1.A balance beam scale uses which of the following units?
2.Which of the following would be about the height of the average doorway?
3.A series dissipative regulator is an example of a:
4.Which of the following is a unit of mass in the metric system?
5.What device is similar to an RTD but has a negative temperature coefficient?
6.The resistive change of a strain gauge
is based on the weight placed upon it, but can be many thousands of ohms
is usually no more than 100 omega.
is based on the gauge factor, but is typically less than an ohm
has a positive temperature coefficient
7.The output voltage of a typical thermocouple is
Less than 100 mV
Greater than 1 V
Thermocouples vary resistance, not voltage.
None of the above
8.The connections to a thermocouple
Can produce an unwanted thermocouple effect, which must be compensated for
Produce an extra desirable thermocouple effect
Must be protected, since high voltages are present
Produce an extra desirable thermocouple effect and must be protected, since high voltages are present
9.What is the zero-voltage switch used for?
To reduce radiation of high frequencies during turn-on of a high current to a load
To control low-voltage circuits
To provide power to a circuit when power is lost
For extremely low-voltage applications
10.Temperature sensing can be achieved by the use of
All of the above
11.The purpose of compensation for a thermocouple is
to decrease temperature sensitivity
to increase voltage output
to cancel unwanted voltage output of a thermocouple
used for high-temperature circuits
12.The change in value of an analog signal during the conversion process produces what is called the
13.Which of the following performance specifications applies to a sample-and-hold circuit?
14. RTDs are typically connected with other fixed resistors
In a pi configuration
In a bridge configuration
And variable resistors
And capacitors in a filter-type circuit
15.Holding current for an SCR is best described as
The minimum current required for turn-off
The current required before an SCR will turn on
The amount of current required to maintain conduction
The gate current required maintaining conduction
16.What is the moving part of a linear variable differential transformer?
17.What does a Hall Effect sensor sense?
18.What causes the piezoelectric effect?
Heat or dissimilar metals
Pressure on a crystal
Water running on iron
A magnetic field
19.What type of application would use a photo voltaic cell?
An automobile horn
A TI 92 calculator
A magnetic field detector
A remote power source
20.A transducer’s function is to:
Transmit electrical energy
Produce mechanical energy
Prevent current flow
21.Derived units are obtained from various combinations of
22.Scientific notation is a method
of expressing a very large number
of expressing a very small number
used to make calculations with large and small numbers
All of the above
23.If you drop a 5 when rounding a number, you are using the
significant digit rule
retained digit rule
24.A measure of the repeatability of a measurement of some quantity is
25.Another name for “fundamental units” is
the metric system
26.When using the terms “accuracy” and “precision” for measurements
“precision” implies less measurement error than “accuracy”
“accuracy” implies less measurement error than “precision”
“precision” measures the repeatability of a measurement
both terms mean the same thing
27.For what purpose Pirani gauge is used?
Ans. Low pressure measurement.
28.Psi stands for ____?
Ans. Pounds per square inch.
29.Low pressure is expressed in ________?
30.What is the disadvantage of orifice meter & why they are preferred in the industries?
Ans. It gives a permanent pressure loss but due to their low cost they are preferred in the industries.
31.Bolometer is used as ________?
Ans. Radiation detector.
32.Stroboscope is used for the measurement of____?
Ans. Angular velocity.
33.Electromagnetic flow meter is not suitable for which kind of fluids?
Ans. Non conductive fluids.
34.What is measurement?
Ans. It is an act of comparison between the unknown quantity and the standard resulting in knowing the magnitude of the unknown quantity in terms of the standard being used for the purpose of comparison.
35.What is an instrument?
Ans. It facilitates this process of comparison.
36.What is the difference between accuracy & precision?
Ans. Accuracy: it is the degree of closeness with which an instrumentation reading approaches to the true value of the quantity being measured.
Precision: it is a measure of reproducibility
37.What are the other name of multiplexer and demultiplexer?
