RU2361181C1 - Method of measuring mass of fuel liquefied hydrocarbon gases in reservoir - Google Patents

Abstract

FIELD: physics; measurement.

SUBSTANCE: invention relates to measuring techniques and can be used for accurate measurement of mass of fuel liquefied hydrocarbons (LH). The method is based on an indirect method of statistic measurement, where the level, average density and average temperature of the liquid phase of the liquefied hydrocarbon gas is measured. From the measured values of average density and average temperature of the liquid phase of the liquefied hydrocarbons, the relative component content of the liquid and vapour phase of the liquefied hydrocarbons is determined. The average temperature of the vapour phase of the liquefied hydrocarbons is measured. From the measured values of pressure, average temperature of the vapour phase, the average density of the vapour phase is calculated. The level of the liquid phase of the liquefied hydrocarbons is then corrected. From the measured values of the level and average temperature of the liquid and vapour phases of the liquefied hydrocarbons, the volume of these phases is determined using a calibration table for the reservoir. Mass of the liquid and vapour phases of the liquefied hydrocarbons is calculated, as well as total mass as a sum of the mass of the liquid phase and vapour phase. Relative measurement error is automatically controlled and this involves calculation of relative measurement error, comparison with a given value and determination of recommended values of the minimum permissible level when storing the liquefied hydrocarbons and the minimum change in the level during intake and outlet of the liquefied hydrocarbons, providing for meeting requirements for the given value of measurement error. The obtained results are displayed on a display device for the convenience of the operator.

EFFECT: more accurate measurement of the mass of liquefied hydrocarbons, and effective dynamic control of measurement error during intake and outlet.

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Description

The invention relates to the field of measuring equipment and can be used in systems for measuring the mass of fuel liquefied hydrocarbon gases (LPG) in a tank during reception, dispensing and storage.

Known indirect method for static measurements of the mass of LPG, according to which the mass is determined by the formula

where V is the volume of the product in the tank;

ρ is the average density of the product in the tank.

The volume of liquid in the tank is determined using the calibration tables of the tanks according to the values of the filling level.

However, the LPG in the tank is a closed two-phase system: the liquid phase is the vapor phase. This necessitates the determination of the mass of both the liquid phase and the vapor phase according to the specified formula. It should be noted that a change in temperature, pressure, component composition or amount of LPG in the tank is accompanied by a redistribution of LPG between the liquid and vapor phases, which significantly complicates the measurement of LPG mass.

Known methods for measuring the mass of LPG in the tank, based on measuring the volume of the liquid phase of LPG using calibration tables of tanks and level sensors on capacitive, radio wave and float magnetostrictive principles for the construction of measuring devices. These methods give a large error due to the fact that part of the LPG is in the form of a vapor phase. In addition, a change in temperature, pressure or LPG composition leads to distortions in the readings of the float magnetostrictive level sensors associated with a change in the density of the LPG vapor phase and, as a result, the depth of immersion of the float in the liquid phase.

A non-invasive method for determining the level and density of a liquid inside a container is known, based on the use of an acoustic wave that passes through the container body and measures the phase difference from the sensor to the receiver. Such a method is described in US patent US 6053041. When using this method, the measurements are independent of the shape of the tank and its size. Fluctuations in the phase difference make it possible to measure not only the level, but also the density of the liquid. But this method has a very significant drawback - the low accuracy of density measurements (10 kg / m ³ ), which does not allow it to be used for commercial accounting.

Known methods for using differential pressure sensors to determine the volume and density of liquefied gases. For example, such a method is described in French patent FR 2811752 (G01F 22/00), which allows to display the volume of liquid in a tank with compressed gas. However, this method provides reliable measurements only in tanks with the correct geometric shape, which is a significant drawback. Another measurement method using a differential pressure sensor is described in US Pat. No. 6,944,470. This patent describes a differential pressure sensor that determines the density of a liquid and gas based on a measured pressure. However, these measurements will be valid for single-component products, for mixtures this sensor will give a large error due to fluctuations in the composition of the product.

