There are three methods for natural gas flow metering: volume flow metering, mass flow metering, and energy flow metering. Conventional natural gas metering uses volumetric units. In recent years, measuring in terms of quality and energy has become a trend. The use of energy or mass units is preferred in high volume, high pressure natural gas metering. China's early attention to the measurement of natural gas is insufficient, and the development of natural gas measurement technology is slow. Until now, natural gas commodity measurement still uses volume measurement. At present, natural gas volume flow metering instruments mainly include orifice plate flowmeters, vortex flowmeters, turbine flowmeters, and positive displacement flowmeters. We use the standard orifice plate flowmeters more widely.
1. Problems in measurement
The standard orifice flowmeter is an indirect, comprehensive technical parameter measurement with many instruments and complex factors. Under normal circumstances, its measurement accuracy can meet the requirements of GB2624-93 standard and SY/T6143-1996 standards for standard orifice plate measurement methods for natural gas flow. In actual work, deviations from the standard's conditions affect the accuracy of measurement, and some may be quantitatively estimated and corrected. Some may only qualitatively estimate the amplitude and direction of uncertainty, but some are simultaneous deviations of multiple conditions. A very complicated situation has arisen. Because the general literature only describes errors caused by a certain conditional deviation, there is a lack of relevant information on a variety of conditions simultaneously deviating from the measurement error. A large number of on-site investigations and practical experiences have shown that the main reason for measurement problems is that the design, manufacture, installation and use of throttle devices and working conditions have deviated from the scope of the standard. Its main performance is as follows:
No strict design, manufacture and installation according to SY/T6143-1996 standard. Select measuring pipe diameter is too large, long-term low Reynolds integer, upstream and downstream pipe sections are not installed according to standard requirements, pipe diameter is not measured.
Orifice flowmeters use a more stable flow rate parameter as a design basis. If the flow rate is too small or too large, the measurement error will increase. To properly select and use a differential pressure gauge, if the differential pressure gauge's working range is below 30%, the flow measurement accuracy will be greatly reduced. When the flow of natural gas decreases, the differential pressure gauge range or orifice plate specifications must be promptly replaced. Otherwise, the measurement error due to differential pressure will increase exponentially. When selecting the differential pressure range of the meter, it is necessary to consider the aperture ratio, and also whether the pressure after the pressure loss of the orifice plate meets the production needs. Flange pressure and angle pressure ratio of the angle should be between 0.2 ~ 0.75, generally should be selected between 0.4 ~ 0.6, so that can not only ensure measurement accuracy, but also reduce pressure loss.
2. Accumulation scheme using dedicated flow computer
The dedicated calculation program uses modern mathematics computer technology to accurately calculate various flow rates (volumetric flow and mass flow) according to the user's requirements, and automatically or continuously compensates the flow for full or partial compensation, as long as the individual coefficients of the flow equation are used. Formulas are programmed according to certain requirements to achieve accurate total flow.
3, the use of exchangeable orifice device and constant throttle device
The replaceable orifice throttling device is a new type of throttling device with a major structural breakthrough. The throttling element is accurately installed in a fixed seat body (the seat body is connected with the pipe through the flange), and the pipe is not removed or kept In the case of delivery, the orifice plate can be easily lifted for inspection, cleaning or replacement, thus ensuring measurement accuracy. The use of hydraulic lifting device orifice plate to enhance light, especially suitable for large-caliber orifice plate. This throttling device is also equipped with a cleaning mechanism that eliminates dirty media and provides an effective means for accurate measurement of single well natural gas.
The fixed throttling device changes the existing throttling device. According to the calculation results of the method of processing its aperture, each nominal diameter pipe is equipped with a limited-aperture throttling element, and the aperture series is selected according to the priority coefficient. The use of fixed throttling devices facilitates mass production of products and reduces production costs. The exchangeable orifice throttling device is matched with the fixed value orifice plate, which will change the traditional production mode, realize the serialization, generalization and standardization of the throttling device, and help improve the accuracy of throttling device measurement.
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