ch. 4 level measurement

Chapter 4Chapter 4LEVEL MEASUREMENT

Level Measurement DefinitionLevel Measurement Definition Level measurement is a single dimension from a referenceg

point. As shown tank level is measured, either by Inagemethod or Outage method. Each Manufacturer has a differentInstruction Set.Instruction Set.

Level Measurement PrincipleLevel Measurement PrincipleLevel devices operate under three main different

principles:

1 The position (height) of the liquid surface1. The position (height) of the liquid surface

2. The pressure head

3. The weight of the material

There are two methods used to measure the level of a liquid:

1) Direct Methods

2) Indirect or inferential Methods

1. Direct Methods (Visual Methods)( )

The direct method measures the height above a zero point by any of the following methods point by any of the following methods.

Direct methods for level measurement are mainly d h l l h ll d l h used where level changes are small and slow such as;

Sump tanks and Bulk storage tanks.

Direct methods are simple and reliable. There are four types of direct level measurement devices:

1.Dip-sticks & Dip-Rods

2.Weighted gauge tapeg g g p

3.Sight Gauges, and

4 Floats4.Floats.

1.1) Dip-Sticks & Dip-Rods) p p

1.2) Weighted gauge tape) g g g pAnother variation is the weighted gauge tape. This is used in a

similar fashion to the dipstick but on deep vessels and tanks similar fashion to the dipstick, but on deep vessels and tanks

where a solid rod would be inappropriate.

1.3) Sight Gauges) g g

There are various types of sight gauges, the two most

common types being used are:

1 Sight glass; flat tubular and reflex1.Sight glass; flat, tubular and reflex

2.Magnetic sight gauges. g g g g

The flat glassThe flat glass

oThe flat glass type, is used for non pressurised vessels.

oIt is a glass window or windows that forms part of the vessel.

A t pi al appli ation is in hot oil tanks he e e essi e foam oA typical application is in hot oil tanks, where excessive foam

contaminated oil may be easily detected.

Tubular Sight Glassesg Light is refracted from the vapour portion of the column and is

shown generally as white colour Light is absorbed by the liquidshown generally as white colour. Light is absorbed by the liquid

portion in the column and is shown generally as a dark colour.

h d l f l d They are used mainly for non-corrosive, non-toxic inert liquids

at moderate temperatures and pressures.

Reflex sight glassesReflex sight glasses

Magnetic type Sight gauges The magnetic level gage, consisting of a magnetic float that

travels up and down on the inside of a long non-magnetictravels up and down on the inside of a long, non magnetic

(usually stainless steel) pipe. The pipe is connected to flanged

nozzles on the side of the tank The pipe column is providednozzles on the side of the tank. The pipe column is provided

with a visual indicator, consisting of triangular wafer

l telements.

These elements flip over (from green to red, or any other

color) when the magnet in the float reaches their level. Alarm

switches and transmitter options are available with similar

magnetic coupling schemes.

Operational considerations for Sight Glasses

o Sight glasses are usually installed with shutoff valves and a drain valve for the purpose of maintenance, repair and replacement.

o An important safety feature of these external sight glasses is o An important safety feature of these external sight glasses is the inclusion of ball check valves within the isolation valves. The purpose of these check valves is to prevent the escape p p p pdangerous fluids if the glass breaks. Therefore it is important that the isolation valves are left fully open when the sight glass is in use, otherwise the operation of the check valves may be inhibited.

o Sight gauges must be accessible and located within visual range.

They are not suitable for dark liquids Dirty liquids will prevent o They are not suitable for dark liquids. Dirty liquids will prevent the viewing of the liquid level.

l h h b d d f b f l d lo Glass has the obvious disadvantage of being fragile and easily damaged or broken. Therefore, this type should not be used for measuring hazardous liquids On safe applications tubular for measuring hazardous liquids. On safe applications, tubular gauge glasses can be used.

o Reflex gauges are permissible for low and medium pressure applications. For high-pressure applications or where the fluid i t i d ith ti di l h ld b d is toxic, armoured gauges with magnetic dials should be used for safety reasons, gauge glass lengths between process connections should not exceed 4 ft connections should not exceed 4 ft.

1.4) Floats Floats give a direct readout of liquid level when they are

connected to an indicating instrument through a mechanical connected to an indicating instrument through a mechanical

linkage.

A simple example of this is the weighted tape tank gauge, the

position of the weighted anchor against a gauge board gives an

indication of the liquid level in the tank.

