WO2014118069A1

WO2014118069A1 - Method for determining an electrical resistance

Info

Publication number: WO2014118069A1
Application number: PCT/EP2014/051287
Authority: WO
Inventors: Steffen BÜHLMEYER; Axel FELDERER; Michael Rollmann
Original assignee: Siemens Aktiengesellschaft
Priority date: 2013-01-30
Filing date: 2014-01-23
Publication date: 2014-08-07

Abstract

The invention relates to a method for determining an electrical resistance (R_M) between at least two electrical conductors (1, 2, 7, 8) arranged on the surface of a metallic component (3), which method comprises the steps: a) fastening at least four electrical conductors (1, 2, 7, 8) on the surface of the component (3) in a defined areal or spatial arrangement; b) connecting in each case two electrical conductors (1, 2) to a current source (4) and measuring the voltage drop or a physical variable representing same between the conductors (1, 2), wherein this process is carried out for all the possible conductor pairs (1, 7; 2, 7; 7, 8; 1, 8; 2, 8) of at least four electrical conductors (1, 2, 7, 8), and c) computationally determining the electrical resistance (R_M) between at least two conductors (1, 2, 7, 8) based on the measurement results determined in step b).

Description

Method for determining an electrical resistance

The present invention relates to a method for determining an electrical resistance between at least two arranged on the surface of a metallic component electrical conductors based on the potential probe method. In the operation of a machine, in particular a gas or steam turbine, cracks can occur in components that grow during operation and possibly lead to failure of the component. Most components can tolerate cracks up to a critical size, with the critical crack size generally being conservatively estimable, for example, based on finite element analysis performed in consideration of material data, operational stress fields, and the like. On the other hand, the timing of crack initiation and subsequent crack growth at the initial stage can not be predicted with certainty because, on the one hand, cracking and crack growth are highly variable and, on the other hand, crack growth simulations associated with finite element analysis are complex and not always reliable , For reasons of safety, components are usually replaced preventatively in the event of a crack diagnosis during a revision, even if it can be assumed from experience that a component will continue to be reliable for a longer period of time. An early and sometimes unnecessary replacement of components is associated with high costs, which is not desirable in principle.

The presence of cracks is determined during a revision using conventional nondestructive testing methods, for example by means of magnetic particle testing,

Injection testing or the like, to name just a few examples. After a crack diagnosis, the crack geometry is to be determined in order to determine the remaining service life to meet the component. For this purpose, the detected cracks are usually measured in a first step on the surface. In this way, the length and width of a crack are recorded. In addition, the crack depth must be determined using a suitable method.

One way to get information about the crack depth is to remove material in stages until the crack no longer exists. Alternatively, test wells may be performed, whereupon the crack depth in the generated wells is determined. A disadvantage of these methods, however, is that they are only partially or not applicable in cases where the crack depth is very large or the crack is located at an unfavorable position of the component. In addition, these methods can only be performed during a revision and not during operation.

An alternative is provided by ultrasonic methods. But their use is not possible in many cases. In addition, ultrasound methods can usually only be carried out as part of a revision, in particular if the operating temperature exceeds the maximum operating temperature of ultrasound heads.

Another variant for crack depth determination is the so-called AC potential method, which is used, for example, in the RMG4015 measuring instrument from Karl Deutsch. The meter has four probes in series, which are placed on the surface of a component. The outer two probes introduce a constant current of known frequency into the device. The voltage dropping in the material is then tapped between the two inner probes. If there is a crack between the two inner probes, a higher voltage is measured than if there were no crack, since the path for the current in the case of a crack is two times the crack depth. However, a major disadvantage of the meter is in that it can only be used at low temperatures, which are often below the temperatures to which a component is exposed during operation. For this reason, it is often only possible to carry out measurements within the scope of a revision.

Based on this prior art, it is an object of the present invention to provide an alternative method of the type mentioned, which can also be used at high operating temperatures.

To achieve this object, the present invention provides a method for determining an electrical resistance between at least two arranged on the surface of a metallic component electrical conductors, comprising the steps:

a) securing at least four electrical conductors on the surface of the component in a defined planar or spatial arrangement;

b) Connect two electrical conductors to one

Current source and measuring the voltage drop or a physical size between the conductors representing them, wherein this process is performed for all of the possible conductor pairings of at least four electrical conductors, and

c) computational determination of the electrical resistance between at least two conductors based on the measured results determined in step b). The arrangement of the at least four electrical conductors on the surface of the component in a defined planar or spatial arrangement enables a reliable determination of the electrical resistance between at least two of the electrical conductors, regardless of the line resistance of the electrical conductors and / or of the contact resistance between the electrical conductors and the component. On the one hand, this opens up the possibility of selecting the electrical conductors so long that the temperature-sensitive sensitive measuring technology can be arranged outside a machine so that it is not exposed to operating temperatures. On the other hand, the connections between the electrical conductors and the component can be chosen such that they can withstand high temperatures. In this way it is possible to carry out measurements even at high temperatures. Accordingly, a component exposed to high operating temperatures can be monitored during operation, for example with regard to the formation or growth of cracks. A component affected by a crack may accordingly be left in the machine until a critical crack geometry has been reached. Premature removal of components subject to cracking for reasons of safety can thus be prevented.

