Application of mass flow meters in liquefied gas flow measurement

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Application of Mass Flow Meters in Liquefied Gas Flow Measurement Authors: Yang Xinan, Wang Yunhui 【Abstract】This paper introduces the working principle of mass flow meters, as well as the problems that arise in their use for measuring liquefied gas flow and the solutions to those problems. The problems discussed are typical, and the solutions are practical and reliable, providing a reference for designers, maintenance personnel, and operators. Keywords: mass flow meter ； Principle ； Liquefied gas ； Gas ； Introduction to Pressure 1 Mass Flow Meters: When a fluid flows within a rotating tube, it exerts a force on the tube walls; this force was discovered by Coriolis in 1832 while studying water wheels, and is commonly referred to as the Coriolis force. Mass flow meters operate on the principle of the Coriolis force. Inside the sensor, there are two parallel T-shaped vibrating tubes; a drive coil is located in the middle of these tubes, while pickup coils are situated at each end. When an excitation voltage supplied by the transmitter is applied to the drive coil, the vibrating tubes begin to oscillate back and forth periodically. As the fluid flowing through the sensor passes by these vibrating tubes, the Coriolis force comes into play, causing the tubes to vibrate torsionally. The pickup coils at the ends of the tubes generate two sets of signals with different phases, and the difference between these two signals is proportional to the mass flow rate of the fluid passing through the sensor. The computer calculates the mass flow rate flowing through the oscillating tube. When different media flow through the sensor, the fundamental oscillation frequency of the vibrating tube varies, and the density of the medium is determined based on this. The platinum resistor mounted on the sensor’s vibrating tube can indirectly measure the temperature of the medium. A mass flow meter directly measures the mass flow rate of the medium passing through it; it can also measure the density of the medium and indirectly determine its temperature. Since the transmitter is an intelligent instrument based on a microcontroller, more than a dozen parameters can be derived from these three basic quantities for use by users. The mass flow meter features flexible configuration, powerful functions, and a high performance-to-price ratio, making it a next-generation flow meter. Main technical specifications of the mass flow meter:
(1) Key parameters:
– Mass flow accuracy: ±0.002×flow rate ± zero drift
– Density measurement accuracy: ±0.003 g/cm3
– Density measurement range: 0.5–1.5 g/cm3
– Temperature measurement range: ±1°C

(2) Sensor-related data:
– Ambient temperature: –40–60°C
– Medium temperature: –50–200°C
– Explosion protection rating: iBⅡBT3
– Associated equipment: Companion transmitter

(3) Transmitter-related data:
– Operating temperature: 0–60°C
– Relative humidity: below 95%
– Power supply: 220±10% VAC, 50 Hz or 24±5% VDC, 40 W

2. Installation and use of the mass flow meter
Example 1: In order to support the expansion and renovation of our company’s gas distillation facility, we carried out the design and construction of the propylene loading system at the beginning of 1998. For measuring the propylene flow rate, mass flow meters produced by Taihang Instrument Factory were used. After the facility started operating, the flow meters functioned properly; however, there was a significant discrepancy between the values indicated by these meters and those recorded by the electronic scales. Cause analysis: (1) Through discussions with process engineers, it was ruled out that an excessive amount of dry gas in the liquefied gas (5%) could be responsible for the inaccurate readings of the instruments. (2) A set of instruments with the same specifications was removed from the gas fractionation unit and installed for observation; the phenomenon remained unchanged, ruling out the possibility that the flow meter was damaged. (3) To this end, we sent people to inspect the Zhongyuan Oilfield Refining and Chemical Company; the flow meters used in its liquefied gas loading system are all mass flow meters. Since they were put into use over two years ago, they have been functioning properly with accurate readings. Its process flow and instrument installation are similar to those in our plant. The flow diagram shows that this plant places special emphasis on the sequence of operations: once loading begins, valve d on the loading line is opened first; once the readings on pressure gauges c and f are equal, then valve g on the gas return line is opened. In our plant, however, both valves d and g were opened right from the start. As a result, the pressure on the loading line became much higher than the pressure inside the tank truck, which made it easy for some of the remaining propylene in the tank truck to be pushed back into the propylene tank, thereby causing measurement errors. With the improved operating method, the system is working properly, achieving accurate quantified loading and thus improving work efficiency. Example 2: To support the renovation of our company’s gas fractionation unit, we carried out the design and construction of a liquefied gas desulfurization unit at the beginning of 1998. The flow meter used in this unit was a mass flow meter produced by the Flow Meter Branch of Taihang Instrument Factory. After the unit started operating, the flow meter did not function properly; it frequently returned to zero in terms of instantaneous flow, and an alarm signal indicating the presence of entrained gas was also displayed. Cause analysis: (1) Through joint discussions among the instrument manufacturers and process engineers, it was ruled out that an excessive amount of dry gas in the liquefied gas (5%) could be responsible for the inaccurate readings of the instruments. (2) A set of instruments with the same specifications was removed from the gas fractionation unit and installed for observation; the phenomenon remained unchanged, ruling out the possibility that the flow meter was damaged. (3) It has also been observed that the zeroing of the instrument is related to the medium pressure. To this end, we sent people to the experimental plant of Luoyang Petrochemical Engineering Company for inspection; the flow meters used in the liquefied gas output systems of its two catalytic units have been functioning properly for over two years, providing accurate readings. The pressure of the medium measured by one set of instruments is 1.0 MPa, while the pressure of the medium measured by the other set of instruments is 2.0 MPa. In other aspects, it is similar to our factory. The pressure at the liquefied gas outlet of the liquefied gas desulfurization unit in our plant is 0.6~0.9 MPa. It has been observed that when the medium pressure falls below 0.75 MPa, the instantaneous reading of the instruments returns to zero. Based on this, it can be concluded that 0.75 MPa is the saturated vapor pressure of the liquefied gas produced by our company (it is difficult to calculate this value due to incomplete data; it can only be determined through chromatographic analysis). At this pressure, the amount of gas evaporated from the liquefied gas exceeds 5% (M/M), which causes the instantaneous flow rate indicated by the mass flow meter to drop to zero. After discussion with the process engineers and operators, it was decided to install the sensor of the mass flow meter at the outlet of the liquefied gas feed pump in the desulfurization unit; the pressure of the fluid at this location is at least 1.3 MPa, and it remains stable. After the modification, the mass flow meter operates stably and with high accuracy, achieving the desired results.