Ans. Multiplexer: Data selector
Demultiplexer: Data Distributer
38.What is the range of LVDT?
Ans. 1.25mm to250mm
39.What is the frequency range of AC supply that can be applied to LVDT?
Ans. 50 Hz to 20 kHz.
It is basically moisture content in air or it is the quantity of water vapour retained by gas.
Used to measure the moisture content in air. It also used to measure humidity
42.What is the basic principle of Hygrometer.
It consist of mechanical device measuring the dimension change of humidity sensitive materials like animal hair, animal membrane , paper etc.
Defined as the amount of water absorbed by solids or liquids.
44.What is Psychrometer?
Psychrometer is a device that uses the bulb thermometers to measure humidity. It is also used in air conditioning systems for maintaining humidity.
45.What is Ambient Temperature?
The average or mean temperature of the surrounding air which comes in contact with the equipment and instruments under test.
46.What is Negative Temperature Coefficient?
A decrease in resistance with an increase in temperature
47.what is an actuator?
A device that creates automatic motion by converting various forms of energy to rotary or linear mechanical energy. Its a device to convert an electrical control signal to a physical action.
48.What is flashing in control valve?
When a liquids enters a valve and the static pressure at the vena contracta
less than the fluid vapour pressure and the valve outlet pressure is also less the
fluid vapour pressure the condition called flashing exists
49.When do you use a valve positioner?
If the diaphragm actuator does not supply sufficient force to position the
valve accurately and overcome any opposition that flowing conditions create a positioner may be required
50.What is the need of I/P converter in a control system?
In some process loop the controller is electronic and the final control
element is electronic one. To interconnect these two we need a device that should
linearly converts electric current in to gas pressure (4-20mA-315 psi). such device is called I/P converter
51.What is meant by cavitations in control valve?
When a liquid enters a valve and the static pressure at the vena contracta
drops to less than the fluid vapor pressure and the recovering to above fluid vapour
pressure, this pressure recovery causes an implosion or collapse of the vapour
bubbles formed at the vena contracta. This condition is called cavitation
52.What is “equal percentage” in the equal percentage valve?
For equal increment of stem travel at constant pressure drop an equal
percentage change in existing flow occurs.
53.What is “quick opening” control valve.
For smaller movement of the stem, there is maximum flow rate.
54.What is “Linear” control valve.
If stem position varies linearly with flow rate, then it is linear.
55.Define Control Valve sizing.
P-pressure difference across valve.
Sg-Specific gravity of liquid.
56.WHAT ARE THE PRIMARY ELEMENT USED FOR FLOW MEASUREMENTS?
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)
89.What are the primary elements used for flow measurement?
The primary elements used for flow measurement are:
Weir & Flumes.
90.What are the different types of orifice plates and state their uses?
The different types of orifice plates are:
The concentric orifice plate is used for ideal liquid as well as gases and steam service. This orifice plate beta ratio falls 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)
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 centre to minimize problems in services of solids-containing materials.
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.
It common use in Europe and are particularly useful for pipe sizes less than 2 inches.
Quadrant edge orifices produce a relatively constant
coefficient of discharge for services with low
Reynolds numbers in the range from 100,000 down
91.How do you identify an orifice in the pipeline?
An orifice tab is welded on the orifice plate, which extends out of the line giving an indication of the orifice plate.
92.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.
93.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.
94.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.
95.How do you calibrate a D. P. transmitter?
The following steps are to be taken which calibrating:
Adjust zero of the Xmtrs.
Static pressure test: Give equal pressure on both sides of the transmitter. Zero should not shift. If it is shifting carry out static alignment.
Vacuum test: Apply equal vacuum to both the sides. The zero should not shift.
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.
Apply required pressure to high-pressure side of the transmitter and adjust the span.
Adjust zero again if necessary.
96.What is the seal liquid used for filling impulse lines on crude and viscous liquid?
97.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.
