Orifice plate flowmeter problems and solution measures

Time: 2025-01-07
Equipment Configuration:Orifice plate flowmeter

The proportion of natural gas in the world's energy structure continues to rise.

Project Overview

The proportion of natural gas in the world's energy structure continues to rise. According to statistics, it was 17.7% in 1970, 18.8% in 1980, 21.5% in 1990, 22.3% in 2000, and is expected to be 24.3% in 2010. At present, the annual production of natural gas has reached about 2.3 trillion m3. The distance between natural gas production areas and consumption areas determines the huge international trade volume of this energy. This number has been rising since the 1990s and has now reached 420 billion m3/year, of which more than 75% is transported by pipelines. , the rest is liquefied natural gas.

my country is one of the countries rich in natural gas resources, with prospective natural gas reserves reaching approximately 38 trillion m3, of which 79% is onshore and 21% is offshore. The production and utilization process of natural gas is a system with complex processes, large scale, fast speed and continuous operation. In this system, various flow measurement instruments for different purposes can be seen at well sites, gas gathering stations, gas distribution stations, or in users' homes. The accuracy of its measurement is generally valued by people. Therefore, measurement accuracy is an important indicator that must be considered when selecting any type of flow meter.

In practical applications, there are many factors that affect the measurement accuracy of flow meters. Several aspects will be discussed below.

1 Current problems existing in the measurement of orifice flowmeters

There are many types of flow measurement instruments and various calculation models, such as standard orifice flowmeters, vortex flowmeters, and vortex flowmeters. , ultrasonic flowmeters, etc., except for standard orifice flowmeters, lack necessary standards or specification support.

The orifice plate flowmeter was used to measure natural gas flow in the early 20th century. After a century of development, it has become the most important natural gas flowmeter in the world. It is estimated that it currently accounts for about Accounting for 60%, and more than 90% in China. The American AGA (American Gas Association) Report No. 3 (Orifice Plate Flowmeters for Measuring Natural Gas and Other Hydrocarbon Fluids) is a standard for natural gas measurement. It has been revised many times since its release in 1955 (first edition in 1955, first edition in 1969) The first edition was revised, the second edition was in 1985, the third edition was in 2000, and the fourth edition was in 2000), especially the third edition which had substantial revisions.

It was the first international review of orifice plates in the 1980s. Based on large-scale research and experiments on flowmeters, AGANO3 has summarized dozens of special studies on natural gas measurement. Most of the experimental media are natural gas. It can be said that the use of orifice flowmeters to measure natural gas flow has been very in-depth. Careful research and testing, and accumulated practical experience have also produced a qualitative leap on the basis of quantity. The symbol is standardization. Even if a standard orifice plate flowmeter is used, the signal (differential pressure) and flow rate can be determined without actual calibration. relationship, and estimate its measurement error. Currently, it is the only one among all flowmeters that reaches this standard.

The main features of the orifice flow meter are that the structure is easy to copy, simple and solid, the performance is stable and reliable, and the service life is long.long, low price, etc. The complete flow meter consists of a throttling device, a differential pressure transmitter and a flow display (or flow computer). They can be produced by different manufacturers, are easy to form large-scale production, have high economic benefits, and the combination of each part is very flexible. Even the integrated orifice flowmeter currently launched can be produced separately and then assembled flexibly.

But an orifice flowmeter generally consists of three parts: a throttling device, a differential pressure transmitter and a flow indicator. Under the harsh working conditions on site, the throttling device, differential pressure transmitter and its connecting pressure pipeline are the focus of use and maintenance. "Standard Orifice Plate Measurement Method for Natural Gas Flow" (SY/) points out that natural gas is extracted from the formation. Although it has been separated, dusted and filtered, its components are very complex. From single well calculations, gas gathering and metering to gas distribution Metering and gas composition are different, so the corrosion of the throttling device during use is also different. Especially the erosion and corrosion of the right-angle entrance edge of the orifice plate and the inner wall of the measuring tube will affect the right-angle entrance edge of the orifice plate. According to the prescribed standards of arc radius rk and relative roughness k/d of the inner wall of the measuring tube, the outflow coefficient C will change, and the flow measurement uncertainty exceeds the estimated number. The output signal is an analog signal, and the repeatability is not high, which is not accurate for the entire flow meter. There are many and complex factors that affect the accuracy, so it is very difficult to improve the accuracy.

