Fundamental of Instruments

8/8/2019 Fundamental of Instruments

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SHRI RAM FIBRES LIMITED

FUNDAMENTALS OF INSTRUMENTATION

CHAPTER I

1. Why Instruments Are Required?

Instruments Are Required For :

a. Accurate Measurement

b. Reliability

c. Repeatability

2. What is the function of the Instruments in Process Industries?

To MeasureIndicate

Record and

Control

The process variables such as fluid:

a. Pressure

b. Level

c. Flowd. Temperature

3. Measurement Of Fluid Pressure:There are four main types of fluid pressure instruments .

a. Differential pressure : that is difference between two pressures

b. Gauge pressure : which is the pressure measured above the localatmospheric pressure

c. Absolute pressure : which is the total of the atmospheric pressure

and gauge pressured. Vacuum : a condition in which the pressure is always less than that

Of the local atmospheric

Static Pressure :

The term static pressure is widely used in the measurement of pressure liquid level and rate

of flow. Where the fluid in a process is still or static the measuring of the static pressure is

reasonably clear . when a fluid in motion is en countered however the meaning may not be soobvious. For example, consider a pipe full of a fluid in motion . if a hole is drilled in the wall of

the pipe and a pressure gauge is installed to it, then the pressure indicated on the gauge is the

static pressure at the tapping.

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3.1 Instuments used for differential pressure measurements

a. Single U Tube Monometerb. Single Column Monometer

c. Inclined Tube Monometer

d. Diaphragm, Bellows Etc.( In Of H2o Or Inches Of Hg )

3.2 Instruments Used For Gauge Pressure Measurement :

a. Bourdon Type

b. Helical Type Bourdon Tube

c. Diaphragm Stacks And Bellows

Range f Pressure Measurable By Bourdon Tube :

0 5 lbs / sq. inch

0 80000 lbs / sq.inch

Material of bourdon tube:

Phosphor bronze , bronze alloys

Beryllium copper

Steel .

The material of bourdon tube depend upon the pressure value and type of fluid.

For lower pressure measurementDiaphragm stacks and bellows are used.

3.3 Absolute Pressure Measurementa. Liquid Column Instrument

b. Diaphragm Stacks Or Bellow Instruments

Range : From A Few Inches Of Water To About 80 Lbs / Sq. Inch.

3.4 Vacuum Measurement

Medium High Vacuum 1 To 10 3 Mm Hg.High Vacuum 1 To 10 7 Mm Hg.

Ultra High Vacuum 10-7 Mm Hg And Lower Absolute Pressure

The Term TORRIs Given To The Unit 1mm Of Hg.

1. Mecleod Gauge

2. Pirani Gauge3. Cold Cathode Magnetron Ionization Gauge

4. Hot Cathode Magnetron Ionization Gauge

5. Alpha Particle Ionization Gauge

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4.0 Liquid And Solids Level Measurement

Instruments for the measurement of liquid level in a tank can be classified under thefollowing headings :

1. Mechanical And General2. Pneumatic

3. Electrical

4. Electronic

5. NucleonicThe type of instruments to be employed is dependent on the liquid level range, the working

pressure s and the nature of the liquid.

4.1 Mechanical And General Type:1. Sight Glass Used In Boilers

2. Ekstron Gauge ( High Pressure Or Corrosive Fluid )

3. Float Gauge ( Tank Level )

4.2 Pneumatic Type :

1. Liquid Manometer U Tube And U Tube Float Pattern Manometer.H = K / ( D1 D2)

H = P

D1 = Density Of The Tank LiquidD2 = Density Of The Manometer Liquid

2. Diaphragm And Bellows Instruments

3. Dip Tube Instruments4. Buoyancy Type And Torque Tube Instrument

4.3 Electrical Type Instruments:1. Float And Magnetic Type

2. Electrode Type High Pressure Manometers

3. Paddle Type ( Bindicators Solid Level )

4.4 Electronic Type Instruments:

1. Capacitor Type Instruments ( Liquid & Solid )This type of level instruments is suitable for liquids which can as a dielectric. The

electrode and tank form a capacitor system whose dielectrics are the liquids itself and the

air or gas in the space above the liquids. These form capacitances co and c1 as theprincipal components. Since the dielectric constants of liquids are normally much greater

than these of gases . The liquids dielectric by its rise and fall in the tank can alter the

effective capacity of the system and so afford measurement of the level of the liquid.

