Analysis of the Causes of Errors in Orifice Flowmeters

Orifice flowmeter is the most widely used in natural gas metering. The orifice flow measurement system is generally composed of a throttling device (standard orifice), a differential pressure transmitter and a data processor (square root integrator or computer). The main features of orifice flowmeter: 
Advantages:

(1) Suitable for measuring larger diameter pipelines (currently, flow meters with diameters greater than DN600 can generally only use orifice plates). 
(2) No moving parts, durable. 
(3) Long history of application, most complete standards. 
(4) Relatively easy to manufacture, cheap. 
In order to improve the accuracy of orifice flowmeters in natural gas measurement, analyzing and understanding the causes of errors in the measurement device itself during use is an essential and important task in measurement work.　 
2. Measurement principle 
The flow velocity will form a local contraction at the throttling device, so the flow velocity increases and the static pressure decreases, so a pressure difference is generated before and after the throttling device. The larger the fluid flow rate, the greater the pressure difference generated, so the flow rate can be measured based on the pressure difference. This measurement method is based on the flow continuity equation (law of conservation of mass) and the Bernoulli equation (law of conservation of energy). The size of the pressure difference is not only related to the flow rate but also to many other factors. For example, when the form of the throttling device or the physical properties of the fluid in the pipeline (density, viscosity) are different, the pressure difference generated at the same flow rate is also different. Based on the Bernoulli equation and the continuity equation of fluid flow. 
3. Analysis of error factors 
1. Basic error 
is the error determined by the accuracy of the measuring device itself. 
Since the flow measured by the orifice flowmeter is indirectly obtained based on the differential pressure signal. As can be seen from the formula, the main factors affecting the measurement accuracy are α, F, ε and other values. α, F, and ε must be kept at constant values ​​to achieve a constant correspondence between differential pressure and flow. However, in actual production, α, F, and ε are all related to certain factors, especially the flow coefficient α, which is an important coefficient with complex influencing factors and a large range of variation. If α cannot be guaranteed to be a constant value during the measurement process, the measurement error will be large. It 
has been proved from data and experiments that the flow coefficient α value is related to the position of the pressure point, the ratio of the opening cross-sectional area of ​​the orifice plate to the cross-sectional area of ​​the pipeline (m= d2 /D2), the Reynolds number, the roughness of the pipe wall, and the sharpness of the edge of the orifice plate entrance. Therefore, the selection of standard orifice plates should meet the following technical requirements:

