Best Instrumentation interview Study Material
INSTRUMENT QUESTION & ANSWERS
NOTE : Diagrams are not clear you can get the clear diagrams in the word file attached in the link at the end.
INTRODUCTION:
It
is the branch of engineering which deals with the measurement, monitoring,
display etc. of the various of energy exchanges which take place during process
operations. "In short Instrumentation is the study of Instrument."
INSTRUMENT:
Instrument
is a device which is used to measure, monitor, display etc. of a process
variable.
1.
What is the process Variable?
The process Variable is:
·
Flow.
·
Pressure.
·
Temperature.
·
Level.
·
Quality i. e. % O2,
CO2, pH etc.
2.
Define all the process Variable and state their unit of measurement. ?
Flow: Defined as volume per unit of time at specified
temperature and pressure
Conditions, is generally measured by
positive-displacement or rate meters.
Units: kg / hr , litter / min, gallon / min , m3
/ hr , Nm3 / hr. ( gases )
Pressure: Force acting per unit Area. P = F/A
Units :
bar, Pascal, kg / cm2, lb / in2.
Level: Different between two heights.
Units: Meters, mm, cm, percentage.
Temperature: It is the degree of hotness or coldness of a body.
Units : Degree Centigrade, Degree Fahrenheit, Degree Kelvin,
Degree Rankine.
Quality: It deals with analysis.( pH, % CO2, % 02,
Conductivity, Viscosity )
FLOW
3. What are the primary elements used for flow
measurement ?
The
primary elements used for flow measurement are :
·
Orifice Plate.
·
Venturi tube.
·
Pitot tube.
·
Annubars.
·
Flow Nozzle.
·
Weir &
Flumes.
4. What are the different types of orifice plates and
state their uses ?
The
different types of orifice plates are :
·
Concentric.
·
Segmental.
·
Eccentric.
·
Quadrant Edge.
Concentric :
The
concentric orifice plate is used for ideal liquid as well as gases and steam
service. This orifice plate beta ratio fall between of 0.15 to 0.75 for liquids
and 0.20 to 0.70 for gases, and steam. Best results occur between value of 0.4
and 0.6. beta ratio means ratio of the orifice bore to the internal pipe
diameters.
|
|||
(45º beveled edges are often used to minimize
friction resistance to flowing fluid )
flow
º
(
fig 1)
Eccentric:
The
eccentric orifice plate has a hole eccentric. Use full for measuring containing
solids, oil containing water and wet steam. Eccentric plates can use either
flange or vena contracta taps, but the tap must be at 180º or 90º to the
eccentric opening.
|
Eccentric
orifices have the bore offset from center to
Minimize
problems in services of solids-containing
materials.
( fig 2
)
Segmental:
The segmental orifice place has the
hole in the form segment of a circle. This is used for colloidal and slurry
flow measurement. For best accuracy, the tap location should be 180º from the
center of tangency.
Segmental orifices provide another
version of plates
useful for solids containing
materials.
( fig 3 )
Quadrant Edge:
It common use in Europe
and are particularly useful for pipe sizes less than 2 inchs.
|
|||
Quadrant edge orifices produce a
relatively constant
coefficient of
discharge for services
with low
Reynolds numbers in the range from 100,000 down
to 5,000.
( fig 4 )
5. How do you identify an orifice in the pipe line. ?
An
orifice tab is welded on the orifice plate which extends our of the line giving
an indication of the orifice plate.
6. Why is the orifice tab provided. ?
The
orifice tab is provided due to the following reasons.
·
Indication of an
orifice plate in a line.
·
The orifice
diameter is marked on it.
·
The material of
the orifice plate.
·
The tag no. of
the orifice plate.
·
The mark the
inlet of an orifice.
7. What is Bernoulli's theorem and where it is
applicable. ?
Bernoulli's
theorem states the "total energy of a liquid flowing from one point to
another remains constant." It is applicable for non compressible liquids.
8.
How do you identify the H. P. side or inlet of an orifice plate in line. ?
The
marking is always done H. P. side of the orifice tab which gives an indication
of the H. P. side.
9.
How do you calibrate a D. P. transmitter. ?
The
following steps are to be taken which calibrating :
1.
Adjust zero of the Xmtrs.
2.
Static pressure test : Give equal
pressure on both sides of the transmitter.
Zero should not shift. If it is shifting carry out static alignment.
3.
Vacuum test: Apply equal vacuum to both the sides. The zero should not shift.
4. Calibration Procedure:
·
Give 20 psi air
supply to the transmitter.
·
Vent the L.P.
side to atmosphere.
·
Connect output of
the Instrument to a standard test gauge. Adjust zero.
5.
Apply required pressure to high pressure side of the transmitter and adjust the
span.
6.
Adjust zero again if necessary.
10.
What is the seal liquid used for filling impulse lines on crude and viscous
liquid ?
Glycol.
11.
How do you carry out piping for a Different pressure flow transmitter on
liquids, Gas and steam services? Why?
Liquid lines: On liquid lines the transmitter is
mounted below the orifice plate because liquids have a property of self
draining.
Orifice plate
Flow direction
Primary isolation valve
(LIQUID SERVICE)
3
way manifold valve
Flow transmitter
+ -
Gas Service: On gas service the transmitter is
mounted above the orifice plate because Gases have a property of self venting
and secondly condensate formation.
+ -
Flow transmitter
3 way manifold valve
(GAS
SERVICES)
Primary isolation valve
Flow direction
Orifice plate
Steam
Service: On steam service the
transmitter is mounted below the orifice plate with condensate pots. The pots should be at the same level.
Orifice
plate
Flow direction
Primary isolation valve
Condensate pot
3
way manifold valve
+ - Flow transmitter
12.
Draw and explain any flow control loop?
c/v
Positioner
Orifice plate
Primary isolation valve
AIR SUPPLY
FRC
3 way manifold valve
Flow transmitter
AIR SUPPLY
13.
An operator tells you that flow indication is more? How would you start checking?
·
First flushing the transmitter. Flush both the
impulse lines. Adjust the zero by equalizing if necessary. If still the
indication is more then.
·
Check L.P. side
for choke. If that is clean then.
·
Check the leaks
on L.P. side. If not.
·
Calibrate the
transmitter.
14.
How do you do a zero checks on a D.P. transmitter?
Close
one of the valve either H.P. or L.P. open the equalizing valve. The O/P should
read zero.
15.
How would you do Glycol filling or fill seal liquids in seal pots 7 Draw and
explain.
·
The procedure for
glycol filling is :
·
Close the primary
isolation valves.
·
Open the vent on
the seal pots.
·
Drain the use
glycol if present.
·
Connect a hand
pump on L.P. side while filling the H.P. side with glycol.
·
Keep the
equalizer valve open.
·
Keep the L.P.
side valve closed.
·
Start pumping and
fill glycol.
·
Same repeat for L.P.
side by connecting pump to H.P. side, keeping equalizer open and H.P. side
isolation valve closed.
·
Close the seal
pot vent valves.
·
Close equalizer
valve.
·
Open both the
primary isolation valves.
16.
How do you calculate new factor from new range using old factor and old range?
Q1
√ ∆P1 Q1 =
Old flow(factor)
= Q2 =
New flow(factor)
Q2 √ ∆P2 √ ∆P1 = Old range
√ ∆P2 =
New range
Q2
× √ ∆P1 = Q1 × √ ∆P2
Q1 × √ ∆P2
Q2 =
√ ∆P1
Old factor × √ New range
New factor =
√ Old range
17.
How will you vent air in the D.P. cell? What if seal pots are used?
·
Air is vented by
opening the vent plugs on a liquid service transmitter.
·
On services where
seal pots are used
isolate the primary
isolation valves and
open the vent valves. Fill the
line from the transmitter drain plug with a pump.
18.
Why flow is measured in square root?
Flow
varies directly as the square root of different pressure F = K square root
of AP. Since this flow varies as the
square root of differential pressure the pen does not directly indicate flow.
The flow can be determined by taking the square root of the pen. Say the pen
reads 50% of chart.
19.
What is absolute pressure?
Absolute
pressure is the total pressure present in the system
Abs.
pressure = Gauge pressure + Atm. pressure.
20.
What is absolute zero pressure?
Absolute
zero = 760 mm Hg Vacuum.
21.
What is the maximum Vacuum?
The
maximum Vacuum = 760 mm Hg.
22.
What is Vacuum ?
Any
pressure below atmospheric pressure is vacuum.
Atm
= 760 mm Zero Vacuum
Zero Gauge
Zero absolute Max.vaccum = 760mmHg
23.
What are the primary elements for measuring pressure?
The
primary elements used for measuring pressure are:
·
Bourdon tube.
·
Diaphragm.
·
Capsule.
·
Bellows.
·
Pressure Springs.
The
above are known as elastic deformation pressure elements.
Type
of Bourdon tubes.
·
‘C ' type.
·
Spiral.
·
Helix.
Diaphragm: The diaphragm is best suited for low pressure
measurement.
Capsules : Two
circular diaphragms are welded together to form a pressure capsule.
Material Used: phosphor bronze,
Ni-spanc stainless steel.
Bellows : Bellows is a one - piece, collapsible, seamless metallic unit with deep
folds
Formed from very tin
walled tubing.
Material used: Brass,
phosphor bronze, stainless steel. Used for high
Pressure.
Pre.spring
: Pressure springs of helical or
spiral shape used for measuring high pressures.
24.How
will you calibrate an absolute pressure transmitter using vacuum manometer Range
0-400mm abs. ?
The
procedure for calibration is as follows:
·
Connect air
supply to the transmitter.
·
Connect a test
gauge of 0-1.4 Kg/cm2 to the output.
·
Connect Vacuum
pump with tee off to the manometer.
