Gas flowmeters are expensive and most of them are imported flowmeters. How to use and maintain them properly is an important part of measurement management. The article adopted the temperature, vibration, noise in the installation environment of the gas ultrasonic flowmeter, the bidirectional flow measurement in the pipeline configuration, the length and steps of the straight pipe section, the requirements of the projections, the inner surface, the installation of the thermometer, the use of the rectifier and The installation of the flowmeter, on-site maintenance and management, and analysis of the measurement uncertainty were discussed in four aspects. It was used to use this new flowmeter to ensure its measurement accuracy and reliability.
1, the installation environment
(1) Ambient temperature
According to the requirements of the AGA·NO·9 report and the processing and manufacturing levels of existing materials and equipment, this standard proposes the use of ambient temperature of -25°C to +55°C for gas ultrasonic flowmeters.
In some special cases, in order to prevent the external temperature from affecting the measurement results, it is recommended to install rain-proof sun-protection facilities or take necessary insulation and insulation measures on the meter and its upstream and downstream measuring tubes.
(2) Vibration
Although the ultrasonic flowmeter has undergone rigorous mechanical vibration test before leaving the factory, as a precision measuring instrument, it should be used away from or eliminate the vibration source.
(3) Electrical noise
Ultrasonic flowmeter as a kind of high-precision measurement equipment based on micro-electronics technology may be placed in the electromagnetic interference environment due to inadvertent design and installation. The accuracy of the instrument measurement and the reliability of the work This may be affected.
2, pipeline configuration
(1) Pipe configuration for bidirectional flow measurement
The so-called "bi-directional flow measurement" refers to the use of the same set of flowmeters to measure the flow of the measured medium when it is being transported and reversed. In other words, the upstream of the instrument that is being lost during this period is the downstream of the instrument at the time of return. Underground gas storages or gas storage tanks, which are currently used as peaking means in large and medium-sized cities, require such meters with bi-directional measurement capabilities. This is the unique feature of ultrasonic flowmeters. Therefore, when the ultrasonic flowmeter is applied to the bi-directional flow measurement, it must treat the “downstream†according to the requirement of “upstreamâ€. This is an important prerequisite for realizing the bidirectional and equal precision measurement of the ultrasonic flowmeter.
(2) Straight pipe length requirements
In order to reduce the influence of bad flow state on the measurement results, it is a common basic measure to install a certain length of straight pipe section upstream and downstream of the flow meter. From the accurate measurement point of view, the longer the length of the upper and lower straight pipe sections, the more obvious the effect of improving the measurement performance of the flowmeter. However, the measurement site is often limited by many objective factors such as site expropriation (especially offshore platforms), material supply, and construction and construction costs, and it is expected that the shorter the length, the better. Therefore, it is also one of the major tasks of the GB/T18604-2001 standard to take into account the wishes of both parties and to propose the minimum length of the straight pipe section.
In the configuration length of the straight section of the flowmeter, AGA·NO·9 reported: “Although the manufacturer's recommended installation practices are not consistent, it generally requires that the upstream of the flowmeter at least require a 5–10D straight pipe section. The downstream needs at least 3D straight sections."
In order to reflect the operability of the standard, according to the above standards or recommendations of the report, combined with the actual situation of the domestic production site, and with reference to the opinions of some ultrasonic flowmeter manufacturers, a relevant one is tentatively given in the standard. The technical regulations or requirements for the length configuration of the upstream and downstream straight pipe sections of ultrasonic flowmeters, that is, the length of the shortest straight pipe section upstream of the multi-channel ultrasonic flowmeter should be 10D and the length of the shortest straight downstream pipe section should be 5D without installing a rectifier. If rectifiers are used, the installation location of the rectifiers and the corresponding piping length should be consulted with the manufacturer.
(3) Quality Requirements for Straight Pipe Sections
Steps and projections: any steps and other protrusions that occur within the length of the minimum straight pipe section required on the upstream and downstream sides of the ultrasonic flowmeter (measurement tube) will cause the flow state of the measured medium to change, thereby increasing the flow measurement. The uncertainty. But in fact, as long as the careful selection of the piping used, or to take appropriate control of the inner wall of the pipeline, or according to the pipeline conditions of the site for the manufacturer to propose the required diameter of the ultrasonic flowmeter must be reached, etc., you can avoid the connection point The emergence of steps to achieve equal-diameter connection or good match between the straight pipe section and the ultrasonic flowmeter; In addition, in the process of construction and assembly, the inner ring of the gasket that smooths the connecting inner wall weld or appropriately expands the flange connection is adopted. Measures such as diameter can also avoid protrusions and other disturbances. Therefore, restrictions on steps and protrusions are both necessary and feasible. 2 Inner surface: If there are rust, oil stains or iron sulfide powder and other appendages inside the flow meter body and the inner wall of the measuring tube, on the one hand, the actual inner diameter of the measuring tube may be changed, and on the other hand, the inner diameter of the measuring tube may increase. The average roughness of the wall may, in turn, cause the divergence and attenuation of the sound wave (pulse signal) when reflected on the inner surface of the watch body. All of these factors may have a serious impact on the measurement results (ISO/WD17089 believes that the resulting measurement deviation may exceed 1%), so the requirements and restrictions on the surface of the meter body and the inner surface of the measuring tube are also essential for accurate measurement. One of the prerequisites.
