Pakarab fertilizers limited multan
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- Reluctance pickups
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| 3.6 VELOCITY FLOW METERS Turbine Flow meter Fluid enters the meter from one side, first passing through the flow straightener. The straightener dampens the turbulence in the fluid. The fluid then strikes the rotor smoothly, causing it to spin. The rate at which the rotor spins, sometimes referred to as rotational velocity, is directly proportional to the velocity of the fluid passing through the meter. As the rotor spins, its blades cause a pulsating electrical signal to be produced in the pickup unit. The pickup unit is mounted inline with the rotor. Each time a blade passes the head of the pickup unit, its presence is sensed and a voltage is produced. As the blade moves past the pickup head, the voltage decreases until another blade moves into position. The voltage signals are called pulses. The velocity of fluid flowing through the meter is proportional to volumetric flow rate. 69 Field Instrumentation GEN Rev) The rotational velocity of the rotor is proportional to fluid velocity. The frequency of the electrical signal produced by the magnetic pickup unit is proportional to the rotor’s speed (rpm. The pickup acts as the transducer. It converts the rotary motion of the rotary vanes into electrical energy. There are two kinds of pickups. Inductance pickups have magnet-tipped rotor vanes. As the vanes pass the coil, a voltage is induced and current flows in the pickup coil. The induced current pulses a receiver, each time a vane passes the coil. A disadvantage is possible weakening of the magnets, causing errors in the measurement. Reluctance pickups use an electromagnet as the coil. As the rotor vanes pass the pickup, they interrupt the magnetic field. The interruptions cause variations in the strength of the field, seen as pulses by the receiver. Both kinds of pickup are mounted at aright angle to the turbine rotor, and both kinds produce a pulse output. The turbine output signal is conditioned and sent to either a flow totalizer or rate indicator. The flow totalizer accepts the conditioned signal and provides readout in liters, gallons, barrels, or other units Advantages of turbine flow meter are High accuracy—most turbine meters are accurate within ±0.5% of reading. Repeatability—±0.25% is normal, and meters with ±0.02% repeatability are available. Wide flow range—turbine meters can accurately meter flows ranging from 0.001 to 40,000 gpm Linear output—the meter’s output varies directly with flow rate. 70 Field Instrumentation GEN Rev) Fast response—a turbine meter responds quickly to changes in the flow rate. A response time of 0.005 seconds is typical. Turbine meters are also small in size and weight and are easy to install and maintain. Disadvantages of turbine flow meter are They are expensive and unsuitable for very dirty or viscous (thick) fluids. Also, they are damaged by over speed and the bearing is subject to wear. In addition, they require a sophisticated signal conditioning and flow readout system and filters are required for almost every application. Download 3.37 Mb. Share with your friends:
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