·
Apply 760 mm
Vacuum (or nearest) and adjust zero.
·
Apply 360 mm
Vacuum adjust span. (760 - 360 = 400 mm abs.)
25.
You are given a mercury manometer range 0 -760 mm? A vacuum gauge reads 60 mm
vacuum. The test manometer reads 50 vacuums? Which of the two in correct?
The
transmitter is correct because 760 - 50 = 710 mm abs.
26.
Why is an inclined manometer used?
It
is used to extend the scale of the instrument. Because the manometer is at an
angle to the vertical.
27.
What is the principle of a pressure gauge?
Pressure
works on Hooks law.
Principle:
"Measuring the stress in an elastic medium"
28.
Draw and explain a pressure gauge ? What is the used of a Hair spring ?
The
parts of a pressure gauge are :
1. 'C' type Bourdon tube.
2. Connecting link.
3. Sector gear.
4. Pinion gear.
5. Hair Spring.
6. Pointer.
7. Dial.
Uses
of Hair Spring : Hair spring serves
two purposes namely
·
To eliminate any
play into linkages.
·
It serves as a
controlling torque.
LEVEL
29.
Briefly explain the different methods of level measurement ?
There
are two ways of measuring level :
1. Direct
2. Indirect.
TAPE MEASURE
1.Direct level measurement :
( a ) Bob and tape:
TANK
Heighest point reached by
A bob weight and measuring liquid
tape
provide the most simple
and direct
method of
measuring liquid livel. Distance to be
measured
LIQUID after tape is taken out of
Tank.
Bob (weight)
(
b ) Sight glass :
This
consists of a graduated glass tube mounted on
the side of the vessel. As the
level of the liquid in the vessel change, so does the level of the liquid in
the glass tube.
Indirect level measurement :
( a ) Pressure gauge :
This
is the simplest method, for pressure gauge is located at the zero level
of the liquid in the vessel. Any rise in level causes an increase of pressure which can be measured by a gauge.
( b ) Purge system :
In
this method a pipe is installed vertically with the open and at zero level. The
other end of the pipe is connected to a
regulated air r supply and to a pressure
gauge. To make a level measurement the air supply is adjusted so that pressure
is slightly higher than the pressure due to height of the liquid. This is accomplished by
regulating the air pressure until bubbles cab be seen
slowly leaving the open end of the pipe.
Pressure
gauge
As level gauge
Air regulator
N2 purge
Air
supply rotameter
The air pressure to the bubbler pipe is minutely in
excess
of the liquid pressure in the vessel, so that
air
pressure indicated is a measure of the level in
the tank.
The
method above are suitable for open tank applications. when a liquid is in a
pressure vessel, the liquid column pressure can't be used unless the vessel
pressure is balanced out. This is done through the use of different pressure
meters.
(
c ) Differential pressure meter :
Connection
are made at the vessel top and bottom, and to the two columns of the D.P.
meter. The top connection is made to the L.P. column of the transmitter and the
bottom to H.P. column of the transmitter. The difference in pressure in the
vessel is balanced out, since it is fed to both the column of the meter. The
difference in pressure deducted by the meter will be due only to the changing,
level of the liquid.
( d ) Displacer
type level measurement :
The
leveltrol is one of the most common instruments used measuring level in closed
tanks. This instrument works of Archimedes principle. The displacer in immersed
in the liquid due to which there is loss of weight depending on the specified
gravity of the liquid. This displacer hangs freely on a knife transmitted to
the pneumatic or electronic counterpart at the other end.
30.
Explain how you will measure level with a different pressure transmitter.
The
bottom connection of the vessel is connected to high pressure side of the
transmitter.
Different
Pressure = H X D
H
d
HP LP
+ -
D / P TRANSMITTER
This
difference pressure is applied to H.P. side of the transmitted and calibrated.
31.
How is D.P. transmitter applied to a close tank ?
In
close tank the bottom of the tank is connected to the high pressure side of the
transmitter and top of the tank in connected to L.P. side of the transmitter.
In this way the vessel pressure is balanced.
32.How
is D.P. transmitter applied to an open tank ?
On
an open tank level measurement the L.P. side is vented to atmosphere. Whatever
pressure acts is on the H.P. side which is a measure of level.
SPAN = ( X ) (Sp. Grav)
ZERO
SUPPRESSION = ( Y ) ( Sp.Grav ) X
HP LP
Y + -
33.
How is D.P transmitter applied to a close tank & open tank with Dry leg?
Span = ( X
) ( GL )
HW
at minimum level = ( Z ) ( GS ) + ( Y
) ( GL )
HW
at maximum level = ( Z ) ( GS
) +
( X + Y ) ( GL )
Where :
GL =
Specific gravity of tank liquid.
GS =
Specific gravity of seal liquid.
HW =
Equivalent head of water.
X, Y & Z are
shown in fig (1.1)
Open tank Close tank with dry leg
|
X
MIN.LEVEL
Y
Z
Z
Example :
Open tank with
X =
300 inches
Y = 50 inches
Z = 10 inches
GL = 0.8
GS = 0.9
Span = ( 300 ) ( 0.8 ) = 240
inches
HW at minimum level = ( 10 ) ( 0.9 ) + ( 50
) ( 0.8 ) = 49 inches
HW at maximum level = (10 ) ( 0.9 )
+ ( 50 + 300 ) ( 0.8 ) = 289
inches
Calibrated
range = 49 to 289 inches head of water
Close tank with wet leg:
Span = ( X ) ( GL )
HW at minimum level = (
Y ) ( GL ) – ( d )( GS )
HW at maximum level = ( X
+ Y ) ( GL ) – ( d ) ( GS )
Where : GL = Specific gravity of tank liquid
GS =
Specific gravity of tank liquid
HW =
Equivalent head of water
X, Y and Z are shown in fig.
d
Example :
X = 300
inches
Y = 50
inches
d = 500
inches
GL = 0.8
GS = 0.9
Span = ( 300 ) ( 0.8 ) = 240
inches
HW
minimum level = ( 50 ) ( 0.8 ) - (
500 ) ( 0.9 ) = - 410 inches
HW
maximum level = ( 300 + 50 )
( 0.8 ) – ( 500 ) ( 0.9 ) = - 170 inches
Calibrated
range =
- 410 to –170 inches head of water.
(
minus sings indicate that the higher pressure is applied to the low pressure
side of the transmitter )
34.
What is purge level system ?
This
method is also known as bubbler method of level measurement. A pipe is
installed vertically with its open end at the zero level. The other end of the
pipe is connected to a regulated air supply and to a pressure gauge or to ^P
transmitter. To make a level measurement the air supply is adjusted so that
pressure is slightly higher than the pressure due to the height of the liquid.
This is accomplished by regulating the air pressure until bubbles can be seen
slowly leaving the open end of the pipe. The gage then measures the air
pressure needed to over come the pressure of the liquid.
/\ P = H X D
USE
: On for corrosive liquids where the
transmitter cannot be directly connected to process eg... Acids, Some organic
liquids.
35.
Explain the working of a leveltrol.
The
leveltrol is used for measuring level of liquids in a closed vessel.
1. PRINCIPLE. : It works on Archimedes principle
"The loss in weight of a body immersed in a liquid is equal to amount of
liquid displaced by the body". The leveltrol basically consists of the
following :
2. DISPLACER : It is consists of a cylindrical
shape pipe sealed and filled inside with sand or some weight. The purpose of
this is to convert change in level to primary motion. The variation in buoyancy
resulting from a change in liquid level varies the net weight of the displacer
increasing or decreasing the load on the torque arm. This change is directly
proportional to change in level and specific gravity of the liquid.
3. RELAY : Amplifies pressure variations at the
nozzles.
4. REVERSING
ARC : It is used for the following purposes.
·
Motion take of
from Torque tube.
·
Means of reverse
control action.
·
Adjustment for specific gravity.
5. PROPERTIONAL UNIT. : Converts primary motion to
a proportional output air pressure.
6. CONTROL SETTING UNIT : Provides a motions of
varying the set point.
36.
Explain the working an electronic leveltrol.
REG
+
RVDT DC MOD
OSC
+ +
- - DC AMPLIFIER
SPAN
_ ZERO
The
variation in buoyancy resulting from a change in liquid level, varies the net
weight of the displacer increasing or decreasing the load on the torque arm.
This change is directly proportional to the change in level and specific
gravity of the liquid. The resulting torque tube movement varies the angular
motion of the rotor in the RVDT (Rotary Variable Differential. Transformer)
providing a voltage change proportional to the rotor displacement, which is
converted and amplified to a D.C. current.
37.
How will you reverse an action of the leveltrol.?
The
reversing are serves as motion take off arm from the torque tube. It is
provided with a slot on each side of the center so that link can be connected
either for reverse or direct action.
38.
What is interface level ? How do you calculate it ?
When
a vessel is filled with two liquids of two different specific gravities the
level measurement refers to as interface level.
DP
= H ( D - d )
DENSITY d
DENSITY D
On
a level set the difference of two specific gravities.
39.
How will you calibrate a leveltrol in the field ?
Displacer
chamber
Level transmitter.
Transparent
P.V.C tube
1.
First close both the primary isolation valves and drain the liquid inside the
chamber.
2.
Adjust the zero to get 0% output.
3.
Connect a transparent PVC tube to the drain point as shown in hook up.
4.
Fill it to the center of the top flange.
5.
Adjust the specific gravity or span adjustment
( Electronic Level ).
6.
Fill it up to 50 %, check linearity.
40.
How will you calibrate on interface level control. ?
On
an interface leveltrol there are two liquid of two different specific
gravities.
1.
The level will be zero when it is full of lighter liquid.
Zero % level =
H X d.
H = Displacer length
d = Specific gravity of lighter liquid.
