In case of indirect method or inferential method, levels are measured in terms of other parameters like pressure. Measuring instrument will be calibrated in terms of level. As discussed previously, direct reading level devices are normally for local indication only. When remote indication like in the control room or a control signal is required, an indirect measurement device must be used. There are different types of indirect level-measurement devices like differential pressure, static pressure, ultrasonic, and radiation level measuring instruments. Some are discussed below.
Differential pressure level measurement
This type of instrument measures the differential pressure resulting from two columns of liquids with unequal height. The height of one column will be always constant and that of the other will be variable. A typical example of this type would be the steam drum water level measuring device, similar to the one shown in Figure.
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| Drum level measurement using differential pressure |
This device is used in thermal power plants. The variable leg of this differential pressure measuring system is the drum itself. The low connection is made at the lowest point that level recording is required near the bottom of the drum. The high connection has a condensing pot attached that condenses steam from the drum to maintain a constant reference level. Because both the variable leg (drum level) and the constant leg are affected equally by the pressure of the steam in the drum, its effect is cancelled, leaving only the water levels as factors. Both legs are attached to a differential pressure instrument. The output of the device is a signal that is a measure of the level in the drum. As the level in the drum increases, the reference and variable leg heights are closer to equal. As the difference in height (H) decreases, their differential pressure decreases, and the level indication in control room increases. As the level in the drum decreases, the reverse happens ie, height increases, differential pressure increases, and the level indication in the control room decreases.
If a high pressure condition develops and a safety valve pops open, a sudden drop in drum pressure will occur as steam flows out of the safety. The, immediate result will be an increase in drum level due to the decreased drum pressure. This decrease in drum pressure also causes an increase in steam generation, which causes a rapid drop in drum level. To control this situation, the unit should be shut down as quickly as possible, and the feed water flow to the drum increased to maintain a water level.
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If a high pressure condition develops and a safety valve pops open, a sudden drop in drum pressure will occur as steam flows out of the safety. The, immediate result will be an increase in drum level due to the decreased drum pressure. This decrease in drum pressure also causes an increase in steam generation, which causes a rapid drop in drum level. To control this situation, the unit should be shut down as quickly as possible, and the feed water flow to the drum increased to maintain a water level.
See Also:
Radiation level detectionlevel measurement using hydrostatic pressure
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