EP2404147A1

EP2404147A1 - Arrangement for mounting a sensor in a heat exchanger wall

Info

Publication number: EP2404147A1
Application number: EP10748389A
Authority: EP
Inventors: Timo Leino; Markku Orjala; Jouko Aalto; Janne KÄRKI
Original assignee: VTT Technical Research Centre of Finland Ltd
Current assignee: VTT Technical Research Centre of Finland Ltd
Priority date: 2009-03-02
Filing date: 2010-03-02
Publication date: 2012-01-11
Also published as: FI20095206A0; KR20120007494A; EP2404147A4; WO2010100335A1; KR101618394B1

Abstract

The invention relates to an arrangement for mounting a sensor in a heat exchanger wall, which is formed of steel tubes (3, 4) welded next to each other with fin plates (11) in between the tubes forming a membrane wall. The sensor chamber (2) and the conductor channel (5) required by it for the sensor leads (13) are located on the furnace (8) side in a thickening (6) of the wall of the steel tube (4). A sensor element (1) to be attached to some other tube wall is formed for the measurement sensor chamber (2) as a homogeneous steel piece, comprising at least one length of steel tube (3, 4), in which the said wall thickening (6) is formed.

Description

ARRANGEMENT FOR MOUNTING A SENSOR IN A HEAT EXCHANGER WALL

The present invention relates to an arrangement for mounting a sensor in a heat exchanger wall, which is formed of steel tubes, of a selected internal diameter and minimum wall thickness, welded next to each other with fin plates in between the tubes forming a membrane wall, which delimit the furnace, from which the heat flux is coming, which is arranged to heat a high-pressure medium travelling in the steel tubes, and in which the sensor chamber, and the conductor channel it requires for the sensor leads, are located on the furnace side, in a thickening of the wall of the steel tube, and in which a sensor element to be attached to some other tube wall is formed for the measurement sensor chamber, comprising at least one length of steel tube, in which the said wall thickening is formed.

By means of the measurement of the heat flux, additional information is obtained on the temperature distribution of a steam boiler furnace, as well of the thermal load of. the e- vaporator, so that excessive heat fluxes and fouling of the evaporator surface can be measured.

Heat flux is measured using a heat-flux sensor, which is mounted in the tube wall. Various solutions are known, by means of which the sensor can be installed without disturbing the flows on the furnace side.

Publication US 6,485,174 discloses a heat-flux measuring ar- rangement to be mounted on a fin plate. Naturally, the fin plate permits a much easier way of mounting the sensor than the external surface of a tube. If a more accurate measurement of heat flux is required through the tube wall, it is made from the tube itself. In publication US 7,249,885, a dent is made on the furnace side, to create a suitable chamber for the sensors and their leads. After creating the sensor chamber and channel the dent is welded over, when the surface becomes uniform with the rest of the heat exchanger surface. This solu- tion contains a considerable drawback. Inside the tube the flow is disturbed due to a dent in an indefinite way. Especially in a boiler with natural circulation, the flow can differ substantially in the measurement tube from the other tubes .

In the article ^λThe measurement of radiant heat flux in large boiler furnaces-II. Development of flux measuring instruments', Neal, S. B. H. et al . , Int. J. Heat Mass Transfer, Vol. 23, pp. 1023 - 1031, a solution is also disclosed, in which the tube is bent inwards to form a bend and the cavity that arises is used as a mounting chamber, the cavity then being welded over evenly. In such a solution, the filler welding creates two kinds of problem. Firstly, its durability against bed material is questionable. Secondly, the extensive welding and non-homogeneous structure weaken the tube and create a particular stress state in it. Its structural durability is difficult to estimate.

The present invention is intended to create an arrangement es- pecially for the measurement of the heat flux of an evaporator surface, which does not have the aforementioned drawbacks. The arrangement according to the invention is characterized by what is stated in the characterizing portion of Claim 1. The homogeneous structure of the steel piece, in which the tube itself, the sensor chamber, and the leads channel can be machined and created in casting in the controlled circumstances, achieves precisely predictable structural and thermo-technical properties. The arrangement is also pre-eminently suitable for the measurement of other variables. By means of the arrangement that has been developed, heat flux in particular can be measured more precisely than previously from the evaporator surface of a fluidized bed boiler.

The measurement method is mounted permanently in the heat exchanger surface of the evaporator, so that the measurement results correspond very well to the real evaporator surface.

In one embodiment, the thickening of the wall faces inwards and the furnace side of the mounted sensor element lies on the plane of the rest of the heat exchanger surfaces, so that the channel formed by the internal diameter is shaped as a gentle bend and the sensor chamber and channel for the leads will fit into the local enlargement of the tube wall thickness . However, the arrangement according to the invention does not preclude the thickening being made as a very gently curved protrusion into the furnace, in which case the internal channel in the tube could be quite straight.

According to the invention, the sensor chamber and lead channel can be formed precisely in a wall thickening outside the minimum wall thickness.

