Abstract : This paper analyzes various factors affecting the calibration accuracy of gas turbine flowmeters and proposes solutions.
1 Introduction
Due to the development of production, the use of coal carbon and fuel oil has increased too rapidly, causing environmental pollution to become more and more serious. To develop the use of clean fuel such as natural gas, coal gas and biogas has become an important means to solve environmental problems and has been achieved. Great results. The development of the gas industry makes the accuracy of gas trade measurement more important. Therefore, the search for high-precision metering instrument has become an important issue in the gas trading industry. The choice of calibration method and calibration system is the key to the actual metering accuracy of the meter. Gas turbine flowmeter (hereinafter referred to as "turbine meter") is a speed-type flow measuring instrument, with a wide range (range ratio Qmax: Qmin up to 10:1 to 20:1), and good repeatability. In the domestic and foreign gas, petrochemical and other industries have been widely used. Although the turbine meter itself has accurate measurement and high accuracy, if it is not accurately calibrated at the factory, it will directly affect its measurement accuracy. Before each turbo table is put into use abroad, it must be strictly calibrated. The standard devices used are mainly "standard gas turbine flowmeter", "sonic nozzle", "bell-type standard gauge", and "standard cylinder gas flowmeter". "Standard Rotating Blade Gas Flowmeters," among which the first three are commonly used standard devices. In order to ensure that it serves as a basis for trade settlement, as well as its notarization and accuracy, it is an imminent task to improve the calibration accuracy of gas turbine flowmeters.
2 Factors Affecting Calibration Accuracy
2.1 Impact of standard devices
Briefly introduce three standard devices:
The standard gas turbine flowmeter has the advantages of high calibration accuracy, high calibration speed, and low cost, but over time, due to the wear of its internal bearings and other moving parts, it will cause the performance of the standard turbine table itself to change, thereby reducing the calibration accuracy. . Therefore, the standard turbine meter itself needs to be calibrated regularly. And the smaller the standard turbine gauge diameter, the smaller the flow through the gas, the greater the impact of the accuracy of the internal moving parts wear. Therefore, when the diameter of the turbine table to be calibrated is small (less than 100mm), it is not appropriate to use a turbine meter as a standard device.
The fluid mechanics principle of the sonic nozzle is: when the gas on the upstream side of the nozzle flows out through the nozzle, if the ratio P1/P0 of the pressure P1 behind the nozzle to the pressure P0 before the nozzle is smaller than a certain critical value (0.528 when using air as medium) ), the airflow reaches the speed of sound at the nozzle outlet cross-section, and when the pressure ratio P1/P0 continues to decrease, the airflow speed remains at the same speed of sound, so that a constant and constant critical flow is generated in the system to which the calibrated flowmeter is connected. Multiplied by the time used, we know the volume of the standard device. Therefore, the flow meter can be calibrated as a flow standard device. Under ideal conditions, the accuracy of the sonic nozzle is very high, but the gas is viscous. Therefore, when the gas passes through the nozzle, energy loss occurs, which results in calibration errors. The size of the error is related to the shape of the nozzle, the geometry, the surface finish, and the Reynolds number of the gas flowing through the throat.
Bell-type metering The flow range of a calibrated flowmeter is limited by the size of the bell. When the bell volume is small, the number of volumes compared to the test table during the measurement is small. The random error will increase, affecting the calibration accuracy of the turbine's large flow test point. When the bell volume is V capacity and the fall time is about 1 minute, the maximum flow rate that can be tested is:
Q max = 60 x V capacity (m3/h)
In addition, factors that affect the calibration accuracy of sonic nozzles and bell jars include the fact that the gas temperature inside the vessel is not uniform for a short period of time, and the water content of the gas is too large. The temperature, humidity, and pressure changes in the environment.
By understanding the characteristics of different standard devices, it is possible to select the appropriate standard device in the actual calibration. As far as conditions permit, high-precision standard devices are used as much as possible because the accuracy and operation procedures of the standard devices themselves directly affect the calibration accuracy of the turbine meters to be tested.
2.2 The effect of calibration pressure and calibration temperature
2.2.1 Turbine meter flow characteristics
When the flow rate is greater than the initial flow rate, the turbine rotation angular velocity will increase as the flow rate increases. In the measurement range, the resistance torque T generated by the fluid will become the main factor affecting the flowmeter characteristics. Relatively speaking, the mechanical resistance torque produced by the friction of bearings and other mechanical transmission components is relatively small. In the following discussion, assuming that the mechanical resistance torque is 0, the meter factor:
K=B - C[T/ÏQ2 ] (1)
Where: Q - the flow of fluid in the pipe;
B, C - constants.
Since the mechanism of fluid resistance is different and the effect is different under different flow conditions, the laminar flow state and the turbulent flow state will be discussed separately.
Differentiating laminar flow states from turbulent flow states must introduce the concept of Reynolds number (Re)
Re=4Q/πdν (2)
Where: Q - the flow of fluid in the pipe;
d - diameter of the pipe;
ν - the kinematic viscosity of the fluid in the tube.