(4) thermometer installation
The thermometer installation should mainly consider the following three points:
1 The temperature sensing element should be of sufficient length to ensure that there is sufficient contact area between the measured medium and the temperature measuring element; 2 the influence of the convection state is as small as possible; 3 the airflow will not be flushed during the normal measurement process. For other reasons, the thermal element breaks or other mechanical damage is caused.
(5) Acoustic noise interference
Ultrasonic flowmeter is a kind of measuring instrument based on the acoustic principle. Therefore, the existing ultrasonic flowmeter is for the noise, especially for the UHF caused by the decompression equipment such as high speed and large pressure difference from the inside of the measured medium. Noise, especially sensitive, affects the normal operation of this type of flowmeter. To ensure the normal operation of the ultrasonic flowmeter, the most effective method is to stay away from noise sources or consult the manufacturer.
(6) The role of the rectifier
In general, the installation of a rectifier can improve the measurement conditions upstream and downstream of the ultrasonic flowmeter to a certain extent, and therefore it also helps to improve the accuracy of the measurement results. Especially for a monophonic ultrasonic flowmeter, the effect may be even greater. obvious. However, if the rectifier is improperly shaped, the processing quality is not high, or the installation and use are not properly maintained, it may increase the measurement deviation and cause the result to be counterproductive. Therefore, in the design and installation process of the ultrasonic flowmeter application site, specific problems must be specifically analyzed. In other words, whether or not to install that type of rectifier should be determined according to the type of flow meter selected, the distortion form of the upstream flow state, the severity, and the level of accuracy required for flow measurement.
(7) How to install the flowmeter
In theory, the gas ultrasonic flowmeter can be installed in any manner such as horizontal, vertical, and tilt, and it can be rotated freely along the side axis of the body pipe. However, in the actual installation process should mainly consider the following three factors: 1 whether the flowmeter (especially the ultrasonic transducer therein) is easy to detect and maintain; 2 whether the ultrasonic transducer is susceptible to impurities in the airflow and The effect of effusion; from 3 o'clock to the present, whether or not an arbitrary installation method can achieve full precision measurement in the strict sense has not been fully verified by experiments.
Therefore, GB/T 18604-2001 uses natural gas flowmeters to measure natural gas flow standards.
3, on-site use, maintenance and management
(1) About periodic inspection
Regular testing is an effective means to prevent and promptly eliminate instrument failures. If the period of periodic inspections is set too long, some problems may not be discovered and handled in time; if the period is set too short, it will increase the workload and may also damage the original performance of some mechanical and electronic components (such as sealing Sex). Therefore, the periodicity of a reasonable periodic inspection should be determined on a case-by-case basis.
As for the contents and methods of regular inspection, it should be carried out according to GB/T 18604-2001 requirements for the measurement of natural gas flow by the gas ultrasonic flowmeter in the standards of 8.3 - 9.2. In some production sites, since it is difficult to dismantle and install the flowmeter from the process pipeline, the zero flow test in this case can be performed with the upstream and downstream valves completely closed. However, it must also ensure that:
1 The valve used must have flexible opening and closing characteristics and good shutdown performance. No internal leakage or leakage may occur between the upstream and downstream valves after shutdown.
2 In order to avoid convection of the fluid inside the body and the pipe, the difference between the temperature of the medium and the ambient temperature should not be too large. Otherwise, insulation or insulation measures should be taken.
3 During the refurbishment test, the pressure and temperature should be stable, and the relative uncertainty of the measurement should be controlled at urel(p) ≤ 0.1% and urel(t) ≤ 0.2%;
4 The negative effects of zero-flow test checks due to gas flow or other mechanical vibrations in the nearby pipelines should be kept to a minimum.
(2) Test report
The ultimate goal of the on-site inspection or testing is to allow the administrator or user of the ultrasonic flow meter to have a full understanding and understanding of the operation and performance of the equipment. Therefore, it is necessary to issue a formal written report based on the specific conditions of the on-site inspection and testing. The specific format and content of the report can refer to GB/T18604-2001 for the measurement of natural gas flow by the gas ultrasonic flowmeter in accordance with the requirements of Section 9.2 of this standard. On the other hand, it can be designed or added or deleted as appropriate according to actual conditions.
The above materials combined with GB/T18604 gas ultrasonic flowmeter to measure the natural gas flow in the gas ultrasonic flowmeter installation environment, management configuration, on-site use of maintenance and management and some other views, in order to provide useful advice in the future gas measurement.
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