Metering device and application of differential pressure gas flow meter selection Hu Haiyan (Shengli Oilfield Hekou Oil Production Plant technical inspection station measurement station, Shandong Hekou differential pressure gas flow meter is a kind of most widely used gas flow meter, in various types of flow The usage of the meter is the highest in the instrument. The article uses the channel flowmeter, V-cone flowmeter, and precession vortex flowmeter as examples to introduce how to select the differential pressure gas flowmeter and what should be noted in the selection. problem.
Differential Pressure Gas Flow Meters; Model Selection; Instrument Performance Natural gas is delivered in pressurized pipelines with flow rates that are directed through a certain cross section of the pipeline. How much natural gas is transported between the supply and demand parties is determined by the measurement data of the flow measuring device installed on the gas pipeline. This flow measurement data is also an important basic data to measure the production technology level of a unit. Accurately carrying out natural gas flow measurement is a key link for economic analysis and reduction of operating costs in the corporate sector. It directly affects the economic efficiency of enterprises and is of great concern to both supply and demand. As the world’s energy supply and demand become increasingly tight, people are very concerned about and seeking a A high-precision, adaptable flow meter to measure natural gas flow.
1 Differential Pressure Flowmeter Although differential pressure gas flowmeters have become more mature today, their types are still extremely varied and their performance varies. Prior to the 1950s, differential pressure flowmeters were the only natural gas flowmeters that accounted for approximately 50% of the total volume of flowmeters and 60% of the total sales volume in China was approximately 40%. It is based on the differential pressure generated by the flow detection device installed in the pipeline, the known fluid condition, and the geometrical dimensions of the test piece and the pipeline to measure the flow rate. It has a long history of application, rich and mature practical experience, and perfect standard specifications (both Except for the speed control tube) A type of flowmeter with complete specifications and specifications, in addition it has the advantages of simple structure, durability, high versatility, low cost, and no need for actual fluid calibration of the standard throttle.
Differential pressure flowmeter also has many shortcomings, it requires high installation conditions on site, requires a long straight pipe section, the accuracy of the entire flowmeter is also determined by the accuracy of the differential pressure transmitter and flow indicator, is a design from the Manufacture, installation and use of very strict instruments, errors in any link will produce a lot of errors; In addition, differential pressure flowmeter output signal and flow is a square relationship, is a non-linear meter, a narrow range, pressure loss is also large One of its weaknesses. In response to these deficiencies, domestic manufacturers have introduced a series of improvements in recent years, such as integrated differential pressure flowmeters, fixed throttles, orifice plate throttling devices, standard nozzles, etc., plus differential pressure after the 1980s. The two components of the flowmeter: the differential pressure transmitter and the flowmeter have outstanding progress. It is estimated that such flowmeters will still occupy an important position in the future. Channel flowmeters, V-cone flowmeters, and precession vortex flowmeters are three representative new flowmeters, all of which are differential pressure flowmeters.
2 differential pressure flowmeter classification Differential pressure flowmeter has a wide variety, according to different classification methods, can be divided into three categories of 20 types. See Table 1 for specific classification. Table 1 Classification of Differential Pressure Flowmeters Classification Principle Classification Type Throttling type; Dynamic pressure head type; According to the differential pressure generated (3) Hydraulic resistance type Principle of action Classification (4) Centrifugal; Dynamic pressure Gain Jet Type Standard Orifice; Standard Nozzle; Classic Venturi Tube; Venturi Nozzle Cone Inlet Orifice; 1/4 Round Hole Plate; Round Needle Plate; Eccentric Orifice Plate; Classified by Structure Type (9) Wedge Hole Plate; Integral (contained) orifice plate; Linear orifice plate; Annular orifice plate; Dole tube; Luoluosi tube; Bend tube; Reversible orifice plate throttling device; Critical flow throttling device Hu Haiyan (1979―) Female, Shandong Zhanhua Ren, an engineer, graduated from China University of Petroleum and worked in metrology technology.
Continuing Table Classification Principle Classification Type Standard throttling device; low Reynolds number throttling device; dirty flow throttling device; classification of low pressure loss throttling device by purpose; small pipe diameter throttling device; wide range throttling device; critical flow Throttling device 3 Differential Pressure Flowmeter Selection With the development of science and technology, there are many new types of flowmeters that continue to emerge. However, since the natural gas flow measurement technology itself involves a wide range of different objects, there are different requirements for the flowmeter, which means that a variety of flowmeters coexist objectively to meet the requirements of different levels and different occasions. Different types of flowmeters have different instrument characteristics and fluid characteristics. The required installation conditions and application conditions are different. Each type of flowmeter has its applicable scope and also has limitations. This requires us to simultaneously select Consider various factors. When selecting the instrument, be sure to be familiar with the measured object, instrument performance, measurement environment and other aspects.