The scale of the gauge board is in reverse order i e the zero The scale of the gauge board is in reverse order, i.e. the zero

level indication is at the top and the maximum level indication is

at the bottom of the gauge boardat the bottom of the gauge board.

2. Indirect (inferential) Methods2. Indirect (inferential) Methods The indirect or inferential method of

measurement uses the changing position of the measurement uses the changing position of the liquid surface to determine level with reference to a datum line. to a datum line.

It can be used for low & high levels where the use of the direct method instruments is use of the direct method instruments is impractical.

H d t ti P M th d1.Hydrostatic Pressure Methods

2.Displacement devices

3.Capacitance Level Instrumentation

4.Radiation-Based Level Gages 4.Radiation Based Level Gages

2.1) Hydrostatic Pressure Methods) y A vertical column of fluid exerts a pressure due to the column’s

weight. The relationship between column height and fluid pressure at the bottom of the column is constant for any particular fluid (density) regardless of vessel width or shape.

This principle makes it possible to infer the height of liquid in a vessel by measuring the pressure generated at the bottom:

Where,

P = Hydrostatic pressure

ρ = Mass density of fluid in kilograms per cubic meter

g = Acceleration of gravity

γ = Weight density of fluid in newtons per cubic meterγ g y p

h = Height of vertical fluid column above point of pressure measurement

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

pressure reading if the weight density of the liquid is constant.

Level measurement involving the

principles of hydrostatics have taken

numerous forms, including:

1.The diaphragm-box system

2 H d t ti diff ti l t2.Hydrostatic differential-pressure meters

3 The air bubble tube or purge system3.The air-bubble tube or purge system

2.1.1) The Diaphragm-box System Pressure sensor / transmitter can make use of this principle for liquid

level measurement. The PSE can be connected to the bottom the vessel so that it's input is related the hydrostatic pressure within the vessel so that it s input is related the hydrostatic pressure within the tank.

A level can be measured using a pressure gauge. These systems are g p g g yemployed on open vessels. They operate by giving an indication of the pressure produced by the static head of the liquid that is related to the actual level in the tank In this case the gauge is calibrated in units actual level in the tank. In this case the gauge is calibrated in units relating to the liquid level in the tank in % value.

2.1.2) Differential-Pressure Level Meters2.1.2) Differential Pressure Level Meters

Differential Pressure (DP) capsules are the most Differential Pressure (DP) capsules are the most

commonly used devices to measure the pressure at

the base of a tank.

Wh DP t itt i d f th f When a DP transmitter is used for the purpose of

measuring a level, it will be called a level

transmitter.

Open Tank Measurement The simplest application is the fluid level in an open tank.

If the tank is open to atmosphere, the high-pressure side of thel l i ill b d h b f h k hillevel transmitter will be connected to the base of the tank whilethe low-pressure side will be vented to atmosphere. In thismanner, the level transmitter acts as a simple pressuretransmitter.

P High = P atm + S . Hg

P Low = P atm

Differential pressure ∆P = P High – P Low = S . Hp g

The level transmitter can be calibrated to output 4 mA when the tank isat 0% level and 20 mA when the tank is at 100% level.

Closed tank - Dry Leg System

o When the low-pressure impulse line is connected directly to the

gas phase above the liquid level it is called a dry leggas phase above the liquid level, it is called a dry leg.

o Dry leg system usually used where the gas phase is non-

condensable.

Closed tank - Wet Leg System In a wet leg system, the low-pressure impulse line is completely filled

with liquid (usually the same liquid as the process) and hence thename wet leg.

At the top of the low pressure impulse line is a small catch tank. Thegas phase or vapour will condense in the wet leg and the catch tank.The catch tank with the inclined interconnecting line maintains aThe catch tank, with the inclined interconnecting line, maintains aconstant hydrostatic pressure on the low-pressure side of the leveltransmitter. This pressure, being a constant, can easily becompensated for by calibration.compensated for by calibration.

2.1.3) Bubbler Level Measurement System) y

If the process liquid contains suspendedsolids or is chemically corrosive orradioactive, it is desirable to prevent it, pfrom coming into direct contact with thelevel transmitter In these cases alevel transmitter. In these cases, abubbler level measurement system,which utilizes a purge gas can be usedwhich utilizes a purge gas, can be used.

a) Open tank application for bubbler system:

A bubbler tube is immersed to the bottom of the vessel in which the liquid level is to be measured. A gas (called purge q g ( p ggas) is allowed to pass through the bubbler tube.

