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Description of the accuracy of the orifice flowmeter in natural gas metering

Measuring principle
In order to improve the accuracy of the orifice meter in natural gas metering, it is an important task in the measurement work to analyze and master the error of the measuring device itself during use. The flow rate will create a local contraction at the throttle, so that the flow rate increases and the static pressure decreases, so that a pressure differential is created before and after the throttle. The larger the fluid flow rate, the greater the pressure difference produced, so that the flow rate can be measured by the differential pressure. This measurement method is based on the flow continuity equation (the law of conservation of mass) and the Bernoulli equation (the law of conservation of energy). The magnitude of the pressure difference is not only related to the flow rate but also to many other factors. For example, when the physical properties (density, viscosity) of the fluid in the form of the throttling device or the pipe are different, the pressure difference generated under the same flow rate is also different. Based on the Bernoulli equation and the continuity equation of fluid flow.
Error factor analysis
The orifice throttling device is installed in the harsh workplace. After long-term operation, some changes will occur in the pipeline or throttling device, such as blockage, scaling, wear, corrosion and so on. The detection piece relies on the shape and size of the structure to maintain the accuracy of the signal, so any geometric shape and size changes will bring additional errors. The trouble is that the change in measurement error is not noticeable from the signal, so it is necessary to periodically check the test piece. The period of the inspection can be determined based on the condition of the measuring medium.
Basic error
The error determined by the accuracy of the measuring device itself.
Since the flow rate measured by the orifice flow meter is indirectly obtained from the differential pressure signal. It can be seen from the formula that the main factors affecting the measurement accuracy are α, F, ε and so on. It is necessary to keep α, F, and ε all constant values in order to achieve a constant correspondence between the differential pressure and the flow rate. However, in actual production, α, F, and ε are related to some factors, especially the flow coefficient α, which is an important factor with complex influence factors and large variation range. If α cannot be guaranteed to be constant during the measurement process, Then the measurement error will be larger.
From the data and experiments, the flow coefficient α value and the pressure point location, the orifice cross-sectional area of the orifice plate and the cross-sectional area of the pipe (m = d2 / D2), Reynolds number, pipe wall roughness, orifice edge sharpness related. Therefore, the selection of standard orifices should meet the following technical requirements:

(1) Technical requirements for standard orifice plates
1. The orifice plate is symmetrical with respect to the axis center of the opening diameter d;
2. The length of the inner cylindrical surface e of the hole diameter d of the orifice plate shall be in accordance with: 0.005D ≤ e ≤ 0.02D;
3, the thickness of the orifice plate should meet: e ≤ F ≤ 0.005D;
4. The outlet edge H of the downstream side of the orifice plate and the outlet edge I of the cylindrical downstream side of the orifice opening should be free of burrs, scratches and visible damage;
5. During the use of the standard orifice plate, deformation will occur due to the erosion of natural gas, causing the flow coefficient to increase and the measurement error 66. The upstream end face A of the orifice plate should be parallel to the downstream end face B, and the diameter of the opening d is The axis is vertical;
7. There should be no visible damage on the A and B faces of the orifice plate;
8. The upstream edge of the hole diameter d of the orifice plate should be sharp, without burrs and scratches.

The accuracy of the entire orifice flow meter is also determined by the accuracy of the differential pressure transmitter and flow meter. However, when the accuracy of other parameters is not high, the use of high-precision differential pressure transmitters does not play much role. The function of the measurement display is mainly in monitoring the stability of the operating parameters, and its data conversion accuracy is generally no problem. Therefore, there should be a comprehensive estimate of the accuracy of the measurement in order to make the best choice for the technical economy. Therefore, the material of the orifice plate is also a factor to ensure reliable operation and accurate measurement of the orifice plate. Orifice for natural gas measurement is generally: Cr17, 1Cr18Ni9TiC and other acid-resistant steel.
2, additional error
During the installation and use of the flowmeter, the errors found in the technical requirements and the data provided by the design are not strictly adhered to. The magnitude of this error is not measurable. However, the error can be reduced by analyzing the error factors. When installing the orifice flow measurement system, the following technical requirements must be met:
1. When the orifice plate is installed, his opening center is concentric with the central axis of the pipe, and its end face is perpendicular to the pipe axis;
2. The installation of the orifice plate requires a straight pipe section before and after. The length of the straight pipe section is related to the form and diameter ratio β of the local resistance part on the upstream side of the orifice plate.
3. The axis of the pressure-receiving hole on the orifice plate and the downstream side meets the requirement of 25.4±0.8 mm from the orifice plate and the downstream end face;
4. The axis of the pressure-receiving hole should be perpendicular to the axis of the inner cylinder of the length of the 2D measuring tube on the downstream side of the orifice plate, and the angle between the axis of the pressure-receiving hole and the outer inclined angle of the two ends of the orifice plate is not more than 3 degrees. The diameter of the pressure tapping hole should not exceed 0.08D;
5. The thermometer for measuring the gas temperature is preferably installed outside the straight pipe section on the downstream side of the orifice plate. When calculating the gas volume, the temperature of the gas flow measured at this time should be converted into the temperature of the gas flow on the upstream side of the orifice plate, if the thermometer is installed upstream of the orifice plate. On the side, the length L of the straight pipe between the thermometer casing and the orifice plate shall meet the following requirements:
When the diameter of the thermowell is not more than 0.03D (D is the inner diameter of the measuring tube), L ≥ 5D;
When the diameter of the thermowell is in the range of 0.03D to 0.13D (D is the inner diameter of the measuring tube), L ≥ 20D;
6, the installation of differential pressure signal pipeline
The differential pressure signal line refers to the pressure guiding line of the throttling device and the differential pressure transmitter (or differential pressure gauge). It is the weak link of the orifice flowmeter. According to the statistics of the fault of the orifice flowmeter, the most pressured pipelines, such as blockage, corrosion, leakage, freezing, false signals, etc., account for about 70% of the total failure rate, so The configuration and installation of differential pressure signal lines should be highly valued.
(1), the pressure port The pressure port is generally set on the flange, when the measuring pipe is horizontal or inclined, the mounting direction of the pressure port is shown in Figure 2. It can prevent liquid or liquid from entering the pressure guiding tube when the liquid enters the pressure guiding tube or when measuring the gas. When the measuring pipe is vertical, the position of the pressure tapping port is on the plane of the pressure taking position, and the direction can be arbitrarily selected.
(2) The material of the pressure guiding tube shall be determined according to the nature and parameters of the medium to be tested. The inner diameter is not less than 6mm, the length is preferably within 16mm, and the inner diameter of the guiding tube is different when the measured medium is of different length. The recommended values are shown in the table below. The pressure guiding tube should be laid vertically or obliquely, and the inclination should be no less than 1:12. The viscosity of the fluid with high viscosity should be increased. When the length of the pressure guiding tube exceeds 30mm, the pressure guiding tube should be inclined in sections, and the collector (or exhaust valve) and the precipitator (or drain valve) should be installed at the highest point and the lowest point. The positive and negative pressure guiding pipes should be laid as close as possible to prevent the two pipes from being different in temperature to make the signal distorted. In the cold area, the pressure guiding pipe should be protected from frost, heat or heat by steam, to prevent overheating, and the fluid vaporization in the pressure guiding pipe will produce a false error. Pressure.

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