Method for determining the presence of corrosion or deposition of a material

Abstract

The invention relates to a method for determining the presence of corrosion or deposition of a material with a mechanically vibrating unit (1) of a vibration sensor. A vibration element (10) of the mechanically vibrating unit (1) has a narrow side (11) and a broad side (12) in two orthogonal directions (R1, R2) and is excited to produce mechanical vibrations in two modes (S1, S2), in which it vibrates in the two directions (R1, R2) within a medium (M). The respective vibration frequencies (F F1 , F F2 ) are determined from the received mechanical vibrations in the two modes (S1, S2). From a determined temperature (t) of the vibration element (10) and the vibration frequencies (F F1 , F F2 ), a value of a change in mass (Δm) is determined, from which it is derived whether it is corrosion or deposition of the material that is present.

Description

[0001] Methods for determining the presence of corrosion or deposition of a material

[0002] The invention relates to a method for detecting the presence of corrosion or deposits of a material using a mechanically vibrating unit of a vibration sensor.

[0003] The tuning fork is an example of a classic vibration sensor that oscillates at different frequencies and / or quality factors depending on medium properties such as density or viscoelasticity. The two fork prongs, which typically have a paddle with a wide and a narrow side, are located on a diaphragm and, in application, contact the medium. The paddles of the mechanically vibrating unit are aligned parallel to each other. By forming a mechanical resonator or closed oscillating circuit, the fork prongs can be used to determine or monitor a variety of process variables based on the oscillation frequency and amplitude, which serve as a measure of the mechanical quality factor. These process variables include, for example, the fill level, density, or viscosity of a medium.

[0004] In process plants in the chemical industry, the food industry, water treatment, steam generation, etc., corrosion and deposits in parts of the plant can lead to serious malfunctions. For example, limescale can build up in pipes or tanks, on valves or pumps, or corrosion can occur due to the media present or process conditions. Therefore, the condition of such vulnerable areas of a plant must be regularly inspected. However, this is time-consuming and costly.

[0005] The invention is therefore based on the objective of realizing continuous monitoring for corrosion or deposits using a vibration sensor.

[0006] The invention solves the problem by a method for detecting the presence of corrosion or deposits in a material using a mechanically vibrating unit of a vibration sensor, wherein the method comprises at least the following steps: that at least one vibrating element of the mechanically vibrating unit, which has different dimensions in two orthogonal directions by having a narrow side and a wide side, is excited to mechanical vibrations in two modes, in which the vibrating element performs mechanical vibrations in a medium in the two orthogonal directions; that the mechanical vibrations of the vibrating element in the two modes are received and at least one respective vibration frequency is determined from them; that a temperature of the vibrating element is determined.that at least on the basis of the determined vibration frequencies and the determined temperature, a value of a mass change of the mechanically vibrating unit is determined, and that from the value of the mass change it is derived whether the case is corrosion or the case is deposition of the material.

[0007] The vibration sensor has at least one oscillating element with different dimensions in two directions. In one embodiment, this element is a paddle. The oscillating element is excited to vibrate in both directions, and the respective vibration frequencies are determined from the received vibrations. Since the oscillating element is located in a medium—preferably a liquid—an interaction occurs between the oscillating element and the medium. The degree of this interaction depends on the effective area of ​​the oscillating element. Because the oscillating element has different dimensions in the two directions of vibration, the vibrations are affected differently, resulting in different vibration frequencies. Based on the two vibration frequencies and the measured temperature of the oscillating element, a value for the mass change of the oscillating element is determined.This value allows us to determine whether the case is corrosion – i.e., a decrease in mass – or the case is deposition of the material – i.e., an increase in mass.

[0008] When a single oscillating element is mentioned here or in the following, the same applies to the case of two oscillating elements, such as those belonging to a tuning fork as a mechanically oscillating unit. In the following descriptions, the dependencies of the mass change on measurement conditions are examined and appropriately used to determine the mass change.