Consider that the tank is empty, the gas will escape freely at Consider that the tank is empty, 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. The hydrostatic pressure of the liquid i ff t t l hi h t i t th f in effect acts as a seal, which restricts the escape of, purge gas from the bubbler tube.

22..2 2 Displacement devicesDisplacement devices The displacement level transmitter is commonly used for The displacement level transmitter is commonly used for

continuous level measurement. It works on the buoyancy principle. The displacer has a cylindrical shape therefore each increment of

submersion in the liquid; an equal increment of buoyancy changewill result. This is a linear and proportional relationship.

When the weight of an object is heavier than an equal volume of the fluidinto which it is submerged, full immersion results and the object neverinto which it is submerged, full immersion results and the object neverfloats. Although the object (displacer) never floats on the liquid surface, itdoes assume a relative position in the liquid. As the liquid level moves upand down along the length of the displacer, the displacer undergoes ag g p , p gchange in its weight caused by the buoyancy of the liquid. Buoyancy isexplained by Archimedes' principle, which states that: "The resultantpressure of a fluid on a body immersed in it acts vertically upwardthrough the centre of gravity of the displaced fluid and is equal to theweight of the fluid displaced".

The upward pressure acting on the area of the displacer creates the force The upward pressure acting on the area of the displacer creates the force called buoyancy.

The immersed displacer is supported by arms or springs that allow some small amount of vertical movement or displacement of the displacer due small amount of vertical movement or displacement of the displacer due to buoyancy forces caused by the change in the liquid level. This buoyancy force can be measured to reflect the level variations.Wh b d i f ll ti ll i d i li id it i d d i When a body is fully or partially immersed in any liquid, it is reduced in weight by an amount equal to the weight of the volume of liquid displaced.

Determining Suspended Weight for Dry Calibration:

To determine the total weight that must be suspended from the displacer rod to simulate a certain condition of fluid level or specific gravity, the following equation can be used:gravity, the following equation can be used:

Ws =Wd – [(0.0361)(V)(Sg)] Ws = Total suspended weight in pounds (apparent weight). Ws Total suspended weight in pounds (apparent weight).

Wd = Weight of displacer, dry, in pounds (determine by weighing displacer).

0.0361= Weight of one cubic inch of water, in pounds (specific gravity =1.0)

V = Volume of the displacer that would be submersed at the level required by the calibration Procedure (in cubic inches) OR,

V = /4 (displacer diameter)2 x (submerged length of displacer) V = /4 (displacer diameter)2 x (submerged length of displacer)

Sg = Specific gravity of the process fluid at operating temperature.

Torque tubeq In this method a displacer body is connected to a torque tube

which twists a specified amount for each increment of buoyancy h change.

As shown, the twisting force can drive a pointer, an indicator or be transferred to a pneumatic or electronic system. It is transferring p y gthe displacer movement from the inside of pressurised vessel to the readout mechanism, which is in atmospheric pressure.

The torque tube rotates around 4° to 6° degrees angular to The torque tube rotates around 4° to 6° degrees angular to establish the 0-100% level readout.

Displacement Devices Applicationsp pp This type of measurement should only be used for liquids:

1. With fixed specific gravity2. Where errors due to process variations are acceptable3. Where a change in process conditions will not create crystallisation or

solids

If the displacer is mounted in an external still-well then the block and drain valves should be installed for maintenance purpose. Trace heating or insulation may by needed to maintain the temperature of heating, or insulation may by needed to maintain the temperature of the liquid in the well.

The piping arrangement should be designed to prevent the p p g g g pformation of sediment on the bottom of the cage as eventually this can build up and affect the displacer movement. Coating build-up or dirt that clings to the displacer may affect the elements’ buoyancy dirt that clings to the displacer may affect the elements’ buoyancy resulting in the accuracy of the measurement.

Continuous Level Displacers Displacers are popular as level transmitters and as local level controllers,

however, they are not suited for slurry or sludge service because coatingof the displacer changes its volume and therefore its buoyant force.They are most accurate and reliable for services involving clean liquids of They are most accurate and reliable for services involving clean liquids ofconstant density.

Displacement Interface Level MeasurementDisplacer level instruments may be used to measure liquid-liquid interfaces just the same as hydrostatic pressure instruments. One important requirement is that the displacer always be fully submerged. If this rule is violated, the instrument will not be able to “tell” the difference between a low (total) liquid level and a low interface levellow (total) liquid level and a low interface level.