Known attempts to solve the problem of determining the level of LPG using capacitive methods. This method is described in the article “Monitoring the level of liquefied hydrocarbon gases using capacitive devices” by Atayants, Peshchenko, Severin (Gas industry, 1997, No. 6, pp. 25-28). According to this method, a vertical capacitive sensor in the form of a cylindrical capacitor partially filled with a controlled substance is placed in a tank with liquefied gas. The degree of filling the tank with liquefied gas corresponds to the degree of immersion of the sensor in it. This method has a significant drawback. Since there is an uncontrolled transition of LPG from the liquid phase to the gaseous phase and vice versa, the determination of the LPG mass requires the use of two density sensors separately for the liquid and gaseous phases, which significantly complicates the measurement process and the design of measuring devices that implement this method.

The disadvantage of the above method is eliminated in the method for determining the physical parameters of LPG in the tank according to the patent of the Russian Federation No. 2262667 authors Sovlukova AS and Tereshina V.I.

The method for determining the physical parameters of LPG in a tank, described in the aforementioned patent, allows you to determine the mass, level, volume, density of the liquid and vapor phases of LPG, while the first measurement of the liquid level is made by the value of the measured electric capacitance C1 of the first radio frequency sensor and the density of the vapor phase is determined by the measured electric capacitance SZ of the corresponding RF sensor, then a second measurement of the liquid level is made in the range of its measurement reduced or increased from below about the measured value of the electric capacitance C2 of the corresponding RF sensor, perform a joint functional transformation of the measured electric capacitances C1, C2 and C3 and determine the dielectric constant of the liquid and vapor phase according to the formulas:

where the sign ″ - ″ corresponds to the reduced range of the second level measurement, the sign ″ + ″ corresponds to the increased range of the second level measurement;

I _O - the value of the range of the level change;

With _O is the electric capacitance per unit length of the sensor;

With _E - the value of the equivalent capacity With ₃ ;

With _Eo - the initial (with ε _g = 1) value of the equivalent capacity of C ₃ ;

the density of the liquid and vapor phases is determined by the formulas, respectively

where A is a constant for each substance, the liquid level is determined by the formula

where I is the range of level measurements;

volume of liquid and vapor phases - according to the formulas, respectively

where S is the cross-sectional area of the tank,

Vo is the volume of capacity

mass of liquid and gas phases - according to the formulas, respectively

where V _W and V _{g the} volume of the liquid and vapor phases, respectively. However, this method has a low accuracy of mass measurement, due to a number of significant disadvantages.

The first disadvantage is the low level measurement accuracy. This is confirmed by the fact that the level measuring system implemented using the patent described above (SU-5D TSSCh.000.115TU) in accordance with the description of the type of measuring instruments registration No. 179292-04 of October 26, 2004, approved by VNIIM them. D.I. Mendeleev, has a limit of permissible absolute basic error of level measurements of ± 3 mm, which, accordingly, entails a proportional error of mass measurements.

The second disadvantage is the large error in density measurements, because in the above formulas for calculating the density there is a function of the properties of the components "A", but the method does not provide a device for determining the component composition of LPG. The measurement of this parameter by classical chromatographic methods in accordance with GOST 10679-76 “Liquefied hydrocarbon gases. The method of determining the hydrocarbon composition ”requires a special equipped laboratory and a long time. Using the average value of the coefficient "A" will lead to large errors, because in accordance with GOST R52087-2003 “Liquefied petroleum hydrocarbon gases. Specifications "the mass fraction of components can vary within ± 10% or more for different grades of LPG, and in GOST 20448-90" Liquefied petroleum hydrocarbon gases for household consumption. Technical conditions ”are generally not regulated by accuracy (a value not less than ... is indicated).

The third disadvantage is the measurement error caused by the presence of water and water vapor in the tank. Water enters the tank with LPG, and also condenses from the air. As experience in operating tanks with LPG shows, more or less water is always present. The effect of water is described in the article "On methodological errors of accounting for LPG in the tank farm", authors V. Tereshin, A. Sovlukov, A. Letunovsky, magazine "Gas filling complex", No. 5, 2006 (the first two authors of the article are the authors of the patent in question ) It should be noted that the capacitive methods used in this method of measuring the mass of LPG are designed to work in a medium of dielectric materials, which are LPG. Water has both dielectric properties and conductivity, and its presence in the tank leads to very large measurement errors, since the dielectric constant of LPG ε _g ≈1.7, and the dielectric constant of water ε _in = 81, which is about 50 times while the measured capacitance will also increase, which will lead to distortion of the measurement results.