3.1 The purpose of the meter's use of the meter should be very clear. That is to know whether the measured volume flow or mass flow; instantaneous flow or cumulative flow to be measured, and the flow meter due to the differences in the structure, principle is often only applicable to a flow measurement or measurement of a certain kind of flow is the most accurate Measuring other flow rate requires increasing the conversion equipment or decreasing the precision. For example, the precession vortex flowmeter can obtain the cumulative flow value by counting or counting the pulse. It is suitable for measuring the total amount. If it is used to measure the instantaneous flow rate, it must be timed. The device and the corresponding calculation circuit can only convert the instantaneous value, and the accuracy will be reduced due to factors such as significant digits in the conversion process.
First, the properties of the fluid should be considered. For example, for incompressible fluids, the volumetric and mass flow can be switched, ie, the speed and density can be obtained by determining speed and density.
There are two kinds of conditions for the measurement of natural gas: one is the measurement of unclean natural gas before processing, and the other is the measurement of clean natural gas after processing. Unclean natural gas is often entrained with liquids, solid particles, and other precipitates that may contaminate the gas flowmeter. This causes troubles in metering. The clean natural gas after processing is unsatisfactory due to poor processing technology and dirty collection pipelines. Natural gas gathering and transportation is also sometimes unclean.
3.2 Measurement Environment Installation conditions often depend on the requirements of site space and orientation. For example, swirling vortex flowmeters often require that the sensor's installation direction and flow direction are horizontal, vertical, and horizontal, respectively, and due to the influence of flow rate distortion and other factors, a certain length of straight pipe needs to be added upstream or downstream to eliminate the interference thereof.
When the flowmeter is used, the environmental factors affecting it are mainly electromagnetic interference. Such as low-frequency electromagnetic interference, that is, when used in the field, from the AC power supply (220V50HZ because of the installation, in order to route specifications, often through the cable bridge or protective tube, so that it may make the AC power and signal lines along the parallel wiring, interference signal It will be coupled to the signal output line, which shows that by increasing the distance between the power line and the signal line, reducing the distribution, and properly selecting the signal line ground point, the influence of the interference voltage on the flow meter can be avoided.
3.3 Instrument Performance In gas flow measurement, temperature and pressure correction is one of the important means to improve gas flow measurement accuracy.
Whether or not it has the temperature and pressure correction is one of the issues that need to be considered in the selection. The deviation caused by the pressure correction is definitely negative, and the deviation caused by the pressure in the actual working condition is also greater. Without temperature correction, when the ambient temperature is higher than 20*C, the deviation caused is positive; when the ambient temperature is lower than 20*C, the deviation caused is negative, so it is very important to carry out temperature pressure correction.
How much does the overall measurement accuracy require, whether it is used at a specific flow rate, or within a certain flow range, what accuracy is maintained within the measurement range, how long the accuracy of the selected meter can be maintained, and whether it is easy to recalibrate Whether or not to verify the accuracy of the instrument on-site online must be carefully considered.
However, the throttling element is a machined metal structural part, and the transmitter and flow computer can effectively correct the accuracy error of various throttling parts, so the system performance depends on the throttling differential pressure transmitter. The sum of the two.
The "Regulations for the Use and Management of Energy-using Units for Energy Measuring Instruments," section 438 of 2006, put forward specific requirements for the accuracy class of energy measuring instruments for energy-using units or equipment. The level of accuracy required for gas and natural gas flow meters is 20; for steam, 2.5.
When comparing the instrument performance specifications of various manufacturers, it should be noted that the percentage of error is the ratio of the reference error (the ratio of the color error to the upper limit of the meter, commonly referred to as S) or the relative error (the ratio of the absolute error to the measured true value). Commonly used).
Also note that the accuracy specified by the manufacturer's product specification refers to the error margin of the basic industry for the year of the year. Changes in the environment, power, and fluid conditions at the site will cause additional errors. The accuracy of on-site use should be composed of the basic error and the error generated by the influence. If the impact is large, the additional error may far exceed the basic error.
Reproducibility is an important indicator in process control applications. It is determined by the principle of the instrument itself and the quality of manufacturing. Accuracy depends on repeatability, but also on the calibration system. Strictly speaking, repeatability refers to environmental conditions. When the parameters of the medium and the medium are constant, the consistency of multiple measurements in the same direction within a certain flow value period is met. However, in practical applications, the excellent repeatability of the instrument is disturbed by many factors including fluid viscosity, density, and the like. However, these changes have not yet reached the point where they need to specifically perform inspection and correction. These effects are often mistaken for the poor repeatability of the instrument. Therefore, there must be a highly repetitive site with general requirements (ie, conditions of use that affect the factors that affect the parameters). Do not choose instruments that affect the sensitivity.
