RU2478919C2 - Calibration method under operating conditions of ultrasonic flow metres - counters of flow rate and volume of liquid single-phase media - Google Patents

Abstract

FIELD: measurement equipment.SUBSTANCE: calibration method involves stages, at which a reference sound velocity in stationary medium is determined for the specified absolute pressure, absolute temperature and component composition (in case of natural gas). Then, using similarity parameters for developed turbulent streams, values of propagation time of ultrasonic signals along acoustic paths of a certain counter in flowing fluid medium are calculated by means of a programme at the specified value of averaged volume flow rate within the measurement range. In compliance with the received time (frequency) values of propagation, time measurement channels of that counter are checked and adjusted by means of a sound pulse generator; after that, fluid medium volume is measured as per the counter readings (actual volume) at the specified point of variation range of averaged volume flow rate (volume) of the medium flowing in the pipeline. By comparing the obtained result to the primary calibration data, correction (as per reference volume) to the counter readings at the specified point of the measurement range is obtained. Correction for the initially chosen averaged volume is determined in the same manner. If difference between corrections meets the established criterion, relative correction (in %) is entered to the counter computer.EFFECT: performing a calibration procedure of flow meters - counters in field in a dynamic mode and reduction of costs owing to reducing the costs for calibration, and namely for flow metres - counter with inner diameter of more than 300 mm, due to refusal to use stationary calibration units of increased height and larger diameters.

Description

The calibration method in operating conditions of ultrasonic flow meters, counters (hereinafter referred to as counters) of the flow rate and volume of liquid single-phase media relates to a flow measuring technique.

When discontinued from production, the only direct way to establish the metrological characteristics of an ultrasonic counter is to calibrate it by comparing the counter with a high-precision volume reference (ie, the ultrasonic counter is "spilled" on the reference unit). Currently, the leading manufacturers of ultrasonic liquid volume meters, in particular oil, water, natural gas, use either bell provers, which are the primary volume standards, or bulky reference devices with pumping the working medium (air, natural gas) as high-precision volume standards. located at the factory and having traceability to primary standards - bell provers. The fleet of the aforementioned reference instruments for measuring the volume of gaseous media (air, natural gas) used in the continuous method of calibration (verification) is calculated in units all over the world.

Despite its obvious advantages, the periodic calibration of ultrasonic liquid (gas) volume meters (especially large diameters of 300 mm or more) during their operation using high-precision volume standards is associated with large labor costs and a significant loss of time. Large labor costs and time losses are associated with the fact that the meter must be removed from the pipeline, delivered to the location of the volume standard, calibrated over the entire measurement range, taken back and installed again on the pipeline.

In this regard, imitation methods and methods of calibration (verification), in particular, ultrasonic gas meters are of particular relevance. A typical example of a simulation verification method is the FSUE VNIIMS approved in 2004 and used in operating conditions for the verification of Qsonic, Check Sonic ultrasonic flow meters and gas meters from Elster Instromet, Belgium. This verification technique is based on measurements of the speed of sound during the propagation of ultrasonic signals along the meter’s acoustic paths in a stationary medium and determination of the gas flow velocity at zero average volumetric flow rate. The results of measurements of the speed of sound are compared with its reference value in a stationary gas, which is calculated by the known component composition and state parameters: absolute pressure and absolute temperature; “Zero flow rate” must also satisfy the established criterion. The performed comparisons allow us to make a judgment on the calibration of the ultrasonic gas volume meter (and only). The considered simulation verification technique does not allow the meter to be calibrated in the dynamic mode of operation, i.e. at an arbitrary point in the measuring range, therefore, it is not possible to calibrate the meter in the entire measuring range. Similar “zero” simulation calibration methods (without using volume standards) are also used for other types of ultrasonic liquid volume meters, however, none of them allows meter calibration to be performed over the entire measurement range.

