CN108548631B - Method for measuring pressure intensity of excitable gas pressure container - Google Patents
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Abstract
The invention discloses an excitable gas (diatomic or polyatomic molecular gas)For example: natural gas, carbon dioxide, chlorine) pressure vessel pressure intensity measuring method, placing frequency f in the gas pressure vessel1And f2The two pairs of ultrasonic probes and the thermocouple, the two pairs of ultrasonic probes respectively obtain two selected frequency points f1And f2Upper sound absorption coefficient α (f)1)、α(f2) And speed of sound c (f)1)、c(f2) For the acoustic relaxation frequency f of the synthesis gasm(ii) a The current gas temperature measured by the thermocouple, and the gas reference acoustic relaxation frequency f at 1 standard atmosphere (i.e. 1atm ═ 101.325kPa) at the current gas temperature are obtained in a table look-up manner0(ii) a Relaxation of frequency f by synthetic soundmAnd a reference acoustic relaxation frequency f0The gas container pressure is obtained. The invention has the advantages of on-line detection, nondestructive detection, quick response, low power consumption, simple composition, stable long-time work, high measurement precision and the like.
Description
Technical Field
The invention relates to the field of pressure monitoring in a gas pressure container, in particular to a method for measuring the pressure of an excitable gas pressure container.
Background
An important thermal parameter of a gas pressure vessel is the pressure (also referred to as pressure in industrial measurements). On one hand, many processes require a specific pressure to achieve the desired effect, and thus require an accurate value of the vessel pressure; on the other hand, in order to improve the production efficiency, a plurality of chemical reactions need to be carried out under high pressure, and monitoring and controlling the pressure in the gas pressure container is an important link in the whole production process; thirdly, some operation errors or violent and abnormal chemical reactions in the production and transportation process can cause the pressure in the container to rise rapidly, and the pressure state in the container is monitored effectively in real time in order to prevent the damage and accidents of the pressure container. According to different sensitive components and conversion principles, the conventional pressure detection methods can be generally divided into four categories, namely liquid pressure gauges, elastic pressure gauges, electric remote transmission pressure meters and physical pressure sensors. The gas pressure vessel has the development trends of high parameter, large-scale, long-period operation and the like. However, the traditional method for measuring the pressure of the gas pressure container has the defects of high cost, complex realization and long response time, and is not suitable for on-site gas detection. Therefore, the research on the non-invasive gas pressure container pressure detection method which is low in cost, simple in composition and capable of monitoring in real time is of great significance.
When sound waves propagate in an excitable gas (diatomic or polyatomic molecular gas), the translational energy of gas molecules is increased firstly during sound compression, and part of the increased translational energy may enter an intra-molecular vibration mode through inelastic collision among the gas molecules to enable the gas molecules to be excited in an energy level. The part of the acoustic excitation energy is lost due to a thermal relaxation process during acoustic expansion, and then acoustic relaxation absorption which changes along with frequency due to acoustic propagation characteristics is influenced. The phase lag time of the vibration mode temperature change relative to the translational temperature or acoustic wave change during relaxation is called the vibration relaxation time. The relaxation time is the macroscopic embodiment of the energy transfer rate of the internal and external freedom degrees of the excitable gas molecules, and the energy transfer rate of the internal and external freedom degrees of the molecules is determined by the external environment temperature and pressure, the vibration frequency, the mass, the geometric structure, the component composition proportion and other factors of the gas molecules. When the relaxation time is far longer than the change time of the acoustic quantity (namely the reciprocal of the acoustic frequency) and when the relaxation time is far shorter than the change time of the acoustic quantity, no energy exchange occurs between the internal and external degrees of freedom, and no relaxation absorption is generated; relaxation absorption occurs only when the two are close to each other, energy exchange occurs between the inner and outer degrees of freedom. This results in the excitable gas appearing to have a "bell-shaped" acoustic absorption spectrum with a peak point. The frequency corresponding to the peak point of the acoustic absorption spectrum is called the acoustic relaxation frequency. The applicant finds that the acoustic relaxation frequency is determined by the heat capacity of the degree of freedom inside and outside the molecule, the matching degree of the change time of the acoustic quantity and the molecular vibration relaxation time, and the temperature and the pressure of the external environment. The acoustic relaxation frequency is directly proportional to the ambient temperature, the acoustic relaxation