[0009] One embodiment provides that a reference oscillation frequency is determined for the oscillating element in each of the two modes at a reference temperature – in particular 0 °C – and in an uncovered state of the oscillating element; that temperature coefficients are determined for the oscillating element in the two modes, whereby the determined temperature coefficients allow an approximate adaptation of the determined reference oscillation frequencies to the reference oscillation frequency applicable to a given temperature via the following relationship: F* Q1 / Q2 = F Q1 / Q2 * (l + a * t + ß * t 2 ), where a, ß are the temperature coefficients in the two modes, where F 01 , F Q2 the determined reference oscillation frequencies are, where F* 01 , F* 02the reference oscillation frequencies applicable to a temperature t, where t is the temperature of the oscillating element, and that the value of the mass change is determined based on at least the oscillation frequencies, the temperature, the temperature coefficients and the reference oscillation frequencies.

[0010] To determine the mass change, this embodiment takes into account the fact that the oscillation frequency has a temperature dependence. In this embodiment, this dependence is specifically addressed for a temperature range between -50 °C and 150 °C using a second-order polynomial: F* 01 / 02 = F 01 / 02 * (1 + a * t + ß * t 2The vibration frequencies in the uncovered state at the given and determined temperature t are derived from the reference vibration frequencies determined at 0 °C. The coefficients a and β depend on the geometry and material of the vibrating elements. Typical values ​​for steel forklift tines at vibration frequencies in the range (400...1500) Hz are: a = -(1.3...2.5) * 10 -4 K' 1 and ß

[0011]

[0012] = -(1,2... 1,8) * IO“ 7 K' 2 .

[0013] One embodiment consists in determining a sensitivity factor for the oscillating element in each of the two modes, and in describing a relationship between the oscillation frequency of the oscillating element and the interaction with the medium of a density, which covers the oscillating element in a covered state, via the following relationship: F P1 / P2 = F 1 / 2 * / - - -, where FP1 , F '' P

[0014]

[0015] AJ 1+SI / 2*PF2 are the oscillation frequencies of the oscillating element in a covered state, where F1, F2 are the oscillation frequencies of the oscillating element in a free state, where S1, S2 are the sensitivity factors of the two modes, where p F the approximately determined value of the density of the medium, and that in the case that the vibration frequencies have been determined in the covered case of the vibrating element, the value of the mass change is determined on the basis of at least the vibration frequencies, the temperature, the sensitivity factors and the density of the medium.

[0016] This design is based on the understanding that the oscillation frequency of the mechanically oscillating unit, e.g., the tuning fork, is a function of the density of the contacting medium (p). F), a sensitivity factor (S, depending on the effective area and the moment of inertia of the oscillating element or elements) and the temperature t as well as the oscillation frequency (F) 1 / 2 ) in a vacuum - i.e., in an uncovered state - at a predetermined reference temperature.

[0017] The following applies: F P1 / P2 = F 1 / 2 * / - - - each for the two modes.

[0018] 7

[0019]

[0020] 7 AJ 1+SI / 2*PF

[0021] From this context, the density of the medium in a fully immersed state of the tuning fork can be determined:

[0022] _ 1 ( F 1 / 2 ~ F F1 / F2'\

[0023] PF ~ \ F

[0024]

[0025] d 5 ■

[0026] l / 2 \ ^Fl / f-2 /

[0027] Secondly, the relationship between the reference oscillation frequencies in the uncovered state allows conclusions to be drawn about adapted reference values ​​in the covered state. The oscillating element is preferably designed such that the sensor sensitivities are not only different (S x S2), but can even differ significantly from each other: S » S2. Here, S1 is the sensitivity resulting from the larger area, e.g., the paddle surface. In this example, S2 is the sensitivity resulting from the edge of a paddle.

[0028] The following embodiment combines the two preceding embodiments by taking into account the temperature dependence and the difference between the covered and uncovered states. In each case, the reference oscillation frequency of the two modes at the reference temperature and in the uncovered state is sufficient.

