Abstract: Comprehensive and systematic analysis of various factors affecting the accuracy of automatic control system for oil filling has important guiding significance for improving filling accuracy. In this paper, the oil quantitative filling control system is used to analyze the source of additional error caused by volume flowmeter and Coriolis mass flowmeter. The main factors affecting the overshoot and the overshoot error are discussed. The filling control is given. The formula for calculating the total error of the system. Through error analysis, the basic ways to improve the accuracy of the filling control system are summarized. The analysis also shows that the filling accuracy of the filling control system depends mainly on the accuracy of the flow meter and the temperature, pressure and other instruments in the case of setting the small flow rate to close the valve in advance. The overshoot error has little effect on the filling accuracy of the system. .
Key words: filling control system; error analysis; flowmeter; overshoot
1 Introduction
The oil filling automatic control system (hereinafter referred to as “filling control systemâ€) is an important part of the oil depot and gas station. The filling control system is generally based on flow measuring instruments, with electromagnetic shut-off valves or pneumatic electric shut-off valves as terminal actuators, and flow calculation, accumulation, display and logic control by flow quantitative controller or computer [1~3]. Therefore, the factors affecting the accuracy of the filling control system include not only the error of the flow measuring instrument, but also the errors of signal transmission, control adjustment, and operation execution. Comprehensive and systematic analysis of various factors affecting the accuracy of the filling control system has important guiding significance for improving the filling accuracy.
2. Error analysis of flowmeter
In the filling control system, the flow rate detection is generally achieved by various flow meters. The basic error of the flowmeter is obtained under standard working conditions. In the actual use process, since the field use conditions often deviate from the standard working conditions, additional errors will inevitably occur. Therefore, the field use error of the flowmeter should be the basic error of the flowmeter. Synthesis of additional errors.
Flow meters can be divided into two major categories: volume flow meters and mass flow meters. The basic error of common volumetric flowmeters is generally ±0.5%R, and the high-precision type can reach ±(0.1~0.2)%R. The amount of influence that causes the volumetric flowmeter to produce additional errors mainly includes the temperature, pressure, and viscosity of the oil.
(1) Temperature
Temperature changes are the primary cause of additional errors in the volumetric flow meter. The change in temperature primarily produces additional errors by varying the viscosity of the oil and the volume of the flow metering chamber. The viscosity of the oil decreases with increasing temperature. The change in temperature changes the volume of the metering chamber of the volumetric flow meter, causing a change in the gap between the flowmeter housing and the rotor, thereby changing the amount of leakage. For example, for a volumetric flowmeter in which the metering chamber cavity material is cast steel, the additional error caused by the operating temperature being 10 ° C above the calibration temperature is about 0.04%.
(2) Pressure
The change in pressure also primarily creates additional errors by varying the viscosity of the oil and the volume of the metering chamber. The viscosity of the oil increases with increasing pressure. For example, for a normal liquid, the viscosity changes by about 0.1 to 0.3 MPa (0.1 to 0.3)% for each 0.1 Mpa increase. The effect of pressure change on the volume of the metering chamber is not as significant as the temperature. For a single-chamber volumetric meter, the increase in pressure will result in an increase in the volume of the metering chamber and an increase in the amount of leakage, thereby causing the flowmeter error curve to move in a negative direction. For the double-shell volumetric flowmeter, since the pressure difference between the inner and outer walls of the metering chamber is zero, it is not affected by the pressure.
(3) Viscosity
Viscosity changes can shift the flowmeter characteristics. For the volumetric flowmeter, if the viscosity increases, the leakage amount decreases and the error is negative. Conversely, if the viscosity decreases, the leakage amount increases and the error is positive.
Figure 1 shows the effect of liquid viscosity on the basic error of a lumbar flowmeter. It can be seen that in the viscosity range of 0.8~11mPa?s, the viscosity has a great influence, the viscosity decreases from 5.65mPa?s to 0.8mPa?s, and the error increases by about 0.5% in the negative direction; in the range of 11~51mPa?s, the viscosity is measured. The error still has a significant effect; when the viscosity is greater than 51mPa?s, the effect of viscosity on the error is not obvious. Therefore, for a volumetric flowmeter with an accuracy of 0.2 or more, the influence of the error caused by the viscosity change of the oil must be considered. During the verification and actual use, when the viscosity of the calibration oil is significantly different from the viscosity of the oil actually used, the corresponding correction must be made.
Mass flow meters can be divided into direct and indirect (derived) categories. The mass flow meter that is currently the most widely used and can directly measure liquid flow is a Coriolis mass flow meter (hereinafter referred to as "Coriolis flowmeter"). The basic error of Coriolis flowmeters is generally ±(0.15~0.5)%R, and special can reach ±0.1%R. For less accurate Coriolis flowmeters, the effects of media temperature and static pressure changes can be neglected, while for Coriolis flowmeters with accuracy up to (0.1~0.15)%R, the effects should be considered.
