On-site comparative calibration of gas flow meters

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I. Introduction: Periodic calibration of measuring instruments is an important aspect of measurement management. Our company uses a CW1—G106—3 type vortex flow meter for the measurement of gas produced at the factory. Since changes in the composition, temperature, pressure, and humidity of gas over different periods can affect the accuracy of measurement by gas flow meters, it is an essential task to perform periodic calibrations of these meters on a monthly basis to ensure reliable measurements. Only by ensuring that the flow meters can measure accurately and stably is it possible to provide an accurate assessment of the differences in gas supply and consumption. To this end, the company stipulates that under normal circumstances, the vortex flow meters used to measure the gas shipped from the factory should be calibrated once per quarter. I. Principle of on-site calibration: In the absence of equipment available for calibrating such gas flow meters, we employ the on-site calibration method. As shown in the figure, theoretically, when the gas inlet is closed, the flow rate of gas output corresponds to the decreasing volume of the gas holder. Since the flow meter performs measurements with temperature compensated to 21 degrees Celsius and pressure compensated to 101325 Pascals under standard conditions, the changes in the volume of the gas tank must also be converted to their values under standard conditions during calibration ； According to the ideal gas law, P_std·V_std/T_std = PV/T; therefore, △V_std = (P·T_std / P_std·T)·△V. As an example of calibration, our company has two gas tanks with capacities of 75,000 cubic meters and 100,000 cubic meters each (with four towers in each tank). To carry out calibration without interrupting production, the 75,000-cubic-meter tank is first depressurized by releasing gas from its two towers. Once space is created, the water seal at the outlet of this tank can be sealed, preventing gas from exiting while allowing the gas produced to continue to flow into the tank for storage. At the same time, the water seal at the inlet of the 100,000-cubic-meter tank is sealed as well, preventing gas from entering while allowing it to exit, thereby enabling the calibration of the flow meter. In this way, 100,000 gas cylinders, compressors, flow meters, and piping networks form a separate system. The measured drop in height of the gas tank is multiplied by the cross-sectional area of the tower section it is located in; then, the temperature and pressure of the gas at that time are taken into account in the calculations to determine the change in volume under standard conditions. By comparing the cumulative flow volume of the flow meter during that period with the obtained volume change value, it is possible to determine the magnitude of the error in the flow meter; the flow meter coefficient is then adjusted until the error is reduced and remains within the allowable range. Note: ① Before formal calibration, the resolution of the readings on the indicating instrument should be set as low as possible (the minimum resolution displayed on the instruments used by our company is 1×10 cubic meters), so that the time taken for the reading to change corresponds as closely as possible to the time it takes for the gas tank to descend by 1 meter. ②During on-site calibration, it is impossible for the reading of the gas tank’s height to be perfectly synchronized with the readings from the instruments (the distance between the gas tank and the instruments is approximately 100 meters along the pipelines), and the descent of the gas tank cannot be completely uniform; abnormalities such as oscillations, sudden movements, and sticking can occur. Additionally, random errors are inevitable when measuring the height. Therefore, at the beginning, each measurement should be based on a descent of 1 meter; once the values become more accurate, a larger descent distance should be used as the basis for measurement, so that the errors can offset each other and the true value can be determined as accurately as possible. 　Although we have accumulated some experience through numerous on-site calibration exercises, our work still needs continuous improvement.