What is the reason for the inaccurate measurement of the orifice flowmeter? Methods and techniques
Author: Source: Date: 2021-05-25 14:37:06
1. Reasons for inaccurate measurement of orifice flowmeter
1. The differential pressure pipeline is blocked, and the differential pressure pipeline is cleared;
2. When the differential pressure gauge fails, check the differential pressure gauge;
3. When the display value of the differential pressure transmitter is obviously shifted, check the size display value;
4. When the installation direction of the orifice parts is wrong, reinstall the orifice parts;
5. The working condition parameters of the measured medium are inconsistent with the parameters used in the design of the throttling device. Correct according to the relevant formula, and recalculate the differential pressure value if necessary;
6. When the length of the straight pipe before and after the throttling device is insufficient, the length of the straight pipe should be adjusted;
7. The inner diameter of the straight pipe section is out of tolerance, the inner diameter of the straight pipe section is actually measured, and the maximum flow rate is recalculated;
8. The throttle aperture is extremely poor, the actual throttle aperture is measured, and the maximum flow rate is recalculated;
9. The orifice component is deformed, replace the orifice component;
10. When there are attachments to the orifice, clean and replace the orifice;
11. The sharp side of the orifice is the inlet end facing the fluid flow, and the flared side is the outlet end. If you install it the other way round, the display will become much smaller. Check the installation direction of the orifice plate, and install the orifice plate correctly;
12. The entrance edge of the orifice plate is worn and the orifice plate has been used for a long time, especially when the measured medium is mixed with solid particles and other foreign objects, the geometry and size of the orifice plate will change, the orifice plate will become larger, or the edge of the orifice plate will change Blunt, the pressure difference is measured so it is necessary to rework the orifice plate;
13. The zero drift of the transmitter: long use time may cause the zero drift of the transmitter. When negative drift, the displayed pressure difference will decrease and the displayed flow will also decrease. Long-term calibration of the zero point of the transmitter
14. The length of the upstream and downstream straight pipes is not enough. If the length of the upstream and downstream straight pipe sections is not enough, the gas cannot be fully developed, and the measurement results will produce large errors. When there are multiple elbows in the upstream specified straight pipe section, the measurement result will be lower. The modified steam pipeline requires the length of the upstream and downstream straight pipe sections to meet the specified requirements. Add a rectifier in front of the throttling device
15. The three-valve group of the differential pressure transmitter leaks air. If the high-pressure valve cargo balance valve in the three-valve group leaks, the measured pressure difference value becomes smaller and the measurement result becomes lower. When the high pressure valve of the three-way valve group leaks, tighten the valve and replace it as needed. When the balance valve of the three-way valve group leaks, tighten the valve and replace it as needed. 2. Elimination methods and techniques for inaccurate measurement of orifice flowmeter
1. Eliminate the pulsating flow in the air flow
The pulsating flow is generated in the pipeline due to the rapid change of gas flow rate and pressure, which causes the change of differential pressure. The flowmeter formula of the throttling device is based on the stable flow of the orifice. If there is a pulsation phenomenon at the measuring point, the stability principle is not established. Affect the measurement accuracy and produce measurement errors.
The total uncertainty of the pulse flow is equal to the combination of the measurement error calculated by GB/T2624-93 and the additional uncertainty of the pulse.
Therefore, in order to ensure the measurement accuracy of natural gas, it is necessary to suppress the pulsating flow. General measures include:
1) Under the condition of satisfying the measurement ability, choose a measuring tube with a small inner diameter to increase the differential pressure and aperture ratio.
2) Use short wires to reduce the resistance components in the line, make the length of the upstream and downstream lines equal, reduce the resonance generated in the system and the increase of the pressure pulsation amplitude;
) 3) Remove the free liquid from the production line. The pulsation caused by the liquid storage in the production line can be handled by an automatic cleaning system or by installing a dispenser at a low place.
2. Design and installation of metering device
The fundamental reason that affects the measurement accuracy of the orifice flowmeter is whether the geometry and flow dynamics of the throttling device deviate from the design standard. Therefore, the system needs to be inspected and maintained regularly during use, and it must be modified in time for changes in operating conditions such as pressure, temperature, and flow in actual use. In order to reduce the measurement error and ensure the measurement accuracy, the cumulative plan of the flow computer that can be fully compensated is adopted.
3, avoid human measurement errors
Strengthen metering management, improve the technical quality of operators, and actively introduce and absorb advanced foreign gas metering technologies.
The standard orifice is a thin plate with a circular hole obtained by machining. The cylindrical cylindrical surface of the orifice is perpendicular to the upstream end surface of the orifice, and the edge is sharp, and the thickness of the orifice is smaller than the diameter of the orifice. The inside of the measuring tube of the orifice plate is divided into circles, which should be coaxial with the axis of the measuring tube. The two ends of the orifice plate should always be flat and parallel. However, when used in the field, the following abnormalities often occur.
) 1) Orifice eccentric
According to the provisions of GB2624-81, the orifice should be centered with the straight pipe section of the throttling device. Experiments show that the measurement error caused by the eccentricity of the orifice is usually within 2%. The higher the aperture ratio, the greater the influence of the eccentricity, and the higher the orifice is not used. ) 2) Orifice plate bending
because of improper installation and maintenance. The orifice is bent and deformed, and the flow measurement error increases. For testing with punched flanged orifice plates, the maximum error caused by orifice plate bending is about 3.5%.