The fourth drawback is the lack of control of the magnitude of the current measurement error, which does not allow us to evaluate the accuracy of mass measurements during the reception, tempering and storage of LPG.

The fifth drawback is the lack of temperature compensation of the size of the sensors, which also introduces an additional measurement error.

These shortcomings do not allow us to consider the specified method sufficiently accurate.

Other known technical solutions using radio-frequency methods have similar disadvantages, for example, the method of combining density and dielectric measurements to determine the volume of liquids in tanks according to UK patent GB 2409727 (G01V, 11/00) or the radio-frequency method for measuring the density of liquids and gases by determining complex dielectric properties according to the patent of Finland FI 00102014 (G01N, 22/00).

As a prototype, the indirect method of static measurements of the mass of liquid in the tank based on the magnetostrictive effect, implemented in the measuring system STRUNA TU 4210-001-23434764-2004, is chosen. The choice of the prototype is justified by the fact that according to the description of the type of measuring instruments reg. No. 28116-04 of November 19, 2004, the limits of the permissible absolute error of measurements of the liquid level are ± 1 mm, and the limits of the permissible absolute error of measurements of the liquid density for submersible densitometers is ± 1 kg / m ³ . These parameters make it possible to obtain high accuracy in measuring the mass of the liquid phase of the LPG.

It should be noted the successful design of the density meter for a liquid with low density according to patent RU No. 2308019 (G01N, 9/10), implemented in the measuring system “STRUNA”. The specified density meter for a liquid with a low density contains a balanced vertically moving float immersed in a liquid in the form of a toroid with magnets placed in it, means for balancing the float in the form of chains, a transducer for moving the float into an electrical signal placed in a tubular body made of non-magnetic material, covered by a toroidal float made in the form of a magnetostrictive waveguide with a read coil located on it and a pulse shaper connected to it, to the reading carcass is connected to the input of the driver-amplifier, the output of which is connected to the input of the signal processing unit, the output of which is connected to the input of the pulse shaper, the balancing chains of the floats are attached at one end to the float, and at the other end to a latch, rigidly fixed to the housing of the transducer, and the number of floats and their fastening elements is n pieces with their location at different heights depending on the required number of points for measuring the density of the liquid, They are composite and consist of the heavy and light parts connected in series, the heavy part being attached to a latch rigidly attached to the transducer body, and the light part being attached to the float. This densitometer allows you to measure the density of the liquid phase of LPG with high accuracy. The densitometer created according to the specified patent as a part of the measuring system “STRUNA” in accordance with the description of the type of measuring instruments has limits of the permissible absolute error of density measurements ± 1 kg / m ³ . In addition, it should be noted that this densitometer is not critical to the presence of water in the LPG.

The measurement of the level and density in this system is based on measurements of the propagation time of ultrasound in a magnetostrictive waveguide. The propagation velocity of ultrasound in the waveguide is practically independent of pressure and humidity. The influence of temperature is automatically compensated using a special algorithm for processing time intervals of ultrasound propagation.

An ultrasonic wave is generated in a waveguide made of a special steel wire with magnetostrictive properties and located inside a body of non-magnetic material.

In the interaction of an alternating magnetic field created by a current pulse in the waveguide and a field of permanent magnets located in the floats of the level and density sensors, the crystal structure of the waveguide is deformed, which creates a mechanical wave propagating with ultrasonic speed. An ultrasonic wave arising at the locations of the magnets propagates along the waveguide in both directions from the place of origin. In the upper part of the waveguide, ultrasonic waves are converted by a read coil into electrical pulses due to the inverse magnetostrictive effect and then are damped by a damper. These pulses enter the amplifier-driver, where they are converted into a rectangular shape, and then they enter the density measuring unit and the level measuring unit. The time interval between the moment of generation of the ultrasonic wave and its conversion into electrical pulses is proportional to the measured distance, i.e. position of the floats. At the same time, the moments of the conversion of ultrasonic waves into electric pulses are spaced in time, and the analysis of the number of pulses and the corresponding time intervals allows us to determine the position of each float and thus measure the level and density of the liquid. A more detailed description of the work is described in patents RU No. 2138028 (G01F 23/68, G01N 9/10, 1998) and RU No. 2285908 (G01F 23/68, G01N 9/10, 2005).