frequency is linearly inversely proportional to the relaxation time of the main relaxation process, the increase in pressure increases the molecular collision rate causing a decrease in relaxation time, and thus the acoustic relaxation frequency is linearly proportional to the ambient pressure. In addition, the acoustic relaxation frequency can be synthesized through acoustic absorption coefficients and acoustic velocity measurement values on two frequency points. Therefore, the pressure information in the gas container can be obtained by the acoustic measurements at two frequency points. In particular, the acoustic relaxation frequency f corresponding to the peak point of the acoustic absorption spectrummThe heat capacity of translation and rotation, which is three elements forming the effective heat capacity of the main relaxation processSum of
Vibration coupling heat capacity
The relaxation time τ is determined jointly, i.e.:
wherein, R is 8.31J mol-1K-1Is the sum of mole constant, heat capacity of translation and rotation
Vibration coupled heat capacity dependent only on the geometry of the gas molecules and independent of ambient temperature and pressure
The vibration heat capacity in the gas is formed by V-V energy transfer coupling and is proportional to the temperature and independent of the pressure, and the relaxation time tau is proportional to the temperature and is linearly inversely proportional to the pressure. It can be seen that, on the one hand, for a gas of a certain composition, fmProportional to ambient temperature; on the other hand, for a certain composition of excitable gas, the acoustic relaxation frequency increases linearly with increasing pressure when the ambient temperature is constant.
The ultrasonic probe has the advantages of low cost and power consumption, quick transient response, easy installation and maintenance, durable probe, capability of performing non-invasive measurement and suitability for flammable and explosive gas environment, so that the ultrasonic probe is widely applied to the aspects of ultrasonic flaw detection, distance measurement, thickness measurement, leak detection, medical imaging diagnosis, processing process control and the like in various industries such as metallurgy, ships, machinery, food, petroleum, automobiles, medical treatment, chemistry, bioengineering, environmental engineering and the like.
The invention provides a pressure measurement method of a non-invasive gas pressure container based on ultrasonic relaxation frequency, which is different from the traditional open pore pressure-leading type. The method has the advantages of on-line detection, nondestructive detection, quick response, low power consumption, simple composition, long-time working stability, high measurement precision and the like. The invention is suitable for the real-time monitoring and measurement of the pressure container storing excitable gas (diatomic or polyatomic molecular gas). For example, natural gas is an excitable gas mixture that contains methane as a major component and small amounts of other gases such as ethane, butane, pentane, carbon dioxide, nitrogen, oxygen, water vapor, and the like. The energy content (mole fraction) of methane in natural gas is typically distributed between 70% and 98% from site to site. The pressure sensor can be suitable for real-time monitoring and measurement of the pressure of a natural gas pressure container.
Disclosure of Invention
In order to solve the defects in the prior art, the invention provides a method for measuring the pressure of an excitable gas pressure container, and solves the problems that the traditional method for measuring the pressure of the gas pressure container is high in cost, complex to implement, long in response time and not suitable for field gas detection.
In order to achieve the above purpose, the invention adopts the following technical scheme: a method for measuring the pressure of an excitable gas pressure container is characterized in that:
1) in a gas pressure vessel at a frequency of f1And f2Two pairs of ultrasonic probes and a thermocouple, the two pairs of ultrasonic probes are respectively used for measuring the frequency f1And f2The thermocouple is used for measuring the gas temperature value;
2) two pairs of ultrasonic probes are utilized to respectively obtain two selected frequency points f1And f2Upper sound absorption coefficient α (f)1)、α(f2) And speed of sound c (f)1)、c(f2) A measured value of (a);
3) synthesizing the acoustic relaxation frequency by using the two-frequency point acoustic measurement values, and synthesizing the acoustic relaxation frequency f of the gas by the formula (4)mComprises the following steps:
4) pre-measuring the values of the acoustic relaxation frequencies of the gas in a gas pressure vessel at different temperatures at 1 standard atmosphereAs reference value for table lookup; obtaining the gas reference acoustic relaxation frequency f at 1 standard atmospheric pressure at the current temperature in a table look-up mode according to the current gas temperature obtained by thermocouple measurement in the step 1)0；
5) By the acoustic relaxation frequency f synthesized in step 3)mAnd step 4) obtaining the reference acoustic relaxation frequency f by looking up the table0And calculating to obtain the pressure P ═ f of the gas containerm/f0。
The method for measuring the pressure of the excitable gas pressure container is characterized in that: the step 2) specifically comprises the following steps:
(1) the variation of the sound pressure follows an exponential decay law, i.e.
