Instrumentation Basics -03- Level Measurement

7/30/2019 Instrumentation Basics -03- Level Measurement

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Basic Instrumentation Course

Prepared by

Eng\ Ahmed Mohamed Abdel-Halim

EMC, MIDOR Site

2008


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3. Level Measurements


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3-Level Measurement

Very simple systems employ external sight glasses or tubes to viewthe height of the fluid.

Others utilize floats connected to variable potentiometers or

rheostats that will change the resistance according to the amount of

motion of the float.

This signal is then inputted to transmitters that send a signal to aninstrument calibrated to read out the height or volume.

So, we have

Sight-type Instruments

Pressure-type Instruments

Electrical-type Instruments

Sonic-type Instruments

Radiation-type Instruments


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Glass Gauges The gauges are made of glass, plastic, or acombination of the two materials must be able to withstand the

pressure in the vessel.


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The pressure that the liquid exerts in the tank forces the liquid

in the sight glass to rise to the same level as the liquid in thetank.

For the pressurized tank, the upper end of the tube is

connected to the tank.

This creates an equilibrium pressure in both ends of the tube,and the liquid in the tube rises to the same level as the liquid

in the vessel.

Pressure at the base of a vessel containing liquid is directly

proportional to the height of the liquid in the vessel.

P = g H

The level of liquid inside a tank can be determined from the

pressure reading if the density of the liquid is constant.


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There are two basic types of flat sight gauges: reflex andtransparent.

The reflex-type gauge produces a dark area where liquid is

present and a light area where vapor is present.

The reflex type gauge is normally chosen for liquids thatare colorless, clear, and nonviscous.

The transparent gauge is generally used when the liquid is

colored, viscous, and corrosive.

Sight glass gauges are installed with manual shutoff valvesat both ends so maintenance can be performed.


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Magnetic Level Gauges

An external float chamber

connected to the tank.

The float has a magnetic

property that makes the

indicator goes up and downon a scale according to the

level in the tank.


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Tape Float

A tape is connected to a float on one end and to a counterweight

on the other to keep the tape under constant tension.

The float motion makes the counterweight ride up and down a

direct-reading gauge board.


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Displacers Displacer level gauges operate on Archimedes principle.

A body fully or partially immersed in a fluid is buoyed up by aforce equal to the weight of the fluid displaced.

By measuring the buoyancy force produced by a displacer, youcan determine a level value.

Displacer may used as a level transmitter for single liquid (LT), or

differential level transmitter for interface applications (LDT). For empty tank (or filled with a light liquid), there is no (or

minimum) buoyancy force Maximum weight of the displacer.

For filled tank ( or filled with heavy liquid), maximum buoyancyforce generated on the displacer Minimum weight of the

displacer.

The displacer may be top mounted inside the tank or chamberedunit.


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Displacers


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Displacers Installation


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Displacers (LT)


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Displacers (LDT)


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Pressure-type Instruments

The pressure at the base of a vessel containing liquid is

directly proportional to the height of the liquid in the

vessel.

As the level in the vessel rises, the pressure exerted by

the liquid at the base of the vessel will increase linearly.

Differential Pressure (DP) capsules are the most

commonly used devices to measure the pressure at the

base of a tank.

When a DP transmitter is used for the purpose of

measuring a level, it will be called a level transmitter.


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DP Level Transmitter

When using pressure or differential Pressure

transmitters to measure tank liquid level, determining

the liquid density is important to accurately calculate the

level for a given liquid head pressure.

Tanks may be open (vented), or they may be closed

(pressurized).

With closed tanks that may have a pressure above the

liquid that is different from atmospheric pressure, a dry

leg or wet leg system may be used to port the top-of-

tank pressure to the opposite side of a differentialpressure transmitter that is connected to the tank to

measure level.


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DP Level Transmitter

If it is a dry leg, then density of vapor or noncondensing

gas in dry leg is calculated and used to correct levelmeasurement for vapor density changes.

If it is a wet leg, density of liquid in wet leg is calculated

and used to correct the liquid level measurement.

Transmitter can be mounted at minimum level, or below

minimum level.

If transmitter is mounted below minimum level, the leg to

transmitter from the lower tank connection is assumed to

have liquid in it, and its density calculated to correct theliquid level measurement.