The indirect method of static measurements of the mass of liquid in the tank allows you to measure the mass of liquid in the tank, but it does not take into account the fact that the LPG in the tank is a closed two-phase system: liquid phase - vapor phase. In this regard, the direct use of this method for measuring the mass of LPG leads to large measurement errors.

It should be noted that when using a level float, a methodological error is observed, caused by the dependence of the depth of immersion of the float on the density of the liquid and vapor phase. This fact is mentioned in the above article “On methodological errors in the accounting of LPG in the tank farm”, authors V. Tereshin, A. Sovlukov, A. Letunovsky, magazine “Gas filling complex”, No. 5, 2006

The dependence of the immersion depth of the level float on the density of the liquid is taken into account in patent RU No. 2285908 (G01F 23/68, G01N 9/10) and is implemented in the prototype, but this is only part of the solution to reduce the error.

Known options for installing pressure sensors in the upper part of the tank, for example in the prototype, for monitoring the pressure of the vapor phase of the LPG (practically sensors or manometers are on each tank with LPG according to safety rules), but they are not used to measure the density of the vapor phase of the LPG.

In addition, the prototype does not provide an estimate of the error in the measurement process and the possibilities of minimizing the error by issuing recommendations on the value of the minimum acceptable level during storage of the product and the value of the minimum level change during the reception and dispensing of the product, which allows to provide the specified value of the measurement error.

The problem to which the invention is directed, is to create a method that allows you to measure the mass of LPG with high accuracy, to carry out operational dynamic control of the measurement error during storage, reception and dispensing of LPG and give recommendations on the value of the minimum permissible level during storage of the product and the value minimum level changes during the reception and dispensing of the product, allowing to provide the specified value of the measurement error.

To obtain reliable results of measuring the mass of LPG, it is necessary to know the density of the vapor phase of LPG.

To determine the density of the vapor phase of LPG, it is necessary to know the component composition of LPG. In real conditions, the composition of the LPG coming from the manufacturer is indicated in the passport. But the incoming LPG in each batch is different from previous deliveries. In the storage tanks, the received LPG is mixed with the remainder, and the composition of the mixture becomes uncertain. It can be determined by the chromatographic method described in GOST 10679-76. However, this method requires a specially equipped expensive laboratory and takes a lot of time. As is known from [8], the parameters of the LPG mixture are approximately proportional to the concentrations and parameters of the individual components. Therefore, it is possible to judge the conditional component composition (with sufficient accuracy for operational needs), i.e. Find out the parameters of the mixture, and not each hydrocarbon separately. This makes it possible to combine ethane, ethylene, propane and propylene in the pseudopropane group, and butanes, butylenes and pentanes in the n-pseudobutane group. Then you can determine the pseudo-composition of a real mixture of LPG, considering it a mixture of propane and n-butane. This generalization is accepted in the proposed device for determining the density of the vapor phase of the LPG.

The subject of the invention is a method for measuring the mass of LPG in a tank using the indirect method of static measurements, in which the level of the liquid phase of LPG is measured using a magnetostrictive level sensor, the average density of the liquid phase of LPG using a multipoint magnetostrictive density sensor, the average temperature of the liquid phase of LPG using temperature sensors , the volume of the LPG liquid phase is calculated from the measured value of the level and the average temperature of the LPG liquid phase using the calibration tables of the reservoir, the mass of the LPG liquid phase is calculated as the product of the volume and the average density of the LPG liquid phase, the pressure of the LPG vapor phase is measured using a pressure sensor, the conditional component composition of the LPG and vapor phase of the LPG is determined from the measured values of the average density and average temperature of the LPG liquid phase, the average LPG vapor phase temperature using temperature sensors, the average density is calculated from the measured values of pressure and average temperature, the conditional component composition of the LPG vapor phase the LPG vapor phase, the effect of the LPG vapor phase on the immersion depth of the level sensor float is taken into account and the volume and mass values of the LPG liquid phase are adjusted, the volume of the LPG vapor phase is determined from the measured values of the LPG vapor phase using the calibration table of the tank, the vapor mass is calculated, the mass of the vapor is calculated LPG phases as a product of the volume and average density of the LPG vapor phase, the total mass of LPG is calculated as the sum of the masses of the liquid phase with correction and LPG vapor phases, it automatically dynamic control of the relative measurement error, including calculating the value of the relative measurement error, comparing it with the set value and determining the recommended values of the minimum acceptable level during storage of LPG and the minimum level change when receiving and dispensing LPG, ensuring compliance with the requirements for a given value of measurement error, and received the results are displayed on the display device for the convenience of the operator.

