Selection and Sizing of Turbine Flowmeter
When selecting and sizing Turbine flow meters, the guide listed below can help in ensuring that the right meter is selected and correctly sized for your application:
(a) Turbine Flowmeters are sized by volumetric flow rate; however the main factor that affects the meter is viscosity. Viscosity affects the accuracy and linearity of turbine meters. It is therefore important to calibrate the meter for the specific fluid it is intended to measure. Repeatability is generally not greatly affected by changes in viscosity.
the meter be sized such that the maximum flow rate of the application is about 70 to 80% of the maximum capacity of the flowmeter.
(c) Flow velocity is critical for measurement accuracy and the proper operation of the meter. Low flow velocities are insufficient for proper operation while excessive flow velocities can result in excessive wear of the meter. Care should therefore be taken to select moderate flow velocities that are adequate for optimum meter performance. Most Turbine meters are designed for a maximum velocity of 9m/s.
(d) Temperature variation affects the viscosity of the fluid through the turbine meter which in turn affects accuracy of the meter. Therefore temperature variation must be controlled. Advances in flow computers and digital communication technology (fieldbus) now make it possible to correct for changes in temperature and viscosity.
Best Design and Installation Practice
When designing the Turbine meter set up and eventual installation in the field for actual use, it is important to take note of the following best practices as they will help ensure that the Turbine meter performs optimally throughout the useful life of the metering set up:
(a) Turbine meters are sensitive to upstream piping geometry that can cause vortices and swirling flow that causes linearity problems and introduce measurement inaccuracies. To measure flow accurately, turbine flowmeters require a good laminar flow. To achieve good laminar flow, most specifications recommend 10-15 diameters of straight run upstream and five diameters of straight run downstream piping for the flow meter. However, If there are upstream obstructions – a tee, an elbow, a filter, a strainer, a partially open valve etc - then the upstream pipe runs is significantly increased more than the required 15 diameters.
One strategy to reduce upstream pipe runs requirement is the use of straightening vanes or flow conditioners. The straightening vanes or flow conditioners should be located at a minimum of five diameters upstream of the Turbine meter.
(b) Excessive pressure drop across the Turbine meter can cause flashing or cavitation and may even result in rotor damage. Therefore, Turbine flowmeters should be sized for between 3 and 5 psi pressure differential drop at maximum flow.
(c) One of the requirements of a Turbine flow meter is that the fluid (liquid or gas) passing through it be clean. However in the course of operation, solids may become entrained in the fluid which can lead to turbine meter damage. To reduce the probability of this happening, it is required that a strainer or cartridge filter be installed at least 20 diameters of straight run pipe upstream of the flowmeter to trap any entrained solid in the fluid.
(d) The Turbine flowmeter is a unidirectional device. Flow through the Turbine meter is in only one direction. Therefore care should be taken to install the Turbine meter in the direction of flow specified by the manufacturer otherwise the meter may be damaged or may not perform satisfactorily