[0029] The design therefore provides that a corrosion thickness or corrosion depth - denoted by x - of the mechanically vibrating unit is determined using the following formula:

[0030] m0SIPQ2 2 F

[0031] x = (l + a * t + ß * t 2 ) 2 F1 2

[0032]

[0033] A»p Ä FF2 2 [SIPFI 2 +S2^ -P^]

[0034] where mo is a starting mass of the mechanically vibrating unit, where x is a measure of a build-up thickness or corrosion depth of the mechanically vibrating unit, where A is the surface of the vibrating element coming into contact with the medium, where p AThis is either the density of the vibrating element or the density of the material that could potentially form deposits. The initial mass is thus the known mass of the vibrating unit, and A is the total surface area of ​​the vibrating element(s) on which deposits can form or corrosion can occur. The mass change is relative to the initial mass. For the density p AEither the density of the vibrating element or the density of the material forming a potential buildup is used. The choice depends on whether the case involves buildup (in which case the density of the material forming the buildup) or corrosion (in which case the density of the vibrating element). The buildup thickness is a measure of the thickness of a layer formed by buildup. Conversely, the corrosion depth is a measure of the depth of material loss resulting from a corrosive medium or the corrosive environmental conditions during application. The buildup considered for the invention is, in particular, a deposit extending over a larger area of ​​the vibrating element. The occurrence of corrosion typically manifests itself either as surface corrosion or as pitting. The type of corrosion relevant to the invention is preferably surface corrosion.The focus is therefore particularly on changes in the mass of the vibrating element that manifest themselves over a surface, i.e., either surface deposition or surface corrosion.

[0035] The design formula is based on the following considerations: For a mass-spring oscillator, the following applies: a)0=

[0036] m where ) Q the angular frequency, K 0

[0037] The spring constant and m0 the mass of the oscillator. A change in mass Am leads to a change in the angular frequency: a > = -. In a corrosion case,

[0038]

[0039] -yj mQ + Am

[0040] Am < 0 and in the case of an approach, Am > 0.

[0041]

[0042] The frequency ratio is: — CÜQ = — F = m0 m +°m where F and F* are the corresponding frequencies with and without increment Am.

[0043] The oscillation frequencies in the covered state of the oscillating unit and with a mass change of the oscillating unit can be traced back to the reference oscillation frequencies in the uncovered state at the reference temperature as follows:

[0044] F P1 = F 01 (l + a * t + ß ' * t 2 ) * yl I—m0+&m and

[0045] F F2 = F 02 (l + a * t + ß * t 2 ) * I— * | 1 .

[0046]

[0047] “ '\Jm0+Am A S | 1+S2*PF

[0048] Since F 01 (l + a * t + ß * t 2 ) -\l |- - = fi And F 02 (l + a . t + / ?. t 2 ) -\l P IIIQ^-T l lil = F2,

[0049] applies: F F1 = Fi and F F2 = F2|—.

[0050]

[0051] All l+5i*pp

[0052]

[0053] 'M 1+52*PF

[0054] The sensitivity of the vibrations of the oscillating unit to the density of the medium is altered in both vibration modes by corrosion or by deposits. The sensitivity factors resulting from corrosion or deposits are approximately increased or decreased by the same factor N. This leads to the following relationship:

[0055] ii 1

[0056] - and F F2 = F2- ■

[0057]

[0058] l+S1*N*p F l+S2*N*p F

[0059] The formula for F F1 can be determined according to the density p F solve and insert into the formula for F F2 insert so that the following results:

[0060]

[0061] The difference in oscillation frequencies is: F1- F2= (F O1 - F 02 )(l + a * t + ß * t 2 ) - — —

[0062]

[0063] J 1 + / m0 This results in:

[0064] i xAm / (l + a * t + ß * t 2 ) 2 * (F 01 - F 02 ) 2

[0065]