2. Read Relay Type3. Ultrasonic Type

4. Load Cells ( Strain Gauge Solid Level )

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4.5 Nucleonic Type Instruments

The nucleonic type level instrument involves a radioactive source, a radiation detector and

electronic measuring circuits including amplifier etc.The source is placed externally on one side of the tank and the detector on the opposite

external side of the tank. The liquid rising and falling inside and tank absorbs radiation and

the change in intensity received by the detector is measured and gives an indication of theliquid level.

5. Flow Measurement

Fluid flow in industries under taking occurs in two general forms, either as a flow in apipe or as a flow in an open channel. In both cases the rate of the flow and the totalized

flow are important.

the rate of flow measuring instruments are broadly sub divided into

1. Differential Pressure Flow Meters

Orifice Pattern

Venturi And Nozzle PatternPitot Tube Pattern

Dall Tube Pattern

Miscellaneous Tube Pattern2. Variable Area Flow Meters

3. Displacement And Inferential Flow Meter

4. Electromagnetic Flow Meters5. Ultrasonic Flow Meters

6. Anemo Metres

5.1 Differential Pressure Flow Meters

The Measuring Element May Be Any One of The Following :

1. Orfice Plate

2. Venturi Tube

3. Pitot Tube Etc.

Square Law Effect : When we talk of differential pressure measurement, we should know

the square law effect or square root effect.

This can be explained by taking a simple flow equation at maximum flow q-1Q-1 = k _ h1 q2 = h2

At any other flow q-2 q1 h1

Q-2 = k _ h2 h2 = q2/2 x h1

q2/1

If q1 is 20,000 lbs per hour and h1 is 50 inches of water at maximum flow

100,000 lbs/hr.H2 = ( 10,000 )2 x 50 = 12.5 inches of water.

( 20,000 )2

The square root effect is now apparent since for the maximum flow we have only of thedifferential pressure.

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5.1.1 The different type of flow instruments for differential pressure flow devices :

a. The ledoux bell pattern flow meter

b. The shaped chamber flow meterc. D/p cell

From the above flow instruments, we will get the rate of flow . The total quantity of fluid

passing ( flowing ) in a given period is obtained by having a integrator. The instrumentsperforming the totalizer operation are known as integrators.

Different types integrators :a. Planimeter

b. Continuously Operated Mechanical Integrators

c. Intermittently Operated Mechanical Integrators

d. Pneumatic Integratorse. Electronic Integrators

f. Electrical Integrators

5.2 Variable Area Flow Meters ( Rotameters )

In case of differential pressure flow the differential pressure head h variesWith the flow. Where as in the variable area flow meter the differential pressure head h

is constant.

The differential type of variable area flow meters are given below:

5.2.1 Glass Tube Type

The glass tube type measures from 2cc/min. Up to 3000 liters /min. of gas and 0.5 cc to

225 liters /min. of liquid.

the pressure drop will depend on the type of float being used and the nature of the fluid,but varies between about 0.2 cm ( 0.078 inch water gauge ) for small gas flows and 3.5

cm ( 1.38 inch) water gauge for liquid flow.

Different type of floats

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How to read correctly the rate of flow in Rotameter :

Always read the flow rate corresponding to the top edge of the float.

