Ensuring the Accuracy of Gas Turbine Flowmeters
Gas turbine flowmeters, boasting advantages such as high sensitivity, excellent repeatability, a wide turndown ratio, and high accuracy, have been widely used in natural gas trade settlement metering. They are even employed as standard instruments for quantity transfer. With the full-scale development of urban gas projects in China and the ever-increasing requirements for gas commercial trade and handover metering, gas turbine flowmeters have gradually become one of the preferred instruments for urban gas commercial trade and handover metering in the country.
As a velocity-type flowmeter, the gas turbine flowmeter operates on the principle that gas drives the flowmeter's impeller to rotate. The rotational speed of the impeller is proportional to the volumetric flow rate of the fluid. Based on the electromagnetic induction principle, a magnetosensitive sensor induces pulse signals proportional to the volumetric flow rate of the fluid from the synchronously rotating impeller. After calculation and processing, the volumetric flow rate is obtained. It features high measurement accuracy, with an accuracy class of up to Class 1.0 or Class 1.5; a wide turndown ratio, generally 1:20, and a broad measurement range; a compact and lightweight structure, easy installation and maintenance, and low requirements for straight pipe sections at the front and rear. It can be used for medium and high-pressure metering.
However, turbine flowmeters also have the following drawbacks:
They contain moving parts that are prone to damage. The key component, the bearing, is liable to wear. They have poor anti-contamination capability and high requirements for the cleanliness of the medium. It is difficult to maintain their calibration characteristics for a long time, and regular verification is required.
The causes of measurement errors include: quality issues of the meter itself, unreasonable design and selection, improper installation, and inadequate maintenance during operation.
So, how to control these errors?
Correctly determine the application site and specifications of the flowmeter: Due to the inertia of the turbine in the turbine flowmeter, it is not suitable for use in occasions where the flow rate fluctuates frequently, otherwise, the measurement accuracy will be reduced. It is necessary to relatively accurately estimate the peak and valley values of gas consumption and the pressure of the medium to correctly determine the specifications of the flowmeter.
A filter must be installed in front of the gas turbine flowmeter: The filter should be kept unobstructed. If blockage of the filter is detected (which can be judged by the pressure difference between the inlet and outlet of the filter), the filter should be cleaned in a timely manner. For filters not equipped with a differential pressure gauge, cleaning should be performed once a month.
The requirements for straight pipe sections must be met, especially when there is a reduced diameter or a half-open valve in front of the meter. During installation, the gasket must not protrude into the pipeline. There should be no obvious deviation between the flowmeter and the axis of the pipeline (as observed visually), and no installation stress should be generated. During installation, all impurities in the pipeline must be thoroughly cleaned to prevent the bearing and turbine from seizing.