In one embodiment, the tube is drilled into a steel piece. Alternatively, the sensor element is a cast piece, in which the medium channel is formed with the aid of a core. This can also be applied in the manufacture of the sensor chamber and conductor channel .

In one embodiment, the sensor element includes not only a first tube containing the sensor chamber, but also at least one tube welded to it on the side of the said channel, in which case the field conditions need not particularly endanger the channel and leads possibly inside it. This risk can be further reduced by forming shapes corresponding to the fin and welding in a homogeneous steel piece at least on the channel side, in which case the joint welding will be even further from the leads channel.

The sensor element formed in the homogeneous piece can also be corrosion coated with another material, for example, according to the requirements of a recovery boiler.

The sensor chamber can be under the surface or open out onto the surface, depending on the type of sensor in question. A thermocouple suitable for measuring heat flux is placed under the surface, whereas a sensor measuring corrosion will be placed on the surface.

The method according to the invention permits the measurement of heat flux and corrosion from an evaporator surface. Other benefits and embodiments of the invention are described here- inafter, in connection with an example application.

In the following, the invention is described with the aid of examples and with reference to the accompanying figures.

Figure 1 shows a cross-section of a sensor element according to the invention mounted on the wall of a boiler.

Figure 2 shows a top view of the sensor element of Figure 1, when separate. Figure 3 shows a view from the furnace of the sensor element, when separate.

Figure 4 shows an alternative way of forming the tube of the sensor element. The figures show a sensor element intended for the tubes of a boiler. The boiler can be a hot water boiler or a steam- generating boiler. In the figures, the furnace is marked with the reference number 8, the tube equipped with a bend with the number 4, the parallel tubes with number 3, and the internal channel of the tubes with the reference number 18, 18', of which the latter is the internal channel 18 ' in the location of the bend. A gentle bend do not disturb the flow of the medium 19, which is heated by the heat flux transferred from the furnace through the tube wall.

According to Figure 1, in the homogeneous structure of the tube 4 on the furnace 8 side there is a thickening 6, in which a sensor chamber 2 can be formed, and also a channel 5 (Figure 2) leading to it for the sensor leads.

In Figure 1, there is insulation 23 in the boiler wall on the boiler-room side of the tube wall and a skin plate 22 on the boiler-room side. The leads 13 (Figure 2) are run to the out- side of the wall in a conduit 14 and from there through a connector component 25 to a junction box 16. The tube 14 is supported on angle supports 17 and it should have a sufficient length to reduce heat conduction, though it can turn through a 90° bend. At the base of the junction box 16, there is a sili- con seal 26, in order to achieve steam tightness.

According to Figure 2, there are tubes 3 in the sensor element parallel to the special tube 4, attached to it by fin plates 11 and welding 12. In the tubes 3, 4, there is a common flow cross-sectional area, i.e. a channel, which is otherwise marked with the reference number 18, but at the bend location the channel is marked with the reference number 18 '. At the conductor channel 5, the fin plate 11 is first welded to the homogeneous steel piece, i.e. tube 4 with welds 12 and 12". After that the conductor channel 5 is drilled from the boiler- room side. On the boiler-room side, the weld 12" can be thicker, in order to give this area heat-transfer conditions similar to those of the other tubes.

The weld 12 ' of the fin plate 11 in the vicinity of the channel 5 to the adjacent tube is quite critical and is best made in factory conditions.

It is easy to see from this Figure 2 that it could be advantageous if the homogeneous piece also included in its shape the fin plate on the side of the channel 5, together with its welds. When the sensor element 1 is attached to the rest of the boiler's tube wall, the edge-most fin plates II¹ are welded onto the adjacent tubes. Correspondingly, the bevelled joint ends 3' and 4' (Figure 3) of the tubes 3, 4 are welded endwise to the tubes of the wall.

In Figure 2, the drill-hole direction 9 of the leads channel 5 is marked on the furnace side. This drill hole 9 connects to a drill hole made from the other side. Instead of drill holes, it is possible to use other machining procedures, such as electro-discharge machining. After the machining of the sensor cavity and the installation of the sensor itself, the cavity is closed by welding, unless the sensor is left on the surface intentionally. The leads 13 run on the hot side in the conductor channel 5. On the other side, the leads 13 can also be protected on the surface of the tube by welding on the channel 28 up to the connection conduit 14.

According to Figure 3, the sensor element 1 to be installed is very compact and can withstand installation without endangering the sensor in the sensor chamber 2 and its leads. The sensor element, together with the sensors and leads, can be cali- brated prior to delivery to the installation site, where it can be welded on, using conventional work procedures.

A homogeneous piece makes possible the best durability of the sensor element in terms of pressure resistance, while also giving excellent measurement precision.

A homogeneous piece according to Figure 4 can comprise not only a tube 4, but also a fin 11 integrated with it, at least on the side of the conductor channel 5. In this case, the fin 11 and the shapes corresponding to its welds are formed in same piece.