(1) Temperature
Changes in media temperature or ambient temperature change the Young's modulus and geometry of the Coriolis flowmeter to measure the flow tube, thereby changing the flow ratio factor [4]. Most models use electronic circuits to compensate the temperature coefficient of Young's modulus to reduce its influence. However, the temperature coefficient of Young's modulus is a statistic. Due to the inconsistency of the process tube material batch number and heat treatment process, it is impossible. All compensation is zero. In addition, temperature changes can also affect the stress distribution of the measuring tube, causing asymmetry in the mechanical vibration, resulting in zero drift. Since the zero drift is caused by the imbalance of the vibration tube geometry and material, it cannot be reduced or eliminated. The actual flow test on the Coriolis flowmeter shows that the influence of temperature change on the meter constant can reach ±(0.014~0.057)%/°C.
(2) Pressure
The increase of hydrostatic pressure will cause the measuring vibration tube to be tight, and the bending tube also has the Buden tube effect, resulting in negative deviation [5]. For high-precision instruments, when the static pressure and the calibration are very different when used, the influence of these two pressure effects can not be ignored. The influence of pressure depends on the pipe diameter, wall thickness and shape of the measuring pipe. The small-diameter instrument has a small influence on the pipe diameter ratio, and the influence amount is small. The large-diameter instrument has a small influence on the pipe diameter ratio. According to the data provided by MicroMotion, the pressure influence of the CMF100 meter is -0.03%R/Mpa, the CMF200 type is -0.12%R/Mpa, and the D300 type is -1.35%/Mpa based on the calibration pressure of 0.2Mpa. The D600 type is -0.75%/Mpa.
(3) Pulsation and vibration
Coriolis flowmeters are based on vibration principles and are therefore sensitive to external vibration disturbances. The experimental results show that when the frequency of the pulsating flow or external mechanical vibration is close to or equal to the resonant frequency of the Coriolis flowmeter, a large measurement error will occur, even making it impossible to work properly.
3. Analysis of overshoot error
In the filling control system, it takes a certain time from the controller to issue the valve closing command until the valve is completely shut off, and the accumulated flow flowing during this period is called the overshoot. The magnitude of the overshoot is related to the system response speed, the valve closing stroke time, the instantaneous flow rate during the valve closing process, etc. In order to overcome the error caused by the overshoot, the method of closing the valve in advance is usually adopted, that is, the valve advance amount ΔL' is set. When the difference between the target filling amount and the accumulated oil amount is equal to ΔL', a valve closing command is issued. During the actual filling process, the pump/pipe pressure will fluctuate within a certain range, causing a change in Δp during valve closing. In addition, changes in temperature and pressure can also have an effect on fluid density. Therefore, the actual overshoot during each valve closing process is not exactly the same. If a fixed ΔL' is used each time, a large overshoot error may be caused.
4. Total error of the filling control system
For example, the oil drum quantitative filling control system of a certain oil depot uses a volumetric flowmeter with an accuracy of 0.2%. The temperature and pressure measurement accuracy is 0.5, and the mass flow measurement error is about ±0.27% considering various additional errors. The target filling amount is set to 100kg, and the early closing method is adopted, and the overshooting error is about ±3%. If the segmentation valve is not used, the overshoot is about 1kg, and the total error of the filling system is ±0.2973%.
If a small flow section is used to close the valve, the overshoot in the final valve closing process is about 0.2 kg, and the total error of the filling system is ±0.2755%.
For another example, the quantitative fueling control system of a transit oil depot uses a mass flowmeter with an accuracy of 0.15% (the on-site measurement error is about ±0.18% considering the additional error), and the target filling amount is set to 3t, the overshoot error It is about ±3%. If the segmentation valve is not used, the overshoot is about 30kg, and the total error of the filling system is ±0.2082%.
If a small flow section is used to close the valve, the overshoot in the final valve closing process is about 5 kg, and the total error of the filling system is ±0.1847%.
If the small flow segmentation valve is still used, the target filling amount is set to 6t. Since the overshoot and overshoot error during the final valve closing process is independent of the target filling amount, the total error of the filling system is ±0.1823%.
5 Conclusion
Through the above analysis, the error of the filling control system is mainly composed of two parts: the flow measurement error and the overshoot control error. The flow measurement error mainly includes the error of the flow meter and the temperature, pressure and other instruments. The overshoot control error mainly includes the error of control adjustment and operation execution. In addition, the conversion error, resolution error, etc. in the signal processing process will also have a certain impact on the system filling accuracy. Therefore, the ways to improve the accuracy of the filling control system mainly include: (1) linearization of the flow meter factor; (2) temperature, pressure and viscosity correction of the flowmeter additional error; (3) real-time temperature and pressure compensation of the fluid density; (4) Small flow rate to close the valve in advance; (5) Real-time correction of overshoot error; (6) Improve signal conversion accuracy.
Through the previous analysis, it is also found that the accuracy of the filling control system mainly depends on the accuracy of the flow meter and the temperature, pressure and other instruments in the case of setting the small flow rate to close the valve in advance. The influence of the overshoot error on the filling accuracy of the system is not large. .
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