Wherein α is the sound absorption coefficient or sound pressure absorption coefficient in Nepeh/m, x is the propagation distance, i.e. the distance between each pair of probes, p0For transmitting the sound pressure of the acoustic probe, p (x) is the sound pressure when the sound wave reaches the receiving acoustic probe after propagating through the distance x; by recording the peak value A of the electrical signal of the transducer of the transmitting probe0And receiving an acoustic probe transducer electrical signal peak a (x) after propagation over a distance x, using the formula α ═ ln (a)0/A(x))/x＝ln(p0The/p (x) and the/x can be calculated to obtain two selected frequency points f1And f2Upper sound absorption coefficient α (f)1)、α(f2)；
(2) Measuring sound velocity c by adopting a time difference method, recording the propagation time t of sound waves between a sound receiving probe and a sound emitting probe by utilizing a timer, and calculating by utilizing a formula c as x/t to obtain two selected frequency points f1And f2Upward sound propagation velocity c (f)1) And c (f)2)。
The method for measuring the pressure of the excitable gas pressure container is characterized in that: the step 3) of synthesizing the acoustic relaxation frequency by using the two-frequency point acoustic measurement values comprises the following specific steps:
(1) according to the Kneser classical acoustic gas vibration relaxation theory, the acoustic absorption spectrum μ (f) as a function of acoustic frequency is expressed as:
where f is the acoustic frequency, λ is the acoustic wavelength, fmIs the acoustic relaxation frequency, mumIs the amplitude of the maximum of the acoustic absorption spectrum, α (f) is the acoustic absorption coefficient as a function of acoustic frequency;
(2) two pairs of ultrasonic probes at two selected frequency points f1And f2After the sound absorption coefficient and the sound velocity are respectively measured, two selected frequency points f are calculated1And f2Sound absorption spectrum value mu (f) of1)、μ(f2) Comprises the following steps:
μ(f1)＝α(f1)c(f1)/f1，μ(f2)＝α(f2)c(f2)/f2(2)
(3) obtained from formulae (1) and (2):
two equations in the joint type (3) can obtain the acoustic relaxation frequency f of the gasm：
The method for measuring the pressure of the excitable gas pressure container is characterized in that: the method for pre-measuring the acoustic relaxation frequency values of the gas in the gas pressure container at different temperatures under 1 standard atmospheric pressure as the reference value required by table lookup utilizes the method of the steps 1) -3) to place two pairs of frequencies in the gas pressure container, wherein the two pairs of frequencies are respectively f1And f2And a thermocouple.
The method for measuring the pressure of the excitable gas pressure container is characterized in that: the gas pressure vessel stores an excitable gas, including a diatomic or polyatomic molecular gas.
The method for measuring the pressure of the excitable gas pressure container is characterized in that: and the acoustic relaxation frequency is the acoustic frequency corresponding to the peak point of the acoustic absorption spectrum, and the synthetic gas pressure of the acoustic relaxation frequency calculated by referring to the acoustic relaxation frequency and the acoustic measurement value is obtained by using a table look-up.
The invention achieves the following beneficial effects: the invention provides a non-invasive gas pressure container pressure detection method which is low in cost, simple in composition and capable of monitoring in real time by measuring relaxation frequency of excitable gas through an acoustic method, is different from a traditional open-pore pressure-leading type pressure measurement method, has the advantages of on-line detection, nondestructive detection, quick response, low power consumption, simple composition, long-time working stability, high measurement precision and the like, and is suitable for storing the pressure of a pressure container of excitable gas (diatomic or polyatomic molecular gas, such as natural gas, carbon dioxide and chlorine) to monitor and measure in real time.
Drawings
FIG. 1 is a schematic diagram of the system design of the present invention;
FIG. 2 is a flow chart of a method of the present invention;
FIG. 3 is 2% N2-98％CH4And (3) a curve of the acoustic relaxation frequency value at 1 standard atmospheric pressure and at a temperature of 270K-350K.
