. In this embodiment an intermediate support 17 is not necessary unless it is necessary to adjust the specific frequency to a certain frequency range. A measuring solution using the same principle is disclosed in publication FI20080395, where change of the specific frequency of hanger rods is directly measured. However, the solution according to the publication has the same problem linked to the resolution as exists with other measurement solutions where tension of a hanger rod 7 is measured directly.

[0031] FIG. 4 shows an example of the measuring instrument 15, which also functions as an elastic member 16. The frame 18 of it is a sleeve-like piece provided with fixing holes 19 in order to attach it to the connecting member, to the measuring element 9, 10 or to the support beam 8. Fastening the sleeve to the measuring arrangement may be accomplished with any other robust fasteners or joining means. At least one strain gage 20 is attached to the frame 18 for measuring deformations of it. The strain gage 20 is connected to a measuring device (not represented) which transforms changes of it's electric properties into measurement results. The obtained results can be calibrated to be proportionate to changes in X, whereby heat-exchangers' mass changes can be calculated using the coefficient of elasticity and the cross-sectional area of the hanger rods.

[0032] Advantageously the measuring arrangement is pre-stressed so that the frame 18 is under a tensile force in all circumstances and it will not be subjected to compressing forces. Thereby any possible non-linearity in the measurement results caused by clearances of the structure are avoided. The frame 18 can also be a piece of profile of some other closed or open shape than a circular sleeve, or it can be a straight metal sheet thinned at the location of the strain gage. In connection with a straight metal sheet, it may be necessary to use a separate elastic member 16. Because the changes in the stretching of the hanger rods 7 are very small, the structure of the measuring instrument 15 can be very slim. Thereby even the smallest changes in the measurement length X can be indicated with high resolution by the strain gage measurements and the structure of the arrangement is not subjected to significant forces.

[0033] A solution according to the invention could also be realized so that very long, strain gages 20 reaching the mentioned advantageous lengths or subsequent combinations of them thereof are fixed directly onto the surfaces of the hanger rods 7. Thereby the highest and lowest fixing points of the strain gages would function as the measuring elements 9, 10, from which the changes of the measuring length X is measured. Long strain gages normally would require thermal sensors aside them for accomplishing thermal compensation.