Brief Analysis on Research and Application of Flow Measuring Instruments

Flow measurement technology is very different from the application of traditional measurement instruments. It is not a simple matter of installing the flow meter and putting it into operation to achieve the measurement purpose. Two experts once conducted a survey on more than a thousand flow meters installed on site and found that about 60% of the selected measurement methods were not the most appropriate or incorrect. Among the remaining 40%, about half of them had appropriate measurement methods but unreasonable on-site layout and installation. These inappropriate, incorrect and unreasonable phenomena brought corresponding measurement errors. Therefore, flow measurement is a measurement that strongly depends on the conditions of use. In the laboratory, the flow meter can achieve extremely high accuracy, but at the site of use, once the fluid conditions or environmental conditions change greatly, not only the accuracy cannot be guaranteed, but even normal measurement cannot be performed.

The error of a flow meter is better than ±0.5% after factory calibration, but it is not uncommon for the error of a new meter to increase to ±5% to ±10% after it is installed on site. There are many reasons for this, such as unreasonable selection, inappropriate range, insufficient length of upstream and downstream straight pipe sections, incorrect installation, too much deviation of fluid properties from the design state, operating conditions exceeding the allowable value, pulsating flow, harsh environmental conditions such as vibration, etc., and many more can be cited. Therefore, flow measurement is a system problem, including detection devices, display devices, front and rear straight pipe sections, and auxiliary equipment. The research on application technology also includes the measurement object itself. Only the good performance of the flow meter itself cannot guarantee the required measurement effect.

The goal of research on the application technology of flow measurement instruments is to use them correctly, which mainly includes the following specific contents.

1. Improve the opening rate

In the management of instrumentation equipment, the definition of the opening rate is: (total number of instruments minus the number of instruments not in normal use)/total number of instruments. Therefore, to improve the opening rate is to reduce the number of instruments that cannot be put into normal use. In the design institute, the opening rate of the measurement system designed by the automatic control profession is one of the important indicators reflecting the work quality and technical proficiency of the designers. Experienced and responsible designers can make the opening rate reach more than 95%, and the remote pressure gauge can reach more than 95% through rectification. However, under the conditions of the market economy, engineering companies often implement turnkey contracting for owners, and the opening rate required is not 95%, but 100%. If the designed instrument system cannot be put into normal use, it is the responsibility of the engineering company, that is, to rectify or replace the instrument, which means economic loss. Therefore, the research on instrument application technology has practical economic significance.

The opening rate is a comprehensive expression of the instrument application technology level and the instrument quality itself. The matching, coordination, and optimization of the measurement method and instrument to the measurement object and the use environment, as well as the design selection and installation and commissioning before that are all important factors affecting the opening rate.

Over the years, my country's flow measurement instrument application technology has made great progress, and the installation rate of flow measurement instruments has greatly increased. On the one hand, this is because the overall technical level of instrument personnel has been significantly improved and their sense of responsibility has been enhanced. More importantly, the quality of instruments has been greatly improved compared with the previous planned economy era. The proportion of imported instruments and instruments assembled with advanced foreign technology is increasing. Especially after the instruments are generally intelligent, the adjustable measuring range has been greatly expanded. Instruments that could not be put into normal use before due to inappropriate measurement range selection can generally be put into use by changing the range.

2. Ensure measurement accuracy

Flow measurement accuracy refers to the accuracy obtained by the flow measurement system, which is different from the accuracy of the flow meter itself. Just because the flow meter itself has good performance and high accuracy does not necessarily mean that high measurement accuracy can be achieved.

To ensure the accuracy of the flow measurement system, in addition to reasonable selection, correct installation and commissioning, and timely maintenance and care, the application of intelligent technology to properly compensate and correct the errors that may be introduced by the measurement part is also an effective method. For example, compensation for the temperature expansion coefficient of the liquid, compensation for the temperature, pressure and compression coefficient of the gas by the stainless steel pressure gauge, compensation for the Reynolds number effect and flow expansion coefficient of the differential pressure flowmeter, compensation for the nonlinearity of the flow coefficient of various flowmeters, compensation for the temperature effect of the volumetric flowmeter and vortex flowmeter, compensation for the velocity distribution of the ultrasonic flowmeter, etc. This compensation and correction is to use a systematic method to deal with the errors inherent in the detection part that cannot be overcome by itself, so as to eliminate or basically eliminate them. Practice has shown that this method is simple, effective and promising.

