Turbine flow meter
Principle:
Theturbine flowmeter is used for the measurement of liquid, gas and very low flow rates. It works on the basic principle of turbine.
Construction:
- It consists of a multi-bladed rotor (turbine wheel) which is mounted at right angles to the axis of the flowing liquid, as shown in Fig.
- The rotor is supported by ball or sleeve bearings on a shaft which is retained in the flowmeter housing by a shaft-support section. The rotor is free to rotate about its axis.
Working:
- The flowing fluid impinges on the turbine blades (rotor), imparting a force to the blade surface which causes the rotation of the rotor.
- The speed of rotor is directly proportional to the fluid velocity, and hence to volumetric flow rate when it is at a steady rotational speed.
- The speed of rotation is monitored in most of the meters by a magnetic pickup coil, which is fitted to the outside of the meter housing.
- The magnetic-pickup coil consists of a permanent magnet with coil windings which is mounted in close proximity to the rotor but internal to the fluid channel.
- As each rotor blade passes the magnetic-pickup coil, it generate a voltage pulse which is a measure of the flow rate, and the total number of pulses give a measure of the total flow.
- The electrical voltage pulses can be totalled; subtracted and manipulated by digital techniques so that a zero error characteristic of digital handling is provided from the pulse generator to the final read out.
The K factor (i e, the number of pulses generated per gallon of flow) is given as
K = (Tk x f) / Q
where,
K = pulses per volume unit, Tk =a time constant in min,
Q =a volumetric flow rate in gpm and f= frequency in Hz
Advantages:
- No external power supply for Rotating vane and Woltman meters
- Turbine flowmeters suitable for cryogenic liquids
- Turbine flowmeters usable at extreme temperatures and pressures
Disadvantages:
- Limited choice of materials
- Only for low viscosities
- Moving parts, wear
- Sensitive to contamination
- Axial flow totalizers are flow profile sensitive
- Inlet and outlet sections required (not for rotating vane meters)
- Affected by overloading and quick changes at high differential pressure, danger of over speeding
- Vibration sensitive
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