The present invention: a method for calibrating ultrasonic volume meters of liquid single-phase media allows calibration without direct use of the volume standard under operating conditions in the entire measurement range, or at least at one point in this range, due to operating conditions.

According to publicly available information currently known, there are no analogues to this invention.

As noted above, during the operation of ultrasonic meters, it is often necessary to verify their metrological characteristics. Without such a check, it is impossible to make a judgment about the serviceability of the device and, therefore, about the correctness of measurements of the volume of a single-phase liquid medium.

In accordance with practical needs, an object of the present invention is a calibration method in operating conditions of ultrasonic flow meters, flow meters and volume of liquid single-phase media. As applied to an ultrasonic volume meter of a liquid single-phase medium, which has “m” acoustic paths, this calibration method includes the following steps:

- checking the correctness of measurements of the propagation times of ultrasonic signals along the meter’s acoustic paths by comparing the sound velocities measured in the meter for each acoustic path with a reference sound velocity for the same stationary medium with the same state parameters (in the case of natural gas, the reference value of the sound velocity in a stationary the environment is found using a computer program that implements an algorithm for calculating the thermodynamic parameters of natural gas given in the international one ISO 20765-1 standard, "Natural gas - Calculation of thermodynamic properties - Part 1: Gas phase properties for transmission and distribution applications".

- upon detection of unacceptable differences in the speed of sound in a stationary medium, characteristic of each acoustic path, from each other and from the reference speed of sound, it is necessary to make adjustments to the procedure for measuring and recording the propagation times of ultrasonic signals along the acoustic paths of the counter for different points of the measuring range;

- to adjust the built-in channels for measuring the propagation times of ultrasonic waves along the meter’s acoustic paths, set the value of the averaged volumetric flow rate (volume) Q _i at an arbitrary ith point within the measurement range near the nominal value of the averaged volumetric flow rate known from previous measurements of this quantity with the existing pipeline operation mode;

- for a given volume value Q _i , the Reynolds number Re is determined and, depending on the value of the number Re in the central cross section of the ultrasonic counter, the distribution of the longitudinal mass flow velocity is determined using representations of the power-law distribution of the developed turbulent flow velocity in the pipe cross section according to [1]: G. Schlichting. "Theory of the boundary layer", publishing house "Science", Main edition of the physical and mathematical literature, Moscow, 1974;

- based on the known: the reference speed of sound in a stationary medium and the distribution of the velocity of the developed turbulent flow in the cross section of the meter’s measuring channel, programmatically calculate the propagation times of ultrasonic waves in a moving medium along the acoustic paths for the ith selected point by the average volumetric flow rate (volume) ;

- by means of stop valves, the flow passing through the meter is blocked, while preserving the pressure and temperature of the medium in the measuring channel;

- using the frequency counter, the output of the sound pulse generator is adjusted to a frequency corresponding to one of the above-defined propagation times of the ultrasonic wave along any acoustic path of the counter; further, by connecting the output of the sound generator to the emitter of ultrasonic pulses along the selected acoustic path, the channel for measuring the propagation time for this path is adjusted; then the adjustment of the measurement of the propagation time of the ultrasonic signals is performed for each of the remaining "m-1" channels (acoustic paths of the counter);

- after adjusting each channel for measuring the transit time by an ultrasonic signal of the corresponding acoustic path for the i-th selected point in the range of volume measurements, stop valves are opened, again the medium flows through the counter and the result of measuring the volume of medium Q _f according to the meter readings (actual volume);

- using the calibration certificate of the ultrasonic meter, issued during its release from production, and using linear interpolation of the experimental data shown in the certificate obtained using the volume standard, find the correction e _f to the actual average volumetric flow rate Q _{f of the} single-phase current medium;

- as in the previous paragraph, on the basis of the calibration certificate of the ultrasonic counter, the correction e _{i is} calculated for a given average volumetric flow rate of the medium Q _i ; if these corrections (e _i , e _f ) satisfy the conformity criterion, then the correction e _f can be entered into the counter software package;

- to complete the calibration procedure, it is necessary to re-calculate, taking into account the received corrections e _f and e _{i, the} average correction factor of the current readings of the ultrasonic counter at all points of the measuring range.

