# Orifice flowmeter in use K coefficient for the calculation of flow

by：Kaidi Sensors     2020-09-28
Orifice flowmeter is a kind of process flow instrumentation throttling device, is the standard orifice differential pressure transmitter with multiple parameters ( Or differential pressure transmitter, temperature transmitter and pressure transmitter) Form a complete set of high contrast range pressure flow device, capable of measuring gas, steam, liquid and flow, widely used in petroleum, chemical industry, metallurgy, electricity, heating, water supply in areas such as process control and measurement. Because of its applications has a long history, in the use of the establishment of a perfect design, calculations, specifications, selection of database table and formed the standard specification. Engineering design as a kind of standard orifice throttling element has been widely used, traffic detection is engineer design scheme of * *. Orifice flowmeter measurement principle of orifice plate is used to process flow testing, is based on fluid mechanics and made the ideal fluid transmission characteristics of throttling element. Ideal fluid Bernoulli equation: Z1 + P1 / & rho; 1 g + V12/2g = Z2 + P2 /ρ 2 g + + hfZ1 V22/2 g, Z2 is throttling element before and after taking pressure point distance benchmark platform produced by the vertical height of the pressure head; P1, P2 is throttling components before and after the static pressure of medium inside pipeline; V1, V2 is throttling element medium before and after the average flow velocity; ρ 1、ρ 2 is the medium density before and after the throttle, ideal fluid can be regarded as density before and after the throttle is not a & rho; ; Hf for fluid head loss caused by the throttling element in the standard throttling element standardization installation process can be ignored. According to the above formula and symbol definition available: ( V22 — V12） / 2 g = ( P1 - P2) /ρ g + ( Z1 — Z2 ） According to the law of conservation of the flow of fluid as throttling element before and after the fluid flow is equal to: Q1 = Q2; V1S1 = Q1, Q2 = V2S2, thus throttling element of average flow velocity is: V2 = V1 S1 / S2 of S1, S2 separately and throttling element circulation cross-sectional area of the pipeline, the pipeline of the size and the throttling orifice plate hole size can be calculated. Thus it is concluded that the average flow velocity of fluid before throttling element: V12 = [ ( P1 - P2) /ρ g + ( Z1 — Z2 ） ] 2g S22/( S12- S22 ) ; In practical use, take pressure before and after the throttling element points to the benchmark platform is consistent with vertical distance, therefore, Z1 — Z2 ） = 0, thus the above formula can be simplified as: V1 = & radic; ( P1 - P2 ） √ ( 2S22/( S12- S22 ) ρ =K√ ( P1 - P2 ） ; Type in the K = & radic; ( 2S22/( S12- S22 ) ρ In the use of standard orifice plate installed in maintaining basic constant can be regarded as a constant as the flow coefficient, calculated in the engineering design and calculation. By a formula on the fluid flow velocity and throttling element before and after the pressure differential in the direct ratio of kaiping, as long as we detect the throttling element before and after the pressure difference value, average flow velocity can be medium, which can calculate the volume flow of the fluid, which is the principle of the throttling orifice plate to measure the fluid flow. Orifice flowmeter in use error influence by the principle of measurement of the orifice meter above you can see, in the assumption of constant conditions remain the same circumstances, to achieve orifice differential pressure before and after the opening square is proportional to the velocity of the fluid. So in practical use, whether this restriction conditions keep constant orifice meter is * to factors of measurement is accurate. For that we used in design, installation and maintenance are required to work accordingly. K to maintain constant flow coefficient K, and the influence of the engineering design for medium density requires accurate, therefore in the process of selection of the orifice plate to provide fluid in the normal use of parameters such as temperature, pressure, density and composition. If these parameters deviating from the selection of parameters in real use is too big, may need to be done on the discharge coefficient to numerical correction. Most production equipment according to the engineering design of parameters, and online testing flow process for the precision of the parameter is not very demanding, therefore this revision of the orifice discharge coefficient does not see more in the process of production. If the orifice meter used in material measurement test, the need for fluid flow coefficient compensation measures, common is the use of temperature compensation measures for pressure flow coefficient value revision.
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