The technical result is the measurement of the mass of LPG with high accuracy, obtaining during the storage, reception and dispensing of LPG the value of the error in measuring the mass of LPG, the recommended values of the minimum permissible level during storage of LPG and the minimum level change during reception and dispensing of LPG, ensuring compliance with the requirements for the value of the specified error measurements.

Figure 1 presents the algorithm of the method for measuring the mass of LPG in the tank.

Figure 2 presents the algorithm for calculating the average density of the vapor phase of the LPG.

Figure 3 presents an example implementation of a method for measuring the mass of LPG in a tank.

Figure 4 presents the location of the sensors in the tank as an example implementation.

As can be seen from figure 1, the sequence of actions when implementing the method of measuring the mass of the product in the tank is as follows:

- after the start of work, the calibration tables of the tank are read;

- the parameters obtained from the measuring system used are read: level, density of the liquid phase of the LPG, temperature of the liquid and vapor phase of the LPG, pressure of the vapor phase of the LPG;

- the average density of the vapor phase of the LPG is calculated;

- a correction is made to measure the level of the liquid phase of the LPG (depth of immersion of the float);

- a cyclical calculation of the mass of the product and the error of its measurement, a comparison of the resulting error with a given value;

- a cyclical calculation of the recommended values of the minimum acceptable level during storage of the product and the minimum level change during the reception and dispensing of the product, at which the magnitude of the measurement error of the mass does not exceed the specified;

- the obtained calculation results are cyclically displayed on the display device.

In calculating the average density of the vapor phase, the algorithm of which is shown in figure 2, the following notation:

t _ж.cp.i - average temperature of the liquid phase of LPG;

ρ _ж.cp.i - average density of the liquid phase of LPG;

g _np is the mass content of propane in the liquid phase of the LPG;

x _np.i - molar propane content in fractions of a unit for the liquid phase of LPG;

r _np.i is the molar content of propane in fractions of a unit for the vapor phase of the LPG;

R _{see i} - specific gas constant of a mixture of propane and butane;

R _{sred.i. - the} average pressure of the vapor phase of the LPG;

T _cf.i - the _{average critical} temperature of the vapor phase of the LPG;

P _priv.i - reduced pressure of the vapor phase of the LPG;

T _priv.i - reduced temperature of the vapor phase of the LPG;

k _i - compressibility factor of the vapor phase of the LPG.

In the process of calculation, tuning parameters are used, which include the value of the specified permissible relative measurement error and the necessary technological constants (parameters) of the product, tank, and measuring system.

Compliance with the recommended level values will allow the operator to avoid errors during storage, reception and dispensing of LPG in terms of exceeding the permissible values of the measurement error.

A method for measuring the mass of LPG in the tank can be implemented using a measuring device or system that measures the level, average density and temperature, as well as the pressure of the LPG in the tank, the dip float corrector, made on a microprocessor, a device for calculating the mass of LPG and dynamic control of measurement error made on a microprocessor, and a display device, which can be either an independent structural element, or combined with the display of measuring systems . An example implementation is schematically presented in figure 3. An example of the location of the sensors in the tank is presented in figure 4.

The numbers in figure 3 and figure 4 indicate:

1 - reservoir;

2 - measuring system;

3 - device for calculating mass and dynamic control of measurement error;

4 - level corrector;

5 - file "Precision characteristics of measuring instruments";

6 - file "Calibration parameters of the tank";

7 - display device.

8 - liquid phase LPG;

9 - vapor phase LPG;

10 - level and temperature sensor;

11 - multi-point immersion density sensor;

12 - pressure sensor vapor phase LPG.

