Selection of Differential Pressure Flow Meters with Large Range Ratio

Many process personnel at industrial sites may know that the range ratio of standard throttling devices is more suitable at 3:1. If the flow rate is below 30% of the full range, it should not be used. If the range ratio is too large, it will seriously affect the accuracy. Now many differential pressure flowmeters can have a large range ratio. For example, the widely used V-cone flowmeter can reach 13:1, and the wedge flowmeter that is good at measuring dirty media has a larger range ratio of 15:1. These differential pressure flowmeter products make up for the shortcomings of the small range ratio of standard throttling devices. So why is there only such a small flow when the throttling device measures the flow rate on site? If you study its principle, you will find that it is mainly determined by the flow calculation formula of the throttling device. Here is a detailed introduction to the reason:

In flow measurement, the national standard stipulates that the applicable flow ratio of the throttling device is 30% (that is, maximum flow: minimum flow = 3:1). Because the differential pressure is proportional to the square of the flow, when the flow ratio is lower than 30%, the accuracy cannot be guaranteed. In addition, when the flow is lower than 30%, the Reynolds number is often lower than the limit Reynolds number, and the flow coefficient α is not a constant, resulting in inaccurate flow measurement.

So how to deal with such a situation?

1. First, negotiate with the process personnel to reduce the maximum flow. If the Reynolds number is large enough, it can be solved by changing the orifice plate or differential pressure.
2. Use other differential pressure flowmeters instead. If the on-site operating parameters cannot be changed and the flow range is very large, we can choose V-cone flowmeters. V-cone flowmeters are currently widely used. The main features are: high accuracy, large range, small pressure loss, and energy saving. For on-site corrosive media, wedge flowmeters can be used. The main features are: low Reynolds number lower limit, wide application range, and good stability.
3. For on-site large-caliber measurements, from the perspective of economy and installation, you can choose an inserted differential pressure flowmeter, such as: Annubar, Willbar, Deltabar and other average velocity tube flowmeters, inserted dual Venturi, inserted V-cone flowmeters, etc. These flow meters have absolute advantages when measuring large diameters. First of all, they are low cost and simple to install. More importantly, they can guarantee on-site measurement results.

No matter which flowmeter is selected, it has certain limitations, and we should also use other methods to make up for the shortcomings.