Orifice plate

The measurement diaphragm is provided as part of a shutter, a distance measuring sensor, with which the flow rate of a pipeline can be measured by the differential pressure method. To measure itself is a differential pressure meter and the knowledge of the material properties (viscosity, density, and isentropic exponent ) are required. The whole thing is a system for flow measurement by the differential pressure method. The details are specified in ISO 5167-1 and 2:2003 ( formerly DIN 1952) and defined for special applications in VDI 2041.

Function

The steady flow of a fluid in a pipe is constricted by the diaphragm ( cross-sectional narrowing ), thereby increasing the speed at this point. The increase in velocity at the constriction caused in accordance with the Bernoulli equation, a power reduction of the static pressure. The resulting pressure difference is referred to as differential pressure, and is a measure of the flow rate (volume or mass flow ). The essential features of a standard orifice plate are a sharp leading edge, a concentric arrangement of the bore and a cylindrical bore of defined length. The possible measurement range ( min / max ) is 1 to 10 For flow measurements for commercial invoices (English: fiscal metering ) a range of 1 is used only to 3. The flow measurement with a diaphragm or a diaphragm measuring section is verifiable, but need not be calibrated. Do the devices the high geometric requirements of ISO 5167, can be calculated via the throttle element of the flow from the geometry of the throttle element, the respective physical properties of the fluid and the differential pressure. It will reach up to ± 0.2 % accuracy. The respective measurement error is primarily determined by the respective errors of the differential pressure measurement, since the flow rate is proportional to the square root of the differential pressure. At higher accuracy requirements of the influence of the temperature and the change in density of the fluid is also considered. Decisive influence on measuring accuracy have continued the inlet and outlet sections are described in detail in ISO 5167. The required here undisturbed flow profile (from 6 - to 44 - times the pipe inside diameter ) is often associated with the available space in conflict. Due to the increased friction has an aperture measuring section in comparison with other flow measurement devices have a higher pressure loss ( the so-called constant pressure differential). This is dependent upon the fluid properties and the diameter ratio and is smaller than the differential pressure, but is usually at least 40%. Measuring orifices therefore are preferred especially for the calibration of flow meters and used in testing equipment.

Application Examples

  • Measure flow, which are the basis of a settlement
  • Measurement of fluid at very high temperatures (eg power plants)
  • Calibration of flow measuring devices
  • Test benches that provide very high demands on the accuracy of the volume flow
  • Fan - test according to DIN 24163 and ISO 5801

Parameters for the calculation

The mathematical basis provides the fluid mechanics, in particular the Bernoulli law. After this basis flow coefficients were determined empirically. According to the law of similars ( Reynolds number ), these flow coefficients are universal and can thus be transferred to the concrete installation. Due to the geometric constraints of the standard ISO 5167, the volume flow measurement for air with aperture measuring sections is usually in the range between 11 m³ / h and 100,000 m³ / h. The lower limit is defined by the requirement of the standard that the tube inside diameter is not less than 50 mm and the smallest permissible Reynolds number must be based on the pipe inner diameter above 5,000. The smallest allowable Reynolds number is selected depending on the diameter ratio and the design of the taps for the differential pressure (according to norm this as Eckdruckentnahme than Flanschdruckentnahme and when; possible D / 2 pressure taps D ).

  • D inside diameter of the pipe at operating
  • D inside diameter of the aperture at the operating temperature
  • Diameter ratio ()
  • Re Reynolds number relative to the internal tube diameter
  • Flow coefficient ()
  • Isentropic exponent (for gases )
  • Expansion factor (for compressible fluids )
  • Plus pressure (absolute pressure upstream of the orifice )
  • Density of the fluid upstream of the orifice at the operating temperature
  • Differential pressure ()
  • Mass flow
  • Volume flow
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