Turbine Measuring Principle
A turbine flow meter is constructed with rotor and blades that use the mechanical energy of the fluid to rotate the rotor in the flow stream. Blades on the rotor are angled to transform energy from the flow stream into rotational energy. The rotor shaft spins on bearings: when the fluid moves faster, the rotor spins proportionally faster. Shaft rotation can be sensed mechanically or by detecting the movement of the rotor blades.
Rotor movement is often detected magnetically, where movement of the rotor generates a pulse. When the fluid moves faster, more pulses are generated. Turbine flow meter sensors detecting the pulse are typically located external to the flowing stream to avoid material of construction constraints that would result if wetted sensors were used. The RPM of the turbine wheel is directly proportional to the mean flow velocity within the tube diameter and corresponds to the volume flow over a wide range.
A flow transmitter processes the pulse signal to determine the flow of the fluid. Flow transmitter and sensing systems are available to sense flow in both the forward and reverse flow directions. High accuracy turbine flowmeters are available for custody transfer of hydrocarbons and natural gas. This fuel flow meter often incorporates the functionality of a flow computer to correct for pressure, temperature, and fluid properties in order to achieve the desired accuracy for the custody transfer application.
Care should be taken when using a turbine flow meter on fluids that are non-lubricating because the flowmeter can become inaccurate and fail if its bearings prematurely wear. A turbine flow meter can be outfitted with grease fittings for applications with non-lubricating fluids. In addition, a turbine flow meter that is designed for a specific purpose, such as natural gas service, can often operate over a limited range of temperatures (such as up to 140oF or 60ºC) whereby operation at higher temperatures can damage the flowmeter.
A turbine flow meter is less accurate at low flow rates due to rotor/bearing drag that slows the rotor. Care should be taken when operating these flowmeters above approximately 5 percent of maximum flow. A turbine flow meter should not be operated at high velocity because premature bearing wear and/or damage can occur. When measuring fluids that are non-lubricating, bearing wear can cause the flowmeter to become inaccurate and fail. Bearing replacement may be needed in some applications to maintain good accuracy. Application in dirty fluids should generally be avoided so as to reduce the possibility of flowmeter wear and bearing damage.
Turbine flow meters have moving parts that are subject to degradation with time and use. Abrupt transitions from gas flowmeter applications to liquid flowmeter use should be avoided because they can mechanically stress the flowmeter, degrade accuracy, and/or damage the flow meter. These conditions generally occur when filling the pipe and under slug flow conditions. Using the turbine flow meter for two-phase flow conditions such as steam flow metering applications can also cause a turbine flow meter to measure inaccurately.
A turbine flow meter measures the velocity of liquids, gases and vapors in pipes, such as hydrocarbons in fuel flow measurement, chemical flow metering, water flow metering, cryogenic liquid flow metering, air or gas flow metering, and general industrial flow metering. High accuracy turbine flowmeters are available for custody transfer of hydrocarbons and natural gas. A mass flow computer is often used in custody-transfer applications to correct for pressure, temperature and fluid properties in order to achieve the desired accuracy. Other low viscosity applications are tap and demineralized water, fuel flow meter solvents, and pharmaceutical fluids.
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