Reasons for inaccurate flow meters

A standard orifice plate is a thin circular perforated plate obtained by mechanical processing. Its throttle hole cylindrical surface is perpendicular to the upstream end face of the orifice plate, and its edge is sharp. The ratio of orifice plate thickness to orifice plate diameter is relatively small. The part of the orifice plate inside the measuring tube should be circular and coaxial with the axis of the measuring tube. The two end faces of the orifice plate should always be flat and parallel
3.1 Eccentricity of orifice plate
According to GB2624-81, the orifice plate should be aligned with the straight pipe section in the throttling device. Experiments have shown that the measurement error caused by eccentricity of orifice plates is generally within 2%. The higher the aperture ratio β value, the greater the influence of eccentricity, and orifice plates with higher values should not be used.

3.2 Bending of orifice plate
Due to improper installation or maintenance. Causing the orifice plate to bend or deform, resulting in significant measurement errors in flow rate. The maximum error caused by the bending of the orifice plate during testing on the flange pressure tapping orifice plate is about 3.5%,
3.3 Edge sharpness of orifice plate
The wear and tear of the inlet edge of the orifice plate, or the occurrence of notches due to corrosion, or the welding seams or measuring flange gaskets inside the orifice plate pipeline, will increase the actual flow coefficient and decrease the differential pressure, resulting in a smaller calculated gas volume.
2、 Measures to improve measurement accuracy
1. Eliminate pulsating flow in the airflow
Due to sudden changes in gas flow velocity and pressure in pipelines, pulsating flow can occur, which can cause fluctuations in differential pressure. The flow calculation formula of throttling devices is based on the stable flow of the orifice plate. When there is pulsation at the measurement point, the stability principle cannot be established, which affects the measurement accuracy and generates measurement errors.
The total uncertainty of pulse flow is equal to the combination of measurement error calculated according to GB/T2624-93 and additional uncertainty of pulsation.
In the formula: ET - additional uncertainty of pulsation, dimensionless; -Axial time averaged velocity, m/s； -Root mean square value of velocity pulsation component, m/s。 (Formula application condition ≤ 0.32)
Therefore, in order to ensure the accuracy of natural gas metering, it is necessary to suppress pulsating flow. Common measures include:
(1) Under the condition of meeting the measurement capability, a measuring tube with a smaller inner diameter should be selected to increase the differential pressure and aperture ratio;
(2) Using short pressure pipelines to reduce resistance components in the pipeline, and making the length of upstream and downstream pipelines equal, to reduce resonance and increase pressure pulsation amplitude in the system;
(3) Eliminate free liquid from the pipeline, and the pulsation caused by the accumulation of liquid in the pipeline can be treated by using an automatic cleaning system or installing a liquid separator at a lower position.
2. The design and installation of measuring devices shall comply with SY/T 6143-1996
The fundamental reason affecting the measurement accuracy of orifice flow meters is whether the geometric shape and flow dynamics of the throttling device deviate from the design standards. Therefore, regular calibration and maintenance of the system must be carried out during use, and timely corrections should be made for changes in operating parameters such as pressure, temperature, and flow rate in actual use. A fully compensated flow computer integration scheme can be used to reduce measurement errors and ensure measurement accuracy.
3. Avoid human measurement errors
Strengthen measurement management, improve the technical quality of operators, and actively introduce and absorb natural gas measurement technology.
A standard orifice plate is a thin circular perforated plate obtained by mechanical processing. Its throttle hole cylindrical surface is perpendicular to the upstream end face of the orifice plate, and its edge is sharp. The ratio of orifice plate thickness to orifice plate diameter is relatively small. The part of the orifice plate inside the measuring tube should be circular and coaxial with the axis of the measuring tube. The two end faces of the orifice plate should always be flat and parallel
3.1 Eccentricity of orifice plate
According to GB2624-81, the orifice plate should be aligned with the straight pipe section in the throttling device. Experiments have shown that the measurement error caused by eccentricity of orifice plates is generally within 2%. The higher the aperture ratio β value, the greater the influence of eccentricity, and orifice plates with higher values should not be used.
3.2 Bending of orifice plate
Due to improper installation or maintenance. Causing the orifice plate to bend or deform, resulting in significant measurement errors in flow rate. The maximum error caused by the bending of the orifice plate during testing on the flange pressure tapping orifice plate is about 3.5%,
3.3 Edge sharpness of orifice plate
The wear and tear of the inlet edge of the orifice plate, or the occurrence of notches due to corrosion, or the welding seams or measuring flange gaskets inside the orifice plate pipeline, will increase the actual flow coefficient and decrease the differential pressure, resulting in a smaller calculated gas volume.
2、 Measures to improve measurement accuracy
1. Eliminate pulsating flow in the airflow
Due to sudden changes in gas flow velocity and pressure in pipelines, pulsating flow can occur, which can cause fluctuations in differential pressure. The flow calculation formula of throttling devices is based on the stable flow of the orifice plate. When there is pulsation at the measurement point, the stability principle cannot be established, which affects the measurement accuracy and generates measurement errors.
The total uncertainty of pulse flow is equal to the combination of measurement error calculated according to GB/T2624-93 and additional uncertainty of pulsation.
In the formula: ET - additional uncertainty of pulsation, dimensionless; -Axial time averaged velocity, m/s； -Root mean square value of velocity pulsation component, m/s。 (Formula application condition ≤ 0.32)
Therefore, in order to ensure the accuracy of natural gas metering, it is necessary to suppress pulsating flow. Common measures include:
(1) Under the condition of meeting the measurement capability, a measuring tube with a smaller inner diameter should be selected to increase the differential pressure and aperture ratio;
(2) Using short pressure pipelines to reduce resistance components in the pipeline, and making the length of upstream and downstream pipelines equal, to reduce resonance and increase pressure pulsation amplitude in the system;
(3) Eliminate free liquid from the pipeline, and the pulsation caused by the accumulation of liquid in the pipeline can be treated by using an automatic cleaning system or installing a liquid separator at a lower position.
2. The design and installation of measuring devices shall comply with SY/T 6143-1996
The fundamental reason affecting the measurement accuracy of orifice flow meters is whether the geometric shape and flow dynamics of the throttling device deviate from the design standards. Therefore, regular calibration and maintenance of the system must be carried out during use, and timely corrections should be made for changes in operating parameters such as pressure, temperature, and flow rate in actual use. A fully compensated flow computer integration scheme can be used to reduce measurement errors and ensure measurement accuracy.
3. Avoid human measurement errors
Strengthen measurement management, improve the technical quality of operators, and actively introduce and absorb natural gas measurement technology