2
The level will be 100 % when it is full of heavier liquid.
100 % level
= H X D.
D =
Specific gravity of heavier liquid.
Calibration
with water :
1.
Fill H X d level with water adjust zero.
2.
Fill H X D level with water adjust Sp. gravity or span.
3.
Check linearity.
41.
How will you apply wt. lest calibration to a leveltrol.
Wt. test calibration method :
- Remove the displacer from the torque arm.
- Apply equivalent weight on the torque arm that is equal to the wt. of the displacer. Adjust zero % output.
- For Span : V = πr2h
Loss in weight = Wt. of float - wt. of the float
immersed in liquid
Loss in weight = [ wt. of float - Vol. x d ]
Span wt. = (wt. of float - Loss
in wt.)
r = radius of the displacer.
h = ht. of displacer.
- Apply equivalent wt. equal to the (Wt. of float - Loss in weight). Adjust Span to get 100 % out put.
- To check linearity apply average of the two weights.
42.
What will happen if the displacer has fallen down while in line ?
The
output will be maximum.
43.
What will happen if the displacer has a hole in it while in line ?
The
output will be minimum.
44.
What is the used of Suppression and
elevation ?
Suppression
and elevation are used on Level applications where (1) transmitters are not
mounted on some level (2) Wet leg. i.e. condensable vapors are present.
45.
What are the limitations of leveltrol ?
The
limitations of a level control that it cannot be used for lengths more than 72
inches.
46.
How will you commission D.P. transmitter in field in pressurized vessel.
1. Close both the isolation valves, Vent the H.P. side.
2. Fill it with the sealing liquid.
3. Open the L.P. side vent valve.
4. Adjust zero with suppression spring.
5. Close
the L.P. side vent valve.
6. Open both the isolation valves.
47.
How will you check zero of a level D.P. transmitter while is line ?
1. Close both the isolation valves.
2. Open the vent valve on L.P. leg and H.P. leg drain.
3. Check and adjust zero if necessary.
48.Explain
the working of an Enraf level gauge?
The
Enraf precise level gauge are based on
servo powered null-balance technique. A displacer serves as a continuous level
sensing element.
Principle:
A
displacer with a relative density higher than that of the product to be
measured, is suspended from a stainless steel wire B, that is attached to a
measuring drum. A two phase servo meter controlled by a capacitive balance
system winds or unwinds the measuring wire until the tension in the weighing
springs is in balance with the weight of the displacer partly immersed in the
liquid. The sensing system in principle measures the two capacitance formed by
the moving center sensing rod E provided with two capacitor plates and the side
plates. In balance position the capacitances are of equal value. A level
variation will a difference in buoyancy of the displacer. The center sensing
rod will move in the direction of one of the side capacitor plates. This causes
a difference in value of these capacitances. By an electronic circuit this
change is detected and integrated. During the rotation of the servo motor the
cam driven transmitter continuously change the voltage pattern to a remote
indicator of which the receiver motor drives a counter indicating level
variation.
TEMPRETURE
49.
What are the different methods of temperature measurement ? Explain.
The
different methods of temperature measurement are :
1. Mechanical 2.
Electrical.
Mechanical methods:
1.
Mercury in glass thermometers : This
consists of a glass tube of very fine bore joined to a reservoir at the bottom
and sealed at the top. A measured quantity of mercury is the enclosed. When the
thermometer is heated the mercury expands much more than the glass and is
therefore forced to rise up in the tubing A scale is fixed at the side.
2.
Bimetallic Thermometer : Two metals
whose coefficient of linear expansion is different are welded and rolled
together to the desire thickness. The actual movement of a bimetal is its
flexivity with one end fixed, a straight bimetal strip deflects in proportion
to its temperature, to the square of its length and inversely with its
thickens.
3.Pressure Spring Thermometers :There are four
classes of pressure spring thermometers.
1.
Liquid filled = class 1
2.
Vapor pressure = class 2
3.
Gas filled = class 3
4.
mercury filled = class 4
Liquid filled & Mercury filled :
Both type, operate on the principle of thermal expansion.
Where the bulb is immersed in a heated substance. The liquid expands causing
the pressure spring to unwind. The indicating, recording or controlling
mechanisms are attached to pressure spring.
Compensated Thermometer System :
Compensations are provided in order to nullify the effect
of changes in ambient temperature. The compensation in liquid filled expansions
thermal system consists of the second tubing and helical element, both liquid
filled. The two elements are so constructed that the measuring helical floats
on a movable base the position of which is governed by the compensating
helical. The two tubing and helicals are matched in volume so that variation in
temperature at the instrument case and along the capillary tubing produce equal
motion from both helicals. Such motion nullity each other so that only motion
produced by varying the bulb temperature actuates the recorder pen.
Gas
filled Thermometers :
This
type depends upon the increase in pressure of a confirm gas (constant volume)
due to temp. increase. The relate between temp. and pressure in this kind of
system follow Charles law and may be expressed.
P1 T1
P2 T2
The system is filled under high pressure. The increase
pressure for each degree of temperature rise is therefore greater than if the
filling pressure were low. Nitrogen the gas most after used for such systems,
because it chemically insert and possesses a favorable coefficient thermal
expansion.
Vapor
- Pressure Thermometers :
Vapor pressure
thermometers depend upon vapor pressure of liquid which only partially fills
the system. At low temperatures the vapor pressure increase for each unit
temperature charge is small, at higher temperature the vapor pressure change is
much greater.
Electrical
method of temperature measurement:
1. Thermocouples: It is a simple device consisting of a
dissimilar metal wires joined at their ends. when an of each wire is connected
to a measuring instrument thermocouples becomes an accurate and sensitive
temperature measuring device.
cold or reference
junction
Hot or measuring
junction
Thermocouples Types and Range:
Type T/C
|
Positive wire & color
|
Negative wire &
color
|
Range °F
|
( J )
|
Iron & White
|
Constantan & Red
|
-300 to1400
|
( K )
|
Chromel & Yellow
|
Alumel & Red
|
-300 to2300
|
( E )
|
Chromel & Purple
|
Constantan & Red
|
-300 to1600
|
( T )
|
Copper & Blue
|
Constantan & Red
|
-300 to 650
|
( R )
|
Platinum and 10%Rhodium
& Black
|
Platinum & Red
|
32
to 2700
|
( S )
|
Platinum and 13%Rhodium
& Black
|
Platinum & Red
|
32
to 2700
|
Resistance – Temperature Detectors (RTD):
RTD's
are generally used for precise temperature measurement. It consists of a five
wire wrapped around an insulator and enclosed in a metal. The most sheath of a
resistance thermometer resembles that of bimetallic thermometer bulb.
PRINCIPLE
: "Resistance increases as temperature increase"
Rt.
= Ro (1 + α t )
Rt.
= Resistance of Temperature to measured.
Ro. = Resistance of zero temperature.
α = Co. off of thermal
(expansion).
t = Temperature to be measured.
These
metals have a positive temperature co-efficient of expansion. Therefore
resistance increases as the temperature
increases.
Types of material used: (1) Platinum (2) Nickel
These
metals have a positive temperature co-efficient of expansion. Therefore
resistance increases as the temp. increases.
Calculation of
Resistance or Pt100.
Ro. =
100
x
for platinum = 0.00385 /c.
To
calculate Resistance at 100'c.
R100 = 100 [ 1+ ( 38.5 x 10
4 x 100 ) ]
= 100 + (100 x
0.385)
R100 = 138.5
Resistance
at 100'c = 138.5
50.
What is Pt 100 mean. ?
Pt100
means 100 OHMS at 0'C for a platinum resistance bulb.
51.
What is two wire and three wire R.T.D. system ?
Two
wire R .T .D. system :
Two
wire RTD system use for short distance like a compressor field local panel.
Three
wire System :
Three
wire system use for long distance coke a field to control Run.
The
third wire is used for compensation of lead wire resistance.
Two-wire R.T.D Three-wire R.T.D
+ - + -
R1 R2 R1 R2
R3 R3
R.T.D R.T.D
52.
Draw a potentiometer temp. measuring circuits and explain its?
Thermo
couple
Two different signal
AMP
Un know
signal
Servo
balancing motor
Meas. Constant voltage Known
circuit signal signal
OPERATION
:
The input to the instrument is a measurement of some in
the processes using a sensing element ( such as thermocouple ) or a device to
produce direct voltage, which is the voltage (signal). This voltage is
subtracted from a voltage developed by a known constant voltage in a potentiometer
measuring circuit. The subtraction occurs by connecting two voltages in series
with the opposing polarity, difference between these two voltages produces
signal, the voltage going to the amplifier. The error will positive or negative
depending on which of the two voltages greater. When amplified, the error
signal will drive servo balancing motor in appropriate direction to adjust
circuit ( actually drive the slide wire ) until the difference between the
feedback voltage and the input voltage is balance out. An error signal equal to
zero results ( null point ) the balancing ( servo motor is be longer driven )
53.
What is the constant voltage unit ?
R2=29.4 RM Rc
Cr1 R1 +
2k Cr4
C1
R3 Cr3 -
764Ω Resistance lead of
CRT = 343.33
The
constant voltage circuit consists of a rectifier, CR, a filter capacitor C1,
followed by two stages of zener regulation. Abridge configuration is provided
to 1amp line voltage regulation zener CR3, R1 and R2 combine provide relatively
constant current to zener CR4, Thus variations. Resisters R2 and R3 form a
bridge that any remoment line voltage effects.
54.
Explain the working of a balancing motor.
Signal
in control winding appears as due tank circuit formed
by winding and capacitor of amp. board.
RED
Control signal
winding
from
amplifier
GREEN
+ve
signal :
It lags 90' from due to line phase capacitor amp. board.
-ve
signal : Leads 90' from line due to line phase
capacitor of amp. board.