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Open Tank (Vented) or Closed Tank With Dry Leg

Assume dP cell below bottom tap

we make zero suppressioncalculation.

The span points for the dP cell

are calculated as follows

DP0% = d g

DP100% = (Lmax+d) g The DP transmitter must be

calibrated from DP0% to DP100%

Any condensation in the leg will

create an error in the level

measurement.

We can prevent condensation in most cases by doing steamtracing of the reference leg.


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Closed Tank With Wet Leg

Assume the leg filled with liquid that ( w > ) to prevent theserviced liquid goes to the transmitter cell.

Assume dP cell below bottom tap

we make zero elevation calculation.

The span points for the dP

cell are calculated as follows

DP0%

= d g h w

g

DP100% = (Lmax+ d) g(hwg)

Any change in the height or

density of the wet leg fluid

influences the accuracy of the

level measurement.


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Bubbler Level Measurement System

Used If the process liquid contains suspended solids or is chemically

corrosive or radioactive. It is desirable to prevent it from coming into direct contact with the

level transmitter.

A bubbler tube is immersed to the bottom of the vessel in which the

liquid level is to be measured.

A gas (called purge gas) is allowed to pass through the bubbler

tube.

Consider that the tank is empty, so, the gas will escape freely at the

end of the tube and therefore the gas pressure inside the bubbler

tube (called back pressure) will be at atmospheric pressure. As the liquid level inside the tank increases, pressure exerted by the

liquid at the base of the tank (and at the opening of the bubbler tube)

increases.


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As a result, the gas pressure in the bubbler tube will continue to

increase until it just balances the pressure of the liquid & anyexcess supply pressure will escape as bubbles through the liquid.

A level transmitter (DP cell) used to monitor this backpressure.

The bubbler tube is connected to the high-pressure side of the

transmitter, while the low pressure side is vented to atmosphere.

The output of the transmitter will be proportional to the tank level.

Note that bubbling action has to be continuous or the measurement

signal will not be accurate.

The purge supply pressure should be at least 10 psi higher than the

highest hydrostatic pressure the process will encounter. You should keep the purge rate small so no significant pressure

drop occurs in the dip tube.

The purge medium is air or nitrogen, although you can use liquids.


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Diaphragm Level Detectors

Diaphragm detectors operate by the simple principle of

detecting the pressure that the process material exerts

against the diaphragm.

The diaphragm box instrument consists of an air-filled

diaphragm that is connected to a pressure detector viaair tubing.

As the level rises above the diaphragm, the liquid head

pressure compresses the captive air inside.

A differential pressure element senses the air pressure,which is displayed as level.


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Diaphragm Level Detectors


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Servo Tank Gauge Its level measuring principle is based on the detection of

variations in the weight of a displacer suspended in theprocess fluid.

The displacer on level is partly immersed in the liquid.

The apparent weight is the weight of the displacer minusthe weight of the displaced product.

The hart of the servo gauge is an accurate forcetransducer that continuously measures the apparentweight of the displacer.

A weight corresponding with the apparent weight of thedisplacer on level is programmed by software settings.

In equilibrium condition, the weight of the partly immerseddisplacer balances against the weight programmed to bemeasured by the force transducer.


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Servo Tank Gauge


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Servo Tank Gauge

When emptying the tank, the liquid level starts moving

downwards. The force transducer will experience an increasing

weight, as upward force is no longer acting on thedisplacer.

Smart communication between the force transducer and

the servo controller will ensure that the displacer islowered.

The servo motor drives the measuring drum to unwindmeasuring wire until the displacer is partly immersed inliquid again until the measured or apparent weight

equals the programmed weight. The smart processing unit then can detect the level of

the liquid inside the tank corresponding to the drumapparent weight.


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Servo Tank Gauge


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Electrical-type Instruments Capacitance Probes one side of the process tank acts as one

plate and an immersion electrode is used as the other. The dielectric is either air or the material in the vessel & the

dielectric varies with the level in the vessel.

With the tank empty, the insulating medium between the twoconductors is air.

With the tank full, the insulating material is the process liquid orsolid.

