Standard and non-standard differential pressure flowmeters

The differential pressure flowmeter (DPF or flowmeter for short) is an instrument that measures flow based on the differential pressure generated by the flow detection device installed in the pipeline, the known fluid conditions, and the geometric dimensions of the detection device and the pipeline. The development of the standard throttling DPF has gone through a long process. As early as the 1920s, the United States and Europe began to conduct large-scale experimental research on throttling devices. The most commonly used throttling devices, orifice plates and nozzles, began to be standardized. The geometric shape of the ISA l932 nozzle, a type of standard nozzle, was standardized in the 1930s, and the standard orifice plate was also called the ISA l932 orifice plate. The standardization of the structural form of the throttling device has far-reaching significance, because only when the structural form of the throttling device is standardized can it be possible to bring together many international research results. It promotes the expansion of the theory and practice of detection parts in depth and breadth, which is beyond the reach of other flow meters.

       In 1980, ISO (International Organization for Standardization) officially adopted the international standard ISO 5167, and the first international standard for flow measurement throttling devices was born. ISO 5167 summarizes the theoretical and experimental research results of a limited number of throttling devices (orifice plates, nozzles and venturi tubes) in the world over the past few decades, reflecting the contemporary scientific and production technology level of such test parts. However, since the official promulgation of ISO 5167, it has exposed many problems that need to be solved. These problems are mainly in the following aspects.

   The obsolescence of the test data of ISO 5167. Most of the data used in ISO 5167 are test results from the 1930s. Today, there have been great advances in the manufacturing technology of throttling devices, flow test equipment and experimental technology. It is necessary to re-test systematically to obtain more accurate and reliable data. In the 1980s, the United States and Europe conducted large-scale tests to lay the foundation for the revision of ISO 5167.
   The problem of the straight pipe length regulations in ISO 5167. When ISO voted to pass ISO 5167, the United States voted against it. The main reason was that it had different opinions on the regulations on the straight pipe length. This issue should be one of the main issues in the revision of ISO 5167. The
   scientific issues of various regulations in ISO 5167. There are many factors that affect the outflow coefficient of the throttling device, mainly the ratio of the aperture to the pipe diameter β, the pressure device, the Reynolds number, the eccentricity of the throttling device installation, the type of front and rear flow-blocking devices and the length of the straight pipe section, the sharpness of the edge of the orifice plate inlet, the roughness of the pipe wall, the turbulence of the fluid flow, etc. The influence of many factors is complicated, and some parameters are difficult to measure directly. Therefore, some regulations in the standard are not scientifically determined, but have to be determined artificially in order to achieve consistency. The famous flow expert Spencer (E.A. Spencer) proposed a series of issues that should be reviewed, such as the straightness, concentricity, sharpness of the right-angle edge, pipe roughness, upstream flow velocity distribution and the role of the flow regulator.
   Regarding the problem of improving the measurement accuracy of the throttling DPF. Given that the throttling DPF occupies an important position in the flow meter, it is of great significance to improve its measurement accuracy. All previous international academic conferences believe that it is necessary to make flow measurement workers, fluid mechanics and computer technology workers work closely together to tackle the problem in order to solve this problem.

　　In the 1980s, the United States and Europe began to conduct large-scale orifice flowmeter test research. Europe was the European Community Experimental Program (EEC Experimental Program), and the United States was the API Experimental Program (API Experimental Program). The purpose of the test was to use the latest modern test equipment and statistical processing technology of test data to conduct a new round of extensive test research, laying a technical foundation for the revision of ISO 5167. In 1999, ISO issued a revised draft of ISO 5167 (ISO/CD 5167-1-4). This document is a committee draft. It has major changes in technical content and editing. It is a brand new standard. It was originally scheduled to be reviewed and approved as DIS (Draft Standard) at the ISO/TC30/SC2 meeting held in Denver, USA in July 1999, but the meeting believed that there were still details that should be discussed and failed to pass. It is still unknown when the new ISO 5167 standard will be officially promulgated. The new ISO 5167 standard has substantial changes in two core contents. One is the orifice plate discharge coefficient formula, which uses the Reader-Harris/Gallagher calculation formula (RG formula) instead of the Stolz calculation formula. The other is the regulation of the length of the straight pipe section on the upstream side of the throttling device and the use of flow conditioners.

　　We usually call the throttling devices listed in ISO 5167 (GB/T2624) standard throttling devices, and the others are called non-standard throttling devices. It should be pointed out that non-standard throttling devices not only refer to those throttling devices whose structures are different from standard throttling devices, but also if the standard throttling devices work under conditions deviating from the standard, they should also be called non-standard throttling devices, for example, standard orifice plates working in mixed flow or standard Venturi nozzles working under critical flow.

　　At present, there are roughly the following types of non-standard throttling devices:

   1/4 circular orifice plate, conical inlet orifice plate, double orifice plate, double inclined orifice plate, semicircular
   orifice plate, etc. are used for low Reynolds number; circular segment orifice plate, eccentric orifice plate, annular orifice plate, wedge-shaped orifice plate, elbow throttling piece , etc. are used for dirty media;
   Rollos tube, Doyle tube, Doyle orifice plate, double Venturi nozzle, universal Venturi tube, Vasy tube, etc. are used for low pressure loss ;
   integral (built-in) orifice plate is used for small diameter;
   end throttling device, end orifice plate, end nozzle, Borda tube, etc.;
   wide range throttling device, elastically loaded variable area variable pressure head flowmeter (linear orifice plate);
   capillary throttling piece
   , laminar flowmeter; pulsating flow throttling device;
   critical flow throttling device, sonic Venturi nozzle;
   mixed phase flow throttling device.

　　The continuous expansion of the field application of throttling DPF will inevitably put forward the requirement of developing non-standard throttling devices. For many years, ISO has also been continuously formulating technical documents on non-standard throttling devices and publishing them as technical reports before they can become formal standards. It is foreseeable that in the future, some relatively mature non-standard throttling devices may be promoted to standard types. In the mid-to-late 1990s, the sales volume of various types of DPFs in the world accounted for 50%-60% of the total flow meters (about one million units per year), and the amount accounted for about 30%. The sales volume in my country accounts for about 35%-42% of the total flow meters (excluding household gas meters, household water meters and glass tube float flowmeters) (60,000-70,000 units per year).