RU2550306C1 - Method of measurement of volume concentration of hydrogen - Google Patents

Abstract

FIELD: instrumentation.

SUBSTANCE: temperature measurement and ultrasound speed are measured in the measured gas, the speed in pure hydrogen is determined at the same temperature, and the concentration of hydrogen in gas mix is calculated using the mathematical expression considering the ratio of ultrasound speed in pure hydrogen squared to ultrasound speed in the measured mix of gases squared and the ratio of molar mass of admixtures in hydrogen to the molar mass of pure hydrogen are determined.

EFFECT: simplification of hydrogen volume concentration measuring system, increase of its long-term stability and decrease of measurements errors.

2 cl, 1 dwg

Description

The invention relates to measuring technique, in particular to measuring the volumetric concentration of hydrogen in a mixture of gases in powerful turbo-generators with hydrogen cooling.

For the removal of Joule heat from the internal volume of powerful turbogenerators, hydrogen is used, which has high thermal conductivity and low viscosity. At an average temperature of 60 ° C in turbogenerators, the thermal conductivity of hydrogen is λ _H = 210 · 10 ^-3 W / (m · K), and the dynamic viscosity is µ _H = 8.5 · 10 ^-6 Pa · s. At the same temperature, the thermal conductivity of air λ _B = 29 · 10 ^-3 W / (m · K), water vapor λ _P = 21 · 10 ^-3 W / (m · K); dynamic viscosity coefficients, respectively, µ _B = 25 · 10 ^-6 Pa · s and µ _P = 500 · 10 ^-6 Pa · s.

It can be seen from the above data that hydrogen is an order of magnitude more efficient as a coolant and the loss of viscous resistance is 3 times less than that of air and almost 60 times less than water vapor.

The indicated feature of hydrogen makes it possible to efficiently cool the windings of stators and rotors of powerful turbo-generators with low hydrodynamic friction losses. At the same time, the appearance in hydrogen of impurities of other gases or water vapor dramatically reduces the heat transfer efficiency. For this reason, it is not allowed to reduce the purity of hydrogen in turbogenerators below 97% [1, clause 5.1.11].

A known method of measuring the volumetric concentration of hydrogen by measuring the thermal conductivity of the gas mixture [2]. According to the description, the measured gas is passed through a cell containing a conductor heated by electric current, the temperature of which is functionally related to the thermal conductivity of the measured gas, which transfers part of the heat to the cold wall of the measuring cell. In such cells there are a large number of sources of error:

- Methodological, associated with the adoption of a two-component model of the measured gas, for example, air in hydrogen or hydrogen in argon (real gas contains more components); acceptance of the condition of constant temperature of the cell wall, constant gas flow through the cell, the absence of heat loss due to radiant and convective heat transfer.

- Instrumentation related to the cooling of the conductor due to heat transfer in the area of the conductor fastening to the cell body, temperature gradient along the measuring cell, changes in the supply current, changes in the properties of the heated conductor, etc.

The parry of these errors leads to a complication of the design of the concentration meter, namely:

- installation of a gas dryer in front of the measuring cell;

- introduction to the gas path of the flow meter and stabilizer gas flow through the cell;

- completing the design with a second measuring cell filled with gas of precisely known composition and connecting the heated conductors of two cells to a bridge measuring circuit;

- Complementing the design with a thermostat to stabilize the temperature of the inlet gas and the housing of the measuring cells.

Despite all the measures taken, the measurement error cannot be reduced less than 2% -5% (depending on the measurement range) and the long-term stability of the conversion function of the measuring instrument is ensured, which necessitates repeated calibrations of the instruments every 3 months [2, p. 39] .

A known gas analyzer of hydrogen containing palladium wire (or from a palladium alloy wire), heated to a temperature above 170 ° C [3]. The measurement method is based on the fact that palladium (and its alloys) selectively absorbs hydrogen from a mixture of gases; in this case, the ohmic resistance of the sample changes, by measuring which one can judge the concentration of hydrogen in the gas mixture.

The disadvantages of the method according to the patent [3] are related to the fact that the equilibrium absorption of hydrogen by palladium is described by an equation of the form [4, p. 323, expression (50)]

where [H] _Pd is the concentration of hydrogen in palladium;

P is the partial pressure of hydrogen in the gas;

T is the absolute temperature of the gas and palladium.

The expression shows, firstly, a very strong dependence of the hydrogen concentration in palladium on temperature, the vaporization of which requires careful thermal stabilization of both the palladium wire and the measured gas. Secondly, since the hydrogen pressure enters the expression under the square root, with the increase of the partial pressure (concentration) of hydrogen in the gas mixture, the relative increase in the hydrogen concentration in palladium decreases, i.e. the sensitivity of the transmitter decreases with increasing hydrogen concentration. If, at a partial pressure of hydrogen of 50 kPa, the relative sensitivity of the converter is taken as 1, then at 400 kPa it turns out to be 0.1. A decrease in the conversion sensitivity leads to an equivalent increase in the measurement error of high hydrogen concentrations characteristic of turbogenerators.

