Turbine Flowmeter


    Turbine flow meters are a type of velocity flow meter that have found widespread use in a variety of industrial applications – including aerospace, cryogenic, and custody transfer – for high accuracy measurements. Invented by Reinhard Woltman in the 18th century, the turbine flow meter is reliable for use with both liquids and gases. It consists of a multi-bladed rotor mounted at right angles to the fluid flow and suspended in the fluid stream on a free-running bearing. The diameter of the rotor is very slightly less than the inside diameter of the metering chamber, and its speed of rotation is proportional to the volumetric flow rate. Turbine rotation can be detected by solid state devices (reluctance, inductance, capacitive and Hall-effect pick-ups) or by mechanical sensors (gear or magnetic drives).

    In the reluctance pick-up, the coil is a permanent magnet and the turbine blades are made of a material attracted to magnets. As each blade passes the coil, a voltage is generated in the coil. Each pulse represents a discrete volume of liquid. The number of pulses per unit volume is called the meter's K-factor. In the inductance pick-up, the permanent magnet is embedded in the rotor, or the blades of the rotor are made of permanently magnetized material. As each blade passes the coil, it generates a voltage pulse. In some designs, only one blade is magnetic, and the pulse represents a complete revolution of the rotor.

    The outputs of reluctance and inductive pick-up coils are continuous sine waves with the pulse train's frequency proportional to the flow rate. At low flow rates, the output (the height of the voltage pulse) may be on the order of 20 mV peak-to-peak. It is not advisable to transport such a weak signal over long distances. Therefore, the distance between the pickup and associated display electronics or preamplifier must be short. Capacitive sensors produce a sine wave by generating an RF signal that is amplitude-modulated by the movement of the rotor blades. Instead of pick-up coils, Hall-effect transistors also can be used. These transistors change their state when they are in the presence of a very low strength (on the order of 25 gauss) magnetic field.

    In these turbine flow meters, very small magnets are embedded in the tips of the rotor blades. Rotors are typically made of a non-magnetic material, like polypropylene, Ryton, or PVDF (Kynar). The signal output from a Hall-effect sensor is a square wave pulse train, at a frequency proportional to the volumetric flowrate. Because Hall-effect sensors have no magnetic drag, they can operate at lower flow velocities (0.2 ft/sec) than magnetic pick-up designs (0.5-1.0 ft/sec). In addition, the Hall-effect sensor provides a signal of high amplitude (typically a 10.8-V square wave), permitting distances up to 3,000 ft. between the sensor and the electronics without amplification.

    In the water distribution industry, mechanical-drive Woltman-type turbine flow meters continue to be the standard. These turbine meters use a gear train to convert the rotation of the rotor into the rotation of a vertical shaft. The shaft passes between the metering tube and the register section through a mechanical stuffing box, turning a geared mechanical register assembly to indicate flow rate and actuate a mechanical totalizer counter. More recently, the water distribution industry has adopted a magnetic drive as an improvement over high maintenance mechanical-drive turbine meters. This type of meter has a sealing disc between the measuring chamber and the register. On the measuring chamber side, the vertical shaft turns a magnet instead of a gear. On the register side, an opposing magnet is mounted to turn the gear. This permits a completely sealed register to be used with a mechanical drive mechanism.

    In the United States, the AWWA sets the standards for turbine flow meters used in water distribution systems. Standard C701 provides for two classes (Class I and Class II) of turbine flow meters. Class I turbine meters must register between 98-102% of actual rate at maximum flow when tested. Class II turbine meters must register between 98.5-101.5% of actual rate. Both Class I and Class II meters must have mechanical registers.

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