The impact of zero point on mass flow meters

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This post was last edited by Wang Tianze on 2016-11-16 at 12:05. Using the HK-CMF Coriolis mass flow meter to measure the mass flow rate of liquid hydrocarbons is a method that is widely recognized today. It effectively reduces the impact of changes in parameters such as medium temperature and pressure on the measurement results, thereby raising the accuracy of mass measurement for liquid hydrocarbons to a new level. 　　However, in daily use, factors such as the installation location, operating environment, and process conditions severely affect the stability of the measurement performance and the accuracy of the results of the HK-CMF Coriolis mass flow meter, due to issues like zero drift and air entrapment. The air entrainment can be eliminated through means such as oil-gas separators or the installation of pressure valves, and this will not be discussed in detail in this article. Additionally, zero drift becomes the main factor affecting the measurement results of CMF, and maintaining the zero point of CMF is an important way to ensure the stable metering performance and accurate measurements of the HK-CMF Coriolis mass flow meter. 　　Zero-point maintenance usually only involves on-site zeroing as a method. Since zero drift has a significant impact on the measurement accuracy of HK-CMF Coriolis mass flow meters, especially at low flow rates, care must be taken when using zeroing methods. Only when it is confirmed that zero drift indeed exists and zeroing is necessary should it be carried out strictly in accordance with the specifications. This article discusses the impact of the zero point on the accuracy of CMF measurements, zeroing methods, and simple maintenance of the daily zero point. 　　I. Case Analysis of the Impact of Zero Point Below, we cite a case that occurred in our company in 2013 to illustrate the significant impact that the zero point has on measurements in practical applications, and to emphasize the importance of zero point adjustment. 　　1. Introduction to the case: In November 2012, our company’s new coking plant with a capacity of 1.6 million tons was put into operation, with a designed liquid yield of 65%. However, within a few months of the unit coming online, the liquid recovery rate was only around 63%, falling short of the design requirements. The data are shown in Table 1. http://106.37.166.86:8009/zgjl/inset/15/151007201.JPG Table 1: Material balance sheet for the new coking unit from June to August. The raw material for this unit is residue oil, while the liquid products are diesel, gasoline, wax oil, and a small amount of liquefied gas. The flow meters used to measure these five liquid substances are all HK-CMF Coriolis mass flow meters that have passed calibration tests, and their measurement performance meets all the requirements of the unit’s metering needs. To help the workshop identify the reasons for the substandard liquid recovery, flow meter maintenance staff and technicians conducted thorough investigations, and the wax oil flow meter was disassembled for inspection; the inspection results showed it to be still in good condition. At the same time, the zero points of other flowmeters were checked; by closing and opening the manual valves, all the flowmeters returned to zero, which ruled out the possibility that the zero point was causing deviations in the flowmeter readings (yet this is precisely where the problem lay). Subsequently, Emerson’s professional engineers were invited to the site to conduct a thorough inspection of all flow meters. It was found that for the meter used to measure crude oil and residue oil, of model CMFHC2G, the static zero value of the meter’s core processor at the time of manufacture was Kzero = -0.01188947 μs; however, the current zero value was -1.059366 μs. In other words, Kzero was no longer the default zero value, but had changed significantly. 　　2. Case Analysis Since inaccuracies in the zero point can have a significant impact on the measurement accuracy of the HK-CMF Coriolis mass flow meter, in this case the zero point value varies considerably. By ignoring the effect of temperature and using the relevant formulas, the dynamic zero point of the current flow meter should be 10.7 t/h. Based on a current CMF flow rate of 156 t/h, the error caused by this zero point amounts to 10.7/156 = 6.86%. After the zero point was restored, the measurement values of this HK-CMF Coriolis mass flow meter returned to normal, and the liquid recovery rate of the device reached 68%, meeting the design requirements. 　　So what causes such a large zero drift? First, let’s analyze the reasons for this significant zero drift: (1) The HK-CMF Coriolis mass flow meter was calibrated before installation, and the calibration results were satisfactory; therefore, this should not cause any zero drift. 　　(2) Manually modifying the static zero point requires a professional handheld device or laptop software, which is only available to specialized metrology maintenance personnel, and they are unlikely to make such modifications. 　　