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.
Steam Service: On steam service the transmitter is mounted below the orifice plate with condensate pots. The pots should be at the same level.
98.Draw and explain any flow control loop?
99.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.
100.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.
101.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.
102.How do you calculate new factor from new range using old factor and old range?
103.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.
104.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.
105.What is absolute pressure?
Absolute pressure is the total pressure present in the system
Abs. pressure = Gauge pressure + Atm. pressure.
106.What is absolute zero pressure?
Absolute zero = 760 mm Hg Vacuum.
107.What is the maximum Vacuum?
The maximum Vacuum = 760 mm Hg.
108.What is Vacuum?
Any pressure below atmospheric pressure is vacuum.
109.What are the primary elements for measuring pressure?
The primary elements used for measuring pressure are:
The above are known as elastic deformation pressure elements.
Type of Bourdon tubes.
‘ C ‘ type.
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, and stainless steel. Used for high pressure.
Pre.spring: Pressure springs of helical or spiral shape used for measuring high pressures.
110.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.)
111.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.
112.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.
113.What is the principle of a pressure gauge?
Pressure works on Hooks law.
Principle: “Measuring the stress in an elastic medium”
114.Draw and explain a pressure gauge? What is the used of a Hairspring?
The parts of a pressure gauge are:
‘C’ type Bourdon tube.
Uses of Hair Spring: Hairspring serves two purposes namely
To eliminate any play into linkages.
It serves as a controlling torque.
115.Briefly explain the different methods of level measurement?
There are two ways of measuring level:
Direct level measurement:
Bob and tape:
Highest point reached by a bob weight and measuring liquid tape provide the most simple and direct method of measuring liquid level.
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:
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.
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
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 methods 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.
Differential pressure meter:
Connections 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.
Displacer type level measurement:
The level troll 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.
116.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
This difference pressure is applied to H.P. side of the transmitted and calibrated. 117.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.
118.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)
119.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 )
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)
120.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 overcome 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.
121.Explain the working of a leveltrol.
The leveltrol is used for measuring level of liquids in a closed vessel.
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:
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.
RELAY: Amplifies pressure variations at the nozzles.
REVERSING ARC: It is used for the following purposes.
Motion takes off from Torque tube.
Means of reverse control action.
Adjustment for specific gravity.
PROPORTIONAL UNIT. : Converts primary motion to a proportional output air pressure.
CONTROL SETTING UNIT: Provides a motion of varying the set point.
122.Explain the working an electronic leveltrol.
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.
123.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 centre so that link can be connected either for reverse or direct action.
124.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 )
On a level set the difference of two specific gravities.
125.How will you calibrate a leveltrol in the field?
First close both the primary isolation valves and drain the liquid inside the chamber.
Adjust the zero to get 0% output.
Connect a transparent PVC tube to the drain point as shown in hook up.
Fill it to the centre of the top flange.
Adjust the specific gravity or span adjustment (Electronic Level).
Fill it up to 50 %, check linearity.
126.How will you calibrate on interface level control. ?
On an interface leveltrol there are two liquids of two different specific gravities.
-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.
-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:
-Fill H X d level with water adjust zero.
-Fill H X D level with water,adjust Sp. gravity or span.
– Check linearity.
127.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 % output.
To check linearity, apply average of the two weights.
128.What will happen if the displacer has fallen down while in line?
The output will be maximum.
129.What will happen if the displacer has a hole in it while in line?
The output will be minimum.
130.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 vapours are present.
131.What are the limitations of leveltrol?
The limitations of a level control that it cannot be used for lengths more than 72 inches.
132.How will you commission D.P. transmitter in field in pressurized vessel.
Close both the isolation valves, Vent the H.P. side.
Fill it with the sealing liquid.
Open the L.P. side vent valve.
Adjust zero with suppression spring.
Close the L.P. side vent valve.
Open both the isolation valves.
133.How will you check zero of a level D.P. transmitter while is line?
