Device for determination of mass of liquefied gas

FIELD: measurement technology; high-accuracy determination of liquefied gas mass in reservoir irrespective of its phase.

SUBSTANCE: three RF sensors of different length are placed in reservoir filled with liquefied gas. Sensors are connected to secondary converter. Length of first sensor vertically placed in reservoir corresponds to height of reservoir. Length of second vertical sensor is reduced from below by value no more than 0.35 of length of first sensor. Third sensor is located in gas phase of liquefied gas and its length is no more than 0.3 of length of first sensor. Each sensor may be connected to frequency-setting circuit of respective self-excited oscillator.

EFFECT: enhanced measurement accuracy; extended field of application.

2 cl, 3 dwg

 

The invention relates to the field of measurement technology and can be used for high-precision determination of the mass of liquefied gas in the tank regardless of its phase state.

The known device for determining the amount (level, volume, mass) of the substance in the vessel, based on the electric - capacitive, radio wave, the principles of measuring instruments (monographs: 1) beaver G.N., Rollers A.G. Methods of level measurement. M: Engineering. 1977. S-141; 2) V.A. Viktorov, Lunkin BV, Sablukov A.S. radio-wave measurement of parameters of technological processes. M.: Nauka. 1989. P.84-117). Thus, in particular, the measurement of electrical capacitance using capacitive measurement methods, or resonant frequency of the electromagnetic oscillations of the RF or microwave resonator partially filled of controlled substances, to determine the level of the controlled fluid. Such devices are applicable in cases when the electrophysical parameters of the liquid and gas environment above it unchanged. When solving the problem of measuring the amount (mass) of liquefied gas such methods are very methodical measurement error due to variability ratio of liquid and gas phases, its arbitrary and uncontrolled changes due to temperature changes and other factors.

is the most closest to the technical essence and purpose is a capacitive device for measuring the amount (level) of the liquid, described in the patent US 3862571 and adopted as a prototype. The device prototype contains three have vertically in the tank with the liquid coaxial capacitive sensor having a different length. The degree of lling of liquid corresponds to the degree of immersion in his first sensor, and the other two sensors have a shorter bottom length compared to the length of the first sensor.

This device, however, has some significant drawbacks. The ratio of the lengths of the sensors is that it ensures the achievement of the objectives of the present invention, namely, the measurement of the level of the dielectric liquid regardless of its density. However, this solution does not allow to determine the mass of liquid in a tank, in particular, the mass of liquefied gas. In this known device, the length of the second sensor is about 0.95 length of the first level sensor, that is, the second sensor is truncated from below at length about 0.05 length of the first level sensor. The length of the third level sensor is about 0,85 length of the first sensor, the third sensor truncated from below at length about 0.15 length of the first sensor. These sensors are connected to the electronic module (bridge circuit). Schematic solution, providing the balance of the bridge circuit, allows to determine the level of liquids with different density values.

p> However, this device has a limited scope. The ratio of the lengths of these sensors and the joint transform their informative parameters implemented in this known device, do not allow to measure the mass of liquid in a tank, in particular, the mass of liquefied gas.

To determine the mass of liquefied gas, which is the most objective parameter of the content of this two-phase substance in the tank, you want to make additional density measurements of the special sensor density. Because it takes place in the vessel uncontrolled transition gas depends on its temperature, composition) from a liquid phase to a gas and Vice versa, the readings of this sensor density is inaccurate; the use of two sensors density separately for liquid and gas phases significantly complicates the measurement process and the construction of the measuring device implementing this method. Since the density of vapors of liquefied gas depends on temperature, pressure and composition (in particular, the ratio of propane and butane), then measuring the mass of the vapor phase may pay an additional truncation error (3-7%). Added to this is also the error due to the variation in pressure when pumping gas. Thus, for high-precision measurement of the mass of liquefied gas neo what should be the measurement channel pair and correction of errors due to changes in the density of steam. Next, note that the liquefied gas is boiling, resulting in lost the "liquid mirror". Therefore, various known measuring device (a radio wave, ultrasonic, capacitive, which allows to determine only the level or interface cannot provide high accuracy measurements. They do not provide stable and reliable determination of the actual value of the quantity (mass) of liquefied natural gas. Besides having a place boiling liquefied gas change as the level and density of gas. Intensive boiling (increase by 5-10%) occurs at the end of the process of filling the tank with liquefied gas, the adjustment pressure vapor phase; it lasts a few minutes. More low boil (increase by 1-3%) was observed after an intense boiling as fading process, and also when selecting a pair of the compressor, when mixing liquefied gases of different composition or different temperatures and can take in this case a few hours.