The
servo ( balancing ) motor is an induction motor that functions by creating a
rotating magnetic field in the stator.
The
rotor ( armature ) turns by following this field. The field is developed by the
use of two windings in the stator.
It
has got two windings, one of which is continuously energized by the line
voltage. The other winding is energized by the power amplifier, with a current
whose phase with respect to line current determines the direction of rotation
of motor.
55.
What is burnout feature ? Explain.
Burnout
provides the warning feature of driving indicator the end of scale if the input
circuit should open.
A
burnout resistor is provided which develops a voltage drop between the
measuring circuit and the amplifier. The polarity of the signal determines the
direction of the servo drive upon an open circuit in the input.
Upscale
burnout : R value 10 M
Downscale
burnout : R value 2.2 M
57.
Why is a converter used in a temp. recorder ?
The
converter is designed to convert D. C. input voltage into an A. C. input
voltage proportional in amplitude to the input.
58.
Why are Thermowells used ?
In
numerous application it is neither desirable nor practical to expose a
temperature sensor directly to a process material. Wells are therefore used to
protect against damage corrosion, erosion, abrasion and high pressure
processes. A thermowell is also useful in protecting a sensor from physical
damage during handling and normal operation.
Selecting a thermowell :
The
significant properties considered in selecting a material for the well are as
follows :
1.
Resistance to corrosion and oxidation.
2.
Resistance to mechanical and thermal shock.
3.
Low permeability (Resistance to gas leakage).
4.
Mechanical strength.
5.
Thermal conductivity.
Material for Wells :
1.
Stainless steal.
2.
Inconel.
3.
Monel.
4.
Alloy steal.
5.
Hastelloy 'C'.
59.
How will you calibrate a temp. recorder using a potentiometer ?
Connect
the potentiometer output to the input of temp. recorder.
1.
Connect the ( +ve ) to the ( +ve ) and ( -ve ) to the ( -ve ).
2.
If ambient compensation is provided in potentiometer set it to the correct
ambient temp.
3.
If no ambient compensation is provided take a thermometer and measure the
correct ambient temp.. Find out the corresponding mV s for that temp. for the
given input type of thermocouple.
4.
While feeding subtract the ambient temp. m v s from the corresponding temp.
every time.
5.
Adjust the necessary adjustments.
Measuring Temperature With a Potentiometer:
1.Connect the input of the potentiometer to the
thermocouple.
2.If
no ambient compensation is provided find out the corresponding millvolts for
that
ambient temp. for the type of thermocouple
used.
3.Add
the ambient temp. millvolts to the corr. input millvolts measured. Find out
from the
chart the corresponding temperature.
60.
What type of sensing element would you use to measure very low temperature ?
The
sensing element used for measuring very low temperature is R. T. D.
( Resistance Temperature Detector )
61.
What are skin temperature thermocouples ?
Skin
thermocouples are those which are directly connected to the process without any
thermowell.
Used
for measuring the skin temperature of heaters furnaces, flue gas etc.
62.
What is the specialty of thermocouples lead wires ?
They
should be of the same material as the thermocouple.
63.
What is the difference the a wheatstone bridge and a potentiometer ?
The
difference between a potentiometer and a wheatstone bridge measuring instrument
is that potentiometer is a voltage measuring instrument and wheatstone bridge
is a current measuring instrument.
64.
Explain the continuos balance potentiometer system using R. T. D.'s.
Voltage Power
Converter
amplifier amplifier
Energizing
coil
Slide
wire
Balancing
motor
R.T.D
In
a balance wheatstone bridge resistance thermometer a resistance bulb is
connected into one branch of a d.c.
bridge circuit; in another branch is a variable resistance in the form of a
calibrated slidewire. Variations in temp. of the measured medium cause a change
in resistance of the bulb and a consequent unbalance of the bridge circuit. A
self balancing wheatstone bridge recognizes the condition of unbalance,
determines its direction and magnitude and position the slidewire contractor to
rebalance the bridge and indicate the
temp. on the scale.
The D. C. potential
appearing at AA is converted by the converting stage to an A. C. voltage
appearing at BB and is multiplied by the voltage amplifier to a large value at
cc. It is then used to control the power amplifier output DD which drives the
balancing motor in the proper direction to balance the bridge.
The polarity of the signal at AA determines the phase of
the alternating voltage at BB which in turn determines the direction of
rotation of the balancing motor.
65.
How is automatic Reference junction compensation carried out in temp. recorders
?
Rheostat
+ -
scale
Variable
Resistor
+ -
Reference Hot
junction
junction
∆P
For
automatic reference junction compensation a variable nickel resister is used.
As the temperature changes, so does its resistance. This reference junction
compensatory is located, so that it will be at the temperature of the reference
junction. The reference junction is at the position where the dissimilar wire
of the thermocouple is rejoined, which invariably is at the terminal strip of
the instrument.
CONTROL SYSTEMS
66.
Explain the application of proportional integral and derivative action?
Proportional control only :
Proportional
control only attempts to return a measurement to the set point after a load
upset has occurred. How ever it is impossible for a proportional controller to
return the measurement exactly to the set point.
Use : It is normally used for level controls. It reduces
the effect of a load change but it can not eliminate it.
Proportional
plus reset control:
Reset action is introduced to eliminate offset. It will
integrate any difference between measurement and setpoint and cause the
controller's output to change until the difference between the measurement and
set point is zero. Reset will act as long as the error exists.
Use:
Proportional + Reset controllers are
by far the common types used in industrial process control and where
predominate dead times occur.
Proportional
plus reset plus derivative:
Derivative or rate action helps the controller overcome
system inertia and result in faster, more precise control. Derivative action
occurs whenever the measurement signal changes. Under study conditions the rate
action does not act. Derivative allows the controller to inject more corrective
action.
Use
: On temperature controls.
67.
What is difference gap control ?
Differential
gap control is similar to on off control except that a band or gap exists
around the control point.
Use:
In industry differential gap control
is often found in non critical level control applications where it is desirable
only to prevent a tank from flooding or drying. When a measured variable
exceeds the upper gap the control valve will open fully or be closed fully.
Similarly when it exceeds the lower gap it will open or close fully.
68.
Where is on off control used ?
On
off control is used when
1. Precise control is not needed.
2. Processes that have sufficient capacity to allow the
final operator to keep up with the measurement cycle.
3. It is mainly used in refrigeration and are
conditioning systems.
69. What is reset-wind up?
When
reset action is applied in controllers Where the measurement is away from the
set point for long periods the rest may drive the output to its maximum
resulting in rest wind up. When the process starts again the output will no
come off its maximum until the measurement crosses the so point causing large
overshoots. This problem can be avoid by including anti-reset wind up
circuit which eliminates the problem of output saturation.
70.
Why is reset called integral and Rate derivative ?
Reset
is called integral because of the mathematical relationship to the output.
Rate
is called derivative because
t = i
Oi = f ∫ e ( dt ) + O0 Oi = r (
de / dt ) + O0
t = o
r = is
the rate time
Oi
= is the Output at any given time
O0
= is the out put at time zero or zero
error.
e = is
the error signal
t = is time.
f = is the reset rate in respects per
minute.
71.
Explain tuning of controllers.
Tuning
basically involves adjustment of proportional. Integral and derivative parameters to achieve good
control. The gain, time constants, and dead times around the loop will dictate
the settings of various parameters of the controller.
Tuning
methods are broadly classified into two :
1.
Closed Loop Method : e.g. Ultimate Gain
Method.
2.
Open Loop Method : e.g. process Reaction curve.
Ultimate
gain method:
The term ultimate gain was attached to this method because
its use require the determination of the ultimate gain (sensitivity) and
ultimate period. The ultimate sensitivity Ku is the maximum allowable value of
gain (for a controller with only Proportional mode) for which the system is
stable. The ultimate period is the period of the response with the gain set at
its ultimate value.
Process
reaction curve :
To deter mine the process reaction curve, the following
steps are recommended. :
1. Let
the system come to steady state at the normal load level.
2. Place
the controller on manual.
3. Manually
set the output of the controller at the value at which it was operating in the
automatic mode.
4. Allow
the system to reach the steady state.
5. With
controller on manual, impose a step changes in the output of controller, which
is an signal to value.
6. Record
the response of controlled variable.
7. Return
the controller output to its previous value and return the controller to auto
operation.
72.
Explain the working of an electronic P.I.D. controller.
Input from the measurement transmitter is compared with
the set point voltage to produce a deviation signal. The deviation signal is
combined with a characterized feed back signal to provide the input for the
function generator amplifier. This amplifiers output is delivered to the feed
back network, and to the final output which is a 10-50m.a. do signal for
actuation of final operators.
Set
Manual
Measurement Deviation Amplifier Manual
Automatic
Gain To
final operator
Proportional
action: It is a obtained by adjusting
the magnitude of feed back signal. An increase in negative feed back means less
effective gain and thus a broader proportional band.
Reset
actions: It is obtained by charging
the reset capacitor at a rate determined
by the value of reset resister. The reset resister is variable, and constitutes
reset adjustment.
Derivative
action: The connection of a
derivative capacitor across the feedback circuit delays feedback until the
capacitor is charged to a value approaching amplifier output. This delay is
controlled by value of derivative resister. This resister is variable and
constitutes derivative adjustment.
73.
What is an analogue integrator and an analogue differentiator ?
Analog
integrator:
R C
Vin
+
AV
-1
- Vout
V0 = Vindt
RC
Analog differentiator:
R
C
d
+
V0
= - RC Vin Vin AV
dt
- Vout
74.
What is anti reset wind up ?
If
the limit acts in the feed back section of the control amplifiers integral
circuit, the controller output will immediately begin to drive in the opposite
direction as soon as the process signal crosses the set point. This approach is
referred to as antireset wind up.