Close both the isolation valves.
Open the vent valve on L.P. leg and H.P. leg drain.
Check and adjust zero if necessary.
134.Explain the working of an Enraf level gauge?
The Enraf precise level gauges are based on servo powered null-balance technique. A displacer serves as a continuous level-sensing element.
A displacer with a relative density higher than that of the product to be measured is suspended from a stainless steel wire B, which 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. Level variations 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 servomotor the cam driven transmitter continuously change the voltage pattern to a remote indicator of which the receiver motor drives a counter indicating level variation.
135.What are the different methods of temperature measurement? Explain.
The different methods of temperature measurement are:
Mechanical 2. Electrical.
-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.
-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 flexibility 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.
-Pressure Spring Thermometers: There are four classes of pressure spring thermometers.
Liquid filled = class 1
Vapour pressure = class 2
Gas filled = class 3
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 helical are matched in volume so that variation in temperature at the instrument case and along the capillary tubing, produce equal motion from both helical. 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 temperature increase. The relation between temperature and pressure in this kind of system follow Charles law and may be expressed.
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.
Vapour – Pressure Thermometers:
Vapour pressure thermometers depend upon vapour pressure of liquid, which only partially fills the system. At low temperatures the vapour pressure increase for each unit temperature charge is small, at higher temperature the vapour pressure change is much greater.
Electrical method of temperature measurement:
-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.
Thermocouples Types and Range:
-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 temperature 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
136.What is Pt 100 mean?
Pt100 means 100 OHMS at 0’C for a platinum resistance bulb.
137.What is two wires and three wires 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.138.Draw a potentiometer temperature measuring circuits and explain its?
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 errors 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)
139.What is the constant voltage unit?
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 re moment line voltage effects.
139.Explain the working of a balancing motor.
Signal in control winding appears as due tank circuit formed by winding and capacitor of amp. board.
+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.
140.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
141.Why is a converter used in a temperature recorder?
The converter is designed to convert D. C. input voltage into an A. C. input voltage proportional in amplitude to the input.
142.Why are Thermo wells used?
In numerous applications 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 thermo well is also useful in protecting a sensor from physical damage during handling and normal operation.
Selecting a thermo well:
The significant properties considered in selecting a material for the well are as follows:
Resistance to corrosion and oxidation.
Resistance to mechanical and thermal shock.
Low permeability (Resistance to gas leakage).
Material for Wells:
143.How will you calibrate a temperature recorder using a potentiometer?
Connect the potentiometer output to the input of temperature recorder.Connect the (+ve) to (+ve) and (-ve) to (-ve).
If ambient compensation is provided in potentiometer set it to the correct ambient temp.If no ambient compensation is provided take a thermometer and measure the correct ambient temperature. Find out the corresponding mV s for that temperature for the given input type of thermocouple.
While feeding subtract the ambient temp minus from the corresponding temperature every time.Adjust the necessary adjustments.Measuring Temperature with a Potentiometer:
-Connect the input of the potentiometer to the thermocouple.
-If no ambient compensation is provided find out the corresponding mill volts for that
Ambient temperature for the type of thermocouple used.
-Add the ambient temperature mill volts to the corr. input mill volts measured. Find out from the chart the corresponding temperature.
144.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)
145.What are skin temperature thermocouples?
Skin thermocouples are those, which are directly connected to the process without any thermo well. Used for measuring the skin temperature of heaters furnaces, flue gas etc.
146.What is the specialty of thermocouples lead wires?
They should be of the same material as the thermocouple.
147.What is the difference the wheat stone bridge and a potentiometer?
The difference between a potentiometer and a wheat stone bridge measuring instrument is that potentiometer is a voltage measuring instrument and wheat stone bridge is a current measuring instrument.
148.Explain the continuous balance potentiometer system using R. T. D.’s.