(1) Technical requirements for standard orifice plates 
①. The orifice plate is symmetrical with respect to the axis of the opening diameter d; 
②. The upstream end face A of the orifice plate should be parallel to the downstream end face B and perpendicular to the axis of the opening diameter d; 
③. There should be no visible damage on the A and B surfaces of the orifice plate; 
④. The upstream right-angle entrance edge G of the orifice opening diameter d should be sharp, without burrs and scratches. 
⑤. The length e of the inner cylindrical surface of the orifice opening diameter d should meet the following conditions: 0.005D≤e≤0.02D; 
⑥. The thickness of the orifice plate should meet the following conditions: e≤F≤0.005D; 
⑦. The downstream outlet edge H of the orifice plate and the downstream outlet edge I of the orifice opening cylinder should be free of burrs, scratches and visible damage; 
⑧. During use, the standard orifice plate will be deformed due to the erosion of natural gas, which will cause the flow coefficient to increase and produce measurement errors. Therefore, the material of the orifice plate is also a factor in ensuring the reliable operation and accurate measurement of the orifice plate. Orifice plates used for natural gas measurement are generally made of: Cr17, 1Cr18Ni9TiC and other acid-resistant steels. 
The accuracy of the entire orifice flowmeter is also determined by the accuracy of the differential pressure transmitter and the flow display. However, when the accuracy of other parameters is not high, the use of high-precision differential pressure transmitters will not play a big role. The role of the measurement display is mainly to monitor the stability of operating parameters, and its data conversion accuracy is generally not a problem. Therefore, to improve the accuracy of the measurement, there should be a comprehensive estimate, so that the best technical and economic solution can be selected. 
2. Additional error 
is the error found during the installation and use of the flowmeter without strictly complying with the technical requirements and design data. The size of this error cannot be estimated. However, by analyzing the factors causing the error, the error can be reduced. When the orifice plate flow measurement system is installed and used, it must meet the following technical requirements: 
①. When the orifice plate is installed, its opening center is concentric with the central axis of the pipeline, and its end face is perpendicular to the pipeline axis; 
②. The axis of the pressure holes on the upper and lower downstream sides of the orifice plate meets the requirement of 25.4±0.8mm from the upper and lower downstream end faces of the orifice plate; 
③. The axis of the pressure hole should be perpendicular to the axis of the inner cylinder of the 2D measuring tube length on the upstream and downstream sides of the orifice plate, and the angle between the axis of the pressure hole and the outward inclination angle of the two ends of the orifice plate should not exceed 3 degrees. The diameter of the pressure hole should not be greater than 0.08D; 
④. The installation of the orifice plate requires straight pipe sections before and after, and the length of the straight pipe section is related to the form and diameter ratio β of the local resistance member on the upstream side of the orifice plate. 
⑤. Installation of differential pressure signal pipeline 
The differential pressure signal pipeline refers to the pressure pipeline between the throttling device and the differential pressure transmitter (or differential pressure gauge). It is the weak link of the orifice flowmeter. According to statistics, the pressure pipeline is the most common fault in the orifice flowmeter, such as blockage, corrosion, leakage, freezing, false signals, etc., accounting for about 70% of the total failure rate. Therefore, the configuration and installation of the differential pressure signal pipeline should be given great attention. 
(1) Pressure tapping port The pressure tapping port is generally set on the flange. When the measuring pipeline is horizontal or inclined, the installation direction of the pressure tapping port is shown in Figure 2. It can prevent gas from entering the pressure pipe when measuring liquid or droplets or dirt from entering the pressure pipe when measuring gas. When the measuring pipeline is vertical, the position of the pressure tapping port is on the plane of the pressure tapping position, and the direction can be selected arbitrarily. 
(2) Pressure pipe The material of the pressure pipe should be determined according to the properties and parameters of the measured medium. Its inner diameter should not be less than 6mm, and the length should preferably be within 16mm. The recommended values ​​of the inner diameter of the pressure pipe for various measured media at different lengths are shown in the following table. The pressure pipe should be laid vertically or inclined, with a starting inclination of not less than 1:12. For fluids with high viscosity, the inclination should be increased. When the length of the pressure pipe exceeds 30mm, the pressure pipe should be tilted in sections, and gas collectors (or exhaust valves) and precipitators (or sewage valves) should be installed at the highest and lowest points. Positive and negative pressure pipes should be laid as close as possible to prevent signal distortion due to different temperatures of the two pipes. In cold areas, the pressure pipe should be protected from freezing and heated with electricity or steam to prevent overheating. The vaporization of the fluid in the pressure pipe will produce false differential pressure. 
⑥. The thermometer for measuring gas temperature is preferably installed outside the straight pipe section on the downstream side of the orifice plate. When calculating the gas volume, the measured air flow temperature at this time should be converted into the air flow temperature on the upstream side of the orifice plate. If the thermometer is installed on the upstream side of the orifice plate, the straight pipe section length L between the thermometer sleeve and the orifice plate should meet the following requirements: 
when the diameter of the thermometer sleeve is not greater than 0.03D (D is the inner diameter of the metering tube), L ≥ 5D; 
when the diameter of the thermometer sleeve is in the range of 0.03D to 0.13D (D is the inner diameter of the metering tube), L ≥ 20D; The 
orifice throttling device is installed in a harsh workplace on site. After long-term operation, both the pipeline and the throttling device will undergo some changes, such as blockage, scaling, wear, corrosion, etc. The detection part relies on the structural shape and size to maintain the accuracy of the signal, so any change in geometric shape and size will bring additional errors. The trouble is that the change in measurement error cannot be detected from the signal, so it is necessary to check the detection part regularly. The inspection cycle can be determined according to the situation of the measured medium. 
The above is the reason for the measurement error analyzed, the purpose is to eliminate or reduce the error as much as possible. We should take appropriate measures according to the cause of the error to improve the accuracy of natural gas flow measurement.