level measurement - kmuttinc.kmutt.ac.th/course/inc331/inc331_level_measurement_all_1_59.pdfa....

Level MEASUREMENT

1/2016

AGENDA

A. Introduction

B. Float method

C. Displacer method

D. Hydrostatic pressure method

E. Capacitance method

G. Ultrasonic method

H. Radar method

I. Laser method

J. Level detections

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A. Introduction

1. Continuous Liquid Level Measurement and Control

Level measurement technologies are made available in different versions to

address a wide range of measurement needs or sometimes to address just

one specific application. The family of level measurement systems can be divided into 3 main groups as follows:

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2. Point Liquid Level Detection

3. Solids & Dry Products Level Capability

A. Introduction 4

2. Point Liquid Level Detection

3. Solids & Dry Products Level Capability

A. Introduction 5

A. Introduction

General considerations in level measurement technology selection

• Density and viscosity

• Chemical composition

• Ambient temperature

• Process temperature

• Process pressure

• Vapor, mist, and dust

• Process conductivity and dielectric constant

• Vibration

• Humidity/moisture

• Repeatability and accuracy requirement

•Cost

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B. Float Methods

The level gauge consists of a float chamber, a float, and an external indication

device. The float chamber is basically a column with process connections to

match those of the storage tank, reactor, drum, column or other equipment

where level is to be measured. These connections may be side couplings or

flanges, or top and bottom flanges.

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C. Displacers

Displacer level instruments exploit

Archimedes’ Principle to detect liquid level by

continuously measuring the weight of a rod

immersed in the process liquid. As liquid level

increases, the displacer rod experiences a

greater buoyant force, making it appear lighter

to the sensing instrument, which interprets the

loss of weight as an increase in level and

transmits a proportional output signal.

In physics, buoyancy is an upward force exerted by a liquid, gas or other fluid, that opposes the weight of an immersed object.

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The displacement method is based on the

difference between the weight of the displacement

body and the upward force exerted by the medium

on this body (buoyancy force).

The upward force depends on the volume of

displacement body, the relative density of medium,

and the level of medium.

For a given volumes and relative density, the

upward force will depend on only the level of the

medium.

Displacers work well with clean liquids and are

accurate and adaptable to wide variations in fluid

densities. Once commissioned, however, the

process fluid measured must maintain its density

if repeatability is required.

Mounting may be either directly into a vessel or externally mounted in a chamber.

C. Displacers 9

D. Hydrostatic pressure methods

D.1 Bubbler

The bubbler system supplies a constant rate of air

flow through a small diameter tube anchored near

the bottom of the tank. The amount of pressure

required to force the air bubble out of the bottom

of the tube is equal to the hydrostatic pressure at that point (i.e. the deepest point in the tank).

This is calculated using the formula

Relative density, or specific gravity, is the ratio of the density (mass of a unit

volume) of a substance to the density of a given reference material. Specific gravity usually means relative density with respect to water.

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Simplicity of design and low initial purchase cost are frequently given as

advantages of bubblers. The system consists of a pipe, an air supply, a

pressure transmitter and a differential pressure regulator. The regulator produces the constant gas flow required to prevent calibration changes.

The higher level , the higher pressure.

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Among the level measurement methods, the

measure based on differential pressure(DP) has

become the most popular type. A DP is used to

transmit the head pressure that the diaphragm

senses due to the height of the material in the vessel multiplied by a density variable.

The primary benefit of DP’s is that it can be

externally installed or retrofitted to an existing

vessel. It can also be isolated safely from the

process using block valves for maintenance and

testing. There are certain measurements such

as total level in separator vessels that due to wide

variations in material composition of the upper phase DP is the only viable if not ideal option.

d/p cell

Close tank

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DP transmitters are subject to errors due to

changes in liquid density. Density variations are

caused by temperature changes or change of product.

Fluid density must be stable if readings are to be

accurate. If liquid density is subject to change a

second d/p transmitter is required to measure density and then used to.

It should be noted that the change in specific

gravity will affect the accuracy of the measurement using d/p cell.

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D. Hydrostatic pressure methods 14


Example: If P = 0 – 20 psi , P = 0 => O/P 4 mA and P = 20 => O/P 20 mA

At the minimum level=> O/P 0 % and at maximum level=> O/P 100%

Z

YX

d/p cell

Min level

PH = GsZ+GLY+Pa , PL = Pa

Pmin = GsZ+GLY

Assume liquid in the tank has a specific gravity of GL and liquid in the tube has a specific gravity of Gs .

Max levelPH = GsZ+GL(Y+X)+Pa , PL = Pa

Pmax = GsZ+GL(Y+X) Span = XGL, Elevation = YGL+ZGS

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E. Capacitance method

Capacitance is the ratio of the electric charge on one of

a pair of conductors to the potential difference between the conductors.

A capacitance level probe determines the level of liquid

in a column or receiver by measuring the combined capacitance of the liquid and gas (vapor) in the column.

“RF” level measurement

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Non-conductive material

For non-metallic tank or horizontal cylindrical tank

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Conductive material

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A. Introduction

As the liquid level rises in the column, the total capacitance value increases.