5.2.2 Metal Tube Type Rotameter

For larger flows than the glass type tube can accommodate, a conical

metal Tube pattern is introduced. Here the metal body is of gun metal, cast iron or

stainless steel with a stainless steel float. This type of meter has typically ranges

250 to 120,000 liters / min. for gas flow and 20 to 7000 liters / min. for liquid . Themaximum fluid working pressure is 500 lbs/ sq. Inch.

5.2.3 Plug Type Area Meter

Another forms of area meter is the loaded plug type . This area meter is designed for

incorporation in a pipe line and comprises a flanged body casting through which theliquid being metered passes. The metering plug and port assembly is designed in

such a manner that a constant pressure drop is maintained with different rate of flow.

5.2.4 Magnetically Coupled Variable Area Meter

Where working pressures higher than 500 lbs/ sq. inch or for corrosivefluid a totally enclosed metal tube type of instruments with a magnetic coupling is

used. Typical flow ranges are from 30 to 120,000 liters / min. Of gas and 0.75 to

7000 liters / min. of liquids.

5.3 Mechanical Meters Can Be Divide Into Main Groups :

1. Displacement And 2. Inferential Type.

In the displacement type of flow meters which are volumetric in operation, the

stream of fluid can operate cyclically a movable member of the meter , provide ameans for measuring the total flow over a given period.

The Different Type Of Displacement Type Meters Are :

a. Reciprocating Piston Typeb. Semi Rotary Piston Type

c. Nutating Disc Type

d. Helix And Other Type

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Inferential type of flow meter measure the velocity of flow and from this measureinfer the volume of flow.

The Different Type Of Inferential Flow Meters Are :a. Single Or Multi Jet Fan

b. Turbine

c. Horizontal Or Vertical Propeller Type

d. Electronic Type Inferential Meter

Ranges :

In view of the extensive use of these type of meters in the water and fuel supplyfields the range are extremely high. The range of the meter varies with meter size.

For example a 4 inch diameter meter has a range 0 7000 gallon/hour. The meter are

also suitable for very low liquid flow rates, eg. 4 gallons/hour.

General Remarks On Mechanical Displacement Meters :

a. Part or whole of the gear assembly may be in the fluid . It is therefore, necessary

to use material which are non corrosive and wear resistant.b. These meters required protection against freezing, if the metered liquid is likely

to freeze in cold weather.

c. These meters have a pressure drop between inlet and outlet .the value dependingon the type and the rate of flow through the meter.

5.4 Electromagnetic Flow Meter :

In the electromagnetic flow meter the theory of faradays law of induction is used,i.e. When an electrical conductor of length l moves through a magnetic field ofstrength h at a velocity in a direction at right angles both to the magnetic field and

its length an e.m.f. Is generated of value :

E= KHLV

If we consider a very thin of an electrically conducting liquid, then l is replacedby d the internal diameter of the pipe.

E = KHVd. K, d and h are constant.

E = KVQ = VA. A is the area of the pipe.

5.4.1 Advantages of the electromagnetic flow meter :

a. linear relation between flow rate and measuring signal

b. the only pressure loss is that due to the length of tube forming the meter.

c. There is no obstruction to flowd. Suitable for corrosive fluids

e. It is not affected by velocity profiles

f. Suitable for slurry flow measurement.

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5.4.2 Disadvantages of the electromagnetic flow meter :

a. it is not suitable for measuring gases or vapourb. liquids to be metered and must be conductors of electricity

c. not suitable for hazardous areas

Ranges :

A meter has been constructed of 1/10 inch diameter with a flow range 0.002 to 0.2

gal/min. a typical large diameter meter has been 71 inches covering a range of 500

to 50,000 gal/min.

6.0 Measurement of temperature :

temperature measurement can be divided into two main groups :

a. non electrical thermometers

b. electrical thermometers

Non- electrical thermometers

The most important classes of non electrical thermometers are :

a. liquid expansion typeb. gas expansion type

c. vapour expansion type

d. bi-metallic type

6.1 Liquid expansion type thermometer :

in a typical liquid expansion thermometer a bourdon tube is connected to a metalbulb by small bore metal tube known as capillary. The whole of the internal volume

formed by the bulb, capillary and bourdon is filled with a liquid with as great a rangeas possible between freezing and boiling points.

