Instrumentation Level Measurement with Calculation Examples

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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:

  1. Sight glass
  2. Dip-stick
  3. Float based device
  4. Hydrostatic devices
  5. Conductivity probes
  6. Bubble method
  7. Ultrasonic level gauge
  8. Radiation methods
  9. Hot-wire element
  10. Radar
  11. Laser
  12. Radio frequency
STRATEGIES
To be able to calculate level from level sensors.
To be able to decide the type of sensors for a given application.

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

The direct methods involve a direct measurement of the distance fromthe liquid level to a datum line, which can be accomplished by:
  • 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

  1. Sight glass technique:

This is a simple, inexpensive and reliable device with the major advantage that the
liquid level can be seen.
Sight glass technique

  1. Dip-sticks

This has been used quite extensively in fuel oil tank and engine oil level measurements
Dip-sticks

  1. Optical dipsticks

Optical dipsticks


  1. Float based devices


Float based devices

Inferential Methods

  • Conductivity probes

This is a very effective technique to control the level of a tank at different fixed values
Fig


  • Bubble method

A constant air supply is applied to push the bubbles from the bottom of the tank. The pressure used to do so will depend on the level of the the liquid.
bubble method


  • Vibrating Probe Type:

This type of level switch is suitable for operating temperatures between –40 to +200oC.

VIBRATING FORK / PROBE LEVEL SWITCH

The system consist of a rid fixed to a membrane with a natural frequency of approximate 120Hz (vibrating fork principle).
The difference between vibrating fork switch for solids and a vibrating pork switch for liquids us a resonance frequency and the energy difference.
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

Ultrasonic level gauge
One Liquid
Fig

Liquid to liquid liquid to precipitate

  • Radiation methods

This is an expensive technique in which gamma-ray source is used
Fig


  • Hot-wire element techniques

This is based on the principle that the temperature of the liquid is different from that of the air. Therefore, one can observe the temperature change along the wall of the tank to obtain the information about the level.
Hot-wire element techniques

  • Radar techniques


Radar techniques

Radar techniques

  • Laser techniques

Laser techniques


  • Radio frequency technique

Whenever there is a change in permittivity, ε, a wave is reflected. By measuring the time of those reflected waves, one can determine the level of different liquid in the tank.
Radio frequency technique
  • Radiation methods: This is an expensive technique in which gamma-ray source is used
Fig

Usage

Most nuclear gauges respond to the total mass of anything and every thing between the source and the detector. This basic principle permits a great deal of flexibility in application. With minor variations in size, shape, and design, the basic sensor can be used to measure a variety of process conditions.
A wide range of industry operations profit from the simplicity and accuracy of quality control systems using Ohm art/VEGA Nuclear Level gauges. A level gauge may be used in a number of applications, such as the following:
• Pulp and Paper
• Liquors • Bleach plant chemicals• Coating chemical storage• Lime mud• Wastewater treatment tanks
• Chemical• Low pressure / low vapor chemical storage• Surge tanks Etc.

  • Buoyancy Type

Principle (Archimedes Principle) When a body is partly or completely submerged in liquid, it losses weight equal to the weight of the liquid displaced.
Buoyancy Type
F = PI.D (H-d)Sp. Gr
D = Cylinder Dia.
H = Arbitrary Level
d = Upward Movement of float
Sp. Gr. = Specific Gravity
Fig
Level measurement using pressure information
  • By Pressure
Principle:
Liquid level can be measured by measuring the static pressure called by the liquid.
h = P / Sp.Gr
h = Arbitrary Height
P = Static Pressure
Sp. Gr. = Specific Gravity
Fig


  • Pressure(Open Tank)

P = h1 x Sp.Gr.+ h2 x Sp.Gr.
h1 x Sp.Gr. = Req. head Press
h2 x Sp.Gr. = Req. head press. to be corrected
Often referred to as elevation

  • Pressure(Closed Tank)

P1 =S.G. x h1 + P0
P2 = P0
dp = P1-P2
= (S.G. x h1 +P0) – P0
= S.G. x h1

  • Pressure(Liquid Filled)
P1 =S.G. x h1 + P0
P2 = P0 + S.G.1(h + h1)
dp = P1-P2
= [S.G. x h1 +P0] – [P0 +S.G.1(h + h1)]
= [S.G. x h1] – [S.G.1(h + h1)]

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

  1. A single tap is required on the tank. By this tap, the pressure is transmitted to the gauge pressure transmitter.
  2. 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

In this section, different level measurement techniques have been examined.
Q. An ultrasonic measurement system is used to measure the level of liquid in a container. The angle of the transmitter
(receiver) is 30o. It is found that it takes 6 ms to receive the reflection. Determine the vertical distance between the transmitter and the top of the liquid.

Solution

We can use the following relationship to calculate the length.

The speed of ultrasound wave is 340 m/s.

Therefore

Examples

In this section, different level measurement techniques have been
examined:
1) Measured Range (dp) (H) (Span)
Y x dcon = 2000 x 0.8 = 1600 mmH2O
2) Zero separation (S)
(X + Y + Z) x d.water = 2150 x0.9 = 1935 H2O
3) Zero elevation (E)
Z x d con = 50 x .08 = 40 mmH2O
Calibration Range :
(E-S) to [ H+(E+S)]
(40 – 1935) to [1600+(40-1935)]
(-1895 ) to [1600+(-1895)]
-1895 to -295
LRV – URV;
Span = URV – LRV ; -295 – (-1895) = 1600mm

Question???????????

In this section, different level measurement techniques have been examined:
1) Measured Range (dp) (H) (Span)
Y x dcon = 2000 x 0.8 = 1600 mmH2O
2) Zero separation (S)
(X + Y + Z) x d.water = 2150 x0.9 = 1935 H2O
3) Zero elevation (E)
Z x d con = 50 x .08 = 40 mmH2O
Calibration Range :
= (E-S) to [ H+(E+S)]
=(40 – 1935) to [1600+(40-1935)]
= (-1895 ) to [1600+(-1895)]
= -1895 to -295
= LRV – URV;
Span = URV – LRV ; -295 – (-1895) = 1600mm

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