If it is possible to change the average volumetric flow rate of the liquid medium under operating conditions, then, if necessary, the described calibration method can be implemented at several points in the measuring range.

In the practical implementation of the above invention, the verification of the measurement of the propagation times of ultrasonic signals in a moving medium along the acoustic paths of the meter, for example, in the case of measuring the volume of natural gas, is carried out as follows:

- using the appropriate computer program using known state parameters: absolute temperature and pressure and component composition of the gas, calculate the reference speed of sound in stationary natural gas of the same composition and at the same state parameters; to calculate the reference speed of sound in a stationary gas, the fundamental equation of state of natural gas AGA8 is used, which in the form of Helmholtz free energy is given in ISO 20765-1, an authentic translation of GOST R 8.662-2009;

- compare among themselves the values of the speed of sound in a stationary medium, obtained by measuring the propagation times of ultrasonic waves in - and against - the flow along each acoustic path of the counter; the same values of the speed of sound are compared with the reference speed of sound in a stationary medium; sound speeds obtained experimentally, in the case of natural gas, should not differ from each other by more than 0.2% and from the reference speed of sound by more than 0.4%.

For the experimental determination of the speed of sound in a stationary medium, the counter calculator uses an expression obtained from the equations of the problem of measuring the volume of a liquid single-phase medium, which reflect the measurement principle underlying the operation of the ultrasonic counter; these equations of the measurement problem lead to the following formula for the speed of sound in a stationary medium:

,

,

Where

l _k is the length of the kth acoustic path of the counter;

t _fk is the travel time of the distance l _k by the ultrasonic wave during propagation along the stream;

t _rk is the travel time of the distance l _k by the ultrasonic wave when propagating upstream.

Substituting into the above formula the results of measurements of the times t _fk and t _rk for a given length l _k , we experimentally find the speed of sound in a stationary medium. These calculations, like the processing of primary experimental data, are carried out automatically by the counter calculator for each acoustic path.

If necessary, adjust the built-in channels for measuring the propagation times of ultrasonic signals along the acoustic paths for a given average volumetric flow rate Q _{i and} calculate the Reynolds number using its definition:

,

,

where ν _z is the mass-average longitudinal velocity of the flow (hereinafter, to reduce the notation, we will use the name "average velocity", referring to the average velocity of the liquid medium over the cross section, it is the mass-average velocity);

d is the diameter of the pipe (measuring channel of the counter);

ρ, µ is the density and dynamic viscosity of a single-phase liquid medium under operating conditions.

Depending on the obtained value of the Reynolds number, in accordance with the experimental results given in [1], a power-law distribution of the longitudinal flow velocity in the central cross section of the meter’s measuring channel is built:

where u = u (r) is the current value of the longitudinal flow velocity when the observer moves along the coordinate axis across the flow (pipe);

- радиус-вектор точки в поперечном сечении трубы; (ось z цилиндрической системы координат ориентирована вдоль центральной продольной оси трубы в направлении потока):

- the radius vector of the point in the cross section of the pipe; (the z axis of the cylindrical coordinate system is oriented along the central longitudinal axis of the pipe in the direction of flow):

R is the radius of the cross section of the pipe; d = 2R;

n is an integer (6, 7, 8, 9, 10) that is included in the exponent of the distribution of flow velocity.

The above distribution of the flow velocity meets all the requirements for a turbulent flow of a viscous fluid (although the viscosity of the medium can be very small). In the center of the cross section of the pipe, the flow rate reaches its maximum value, i.e. u (0) = U for r = 0. On the pipe wall there is a sticking condition for a viscous fluid, which is fulfilled both under laminar and turbulent flow regimes, i.e. u (R) = 0 for r = R.