Liquids used in thermometers :

Liquid Temperaturemin.

Temperaturemax.

Mercury 38 f 1000 f

Xylene 40 f 750 f

Alcohol 50 f 300 f

6.2 Gas expansion type thermometers :

The system formed by a bulb capillary and bourdon being filled with a gas at high

pressure . gas expansion type thermometers work on the principle of gas lawPV = RT

Nitrogen is almost universally used as the filling gas. It is inert, has a highcubical coefficient of expansion and is easily available. With gas expansion

thermometers the min . and max. measuring temperature is 60 f to 800 f.

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6.3 Vapour pressure thermometers :

In a confined space , the vapour from a liquid exerts its own pressure ,increasing with temperature . this characteristic of the vapour is used in the vapour

pressure thermometer.

The two important things to consider in the vapour pressure thermometers are:

1. the maximum temperature of the instrument range must be below the

critical temperature of the liquid.

2. the minimum temperature of the instruments range must be above theboiling point of the liquid.

Liquids used for vapour pressure thermometer :Methyl chloride, freon , sulphur dioxide, alcohol, toluene and ether.

The range of the vapour pressure thermometer is from 60 f to 500 f .

6.4 Bi- metallic thermometers :

A type of thermometer widely used is bi-metal thermometer . this consists

of two metals welded together . one of the pair has an expansion co efficient muchgreater than the other . consider one end of the strip to be fixed and the other left

free. The application of heat causes the free end to deflect due to the unequal

expansion of the metal forming the strip. If the strip is wound into helical form, thenit gives a rotary movement to a spindle , when heat is applied.

For the lower expansion member of a bi-metal strip INVAR is commonly

used. The high expansion member may be brass or one of the nickel alloys.

The range covered is approximately from 40 f to 600 f.One great advantage of this pattern of thermometer is that it does not

suffer from head of barometric pressure errors.

6.5 Electrical thermometers:

The most important classes of electrical thermometer are :

a. Thermocouple

b. Resistance thermometerc. Thermistor thermometer

d. Semiconductor thermometer

e. Pyrometers or radiation thermometer

6.6 Thermocouples :

One of the most generally useful temperature sensing devices is thethermocouple. The history of this device dates back to the discovery by see back in 1821

that an electric current flows in a continuous circuit of two different metals if the two

junctions are at different temperatures.

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Positive & negative errors :

The current produced in the thermocouple circuit will flow from one wire to the other

at the cold junction .the wire from which the current flows is termed the positive wireand the wire into which the current flows is the negative one.

Thermocouple material : These are two broad groups of thermocouples

a. rare or precious metal thermocouple

b. base metal thermocouple

Rare metal thermocouple : range

a. 90 % platinum, 10 % rhodium platinum 1400 c

b. 87% platinum , 13% rhodium platinum 1300 c

Base metal thermocouple :

a. copper constantan 400 c

b. iron constantan 850 cc. chromel - alumel 1100 c

6.7 Resistance thermometer :

The variation of electrical resistance of materials with temperature offers a very

convenient basis for temperature measurement. Three metals are used in industries forresistance thermometers :

material rangea. platinum 190 to 540 c max.

b. nickel 200 to 300 c max.c. copper 50 to 120 c max.

The difference in resistance of the thermometer between 100 c and 0 c is

termed fundamental interval. This is used in specifying the resistance thermometers.

E.g. pt.100 ohms. This means that the resistance value of the thermometer is 100 ohms at0 c at 760 mm hg atmospheric pressure.

There are three methods in connection with the resistance thermometer to

instruments, they are .