The device comprises a measuring system 2, for example, a measuring system “STRUNA” (SI) with the densimeter according to RU patent No. 2308019 described above, measuring the level, temperature (using a level and temperature sensor 10), the average density of the liquid phase of LPG (using a multi-point submersible density sensor 11), LPG pressure in the tank 1 (using the LPG vapor pressure sensor 12), a display device 7, a level 4 corrector and a device for calculating mass and dynamic control of measurement error 3, the first input of which is connected to the output SI and the first input of the level corrector, the second input is connected to the output of the level corrector, the first output is connected to the second input of the level corrector, the second output is connected to the input of the display unit, on the third input of the device for calculating mass and dynamic control of measurement error 3, accuracy characteristics of measuring instruments are entered with using file 5 and calibration parameters of the tank using file 6.

Temperature sensors are installed inside the level sensor pipe, and those that are in the liquid phase of LPG 8 are used to determine the average temperature of the liquid phase of LPG 8, and those that are above the level of liquid phase 8, i.e. in the vapor phase 9, are used to determine the average temperature of the vapor phase of the LPG 9.

The following is the procedure for calculating the mass of LPG in a steel tank and cyclically comparing the results with the value of the specified error based on the proposed method for measuring the mass of LPG.

To ensure reliable readings of measurements of the level of the liquid phase of the LPG, a correction is performed as follows.

The equilibrium equation of the level float without correction has the following form:

where M is the mass of the float level;

Н ′ _П - immersion depth of the level float without correction;

S is the cross-sectional area of the float;

ρ _Zh.sr.i - the average density of the liquid phase of the LPG.

The immersion depth of the level float without correction is calculated by the formula:

The equation of equilibrium of the level float with vapor phase correction is:

where N _P - the immersion depth of the level float with correction;

ρ _n.cp.i is the average density of the vapor phase of the LPG, (determined by the method described in paragraph 4 below, and the algorithm for calculating the average density is presented in figure 2);

V is the volume of the float.

Adjustment level float immersion depth:

In the level reading, you must enter the correction calculated by the formula:

The density, temperature and pressure parameters from the SI sensors located in the tank, and the adjusted level value are supplied to the mass calculation and dynamic control unit of the measurement error, where they are processed as follows:

1. The volume of the liquid phase of the LPG is calculated by the formula:

where V _g.20i is the volume of the liquid phase of the LPG in the tank at a measured level of N _i , determined by the calibration table of the tank, compiled at a temperature of 20 ° C;

α _st - temperature coefficient of linear expansion of the material of the wall of the tank (from the passport to the tank), for steel α _st = 12.5 × 10 ^-6 1 / ° C;

t _J.cp.i is the average temperature of the liquid phase of LPG in degrees Celsius.

2. The volume of the vapor phase of the LPG is calculated by the formula:

where V _20max is the total volume of the tank at a temperature of 20 ° C (from the passport to the tank);

t _n.cp.i is the average temperature of the vapor phase of LPG in degrees Celsius.

The value of t _{n.cp.i is} defined as the arithmetic mean temperature of the point sensors located in the vapor phase of the LPG.

The mass of the liquid phase of the LPG is calculated by the formula:

where ρ _ж.cp.i is the average density of the liquid phase of LPG.

3. The mass of the vapor phase of the LPG is calculated by the formula:

where ρ _n.cp.i is the average density of the vapor phase of the LPG.

The average density of the vapor phase of the LPG is calculated by the formula:

where P _i is the pressure in the tank;

P _A - atmospheric pressure, taken equal to 101.3 kPa;

R _{see i} - specific gas constant of a mixture of propane and butane;

k _i is the dimensionless compressibility factor;

t _A = 273.15 ° C.

The calculation of R _{see i} and k _i as follows:

4.1. The mass content of propane in the liquid phase of the LPG g _{np is} calculated by the formula:

where ρ _np.i and ρ _δ.i are the densities of liquefied propane and n-butane, respectively, at a temperature t. _{c.p.i are} determined according to table 1 by linear interpolation.

Table 1 - the Density of hydrocarbons in the liquid state [2] Temperature ° C Density, kg / m ³ Propane n-butane -fifty 590.9 651.1 -45 585.2 646.4 -40 579.4 641.5 -35 573.7 636.7 -thirty 567.7 631.7 -25 561.6 626.8 -twenty 555.5 621.8 -fifteen 549.3 616.6 -10 542.9 611.5 -5 536.4 606.6 0 529.7 601.0 5 522.8 595.7 10 515.8 590.2 fifteen 508.6 584.6 twenty 501.1 578.9 25 493.4 573.2 thirty 485.5 567.3 35 477.5 561.3 40 468.9 555.2 45 460.4 549.0 fifty 451.3 542.6

4.2. The molar content of propane in fractions of a unit for the liquid phase of LPG is calculated by the formula:

wherein M _pr - molecular weight of propane equal to 44.097 kg / kmol;

M _b - the molecular weight of n-butane is equal to 58.124 kg / kmol.

