How to Improve the Measuring Accuracy of Turbine Flowmeter

Gas turbine flowmeter has the advantages of high sensitivity, good repeatability, wide range ratio and high precision. It has been widely used in natural gas trade settlement measurement and even as a standard instrument for value transfer. With the full development of urban gas projects in China and the continuous improvement of gas commercial trade and handover measurement requirements, gas turbine flowmeter has gradually become one of the preferred instruments for urban gas commercial trade and handover measurement in China.

1. Principle of turbine flowmeter

The turbine flowmeter is a velocity flowmeter that uses gas to drive the flowmeter impeller to rotate. The speed of the impeller rotation is proportional to the volume flow of the fluid. According to the principle of electromagnetic induction, a magnetic sensor is used to sense a pulse signal proportional to the volume flow of the fluid from the synchronously rotating impeller, and the volume flow is obtained through calculation and processing. It has high measurement accuracy, and the accuracy level can reach 1.0 and 1.5; the range ratio is wide, generally 1:20, and the measurement range is wide; the structure is compact and lightweight, and the installation and maintenance are convenient. The requirements for the front and rear straight pipe sections are low, and it can be used for medium and high pressure measurement.

2. Causes of Errors

Turbine flowmeters also have the following disadvantages: there are movable parts that are easy to damage, key bearings are easy to wear, poor anti-fouling ability, high requirements for the cleanliness of the medium, it is difficult to maintain calibration characteristics for a long time, and regular calibration is required. The causes of errors are: quality problems of the meter itself, unreasonable design and selection, inadequate installation, improper maintenance during operation, etc.

3. How to control errors

(I) Correctly determine the place and specifications of the flowmeter.

Due to the existence of turbine inertia of the turbine flowmeter, it is not suitable to be used in occasions where the flow fluctuates frequently, otherwise the measurement accuracy will be reduced. It is necessary to estimate the peak and valley values ​​of gas consumption and the pressure of the medium more accurately and correctly determine the specifications of the flowmeter. It can be seen from the error characteristic curve of the turbine flowmeter that the working flow range of the flowmeter should be 20%Qmax-80%Qmax (Qmax is the maximum flow of the flowmeter)

(II) Turbine flowmeter installation requirements

1. A filter must be installed in front of the gas turbine flowmeter; the filter should be kept unobstructed. If the filter is found to be blocked (which can be judged by the pressure difference between the inlet and outlet of the filter), the filter should be cleaned in time. If a differential pressure gauge is not equipped, it should be cleaned once a month.

2. The requirements of the straight pipe section must be met, especially when there is a reduced diameter or half-open valve in front of the meter.

3. During installation, the gasket must not protrude into the pipeline, and there must be no obvious deviation between the flow meter and the pipeline axis when visually observed, and no installation stress must be generated.

4. During installation, all impurities in the pipeline must be cleaned to prevent the bearings and turbine from getting stuck.

(III) Operation management and maintenance requirements

of turbine flowmeters 1. Ventilation and shut-off requirements for turbine flowmeters. Ventilation sequence: Ensure that the valve at the rear end of the flowmeter is in the closed state; then slowly open the valve at the front end of the flowmeter to ensure that the pressure increase rate is ≤35KPa/S; finally, slowly open the valve at the rear end of the flowmeter to allow it to run at a small flow rate until it is adjusted to the required value. The entire process is started under pressure. Shut-off sequence: first slowly close the valve at the rear end of the flowmeter; then slowly close the valve at the front end of the flowmeter.

2. Prevent long-term overload operation. Over-flow operation will seriously affect the service life, reduce the measurement accuracy, and cause the error to increase; (pay attention to the meter head working condition flow percentage should not exceed 100% for a long time) Instantaneous flow: from the observation of instantaneous flow, combined with the user's gas usage at the time, judge whether there is a small fire that does not go away, and a large fire that exceeds the range. The flow range when the meter is running should be between 20%-70%. If it is operated at a low limit or a high limit for a long time, it will affect the measurement; whether the user's gas load or gas equipment has changed, it should be solved in time.