As the level rises in the tank to start covering the probe, some ofthe insulating effect from air changes into that from the processmaterial, producing a change in capacitance between the sensingprobe and ground.

This capacitance is measured to provide a direct and linearmeasurement of tank level.

Noting that this type of instrument used only with NON-conductingservice to act as a dielectric between capacitance plat.


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Capacitance Probes


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Resistance Tapes In these devices, resistive material is spirally wound around a steel

tape & mounted vertically from top to bottom on a process tank.

The pressure of the fluid in the tank causes the resistive tape to beshort-circuited, thus changing the total resistance of the measuring

tape.

This resistance is measured by an electronic circuit and is directlyrelated to the liquid level in the tank.


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Ultrasonic Type Instruments

These system consist of a transmitter mounted on top ofthe tank and receiver as well as associated electronicsequipment.

The transmitter antenna radiates the signal toward thesurface of the process liquid in the tank.

A portion is reflected back to the antenna, where it is

collected and routed to the receiver. The strength of the reflected signal is directly related to the

dielectric constant of the liquid.

The instrument measures the time that elapses betweenthe transmitted burst and the echo signal which is

proportional to distance between the transducers and thesurface of the liquid inside the tank.


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Ultrasonic level measurement works on a "time of flight" principle.

Ultrasonic Level Measurement Instruments use sound waves todetermine level.

A piezoelectric crystal inside the transducer mounted on top of thetank converts electrical pulses into sound energy that travels in theform of a wave at the established frequency and at a constant speedin a given medium.

The sound waves are emitted in bursts and received back at thetransducer as echoes.

The Ultrasonic Level Measurement Instrument measures the time forthe bursts to travel down to the reflecting surface and return.

This time will be proportional to the distance from the transducer tothe surface and can be used to determine the level in the tank.

Sound waves are a form of mechanical energy that uses themolecules in the atmosphere to propagate, so, any changes in thechemical makeup of the atmosphere cause the speed of sound tovary.


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Affected by powders, heavy vapors, surface turbulenceand foam.

Cannot operate in vacuum or high pressure.

Limited temperature range

More position sensitive than other technologies


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Radar And Microwaves Instrument

Radar and Microwavesinstruments are similar to the Sonic-type, but

have advantage that they dont affected by the molecules in theatmosphere to propagate unlike the sonic-type.

Both radar signals and microwaves travel at the speed of light, but aredistinguished by their frequencies and by their power levels.

The transmitter is a microwave oscillator and directional antenna (aparabolic dish, horn-type antenna or rod antenna).

The receiver consists of an antenna, a high gain, a pulse-decodingcircuit, and an output circuit.

Microwave signals are absorbed almost entirely by water and tovarying degrees by water-based liquids or by products that have ahigh moisture content.

Microwave signals cannot penetrate metals but are reflected by them,so metal storage tanks or hoppers must have a detector window thatis transparent to the microwave signals.


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There are two types of radar level instruments: noncontact and

guided wave. In the noncontact type the output electromagnetic energy of the

radar antenna is very weak, typically about 1 mW.

On the return to the instrument, this weak signal loses more

energy.

Liquid turbulence and some foams can further complicate the

measurement by scattering or absorbing the radar pulse.

The guided-wave radar unit can overcome these problems by

using pulses of electromagnetic energy that are transmitted down

the probe tube. The wave guided offers a highly efficient path for the signal to

travel down to reach the surface of the liquid and then bounce

back to the receiver.


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Nuclear Level Measurement

Nuclear radiation systems have the ability to see through tank walls,

and thus they can be mounted on the outside of process equipment. Suitable for liquid or solid material detection.

Composed of a radioactive source material and a radiation detector,the two are mounted across the diameter of a storage vessel foreither solid or liquid material.

The product to be measured is attenuating the radiation coming from

the radioactive source and according to the height of the product inthe vessel, more or less of the original radiation is reaching thedetector.

This measuring signal is then transferred to an output signal whichdirectly correlates to the actual Level of the product.

Two typical nuclear level instruments:

Using a single low-level gamma-ray source on one side of theprocess vessel and a radiation detector on the other side of thetank.

Using several gamma sources at different heights on the tank.