In balance wheat stone 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 slide wire. Variations in temperature of the measured medium cause a change in resistance of the bulb and a consequent unbalance of the bridge circuit. Self-balancing wheat stone bridges recognizes the condition of unbalance determines its direction and magnitude and position the slide wire contractor to rebalance the bridge and indicate the temperature 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.
149.How is automatic Reference junction compensation carried out in temperature recorders?
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.
150.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. However 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 cannot eliminate it.
Proportional plus reset control:
Reset action is introduced to eliminate offset. It will integrate any difference between measurement and set point 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.
151.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.
152.Where is on off control used?
On off control is used when
Precise control is not needed.
Processes that have sufficient capacity to allow the final operator to keep up with the measurement cycle.
It is mainly used in refrigeration and in conditioning systems.
153.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 not come off its maximum until the measurement crosses the so point causing large overshoots. This problem can be avoided by including anti-reset wind up circuit, which eliminates the problem of output saturation.
154.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:
Closed Loop Method: e.g. Ultimate Gain Method.
Open Loop Method: e.g. process Reaction curve.
Ultimate gain method:
The term ultimate gain was attached to this method because their uses 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. :
-Let the system come to steady state at the normal load level.
-Place the controller on manual.
-Manually set the output of the controller at the value at which it was operating in the automatic mode.
-Allow the system to reach the steady state.
-With controller on manual, impose a step changes in the output of controller, which is an signal to value.
-Record the response of controlled variable.
-Return the controller output to its previous value and return the controller to auto operation.
155.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 feedback signal to provide the input for the function generator amplifier. This amplifiers output is delivered to the feedback network, and to the final output which is a 10-50m.a. do signal for actuation of final operators.
Proportional action: It is a obtained by adjusting the magnitude of feedback signal. An increase in negative feedback 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.
156.What is anti reset wind up?
If the limit acts in the feedback 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 anti reset wind up.
157.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.
158.Explain the working of Rotameter?
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.
159.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 is no location for solids to collect except the walls of the tube itself.
160.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 centreline 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.
161.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. 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.
162.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 advantage 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.
163.Where is a quadrant orifice used?
If the fluid is viscous and the operating Reynolds number is low quadrant orifice is preferred
164.What are types of taps used for orifices?
These 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.
On pipe sizes less than 2 inches corner taps located directly at the face of the orifice plate.
-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 is a close approximation to vena contracta taps up to 0.72 d / D.
-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.
165.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 behaviour of fluids. Some of these are inertial viscous, gravitational, compressibility, pressure and elastic forces. Certain dimensionless ratio is developed based on fluid properties. Velocities and dimension, which are essentially force ratio.
The more important of these are Reynolds number
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.
166.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.
167.What is a positive Displacement meter?
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:
-Reciprocating Piston type.
-Rotating or Oscillating Piston type.
-Nutating Disc type.
-Fluted Spiral Rotor type.
-Sliding vane type.
-Rotating vane type.
-Oval Gear type.
168.Why are two plugs provided on a D.P transmitter?
The top plug is a vent plug for venting the air entrapped inside the cell.
The bottom plug is a drain plug for draining the liquid accumulated inside the cell.
169.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.
170.What are the different types of control valves?
The commonly used control valves can be divided as follows.
– Depending on Action.
-Depending on the Body.
Depending on action:
Depending on action there are two types of control valves, (1) Air to close, (2) Air to open.
Depending on body:
Globe valves single or double seated.
Three way valves.
171.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.
172.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.
173.What are the different types of actuators?
The different types of actuators are:
174.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 freeze.
175.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.
176.What is the use of a valve positioner?
The valve positioner is used for following reasons:
Quick Action control valve.
Valves used on viscous liquids.
Line pressure changes on valve.
Valve Bench set not standard.
Reversing valve operation.
177.When can a bypass be not used on a positioner?
A bypass on a positioner cannot be used when:
Split Range operation.
Reverse Acting Positioner.
Valve bench set not standard.
178.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.
179.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.
