RU2688876C2 - Asymmetric bending moment sensor for high-temperature vortex flow meters - Google Patents

Abstract

FIELD: measuring equipment.SUBSTANCE: claimed sensor is used in liquid, gas and steam vortex flowmeters when measuring their flow in pressure pipelines. Claimed solution distinctive feature is the use of high-temperature piezoelectric materials, which are characterized by the transverse piezoelectric modulus dextremely low values, but acceptable values of dIn this device, an asymmetric construction is proposed, which allows to transform the bending moment of the force acting on the side of the vortices on the sensor wing, into compressive-tensile stresses along the axis of the piezo-element package generating an electrical signal proportional to the longitudinal piezoelectric modulus d. Sensing element has the form of a coaxial piezoelectric disks set polarized in thickness and mounted in a cylindrical cavity, whose axis is offset relative to the plane of the outer plate, thanks to which flexural deformations of this plate, transmitted through the membrane, cause compressive-tensile stresses along the axis of piezoelectric disks.EFFECT: increase in the flowmeter working temperature range upper limit.1 cl, 5 dwg

Description

The invention relates to a measuring technique, in particular to piezoelectric sensors for recording the frequency of vortex formation in vortex flow meters of gas, steam and other multiphase media containing a liquid and gaseous fraction.

It is known that a chain of vortices, the so-called Karman vortex path, is formed behind the body of flow in a stream of liquid, gas or steam, and the frequency of the vortices formed is proportional to the flow rate of the liquid, gas or vapor, therefore the registration of the frequency of the vortices allows measuring , gas or steam in the pipeline.

Known pressure sensors for vortex flowmeters [1, 2] with a working temperature of up to 500 ° C, containing a piezoelectric element housed in a cylindrical metal housing. The disadvantage of these flow meters is that the sensors are mounted flush with the pipe wall in the flow path of the flow meter. When measuring the flow of saturated steam in the stream, the liquid phase is inevitably present, which is localized at the pipe wall, which prevents normal operation of the pressure sensors and leads to a significant measurement error of the flow meter, reaching 10% or more.

The closest in technical essence is the bending moment sensor [3, 4], FIG. 1, 2, comprising a case, to the end of which an outer plate (wing) is attached, whose thickness decreases from the end of the case to the free end of the plate so that the angle between the planes of the plate is 1.5 ... 4 ° (beam of equal resistance). This plate is placed in the measured flow behind the flow body so that its plane of symmetry lies on the axis of the pipeline and is parallel to the flow (Fig. 1). On the part of the vortices, a variable pressure force acts on the wing with a frequency

where ν is the flow velocity, d is the characteristic size (width) of the wrap body, and Sh is the Strouhal number, which in a wide range of Reynolds numbers 200 <Re <200000 changes slightly and close to 0.2. The pressure forces on the wing cause periodic bending deformations of the wing transmitted to the hull. Inside the case there is a piezoelectric element in the form of a hollow cylinder polarized in the radial direction; the outer cylindrical surface is covered with a solid electrode, and on the inner surface the electrode is cut into two parts, with the incision plane coinciding with the plane of the wing. When bending the wing, for example, to the right, the right half of the piezoelectric element undergoes axial compression, and the left stretches, and between the internal electrodes of the piezoelectric element arises due to the piezomodule of the piezoelectric ceramics d ₃₁ electric signal - voltage (in idle mode) or charge (in short circuit mode), whose frequency is equal to the frequency of external influence on the wing. The cable, the ends of which are soldered to the electrodes of the piezoelectric element, the signal is transmitted to the recording devices.

The disadvantage of this technical solution is the limitation on the limiting temperature of 290 ° C. This is due to the following reasons:

- the piezoelectric material PZT-83G has a Curie point of about 360 ° C and maximum temperatures during long-term operation of 300 ° C. Other piezoelectric materials of the PZTS system (lead zirconate titanate) have a Curie point temperature not exceeding 420 ° C (PZT-21), and a maximum operating temperature of 350 ° C;

- PSS-2.5 solder used has a melting point of 294 ° C;

- AVKT-6 cable has a heat resistance of 300 ° C.

In order to increase the boundaries of the working temperature range, piezoelectric materials based on bismuth titanate (TB-2, TV-3) with a Curie point of more than 600 ° C and operating temperature up to 500 ° C, as well as bismuth niobate titanate (TNB-1 and others) with a Curie point of 900 ° C and an operating temperature of up to 700 ° C.