4.3. The molar content of propane in fractions of a unit for the vapor phase of the LPG is calculated by the formula:

where P _np.i is the pressure of saturated propane vapor at a temperature t _ж.cp.i , is determined according to table 2 by linear interpolation;

P _δ.i is the saturated vapor pressure of n-butane at a temperature t _ж.cp.i , is determined according to table 2 by linear interpolation.

Table 2 - Saturated vapor pressure [1]

Temperature ° C Saturated vapor pressure, kPa (abs.) Propane n- Bhutan -40 109 16* -35 134 19* -thirty 164 27 * -25 197 35 * -twenty 236 43 * -fifteen 285 56 -10 338 68 -5 399 84 0 466 102 5 543 123 10 629 146 fifteen 725 174 twenty 833 205 25 951 240 thirty 1080 280 35 1226 324 40 1382 374 45 1552 429 fifty 1740 490 55 1943 557

* - data in [1] are absent, obtained by interpolation.

4.4. The specific gas constant of the vapor phase of the LPG is calculated by the formula:

- универсальная газовая постоянная, равна 8314

Where

- universal gas constant, equal to 8314

4.5. The average critical pressure and temperature of the vapor phase of the LPG are calculated by the formulas:

where R _ave . _cr = 4246 kPa is the critical pressure of propane;

R _b.kr = 3800 kPa — critical pressure of n-butane;

T _av.kr = 369.8 K - critical temperature of propane;

T _b.kr = 425.2 K is the critical temperature of n-butane.

Note. The critical parameters are taken from [3].

4.6. The pressure and temperature of the vapor phase of the LPG are calculated by the formulas:

4.7. The compressibility factor k _{i of the} vapor phase of the LPG is calculated according to table 3 by linear interpolation.

5. The mass of LPG in the tank is calculated by the formula:

where m _{w. i} - the mass of the liquid phase of the LPG;

m _ni is the mass of the vapor phase of the LPG.

6. The mass of the issued or accepted LPG, kg, is calculated by the formula:

where m _i is the mass of LPG at the beginning of the reception or vacation;

m _{i + 1} - the mass of LPG at the end of the operation of reception or vacation.

7. The relative error of measurements of the mass of LPG in the mode is calculated by the formula:

где

Where

δN is the relative calculation error;

δK is the relative error in compiling the calibration table;

To _f - coefficient taking into account the geometric shape of the tank, is calculated by the formulas:

a) for the liquid phase of the LPG

where ΔV _20i is the volume per 1 mm of the filling height of the tank at the measured filling level H _i (determined by the calibration table of the tank);

b) for the vapor phase of the LPG

where N _max - the maximum height of the tank (for a horizontal tank is equal to its diameter);

δH _J.i - the relative error in the measurement of the level of the liquid phase of the LPG, is calculated by the formula:

where ΔН is the absolute error of level measurements;

δH _ni is the relative measurement error of the LPG vapor phase level (counting from the upper edge of the tank), is calculated by the formula:

δρ _ж.cp.i - relative error of measurements of the average density of the liquid phase of LPG, calculated by the formula:

where Δρ is the absolute error of measurements of the average density of the liquid phase of the LPG;

δρ _n.cp.i is the relative error of measurements of the average density of the vapor phase of the LPG, calculated by the formula:

where δP _{i is the} relative error of pressure measurements of the vapor phase of the LPG, is calculated by the formula:

where γ _P is the reduced error of pressure measurements;

P _B is the upper limit of pressure measurements;

δt _n.cp.i - the relative error of the temperature measurement of the vapor phase of the LPG, is calculated by the formula:

where Δt is the absolute error of the temperature measurement of the vapor phase of the LPG;

δN _K is the relative error in the calculation of the component composition of the vapor phase of the LPG.

8. The relative error of the mass measurements of the received or released LPG is calculated by the formula:

9. The minimum level of LPG H _min , for the storage mode, above which the error in the measurement of mass is within the limits specified by the user, is calculated as follows.