The exponent in the formula for the distribution of velocity in the cross section of the pipe is chosen in accordance with the inequalities:

if 4.0 · 10 ³ ≤Re≤5.0 · 10 ⁴ , then n = 6;

if 5.0 · 10 ⁴ ≤Re≤5.5 · 10 ⁵ , then n = 7;

if 5.5 · 10 ⁵ ≤Re≤1.5 · 10 ⁶ , then n = 8;

if l, 5 · 10 ⁶ ≤Re≤2,8 · 10 ⁶ , then n = 9;

if 2.8 · 10 ⁶ ≤Re≤3.24 · 10 ⁶ , then n = 10;

if Re≥3.8 · 10 ⁶ , then n = j, j is an open parameter, a real number.

The maximum speed of the developed turbulent flow on the central longitudinal axis of the pipe and the average flow rate are related by the relation

,

,

Accordingly, for the averaged volumetric flow rate, the expression

The last equality is used to calculate the maximum flow velocity U in the center of the cross section of the pipe when constructing the distribution of the flow velocity in this section for a given average volumetric flow rate (volume) of the medium Q _i .

After the distribution of the flow velocity in the cross section having radial symmetry is constructed and its parameters are determined, the points of intersection E _{k of the} acoustic paths l _k with the plane z = z ₀ for which the surface u _i (x, y, z ₀ ) is given by the equation

, z=z₀

, z = z ₀

This surface corresponds to the cross section of the counter z ₀ = 0, which coincides with the transverse plane of symmetry of its flanges. The intersection points of the acoustic paths l _k , k = 1, 2, ..., m with the velocity distribution surface give the flow velocity values corresponding to the paths l _k and the selected point of the averaged volumetric flow rate Q _i , i.e. it is precisely those flow velocities v _ik that should be measured during the propagation of ultrasonic pulses along predetermined paths. The problem of determining the points of intersection of paths with the velocity distribution surface should be solved taking into account the geometry and number of acoustic paths of a particular type of ultrasonic meters.

The known values of the flow velocities v _ik for each k-th acoustic path and the known reference sound velocity “c” in a stationary gas of known composition with given thermodynamic state parameters allow us to calculate the propagation times of ultrasonic waves along each acoustic path at a distance l _k in and against flow. The propagation time of ultrasound along the kth path to its length along the flow for a given volume Q _{i is} denoted by t _fik versus - t _rik . For these times, in the framework of the measurement problem under consideration, there are expressions

The frequencies ν _fik = 1 / t _fik and ν _rik = 1 / t _rik correspond to these propagation times of ultrasonic signals along the kth path. By applying these frequencies with the help of an audio signal generator and a frequency meter to the outputs of the emitters (or receivers) of ultrasonic waves, check the correct operation of the channels for measuring the times of the ultrasonic counter and, in case of inconsistencies detected, set them to measure the frequencies found above.

The setting (adjustment) of the channels for measuring the propagation times of ultrasonic signals along the acoustic paths of the counter corresponds to a priori given average flow rate Q _i .

After adjusting the channels for measuring the propagation times of ultrasound in a moving medium, the stop valves are opened, the flow of the medium through the counter is resumed, and the measurement result Q _{f of the} volume of the moving medium is obtained according to the counter (actual volume). On the basis of the certificate of calibration of the meter, issued upon discontinuation of production, using linear interpolation of the data given, the corrections to the set and actual volumes of the medium e _i and e _f are calculated. To calculate the corrections in% of the meter reading, use the formula

,

,

where Q _w , w = f, i is the actually measured Q _f or the given Q _i volume;

Q _ref is the actual value of the volume reproduced by the standard.

The calculated corrections are compared with each other. If the inequality holds

,

,

then the correction e _f can be entered into the counter software package.