However, the disadvantage of these materials is that they have an extremely small piezomodul d ₃₁ . For example, the material NTV-1, which has a maximum operating temperature of 500 ° C, is characterized by a very small piezomodule d ₃₁ - two orders of magnitude lower than that of PZT-83G, whereas the piezomodul d _{33 is} only an order of magnitude lower than that of PZT-83G. Therefore, to use it, it is necessary to change the design of the sensor so that it works on the longitudinal piezomodule d ₃₃ .

The proposed construction below (Fig. 3) has a sensing element in the form of a set of coaxial piezoelectric disks polarized in thickness, which is installed in a cylindrical cavity in the housing, the axis of which is offset relative to the axis of the housing and the plane of the outer plate (wing). The outer plate of the wedge-shaped section (beam of equal resistance) with an angle between the planes of 2 ... 4 ° is attached with a thicker end to the membrane covering the end of the body. The sensing element rests on a metal heel in the form of a cylinder, freely sliding along the walls of the cavity and ending with a truncated cone, the narrow end of which rests on the membrane. Due to the displacement of the axes, the bending vibrations of the plate transmitted to the membrane and the heel cause compressive-tensile stresses along the axis of the sensing element, and an alternating electrical signal arises at its electrodes, the amplitude of which is proportional to the piezoelectric module d ₃₃ and the frequency is equal to the external influence frequency from the vortices. This signal through the cable is output to measuring instruments that fix the frequency, which is proportional to the flow velocity.

Using the finite element method in the ANSYS software package, the sensor was modeled [3-5]. The question of choosing the optimal geometric characteristics and, above all, about the optimal amount of displacement of the axis of the sensor cavity relative to the axis of the housing was investigated. FIG. 4 shows the calculated dependences of the transducer conversion coefficient on the magnitude of the displacement of the axes x ₀ for various thicknesses of the membrane. The case diameter is 14 mm, the cavity diameter is 10 mm. As can be seen from the figure, the maximum sensitivity of the sensor (KP conversion coefficient) is achieved when the axes x ₀ = 1.4 mm are displaced. The sensitivity of the sensor increases with increasing diameter of the cavity D ₀ (Fig. 5) and with decreasing membrane thickness, but its strength characteristics may decrease.

The strength characteristics of the sensor include the strength characteristics with respect to temperature stresses, with respect to external hydrostatic compression, with respect to the combined effects of temperatures and pressures, as well as the strength characteristics with respect to variable dynamic loads on the side of vortices.

The task of ensuring strength in a wide temperature range dictates the choice of materials that agree on the values of the coefficients of linear thermal expansion (CTE) α.

Heat-resistant piezoceram NTV-1 in a polarized anisotropic state has expansion coefficients

α _x = 8.6⋅10 ^-6 K ^-1 ; α _z = 6,4⋅10 ^-6 K ^-1 ,

but in non-polar state (insulator material)

α = (2α _x + a _z ) / 3≈7.9⋅10 ^-3 K ^-1 .

The CTE of OT4-1 titanium alloy is equal to 810-6 K ^-1 . The proximity of CTE materials provides relatively low internal thermal stresses in a wide temperature range. The calculation by the finite element method allows us to estimate these stresses and compare them with the ultimate stresses of the materials. For a titanium alloy, the ultimate stress is characterized by a stress intensity value S _int ≈ 600 MPa. The strength of brittle ceramic materials is characterized by the values of the maximum tensile stress S ⁺ ≈40 MPa and the maximum modulus of the compressive stress S ^- ≈400 MPa [2].

We present estimates of internal mechanical stresses when choosing an assembly temperature of 220 ° C.

The highest stresses in the metal S _{int are} achieved, as a rule, in the vicinity of the heel-membrane contact. In ceramics, the largest tensile stresses S ^{+ are} achieved in insulators.

Temperature stresses at a membrane thickness of 0.2 mm at T = 20 ° C:

S _int = 61.3 MPa (near the contact of the heel-membrane)

S ⁺ = 19.2 MPa (insulator)

S ^- = -25.4 MPa (insulator).

At T = 500 ° C:

S _int = 283 MPa (near the contact of the heel-membrane)

S ⁺ = 35.9 MPa (insulator)

S ^- = -23.8 MPa (insulator).

Temperature stresses at a membrane thickness of 0.5 mm at T = 20 ° C:

S _int = 61.7 MPa (near the contact of the heel-membrane)

S ⁺ = 18.8 MPa (piezoelectric elements)

S ^- = -25.4 MPa (insulator).

At T = 500 ° C:

S _int = 86.4 MPa (near the contact of the heel-membrane)

S ⁺ = 36.8 MPa (insulator)

S ^- = -26,0 MPa (piezo elements).

