Velocity Meters

When using velocity to measure a fluid flow rate, the primary device generates a signal proportional to fluid velocity. The equation QV = A
V illustrates that the generated signal is linear with respect to the volume flow rate. Velocity meters are usually less sensitive than head meters to velocity profile, some are obstructionless, and because they provide linear output with respect to flow, there is no square-root relationship as with differential pressure meters. This eliminates the potential inaccuracies associated with square-root extraction and explains the greater rangeability of velocity meters in comparison to most head meters.

Turbine Meters

A turbine meter uses a multi-bladed rotor that is supported by bearings within a pipe section perpendicular to the flow (Figure 7). Fluid drives the rotor at a velocity that is proportional to the fluid velocity and, consequently, to the overall volume flow rate. A magnetic coil outside the meter produces an alternating voltage as each blade cuts the coil’s magnetic lines of flux. Each pulse, therefore, represents a discrete volume of liquid. Since the rotor is usually made of stainless steel, it is compatible with many fluids. However, the bearings, which are necessary to support the rotor and which must allow it to spin freely at high speeds, require a fairly clean process. Turbine meters are typically available in pipeline sizes from less than 1/2 inch through 12 inches. They have fast response and good accuracy.

Electromagnetic Flowmeters

The operating principle of magnetic flowmeter system is base upon Faraday’s Law of electromagnetic induction, which states that a voltage will be induced in a conductor moving through a magnetic field.

The magnitude of the induced voltage E is directly proportional to the velocity of the conductor V, conductor width D, and the strength of the magnetic field B. Figure 8 illustrates the relationship between the physical components of the magnetic flowmeter and Faraday’s Law. Magnetic field coils placed on opposite sides of the pipe generate a magnetic field. As the conductive process liquid moves through the field with average velocity V, electrodes sense the induced voltage. The width of the conductor is represented by the distance between electrodes. An insulating liner prevents the signal from shorting to the pipe wall. The only variable in this application of Faraday’s law is the velocity of the conductive liquid V because field strength is controlled constant and electrode spacing is fixed. Therefore, the output voltage E is directly proportional to liquid velocity, resulting in the linear output of a magnetic flowmeter.

Vortex Meters

The operating principle of a vortex flowmeter is based on the phenomenon of vortex shedding known as the von Karman effect. As fluid passes a bluff body, it separates and generates small eddies or vortices that are shed alternately along and behind each side of the bluff body (Figure 9). These vortices cause areas of fluctuating pressure that are detected by a sensor. The frequency of vortex generation is directly proportional to fluid velocity.

The output of a vortex flowmeter depends on the K-factor. The K-factor relates the frequency of generated vortices to the fluid velocity. The formula for fluid velocity is as follows:

The K-factor varies with Reynolds number, but it is virtually constant over a broad flow range (Figure 10). Vortex flowmeters provide highly accurate linear flow rates when operated within this flat region.

Ultrasonic Meters

Ultrasonic flowmeters use sound waves to determine the flow rate of fluids. Pulses from a piezoelectric transducer travel through a moving fluid at the speed of sound and provide an indication of fluid velocity. Two different methods are currently employed to establish this velocity measurement. The first ultrasonic meters used a transit-time method, in which two opposing transducers are mounted so that sound waves traveling between them are at a 45 degree angle to the direction of flow within a pipe. The speed of sound from the upstream transducer to the downstream transducer represents the inherent speed of sound plus a contribution due to the fluid velocity. In a simultaneous measurement in the opposite direction, a value (determined electronically) is representative of the fluid velocity, which is linearly proportional to the flow rate. While the transit-time method works well in most fluids, it is essential that they be free of entrained gas or solids to prevent scattering of the sound waves between transducers. Another type of ultrasonic meter uses the Doppler effect.

This type of ultrasonic meter uses two tranducer elements as well, but each is mounted in the same case on one side of the pipe. An ultrasonic sound wave of constant frequency is transmitted into the fluid by one of the elements. Solids or bubbles within the fluid reflect the sound back to the receiver element. The Doppler principle states that there will be a shift in apparent frequency or wavelength when there is relative motion between transmitter and receiver. Within the Doppler flowmeter, the relative motion of the reflecting bodies suspended within the fluid tends to compress the sound into a shorter wavelength (high frequency).

This new frequency measured at the receiving element is electronically compared with the transmitted frequency to provide a frequency difference that is directly proportional to the flow velocity in the pipe. In contrast to the transit-time method, Doppler ultrasonic meters require entrained gases or suspended solids within the flow to function correctly. While ultrasonic meters have several advantages, including freedom from obstruction in the pipe and negligible cost-sensitivity with respect to pipe diameter, their performance is very dependent on flow conditions. A fair accuracy is attainable with ultrasonic flowmeters when properly applied to appropriate fluids.

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