1.1 Influence of the characteristics of the fluid being measured

Due to the nature of natural gas itself, it will change with the temperature of the external environment. Complex changes will occur due to changes, which will affect the measurement accuracy of the flow meter. For the measurement of natural gas, the operating temperature and pressure of the natural gas must first be determined, because changes in the external temperature will also cause changes in the pressure and temperature of the natural gas itself. Low-density gases may cause excessive density changes and compression coefficient changes, which may cause difficulties for certain measurement methods. At this time, the selected measurement method must be changed, or temperature and/or pressure corrections must be made to ensure accurate measurement. . Therefore, when evaluating the adaptability of the flow meter, it is necessary to grasp the temperature-viscosity characteristics of the gas. Although the viscosity of the gas changes due to temperature and pressure, it still has a certain impact on the accuracy of the flow measurement.

In mine measurement or when measuring natural gas containing impurities, the impurities will corrode the meter contacts, causing the contact surface to scale or precipitate and crystallize, which will reduce the gap between the moving parts, thereby changing the performance parameters of the flow meter and reducing the The sensitivity or measurement performance of sensitive components affects the measurement accuracy.

For oil field moisture (associated gas), it contains a large amount of saturated water vapor, and water will condense when the temperature decreases; the moisture that is not separated is clean. The gas contains oil droplets or oil stains and is a dirty medium; intermittent measurement will cause oil and water to be deposited in the pipeline; they are multi-component flows and should be treated with caution when measuring. Experience shows that single-phase universal flow meters are used for multiple applications. Components or multi-phase fluids, the measurement performance will change (or change significantly). Single-phase flowing gases may sometimes show two phases. For example, water particles in wet gas flow with natural gas, and the ambient temperature or natural gas pressure deviates from the original state.state, the instrument may not adapt. When measuring gas-liquid two-phase flow, phase separation measurement after separation should be used as much as possible to ensure the minimum measurement uncertainty. However, in some cases this method is not feasible or does not meet the requirements.

1.2 Influence of instrument performance

The main causes of errors caused by the orifice plate flowmeter itself are: the sharpness of the right angle of the orifice plate inlet; the pipe diameter size does not match the calculation; the orifice plate thickness error; The accessory of the throttling part produces steps and eccentricity; the flatness of the upstream end face of the orifice plate; the size of the annular chamber produces steps and eccentricity; the pressure-taking position; welding and welding seams protrude; the pressure-tapping holes are not processed in a standardized manner or are blocked; the throttling parts are not axial etc. These factors may affect the repeatability of the orifice plate. Repeatability is determined by the principle and manufacturing quality of the instrument itself. It is an important indicator in process control applications. In addition to repeatability, accuracy is also related to the value calibration system. In practical applications, the excellent repeatability of the instrument is disturbed by many factors including changes in fluid viscosity, density, etc., which will affect the measurement accuracy. If the instrument output characteristics are nonlinear, this effect will be more prominent.

There are two main types of flow meter output: linear and square root nonlinear. The nonlinear error of most flow meters does not list separate indicators, but is included in the basic error. However, for instruments whose pulse output is used for total accumulation in a wide flow range, linearity is an important indicator, which makes it possible to use the same instrument constant within the flow range. A difference in linearity will reduce the accuracy of the instrument.

When applied to pulsating flow sites, attention should be paid to the response of the instrument to step changes in flow. Some applications require the instrument output to follow the flow, while others only require an output with a slower response in order to obtain a comprehensive average. Transient response (transient response) is often expressed in terms of time constant or response frequency. The former ranges from a few milliseconds to a few seconds, and the latter is below hundreds of hertz. Equipped with a display instrument, the response time may be greatly extended. Red medloc believes that asymmetrical flow rise and fall dynamic response of the meter can drastically increase measurement error.

1.3 Impact of flow meter installation

Installation errors caused by deviations in pipeline layout are common. The main reason is that the required length of the straight pipe section cannot be met on site.

Natural gas flow meters can measure natural gas only when installed on pipelines. When installing, you should first consider the layout of the pipeline and the flow direction of natural gas. Although some meters can work in both directions, they must also be installed during installation. Consider whether there is a difference in measurement performance between forward and reverse directions.