The final step in the calibration procedure for the ultrasonic volume meter is to calculate the total correction factor to the meter reading over the entire measurement range. The correction factor K _{f is} calculated through the weighted average correction as follows:

- first calculate the weighted average correction e _wt according to the formula

,

,

- весовой коэффициент;Where

- weight coefficient;

Q _k is the volume measured by the counter; the index k runs from 1 to N + 2;

N points are the data of the factory certificate of calibration of the meter, to which points Q _i and Q _f determined during the implementation of this calibration method are added; e _k - corrections to the meter readings contained in its calibration certificate and obtained during the implementation of this calibration method;

Q _max - the maximum value of the volume indicated in the factory certificate of calibration of the meter;

- the total correction factor K _{f is} found according to the ratio

The calculated correction factor is also entered into the software package of the calibrated ultrasonic counter.

After introducing into the software package corrections to the meter readings at the operating point and the general correction factor within the measurement range, this calibration method should be considered complete. If it is possible to change the average volumetric flow rate in the pipeline, this calibration method can be carried out for operating conditions at several points of the meter’s measuring range, while its arithmetic mean value over the meter calibration points under operating conditions is taken as a general correction factor.

Claims (1)

The calibration method in the operating conditions of ultrasonic flow meters-counters (hereinafter referred to as the counters) of the flow rate and volume of liquid single-phase media, which consists in the following: when calibrating the meter: the reference value of the speed of sound in a stationary medium (when calibrating the counter of the volume of natural gas, the reference value of the speed of sound is calculated along with thermodynamic parameters at a given absolute pressure, absolute temperature, known component composition of the gas using a computer program that implements a the algorithm of ISO 20765-1 standard "Natural gas - Calculation of thermodynamic properties - Part 1: Gas phase properties for transmission and distribution applications"), measurement equations for the speed of sound in a stationary medium and the gas flow velocity depending on the time it takes an ultrasonic signal (wave ) an acoustic path (beam) directed at an angle to the central axis of the pipeline, both downstream and against it, as well as information for this counter on the number, length of acoustic paths and angles between the rays and the central axis of the pipeline, which allows time measurements races rostraneniya experimentally determine the speed of sound in a stationary gas and the gas flow rate for each beam of the counter,
characterized in that its practical implementation does not require the direct use of a standard volumetric flow rate (volume) of a liquid single-phase medium, while the first step in the implementation of this method for any selected point in the range of volumetric flow rate of a liquid medium is the Reynolds number assuming a uniform distribution of the flow velocity in the transverse the cross-section through which the actual axisymmetric distribution of the flow velocity in a straight circular pipe with a developed t turbulent flow (observing the lengths of straight sections) and the coordinates of the points of intersection of the rays with the surface that describes the transverse distribution of the longitudinal flow velocity, knowing the flow velocity for each ray and the speed of sound in a gas of known composition, calculate the propagation times of ultrasonic waves along the rays, and therefore the corresponding frequency values , and then, setting these frequencies using a pulse generator and applying them to the inputs of ultrasonic wave receivers, the channels are checked and tuned (if necessary) measurements of the counter times, which allows you to get the result of volume measurements according to the readings of the counter (actual volume) at the selected point in the range, while the actual volume is compared with the reference, which in this case is set on the basis of the certificate of calibration of this counter when discontinued, with this reference volume and compare the average volume flow rate (volume) specified at the first step at a selected point in the measurement range, then the corrections are calculated as the relative differences (in%) to the meter readings ( the difference between the actual volume and the reference, averaged and reference, referred to the reference volume), which are compared modulo each other, after which, if the module meets the established criterion, an amendment to the meter at a selected point in the range (points) is entered into its calculator, then using the data of the calibration certificate, which was issued when the meter was discontinued, and the corrections found to its readings as a result of applying the present calibration method at a selected point, calculate correction coefficient to the calibration curve over the entire measurement range, which is also introduced into the counter software package.