Help: Calculation of the rated flow rate for orifice plate flow meters
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As shown in the figure, this is the calibrated flow rate for the orifice plate as calculated by the orifice plate manufacturer; however, it differs from the flow rate calculated by the flow meter by eight to nine hundred cubic meters... After investigation, the reason lies in the expansion coefficient: the orifice plate manufacturer used the commonly used expansion coefficient of 0.997 as stated in the calculation formulas, while the flow meter used an expanded coefficient of 0.988. Moreover, the orifice plate manufacturer explicitly stated that the expansion coefficient applicable for calculating the calibrated flow rate should be the one used under normal flow conditions; yet I could not find such a statement in GB/T 2624. May I ask everyone, where can I find the calculation formulas or instructions for flow rate based on orifice plate markings?
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Shouldn’t the coefficient of expansion of a material be determined based on the operating temperature? What is the relationship between the coefficient of expansion and flow rate?
This expansion coefficient does not refer to the material’s expansion coefficient; rather, it represents the proportion of volume expansion before and after the orifice plate, and is used to correct the gas density.
What is measured is volumetric flow rate; what is its relationship to expansion?
The principle of throttling is Bernoulli’s equation. Unlike liquids, when a gas is throttled, the static pressure decreases which leads to a reduction in density; as a result, P1/ρ1 and P2/ρ2 on both sides of the equation cannot be directly canceled out using the pressure difference ΔP (P1–P2). Therefore, the gas expansion coefficient is introduced to \"simulate\" the value of ρ2 at the downstream side, thereby allowing the equation to be balanced. Based on the above measurement principles, gas expansion actually occurs, regardless of the volume unit used. If you ignore the impact of the coefficient of expansion on measurement accuracy and simply set the coefficient of expansion to 1 (treating it as a liquid), the accuracy will decrease by approximately 0.5% to 1%. The larger ΔP/P1, the greater the deviation.