Preferably, the fastening of the electrical conductors on the surface of the component takes place in step a) using a cohesive method, in particular by means of hard soldering or welding. In this way, a secure and temperature-resistant attachment of the electrical conductors is achieved on the component surface, the temperature resistance is naturally dependent on the temperature properties of the materials involved in the cohesive connection materials. At very high operating temperatures, which may for example be in the range of several hundred degrees, welded joints are preferable to the solder joints due to their higher temperature resistance. The electrical conductors advantageously have a length of several meters in order to be able to set up the measuring technique remotely from a machine in which a component to be monitored is installed. In this way it can be prevented, in particular, that the measuring technology is exposed to high operating temperatures of a machine. For example, the length of the electrical conductors may be in the range of 5 to 10 meters. According to one embodiment of the method according to the invention, the at least four electrical conductors are arranged in step a) in the form of a square. A square is advantageous in that the electrical resistance between those conductor pairs that define the edges of the square is identical when the component surface within the square is substantially error free. The same applies to the electrical resistances between those pairs of conductors which define the diagonals of the square. In this way, the electrical resistance can be calculated with only a few electrical conductors, as will be explained in more detail below.

According to a further embodiment of the present invention, in step a) at least six electrical conductors are arranged in the form of two adjacently positioned squares, the two squares having a uniform edge length and a common edge. In this variant, the electrical resistance between at least two conductors can be determined in an alternative way.

In accordance with yet another embodiment of the method according to the invention, the electrical conductors in step a) are arranged in the form of a matrix, it being possible for the matrix to have any desired dimensions. In this way, the surfaces of entire components can be monitored.

Advantageously, the voltage drop in step b) is measured using a Wheatstone bridge, whereby the voltage measurement signal is amplified. As a result, the voltmeter used must dissolve much less accurately than would be the case without the Wheatstone bridge. Also can be dispensed with a measuring amplifier. Advantageously, in step b) the electrical conductors are connected to an AC power source. In this way, the flow of electricity to the surface of the skin thanks to the skin effect Component displaced, whereby the measurement resolution is significantly improved.

According to one embodiment of the method according to the invention, the electrical resistance between at least two conductors is determined at different times. Subsequently, the determined electrical resistances are compared with each other, whereupon, based on the comparison result, conclusions are drawn about a change of the component in a region between at least two electrical conductors, in particular conclusions about a crack formation and / or a crack development. In other words, the inventive method is used in this embodiment to determine the life of the component impairing changes, in particular changes a crack geometry of a present on the surface of the component crack.

Preferably, a crack depth is determined based on the comparison result. Crack depth growth is one of the key factors in determining the remaining life of a part.

Advantageously, the inventive method for monitoring a stationary component of a machine, in particular a gas or steam turbine, is used during its operation, in particular for monitoring a high temperature during operation component, ie operating temperatures of several hundred degrees Celsius. Further features and advantages of the invention will become apparent from the following description of a method according to an embodiment of the present invention with reference to the accompanying drawings. 1 is a schematic view of an arrangement for the

Measuring an electrical resistance between two electrical conductors arranged on the surface of a metallic component; and FIG. 2 shows a schematic view of an arrangement for measuring an electrical resistance between a plurality of electrical conductors arranged on the surface of a metallic component in accordance with FIG

Embodiment of a method according to the invention.

FIG. 1 shows two electrical conductors 1 and 2, which are fastened on the surface of a metallic component 3 at the measuring points A and B which are arranged at a distance a) from one another and on the other hand are connected to an alternating current source 4. In this way, a circuit with the line resistances R _L i and R _{L2 of} the electrical conductors 1 and 2, the _{contact resistances} R _Üa and R _{ÜB of} the welded joints and the material _resistance R _{M is} formed. Furthermore, a voltage measuring device 5 is provided, which measures the total voltage drop caused by the resistances RLI, RÜA, RM, RÜB and RL2. With the arrangement shown in Figure 1, the material resistance R _M can be determined in principle, if the line resistances R _L i and R _L 2 and the contact resistances RÜA and R _{ÜB are} known. The material resistance R _M is of interest, for example, if a change of the component 3 between the measuring points A and B is to be detected, such as cracking and / or crack development. Thus, the material resistance R _{M increases} with increasing crack depth, so that, for example, based on a comparison of the material resistances R _MO R without crack and R _MM R with crack on the crack depth can be concluded.

