Principle and Instructions of Turbine Flowmeter

Gas turbine flowmeter has the advantages of high sensitivity, good repeatability, wide range ratio and high precision, and 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 my country and the continuous improvement of gas commercial trade and handover measurement requirements, gas turbine flowmeter has gradually become one of the instruments for urban gas commercial trade and handover measurement in my country.

1. The 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 fluid volume flow rate. According to the principle of electromagnetic induction, a magnetic sensor is used to sense a pulse signal proportional to the fluid volume flow rate from the synchronously rotating impeller, and the volume flow rate 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 Error

Turbine flowmeters also have the following disadvantages: there are moving 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 include: quality problems of the meter itself, unreasonable design and selection, inadequate installation, improper maintenance during operation, etc.

3. How to control errors

1. Correctly determine the location and specifications of the flow meter.

Due to the existence of turbine inertia, the turbine flowmeter should not be used in occasions where the flow rate fluctuates frequently, otherwise the measurement accuracy will be reduced. To estimate the peak and valley values ​​of gas consumption and the pressure of the medium more accurately, the specifications of the flowmeter should be correctly determined. From the error characteristic curve of the turbine flowmeter, it can be seen that the working flow range of the flowmeter should be 20%Qmax-80%Qmax (Qmax is the maximum flow rate 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 filter inlet and outlet), the filter should be cleaned in time. If a differential pressure gauge is not equipped, the filter should be cleaned once a month.

2. Ensure the straight pipe section requirements, especially when there is a reduced diameter or half-open valve before the meter.

3. During installation, the sealing gasket shall not protrude into the pipeline, there shall be no obvious deviation between the flow meter and the pipeline axis, and no installation stress shall be generated.

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

(III) Turbine flowmeter operation management and maintenance requirements

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 with 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 over-flow operation. Over-flow operation will seriously affect the service life, reduce the measurement accuracy, and increase the error; (Note that the flow percentage of the meter head should not be over-flow 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 of the meter should be between 20%-70% when it is running. 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 condition (m3)

Zg——Gas compression coefficient under working conditions

P=Pa+Pg——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——Temperature under standard conditions (293.15K)

T——Temperature under medium working conditions (273.15+t)K

t——Celsius temperature of the medium being measured (e)

FZ——Gas compressibility factor

It can be seen from the formula that the errors are 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 indicated values ​​are unstable, or when there are large deviations from previous empirical values, timely processing is required.

4. During routine maintenance or meter reading, check whether there are any 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 or abnormal warning symbols, they should be discovered in time to help 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 rate on the corrector in a timely manner to see if they are consistent. Under normal circumstances, the two should not differ much.

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

7. To ensure the normal operation of the turbine flowmeter for a long time, 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 in time to check. If the turbine bearing is severely worn or the blades are damaged, they 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 shorten service life;

4. Carry out weekly inspection of turbine flowmeter

Each flow sensor should have an instrument coefficient given by the standard device calibration, and its accuracy and flow range must meet the requirements of the flow meter technical specifications. The correct instrument coefficient value is the most basic guarantee to ensure the flow meter's measurement accuracy. To ensure the flow meter's measurement accuracy, 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 inspections and maintenance during use. Pay attention to periodic maintenance and calibration: The flow meter in use should be periodically calibrated and calibrated according to relevant standards, generally once every two years, which is a necessary measure to ensure that the flow meter maintains its accuracy; JJG198 "Velocity Flow Meter Calibration Procedure": The calibration cycle for flow meters with an accuracy of 0.1, 0.2, and 0.5 is half a year. For flow meters with accuracy lower than level 0.5, the calibration cycle is determined according to their working principle: 1 year for split-flow rotor flow meters; 2 years for turbine flow meters, vortex flow meters, swirl flow meters, and electromagnetic flow meters; 3 years for ultrasonic flow meters and laser Doppler flow meters; insertion flow meters are carried out according to the calibration cycle of flow meters with the same working principle as their measuring heads.

5. Determination and handling of common practical problems.

1. 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 filter is clogged with debris and the filter should be cleaned.

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

(ii) When the fluid flows normally, there is no display and the total amount counter does not increase.

You should check:

1. Check if there is any foreign matter stuck on the impeller, or if there is any foreign matter stuck on the shaft and bearing or if there is any breakage. Remove the foreign matter, and clean or replace the damaged parts. After restoration, blow the impeller with air or move it by hand, and there should be no friction sound.