The proposed device for determining the mass of liquefied gas is free from the above disadvantages. It provides precision definition: the mass of liquefied gas in real conditions of its storage capacity, regardless of its phase composition and the ratio of liquid and gas phases, the presence of boiling; the mass of steam (above the liquid); density of si the military gas; density of steam; the level of liquefied gas; the volume of liquefied gas.

The aim of the invention is the expansion of the scope.

The goal in the proposed device for determining the mass of liquefied gas contained in the vessel, containing three have in it the RF sensor of different lengths connected to the secondary of the Converter is achieved by the fact that it is the length of the first sensor is placed vertically in the tank corresponds to the vessel height and the length of the second sensor, also have vertically in the tank, is reduced below a value of not more than 0.35 length of the first sensor and the third sensor located in the gas phase LPG, has a length of not more than 0.3 length of the first sensor.

In this device, each sensor may be included in customizados circuit corresponding oscillator included in the secondary of the Converter.

A distinctive signs, according to the authors, are: the choice of the length of the second level sensor is reduced below a value of not more than 0.35 length of the first level sensor; the choice of the length of the third sensor, radio frequency sensor, the gas density is not more than 0.3 length of the first level sensor.

The set of distinctive features of the proposed device makes it a new property: the possibility of the distribution of the real mass quantity of liquefied gas in the tank regardless of its phase state. This property provides a positive effect, formulated in the objectives of the proposals.

Figure 1 shows a diagram of the proposed device. Figure 2 - graphs of the equivalent electrical capacitance sensors level. Figure 3 - measuring channel containing the oscillator with included sensor.

Here we have introduced the following notation: 1 - first level sensor; 2 - second level sensor; 3 - sensor gas density; 4 - secondary Converter; 5 - capacity; 6 is a controlled substance; 7 - oscillator; 8 - inductance; 9 and 10 - the inner and outer conductors of the coaxial lines, respectively; 11 and 12 - dielectric bearings (bushings).

The device operates as follows.

In this device produce a measurement of any informative parameter of each of the radio-frequency sensors, included in its composition. These RF sensors operating in the frequency range within typically 0-100 MHz, include capacitive sensors, sensors based on segments of homogeneous or heterogeneous long lines; the latter is the propagation of electromagnetic waves THAT - or quasi-FACT-type (Viktorov, V.A., Lunkin BV, Sablukov A.S. high-Frequency method for measuring non-electrical quantities. M.: "Nauka". 1980. 280 C.).

So, when you implement this device applies is of capacitive sensors as measured informative parameter of each sensor may be the resonant frequency of the electromagnetic oscillations f poscillating circuit (resonator), containing a capacitive sensor as a frequency control element. When implementing this based devices with the use of long line segments as informative parameter can also be used resonant frequency of electromagnetic oscillations fpthis section of the line, which is the resonator with fluctuations of THE type. In addition, the possible measurement and other informative parameters, in particular in the segment of long lines - measurement of phase shift ϕtransmitted and reflected from the end of a segment of a long line of electromagnetic waves of a fixed frequency, and other

Without loss of generality, to simplify the consideration of the essence of the present device will conduct further description with regard to its implementation using capacitive sensors. In this case, it is possible representation of such sensors in the form of an equivalent electric capacitance. The resulting conclusions are fully apply to the sales of this device based on the lengths of the long lines. In the latter case, the informative parameters used here radiowave sensor (resonant frequency fpthe phase shift ϕ and others) depending on the determined physical parameters are described in more complicated expressions, which, however, does not change on usaamah conclusions.