75.
What are De-saturators ?
When,
in some processes, e.g. batch process, long transient responses are expected
during which a sustained deviation is present the controller integral action
continuously drives the output to a minimum or maximum value. This phenomenon
is called "integral saturation of the control unit". When this
condition.
76.What
is the effect of weep hole on calculation of orifice bore?
(dm)2 where: dm = Measured diameter of orifice.
d
= dm { 1 + 0.55 } dh = Drain hole diameter.
dh d
= Corrected diameter orifice size.
77. Explain the working of Rotameter?
OUT
LET
The
flow rate varies directly
Tapered glass
tube as the float rises and falls in
Scale the tapered tube.
IN LET
Variable
area meters are special form of head meters. Where in the area of flow
restrictor is varied. So as to hold the differential pressure constant. The
rotameters consists of a vertical tapered tube through which the metered fluid
flows in upward direction. A "float" either spherical or cone shaped,
actually more dense than the fluid being measured, creates an annular passage
between its maximum circumference and the weight of the tapered tube. As the
flow varies the "float" rises or falls to vary the area of the
passage so that the differential across it just balances the gravitational
force on the "float" i.e. the differential pressure is maintained
constant. The position of the "float" is the measured of the rate of
flow.
78.
Explain the working of a magnetic meter.
An
electric potential is developed when a conductor is moved across the magnetic
field. In most electrical machinery the conductor is a "wire"; the
principle is equally applicable to a moving, electrically conductive liquid.
The primary device of commercial magnetic meters consists of a straight
cylindrical electrically insulated tube with a pair of electrodes nearly flush
with the tube wall and located at opposite ends of a tube diameter. A uniform
a.c. magnetic field is provided at right angles to electrode diameter and to
the axis of the tube. The a.c. voltage developed at the electrodes is
proportional to the volume flow rate of fluid, and to a magnetic field
strength. This device is limited to electrically conducting liquids. The
magnetic meter is particularly suited to measurement of slurries and dirty
fluids, since there are no location for solids to collect except the walls of
the tube itself.
79.
Explain the working of a turbine meter.
Turbine
meters consist of a straight flow tube within which a turbine or fan is free to
rotate, about its axis which is fixed along the center line of the tube.
Straightening vanes upstream of the turbine minimizes possible rotational
components of fluid flow. In most units a magnetic pick-up system senses the
rotation of the rotor through the tube wall. The turbine meter is a flow rate
device, since the rotor speed is directly proportional to flow rate. The output
is usually in the form of electrical pulses from the magnetic pick-up with a
frequency proportional to flow rate. Turbine meter are primarily applied to
measurement of clean and non-corrosive hydrocarbons.
80.
Explain the working of a Pitot tube.
The
pitot tube measures the velocity at point in the conduct. If quantity rate
measurement is desired, it must be calculated from the ratio of average
velocity to the velocity at the point of measurement.
Principle
: If a tube is placed with its open
and facing into a stream of fluid, then the fluid impinging on the open end
will be brought to rest, and the kinetic energy converted to pressure energy.
This the pressure built up in the tube will be greater than that in the
free stream by the impact pressure or pressure produced by loss of kinetic
energy. The increase in pressure will depend upon the square of the velocity of
the stream. The difference is measured between the pressure in the tube and
static pressure of the stream. The static pressure is measured by a tapping in
the wall of the main or by a tapping incorporated in the pitot static tube
itself. The difference between the pressure in the tube and static pressure
will be a measure of the impact pressure and therefore of the velocity of the
stream oil.
81.
Where is the integral orifice used ?
Integral
orifice is used to measure small flow rates. It is mounted directly on the
secondary device. The integral orifice diameter varies between 0.020 inch and
0.250 inch diameter. The integral orifice finds considerable use in laboratory
and pitot plants.
Calculation of flow rate :
Qn / Fc = Ks x
Cwi x Fa
x Fm x
Gp / Ge , hw
82.
Explain the working of a target meter.
The
target meter combines in a single unit both a primary element and a force
balance flow rate transmitter. A circular disc (or target) supported
concentrically in the pipe carrying the flowing fluid results in an annular
orifice configuration. Pressure difference developed by the fluid flow through
this annular orifice produces a force on target proportional to the square of
the flow rate. This force is carried out of the pipe through a rod passing
through a diaphragm seal, and is measured by a pneumatic or electronic force
balance system identical with the mechanism of the force balance D.P. cell. The
advantages of the target meter lies primarily in its single unit construction
the primary device and responsive mechanism in a single structure. This eliminates
the diff. pressure fluid connections in most heads meters. This is particularly
used for sticky and dirty material which may plug up differential connections
and for liquids which require elevated temperatures to avoid solidification,
this elimination of liquid connection is useful.
Wm 2
F =
Cst Fa Fm Fc rf
83.
Where is a quadrant orifice used ?
If
the fluid is viscous and the operating Reynolds number is low quadrant orifice
is preferred
84.
What are types of taps used for orifices ?
1.
Flange taps:
This are most commonly used on pipe sizes of 2 inches or
larger. They are located in the orifice flange 2 inch from upstream and 1 inch
downstream from the faces 0 orifice plate.
2.
Corner taps:
On pipe sizes less than 2 inches corner taps located
directly at the face of the orifice plate.
3.
Vena contracta and radius taps:
Vena
contracta taps located at 1 pipe diameter upstream and at point of minimum
pressure downstream. There are mostly widely used for measurement of steam.
Radius
taps are located 1 pipe diameter upstream and ½ pipe diameter downstream for the inlet face of the orifice are a close
approximation to vena contracta taps upto 0.72
d
/ D.
4.
Full flow taps:
Face flow taps are located at 2½ pipe diameter upstream and B pipe diameter
downstream. Full flow taps at 2½ and B
pipe diameter have the same advantage as vena contracta or radius taps.
85.
What is Reynolds number ?
Dynamic
similarity implies a correspondence of fluid forces in two systems. In general
situation there are many classes of forces that influence the behavior of
fluids. Some of these are inertial viscous, gravitational, compressibility,
pressure and elastic forces. Certain dimensionless ratio are developed based on
fluid properties. Velocities and dimension, which are essentially force ratio.
The
more important of these are Reynolds number
V
= velocity
v D ρ D = inside diameter of
pipe
R = ρ = fluid density
μ μ = viscosity
For
most applications in practical flow measurement the Reynolds number is taken to
be sufficient criterion of dynamic similarly. The magnitude of Reynolds number
not only indicates whether the flow is laminar or turbulent but also furnishes
the probable shape of velocity profile. Due to the strong role it plays as an
indicator of varying flow characteristics, many of the deviation from the
theoretical equations are called Reynaldo number effects.
86.
How would you choose differential range ?
The
most common diff. range for liquid measurement is 0-100" H2O.
This range is high enough to minimize the errors caused by unequal heads in the
seal chambers, differences in temps. of load lines etc. The 100" range
permits an increase in capacity up to 400" and a decrease down up to
20" by merely changing range tubes or range adjustments.
87.
What is positive Displacement meters ?
principle:
The principle of measurement is that
as the liquid flows through the meter it moves a measuring element which seals
off the measuring chamber into a series of measuring compartments each holding
a definite volume. As the measuring element moves, these compartments
are successively filled and emptied. Thus for each complete of the measuring
element a fixed quantity of liquid is permitted to pass from the inlet to the
outlet of the meter. The seal between measuring element and the measuring
chamber is provided by a film of measured liquid. The number of cycle of the
measuring element is indicated by means of a pointer moving over the dial, a
digital totalizer or some other form of register, driven from the measuring
element through an adjustable gearing.
The
most common forms of positive displacement meters are :
1. Reciprocating
Piston type.
2. Rotating
or Oscillating Piston type.
3. Nutating
Disc type.
4. Fluted
Spiral Rotor type.
5. Sliding
vane type.
6. Rotating
vane type.
7. Oval
Gear type.
88.
Why are two plugs provided on a D.P transmitter?
1.The top plug is a vent plug for venting the air
entrapped inside the cell.
2.The
bottom plug is a drain plug for draining the liquid accumulated inside the
cell.
CONTROL VALVES
89. What is a control valves ?
A
control valve is the final control element, which directly changes the valve of
the manipulated variable by changing the rate of flow of control agent.
A
control valve consists of an operator and valve body. The operator provides the
power to vary the position of the valve plug inside the body. The plug is
connected to the operator by a stem, which slides through a stuffing box. The
air signal from the controller is applied above the diaphragm. The increasing
air signal from the controller is applied above the diaphragm. An increasing
air signal will push the operator stem downwards against the force exerted by
the spring on the diaphragm plate. The valve is adjusted in such a way that the
plug starts moving when 3 psi is applied to the diaphragm and touches the seat
when 15 psi is applied to the diaphragm. Thus an increase in air pressure will
close the valve. Hence the home "Air to Close". Another type is
"Air to open", such that 3 psi on the diaphragm the value is closed
and 15 psi air signal it in fully open.
90.
What are the different types of control valves ?
The
commonly used control valves can be divided as follows.
1.
Depending on Action.
2.
Depending on the Body.
1.
Depending on action:
Depending on action there are two types of control valves,
(1) Air to close, (2) Air to open.
2.
Depending on body:
1.
Globe valves single or double seated.
2.
Angle valves.
3.
Butterfly valves.
4.
Three way valves.
91.
What is the use of single seated valve ?
The
single seated valve is used on smaller sizes, and in valve of larger sizes,
where an absolute shut off is required. The use of single seated valve is
limited by pressure drop across the valve in the closed or almost closed
position.
92.
What is the use of double seated valve ?
