Attività

Turbine flow meters are employed for both lower viscosity gases and liquids with turn down ratios from 7 to 30:1. The use of an appropriate zero drag electronic pick off in place of the standard magnetic type is the reason behind achieving this extended range. The turbine experiences too much extra drag at low fluid velocities due to the standard magnetic type. As a result, the rotational speed is slowed down.

The typical accuracy and repeatability of turbine flow meters are ±0.5% and ±0.1%, respectively. However, it is possible to achieve accuracy of ±0.25% and repeatability of ±0.05%.

Turbine flow meters (Figure 2) are inherently sensitive to Reynolds number. They will not be linear at Reynolds numbers nearing or in the laminar region. Therefore, they should be carefully used in the case of fluids with a broad temperature and viscosity range such as some oils. It is not a problem at constant conditions, but is often hard to achieve practically.

Meter Design

Designing an axial turbine in principal is simple, i.e., putting a propeller in a tube. With years of experience, the basic design has been developed into a relatively complex assembly. The key aspect of any design is keeping the turbine spinning freely. Hence, bearing design and sources of drag within the assembly have been given special focus.

Due to this, it is difficult to make miniature axial turbines as the relative turbine energy is largely negated by the bearing and sensor drags. For rotational efficiency, plain bushes or roller/ball bearings are employed. A ball is generally employed to handle the end thrust. The presence of hydrodynamically designed turbine and bearing supports in some designs creates a low pressure zone in front of the turbine. As a result, the turbine is pulled forward, thus mitigating or removing the end thrust.

The bearing supports lower pressure drop but increases the fluid velocity over the turbine blades. The sensor is typically magnetic and provides a low voltage sine wave output. An electronic pick-up is generally employed for extended flow range, extra accuracy or in the case of operating the flow meter in an electrically noisy environment.

What is an Ultrasonic Flow Meter : Types and Its Applications