1. two lead wire system

2. three lead wire system3. four lead wire system

If the thermometer is considerably away from the instruments the resistance of the

connecting lead is of some significance since the measuring instrument bridge measuresthe total resistance of the thermometer as well as lead wire resistance. By using three

leads from the thermometer, it is possible to compensate the lead wire resistance . this

was shown by sir . w. siemens.

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Thermistor thermometer :

Thermistor are composed of metallic oxides of cobalt, copper , nickelmanganese, iron, tin, magnesium, titanium, uranium, and zinc, which are compressed

and fired at the elevated temperature . they may be produced in the form, of small beads,

discs, rods and washers. They passes negative temperature resistance relations. A typicalvalue of which is about 4 % per degree at 20 c.

The temperature range over which the thermistors are satisfactorily function is

100 300 c .

Thermistors are available in various forms of beads as small as 0.015 indiameter, disc ranging from 0.2 1.0 inch diameter and rods from 0.03 0.025 inch

diameter upto 2 inches length.

6.8 Semi conductor thermometer :

This is the name loosely given to a class of thermometer constructed from pure

germanium or pure silicon crystals . the range can be 320 f to 600 f for a pure crystalof silicon. The characteristics temperature resistance is positive and it is only slightly

non linear. The response to the temperature is extremely fast. To reach 63 % of the

final temperature value , when a change take place required only 15 milli seconds and toreach 90 % of the final value requires 40 milli seconds.

Radiation thermometers or pyrometers :The basic law of radiation :

All hot bodies emit radiate energy, the intensity of which bears a relation to the absolutetemperature of the emitting surface. Heat radiation is a wave phenomena analogous to

light and occupies the definite space in the spectrum. Extending from about 1/ 100 th ofa micron to 100 microns.

There are the different types of pyrometers such as total radiation pyrometers, photocell

pyrometers, optical pyrometers and two colour or ratio pyrometers.

With optical pyrometer, the temperature can be measured with the error of :

3 c at 1000 c6 c at 2000 c

40 c at 4000 c

Why pyrometers are required :

All the thermometers are having one factor in common, i.e. they require the element

itself to be inserted in the medium , whose temperature is to be measured. This notpossible to measure any temperature with thermocouples above 2000 c . this brings us

to the consideration of radiation pattern thermometer of pyrometers.

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CHAPTER - II

1.1.1 Basic control consideration :

In the process industry automatic control is justified by :

1. increased productivity2. reduced costs

3. better and more uniform products

4. safety

An automatic control system consists of five parts :

a. the process b. measuring means

c. error detector d. controller

e .final control element

These five elements can be inter connected in two ways . one in open loop, the other in

closed loop .

1.2.1 Open loop :

In the open loop control, the output has no effect on the input . in other words, no feedback from output to input. E.g. if a home thermostat is installed outside, the roomtemperature due to furnace heat i.e. input would have no effect on the furnace

temperature . however the temperature of furnace will not be controlled by the open loop

system if the room temperature is changed. Another eg. is automatic traffic signal .

1.2.2 Closed loop :

If we close the system by bringing the thermostat inside the room, furnace heat

( output ) now does affects the in door temperature ( input ) this closed loop thermostatsystem continue to operate until the indoor temperature reaches the desired value . in this

closed loop system , there is a feed back information from the output to the input and

there is the possibility of in stability through self excitation, where as open loop is

inherently stable because there is no self excitation.

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1.3.1 Manual and Auto control :

What is manual control ?controlling the liquid level in a tank to a desired height manually by closing or opening

the inlet valve to the tank when ever there is a fall or raise in level by an operator is the

oldest method adopted called manual control.

1.3.2 Why automatic control is required ?

In the chemical industries, because of the completely of process, the operator has todecide his attention over many controlled condition s. the task of the operator is made

easy by introducing automatic control. The automatic control relieve the operator from

dual duties and to give more opportunity to do their work effectively and intelligently .