A problem of determining the material resistance R _M with the circuit shown in Figure 1, however, occurs when the contact resistances RÜA and RÜB are not known. This is the case when the contact resistances are formed for example by welded joints. The use of welded joints is particularly advantageous if the component 3 during the implementation of Measuring method is exposed to temperatures of several hundred degrees Celsius, as welded joints withstand such high temperatures easily. Another problem occurs when the electrical conductors 1 and 2 have a very large length, for example in the range of 5 to 10 meters, since the material resistance R _{M is} then very low compared to the line resistances R _LI and R _L2 , for which reason only a precision measurement would reach the necessary resolution to determine the material resistance R _M. Heads 1 and 2 with long lengths are also used when measurements on a component are to be made during machine operation and the component is subjected to operating temperatures of several hundred degrees Celsius in order to set up the temperature-sensitive AC source 4 as well as the voltmeter 5 away from the measurement location ,

In the cases mentioned, the use of the circuit referred to in FIG. 1 for determining the material resistance R _{M is} not possible or only possible with disproportionately high expense.

FIG. 2 shows an arrangement for determining the material resistance R _M according to an embodiment of a method according to the invention, wherein the method can also be used if the contact resistances RÜ between the electrical conductors and the component are unknown and / or the line resistances of the electrical conductors due to A very large length of the electrical conductors compared to the material resistance R _{M are} very high, as described below.

FIG. 2 shows a component 3, on the surface of which, analogously to FIG. 1, electrical conductors 1 and 2 are fastened at the distance a at the measuring points A and B. In addition to the electrical conductors 1 and 2, two further electrical conductors 7 and 8 are connected at the measuring points C and D to the surface of the metallic component 3, wherein the measuring points B and D, D and C and C and A also have the distance a to each other, whereby an overall arrangement in the form of a square is formed. The diagonals of the square, which extend between the measuring points B and C and A and D, each have the distance b. The electrical conductors 1, 2, 7 and 8 are connected at the measuring points A, B, C and D respectively materially connected to the component 3, for example by means of corresponding welded joints, and each have long lengths, for example in the range of 5 to 10 meters.

If the component 3 has no component defects in the region of the square, such as cracks or the like, it can be assumed that the material resistance R _M between the measuring points A and B, B and D, D and C and C and A due to the uniform Distance a is the same. Furthermore, it can be assumed that the material resistance R _M * between the diagonally opposite measuring points A and D as well as B and C is the same. Summarizing also the line resistance R _Li and the contact resistance R _ÜA to a resistance R _A, the lead resistance R _L2 and the contact resistance R _{recommendation} to a resistance R _B, the line resistance R _L7 and the contact resistance R _ÜC to a resistance R _c and the line resistance R _L8 and the _contact resistance R _{ÜD together} to form a resistor R _D , the entire system has six unknown resistors, namely the resistors

To determine the material resistance R _M or the material resistance R _M * between two conductors, an alternating current is applied in a first step between the electrical conductors 1 and 2 by means of an alternating current source 4. Then, using a voltage measuring device 5, the voltage drop across the resistors R _A , R _M and R _{B is} measured, as shown schematically in FIG. Thus, a first measured value Mi is obtained. In a further step, the electrical conductors 1 and 7 are connected to the AC power source 4, after which the resistors R _A , R _M and R _c falling voltage is detected as the measurement result M ₂ . In further steps, the electrical conductors 2 and 8, the electrical conductors 7 and 8, the electrical conductors 1 and 8 and the electrical conductors 2 and 7 are successively connected to the AC power source 4 and the voltages dropping across the respective resistors are measured, whereby the Measurement results M ₃ , M ₄ , M ₅ and M ₆ result. The connection of the individual conductor pairs with the AC power source 4 via corresponding switches, even if they are presently not shown for clarity. The voltage drop is measured using a Wheatstone bridge. The Wheatstone bridge in this case comprises the resistors Ri and R ₂ , which form a first voltage divider, and the resistors R ₃ and R ₄ , which form a second voltage divider, wherein the voltage measuring device 5 establishes a transverse relationship between the two voltage dividers. Resistor Ri is formed by the previously described switched conductor pairings. The resistor R ₂ is chosen such that it corresponds approximately to the resistor Ri. The resistors R ₃ and R ₄ are each twice as large as the resistor R ₂ selected. The essential advantage of such a Wheatstone bridge is that the voltage measurement signal of the voltage measuring device 5 is amplified.