2.3 Capacitance Level Instrumentation Theory of Operation

» A capacitor consists of two conductors (plates) that are electrically isolatedfrom one another by a nonconductor (dielectric).Wh h d diff i l ( l ) h i» When the two conductors are at different potentials (voltages), the system iscapable of storing an electric charge. The storage capability of a capacitor ismeasured in farads.

» The capacitor plates have an area (A) and are separated by a gap (D) filledi h d i i l f di l i (K)with a nonconducting material of dielectric constant (K).

Capacitance (C) is calculated as:

If the area (A) and the distance (D) between theplates of a capacitor remain constant, capacitanceplates of a capacitor remain constant, capacitancewill vary only as a function of the dielectricconstant of the substance filling the gap betweeng g pthe plates.

If a change in level causes a change in the total If a change in level causes a change in the totaldielectric of the capacitance system, because (asillustrated in Figure B) the lower part of area (A) isillustrated in Figure B) the lower part of area (A) isexposed to a liquid (dielectric Kl) while the upperpart is in contact with a vapor (dielectric Kv, whichp p ( ,is close to 1.0), the capacitance measurement willbe proportional to level.p p

Capacitance Probe as a Level Switch:p In the case of a horizontally mounted level switch, a conductive probe

forms one of the plates of the capacitor (A1), and the vessel wall (assuming it is made from a conductive material) forms the other (A2) (assuming it is made from a conductive material) forms the other (A2). An insulator with a low dielectric constant is used to isolate the conductive probe from the housing, which is connected to the vessel wall wall.

The probe is connected to the level sensor via the conductive threads of the housing.

Measurement is made by applying an supply signal between the conductive probe and the vessel wall.

2.4 Radiation-Based Level Gagesg An entire class of level instrumentation devices is

b d t i l' t d t fl t b b based on a material's tendency to reflect or absorb

radiation. For continuous level gages, the most

common types of radiation used are radar

/microwave ultrasonic and nuclear /microwave, ultrasonic, and nuclear.

The main advantage of a radiation-based level gauge g g g

is the absence of moving parts and the ability to

d t t l l ith t ki h i l t t ith th detect level without making physical contact with the

process fluid.

Ultrasonic level sensorsUltrasonic level sensors

In applications when it is not acceptable for the level

measuring instrument to come into contact with the

process material, an ultrasonic device can by used.process material, an ultrasonic device can by used.

These devices measure the distance from a reference

point in the vessel to the level interface, using

ultrasonic waves ultrasonic waves.

Sonic sensors

o in sonic sensors, the unit uses the echo principle with a frequency in the

audible range. g

o After each pulse, the sensor detects the reflected echo.

o Note this will only work if the surface of the liquid is a good reflector and

that the centre line of the transmitted beam is vertical.

Ultrasonic level system for innage measurement: Ultrasonic level Ultrasonic level system for innage measurement: Ultrasonic level transmitters measure actual distance by issuing an ultrasonic pulse and measuring the time required for the reflected echo to be received

Level Switches Level switches are used to detect the liquid high and

l l l low levels.

The level switch outputs are used for initiating the The level switch outputs are used for initiating the

alarm and shutdown functions.

The outputs are also used for On/Off controls, such

as in the starting and stopping of pumpsas in the starting and stopping of pumps.

a) Float Level Switches

Floats should always be lighter than the minimum expected specific gravity (SG) of the process fluid.

h b d f l l fl ll l d They can be made out of stainless steel, Teflon, Hastelloy, Monel, and various plastic materials.

Typical temperature and pressure ratings are -40 to 80°C and up to Typical temperature and pressure ratings are -40 to 80 C and up to 150 psig for rubber or plastic floats, and -40 to 260°C and up to 750 psig for stainless steel floats. Standard float sizes are available from 1 to 5 inches in diameterto 5 inches in diameter.

Float Switches Applications & Installationspp A simplex (one pump) system will use a single switch wired in

series with the motor leads so that the switch directly starts and stops the pump motor.

side-mounted, Top-mounted, and external-mounted float switches.

Float Level Switch Configuration

Displacer Switchesp 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, or f li ti foamy applications.

These switches are interchangeable between tanks because differences in process density can be accommodated by changing the tension of the support by changing the tension of the support spring.