Detailed Description
The invention is further described below with reference to the accompanying drawings. The following examples are only for illustrating the technical solutions of the present invention more clearly, and the protection scope of the present invention is not limited thereby.
As shown in fig. 1 and 2, a method of measuring pressure of a gas pressure vessel includes the steps of:
1) in a gas pressure vessel at a frequency of f1And f2Two pairs of ultrasonic probes and a thermocouple, the two pairs of ultrasonic probes being respectively used for measuring a frequency f1And f2Obtaining acoustic measurement values for synthesizing acoustic relaxation frequencies by using the acoustic absorption coefficient and the sound velocity value in the gas; the thermocouple is used for measuring the gas temperature value; f. of1And f2The specific setting is carried out according to the type of the gas and the range of the measured pressure. For example, for containing CH4、CO2、Cl2Etc. can excite single or mixed gasIf the measured pressure is in the range of 0.01-50atm, f1And f2Can be selected between 20 kHz and 200 kHz; if f is in the range of 50atm to 250atm for the measured pressure1And f2Can be selected between 200kHz-1000 KHz.
2) Two frequency points f in gas1And f2Measurement of upper sound absorption coefficient and sound velocity:
the variation of the sound pressure follows an exponential decay law, i.e.
Wherein α is the sound absorption coefficient or sound pressure absorption coefficient in Nepem/m, x is the propagation distance, i.e. the distance between each pair of probes, p0For transmitting the sound pressure of the acoustic probe, p (x) is the sound pressure when the sound wave reaches the receiving acoustic probe after propagating through the distance x; by recording the peak value A of the electrical signal of the transducer of the transmitting probe0And receiving an acoustic probe transducer electrical signal peak a (x) after propagation over a distance x, using the formula α ═ ln (a)0/A(x))/x＝ln(p0The/p (x) and the/x can be calculated to obtain two selected frequency points f1And f2Upper sound absorption coefficient α (f)1)、α(f2)。
The sound velocity c can be measured by adopting a time difference method, a timer controlled by a single chip microcomputer is used for recording the propagation time t of sound waves between a sound receiving probe and a sound emitting probe, and two selected frequency points f can be calculated by using a formula c as x/t1And f2Upward sound propagation velocity c (f)1) And c (f)2)。
3) Synthesizing acoustic relaxation frequency by using two-frequency point acoustic measurement values: according to the Kneser classical acoustic gas vibration relaxation theory, the acoustic absorption spectrum μ (f) as a function of acoustic frequency can be expressed as:
where f is the acoustic frequency, λ is the acoustic wavelength, fmIs the acoustic relaxation frequency, mumIs the amplitude of the maximum of the acoustic absorption spectrum, α (f) is the acoustic absorption coefficient as a function of acoustic frequency;
two pairs of ultrasoundThe probe being at two selected frequency points f1And f2After the sound absorption coefficient and the sound velocity are respectively measured, two selected frequency points f are calculated1And f2Sound absorption spectrum value mu (f) of1)、μ(f2) Comprises the following steps:
μ(f1)＝α(f1)c(f1)/f1，μ(f2)＝α(f2)c(f2)/f2(2)
obtained from formulae (1) and (2):
two equations in the joint type (3) can be obtained:
equation (4) indicates that two bin acoustic measurements, i.e., two selected bins f, can be utilized1And f2Upper sound absorption coefficient α (f)1)、α(f2) And speed of harmonic propagation c (f)1) And c (f)2) Synthesizing to obtain acoustic relaxation frequency fm。
4) The ambient temperature also influences the magnitude of the gas acoustic relaxation frequency, and reference acoustic relaxation frequency values of the gas in the cavity of the gas pressure container at different temperatures under 1 standard atmospheric pressure are obtained in advance through the steps 1) -3); obtaining the gas reference acoustic relaxation frequency f at 1 standard atmospheric pressure at the current gas temperature measured by the thermocouple in the step 1) in a table look-up mode0As a reference value for the gas chamber pressure measurement at the current temperature.