By means of the arrangement according to the invention, the heat flux, for example, can be measured from the evaporator surface of a fluidized bed boiler. By means of measurement of the heat flux, additional information is obtained on the heat load of the evaporator at the measurement location, making it possible to investigate both excessive heat fluxes and fouling on the evaporator surface. In addition, information on boiler's temperature distribution is obtained with different fuels. On the basis of the measurement results, additional information is obtained for the design of the boiler's evaporator and also information for the sootblowing.

Furnace corrosion measurements that have been developed earlier have been made with special probes, which are installed in the furnace through openings. Corresponding corrosion measurement methods do not exist.

Hot corrosion is a problem, especially on the evaporator and superheater surfaces of recovery and waste-incineration boilers. Using the arrangement according to the invention, the state of corrosion on the evaporator surface can be monitored at the measurement location, without disturbing the flow properties. The hot-corrosion sensor can itself operate in many different ways. The following are among the known types: Electrical Resistance (ER) , Electrochemical Noise (EN) , and Linear Polarization Resistance (LPR) . Each technique has its own specific sensor type, which is fitted to the sensor element according to the invention. The sensor is insulated from the steel tube by means of a suitable cast mass.

The corrosion monitoring can be used, for instance, to control the use of additives preventing corrosion (fuel, chemicals), as well as sootblowing as required, so that the material of the evaporator will last longer and the replacement interval will be lengthened. Based on the measurement results, addi- tional information is also obtained for the selection of the material of the boiler evaporator, as well as on the effects of different fuels on the corrosion of the evaporator surface.

By means of corrosion measurement, additional information will be obtained on the hot corrosion of the furnace of a steam boiler. The information can be used, for instance, in the selection of fuel blend, in the use of additives, and in material selection. Using the measurement method developed, hot corrosion can be measured from the evaporator surface of a power-plant boiler, without disturbing the flow of the furnace or steam. The measurement method is mounted permanently on the heat exchanger surface of the evaporator, so that the measurement results correspond well with the real evaporator surface.

The shape of the sensor chamber can be any whatever and be designed according to the sensor or sensors at the time. It can be a simple drill hole, for example, if the sensor is an optical fibre extending to the surface, for optical observation of the furnace . The arrangement according to the invention can be used in connection with nearly all types of sensor.

Claims

1. Arrangement for mounting a sensor in a heat exchanger wall, which is formed of steel tubes (3, 4), of a selected internal diameter and minimum wall thickness, welded next to each other with fin plates in between the tubes (11) forming a membrane wall, which delimit the furnace, from which the heat flux is coming, which is arranged to heat a high-pressure medium travelling in the steel tubes (3, 4), and in which the sensor chamber (2) and the conductor channel (5) it requires for the sensor leads (13) are located on the furnace side (8), in a thickening (6) of the wall of the steel tube (4) , and in which a sensor element (1) to be attached to some other tube wall is formed for the measurement sensor chamber (2), comprising at least one length of steel tube (3, 4), in which the said wall thickening (6) is formed, characterized in that the sensor element (1) is essentially a homogeneous steel piece, in which a channel (18, 18'), corresponding in internal diameter to the rest of the tube wall, is formed.

2. Arrangement according to Claim 1, characterized, in that the said thickening (6) of the wall faces inwards and the furnace-side (8) part of the installed sensor element (1) lies on the plane of the rest of the heat exchanger surface, in which case the channel (18, 18') forming the internal diameter is shaped in a gentle inward curve.

3. Arrangement according to Claim 1 or 2 , characterized in that the said sensor chamber (2) and conductor channel (5) are formed in the wall thickening (6) outside the minimum wall thickness.

4. Arrangement according to any of Claims 1 - 3, characterized in that the tube (18, 18') is drilled in the steel piece.

5. Arrangement according to any of Claims 1 - 3, characterized in that the sensor element (1) is a cast piece, in which the tube (18, 18') is formed with the aid of a core.

6. Arrangement according to any of Claims 1 - 5, characterized in that the sensor element (1) includes not only a first tube (4) containing the sensor chamber, but also at least one tube (3) welded to it on the said channel side.

7. Arrangement according to any of Claims 1 - 6, characterized in that the sensor element (1) includes a corrosion coating on the side next to the furnace (8) .

8. Arrangement according to any of Claims 1 - 7, characterized in that the sensor element (1) includes a thermocouple fitted in the sensor chamber (2) .

9. Arrangement according to any of Claims 1 - 8, character- ized in that the sensor element (1) includes a corrosion sensor fitted in the sensor chamber (2) .

10. Arrangement according to any of Claims 1 - 9, characterized in that the sensor element (1) includes a channel tube (14) attached rigidly to one of the said tubes (3) .

11. Arrangement according to any of Claims 1 - 10, characterized in that the homogeneous steel piece includes an external shape, which corresponds to at least one fin together with its welds .