In
double seated valves the upward and downward forces on the plug due to
reduction of fluid pressure are nearly equalized. It is generally used on
bigger size valves and high pressure systems. Actuator forces required are less
i.e. A small size actuator.
93.
What is Cv of a valve ?
Cv
is the capacity of a valve and is defined as :
"No
of gallons per minute of water which passes through a fully open valve at a
pressure drop of 1 psi.
CV =
q ( ▲P / G )
Where: Cv =
Valve co-efficient
q =
Volumetric flow rate ( gallons
minute )
▲P = Pressure drop across the valve in psi.
G =
Specific gravity of flowing fluid.
The
valve coefficient Cv is proportional to the area 'A' between the plug and valve
seat measured perpendicularly to the direction of flow.
94.
What are the different types of actuators ?
The
different types of actuators are :
1.
Diaphragm Operated.
2.
Piston Operated.
95.
What types of bonnets would you use of high temp. and very low temp. ?
High
temperature: Bonnets are provided
with radiation fins to prevent glad packing from getting damaged.
On
very low temperature: Extended
bonnets are used to prevent gland packing from getting freezed.
96.
How will you work on a control valve while it is line ?
While
the control valve is in line or in service, it has to be by passed and secondly the line to be depressurized and drained.
97.
What is the use of a valve positioner ?
The
valve positioner is used for following reasons :
1.
Quick Action control valve.
2.
Valve hysteresis.
3.
Valves used on viscous liquids.
4.
Split Range.
5.
Line pressure changes on valve.
6.
Valve Bench set not standard.
7. Reversing valve operation.
98.
When can a by pass be not used on a positioner ?
A
by pass on a positioner cannot be used when :
1.
Split Range operation.
2.
Reverse Acting Positioner.
3.
Valve bench set not standard.
99.
What is the use of butterfly valves ?
Butterfly
valves are used only in systems where a small pressure drop across the valve is
allowed. The butterfly is fully open when the disc rotates by 90. A drawback
of this valve is that even a very small angular displacement produces a big
change in flow.
100.
What is the use of three way valves ?
Three
way control valves are only used on special systems, where a dividing or
mixture of flows according to a controlled ratio is required.
101.
What are the different types of plugs ?
The
different types of plugs are generally used are :
(
1 ) V. port plug ( 2
) Contoured plug
V-port
plug:
Ported plug are generally used on double seated valves.
This is because ported plugs, have a more constant off balance area.
Contoured
plug:
Contoured plugs are generally used on single seated valve
with small trim sizes.
102.
What is a cage valve?
A
cage valve uses a piston with piston ring seal attached to the single seated
valve "plug". Here the hydrostatic forces acting on the top or the
piston or below the valve plug tend to cancel out. The seat ring is clamped in
by a cage. Cage valves are generally used for noise reduction.
103.
What are the advantages of Cam flex valves?
Cam
flex valves are intermediates between globe valve and butterfly valve. The plug
rotates 60' for full opening.
Advantages:
1.
Actuator forces required are very less.
2.
Extended bonnet and hence can be used on any service i.e. on high temp. and
very low
Temperature.
3.
Variations in flow.
4. Light weight.
104.
What is the use of link connected to the valve positioner ?
The
link serves as the feed back to the value. Ant valve movement is sensed by this
link. Sometimes due to line pressure changes on H.P. service the valve position
may be changed, the link in turn senses this change and the positioner will
produce an output which will operate the valve to the original position.
105.
What is the use of booster relays?
Booster
relays are essentially air load, self contained pressure regulators. They are
classified into three broad groups:
1.
Volume Boosters : These are used to multiply the available volume of air
signal.
2.
Ratio Relays : Use to multiply or divide the pressure of an input
signal.
3.
Reversing Relays : This produces a decreasing output signal for an
increasing input signal.
106.
What is the use of Angle valves ?
Angle
valves are used where very high pressure drops are required and under very
severe conditions. Where the conventional type of valve would be damaged by
erosion.
107.
What are the different valve characteristic ?
The
different types of valve characteristic are :
1.
Linear 2. Equal
Percentage 3. Quick
Opening.
1.Linear:
The valve opening to flow rate is a
linear curve
2.Equal
percentage: For equal increments of
valve opening it will give equal increment in flow rate range. At small opening
the flow will also be small.
3.Quick
opening: At small opening the increments in flow rate is more. At
higher opening the flow rate becomes steady.
108.
What is a solenoid valve ? Where it is used ?
A
solenoid is electrically operated valve. It consist of a solenoid ( coil ) in
which a magnetic plunger moves which is connected to the plug and tends to open
or close the value. There are two types of solenoid valves :
1.
Normally open 2.
Normally closed
USE
: It is used for safety purpose.
109.
How will you change the valve characteristics with positioner ?
The
positioner contains different types of came in it. selection of the proper cams
in it.By selection of the proper cam the valve opening characteristics can be
changed.
110.How
will you change the action of a control valve ?
1.
If the control valve is without bottom cap. The actual needs to be changed.
2.
If bottom cap is provided.
a)
Disconnect the stem from the actuator
stem.
b)
Separate the body from the bonnet.
c)
Remove the bottom cap and the plug from body.
d)
Detach the plug from the stem by removing the pin.
e)
Fix the stem at the other end of the plug and fix the pin back.
f)
Turn the body upside down. Connect it to the bonnet after inserting the plug
and stem.
g)
Connect back the stem to the actuator stem.
h)
Fix back the bottom cap.
i) Calibrate the valve.
111.
How will you select the control valve characteristics ?
The
graphic display of flow various lift shows then the Desired or inherent
characteristic is changed by variations pressure drop. This occurs as the
process changes from condition where most of pressure drop takes place at the
control valve is a condition where most of the pressure drop is generally
distributed through rest of the system.
%
Flow : This variation in where most
of the total drop take place is one of the most important aspects is choosing
the proper valve characteristics for give process.
Flow
control : Normally Equal percentage
valve is used.
Pressure
Control : Normally linear valve
is used to maintain a constant pressure drop.
Temp.
Control : Normally equal percentage
valve is used.
Liquid
Level Control : Normally linear valve
is used.
Basically
in selecting a valve characteristic two important point have to be taken into
account.
a)
There should be a linear relationship between the position of the plug and the
flow through the valve in a wide range of change in the pressure drop across the
valve.
b)
The pressure drop across a valve should be as low as possible.
Control valve sizing
112.
What is the effect of pipe reducers on valve capacity?
When control valves are mounted between pipe reducers,
there is a decrease in the actual valve capacity. The reducers create an
additional pressure drop in the system by acting as contractions of
enlargements in series with the valve.
Metric
formula: for inlet and outlet reduces.
2
d2 Cv2
R =
1 – 1.5 1 -
D2 0.04d2
For outlet reducer only or inlet reducer with entrance
angle less than 40 inches.
2
d2 Cv2
R2 =
1 – 1.5 1 -
D2 0.04d2
d
= valve size mm. D = line size mm. Cv
= required valve co-efficient.
To
compensate for reducer losses at sub-critical flow, divide Cv calculated by R.
113.
An operator tells you that a control valve in a stuck ? How will you start
checking ?
1. First of all get the control valve is passed
from operation.
2. Check the lingual to the diaphragm of the
control valve.
3. Disconnect it possible the actuator stem
from the control valve stem.
4. Stroke the actuator and see whether the actuator
operates or not. It not then the
diaphragm may be punctured.
5. If the actuator operates connect it back to
the plug stem stroke the control valve. If it
does not operate loosen the gland nuts a
bit and see if it operates. If it does not then
the control valve has to be removed from
the line to w/shop.
114.
Where is an Air to close and Air to open control valves used ?
Air
to close:
1.
Reflux lines.
2.
Cooling water lines.
3.
Safety Relief services.
Air
to open:
1.
Feed lines.
2. Steam Service.
115.
Why does control valve operate at IS psi ?
On
higher pressure the actuator sizes becomes bigger in area. The actual force
produced by the actuator.
Force
= Pressure x Area.
= 15 psi x Area,
If Area = 15"
Force produced = 15 psi
x 25 in2 = 375 pounds.
Actual force acting on a control valve = 375 pounds.
GENERAL QUESTIONS
116.
Explain Cascade Control system with a diagram. What would happier if a single
controller were used ?
TT TIC
Master
controller Slave or
secondary
Steam
Controller
PT
Feed water
Fuel gas
PV
Cascade
means two controllers is series. One of them is the Master or Primary and the
second is the secondary of slave controller. The output of the secondary
controller operates the final control element, that is the valve.
Loop
explanation:
The
output of the temp. transmitter goes as measurement signal to the TIC which is
the master controller. Similarly the output of pressure transmitter goes as measurement
signal to the PIC which is the secondary controller.
The
output of TIC comes at set point to PIC which is turn operates the valve. The
reqd. temp. is set on the TIC.
Use
of cascade system:
Cascade loops are invariably installed to prevent outside
disturbances from entering the process. The conventional single controller as
shown in the diagram cannot responds to a change in the fuel gas pressure until
its effect is felt by the process temp. sensor. In other words an error in the
detected temperature has to develop before corrective action can be taken. The
cascade loop in contrast responds immediately correcting for the effect of
pressure change, before it could influence the process temperature. The
improvement in control quality due to cascading is a function of relative
speeds and time lags. A slow primary (Master) variable and a secondary (Slave)
variable which responds quickly to disturbances represent a desirable
combination for this type of control. If the slave can respond quickly to fast disturbances
then these will not be allowed to enter
the process and thereby will not upset the control of primary (master)
variable. It can be said that use of cascade control on heat transfer equipment
contributes to fast recovery from load changes or other disturbances.
117.
Explain ratio control system.
