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.
Turbine Flow Measurement: Turbine Flowmeter design
In principle designing an axial turbine is easy, put a propeller in a tube. In practice years of experience have developed the basic design to a quite complex assembly. Keeping the turbine spinning freely is the main object of any design so particular attention is made to bearing design and sources of drag within the assembly. This is one of the reasons that miniature axial turbines are difficult to make as the relative turbine energy is largely cancelled out by the bearing and sensor drags. Either plain bushes or roller/ball bearings are used for the rotational efficiency and usually a ball is used to take the end thrust. Some designs have hydrodynamically designed turbine and bearing supports which create a low pressure area in front of the turbine which “pulls” it forward so reducing or eliminating the end thrust. The bearing supports are designed to reduce the pressure drop whilst increasing the fluid velocity over the turbine blades. As mentioned previously the sensor is usually magnetic giving a low voltage sine wave output. When extra accuracy, extended flow range or the meter is operating in an electrically noisy environment an electronic pick-up is often used. There are modern miniature variants where the turbine has plain blades in-line with the flow and the actual fluid is rotated by the upstream bearing support which is twisted to swirl the flow onto the plain blades. I have also worked on the original design for a double ended bearing less turbine where the axle was fully supported by the fluid with no turbine contact with the body or bearing drag. A 40 year old prototype turbine assembly is currently used as a paper weight on my desk (see image below). This flowmeter was developed to be an all polymer meter for very aggressive chemicals.
Advantages of turbine flow measurement
- Simple well understood technology.
- Low cost.
- Relatively wide operational envelope.
- Easy to install and operate.
- Good performance.
- Suitable for gases and liquids.
Disadvantages of turbine flow measurement
- Fluids must be clean.
- Require careful installation to avoid errors.
- Cavitations cause problems.
- Require frequent calibration checks.
- Bearing degradation affecting accuracy.
- Viscosity changes cause errors.
Applications for turbine flowmeters are found in monitoring clean liquid flows in the water, petroleum, and chemical industries. Water applications include distribution systems within and between water districts. Petroleum applications include the custody transfer of hydrocarbons. Miscellaneous applications are found in the food and beverage, and chemical industries.
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