5) The acoustic relaxation frequency f is obtained by calculation in 1) -4) by utilizing the property that the acoustic relaxation frequency is linearly proportional to the environmental pressuremAnd the acoustic relaxation frequency f of the gas at the current gas temperature of 1 standard atmospheric pressure obtained by table lookup0And synthesizing to obtain the pressure P ═ f of the gas containerm/f0. Two frequency points f selected by acoustic relaxation frequency algorithm adopted by the patent1And f2As long as the absorption is significant in the acoustic relaxationThe pressure value can be accurately synthesized in the frequency domain range.
Example (b):
suppose that 98% CH is stored in the gas pressure vessel4-2％N2Of natural gas, the frequencies of the two pairs of ultrasonic probes are respectively f140kHz and f2＝125kHz。
Example 1 (positive pressure environment, assumed to be 10 atm):
1) under the current pressure environment, two pairs of ultrasonic probes respectively measure two frequency points f140kHz and f2＝125kHz；
2) The acoustic absorption coefficient α (f) was obtained1)＝0.191m-1、α(f2)＝1.846m-1And speed of sound c (f)1)＝443.7m/s、 c(f2)＝443.8m/s；
3) The two-frequency point sound measurement value synthesis algorithm utilizing the sound relaxation frequency can be calculated to obtain:
4) measuring the gas temperature T to 295K by a thermocouple; FIG. 3 is 2% N2-98％CH4Under 1 standard atmospheric pressure (1atm ═ 101.325kPa), the curve of the acoustic relaxation frequency value at the temperature of 270K to 350K, when the temperature T is 295K, the acoustic relaxation frequency f is known by a table look-up method0＝1.190×105Hz；
5) From f obtainedmAnd f0The pressure P ═ f of the gas container can be calculatedm/f0≈10atm。
Example 2 (positive pressure environment, unlike the chamber pressure of example 1, assuming 5 atm):
1) under the current pressure environment, two pairs of ultrasonic probes respectively measure two frequency points f140kHz and f2＝125kHz
2) The acoustic absorption coefficient α (f) was obtained1)＝0.3806m-1、α(f2)＝3.573m-1And speed of sound c (f)1)＝443.8 m/s、c(f2)＝443.9m/s；
3) The two-frequency point sound measurement value synthesis algorithm utilizing the sound relaxation frequency can be calculated
4) Measuring the gas temperature T to 295K by a thermocouple; from FIG. 3, it can be known that when the temperature T is 295K, the acoustic relaxation frequency f is obtained by looking up the table0＝1.190×105Hz；
5) From f obtainedmAnd f0The pressure P ═ f of the gas container can be calculatedm/f0≈5atm。
Example 3 (negative pressure environment, assumed to be 0.1 atm):
1) under the current pressure environment, two pairs of ultrasonic probes respectively measure two frequency points f140kHz and f2＝125kHz
2) The acoustic absorption coefficient α (f) was obtained1)＝1.555m-1、α(f2)＝1.678m-1And speed of sound c (f)1)＝447.9m/s、 c(f2)＝448.2m/s；
3) The two-frequency point sound measurement value synthesis algorithm utilizing the sound relaxation frequency can be calculated
4) The gas temperature T is measured by a thermocouple to be 295K, and the acoustic relaxation frequency f is known by looking up the table from FIG. 3 when the temperature T is 295K0＝1.190×105Hz；
5) From f obtainedmAnd f0The pressure P ═ f of the gas container can be calculatedm/f0≈0.1atm.
The pressure measurement method provided by the invention is different from the traditional open-hole pressure-leading type pressure measurement method, and has the advantages of online detection, nondestructive detection, quick response, low power consumption, simple composition, long-time working stability, high measurement precision and the like. The invention is suitable for the pressure real-time monitoring and measurement of the pressure vessel storing excitable gas (diatomic or polyatomic molecular gas, such as natural gas, carbon dioxide and chlorine).
The above description is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, several modifications and variations can be made without departing from the technical principle of the present invention, and these modifications and variations should also be regarded as the protection scope of the present invention.
