Machinery's Handbook, 31st Edition
Flow Measurement 2759 Turbine Flow Meters: These meters allow fluid flowing through to turn a turbine rotor. The speed at which the rotor turns is indicative of the fluid velocity. The speed of the rotor is mechanically or electronically monitored. Because the turbine obstructs the fluid stream to some extent, some energy is lost due to restriction. These meters can be used for liquids or gases. Care must be taken when using these meters in a fluid that is non-lubricat - ing, since the bearings that support the rotor may prematurely wear. Turbine flowmeters are less accurate at low flow rates due to drag, and should be operated above 5 percent of rated flow. Use in dirty fluids should generally be avoided to reduce the possibility of wear and damage. Two-phase flow and abrupt transitions from gas to liquid should be avoided because this can damage the mechanism. Vortex Shedding Flow Meters: These are flow-through meters used for both liquids and gases. Placement of a blunt obstruction, called a bluff body , in the fluid stream causes vortices to be shed at a frequency dependent on the flow velocity. The shedding fre - quency is independent of fluid properties such as density, viscosity, conductivity, etc., however the flow must be turbulent for vortex shedding to occur. Sensors electronically monitor the shedding vortices. The obstruction need not be very large, and varies between manufacturers. Therefore, the energy lost by the fluid varies greatly from device to device. The advantage of this type of meter is that it is suitable for a wide range of fluids and gases. However, if a fluid includes substances likely to stick to or coat the bluff body, the meter’s characteristics will change and accuracy will be compromised. The minimum measured flow rate should be at least twice the minimum rate detectable by the meter. The maximum flow capacity of the meter should be at least five times the anticipated maximum flow rate. The energy loss through a vortex meter is about half that of an orifice plate. The installed cost of vortex meters is competitive with that of orifice meters in sizes under six inches. Magnetic Flow Meters: For use with conducting fluids only, magnetic flow meters use electrodes to detect the voltage generated in a fluid as it moves between two electromag netic coils. The voltage generated is dependent on fluid velocity. Since there is nothing obstructing flow, these flow-through meters cause less energy loss than obstructing types. Another advantage of these meters is their suitability for a wide variety of fluids because their function is not affected by temperature, viscosity, or specific gravity. Ultrasonic Flow Meters: These flow-through devices are attached to the outside of a pipe or tube, and therefore do not restrict flow or cause energy loss in the fluid. There are meters for gases and for fluids. Ultrasonic signals are transmitted through the pipe and fluid. The time it takes the signal to reach the receiver depends on the flow rate of the fluid. Some ultrasonic flow meters measure across the pipe, while others measure axially. A special type of ultrasonic flow meter, called a “Doppler” meter, is used for fluids with particulate content which may obstruct traversing ultrasonic signals. These meters measure the time it takes for signals to be reflected back from the fluid. Positive Displacement Flow Meters: These are not flow-through devices, but instead allow fluid to fill a volumetrically calibrated chamber before the fluid is released. These meters can utilize a wide variety of mechanisms to capture and release volumes of fluid, one of the most common being diaphragm types typical to home gas meters. Other de- signs incorporate gears, pistons, and nutating disks (wobble plates). These meters are most often used for liquids, but can be used for gases as well. Coriolis Mass Flow Meters: These flow-through devices measure mass flow rate di - rectly, as opposed to measuring volume flow rate or velocity. By routing fluid through looping geometry, Coriolis force is generated and measured with sensors. Measurement results depend on the vibration of the tubes, so changes in tube rigidity due to temperature and pressure changes will affect calibration of the meter. These factors can be compen sated for, but pitting, cracking, coating, erosion, or corrosion of the tubes cannot. Some energy is lost as the looping geometry causes changes in direction and velocity. These devices can be outfitted to monitor density of the fluid.
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