Titan Enterprises first radial flow turbine flow meter was introduced in 1981. Derivatives of this meter are still in production and being used for a wide range of applications.
Turbine Flowmeters: Axial flow and Radial Flow (Pelton Wheel) flowmeters
Axial Turbine Flowmeters
The two main types of low flow turbine flowmeter types are known as axial and radial flow (Pelton wheel) flowmeters. As the name suggests – in an axial turbine flowmeter the construction is axial and they contain a propeller mounted on a bearing that is central to the tube. The propeller blades are angled so that the passing flow imparts a rotation into the turbine. The rotation is linear to the fluid velocity.
Radial Turbine Flowmeters
Pelton wheel (radial flow) flow meters use a turbine with broad flat blades. The flow is directed at the flat face of the turbine, offset from the centre line, and the spindle is at ninety degrees to the flow. See diagram below.
Turbine Flow meters & Reynolds Number Sensitivity
Axial flow meters are Reynolds number sensitive and usually require fluid velocities in the turbulent region, therefore any changes in fluid density and viscosity will result in measurement inaccuracies. While Pelton wheel radial flow meters are also Reynolds number sensitive the change in geometry from axial to radial flow permits designers to make changes to account for viscosity changes.
Axial Turbine Flowmeter Clearance
Axial turbine flow meters must have a fairly tight clearance with the outer edge of the turbine as the annulus around the periphery of the metering tube accounts for a large proportion of the fluid flow. On a 12mm diameter propeller a 1mm clearance would account for some 16% of the metering area and so lost measurement volume.
Titan Pelton Wheel Flowmeters
With Titan Enterprises Pelton wheel radial flow meters the high velocity inlet liquid stream strikes the “plain” blade face on entering the flow chamber. In this case a 1mm clearance around the turbine reduces the viscous drag on the turbine so that most of the energy is imparted in rotating the turbine. Secondly when the Reynolds number of the metered liquid is smaller, the shape of the chamber is used to generate secondary vortices which act like a fluidic roller bearing – reducing the viscous drag. These meters are still sensitive to changes in viscosity but not as much as their axial counterparts.
Turbine Flowmeter Bearings
The bearings in any turbine flow meter are critical for long term performance. On a miniature axial turbine the applied torque on the rotor is much smaller than with it’s radial flow counterpart. In radial flow turbines there is a lot of energy so the bearings must be robust. Because the available energy is large with radial turbine meters a certain amount of degradation can take place before there is any discernible change in meter performance is seen and this can take very many years of continuous use. The same cannot be said for the miniature axial turbine meters where performance typically degrades faster.