180.What are the different types of plugs?
The different types of plugs are generally used are:
-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 plugs are generally used on single seated valve with small trim sizes.
181.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.
182.What are the advantages of Camflex valves?
Camflex valves are intermediates between globe valve and butterfly valve. The plug rotates 60′ for full opening.
Actuator forces required are very less.
Extended bonnet and hence can be used on any service i.e. on high temperature and very low temperature.
Variations in flow.
183.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.
184.What is the use of booster relays?
Booster relays are essentially air load, self-contained pressure regulators. They are classified into three broad groups:
Volume Boosters: These are used to multiply the available volume of air signal.
Ratio Relays: Use to multiply or divide the pressure of an input signal.
Reversing Relays:This produces a decreasing output signal for an increasing input signal.
185.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.186.What are the different valve characteristics?
The different types of valve characteristic are:
Linear Equalpercentage Quick Opening.
Linear: The valve opening to flow rate is a linear curve
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.
Quick opening: At small opening the increments in flow rate is more. At higher opening the flow rate becomes steady.
187.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:
Normally open Normally closed
USE: It is used for safety purpose.
188.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.
189.How will you change the action of a control valve?
If the control valve is without bottom cap, the actual needs to be changed.
If bottom cap is provided,
Disconnect the stem from the actuator stem.
Separate the body from the bonnet.
Remove the bottom cap and the plug from body.
Detach the plug from the stem by removing the pin.
Fix the stem at the other end of the plug and fix the pin back.
Turn the body upside down. Connect it to the bonnet after inserting the plug and stem.
Connect back the stem to the actuator stem.
Fix back the bottom cap.
Calibrate the valve.
190.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 points have to be taken into account.
-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.
– The pressure drop across a valve should be as low as possible.
191.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.
192.An operator tells you that a control valve in a stuck? How will you start checking?
-First of all get the control valve is passed from operation.
– Check the lingual to the diaphragm of the control valve.
-Disconnect it possible the actuator stem from the control valve stem.
-Stroke the actuator and see whether the actuator operates or not. It not then the diaphragm may be punctured.
– 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.
193.Where is an Air to close and Air to open control valves used?
Air to close:
Cooling water lines.
Safety Relief services.
Air to open:
194.Why does control valve operate at I5 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.
195.Explain Cascade Control system with a diagram. What would happier if a single controller were used?
Cascade means two controllers are 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.
The output of the temperature 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 required temperature 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 respond to a change in the fuel gas pressure until the process temperature sensor feels its effect. 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.
196.Explain ratio control system.
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.
197.Explain fuel to air ratio control of furnaces.
198.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.
199.What is feedback control? What is feed forward control? Discuss its application?
Feed back control:
Feed back control involves the detection of the controlled variable and counteracting of charges its 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:
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 feedback is not. Due to economic impartibility of precision, predicting the amount of correction necessary to achieve satisfactory results with feed forward control, feedback control is most often used. In order to properly choose the type of feedback controller for a particular process application, two factors time and gain must be considered.
200.Explain three element feed water control system?
Feed water control Drum level control Steam control
201.Explain Anti-surge control?
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.
202: What is 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 vapours 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.
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.
203: 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:
-The systems automatically alarms when the signal system becomes inoperative.
-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.
204: What is force balance and motions balance principle ?
FORCE BALANCE PRINCIPLE:
“A controller which generates and output signal by opposing torques”.
The input force is applied on the input bellows which novas the beam. This crackles nozzle backpressure. The nozzle backpressure 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 backpressure will increase. This increase in back pressure acting on the balancing bellows, will expands the bellows, they’re by moving the nozzle upward. The nozzle will move until motion (almost) equals the input (baffle) motion.
Advantages of force Balance:
Moving parts are fever.
Baffle movement is negligible.
Frictional losses are less.
205: 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 continuous level sensing element.