3. Pay attention to the values ​​of temperature and pressure. According to the gas equation:

In the equation: V0 - volume under standard conditions (m3)

V - volume under working conditions (m3)
Zg - gas compression coefficient under working conditions

P = Pa + Pg - absolute pressure at the flow meter pressure detection point (kPa)

Pa - local atmospheric pressure (kPa)
Pg - gauge pressure at the flow meter pressure detection point (kPa)
Po - standard atmospheric pressure (101.325kPa)
To - absolute temperature under standard conditions (293.15K)
T - absolute temperature under medium working conditions (273.15 + t) K
t - Celsius temperature of the measured medium (e)
FZ - gas compression factor

It can be seen from the formula that the error is mainly concentrated in the detection accuracy of pressure and temperature. When it is found that the flow, temperature, and pressure values ​​deviate greatly from the actual values ​​or the indication is unstable, or when there is a large deviation from the previous experience values, it should be handled in time.

4. During daily maintenance or meter reading and inspection, check whether there are abnormal symbols on the display instrument. If the battery symbol flashes, it means that the battery is running out of power and the battery should be replaced in time; if there are abnormal alarm and abnormal warning symbols, it should be discovered in time, which will help to deal with and discover the user's illegal gas use behavior.

5. For imported turbine meters with mechanical readings and correctors, in addition to reading the standard volume value, the base meter reading should be compared with the operating flow on the corrector in time. Under normal circumstances, the two should not differ much.

6. When inspecting the process pipeline, the flow meter should be removed and the two ends should be wrapped with a clean cloth to prevent dirt, iron, etc. from falling into the flow meter and damaging the turbine blades.

7. In order to ensure the long-term normal operation of the turbine flowmeter, the operation inspection of the instrument should be strengthened, and the rotation of the impeller should be monitored. If the sound is abnormal, the internal parts of the sensor should be removed and checked in time. If the turbine bearing is severely worn or the blades are damaged, it must be repaired, replaced, and re-calibrated.

8. For sensors with lubricating oil or cleaning fluid injection ports, lubricating oil or cleaning fluid should be injected regularly as required to ensure the good operation of the impeller. Long-term continuous operation without lubricating oil will inevitably cause fatal wear, increase damping force and cause slower operation, negative difference in measurement results and affect service life;

4. Carry out weekly inspection of turbine flowmeter.

Each flow sensor should have an instrument coefficient given by standard device calibration, and its accuracy and flow range must meet the requirements of flowmeter technical specifications. The correct instrument coefficient value is the most basic guarantee for flowmeter measurement accuracy. To ensure accurate flowmeter measurement, the primary method is to input the instrument coefficient and specification model into the temperature and pressure compensator through calibration, and ensure that the flow sensor is not out of tolerance through weekly inspection and maintenance during use. Pay attention to periodic maintenance and calibration: The flowmeter in use should be periodically calibrated and calibrated according to relevant standards. Generally, it should be calibrated once every two years. This is a necessary measure to ensure that the flowmeter maintains accuracy; JJG198 "Velocity Flowmeter Calibration Procedure": The calibration cycle of flowmeters with accuracy levels of 0.1, 0.2, and 0.5 is half a year. For flowmeters with accuracy lower than 0.5, the calibration cycle is determined according to their working principles: 1 year for split-flow rotor flowmeters; 2 years for turbine flowmeters, vortex flowmeters, swirl flowmeters, and electromagnetic flowmeters; 3 years for ultrasonic flowmeters and laser Doppler flowmeters; and the calibration cycle for insertion flowmeters is carried out according to the flowmeters with the same working principle as their measuring heads.

V. Judgment and treatment of common practical problems.

(I) No flow reduction operation (throttling and load reduction) is performed, but the flow display gradually decreases. Possible situations:

1. The filter is clogged. If the pressure loss at the front and rear ends of the filter is large, it means that the debris has been blocked, and the filter should be cleaned.

2. The sensor impeller is blocked by debris or foreign matter enters the bearing gap, the resistance increases and the deceleration slows down. The impeller and sensor should be removed and cleaned.

(II) There is no display when the fluid flows normally, and the number of words in the total counter does not increase.

Check:

1. Whether the impeller is stuck with foreign matter, and whether the shaft and bearing are stuck with foreign matter or broken. Remove foreign matter and clean or replace damaged parts. After restoration, blow air or move the impeller manually. There should be no friction sound.

2. Is the sensor joint loose?

(III) If the display value is significantly different from the empirical evaluation value, the following checks should be performed.

1. Reasons related to pipeline flow, such as reverse flow without a check valve.
2. Is the bypass valve closed tightly and is there any leakage? (end)