Usually Re≥2320 is the judgment basis for the flow in the tube from the laminar flow state to the turbulent flow state.
In the laminar flow state, the fluid flow resistance torque T is proportional to the hydrodynamic torque (also called viscosity) μ and the fluid flow rate Q, ie, T=C1 μQ. In the formula, C1 is a constant, and it can be known by substituting the formula (1): If the viscosity changes, the meter coefficient K will also change accordingly; if the viscosity does not change. Then K will increase with the increase of traffic.
In the turbulent flow state, the fluid flow resistance torque T is proportional to the fluid density and Q2. At this point can be considered the impact of fluid viscosity. That is, T=C2ÏQ2, where C2 is a constant, and it can be known by substituting the equation (1) that in the turbulent flow state, the meter coefficient K is only related to the structural parameters of the meter, and has nothing to do with the parameters such as the flow rate Q and the fluid viscosity μ. Approximate constant. Only in this state, the meter coefficient K really shows the nature of the constant. The interval of the meter coefficient K is a constant, that is, the measurement range of the flow meter.
2.2.2 Analysis of Effects of Temperature and Pressure Changes on Calibration Accuracy
The kinematic viscosity ν is the ratio of dynamic viscosity to fluid density Ï. According to the relationship between the fluid viscosity and temperature, the definition of Reynolds number and the equation of state of the gas, it can be seen that the Reynolds number increases with the increase of temperature over a certain temperature range at the same flow rate. According to the determination conditions of laminar flow and turbulent flow, it can be known that the increase in temperature can make the fluid reach a turbulent state at a small flow rate, and thus the minimum flow rate Qamin at which the turbine table actually enters the precision range will decrease.
And if the temperature does not change, with the increase in pressure. The Reynolds number also increases at the same flow. According to the conditions of laminar flow and turbulent flow, it can be known that the increase of pressure can make the fluid reach a turbulent state at a small flow, so that the minimum flow Qamin of the flowmeter actually entering the precision range will be reduced.
2.3 Effect of Change of Compression Coefficient on Calibration Accuracy
The compression factor Z is a parameter that measures the degree to which the actual gas is close to the ideal gas. Normally, the calibration temperature is normal temperature, and the calibration pressure will not be too high. The influence of Z may not be considered.
2.4 Effect of Distortion of Velocity Distribution on Calibration Accuracy
Turbine meters are speed-type flow measuring instruments whose meter characteristics are directly affected by the state of gas flow. It is particularly sensitive to the speed distribution at its entrance. Abrupt changes in the inlet flow rate and rotation of the fluid can cause measurement errors to be unacceptable. In calibration, turbine flowmeters typically have straight pipe sections that are ten times the pipe diameter before, but often due to insufficient lengths of the straight pipe sections, the rotating fluids at the inlets are completely eliminated, or the fluids are changed due to the gaskets protruding when the flowmeter is installed. With the angle between the turbine blades, these effects tend to change the meter constant by 2% or more.
2.5 Influence of Auxiliary Measuring Device on Calibration Accuracy
The correct acquisition of data depends on the selection of temperature, pressure sensors, timers, pulse counters and the determination of the installation location. If the accuracy of the sensor is not enough, the accuracy of the measured value cannot be guaranteed. If the sensor is not installed properly, the actual temperature and pressure of the gas passing through the standard device and the turbine meter under test cannot be measured correctly. And may affect the gas flow state, resulting in the speed distribution of the turbine meter inlet is not
Both affect the accuracy of the calibration.
2.6 Impact of Data Analysis on Calibration Accuracy
Because the gas turbine flow meter measures the volume of gas passing through him over a period of time, and the volume of the gas is affected by factors such as pressure and temperature, and the value of the standard device is still slightly different from the true value, so if not fully considered The error caused by the gas passing through the standard device, the state difference in the test table, and the error of the standard device itself will cause great errors.
The calibration for the accuracy of the indicated value of the turbine is generally specified at the following flow rates: Qmin, 0.05 Qmax, 0.1 Qmax, 0.25 Qmax, 0.4 Qmax, 0.7 Qmax, and Qmax. If 0.05 Qmax and 0.1 Qmax are less than Qmin, the flow point is cancelled. Generally, the same flow point is tested at least 3 times, and the average value of 2 similar data is taken as the measured value. If abnormal data appears, increase the number of trials. For each flow, perform the following data analysis.
V', V are the volume of gas actually passing through the standard device and the test table within time t;
V, Vc are the volume indications of the standard device and the test table in time t respectively;
P, Pc are the absolute pressure of gas through the standard device and the test table, respectively;
TN and Tc are the absolute temperature of the gas passing through the standard device and the test table respectively;
ZN, Zc are through the standard device and the gas compression coefficient to be measured, respectively;
â–³P is the gas pressure difference between the meter to be measured and the standard device, â–³P=Pc - PN;
â–³T is the temperature difference between the standard device and the meter to be measured, â–³T=TN - Tc;
fN is the relative error of the indication value VN of the standard device at a certain flow point compared to the volume of gas V' actually passing through it.