Un controlled flow
(
A )
FT Ratio controller
F1
+ F2
Secondary controller
( B ) FT
Controlled flow FV
A
ratio control system is characterized by the fact that variations in the
secondary variable do not reflect back on the primary variable. In the above
diagram 0 a ratio control system the secondary flow is hold in some proportion
to a primary uncontrollable flow.
If
we assume that the output of primary transmitter is A, and the output of the
secondary transmitter is B, And that multiplication factor of the ratio relay
is K, then for equilibrium conditions which means set valve is equal to
measured valve, we find the following relation :
KA
- B = 0
or
B/A = K, where 'K' is the ratio setting of the relay.
118.
Explain fuel to air ratio control of furnaces.
Air
FRC Master steam
FRC FC
FT FRC
RSP primary
secondary
FV
RR FT
Ratio controller
Fuel gas
119.
What is Furnace Draft control ?
Balanced
draft boilers are generally used negative furnace pressure. When both forced
draft and induced draft are used together, at some point in the system the pressure
will be the same as that of atmosphere. Therefore the furnace pressure must be
negative to prevent hot gas leakage. Excessive vacuum in the furnace however
produces heat losses through air infiltration. The most desirable condition is
that the one have is a very slight (about 0.1" H20 ) negative
pressure of the top of furnace.
120.
What is feed back control? What is feed forward control? Discuss its
application ?
Feed
back control:
Controller(FIC)
Measuring
element
Set point Transmitter
Control valve Orifice
plate
Feed
back control involves the detection of the controlled variable and
counteracting of charges its it’s value relative to set point, by adjustment of
a manipulated variable. This mode of control necessities that the disturbance
variable must affect the controlled variable itself before correction can take
place. Hence the term 'feedback' can imply a correction 'back' in terms of
time, a correction that should have taken place earlier when the disturbance
occurred.
Feed
forward control :
|
Output
FT
Controller Additive regulator
Feed
forward control system is a system in which corrective action is based on
measurement of disturbances inputs into the process. This mode of control
responds to a disturbance such that is instantly compensates for that error
which the disturbance would have otherwise caused in the controlled variable
letter in time.
Feed
forward control relies on a prediction. As can be seen from the figure of feed
forward control a necessary amount of input goes to the process. This
measurement goes to the controller which gives output to the control valve. The
control valve regulates the flow.
Feed
back control:
In feed forward control no difference between the desired
result and actual result need exist before corrective action is taken in feed
back control a difference must exist. Hence, open loop or feed forward control
is capable of perfect control, but feed back is not. Due to economic
impartibility of precision , predicting the amount of correction necessary to
achieve satisfactory results with feed forward control, feed back control is most
often used. In order to properly choose the type of feed back controller for a
particular process application, two factors time and gain must be considered.
121.
Explain three element feed water control system?
Feed water control Drum level control Steam control
P2
P1 P4
Square root Square root extractor
Extractor P3
Computing equation:
P3 = R ( P2 - P1 - K )
+ P4 + K0
P3 = Output.
P1, P2
& P4 = Input
K1 = Adjustable
suppression.
K0 = Adj.Bias.
122. Explain Anti-surge
control?
Compressor
PDT
Pinlet
Poutlet
Ratio
FT FY FRC
( Flow converter)
By-Pass valve
This method of surge control
uses the ratio of compressor pressure rise to inlet flow rate to set the flow
in by-bass loop. When the suction pressure drops and discharge shoots up, the
compressor starts surging. The pdt senses this and gives the signal to the FRC
which will open the by-pass valve.
GENERAL QUESTION - II
Que.
: Draw an electronic two wire system
control loop.
Ans.
:
Que.
: What are Intrinsically safe system ?
Ans.
: Intrinsic
safety is a technique for designing electrical equipment for safe use in
locations made hazardous by the presence of flammable gas or vapors in air.
"Defn. :" Intrinsically safe circuit is one in
which any spark or thermal effect produce either normally or under specified
fault conditions is incapable of causing ignition of a specified gas or vapor
in air mixture at the most easily ignited concentration.
HAZARDOUS AREAS :
The specification of products or systems sold as
intrinsically safe must state in what hazardous areas they are infect
intrinsically safe. Universal cooling of hazardous areas has not, unfortunately,
been adopted in all countries. However two sets of codes in common use are.
Que.
: What does a transmitter output start
from 3-15 psi or (0.2 - 1 Kg/Cm2) or 4 - 20 ma. etc. ?
Ans.
: The
transmitter output stance from what is known as "live zero". This
system has specific advantages :
1.
The systems automatically alarms when the signal system becomes inoperative.
2.
The output areas is linear ( Ratio of 1 : 5 ).
DEAD ZERO SIGNAL :
The advantage is that it does not have to be biased to
true zero. A "Live zero" gives the computer additional information,
so that it can takes appropriate alarm action in case of a measurement failure,
because it can discriminate between a transmitter operating, but transmitting a
zero measurement and a failure, in the signal system.
Que.
: What is force balance and motions
balance principle ?
Ans.
:
FORCE BALANCE PRINCIPLE :
"A
controller which generates and output signal by opposing torque’s".
The
input force is applied on the input bellows which novas the beam. This crackles
nozzle back pressure. The nozzle back pressure is sensed by the balancing
bellows which brings the beam to balance. The baffle movement is very less
about 0.002" for full scale output.
MOTION BALANCE PRINCIPLE:
"A
controller which generates an output signal by motion of its parts".
The
increase in input signal will cause the baffle to move towards the nozzle. The
nozzle back pressure will increase. This increase in back pressure acting on
the balancing bellows, will expands the bellows, there by moving the nozzle
upward. The nozzle will move untill motion (almost) equals the input (baffle)
motion.
Advantages of force Balance:
1.
Moving parts are fever.
2.
Baffle movement is negligible.
3. Frictional losses are less.
AUTOMATIC CONTROLLER:
It is a device
which measured the value of variable quantity or condition and operates to
correct or lie it deviation of this measured value from a selected reference.
AUTOMATIC CONTROL SYSTEM:
It is any operable arrangement of one or more automatic
controllers in closed loops with one or more processes.
SELF OPERATED CONTROLLER:
It
is one in which all the energy needed to operate the final control element is
derived from the controlled medium through the primary element.
RELAY OPERATED CONTROLLER:
It is one in which the energy transmitted through the
primary element is either supplemented or amplified for operating the final
control element by employing energy from another sources.
PROCESS:
A process comprises the collective function performed in
and by the equipment in which a variable is to be controlled.
SELF REGULATION:
It is an inherent characteristic of the process which aids
in limiting the deviation of the controlled variable.
CONTROLLED VARIABLE :
The controlled variable is that quantity and condition
which is measured and controlled.
CONTROLLED MIDIUM :
It
is that process energy or material in which a variable is controlled. The
controlled variable is a condition or characteristic of the controlled medium.
For e.g. where temperature of water in a tank is automatically controlled, the
controlled variable is temperature and controlled medium is water.
MANIPULATED VARIABLE :
It is that quantity or condition which is varied by the
automatic controller so as to affect the value of the controlled variable.
CONTROL AGENT :
It
is that process energy or material of which the manipulated variation is a
condition or characteristic. The manipulated variable is a condition or
characteristic of the control agent. For e.g. when a final control element
changes the fuel gas flow to burner the manipulated variable is flow the
control agent is fuel gas.
ACTUATING SIGNAL :
The
actuating signal is the difference at anytime between the reference input and a
signal related to the controlled variable. This basically known as error
signal.
DEVIATION :It is the
difference between the actual value of the controlled variable and the value of
the controlled variable corresponding with set point.
OFFSET :
It
is the steady state difference between the control point and the value of the
controlled variable corresponding with setpoint
CORRECTIVE ACTION :
It is the variation of the manipulated variable produced
by the controlling means. The controlling means operates the final control
element ( control value ) which in turn varies the manipulated variable.
REFERENCE INPUT :
It is the reference signal in an automatic controller.
SET POINT :
It
is the position to which the control point setting mechanism is set.
CONTROL POINT :
It is the value of the controlled variable which under any
fixed set of conditions the automatic controller operates to maintain.
.
DEFINATION
ACCURACY:
A
number or quantity which defines the limit of error under reference conditions.
ATTENUATION:
A
decrease in signal magnitude between two points, or between two frequencies.
DEAD TIME :
The
interval of time between initiation of an impact change or stimulus and the
start of the resulting response.
DRIFT :
As
undesired change in output over a period of time, which change is unrelated to
input, operating conditions, or load.
ERROR :
The
difference between the indication and the true value of the measured signal.
SPAN ERROR :
It
is the difference between the actual span and the specified span and is
expressed as the percent of specified span.
ZERO ERROR :
It is the error of device operating under the specified
conditions of use when the input is at the lower range value.
STATIC GAIN :
It
is the ratio of the output change to an input been change after the steady
state has been reached.
HYSTERESIS :
The
maximum difference between the upscale and downscale indications of the
measured signal during a full range traverse for the same input.
INTERFERENCE :
Interference is any spurious voltage or current arising
from external sources and appearing in the circuits of a device.
COMMON MODE INTERFERENCE :
It
is the form of interference which appears between the measuring circuit
terminals and ground.
NORMAL MODE INTERFERENCE :
It is the form of interference which appears between
measuring circuit terminals.
LINEARITY :
The
closeness to which a curve approximate a straight line.
RANGE :
The
region between the limits within which a quantity is measured received or
transmitted, expressed by stating the lower and upper range values.
REPEATABILITY :
The closeness of agreement among a number of consecutive
measurements of the output for the same value of the measured signal under the
same operating conditions.
REPRODUCIBILITY :
The closeness of agreement among repeated measurements of
the output for the same value of the input made under the same operating
conditions.
RESPONSE :
It is the general behavior of the output of a device as a
function of input both with respect to time.
SIGNAL TO NOISE RATIO :
Ratio of signal amplitude to noise.