A displacer A with a relative density higher than that of a product to be measured is suspended from a stainless steel wire B that 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 positions the capacitance are of equip value. A level variation will cause a difference in buoyancy of the displacer. The centre 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 servomotors the can driven transmitter continuously changes the voltage pattern to remote indicate of which the receiver motor drives a counter indicating low variation.
206: 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.
USE “SEARCH” TAB ON the top right side FOR ALL INSTRUMENTATION RELATED TOPICS
1.Obtain necessary Permit to Work.
2.Override the trip if it is connected to any trip system.
3.Open the top cover of the Temperature Transmitter and open all RTD/TC wires.
4.Connect Multi function calibrator to the transmitter in place of RTD/TC wires as shown below
For RTD calibration the connection is as shown below
For thermocouple calibration the connection is as shown below
5.Select Temperature Simulation mode in Multi function calibrator and select RTD/TC type and feed Temperature input to the temperature transmitter.
6.Apply Temperature input 0%, 25%, 50%, 75% and 100% of the temperature range and accordingly check and note down the readings of HART Communicator and current output of transmitter (mA). Repeat the same for decreasing steps.
7.Calculate the error and compare with allowable limit.
8.If the error is within limit then calibration is ok. If calibration is ok then go-to step 13. Otherwise continue step 9.
9.If the error is not within limit then zero and span calibration required through the HART.
10.For zero calibration apply temperature to lower range and wait for stabilization of the reading. Basic Set up – Calibration –Lower range value trim.
11.For Span calibration apply upper range temperature value to the transmitter and wait for stabilization of the reading. Basic Set up – Calibration –Upper range value trim.
12.Repeat Step No. 4 – 7.
13.Ensure the readings at HMI.
14.After completion of the same, restore the RTD/TC connections of transmitter.
15.Ensure the transmitter output corresponds to atmospheric temperature.
16.If temperature is OK, close the cover. Place the RTD/TC back in service. Go to step no. 15.
17.If temperature is not OK, check RTD/TC, if necessary replace old RTD/TC with New one. Go to step no. 15.
18.Inform production department after completion of the job. Normalize the trip override if
Close the Work Permit.
The temperature transmitter calibration check list is as shown below
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1.Isolate the Pressure Transmitter from the Process.
2.Depressurize the process by slowly open the vent plug and the vent valve.
3.Note down the transmitter output from DMM / HART Communicator. Ensure the transmitter output is 4 mA.
4.Connect Pressure source calibrator to the transmitter as shown beloworor
5.Ensure there is no leak.
6.Apply pressure as per range of transmitter in steps of 25%, 50%, 75% and 100% increasing and decreasing steps
7.Note down transmitter output for each step
8.Calculate the error and compare with allowable limit.
9.If it is within limit then the previous calibration is ok, then go-to step 15. Otherwise continue with step 12
10.If the error calculated is not within the limit then calibration is required.
11.Apply zero pressure and adjust zero of the transmitter
12.Apply 100% pressure and adjust span of the transmitter
13.Repeat the steps 12 and 13 until get the desired result
14.Repeat the steps 7 and 8 and ensure the readings are within limit
15.In case of SMART Transmitter Repeat steps 8 through 16 and follow through the HART Communicator Menu for lower range value trim and upper range value trim. Basic Set up – Calibration – Zero Trim/Sensor Trim —Lower/Upper range value trim.
16.Ensure for the correct indications in HMI at Control Room
17.Depressurize and remove the pressure source
18.Restore the process connection; Ensure all the vent & drain plugs / valves are fully tightened.
19.Take the transmitter on line. Ensure there is no leak
The calibration pressure percentage error check list is as shown below:
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Magnetostrictive LT KTEK AT200 calibration and working principle
A low energy pulse generated by electronics travels through the length of magnetostrictive wire. When this signal encounters a magnetic float having a magnetic field a return signal is generated from the exact location where the float intersects the magnetostrictive wire. A timer measures the time of generation of electric pulse and the return signal. This time difference is used to calculate the level of the float
Calibration of magnetostrictive level transmitter
1.Level Output Calibration
Calibration can be changed with the unit pushbuttons, or with a HART communicator (for units with the HART option), or with the menu driven LCD readout (for units with LCD option).