Fc is the relative error of the indication value of the volumetric indication Vc of the table to be measured at a certain flow point compared with the actual volume V of the gas passing through it.
From the equation of state of the gas available:
Standard device
PNV'/TN=ZNmR (3)
Test table
PcV/Tc=ZcmR (4)
When △P and △T are small, the error calculation after one item is discarded. It can be known from the above equation that ignoring the existence of ΔP results in an error of (ΔP/PN)×100%; the existence of ΔT is ignored. The resulting error is (△T/TN) × 100%; ignoring the presence of the standard device's own error, resulting in an error of fN × 100%, must take into account the differences in temperature and pressure when the gas passes through the standard device and the test table, and the standard The error of the device itself.
3 Methods to Improve Calibration Accuracy
3.1 Reasonable selection of standard devices
Due to the characteristics of the standard turbine meter itself, it needs to be verified every year. When the diameter of the turbine meter to be calibrated is small (less than 100mm), the turbine meter should not be used as a standard meter.
Since the smaller the flow, the smaller the Reynolds number, the more the error caused by the gas viscosity is ignored, so the sonic nozzle standard device is only suitable for calibrating large flow, large diameter turbine meters. To ensure the processing accuracy of the throat diameter, it is necessary to select a high-precision time measurement device. In order to make the container gas temperature as stable as possible, a mixer must be installed in the container.
A bell jar is generally used to calibrate a small-diameter, small-flow turbine watch, and its volume must be large enough to ensure the accuracy of large flow point calibration. In order to ensure the water quality is clean, the water in the bell should be changed every day. When the bell jar swells, in order to make the gas temperature in the bell jar uniform, after the bell jar is full of gas, it should wait 5 minutes before starting the test. In addition, as the standard device, the bell jar and the sonic nozzle also need to use the air conditioner, dryer and other settings to ensure the standard environment required for calibration.
3.2 Elimination of the effect of calibration pressure and calibration temperature
The calibration device must be set up in a test room with sufficient space to ensure that the standard device and the turbine under test are not heated in a single direction (such as solar radiation, heaters, or other heat sources). The temperature change in the laboratory shall not exceed the temperature range of 20±5°C. Since the flow rate range of the turbine meter entering the precision is related to the gauge pressure and the local atmospheric pressure, the gauge pressure and the local atmospheric pressure should be specified.
3.3 Elimination of Pipeline Design Impacts in Calibration Systems
In order to effectively eliminate the rotating flow. The necessary straight pipe section should be installed in front of the turbine meter, and it is better to install a rectifier in front of the turbine meter and ensure that the pipeline and the flowmeter gasket are well positioned so as not to protrude. In order to ensure the normal flow characteristics of the fluid, eliminate the adverse effects of various pipe fittings and valves after the flowmeter, and ensure that the flowmeter should also be at least 5 times the diameter of the straight pipe.
3.4 Eliminating the influence of auxiliary measuring devices
Select high-precision temperature, pressure sensors, timing devices and other ancillary facilities, the accuracy must be higher than the required accuracy of the turbine table to be measured l ~ 2, and the installation location should be appropriate.
3.5 Analysis and Processing of Data
To calculate the error of the meter to be measured, it is necessary to consider the difference in temperature and pressure when the gas passes through the standard device and the meter to be measured, and the error of the standard device itself. If the difference of the gas compression coefficient through the standard device and the test table is ignored, the relative indication error of the turbine table to be measured at a certain flow point can be obtained.
4 Conclusion
(1) To calibrate small-diameter and small-flow turbine meters, bell jar standard equipment should be used. Sonic nozzles should be used to calibrate large-diameter and large-flow turbine meters. Standard turbine meters can also be used for turbine flowmeters with bore diameters of 100mm and greater than 100mm. As a standard device, it needs to be re-verified once a year.
(2) Reasonably design the calibration system to ensure that the gas turbine flowmeter has enough straight pipe sections before and after, if the length of the straight pipe section is not enough due to the restriction of the pipeline, install a rectifier before the flowmeter. When the temperament is dirty, a filter should be installed to avoid the influence of impurities on the system. When the moisture content of the temperament is large. Dehydration drying equipment should be added.
(3) Choosing a suitable design scheme and selecting high-precision temperature, pressure sensors, timing devices, and other auxiliary settings, its accuracy must be higher than the required precision of the turbine table to be measured, and the system error should be minimized.
(4) Calibration shall be carried out in strict accordance with the operating procedures to meet the environmental conditions such as temperature, pressure, and humidity required for calibration.
(5) Correctly process the data and fully consider the differences in the state of the gas passing through the standard device and the test table and the error of the standard device itself. The standard devices should be carefully maintained and regularly tested in accordance with the provisions of the Technical Supervision Bureau.
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