1.3.2 How to achieve the automatic control ?

There are two ways of achieving automatic control

a. by feed back b. by feed forward

In the feed back control system , the output has got the influence over the input, where as

the feed forward control not have a check on the results of its action . however, anintroduction to the feed forward control requires a precise knowledge of the behavior of

the process, otherwise it is impossible to adjust the controlling unit correctly. On the

other hand , the feedback control is not very critical and precise knowledge of theprocess behavior is not required.

1.4.1 The characteristics of automatic control :the basic characteristics of an automatic controller is the manner in which it acts to

restore the controlled variable to the desired value, this is called the mode of control. Thecommon of controls are :

a. two position b. floating c. proportional

d. proportional plus reset e. proportional plus reset plus

1.4.2 Two position control :

In two position control, we get ON /OFF action only, when the controlled variabledeviates from the set point the final control element will either open or close and hence it

is known as ON / OFF controller.

e.g. a float switch used for controlling a tank level.

1.4.3 Floating control :

Floating control is commonly known as single speed floating control. In this the finalcontrol element moves at a constant speed in either direction whenever the controlled

variable deviates from the set point, where as in two position controller changes the

position of the final control element from ON/OFF .floating control has got an importantcharacteristics is that it always works to return the control variable to set point.

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1.4.4 Proportional control :

in proportional controller , the output from the controller is proportional to the input, inother words there is a continuous linear relation between the value of the controlled

variable and the position of the final control element.

Proportional action :

When the controller has proportional action only, then the controller produces a

corrective action proportional to the size of the error signal ( deviation from the setpoint).

Proportional gain :

The variation of the controller output per unit of deviation is called the proportional

again. A large proportional gain means that a small deviation causes a large correction

and a small proportional gain means a large deviation causes a small correction .Proportional band of a controller is defined as the % of input needed to get the 100 %

output of the controller. E.g. if the proportional band dial is set at 10% for a process

deviation of 10% , the controller output will very by 100 %.

If we use proportional controller alone, there will be an off-set if there is a change in

load conditions. This inherent set-back of the proportional controller can be eliminatedonly by adding reset ( integral ) or reset plus rate ( integral plus derivative ) control

action .

What is an off set :

When the load changes, the proportional controller will not maintain the desired value

( set point ). This is called proportional drop or off-set.

Proportional control may be used alone provided the load changes are not great and theoff set that results from these changes does not impair the quality of the product being

processed.

1.4.5 Reset or integral action :

This integral mode of control can be used only with proportional controller.

What is reset ?

Reset or integral action is nothing but integrating the proportional action till the off-set is

eliminated .

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The reset combined with proportional control action, enables the controller always to

maintain variable such as temperature level or pressure at the desired value. The

proportional control mode provides a stabilizing influence while the reset mode providesnecessary action to continue automatically the output correction until the controlled

variable is brought back to the set point .

The combination of proportional plus re set action is most widely used for flow and

pressure control where the process has less time lag and require a wide band proportional

controller for stability.

The reset action is limited in the case of temperature control since the time lag with in

the process is large . hence, only a small amount of reset is permissible otherwise

oscillations will occur .

1.4.6 Rate or derivative action :

This control mode can be used only with proportional or proportional with resetcontroller .

What is the rate or derivative action ?

It performs the action on the valve that a operator would perform if he noted the rapid

change in measurement . e.g. in a a liquid level application, if the level were to raiserapidly, a experienced operator would over correct the valve towards the closed position

to prevent the level from reaching excessive limits. In general, this is the type of action

provided by rate or derivative mode control . in other words the rate action provided byrate or derivative mode control. In other words the rate action provides anticipated

corrective measures and normally over shoot the set point. It is also known as preact orbooster .

1.4.7 Reset and rate actions combined with proportional:

control action gives a three mode controller having the advantages of all control effects.It comes close to duplicating the type of control expected from a good experienced

human operator . if the adjustments of the controller is properly done to suit a process ,

in case of any process disturbance, the controlled variable will not deviate excessivelyand will return to the set point rapidly .

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