Calibration Using the Pushbuttons
Setting the 4mA point:
-Establish a tank level of 0% or move the float to the desired 0% point
-Enter the calibration mode by pressing the UP&DOWN buttons together for 1 second.
-Press the DOWN button for 1 second to set the output at 4.00mA.
Setting the 20mA point:
-Establish a tank level of 100% or move the float to the desired 100% point
-Enter the calibration mode by pressing the UP & DOWN buttons together for 1 second.
-Press the UP button for 1 second to set the output at 20.00 mA
Note: The above steps can be repeated as many times as required
The AT200 will detect and report the position of the float (within the level gauge) on its sensor tube as a level of fluid in the process. In order to measure the fluid in the process properly, the float must move freely up and down the level gauge chamber partially submerged in the liquid. If the float were to become damaged or stuck in the chamber, the transmitter will still report the float position regardless of the actual process fluid level. This, by definition, is a Dangerous Undetectable failure. To prevent this failure the float will need to be inspected for integrity and movement. Some gauges will have two floats mounted in the chamber. This inspection should be done on both floats.
1) Move the float up and down the length of the chamber using the process fluid or some other media. The float should move freely from the bottom of the chamber from one process connection to the other.
2) Remove the float from the level gauge chamber. Inspect the float for signs of excessive wear or damage.
3) Submerge the float in a container of water to check for leaks as air bubbles escaping from the float. The float is a sealed unit and any holes in the shell of the float could allow process fluid to seep inside.
Note: K-TEK floats are designed for different specific gravity ranges. The float may or may not float in the water. It may be necessary to hold the float under the water to perform this test. Upon completion of float inspection, place the float back into the level gauge chamber paying careful attention to the float orientation.
The transmitter of the AT200 is designed to return a level indication and an output based on the position of a float in the level gauge chamber. If the transmitter is equipped with an LCD on the front of the electronics module the level and current output will be displayed.
1) Apply power to the transmitter using the typical loop wiring diagram
2) Move the float up and down the level gauge chamber.
3) Monitor the indication of the level on the LCD to make sure the indication corresponds to the float position
Note: It is possible for the AT200 to continue providing a 4-20mA output if the LCD display is not functioning properly. If the LCD indicator on an electronics module fails during normal operation, it is recommended that the electronics module be replaced at the earliest convenience. It will not be necessary however to shut down a transmitter or remove it from service based on an LCD failure.
The current output of the AT200 transmitter update at least every 110 milliseconds and be filtered through the user adjusted Damping. The maximum response time to a process change will be less than 110 milliseconds or the value of the Damping, whichever is greater.
1.Apply power to the transmitter using the typical loop wiring diagram
2.Connect a multi-meter (set to read milliamps) to the transmitter using the “Meter” connections on the terminal strip.
3.Move the float along the length of the probe and monitor the milliamp output on multi-meter.
4.The output should indicate the float position based on the calibration range of the transmitter.
4-20mA Loop Check
– Without HART
With the transmitter installed, wired and powered in its field location, move the float up and down the length of the probe. Confirm the proper reading at the indication or control room side of the loop. Move the float using the process fluid or some other mechanical means. If moving the float is not possible, the loop may be checked using an independent device such as a loop calibrator.
– With HART communications
With the transmitter installed, wired and powered in its field location and power supplied to the loop, connect a HART handheld device to the loop across a 250 ohm resistor. Using the Loop Test feature of the HART handheld, drive the output of the transmitter to 4mA then 20mA. Confirm the proper reading at the indication or control side of the loop.
Minor adjustments to the output of the transmitter may be made using the DAC Trim (Digital/Analog Converter) feature.