An additional factor that increases the measurement error is the low absolute sensitivity of the conversion. According to the experimental data presented in [5], even in the region of low partial hydrogen pressures in a gas, the conversion sensitivity does not exceed 0.0003 Ohm / (Ohm ·%), and at high hydrogen concentrations it will be even an order of magnitude lower. From the foregoing, it is clear that the metrological characteristics of the gas analyzer according to [3] are many times worse than that of the above-described meter based on thermal conductivity [2].

The closest in technical essence is a method for determining local volumetric concentrations of hydrogen in a vapor-gas medium [6]. According to the invention, simultaneously with the measurement of the partial pressure of hydrogen by the gas analyzer according to the patent [3], the ultrasound velocity in the gas-vapor medium is additionally measured at a frequency f = 0.1-1.0 MHz, the pressure and temperature of the gas-vapor medium and the volume concentrations of hydrogen, water vapor and air are determined in a gas-vapor medium according to the mathematical relations given in the description of the patent.

The proposed method is difficult because it involves, in addition to a hydrogen partial pressure meter, the introduction of channels for measuring the total pressure, temperature of the measured gas and the channel for measuring the speed of ultrasound. It should be borne in mind that each measurement channel introduces an additional error in the measurement result and long-term instability, the parry of which is associated with frequent recalibrations of the measurement system.

The aim of the proposed method is to simplify the measurement system of the volumetric concentration of hydrogen, increase its long-term stability and reduce the measurement error.

This goal is achieved by the fact that in addition to measuring the temperature and speed of ultrasound in the measured gas, the speed in pure hydrogen is determined at the same temperature, and the concentration of hydrogen g in the gas mixture is calculated from the expression

- отношение квадрата скорости ультразвука в чистом водороде $${{\displaystyle C}}_H^2$$

к квадрату скорости ультразвука в измеряемой смеси газов $${{\displaystyle C}}_C^2$$

;where z is $${{\displaystyle C}}_H^2/{{\displaystyle C}}_C^2$$

- the ratio of the square of the speed of ultrasound in pure hydrogen $${{\displaystyle C}}_H^2$$

to the square of the ultrasound velocity in the measured gas mixture $${{\displaystyle C}}_C^2$$

;

α = µ _P / µ _H is the ratio of the molar mass of µ _P impurities in hydrogen to the molar mass of pure hydrogen µ _H.

The drawing shows an embodiment of the proposed method.

The volumetric hydrogen concentration meter is a measuring cell 1 through which the measured gas flows. Emitting ultrasound 2 and receiving 3 elements are installed from two ends of the cell. The gas temperature in the cell is measured by a thermocouple 4 connected to the electronic unit 5.

The electronic unit 5, containing a microcontroller, for example, AT Mega 8535 and amplifiers, performs several functions:

- generates an electrical excitation signal of the radiating element 2;

- amplifies the signal of the receiving element 3, calculates the speed of passage of the ultrasonic signal C _C through the cell;

- converts the signals of the thermocouple 4 into the current gas temperature T in cell 1;

- calculates the speed of ultrasound in pure hydrogen C _H from the value of the measured temperature T;

- calculates the squares of the velocities of ultrasound in cell 1 and in pure hydrogen;

- by expression (1) calculates the concentration of hydrogen in the gas and presents the result to the consumer (on the display and in the form of an electrical signal).

The meter works as follows. The measured gas is supplied to the measuring cell 1, and the ultrasonic signal passes through it from the emitter 2 to the receiver 3. The received signal is transmitted to the electronic unit 5, in which the ultrasound velocity in the measured gas C _{C is} calculated. At the same time, the microcontroller of unit 5 interrogates the thermal converter 4 and converts the received signal into the temperature of gas T. The ultrasound velocity in pure hydrogen C _{H is} calculated from the temperature value by the expression

where k _H is the constant hydrogen adiabat at gas temperature T;

R = 8.314 J / (K · mol) is the universal gas constant;

T is the absolute temperature of the gas;

µ _H = 0.002016 kg / mol - molar mass of molecular hydrogen.

The measured ultrasound velocity in a gas C _C is related to the gas parameters by the expression

where k _C is the adiabatic constant of the measured gas;

r is the volumetric concentration of hydrogen in the gas;

µ _P is the molar mass of impurities in hydrogen (air, water vapor, etc.).