Claims (6)
1. A method for measuring the pressure of an excitable gas pressure container is characterized in that:
1) in a gas pressure vessel at a frequency of f1And f2Two pairs of ultrasonic probes and a thermocouple, the two pairs of ultrasonic probes are respectively used for measuring the frequency f1And f2The thermocouple is used for measuring the gas temperature value;
2) two pairs of ultrasonic probes are utilized to respectively obtain two selected frequency points f1And f2Upper sound absorption coefficient α (f)1)、α(f2) And speed of sound c (f)1)、c(f2) A measured value of (a);
3) synthesizing the acoustic relaxation frequency by using the two-frequency point acoustic measurement values, and synthesizing the acoustic relaxation frequency f of the gas by the formula (4)mComprises the following steps:
4) acoustic relaxation frequency values of gas in a gas pressure container at different temperatures under 1 standard atmospheric pressure are measured in advance and used as reference values required by table lookup; obtaining the gas reference acoustic relaxation frequency f at 1 standard atmospheric pressure at the current temperature in a table look-up mode according to the current gas temperature obtained by thermocouple measurement in the step 1)0；
5) By the acoustic relaxation frequency f synthesized in step 3)mAnd step 4) obtaining the reference acoustic relaxation frequency f by looking up the table0And calculating to obtain the pressure P ═ f of the gas containerm/f0。
2. The excitable gas pressure vessel pressure measurement method of claim 1, wherein: the step 2) specifically comprises the following steps:
(1) the variation of the sound pressure follows an exponential decay law, i.e.
Wherein α is the sound absorption coefficient or sound pressure absorption coefficient in Nepeh/m, x is the propagation distance, i.e. the distance between each pair of probes, p0For transmitting the sound pressure of the acoustic probe, p (x) is the sound pressure when the sound wave reaches the receiving acoustic probe after propagating through the distance x; by recording the peak value A of the electrical signal of the transducer of the transmitting probe0And receiving an acoustic probe transducer electrical signal peak a (x) after propagation over a distance x, using the formula α ═ ln (a)0/A(x))/x＝ln(p0The/p (x) and the/x can be calculated to obtain two selected frequency points f1And f2Upper sound absorption coefficient α (f)1)、α(f2)；
(2) Measuring sound velocity c by adopting a time difference method, recording the propagation time t of sound waves between a sound receiving probe and a sound emitting probe by utilizing a timer, and calculating by utilizing a formula c as x/t to obtain two selected frequency points f1And f2Upward sound propagation velocity c (f)1) And c (f)2)。
3. The excitable gas pressure vessel pressure measurement method of claim 1, wherein: the step 3) of synthesizing the acoustic relaxation frequency by using the two-frequency point acoustic measurement values comprises the following specific steps:
(1) according to the Kneser classical acoustic gas vibration relaxation theory, the acoustic absorption spectrum μ (f) as a function of acoustic frequency is expressed as:
where f is the acoustic frequency, λ is the acoustic wavelength, fmIs the acoustic relaxation frequency, mumIs the amplitude of the maximum of the acoustic absorption spectrum, α (f) is the acoustic absorption coefficient as a function of acoustic frequency;
(2) two pairs of ultrasonic probes at two selected frequency points f1And f2Separately measuring sound absorptionAfter receiving the coefficient and the sound velocity, calculating to obtain two selected frequency points f1And f2Sound absorption spectrum value mu (f) of1)、μ(f2) Comprises the following steps:
μ(f1)＝α(f1)c(f1)/f1，μ(f2)＝α(f2)c(f2)/f2(2)
(3) obtained from formulae (1) and (2):
two equations in the joint type (3) can obtain the acoustic relaxation frequency f of the gasm：
4. The excitable gas pressure vessel pressure measurement method of claim 1, wherein: the method for pre-measuring the acoustic relaxation frequency values of the gas in the gas pressure container at different temperatures under 1 standard atmospheric pressure as the reference value required by table lookup utilizes the method of the steps 1) -3) to place two pairs of frequencies in the gas pressure container, wherein the two pairs of frequencies are respectively f1And f2And a thermocouple.
5. The excitable gas pressure vessel pressure measurement method of claim 1, wherein: the gas pressure vessel stores an excitable gas, including a diatomic or polyatomic molecular gas.
6. The excitable gas pressure vessel pressure measurement method of claim 1, wherein: and the acoustic relaxation frequency is the acoustic frequency corresponding to the peak point of the acoustic absorption spectrum, and the synthetic gas pressure of the acoustic relaxation frequency calculated by referring to the acoustic relaxation frequency and the acoustic measurement value is obtained by using a table look-up.
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