To determine the current value of the mass of liquefied gas in the tank, the proposed device is made, in addition to the first dimension level value of the equivalent electrical capacityE1sensor one of the specified types, the second measurement value of the equivalent capacitance CE2another sensor. The density of the gas phase of the liquefied gas is determined by measuring its dielectric constant εgthat is found by measuring the corresponding equivalent electrical capacityE3 effectsthe third sensor located in the upper part of the tank filled with gas.

The first capacitance (capacitor) or radiofrequency (cut long lines) level sensor 1 (figure 1) provides the definition of level z liquefied gas in the whole range of its dimensions - from zero to full fill (z=l, where l is the height of the tank).

Then for the first measurement channel (sensor 1) will have the following expression for the equivalent capacitance CE1:

Here εWand εg- dielectric permittivity, respectively, the liquid and gas phases of a controlled substance; (C0- capacity per unit length (i.e. linear capacity).

The sensor 2 has a length that is reduced below a value of not more than 0,35l. This is the selection tool length sensor 2 due to the fact, when a smaller value decreases the part of the working range level changes, which determine the current value of εW. For large values of the length of the sensor 2 decreases the value of l0the difference between the lengths of the sensors 1 and 2, leading to the increase of error in the calculation of εWaccording to the procedure described below equation (3): absolute error of the measured values of tanksE1and CE2when divided by the small value of the product of l0C0(C0=const) leads to a significant increase in the relative error of determination εW.

The length of the sensor 3 does not exceed 0,3l. This choice is due to the fact that when the real value of the maximum filling capacity 5 fluid it (liquid) should not reach the level at which is located the lower end of the probe 3. In practice, the maximum liquid level must not exceed the values of zmax=0,7-0,85 value of the measurement range. The choice of a specific maximum level values of zmaxfrom this range and, therefore, the length of the sensor 3 is determined by the regulations for the safe operation of vessels.

For the second measurement channel (sensor 2) is equivalent to capacity WithE2the sensor can be written in the following form:

Here l0the difference between the lengths of tchekov 1 and 2.

Figure 1 shows the secondary Converter 4. It is intended for excitation of electromagnetic oscillations in the sensor, determine the magnitude of his informative parameter and functional processing of signals from sensors 1, 2 and 3 to determine the required mass of liquefied gas 6 in the tank 5, the co conversion of the output signals of these sensors.

If the level z of the liquid above the lower end of the sensor 2, at each level, calculate the dielectric permittivity εWfluid according to the following formula, derived from the joint transformation of the relations (1) and (2):

This calculated value εWput in random access memory (RAM). It is part of the secondary of transformer 4.

The dielectric constant of the gas can be determined by measuring (probe 3) the equivalent capacitance CE3 effects:

where- initial (when εg=1) is equivalent capacityE3 effects.

From the relations (1) and (2) we obtain:

The values here εWand εgare expressed, respectively, by the formulas (3) and (4).

If the liquid level in the tank is less than the location of the lower end of the sensor 2, in which the quality measures of the value of z for this sensor is set to z, which corresponds to the nominal value of the equivalent capacitance CE2for written into the RAM (random access memory device) of the previously calculated value εWLsor (in the absence of such data), it is valued at nominal temperature.

In this case, as a measure to calculate the value of εWaccording to (3) is the previously calculated value. If it was not (the reservoir is filled with liquefied gas for the first time), it is accepted εW=const=εLsi.e. tabular value εWcalculated for the current value of the temperature of the controlled fluid, in particular, propane-butane mixture (this dependence is known and is stored in non-volatile memory). So, for propane-butane mixture εLs=1,7 at a temperature of t=20°C. In the formula (5) to determine the z-substituted-size εLs.

In this case, the level of z is determined by the following formula obtained from the formula (1) and (2) when εWLs- nominal value of εWsubstitute of RAM:

Figure 2 shows graphs of equivalent containersE1(line 1) and CE2(line 2) depending on the level z of the liquid. It also marked the section with a length l0that reduced the n sensor 2 at the second level measurement z. This graph also indicated by dotted lines corresponding to a certain current level value z and the corresponding equivalent capacitors CE1(z) and CE2(z).

