Introduction
Turbine flowmeter have rotor-mounted blades that rotate when a fluid pushes against them. They work on the reverse concept to a propeller system. Whereas in a propeller system, the propeller drives the flow, in this case the flow drives and rotates the propeller. Since it is no longer propelling the fluid, it is now called a turbine.
The rotational speed of the turbine is proportional to the velocity of the fluid. Different methods are used to convey rotational speed information. The usual method is by electrical means where a magnetic pick-up or inductive proximity
switch detects the rotor blades as they turn. As each blade tip on the rotor passes the coil it changes the flux and produces a pulse. Pulse rate is directly proportional to the flowrate.
As the rotation of the turbine is measured by means of non-contact, no tapping points are required in the pipe. Pressure is therefore not a problem, and in fact pressures of up to 9300psi can be applied without any problem, but this of course does depend on pipe diameter and materials of construction.
Temperature limitations are only imposed by the limitations of the materials of construction. To reduce losses or changes in process temperature, turbine flowmeters are available which can be subjected to wide temperature variations.
Turbine meters require a good laminar flow. In fact 10 pipe diameters of straight line upstream and no less than 5 pipe diameters downstream from the meter are required. They are therefore not accurate with swirling flows.
They are not recommended for use with high viscosity fluids due to the high friction of the fluid which causes excessive losses as the turbine becomes too much of an obstruction. The viscosity of the liquid must be known for use of this type of meter. They are also subject to erosion and damage. Each meter must be calibrated for its
application.
Advantages
– High accuracy, repeatability and Rangeability for a defined viscosity and measuring range.
– Temperature range of fluid measurement: -220oC to +350oC.
– Very high-pressure capability: 9300psi.
– Measurement of non-conductive liquids.
– Capability of heating measuring device..
– Suitable for very low flow rates.
Disadvantages
– Not suitable for high viscous fluids.
– Viscosity must be known.
– 10 diameter upstream and 5 diameter downstream of straight pipe is required.
– Not effective with swirling fluids.
– Only suitable for clean liquids and gases.
– Pipe system must not vibrate.
– Specifications critical for measuring range and viscosity.
Application Limitations
As turbine meters rely on the flow, they do absorb some pressure from the flow to propel the turbine. The pressure drop is typically around 20 to 30 kPa at the maximum flow rate and does vary depending on flow rate
Also read
Ultrasonic Flow Measurement
Rotameter basics, variable area flow meter
