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Process Control Sensors Measuring and Monitoring Liquid Flow
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According to the laws of fluid mechanics, an obstruction inserted in a flow stream creates a periodic turbulence behind it. The frequency of shedding the turbulent vortices is directly proportional to the flow velocity. The flow sensor in Fig. 7.30 has a sensing element consisting of a thin metallic obstruction and a downstream metallic bar attached to a multimode fiber-microbend sensor. As illustrated in Fig. 7.31, the vortex pressure produced at the metallic bar is transferred, through a diaphragm at the pipe wall that serves as both a seal and a pivot for the bar, to the microbend sensor located outside the process line pipe. The microbend sensor converts the time-varying mechanical force caused by the vortex shedding into a corresponding intensity modulation of the light. Therefore, the frequency of the signal converted into the electric voltage at the detector provides the flow-velocity information. This flow sensor has the advantage that the measuring
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FIGURE 7.30
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Principle of operation of a vortex-shedding ow sensor.
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FIGURE 7.31
Schematic diagram of a vortex-shedding ow sensor.
accuracy is essentially independent of any changes in the fluid temperature, viscosity, or density, and in the light source intensity. According to the specifications for typical optical vortex-shedding flow sensors, flow rate can be measured over a Reynolds number range from 5 103 to 6000 103 at temperatures from 100 to +600 C. This range is high compared to that of conventional flow meters. In addition, an accuracy of 0.4 and 0.7 percent, respectively, is obtained for liquids and gases with Reynolds numbers above 10,000.
7.16.1 Flow Sensors Detecting Small Air Bubbles for Process Control in Manufacturing
Another optical-fiber flow sensor employed in manufacturing process control monitors a two-fluid mixture (Fig. 7.32). The sensor can distinguish between moving bubbles and liquid in the flow stream and display the void fraction, namely, the ratio of gas volume to the
FIGURE 7.32 Flow sensor for two-phase mixtures.
Analog output TTL output Void-fraction output
LED PIN
Signal processor Vold-fraction display 48.5% Integration Optical fiber Optical sensor
Liquid phase Gas phase
Seven
total volume. The principle of operation is quite simple. The light from the LED is guided by the optical fiber to the sensing element, in which the end portion of the fiber is mounted in a stainless steel needle with a 2.8-mm outer diameter. When liquid is in contact with the end of the fiber, light enters the fluid efficiently and very little light is returned. However, when a gas bubble is present, a significant fraction of light is reflected back. With this technique, bubbles as small as 50 m may be detected with an accuracy of better than 5 percent and a response time of only 10 s. Potential applications of this flow sensor for the control of processes in manufacturing systems are widespread for example, detection of gas plugs in production wells in the oil industry and detection of fermenters and distillers in the blood-processing and pharmaceutical industries. An optical-fiber flow sensor for a two-phase mixture based on Y-guide probes is shown in Fig. 7.33. Two Y-guide probes are placed at different points along the flow stream to emit the input light and pick up the retroreflected light from moving solid particles in the flow. The delay time between the signals of the two probes is determined by the average velocity of the moving particles. Therefore, measurement of the delay time by a conventional correlation technique provides the flow velocity. An accuracy of better than 1 percent and a dynamic range of 20:1 are obtained for flow velocities up to 10 m/s. A potential problem of such flow sensors for two-phase mixtures is poor long-term stability, because the optical fibers are inserted into the process fluid pipes.
Sensor head Bundle fiber LED
Bundle fiber LED PD Signal processor
Output
FIGURE 7.33 technique.
Flow sensor using two Y-guided probes based on a correlation
Industrial Sensors and Control
7.16.2 Liquid Level Sensors in Manufacturing Process Control for Petroleum and Chemical Plants
Several optical-fiber liquid level sensors developed in recent years have been based on direct interaction between the light and liquid. The most common method in commercial products employs a prism attached to the ends of two single optical fibers (Fig. 7.34). The input light from an LED is totally internally reflected and returns to the output fiber when the prism is in air. However, when the prism is immersed in liquid, the light refracts into the fluid with low reflection, resulting in negligible returned light. Thus, this device works as a liquid level switch. The sensitivity of the sensor is determined by the contrast ratio, which depends on the refractive index of the liquid. Typical examples of signal output change for liquids with different refractive indices are indicated in Table 7.2. The output loss stays at a constant value of 33 dB for refractive indices higher than 1.40. The signal output of a well-designed sensor can be switched for a change in liquid level of only 0.1 mm.
Light source Detector (High level) Light source Detector (Low level)
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