Based on the measurement results Mi to M ₆ , six equations for the six unknown resistances R _M , R _M *, R _A , R _B , R _C and R _D can now be set up. These six equations are:

2) M ₂ = R _A + R _{M +} R _c

4) - Rc + RM + RD

6) M ₆ = R _B + R _M * ₊ Rc

However, this system of equations is not solvable in this form, although it has as many equations as unknowns. The reason for this lies in the first four equations, which represent the sides of the square.

This problem can be bypassed in two ways. The first possibility is to expand the system shown in FIG. 2 by two measurement points E and F such that two adjacent positioned squares are formed, wherein both squares have a uniform edge length a and a common edge. The two additional measurement points E and F result in a system of equations with eleven equations and nine unknowns, which can be resolved into R _M or R _M *.

A second way around the problem is to approximate R _M * according to the geometry as R _M * = * R _M

Thanks to this simplification, the remaining five unknowns can be calculated in four different ways, depending on which of the first four equations is not taken into account in the calculation. Correspondingly, a value for R _M for the component 3 without crack, hereinafter referred to as R _MO R, is also obtained. Following the first measurement, the change in the resistance-for example as a result of cracking within the square defined by the measuring points A, B, C and D-can then be followed by the resistance R _MM R determined for the component having a crack the resistance R _MO R determined for the crack-free component 3 is compared. Based on the difference The resistors R _MO R and R _MM R can then be drawn conclusions on the crack depth.

An essential advantage of the method according to the invention is that component monitoring is also possible during operation, even when the component is exposed to high operating temperatures of several hundred degrees Celsius. This advantage is due to the fact that the method is feasible with unknown contact resistance and also with long electrical conductors, which is why temperature-resistant connections can be used to attach the electrical conductors to the component and the temperature-sensitive measuring technology away from the

Machine can be arranged.

The use of alternating current is due to the associated skin effect of advantage, which pushes the flow of current through the component to the component surface, so that the process is easily feasible even with large components. Alternatively, DC can also be used in principle, but AC is preferred.

The arrangement of the measuring points in one or more squares lends itself to the uniform edge length a. However, it should be clear that in principle also other planar or spatial arrangements for the measuring points are possible.

In addition, any number of measuring points can also be arranged in the form of a matrix distributed over the entire surface of a component or at least cover a larger surface area. In this way, even large surfaces can be monitored. It should also be understood that the Wheatstone bridge circuit can be replaced by other circuit elements. Although the invention has been further illustrated and described in detail by the preferred embodiment, the invention is not limited by the disclosed examples, and other variations can be derived therefrom by those skilled in the art without departing from the scope of the invention.

Claims

Patent claims

1. Method for determining an electrical resistance (R _M ) between at least two electrical conductors (1, 2, 7, 8) arranged on the surface of a metallic component (3), which has the steps:

a) attaching at least four electrical conductors (1,

2, 7, 8) on the surface of the component (3) in a defined flat or spatial arrangement;

b) connect two electrical conductors (1, 2) to a power source (4) and measure the voltage drop or a physical quantity representing this between the conductors (1, 2), this process being carried out for all of the possible conductor pairings ( 1.7; 2.7; 7.8; 1.8;

2,8) is carried out by at least four electrical conductors (1, 2, 7, 8),

c) mathematical determination of the electrical resistance (R _M ) between at least two conductors (1, 2, 7, 8) based on the measurement results determined in step b).

2. Method according to claim 1,

in which the electrical conductors (1, 2, 7, 8) are fastened to the surface of the component (3) in step a) using a cohesive process, in particular by soldering or welding.

3. Method according to one of the preceding claims,

in which the electrical conductors (1, 2, 7, 8) are several meters long.

4. Method according to one of the preceding claims,

in which the at least four electrical conductors (1, 2, 7, 8) are arranged in the form of a square in step a).

Method according to one of the preceding claims, in which in step a) at least six electrical conductors are arranged in the form of two adjacently positioned squares,

where the two squares have a uniform edge length (a) and a common edge.

Method according to one of the preceding claims, in which the electrical conductors in step a) are arranged in the form of a matrix.

Method according to one of the preceding claims, in which the voltage drop in step b) is measured using a Wheatstone measuring bridge.

Method according to one of the preceding claims, in which in step b) the electrical conductors are connected to an alternating current source (4).

Method according to one of the preceding claims, in which the electrical resistance (R _M , RM*) between at least two conductors (1, 2, 7, 8) is determined at different times,

the determined electrical resistances are compared with one another and, based on the comparison result, conclusions are drawn about a change in the component (3) in an area between at least two electrical conductors (1, 2, 7, 8),

in particular conclusions about the formation of cracks and/or the development of cracks.

10. Method according to claim 9,

in which a crack depth is determined based on the comparison result.

11. The method according to one of the preceding claims, for monitoring a stationary component (3) of a machine,

in particular a gas or steam turbine,

used during their operation,

especially for monitoring a component exposed to high temperatures during operation.