[0066] 1 + / -» = - -

[0067] And finally, it turns out:

[0068] (l + a * t + ß * t 2 ) 2 * 01 / p — 1J * F 02 2

[0069] Am = m0

[0070]

[0071] p Since the ratios of the oscillation frequencies of the two modes to each other are: — =

[0072] F 2 p

[0073] — which does not change due to mass change, i.e., it remains the same, so it follows:

[0074] (l + a*t+ / ?*t 2 ) *foi / o2 2

[0075] Am = m0

[0076]

[0077] FL / 2 2

[0078] This results in the measure for a corrosion thickness (i.e., more mass) or a corrosion depth (i.e., less mass): x =

[0079]

[0080] In a state of complete immersion in the medium, both frequencies were reduced:

[0081] I i

[0082] FF1 / F2 = ^01 / 02(1 + a * t + ß * t 2 ) L,

[0083] J 1 + S c

[0084] 1 / 2 * p F

[0085]

[0086] As shown above, the following applies to the vibrations in a fully covered state of the mechanically vibrating unit in the case of build-up or corrosion on the vibrating unit for the frequency:

[0087]

[0088] and for the mass change:

[0089] 17 2

[0090] Am = m0(l + a * t + ß * t 2 ) 2 * — 1

[0091]

[0092] Substituting the formula for F2 yields: x = (l + a*t + ß * t 2 ) 2 * — 2r Slf °2 2 Ffi 2— JT - 1

[0093]

[0094] A*PA LF F 2 2 [SIFFI 2 +S2(FI 2 -^I)]

[0095] The following applies to the calculation of the mass change Am = x * A * p A .

[0096] One design includes deriving from the sign of the determined mass change and / or the corrosion depth or the deposit thickness whether the case is corrosion or deposition.

[0097] Therefore, for at least one design for determining corrosion or deposit formation, the following parameters should be known:

[0098] • The sensor constants of the reference oscillation frequencies in the uncovered state at the reference temperature, the temperature coefficients and the sensitivities Si, S2 in the two modes.

[0099] • Specific constants such as the starting mass, the surface area that can come into contact with the medium, and the density of the vibrating element or the attachment, if attachment thickness or corrosion depth is to be determined.

[0100] • The measured process parameters are the vibration frequencies in the covered state and the sensor temperature t to compensate for material temperature drift. For example, in the case of a tuning fork, the temperature preference in the area of ​​the diaphragm is determined.

[0101] The invention also relates to a vibration sensor configured to implement the method according to one of the preceding or following embodiments. The descriptions and explanations also apply to the vibration sensor.

[0102] The invention is explained in more detail with reference to the following figures. They show:

[0103] Fig. 1: a spatial representation of a vibration sensor design and

[0104] Fig. 2: a schematic representation of an application of the invention.

[0105] Fig. 1 shows a so-called vibrating fork as an example of a vibration sensor design. The mechanically vibrating unit 1 has two so-called fork tines or paddles as vibrating elements 10, which are connected to a diaphragm 3. On the inner side of the diaphragm 4 – not shown here – a transducer device is located in a housing 4 (indicated here). This transducer device is supplied with electrical excitation signals by the electronic unit 2 and transmits electrical reception signals to the electronic unit 2.

[0106] The oscillating elements 10 each have a wide side 12 and a narrow side 11. Both oscillating elements 10 are intended to be capable of performing mechanical oscillations in the two directions R1 and R2 (indicated by the arrows). Direction R1 is the direction in which the oscillating elements 10 have their greater extent through the wide side 12. The narrow side 11, which can also be described as the edge of the paddle, is located in the perpendicular direction R2.

[0107] The two modes S1 and S2 in the directions R1 and R2 are each out of phase and thus prevent forces or moments from being uncompensated across the membrane 3.

[0108] Figure 2 indicates that the mechanically oscillating unit 1 of the vibration sensor extends into the medium M and is therefore submerged. A temperature sensor 5 detects the temperature of the medium M and thus the temperature of the mechanically oscillating unit 1.

