Historically, turbine flow meters (Figure 1) were preferred for clean, filtered, low viscosity fluids. The working principles and design of turbine flow meters are well documented. With comparatively low pressure drop for in-line meters, turbine flow meters deliver unprecedented accuracy.
Figure 1. Key components of a turbine flow meter
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.
Please click here if you would like more information on the instrument used in this article or a quote
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.
Modern miniature variants are available with turbines having plain blades in-line with the flow. The upstream bearing support rotates the actual fluid and is twisted to swirl the flow onto the plain blades. In the original design for a double-ended bearing less turbine, the fluid fully supported the axle with no turbine contact with the body or bearing drag. A 40-year-old prototype turbine assembly shown in Figure 2 was designed to be an all polymer flow meter to handle very aggressive chemicals.
Figure 2. A 40-year-old prototype turbine assembly
Advantages and Disadvantages of Turbine Flow Meters
The advantages of turbine flow meters are given below:
- Simple well understood technology
- Suitable for gases and liquids
- Good performance
- Relatively wide operational envelope
- Low cost
- Easy to install and operate
The following are the drawbacks of turbine flow meters:
- Require clean fluids
- Installation must be done carefully to avoid errors
- Problems due to cavitations
- Accuracy is affected by bearing degradation
- Errors caused by viscosity changes
- Require frequent calibration checks
Applications of Turbine Flow Meters
Turbine flow meters find use in monitoring of clean liquid flows in chemical, petroleum and water industries. The custody transfer of hydrocarbons is one of the petroleum applications of turbine flow meters. In the water industry, turbine flow meters are used in distribution systems between and within water districts. Turbine flow meters are also employed in the food and beverage industries.
This information has been sourced, reviewed and adapted from materials provided by Titan Enterprises Ltd.
For more information on this source, please visit Titan Enterprises Ltd.