Accuracy error of the metering throttle element
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Dear seniors at HaiChuan, while reading today I came across a passage that I don’t quite understand; I hope you can help me figure it out. In terms of flow measurement, **the standard specifies that the flow ratio applicable to throttle devices is 30% (maximum flow: minimum flow = 1:3). This is because the differential pressure is proportional to the square of the flow rate, and accuracy cannot be guaranteed when the flow ratio is below 30%. Furthermore, when the flow rate is less than 30%, the Reynolds number is often below the critical Reynolds number, and the flow coefficient is not constant, resulting in inaccurate flow measurement. The two sentences that are not understood are: 1. What does it mean when the traffic ratio is below 30%? 2. When the flow rate is below 30%, does this refer to 30% of the maximum flow rate of the throttling element or 30% of the actual flow rate in the current operation? Please explain
Range ratio, minimum flow: maximum flow. The common range ratios are 1:3 and 1:4.
Answer: The “maximum flow rate” mentioned here refers to the rated maximum flow rate of the throttle element, that is, the nominal flow rate, which is a constant value.
In differential pressure flow calculation, it is necessary to provide the values for rated flow, maximum flow, typical flow, and minimum flow. The 30% mentioned in the standard refers to 30% of the rated flow rate ； A value below 30% means that at low flow rates, the Reynolds number does not reach the turbulent regime; changes in the Reynolds number lead to changes in the discharge coefficient, which in turn means a deviation from the values provided by the initial calculations, and as a result the required measurement accuracy is not achieved. Therefore, flow measurement has range requirements.
I see. It’s a pity that our throttle element specification sheet only lists the operational flow rate and the maximum flow rate of the orifice plate
1/When the flow ratio is below 30%: Maximum flow: Minimum flow = Flow ratio. 2. When the flow rate is below 30%, this refers to being below 30% of the maximum flow rate, not 30% of the actual flow rate used in the process operation