Claims

Patent claims 1. Method for detecting the presence of corrosion or deposits of a material using a mechanically vibrating unit (1) of a vibration sensor, the procedure includes at least the following steps: that at least one oscillating element (10) of the mechanically oscillating unit (1), which has different dimensions in two orthogonal directions (R1, R2) by having a narrow side (11) and a wide side (12), is excited to mechanical oscillations in two modes (S1, S2), in which the oscillating element (10) performs mechanical oscillations in a medium (M) in the two orthogonal directions (R1, R2), that the mechanical oscillations of the oscillating element (10) in the two modes (S1, S2) are received and at least one respective oscillation frequency (F) is derived from them F1 , F F2 ) is determined, that a temperature (t) of the oscillating element (10) is determined, that at least on the basis of the determined oscillation frequencies (F F1 , F F2 ) and the determined temperature (t) a value of a mass change (Am) of the mechanically oscillating unit (1) is determined, and that the value of the mass change (Am) is used to determine whether the case is corrosion or deposition of the material.

2. Method according to claim 1, wherein a reference oscillation frequency (F) is defined for the oscillating element (10) 01 , F 02 ) in each of the two modes (S1, S2) at a reference temperature - in particular 0 °C - and in an uncovered state of the oscillating element (10), wherein temperature coefficients (a, β) for the oscillating element (10) in the two modes (S1, S2) are determined by approximating the determined reference oscillation frequencies (F) with the determined temperature coefficients (a, β).01 , F 02 ) a reference oscillation frequency (F*) applicable to a temperature (t) 01 , F* 02 ) allow the following relationship: F* 01 / 02 = F 01 / 02 * (l + a * t + ß * t 2 ), where a, ß are the temperature coefficients in the two modes (S1, S2), where F 01 , F 02 the determined reference oscillation frequencies are, where F* 01 , F* 02 the reference oscillation frequencies applicable to a temperature t are, where t is the temperature of the oscillating element (10), and that based on at least the oscillation frequencies (F F1 , F F2 ), the temperature (t), the temperature coefficients (a, β) and the reference oscillation frequencies (F 01 , F 02 ) the value of the mass change (Am) is determined.

3. Method according to claim 1 or 2, wherein a sensitivity factor (S1, S2) is determined for the oscillating element (10) in each of the two modes (S1, S2), where the sensitivity factors (S1, S2) represent a relationship between the vibration frequency of the vibrating element (10) and the interaction with the medium (M) of a density (p F ), which in a covered state covers the oscillating element (10), can be described via the following relationship: F F1 / F2 = F 1 / 2 * — - -, where F F1 , F F2 the oscillation frequencies of the oscillating element (10) in a covered state are, where F1, F2 are the oscillation frequencies of the oscillating element (10) in a free state, where S1 and S2 are the sensitivity factors of the two modes, where p F the approximately determined value of the density of the medium (M) is, and where at least the oscillation frequencies (F F1 , F F2), the temperature (t), the sensitivity factors (S x , S2) and the density (p F ) of the medium (M) the value of the mass change (Am) is determined.

4. Method according to claims 2 and 3, where the mass change (Am) of the mechanically oscillating unit (1) is determined using the following formula: Am = m0[(1 + α* t + ß* t 2 ) 2 F F2 2 [S1F F1 2 + S2(F^ - Fk)] and where a layer thickness or corrosion depth (x) is determined using the following formula: x = Ä*PA where m0 is a starting mass of the mechanically oscillating unit (1), where Am is a change in the mass of the mechanically oscillating unit (1), where A is the surface of the vibrating element (10) that comes into contact with the medium (M), where p Aeither a density of the vibrating element (10) or a density of the material forming a possible approach.

5. Method according to claim 4, where, based on the sign of the determined mass change (Am), it is derived whether the case is corrosion or the case is deposition.

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