Among the many methods to ensure measurement accuracy, online real-flow calibration occupies an important position. In the past, flow meters were mostly calibrated using offline methods. Although the flow meters calibrated using this method have higher accuracy after error correction, the reference conditions of the pipeline during calibration are different from those during actual use, the fluid properties during calibration are different from the fluids actually used, and the environmental conditions during calibration are different from the actual environment of the instrument use site, which causes additional usage errors and reduces measurement accuracy. The online real-flow calibration method is an effective way to solve this problem. For example, the standard volume pipe connection port is reserved for the oil metering station during the construction phase. After the standard volume pipe is connected, the online real-flow calibration of each flow meter in the metering station can be achieved by switching the valve. Now, the online real-flow calibration method is also required at natural gas distribution stations.

3. Improve the reliability of flow measurement system

If the flow meter used for safety interlock alarm is unreliable, it will not work when it should be interlocked, which may easily lead to accidents. If it works randomly when it should not, it may easily lead to unnecessary parking and cause losses. If the fuel flow meter in the industrial furnace is unreliable, it will cause blockage of the flow path, which may easily cause the furnace to stall and cause accidents. If the flow meter used for process control is unreliable, it may easily send wrong information to the regulation system, causing the regulation system to be out of adjustment, destroying the stability of the production process, affecting the quality, output and material consumption of the product, and causing losses. If the flow meter used for financial settlement measurement is unreliable, it may easily cause measurement inaccuracy, cause measurement disputes and bring losses to the enterprise. It can be seen that the reliability of flow meters is extremely important.

The main way to improve the reliability of flow measurement is to improve the reliability of the instrument itself and select highly reliable instruments for reliability design. In recent years, the reliability of flow measurement instruments has been significantly improved, mainly in the following aspects.

(1) The reliability of the instrument itself has been significantly improved.

(2) Improve the system reliability by improving the structural design of the instrument. For example, the flow meter can be replaced without affecting the process operation by adopting a continuous flow insertion structure.

The clamp-on structure of the ultrasonic flow sensor, the pressurized replacement structure of the electromagnetic flow sensor electrode, and the structure of the vortex flow sensor with an ultrasonic probe installed outside the pipe can greatly shorten the repair time during the repair process after the instrument is damaged.

(3) Introducing redundant technology. For example, using dual sensors and automatically determining whether the sensors are functioning properly will eliminate the signal from the faulty channel.

(4) Introduce self-diagnosis technology and send the diagnosis results to the operation station or dedicated equipment management system (AMS) through the field bus for display and alarm, so as to detect faults in time and take measures as soon as possible.

4. Save money

The costs mentioned here include not only the instrument purchase cost, but also the accessories purchase cost, installation and commissioning cost, operation cost, spare parts cost, maintenance and regular calibration (verification) cost. The depreciation cost amortized over the average life of the instrument cannot be ignored.

Although some types of flow meters have lower purchase costs, they require the installation of auxiliary equipment such as upstream and downstream cut-off valves and bypass valves. Sometimes the cost of the auxiliary equipment far exceeds the purchase cost of the flow meter itself.

When selecting instruments, we should avoid the one-sided pursuit of high performance and high accuracy, because doing so will not only increase the purchase cost, but also often increase the cost of spare parts. The optimal design selection is the one that meets the use requirements, has the highest reliability, is easy to maintain, and is the most cost-effective.

5. Security

Some of the measured fluids of the seismic-resistant pressure gauge are flammable and explosive media, and some instrument installation sites are flammable and explosive sites. The instrument selection, system design and installation should comply with explosion-proof regulations.

In addition to the above goals, other requirements for use should also be met, such as pressure loss requirements, hygiene requirements, protection requirements, etc. Attention should also be paid to ease of maintenance, and some should also consider the convenience of implementing mandatory inspections.