Turbine Flow Measurement Explained

Turbine flowmeter devices were historically the flowmeter of choice for clean, filtered, low viscosity fluids. Their design and working principles are well understood, they have a relatively low pressure drop for in-line flow meters and can give excellent accuracy. They are used for both lower viscosity liquids and gases with turn down ratios from 7 to 30:1. This extended range is achieved by using a suitable zero drag electronic pick off instead of the standard magnetic type which causes too much extra drag to turbine flow measurement at low fluid velocities slowing the rotational speed. Accuracies of ±0.25% and repeatability of better than ±0.05% are achievable although more typically ±0.5% accuracy and ±0.1% repeatability. The larger the flowmeter the more accurate it is likely to be as designing “miniature” axial flow turbines is plagued by a large number of potential problems. They are inherently Reynolds number sensitive and will not be linear at Reynolds numbers approaching or in the laminar region. For this reason care must be taken when using these turbine flow measurement devices with fluids that have a wide viscosity/temperature range for example some oils. At constant conditions there is no problem but in practice this is often difficult to achieve.