(The capacitance of vapor is very small compared to the capacitance of the liquid.)

This increase is measured by the controlling electronic system and an output control signal is created.

E. Capacitance method 22


Ultrasonic level measurement

devices basically employ sound

waves for detection of liquid level.

They usually work over the frequency

range between 20 kHz to 200 kHz.

Ultrasonic level measurement method

is based on the fact that sound

through a medium with a know

propagation speed, depending on the

density and the temperature of that

medium.

The pulse is generated and then

travels through the medium (typically

air). When the pulse hits the surface

of material, it is reflected back to

transducer to be measure.

The distance to the level surface and

level height can be calculated from

the reflection time and the speed of

sound wave.

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The main advantages of ultrasonic level

instrumentation are that the transducer

does not come into contact with the

process material, they have no moving

parts and a single top of vessel entry

makes leaks less probable than fully

wetted techniques.

There are various influences that affect

the return signal. Things such as

powders, heavy vapors, surface

turbulence, foam and even ambient noise can affect the returning signal.

Temperature can also be a limiting

factor in many process applications.

Ultrasonic devices will not operate on vacuum or high pressure applications.

G. Ultrasonic method 25

H. Radar method

The operation of all radar level detectors involves sending

microwave beams emitted by a sensor to the surface of

liquid in a tank. The electromagnetic waves after hitting

the fluids surface returns back to the sensor which is

mounted at the top of the tank or vessel. The time taken

by the signal to return back i.e. time of flight (TOF) is then

determined to measure the level of fluid in the tank.

This non-contact technology produces highly accurate

measurements in storage tanks and some process

vessels. Radar is an excellent, but fairly expensive technology for continuous level measurements.

There are various influences that affect the return signal.

Things such as powders, heavy vapors, surface

turbulence, foam and even ambient noise can affect the returning signal.

It’s primary disadvantage is cost, which can be justified

for tank gauging and inventory control. The pressure

ratings on radar antenna are limited and these devices cannot measure interfaces.

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H. Radar method

Radar is becoming a rapidly important method

for measuring the level of liquids and some case

of solids. The two technologies on the market are

frequency modulated continuous wave (FMCW)

or pulsed wave time of flight.

Pulsed Wave systems emit a microwave burst

towards the process material, this burst is

reflected by the surface of the material and

detected by the same sensor which now acts as

a receiver. Level is inferred from the time of flight

(transmission to reception) of the microwave signal.

FMCW systems, however, continuously emit a

swept frequency signal and distance is inferred

from the difference in frequency between the

transmit and receive signals at any point in time.

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H. Radar method 28

I. Laser method

Advances in optical technology have reduced

the cost of laser engines to the point where it

has become practical to use infrared lasers as level measurement devices.

The LASERMETER measures the time it takes

for a laser pulse to travel from the instrument to

a target and back. The distance to the target is calculated from this time.

In some cases, they have become competitive in

the same range of applications as microwave

level. They are still susceptible to attenuation of

the beam by vapor, and other forms of beam scattering.

They have been tried on granulars and solids, with wildly varying results.

Disadvantages: cost, dust, dirt, maintenance

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I. Laser method 30

I. Laser method 31

Magnetostrictive Level Sensor

Inside the probe tube there is a rigid wire made of magnetostrictive

material. The sensor circuitry emits pulses of current through the

wire, generating a circular magnetic field. The level transmitter is a

magnet, which is integrated into the float. Its magnetic field

magnetizes the wire axially. Since the two magnetic fields are

superimposed, around the float magnet a torsion wave is

generated which runs in both directions along the wire. One wave

runs directly to the probe head while the other is reflected at the

bottom of the probe tube. The time is measured between emission

of the current pulse and arrival of the wave at the probe head. The

position of the float is determined on the basis of the transit times.

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Magnetostrictive Level Sensor

Automatic tank gauging(Underground oil tank)

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J. Level detections

J.1 Vibrating level switch

The piezo electrically stimulated probe vibrates at its natural resonance frequency.

If the bulk material covers the probe, the damping

thus generated is registered electronically and a corresponding signal output is actuated.

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J. Level detections

J.2 Level paddle switch

Level is detected by the change in inertia of a

rotating paddle depending on whether the paddle

is in the air or in contact with the product.

The location should be selected such that the

product to be monitored is allowed to freely flow

both into and away from the rotating paddle.

However, the paddle should not be placed directly

under the free-falling path of the product.

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J. Level detections

J.4 Level conductivity switch

This method is suitable only for level measure-

ment in conductive liquids. The difference in the

conductivity of partially insulated electrode is

measured when the probe is covered and not

covered with the conductive product.

The advantages of this method are simple,

inexpensive and suitable for dual or multiple point

control.

The disadvantages are probe can not become

contaminated with grease or other deposits and

has limited suitability for products of varying

conductivity.

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SUMMARY

Source: http://www.omega.com/literature/transactions/volume4/images/11_Table.I_l.GIF

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level measurement - kmuttinc.kmutt.ac.th/course/inc331/inc331_level_measurement_all_1_59.pdfa....

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