Level Measurement Techniques

Presented by:Muhammad Owais

Contents• Some basic methods• Differential pressure method• Bubbler method• Displacer method• Capacitance probe• Ultrasonic• Radar• Radiation• Point level detection

• Level measurement is all about measuring the height of a solid or liquid substance in a vessel.

• In a process industry level measurement can be classified into two categories:– Continuous level measurement– Point level detectionPrinciples used for both are almost similar.

• Level is measured in percentage.

• Instruments can also be calibrated to show level in terms of liquid volume.

Some basic methods…..• Glass level gauge

• Float

• Dipstick

Differential Pressure Method

• Liquid in a vessel exerts pressure on its bottom. This pressure is directly proportional to the height of the liquid.

• Level can be measured by measuring this pressure, provided the specific gravity of the liquid is known.

Principle

P = ρ . g . h

• It can also be used to measure level in closed or pressurized tanks. Two different variations are possible in this case.

• Dry leg compensation

Example

• Wet leg compensation:

Span = (X).(SGTANK)

Min. level = (Y).(SGTANK)-(d).(SGWET LEG)

Max. level = (X+Y).(SGTANK)-(d).(SGWET LEG)

Example

• Zero suppressionIf the LRV of the transmitter is a positive number than it is zero suppression.

• Zero elevationIf LRV is a negative number than it is zero elevation.

Example

• Remote Seal:Sealing fluid is used to transmit pressure.

• Use of flushing liquid:For clogging process flushing liquid is continuously purged in the impulse lines.

Application

Advantages:• Can be used for a wide range of pressures and temperatures.• Ease of installation and retrofitting on surface of vessel.• Provided with isolating valves for ease of maintenance.

Disadvantages:• Sensitive to changes in process density.• Must come in contact with the process.

Bubbler Method

Principle• Level is measured using the principle of back pressure.

• It is also known as “purgelevel measurement”.

• Example:01-LI-18T on Drain Collecting Tank

• Advantages:– Ease of installation. – Very cost effective.

• Disadvantages:– Sensitive to changes in process density. – Clogging of dip tube may occur due to dirty process. – Poor results with slurries and corrosive processes.

Displacer Method

Principle• It works on the principle of buoyancy.

• A mechanism is needed to detectthe weight change of the displacer.

• Torque tube:It is an ingenious method of transmitting torque while positively sealing the process.

Principle

• Example:01-LC-28T Urea on V-112

• Advantages:– Works well with clean fluids. – Suitable for high pressure and corrosive fluid applications.

• Disadvantages:– Costly installation. – Sensitive to process changes. – Ineffective for use on slurries. – Available in limited range.

Capacitance Probe

Principle• It works on the principle of changing capacitance with level. It can be used

of both conductive and non-conductive liquids.

• Use on conductive and non-conductive liquids

Principle

• Example:01-LIC-13T ME 103 level transmitter

• Advantages:– No moving parts hence minimum wear and tear. – Relatively low cost. – Can also be used for solids level measurement.

• Disadvantages:– Sticky material coats the probe causing false readings.– Sensitive to changes in dielectric constant of the process.– Reading affected by the geometry of the vessel.

Ultrasonic

Principle• It works by measuring echoes of the transmitted sound.

Fillage = Total height – Ullage

Advantages:– Ease of installation and low cost.– Low maintenance requirements.– Long range and non-contact method.

Dis-advantages:– Errors due to foam formation or vapors.– Temperature sensitive.

Radar Type

Principle• It transmits radio waves and detects the reflections from liquid

surface.• It can use guided as well as unguided waves.

• Advantages:– High accuracy method with no moving parts. – No recalibration is required after installation. – Readings not affected by foam or vapors.

• Disadvantages:– Some materials give poor reflections with the radar.– Relatively high cost.

Radiation Based Method

Principle• It consists of a radiation source and receiver. Less and less radiations are

received as the level increases.

• Advantages:– Works on highly corrosive and hard to handle liquids.– Non-contact method of level measurement.

• Disadvantages:– Associated radiation hazards.– Strict regulatory requirements and cost of installation.

Point Level Detection

• Float switch:

• Tuning fork:

Thank You