Many sites cannot meet the required length of the straight pipe section of the orifice flowmeter, and installation errors caused by deviations in pipeline layout are common.

Most flow meters will be more or less affected by the inlet flow conditions, so good flow conditions must be ensured. Fixed displacement pumps, reciprocating compressors, oscillating valves or regulators installed on the transmission pipeline are common sources of pulsation. The upstream pipeline layout and flow obstructions will cause flow disturbances. In addition, the pipe diameter and directionImproper layout such as sudden changes will cause pulsation. The flowmeter has no time to follow and record the pulsating flow, causing measurement errors. Therefore, consideration should be given to installing supports in the pipes before and after the instrument. Although pulsation dampers can eliminate or reduce the effects of a pump or compressor, it is best to avoid vibration or sources of vibration as much as possible.

The signal output of the flow meter includes flow (volume flow or mass flow), total flow, average flow rate, and point flow rate. It can also be divided into analog quantity (current or voltage) and pulse quantity. Most current instrument systems incorporate electronic devices on or near the instrument. The output signal of the instrument (equipment) is easily affected by the high-power power supply. The high-power power supply will not only cause the instrument output pulse to fluctuate, but also affect the instrument's working performance. For example, the magnetic field of the electromagnetic flowmeter is distorted, affecting the measurement accuracy. Therefore, electrical connections should have the ability to resist spurious level interference, and signal cables should be as far away from power cables and power sources as possible to minimize electromagnetic interference and radio frequency interference, otherwise the signal transmission will be affected. In addition, in addition to considering the above factors, the installation of the flow meter sometimes also depends on the physical properties of the fluid.

1.4 Impact of environmental conditions

Although the flow meter can be used normally after installation, some performance parameters of the meter have changed due to changes in the environmental conditions and expected conditions. There are also changes in hardware that can alter flow meter measurements. For example:

Sudden changes in ambient or medium temperature cause humidity problems. High humidity will accelerate atmospheric corrosion and electrolytic corrosion and reduce electrical insulation, while low humidity will easily induce static electricity.

The electronic components of the instrument and some flow detection parts of the instrument will be affected by changes in ambient temperature. For example, changes in the size of the instrument, changes in fluid density and viscosity through heat transfer through the instrument housing, etc., will reduce measurement accuracy when affecting the electronic components of the display instrument.

At the same time, the factors that affect the measurement of the orifice flowmeter include the gas Reynolds number range that does not meet the standard requirements, the influence of pipeline roughness, the increase of pipeline roughness, the variable variation of pipeline roughness, etc.

2 Ways to solve the measurement problems of orifice flowmeters

2.1 Strengthen management and improve personnel quality.

The reason why the orifice flow meter easily deviates from the standard is the working principle and structural characteristics of the meter itself. The error of the meter itself is caused during manufacturing, while the installation and use errors are caused by the fluid during installation or long-term use. Caused by media corrosion, wear, contamination, etc. Therefore, the flow measurement system should be installed strictly according to the technical requirements to eliminate installation errors.

During use, operators should do a good job in system inspection, maintenance, and upkeep to extend its service life and reduce measurement errors. At the same time, the promotion and implementation of SY/T standards should be strengthened in practical applications. Standard orifice plate measurement method for natural gas flow)) (Methods to reduce errors are mentioned in SY/.

For example, the appendix "Handling partial deviations from the standard regulations when the throttling placement occurs during use" stipulates that in practical applications Take the outflow coefficientAdd two correction coefficients to C, namely the orifice plate inlet sharpness correction coefficient and the pipe wall roughness correction coefficient, or use a replaceable orifice plate throttling device. Therefore, in-depth research should be carried out in the practical application of natural gas measurement, and the standard orifice plate measurement method of natural gas flow should be thoroughly understood (the essence of SY/T). Install, use, and process data in strict accordance with the standards to ensure the accuracy of natural gas measurement.

2.2 Correct selection of measuring instruments

The standard throttling device should be the first choice for measuring instruments. When the flow change range is large, wide-range intelligent differential variable should be considered. Other types of detection instruments should be used in the corresponding measurement of natural gas. The standard comes out of the front desk and should be selected carefully. The measurement model of the secondary instrument should meet the requirements of SY/T to calculate the Fz component and calculate the C value in real time.

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The magnetic inductive flowmeter industry, these are successful cases with comparable and reference value.

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