Natural gas is a clean energy source and occupies an important position in China’s strategy. With the completion of the national West-East gas pipeline and the continuous improvement of the city gas network, the importance of scientific and accurate natural gas trade settlement is highlighted. Because of its significant advantages in accuracy, repeatability, zero-point drift, and range ratio, the turbine flowmeter has become a widely used velocity flowmeter.
Its bearings are well made; the guide plate is specially designed to reduce wear and tear and as well as to ensure the reliability of the measurement under extreme conditions; its output signal is a pulse that is easy to digitize.
Turbine flowmeter coefficient and linearity are less affected by pressure changes, such as a particular function of the secondary instrumentation, quantitative control, over-volume alarm, so it has been widely used in the oil field liquefied gas, natural gas.
This paper mainly introduces the application of gas turbine flowmeter in natural gas metering from 5 aspects: characteristics, principles, types, applications, and shortcomings.
Characteristics of gas turbine flowmeters
As turbine flow meters are simple and easy to install, they are widely used in natural gas metering, both in-line and walk-around.
1.1 Intelligent measurement and compensation
The gas turbine flowmeters can directly measure the standard volume of gas or the mass flow of saturated steam and the acquisition of measurement data on the temperature and pressure of the medium, with automatic compensation of the flow and automatic correction of the compression factor.
1.2 Advanced manufacturing processes
The gas turbine flowmeter uses advanced microcomputer technology and high-performance integrated chip, the use of advanced micro-power high-tech, low power consumption of the whole machine, both by the internal battery for long-term operation, but also by external power supply operation, the use of LCD display for easy reading, output pulse signal, but also output (4 ~ 20) mA standard analog signal.
1.3 High degree of automation
With a communication interface to communicate with the host computer, a host computer can communicate with more than one flowmeter to facilitate centralized management.
The gas turbine flowmeter working principle
The basic principle of the gas turbine flowmeter is that when fluid flowing along the axis enters the flow sensor, the blades force the fluid to rotate and then create a vortex in the center of the vortex trailing body.
The vortex flows in the venturi is accelerated by a sudden throttling when it reaches the constriction section. When the vortex enters the diffusion zone, the return flow forces it to undergo a second rotation. In this case, the frequency of course of the vortex is linearly proportional to the speed of the medium. The weak charge signal detected by the piezoelectric sensor is amplified, filtered, and shaped by a preamplifier and then converted into a pulse signal, the frequency of which is proportional to the flow velocity. Finally, the signal is sent to an integrator for counting and processing.
The microprocessor in the flow meter can obtain the density and pressure of the medium according to the temperature measurement value, and complete the calculation of the volume and mass flow of the medium; it can also compensate for temperature and pressure according to the gas equation, and automatically correct the compression factor.
In the case of mechanical counter turbine flow meters, the counter is rotated and counted by a drive mechanism. In the case of flow meters with electronic flow integrators, a pulse signal representing the turbine speed is output via a rotating disc or signal transducer and an amplifier circuit. The pulse signal’s frequency is proportional to the volumetric flow rate of the fluid.
Types of gas turbine flowmeters
1. Basic turbine flowmeter
The sensor does not have a field display and only transmits the flow signal remotely. The flow signal can be classified as a pulse or current signal. The suitable price, high level of integration, and compact size make the gas turbine flowmeter particularly ideal for use with secondary displays, PLC, DCS, and other computer control systems.
2. Smart turbine flow meters
Intelligent turbine flowmeter combines the turbine flow sensor and advanced ultra-low-power single-chip microcomputer technology to develop a display integrated instrument; it has a compact structure, high reliability, resistance to external power interference, lightning strike, low cost, and other advantages. It also can be on-site meter coefficient three-point correction, intelligent compensation meter coefficient non-linearity, while displaying instantaneous flow, cumulative flow, and temperature, pressure, and other measurement parameters, with real-time data storage function and explosion-proof function, especially suitable for use with PLC, DCS, and other computer control systems.
The use of gas turbine flowmeters
Gas turbine flow meters have been widely used for natural gas flow measurement due to their high sensitivity, good repeatability, large range ratio, and high accuracy.
Gas turbine flow meters are more suitable than differential pressure flow meters when the flow rate varies considerably. With the development of gas metering technology and the increased requirements for gas trading and handover metering, the gas turbine flowmeter can meet trade metering requirements and a wide measurement range. In addition, it is less costly than orifice flow meters and gas ultrasonic flow meters. Therefore, the gas turbine flowmeter is more and more widely used, especially in China’s natural gas market, usage increased significantly. Still, the prevalence of downstream market usage is low, the natural gas pipeline often in the initial low-pressure operation, after a period of process gradually increase the pressure, increase the supply, turbine flowmeter with its wide range ratio can reduce the diameter of the measurement pipe, thus reducing investment.
The accuracy of a gas turbine flowmeter is high level, the accuracy of ordinary flowmeter for (± 1 ~ ± 1 5) %, particular type for (± 0.5 ~ ± 0.2) %, but its accuracy is higher, the more demanding the requirements of the site conditions. Turbine flowmeters have good repeatability, with short-term repeatability of (0.05 to 0.2%). Therefore, if calibrated regularly, it can be achieved a high degree of accuracy.
Gas turbine flowmeter is compact in structure and light in weight, it is easy to install and maintain, it has a large flow capacity, so it is suitable for high-pressure measurement. The meter body does not need to open a hole; it is easy to make high-pressure type meters, it also can meet the requirements of the pipeline pressure. Because of these advantages, it is one of the preferred flowmeters in the natural gas trade settlement.
Defects of gas turbine flow meters
The gas turbine flowmeter is challenging to maintain stable metering characteristics over time; it needs to be regularly checked or calibrated. For trade storage and transportation and high accuracy measurement requirements, the flowmeter needs to be periodically online real flow calibration to maintain its characteristics. At the same time, the gas density has a more significant impact on the meter accuracy, as density and temperature and pressure are closely related, the field temperature and pressure fluctuations are inevitable, according to its impact on the flowmeter metering characteristics to take compensatory measures to maintain a high degree of accuracy. Therefore, while maintaining the accuracy of the turbine flowmeter, it is necessary to regularly calibrate and compensate for the field temperature and pressure, thus ensuring the accuracy of the natural gas pipeline trade measurement.
The flowmeter is strongly influenced by the velocity distribution and the distortion of the rotating flow. Therefore, long straight sections are required at the top and bottom of the sensor. If installation space is limited, a rectifier can be installed to reduce the length of the straight section, which is not suitable for measuring pulsating and miscible flows. In addition, the cleanliness of the gas to be measured is essential. If the gas becomes dirty, a filter can be installed, resulting in higher pressure losses and increased maintenance.
A gas turbine flowmeter has high accuracy, good repeatability, no zero drift, an extensive range ratio, and other advantages. Using advanced ultra-low-power single-chip microcomputer technology to develop the turbine flowmeter, especially suitable for use with computer control systems (such as PLC and DCS) to improve the level of automated natural gas flow measurement, its wide range ratio is ideal for occasions with large changes in flow, can reduce the diameter of the measuring tube, thus reducing investment; short-term repeatability can reach (0.05 ~ 0.2) %, such as frequent calibration or Online calibration can be very high accuracy; easy to install and maintain, high flow capacity for high-pressure measurement, in the natural gas pipeline trade settlement is the preferred flowmeter; however, the gas density has a greater impact on the accuracy of the flowmeter, the need for regular calibration.