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Instrumentation Level Measurement with Calculation Examples
To understand basic principles of different level measurement techniques.In this section, different level measurement techniques have been examined, which includes:
- Sight glass
- Dip-stick
- Float based device
- Hydrostatic devices
- Conductivity probes
- Bubble method
- Ultrasonic level gauge
- Radiation methods
- Hot-wire element
- Radar
- Laser
- Radio frequency
STRATEGIES
DEFINITION OF LEVEL
Level of liquids is defined as the height of the surface of the liquid from some reference point. As such, levels use very basic units of length and can be measured directly, in feet or meters.
General Method of level Measurements
- The position (height) of a liquid surface above or below a datum line.
- The hydrostatic head (pressure) develop by the liquid whose level is being detected.
- An electrical or physical property change because of a change in liquid level.
- Devices which measure the position are said to be direct measuring devices. Devices using hydrostatic head or some
physical or electrical property are said to be inferential measuring devices.
Direct Method
- Visual observation, e.g., with gauge glass, dip stick and dip tape.
- Determination of the position of a detecting member which rides on the liquid surface, such as a ball or other type of float.
- Interruption of a light beam to a photocell or gamma rays to a suitable detector.
- Reflection of sonic waves from a surface
-
Sight glass technique:
liquid level can be seen.
-
Dip-sticks
-
Optical dipsticks
-
Float based devices
Inferential Methods
-
Conductivity probes
-
Bubble method
-
Vibrating Probe Type:
VIBRATING FORK / PROBE LEVEL SWITCH
The vibrating fork switch is completely attenuated by relatively little mass (from 120 Hz to 0 Hz). A shift in frequency only occurs when vibrating fork switches are submerged in liquid.
-
Ultrasonic level gauge
Liquid to liquid liquid to precipitate
-
Radiation methods
-
Hot-wire element techniques
-
Radar techniques
-
Laser techniques
-
Radio frequency technique
- Radiation methods: This is an expensive technique in which gamma-ray source is used
Usage
-
Buoyancy Type
- By Pressure
-
Pressure(Open Tank)
-
Pressure(Closed Tank)
- Pressure(Liquid Filled)
Summary of the principles of level measurement
- Effect of density ————-Float, slight-glass, hydrostatic, radiation, optical dipstick
- Time-of-flight to measure reflection—————– Ultrasonic, radar, laser, radio waves
- Change in physical properties————Conductivity probes, hot-wire
Level Measurement Consideration
Closed tank
- If a dry leg is used, care must be taken to ensure that condensate does not collect in the primary.
- If a wet leg is used, the liquid in this leg must remain at a constant level under all process conditions.
- When the process vapor is not readily condensable, or when the compensating leg is at a higher temperature than the tank interior, a dry leg can be used. A trap should be installed at the bottom of the leg to minimize the possibility of condensate collecting in the primary.
- When the process vapor is condensable, a wet leg is recommended.The leg can be fitted with process liquid or a suitable seal liquid. A filling tee must be installed at the top of the leg.
Connecting requirement:
- Keep the connecting lines as short as possible. Use gate valves wherever shut-off valves are required; gate valves do not restrict the connecting lines.
- All horizontal liquid filled lines should slope at least 1 inch/ft. (85mm/meter) downward toward the primary. This prevents bubbles from collecting in pockets of the connecting piping and assures that the correct differential pressure is applied to the primary.
- Liquid filled connecting lines should be run close together to maintain equal temperature in both the high and low pressure line.
- Avoid the possibility of vaporizing or freezing the liquid in the lines, do not permit the lines to contact extremely hot or cold surfaces.
Open Tank
- A single tap is required on the tank. By this tap, the pressure is transmitted to the gauge pressure transmitter.
- The high pressure side (the other side being open to the atmosphere) thus the total differential pressure is due to the liquid head.
Example of level measurement
Solution
Therefore
Examples
examined:
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