Orifice plate

The orifice plate is as part of a diaphragm measurement path a sensor with which the flow of a pipeline according to the differential pressure can be measured methods. For the measurement itself, a differential pressure measuring device and knowledge of the physical properties ( viscosity , density and isentropic exponent ) are required. The whole is a system for flow measurement according to the differential pressure method. The details are defined in the standard ISO 5167-1 and 2: 2003 (formerly DIN 1952) and for special applications in VDI 2041.

function

Scheme of an orifice measuring section with corner pressure extraction according to DIN EN ISO 5167-2

The uniform flow of a fluid in a pipeline is constricted by the orifice (cross-sectional constriction), so that the speed increases at this point. The increase in speed at the point of constriction causes a decrease in the static pressure in accordance with Bernoulli's energy equation . The resulting pressure difference is referred to as the differential pressure and is a measure of the flow (volume or mass flow). The essential features of a standard orifice plate are a sharp inlet edge, a concentric arrangement of the bore and a cylindrical bore of a defined length. The possible measuring range (min / max) is 1 to 10. Only a range of 1 to 3 is used for flow measurements for fiscal metering . The flow measurement with a measuring orifice or an orifice measuring section can be calibrated, but does not have to be calibrated. If the devices meet the high geometrical requirements of ISO 5167, the flow rate can be calculated from the geometry of the throttle element, the respective physical properties of the fluid and the effective pressure via the throttle element. Accuracies of up to ± 0.2% are achieved. The respective measurement deviation is primarily determined by the respective error in the differential pressure measurement , since the flow rate is proportional to the square root of the differential pressure. With higher accuracy requirements, the influence of temperature and the change in density of the fluid must also be taken into account. The inlet and outlet sections, which are described in detail in ISO 5167, have a decisive influence on the measurement accuracy. The trouble-free flow path required here (from 6 to 44 times the inside diameter of the pipe) is often in conflict with the available space. As a result of the increased friction, an orifice measuring section has a higher pressure loss than other flow measuring devices (the so-called permanent differential pressure). This depends on the fluid properties and on the diameter ratio and is smaller than the effective pressure, but is usually at least 40%. Measuring orifices are therefore preferably used primarily for calibrating volume flow measuring devices and in test equipment. ${\ displaystyle \ beta}$

application

Example of an orifice measuring section with an aluminum orifice body

Due to the high measurement accuracy with low susceptibility to interference, orifice measurement is currently used in a wide variety of application areas for flow measurement of fluids. For reasons of energy efficiency (comparatively high permanent pressure loss), however, the use of orifice measurement in building and process engineering systems has declined significantly in recent years. The orifice measuring section is still very important for applications with very high accuracy requirements, such as in testing and calibration equipment and other special applications.

Examples

Measurement of flow rates, which are the basis for billing
Measurement of fluids with very high temperatures (e.g. power plants)
Calibration of volume flow meters
Test benches that place very high demands on the measurement accuracy of the volume flow
Fan - test according to DIN 24163 and DIN EN ISO 5801

Parameters for the calculation

The mathematical fundamentals are provided by fluid mechanics , especially the Bernoulli equation . On this basis, flow coefficients were determined empirically. According to the law of similars ( Reynolds number ), these flow coefficients are generally valid and can thus be transferred to the specific installation. Due to the geometric restrictions of the ISO 5167 standard, volume flow measurement for air with orifice measuring sections is usually possible in the range between 11 m³ / h and 100,000 m³ / h. The lower limit is formed by the requirement of the standard that the inner pipe diameter must not be less than 50 mm and the smallest permissible Reynolds number, based on the inner pipe diameter, must be above 5,000. The smallest admissible Reynolds number depends on the selected diameter ratio of the design of the pressure tapping for the differential pressure (according to the standard, this is possible as corner pressure tapping, flange pressure tapping and D- ; D / 2 pressure tapping ). ${\ displaystyle \ beta}$

D Inside diameter of the pipeline at operating temperature
d Inner diameter of the orifice at operating temperature
${\ displaystyle \ beta}$ Diameter ratio ( ) ${\ displaystyle \ beta = d / D}$
Re Reynolds number based on the inside diameter of the pipe
${\ displaystyle C}$ Flow coefficient ( ) ${\ displaystyle C = f (\ beta, Re)}$

Various approximation equations have been developed for the software-based calculation. The following Reader-Harris / Gallagher equation , which is also part of ISO 5167-2, is shown here as an example for an orifice measuring section with corner pressure extraction and an internal pipe diameter greater than 71.12 mm .

{\ displaystyle C = 0 {,} 5961 + 0 {,} 0261 \ cdot \ beta ^ {2} -0 {,} 216 \ cdot \ beta ^ {8} +0 {,} 000521 \ cdot (10 ^ { 6} \ cdot \ beta / Re) ^ {0 {,} 7} + (0 {,} 0188 + 0 {,} 0063 \ cdot (19000 \ cdot \ beta / Re) ^ {0 {,} 8}) \ cdot \ beta ^ {3 {,} 5} \ cdot (10 ^ {6} / Re) ^ {0 {,} 3}}

For orifice measuring sections with an internal pipe diameter between 50 mm and 71.12 mm, as well as for alternatives to corner pressure extraction, the equation shown here must be expanded with additional coefficients, which can be used e.g. B. are shown in the international standard ISO 5167-2.

${\ displaystyle \ kappa}$ Isentropic exponent (for gases)
${\ displaystyle \ epsilon}$ Expansion number (only for compressible media, for incompressible media is ) ${\ displaystyle \ epsilon = 1}$
${\ displaystyle p_ {1}}$ Plus pressure (absolute pressure in front of the orifice)
${\ displaystyle \ rho _ {1}}$ Density of the fluid in front of the orifice at operating temperature
${\ displaystyle \ Delta p}$ Differential pressure ( ) ${\ displaystyle \ Delta p = p_ {1} -p_ {2}}$
${\ displaystyle q_ {M}}$ Mass flow ${\ displaystyle q_ {m} = {\ frac {C} {\ sqrt {1- \ beta ^ {4}}}} \ cdot \ epsilon \ cdot {\ frac {\ pi} {4}} \ cdot d ^ {2} \ cdot {\ sqrt {2 \ cdot \ Delta p \ cdot \ rho _ {1}}}}$
${\ displaystyle q_ {V}}$ Volume flow ${\ displaystyle q_ {V} = q_ {m} / \ rho _ {1}}$