TIME CONSTANT :
The time required for the output to complete 63.2 % of the
total rise or decay.
SPAN :
The algebraic difference between upper and lower range
values.
ZERO SHIFT :
Any parallel shift of the input output curve.
PRESSURE
PRESSURE CONVERSIONS:
1psi =
|
27.74 " H2O
|
1 Kg/cm2 =
|
14.223 psi
|
1 Bar =
|
14.504 psi
|
1 Kpa =
|
0.145 psi
|
1 Kg/cm2 =
|
10.000mm of H20
|
1 Bar =
|
1.0197 Kg/cm2
|
1 Kg/cm2 =
|
0.98 Bar
|
1 Torr =
|
1 mm of Hg.
|
Que.
: Explain the working of an Enraf level
gauge ?
Ans.
: The
Enraf level precise level gauges are based on servo powered null balance
technique. A displacer serves as continuos level sensing element.
Principle :
A
displacer A with a relative density higher than that of a product to be
measured, is suspended from a stainless steal wire B tat is attached to a
measuring drum. A two phase servo motor controlled by a capacitive balance
system winds unwinds the measuring wire until the tension on the weight springs
is in balance with the wt. of the displace part immersed in the liquid. The
sensing system in principle measures the two capacitance formed by the moving
central sensing rod E provided with two capacitor plates and the si plates. In
balance position the capacitance are of equip value. A level variation will
cause a difference in buoyancy of the displacer. The center sensing rod will
move in to direction of one of the side capacitor plates. This causes
difference in value of this capacitance. By an electrolyte rotation of the
servo motors the can driven transmitter continuously change the voltage
pattern to remote indicate of which the
receiver motor drives a counter indicating low variation.
PRIMARY FEEDBACK :
It
is the signal which is related to the
bJNtrJHÀå3œìÂdÒÂÄØÈ@À‘_Ÿ___˜‡Ÿ_HŸ€_‚Ò_ÖM_ith the reference input to obtain the
actuating signal. Simply stated primary feedback is the actual measurement of
the controlled variable which when compared with the desired measurement of the
controlled variable produces the actuating signal.
POSITIONING ACTION :
It is that in which there is a predetermined relation
between the value of the controlled variable and the position of the final
control element.
PROPORTIONAL ACTION :
It
is that in which there is a continuous linear relationship between the value of
the actual measurement of the controlled variable and the value position.
FLOATING ACTION :
It is that in which there is a predetermined relation
between the deviation and speed of final control element.
DERIVATIVE ACTION :
It is that in which there is a predetermined relation
between a time derivative of the controlled variable and position of final
control element.
REST ACTION :
It
is the value movement at a speed proportional to the magnitude of deviation.
RATE ACTION :
It is that in which there is a continuos linear relation
between the rate of change of controlled variable and position of final control
element. Rate action produces value motion proportional to the rate of change
of actual measurement.
PROPORTIONAL BAND :
It is the range of values of the controlled variable which
correspond to the full operating range of the final control element.
RESET RATE :
It is the number of times/minute that the effect of
proportional position action upon the final control element is repeated by
proportional speed floating action.
There
are two ways of expressing reset action :
1.
Reset time and 2. Reset Rate
1.
Reset Rate : It is commonly
expressed as a number of "repeats" per minute. It is determined by
dividing.
a) Travel of final control element ( Value stroke ) in one
minute as a result of the effect of proportional speed floating action.
b)
The travel as a result of the effect of proportional position action
with the same deviation in both cases.
2.
Reset Time : It is the time
interval by which the rate is commonly expressed in minutes. It is determined
by subtracting.
a)
The time required for a selected motion of the final control element resulting
from combined effect of the proportional position plus rate action.
b)
The time required for the same motion as
a result of the effect of proportional position action alone with the same rate
of change of controlled variable in both cases or expressed in another way. It
is the time lead in terms of air pressure on the control value produced by rate
action compared with proportional position action for the same rate of change
of actual measurement in both cases.
ELECTRONICS
Que.
: What is a diode?
Ans.
: A
diode consists of two electrodes (1) Anode (2) Cathode. The current flow is
only in one direction.
A
diode is the most basic solid state (semi conductor) device. The above figure
shows a P.N. junction. The P. material
has holes and the N. material has electrons.
FORWARD BIAS :
REVERSE BIAS :
(
1 ) Where the applied voltage overcomes the barrier potential (the p side is
more positive than the n side) the current produce is large because majority
carriers cross the junction in large numbers. This condition is called forward
bias.
(
2 ) When the applied voltage aids the barrier potential (n side + ve
than p side) the current in small. This state is known as Reverse Bias.
Que.
: What is a half wave, full wave and
bridge rectifier ?
Ans.
:
HALF WAVE RECTIFIER :
FULL WAVE RECTIFIER :
BRIDGE RECTIFIER :
PEAK INVERSE VOLTAGE :
Maximum Reverse voltage across the
diode during the cycle.
Que.
: What is a filter used for ?
Ans.
: The
half wave and full wave signals are pulsating D . C . voltages. The use of such
voltages is limited to charging batteries, running D. C. motors, and a few
other applications. What we really have is a D.C. voltage that is constant in
valve, similar to the voltage from a battery. To get a constant voltage from
this, we can use a capacitor input filter.
Que.
: What is Zener Diode ? What is a voltage
Regulator ?
Ans.
: The
breakdown region of a p n diode can be made very sharp and almost vertical
Diodes with almost vertical breakdown region are known as Zever Diodes.
A
Zener diodes operating in the breakdown region is equivalent to a battery.
Because of this current through Zener diode can change but the voltage remains
constant. It is this constant voltage that has made the zever diode
an important device in voltage regulation.
VOLTAGE REGULATOR :
The
output remains constant despite changes in input voltage due to Zever effect.
Que.
: What is transistor ? What are the
different types ?
Ans.
: A transistor is a three lagged semi
conductor device. Basically a transistor means (transfer - resister).
Whether
the transistor is pnp or npn it resembles two diodes (back to back). The one of
the left is called emitter diodes, and the one on the right is the collector
diode. Since two types of charges are involved transistor : are classified as
bipolar devices.
Biasing the transistor :
Emitter - Base - Forward Bias.
Collector
- Base - Reverse Bias.
Que.
: What is CB, CE and CC configuration ?
Ans.
:
Buffer
: A device or a circuit used to isolate two pt'ne circuits or stages. The
emitter follows is a example of buffer.
Alpha = IC / IE
Beta = IC / IB
Que.
: How will you test a transistor with a
multimeter. ?
Ans.
:
1.
Emitter +ve of meter and Base -ve output = Low resistance
2.
Emitter -ve of meter and base +ve output = High resistance.
3.
Collector +ve and Base -ve output = Low.
4.
Collector -ve and base +ve output = High.
Emitter :
Collector = High Resistance.
PNP : Opposite Results.
Que.
: What is a thyristor ? What are its uses
?
Ans.
: A thyristor is a special kind of semi
conductor device that uses internal feedback to produce latching action.
Use : Used for
controlling large amounts of load power in motors, heaters, lighting systems
etc.
Explanation : Because of
the unusual connection we have a +ve feedback also called regeneration. A
change in current at any point in the loop is amplified and returned to the
starting point with the same phase. For instance if the 02 base current
increases, the 02 collector current increases. This force base current through
01. In turn this produces a large 01 collector current which drives the 02 base
harder. This build up in currents will continue until both transistors are
driven in saturation. In this case the latch acts like a closed switch.
On
the other hand , if something causes the 02 base current to decrease, the 02
collector current will decrease. This reduces the 01 base current. In turn, there
is less 01 collector current, which reduces the 01 base current even more. This
regeneration continues until both transistors are driven into cut off. At this
time the latches like a open switch. This latch will always stay in open or
close position.
Que.
: What are logic gates ? Explain with
truth table.
Ans.
:
GATE
: A gate is a logic circuit with one
output and one or more inputs . An output signal occurs only for control
combination of input signals.
1.
OR - GATE :
SYMBOL :
TRUTH
TABLE :
|
A
|
B
|
Y
|
0
|
0
|
0
|
|
0
|
1
|
0
|
|
1
|
0
|
0
|
|
1
|
1
|
1
|
Define
: An OR Gate has one output if any or
all of its input are 1's.
2.
AND GATE :
Define
: An AND Gate has output when all
inputs are present.
SYMBOL :
TRUTH
TABLE :
|
A
|
B
|
Y
|
0
|
0
|
0
|
|
0
|
1
|
0
|
|
1
|
0
|
0
|
|
1
|
1
|
1
|
3.
NOT GATE :
A
not gate is also known as an inverter. This circuit has one input and one out
put . All it does is invert the input signal; if the input is high, the output
is low and vice versa.
TRUTH
TABLE :
|
Input
|
Output
|
0
|
1
|
|
1
|
0
|
Buffer
: This is a non inverting gate, used
to drive low impedance loads.
NOR
GATE : This is an OR GATE followed by
an inverter.
SYMBOL :
TRUTH TABLE :
|
A
|
B
|
Y
|
0
|
0
|
1
|
|
0
|
1
|
0
|
|
1
|
0
|
0
|
|
1
|
1
|
0
|
Exclusive OR GATE
|
A
|
B
|
Y
|
0
|
0
|
0
|
|
0
|
1
|
1
|
|
1
|
0
|
1
|
|
1
|
1
|
0
|
NAND GATE : This is an AND
GATE followed by an inverter.
TRUTH
TABLE :
|
A
|
B
|
Y
|
0
|
0
|
1
|
|
0
|
1
|
1
|
|
1
|
0
|
1
|
|
1
|
1
|
0
|
.
Que.
: Explain the working of TIL - NAND GATE.
Download link : Click here
Leave a Comment