Squaring expression (2) and dividing it by the square of expression (3) gives the equation

where Z = $$\frac{{{\displaystyle C}}_H^2}{{{\displaystyle C}}_H^2}.$$

Consider the ratio of the adiabatic constants in equation (4). The constant of hydrogen adiabat k _H, as well as the constant of air k _Bm , carbon monoxide in the temperature range from 20 to 100 ° C, are close to 1.4. The constant adiabat of water vapor and carbon dioxide in the same temperature range is close to k _Om = 1.3. The hydrogen concentration in the generators under the worst conditions is not lower than r _Hm = 0.95, and the impurities make up air in a volume fraction of not less than r _Bm = 0.04, the rest is water vapor, carbon monoxide and carbon dioxide r _Om . Consequently, the adiabatic constant of the gas mixture will be equal to

Substitution in the last expression of the numerical values of the quantities gives

The result obtained allows us to assume with high accuracy that the ratio of the adiabatic constants in expression (4) is equal to unity, and expression (4) itself can be written as

Determining the volume concentration of hydrogen r from the last expression, we finally

where a = µ _P / µ _H is the ratio of the molar mass of impurities µ _P to the molar mass of the hydrogen molecule µ _H.

Since the gas composition of impurities in hydrogen is not known exactly, but the main partial pressure in the gas is in the air [2], then the molar mass of impurities is taken to be equal to the molar mass of air: µ _P = 0.02898 kg / mol. The assumption made leads to a methodological error in measuring the hydrogen concentration r, since the contributions of water vapor and carbon oxides present in the gas of the operated generators are not taken into account in the molar mass of impurities µ _P.

An assessment of the indicated error gives the following results. According to [1], the limiting value of the partial value of water vapor in the generator cannot exceed 1.7 · 10 ³ Pa at a total gas pressure in the generator of 404 · 10 ³ Pa.

Calculation of the methodical error Δr _{H of} measurements according to formulas (4) and (5) for hydrogen concentrations r _H equal to r _H1 = 0.9; r _H2 = 0.97 (minimum permissible level of hydrogen concentration in turbogenerators); r _H3 = 0.9958 (the concentration of hydrogen at which the entire impurity consists of water vapor) gives the following results:

Δr _H1 = 0.0018; Δr _H2 = 0.002; Δr _H3 = 0.0012

The result shows that the methodological error of measurements by the proposed method does not exceed 0.002 absolute values or 0.2% in relative values.

Other components of the error are associated with the error in measuring the gas temperature and ultrasound velocity in the measuring chamber. The first component of the error does not exceed 0.1 ° C when choosing a platinum primary converter and the corresponding secondary converter, for example, PST-b-Pro converter [7].

The measurement of the speed of ultrasound does not exceed 0.1-0.2% [8] and, therefore, the total error in measuring the hydrogen concentration of the proposed method does not exceed a fraction of a percent with a significant simplification of the device compared to known meters.

The measurement error can be reduced further if the temperature of the measuring cell 1 is stabilized using a temperature converter 4 and an electronic unit 5 in the automatic temperature stabilization system. In this case, the error associated with the gas temperature gradient along the length of the measuring cell is eliminated.

LIST OF LITERATURE sources

1. "Rules for the technical operation of power plants and networks of the Russian Federation", approved by order of the Ministry of Energy of the Russian Federation of June 19, 2003 No. 229.

2. AAB Limited, England. Gas analysis systems for alternating current generators with hydrogen cooling AK 101, AK 104. User manual IM / AK1 / 14 RU. Revision 4.

3. Hydrogen gas analyzer. RF patent No. 2242751, IPC: G01N 27/04.

4. Kurdyumov A.V., Pikunov M.V. and others. Production of castings and non-ferrous alloys. Textbook for high schools. - M.: Metallurgy, 1986 -416 p.

5. A method for determining hydrogen mixed with helium. A.S. USSR No. 238222, IPC: G01N 27/02.

6. A method and a gas analyzer for determining local volumetric concentrations of hydrogen, water vapor and air in a gas-vapor medium using ultrasound. RF patent No. 2374636, IPC: G01N 29/00.

7. NPF KontrAvt Resistance converters - current measuring PST. Passport PIMF 411622.002 PS. Version 1.0.

8. ITO SB RAS “Sound velocity meter in gases and vapors UI-1M” - http://www.nsc.ru/win/elbib/data/show_page.dhtml?56+4

Claims (2)

1. The method of measuring the volumetric concentration of hydrogen, including measuring the temperature and speed of ultrasound in the measured gas, characterized in that they determine the speed in pure hydrogen at the same temperature, and the concentration of hydrogen r in the gas mixture is calculated from the expression
$$r=\frac{z-a}{\mathrm{one}-a},$$

where z = $${{\displaystyle C}}_H^2/{{\displaystyle C}}_C^2$$

- the ratio of the square of the speed of ultrasound in pure hydrogen $${{\displaystyle C}}_H^2$$

to the square of the ultrasound velocity in the measured gas mixture $${{\displaystyle C}}_C^2$$

;
a = µ _P / µ _H is the ratio of the molar mass of impurities in hydrogen µ _P to the molar mass of pure hydrogen µ _H. 2. The method for measuring the volumetric concentration of hydrogen according to claim 1, characterized in that the temperature of the measured gas is stabilized using the results of measuring its temperature.