The values of the LPG level H _{i are set} , with a step of 1 mm, starting from the minimum, and the measurement error of the LPG mass δm _{i is} calculated by formula 21. As soon as the value δm _i falls below the user specified error limits, the calculation ends. The calculation is made for the density ρ _l.s.

10. The level of LPG after completion of the operation of reception (vacation) H _{i + 1} , below (above) which the measurement error of the mass of the received (released) LPG is within the limits set by the user, is calculated as follows.

The value of N _i increases (during reception) or decreases (during vacation) in increments of 1 mm and the value m _{I + 1 is} calculated according to the formula 19, m _O according to the formula 20, δm _O according to the formula 32. As soon as the value δm _O becomes less than the error limits specified by the user, the calculation ends. The calculation is made for the density ρ _l.s.

The results of calculations of the mass of LPG, the magnitude of the measurement error and the recommended values of the minimum acceptable level during storage of the product and the minimum level change during the reception and release of LPG, ensuring compliance with the requirements for the magnitude of the measurement error, are displayed on the display device.

At the applicant enterprise, a prototype was made on the basis of the STRUNA measuring system, which implements the inventive method for measuring the mass of fuel liquefied hydrocarbon gases. The test results fully confirmed the correctness of the proposed solution. In the process of testing, the convenience of the operator during storage, reception and dispensing of LPG was noted.

Information sources

[1] Staskevich N.L., Wigdorchik D.Ya. Handbook of liquefied petroleum gases. L .: Nedra, 1986, 543 pp.

[2] GOST 28656-90. Liquefied hydrocarbon gases. Calculation method for determining the density and pressure of saturated vapors.

[3] Reed R., Prausnits J., Sherwood T. Properties of gases and liquids. L .: Chemistry, 1982, 592 pp.

[4] GOST R52087-3003. Gases hydrocarbon liquefied fuel. Technical conditions

[5] GOST 20448-90. Liquefied petroleum hydrocarbon gases for domestic consumption. Technical conditions

[6] “Monitoring the level of liquefied hydrocarbon gases using capacitive devices” Atayants, Peshchenko, Severin, magazine “Gas industry”, 1897, No. 6, pp. 25-28.

[7] “On methodological errors in the accounting of LPG in the tank farm”, authors V. Tereshin, A. Sovlukov, A. Letunovsky, magazine “Gas filling complex”, No. 5, 2006, pp. 24-28.

[8] Preobrazhensky N.I. Liquefied petroleum gases. L .: Nedra, 1975, 279 pp.

[9] GOST 10679-76. Liquefied hydrocarbon gases. Method for determining the hydrocarbon composition.

Claims (1)

A method of measuring the mass of fuel liquefied petroleum gas (LPG) in a tank using the indirect method of static measurements, in which the level of the liquid phase of the LPG is measured using a magnetostrictive level sensor, the average density of the liquid phase of the LPG using a multipoint magnetostrictive density sensor, the average temperature of the liquid phase of the LPG is using temperature sensors, the volume of the liquid phase of the LPG is calculated from the measured value of the level and the average temperature of the liquid phase of the LPG using a graduated of the tank table, the mass of the LPG liquid phase is calculated as the product of the volume and the average density of the LPG liquid phase, the vapor pressure of the LPG is measured using a pressure sensor, characterized in that the conditional component composition of the liquid and the vapor phase of the LPG, the average temperature of the vapor phase of the LPG is measured using temperature sensors, the average density the LPG vapor phase, the influence of the LPG vapor phase on the immersion depth of the level sensor float is taken into account and the volume and mass values of the LPG liquid phase are adjusted, the volume of the LPG vapor phase is determined from the measured values of the LPG vapor phase and using the calibration table of the tank, the vapor mass is calculated, the mass of the vapor is calculated LPG phases as a product of the volume and average density of the LPG vapor phase, the total mass of LPG is calculated as the sum of the masses of the liquid phase with correction and the LPG vapor phase, automatically dynamic control of the relative measurement error, including calculating the value of the relative measurement error, comparing it with a given value and determining the recommended values of the minimum acceptable level during storage of LPG and the minimum level change during reception and release of LPG, ensuring compliance with the requirements for a given value of measurement error, and received the results are displayed on the display device for the convenience of the operator.

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