(3) During the inspection conducted by the meter maintenance personnel after the flow meter was installed, it was found that the instrument displayed a non-zero flow rate; in the absence of the conditions required for zeroing, they performed zeroing operations in good faith, which resulted in zero drift. 　　Since the exact circumstances at the time are unknown, after analysis, the author believes that the third possibility is the most likely. 　　What then caused such a large zero drift not to be detected during the subsequent inspections of the HK-CMF Coriolis mass flow meter? The author believes that this situation was caused by an improperly set threshold for low-flow cutoff, an issue that wasn’t given enough attention; in other words, the threshold for that HK-CMF Coriolis mass flow meter was set above 10.7 t/h, and due to the zero-cutoff effect, the instrument resetted to zero when the flow rate fell below this set value. During zero-point maintenance, the usual procedure for maintenance personnel is to close the rear hand valve first, then the front hand valve, and check whether the flow rate indicated by the instrument returns to zero. If the instrument shows a value of zero, it is assumed that there is no issue with the instrument’s zero point, even if there was actually an instantaneous flow rate of 10.7 t/h at that time. For this reason, no zero drift issue was detected in subsequent CMF inspections. 　　In fact, this situation is widespread in the application and maintenance of HK-CMF Coriolis mass flow meters in China at present. The zero-cut setting prevents the HK-CMF Coriolis mass flow meter from accumulating an incorrect flow value under abnormal operating conditions; however, if the threshold is set too high, it may prevent the detection of zero-point drift in daily use, a issue that has not received sufficient attention in CMF applications in China. 　　II. Daily zero-point maintenance methods for HK-CMF Coriolis mass flowmeters 1. Appropriate threshold setting Setting reasonable thresholds not only helps to effectively filter out the accumulation of minor fluctuations that occur during abnormal measurement conditions, but also greatly facilitates daily zero-point maintenance, thereby preventing the situations described in the previous examples from occurring. The author believes that it is appropriate to set the threshold at half of the CMF accuracy of the typical flow rate of the medium measured by this HK-CMF Coriolis mass flow meter. For example, in the case mentioned earlier, Emerson’s CMF accuracy is 0.5%, and the normal flow rate is 160 t/h; therefore, the threshold can be set at 160×0.5%÷2=0.4 (t/h). In this way, even if drift occurs, it can be detected in a timely manner by maintenance personnel as long as the value remains above a certain threshold; below that threshold, according to the CMF error calculation formula provided by the manufacturer, the value lies within the acceptable range of ±0.10%±〔(zero point/flow rate)×100〕% = ±0.10%±〔(0.4/160)×100〕% = ±0.35%, so even in the presence of drift, the measurement result remains within the acceptable limits. For discontinuous metering methods, thresholds can also be set in this way, and the cumulative flow during periods when no metering takes place can then be filtered out using the manual reading method for settlement. 　　2. Timely zero-point verification For HK-CMF Coriolis mass flow meters in use, regular zero-point verification should be carried out to detect zero drift in a timely manner and make corrections accordingly. 　　3. Careful zeroing procedure: If it is detected that the HK-CMF Coriolis mass flow meter shows a value other than zero during zero-point verification, it is necessary to first observe the operating condition of the instrument and analyze the reasons for this non-zero value. Factors to consider include whether the fluid in the HK-CMF Coriolis mass flow meter is at rest, whether it is at full flow level (the density value displayed by the instrument can be used as an initial indicator of the fluid condition in the measurement tube), whether there have been significant changes in the process conditions, whether the upstream and downstream valves are properly closed, and whether there are any strong magnetic fields or vibration disturbances in the surrounding environment. Only after excluding the above factors can it be determined that the zero point of the HK-CMF Coriolis mass flow meter may indeed have drifted, allowing for on-site zero adjustment of the HK-CMF Coriolis mass flow meter. 　　4. Proper zeroing procedure (1) Ensure that the HK-CMF Coriolis mass flow meter is in its operating environment, and that the measurement tube is filled with the medium to be measured. 　　(2) If there is a bypass valve, open it first. 　　(3) Close the isolation valve downstream (i.e., the backflow valve) to stop the flow of liquid through the measuring tube, ensuring a good seal. 　　(4) If there is an upstream isolation valve (i.e., a front hand valve), it must be closed. 　　(5) Perform the zeroing operation according to the manufacturer’s specified zeroing procedure. 　　(6) Record the change in the zero point before and after; if the change is significant, consider whether the CMF needs to be retested/calibrated.