Flow meter output mainly has linear and square root nonlinear.
Most of the non-linearity errors of flowmeters are not listed in the individual index but included in the basic error. However, for the meter with a wide flow range pulse output as the totalizer, linearity is an important indicator, making it possible in the measurement range. The same meter factor is used to reduce the accuracy of the meter when the linearity is poor. With the development of microprocessor technology, the signal coefficient is used to correct the meter coefficient nonlinearity, thereby improving the meter accuracy and extending the flow range. If pipeline flow distribution is required, the flow addition or calorimeter should multiply the temperature difference and the flow rate, and a linear output instrument should be selected to simplify the calculation process.
Upper limit flow, lower limit flow and flow range The caliber of the selected flow meter should be selected according to the flow range used by the measured pipe and the upper limit flow and lower limit flow of the selected instrument, instead of simply using the pipe diameter. Although the maximum flow rate of the pipe fluid is usually designed, it is determined by the economic flow rate. Because the flow rate selection is too low, the diameter of the pipe is large and the investment is too large, while if the flow rate is too high, the output power is large and the operating cost is increased. The flow rate of most of the instrument's upper limit flow rate is close to or slightly higher than the economic flow rate of the pipeline. Therefore, there are many opportunities for the instrument to choose the same diameter and diameter, and the installation is more convenient. If it is not the same, it will not be too different, and the adjacent one file size is generally not different. , using different diameter pipe connection. Then, the upper limit flow rate of different types of instruments of the same caliber is restricted by their respective working principles and structures.
When selecting a type, it is often considered that the maximum flow rate of the device does not exceed the flow rate of the upper limit of the flowmeter, and the probability of use of each flow segment of the gas-consuming device and its error region are not taken into consideration. As a result, the flowmeter does not exceed its upper limit of flow in use. However, the error in the small-flow segment or common flow segment is too large.
The lower limit of the flow range shall be selected to meet the minimum flow rate of the flowmeter entering precision (Qi is less than the minimum gas consumption of the pipeline, so as to ensure the measurement accuracy of the small flow.
The range is the ratio of the upper limit flow to the lower limit flow when the accuracy is satisfied, and the larger the value is, the wider the flow range is. The factors restricting the range of differential pressure flowmeters are: the constraints of measurement accuracy, the nonlinearity of the outflow coefficient, and the constraints of the expansion coefficient. Linear instruments have a larger range, usually 10:1 nonlinear instruments are smaller, usually only 3:1 to meet the general process control flow measurement and total business accounting measurement. Differential pressure flowmeters are non-linear instruments. The range of new differential pressure flowmeters has been broadened. Experiments have shown that the range of channel flowmeters can generally exceed 7:1, and energy saving and consumption reduction have become major The extremely important and urgent task that companies must attach great importance to. To achieve the goal of energy saving, energy-saving instruments must be selected. Energy-saving instruments must have two basic conditions. First, they must have high accuracy. Otherwise, it is difficult to use data to explain energy-saving effects, and it is impossible to take further energy-saving measures. Second, permanent pressure loss should not be too large.
In addition to unimpeded flow sensors, most flow sensors either change the direction of flow or place stationary or moving detection elements in the flow path, resulting in irrecoverable pressure losses that vary with flow, sometimes as high as several 1ka. First of all, according to the piping system pumping capacity and the instrument inlet pressure and other conditions, to determine the maximum pressure tolerable pressure loss, according to the selected instrument. Inadequate pressure loss due to improper selection often affects process efficiency. Instruments with a large pressure loss often pay for the cost of pumping over a few years in excess of low-pressure losses and purchases of more expensive instruments.
3.4 Fundamentally, the cost of the flow meter is mainly the cost of purchasing the instrument, and the operation and maintenance costs and the flow meter's flow adjustment and verification costs should also be considered.
The flow measurement is safe and reliable. First of all, the measurement method is reliable. That is to say, the sampling device will not cause mechanical or electrical circuit failures and cause accidents during operation. Second, the measurement instrument will not affect the safety of the production system under normal production or failure conditions. . When there are flammable gases, explosion-proof instruments should be used.
4 Conclusion Differential pressure gas flow meter selection must take into account a variety of factors, but also to identify the primary and secondary, comprehensive analysis, in order to find a suitable type of flow meter can not be one-sided pursuit of flowmeter performance in some aspects and ignore other Negative Effects.
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