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Nuclear Level Measurement


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Level Measurement Errors

Over ranging Damage to the D/P Cell

Faulty Sensing Lines

Loss of Loop Electrical Power

Connections: with an incorrectly connected DP cell theindicated level would go down while the true tank level

increases.

Extra weighted displacer: The sludge formed on thedisplacer makes faulty indication of the level.

Blocked bubbler tube: gives higher level reading thanthe present one.


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Level Switches

A level switch is a device that senses the level of a liquid

in a process tank.

They are also used to control valves or pumps in order to

maintain fluid level at a set value or to prevent tanks from

being overfilled.

Level switch can be

Float type switch.

Ultrasonic type switch.

Rotating Paddle type switch.

Displacer type switch.

Capacitance type switch.


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Float Type Switch The buoyant force is the operating principle of float level switch.

Floats should always be lighter than the minimum expectedspecific gravity of the process fluid.

As the level rises and falls, the float tilts up and down, thus openingand closing its electric contact.

The free length of the cable determines the actuation level.

In most cases, magnetic coupling transfers the float motion to theelectric contact or indicator mechanism.

The switch itself can be mercury or snap-action type switch.

The float, which contains an annular magnet, rises or falls withliquid level and is guided by the tube.

The switch assembly can be either inserted directly into the tank orside-mounted in a separate chamber.

The side-mounted switches are completely sealed and well suitedfor heavy duty industrial applications


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Float Type Switch

Top mounted switch (directly in the tank)


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Float Type Switch

Side-mounted


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Displacer Type Switch

A displacer remains partially or completely submerged.

The apparent weight of the displacer is reduced as it

becomes covered by more liquid.

When the weight drops below the spring tension, the

switch is actuated.

Displacer switches are more reliable than regular floats

on turbulent, frothy, or foamy applications.


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Displacer Type Switch


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Ultrasonic Level Switches Echo transmitter-type level switches contain transmitter and receiver

units.

The transmitter generates pulses in the ultrasonic range, which thereceiver detects.

The transmitter and receiver can be located in the same probe or onopposite sides of the tank

Sound waves are easily transmitted across a transducer gap in thepresence of a liquid medium, but are severely attenuated when the

gap is dry. The level switch will be actuated when the sound beam is

interrupted .

This type of switch is effective for both solid and liquid materialapplications.

Noting that the probes of Ultrasonic switch installation has two

types. Contact type probe: which the probe itself is inserted inside the

tank and surrounded by the media.

Non-Contact type probe: the probe is installed externally of thetank.


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Ultrasonic Level Switches


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Nuclear Level Switch The radioactive source and the radiation detector are

mounted across the diameter of a storage vessel for

either solid or liquid material.

Any height of material beyond the level of the

source/detector arrangement will attenuate the strengthof radiation reaching the detector.

This decrease in radiation at the detector can

provide a switch contact for measurement, alarm point,

or even control of the vessel level.

l l S i h


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Nuclear Level Switch

R i P ddl T S i h


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Rotating Paddle Type Switch

Used to detect the presence or absence of solids in a

process tank.

A Synchronous motor keeps the paddle in motion at very

low speed when no solids are present, there is very low

torque on the motor

drive.

When the level in the tank rises to the paddle, torque is

applied to the motor drive and the paddle stops.

The level instrument detects the torque and actuates a

switch or set of switches.

R i P ddl T S i h


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Rotating Paddle Type Switch

C i T S i h


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Capacitance Type Switches

As media rises and falls in the tank, the

amount of capacitance developed between

the sensing probe and the ground also rises

and falls.

This change in capacitance is sensed by theelectronic circuitry.

At the predefined set point, the electronic

circuit gives the alarm or trip signal.

C i T S i h


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Capacitance Type Switches

Th l l l i h


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Thermal level switch

The sensor head employs two temperature sensors with

a constant heating source physically attached to one ofthe temperature sensors.

The second temperature sensor is isolated from the

heating source and provides compensation for changing

process temperatures. The probe operates by sensing the thermal conductivity

of the product surrounding the probe.

All liquids that form an interface will have a difference in

thermal conductivity that can be sensed.

Th l l l i h


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Thermal level switch

Instrumentation Basics -03- Level Measurement

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