With the combined effect of high temperature (500 ° C) and hydrostatic pressure, pressure is the decisive factor. When the membrane thickness is H _m = 0.5 mm, the intensity of mechanical stresses in the metal increases linearly with increasing pressure and reaches critical values of 600 MPa with a limiting hydrostatic pressure of 28 MPa, while the stresses of S ⁺ in ceramics with increasing pressure increase slightly and remain within acceptable limits 38 MPa, and compressive stresses S - well below the tensile strength. At H _m = 0.2 mm, the limiting pressures are 11 MPa.

The problem of solder heat resistance is solved by the use of elastic contact elements. In this case, it is proposed to use contact welding of piezoelectric electrodes with KTMS cable conductors that withstand temperatures up to 600 ° C and more.

Technical result: increasing the upper limit of the operating temperature range of the sensor to the limits of the operating temperatures of the NTB-1 piezoceramics + 500 ° C, which is achieved by changing the design of the sensor, which allows working on the longitudinal piezomodule.

The invention is illustrated by drawings.

FIG. 1 shows a diagram of a vortex flow meter with a bending moment sensor: 1 - pipeline, 2 - flow body, 3 - sensor, 4 - fitting, 5 - gaskets.

FIG. 2 shows the design of the bending moment type 108 sensor (prototype): 1 - body, 2 - piezoelectric element, 3 - current collector, 4 - nipple, 5 - cable, 6 - outer plate (wing), 7 - guides providing the required orientation of the sensor in the pipeline .

FIG. 3 shows the bending moment sensor in a section along the plane of symmetry: 1 - housing; 2 - membrane; 3 - wing; 4 - heel; 5 - piezoelectric disks; 6 - insulators; 7 - base; 8 - nipple; 9 - cable conductors.

FIG. 4 shows the calculated dependences of the transducer's transform coefficient on the magnitude of the x ₀ axis displacement for various thicknesses of the membrane. The case diameter is 14 mm, the cavity diameter is 10 mm.

FIG. 5 shows the graphs of the coefficient of conversion of the sensor from the diameter of the cavity D ₀ at various thicknesses of the membrane. Offset x ₀ = 1.4 mm.

From the above materials it can be seen that the proposed technical solution provides an increase in the temperature limit of the device operability by changing the sensor design, which allows working on a longitudinal piezomodule and using high-temperature piezoelectric ceramics with a small transverse piezomodule.

Note that in domestic and world practice there are no flow meters operating with saturated steam at temperatures of more than 350 ° C.

Information sources

1. Kremlin P.P. Flow meters and quantity counters. 3rd ed., Pererab. and add. L .: Mechanical Engineering, 1975. 776 p.

2. Bogush M.V. Piezoelectric Sensors for Extreme Operating Conditions // Piezoelectric Instrument Making: Collection. В 3 т. Т. 3. Rostov n / a: Publishing House SKNTS VS, 2006. 346 p.

3. Bogush, MV, Pikalev, E.M. Designing piezoelectric sensors of bending moment for vortex gas and steam flowmeters // Instruments and systems. Management, monitoring, diagnostics. 2008, №3.

4. Bogush M.V., Pikalev E.M. Estimation of permissible operating conditions of piezoelectric sensors of bending moment for vortex gas and steam flow meters // News of higher educational institutions. North Caucasus region. Technical science. 2008, No. 5, p. 50-54.

5. Bogush M.V. Designing piezoelectric sensors based on spatial electroelastic models // Piezoelectric Instrument Making. T. IX. Technosphere, M., 2014. 312 p.

Claims (1)

A bending moment sensor for vortex liquid or gas flow meters mounted in a pipe behind a flow body, generating a chain of vortices in a liquid (gas) flow, whose frequency is proportional to the flow rate of liquid (gas) having an outer plate, one end of which is attached to the end of the cylindrical body, the other end is free, and the plate thickness decreases linearly from the fixed end to the free end with an angle between the planes equal to 2 ... 4 °, which perceives a variable bending moment of pressure force from the side cause and variable deformations of the body, and one or more piezoelectric elements in the body cavity and converting the bending moment into a variable electrical signal whose frequency is equal to the frequency of vortexes, characterized in that, in order to extend the temperature range through the use of high-temperature piezomaterials, characterized by small values of the piezomodul d ₃₁ , but acceptable values of the piezomodul d ₃₃ , the geometry of the transducer is changed so that its sensitive the element had the form of a set of coaxial piezoelectric disks polarized in thickness and installed in a cylindrical cavity, the axis of which is displaced relative to the plane of the outer plate, thanks to which the bending deformations of this plate transmitted through the membrane cause compression-tension stresses along the axis of the piezoelectric disks converted into an electrical signal proportional to the piezomodule d ₃₃ , which is outputted via a cable to the instruments, which fix its frequency.

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