For LPG the most appropriate parameter to determine its reserves in the tank is its mass. For nonpolar substances, including liquefied petroleum gas (LPG), is just the ratio of the Clausius - Mosotti describing the functional relationship between the density of matter (liquid, gas) and dielectric constant (Skanavi GI Physics of dielectrics (the region of weak fields). M. - L., Gill. 1949).

where ε - permittivity substance, μ - its molecular weight, ρ - density of matter, α - polarizability of molecules, N is the Avogadro's number.

From (7) it follows that

where A=4πNα/3μ - constant for each compound value.

Therefore,

Then, knowing the density of the liquid and gas phases of a substance can help determine the appropriate mass values:

It is known that mixtures of gases, mixtures of liquefied gases, in particular, propane-butane mixtures, spravedlivo with high accuracy the following relationship (Rudakov GY and other "Dielectric permeability of gas condensate and its fractions". Scientific and technical. the overview. Series: the Processing of gas and gas condensate. M: Negatron. 1973):

Here ρcmand εcmthe density and dielectric constant of the mixture; nithe number of molecules of the ith species per unit volume, αithe polarizability of molecules of the ith species, μi- molecular weight molecules of the ith species.

For each mixture the value of the

is constant.

From relations (13) and (14) imply the following formula for mixtures of substances:

This ratio is fairly high accuracy for any phase state of a mixture of substances, including liquefied gases. Therefore, in this case on the basis of (15) can be found, as in (11) and (12), the mass of liquid, gaseous phase of the mixture and the total mass M=MW+Mg.

When carrying out measurements using capacitive sensors or sensors in the form of segments of long lines are often used scheme in which such sensors are customizability elements of the oscillating circuit. For circuits with capacitive sensors, the resonant frequency fpcapacitive sensor

where L is the inductance connected to the sensor with e vigalantee capacity With e. For circuits with RF sensors on the basis of long segments of lines based on the designated level describes the exact transcendental equations implicitly or explicitly, close relationships, which, however, accurate enough for the task solution process measurements (see, for example, the monograph: V.A. Viktorov, Lunkin BV, Sablukov A.S. radio-wave measurement of parameters of technological processes. M.: Nauka. 1989. 280 C.).

Figure 3 shows a diagram of the measuring channel, containing each of the sensors 1, 2 or 3, included as a frequency control element in the composition of the oscillator 7. Here the oscillatory circuit formed by the combination of the capacitor (when implementing a device with a capacitive sensor) with the corresponding sensor 1, 2 or 3 the value of the equivalent capacitance and connected inductance 8. Hard mutual arrangement of conductors 9 and 10 sensors is provided with a dielectric, such as Teflon, bushings 11 and 12 (they are really over 0.7 m along the length of the sensors). Figure 3 shows only two such bushings. They have a through hole for the free flow of liquid.

The output signal of the oscillator 7 (frequency up to ten megahertz) is divided to 1-2 kHz, convenient for transmission over the communication line. Unit 7 included in wtorek the CSO Converter 4. In it coming from the sensors 1, 2 and 3, the signals undergo joint functional processing according to the above ratios. Indicator contained in unit 4, shows the current values of the mass of liquefied gas 6 in the tank, including separately the liquid mass MWgas Mgphases and the total mass M

Thus, the proposed device provides a high-precision determination of the actual values of the mass of liquefied gas in the tank regardless of its phase state and the ratio of the component that is in a liquid or gas phase, from having a boiling liquefied gas.

1. Device for determining the mass of liquefied gas contained in the vessel, containing three have in it the RF sensor of different lengths connected to the secondary of the Converter, characterized in that the length of the first sensor placed vertically in the tank corresponds to the vessel height and the length of the second sensor, also have vertically in the tank, is reduced below a value of not more than 0.35 length of the first sensor and the third sensor located in the gas phase LPG, has a length of not more than 0.3 length of the first sensor.

2. The device according to claim 1, characterized in that each sensor included in customizados circuit of the corresponding oscillator, part of the secondary transformed is the Converter.



 

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