150+ Interview Questions for Instrumentation Engineers

150+ Interview Questions for Instrumentation Engineers

INSTRUMENT:

An instrument is a device used for the measurement, monitoring, display, etc., of a process variable.

Que. 1: What are the process variables?

Ans.: The process variables are:

Flow
Pressure
Temperature
Level
Quality, i.e., % D2, CO2, pH, etc.

Que.2: Define all the process variables and state their unit of measurement.?

Ans.:

FLOW: Kg/hr, Liter/min, Gallon/min, M3/NM3/HR (GASES)
PRESSURE: Force acting per unit Area. Units: Bar, Pascal, Kg/CM, Pounds
LEVEL: Difference between two heights. Units: Meters, mm, cm, %.
TEMPERATURE: Degree of hotness or coldness of a body. Units: Degree Centigrade, Degree Fahrenheit, Degree Kelvin, Degree Rankine.
QUALITY: pH, % CO2, % O2, Conductivity, Viscosity.

Que.3: What are the primary elements used for flow measurement?

Ans.: The primary elements used for flow measurement are:

Orifice Plate.
Venturi tube.
Pitot tube.
Annubars.
Flow Nozzle.
Weir & Flumes.

Que.4: What are the different types of orifice plates and state their uses?

Ans.: The different types of orifice plates are:

Concentric: Used for ideal liquid, gases, and steam service.
Segmental: Hole eccentric; used in viscous and slurry flow measurement.
Eccentric: Hole in the form of a segment of a circle; used for colloidal and slurry flow measurement.

Que.5: How do you identify an orifice in the pipeline?

Ans.: An orifice tab, welded on the orifice plate, extends out of the line, providing an indication of the orifice plate.

Que.6: Why is the orifice tab provided?

Ans.: The orifice tab is provided for several reasons:

Indication of an orifice plate in a line.
Marking the orifice diameter.
Displaying the material of the orifice plate.
Indicating the tag number of the orifice plate.
Marking the inlet of an orifice.

Que.7: What is Bernoulli's theorem and where is it applicable?

Ans.: Bernoulli's theorem states that the total energy of a liquid flowing from one point to another remains constant. It is applicable for non-compressible liquids.

Que.8: How do you identify the H.P. side or inlet of an orifice plate in the line?

Ans.: The marking is always done on the H.P. side of the orifice tab, providing an indication of the H.P. side.

Que.9: How do you calibrate a D.P. transmitter?

Ans.: The calibration steps include:

When the transmitter is removed, close both side valves, fully close the equalizing valve, and fully open the isolation valve. Reverse the process during transmitter connection.
Adjust zero of the transmitters.
Static pressure test: Equalize pressure on both sides; zero should not shift. If shifting occurs, carry out static alignment.
Vacuum test: Apply equal vacuum to both sides; zero should not shift.
Calibration Procedure:

Provide 20 psi air supply to the transmitter.
Vent the L.P. side to the atmosphere.
Connect the instrument output to a standard test gauge and adjust zero.
Apply the required pressure to the high-pressure side and adjust the span.
Re-adjust zero if necessary.

Que.10: What is the seal liquid used for filling impulse lines on crude and viscous liquids?

Ans.: Glycol.

Que.11: How do you carry out piping for a Different pressure flow transmitter on liquids, Gas, and steam services? Why?

Ans.: Liquid lines are mounted below the orifice plate because liquids have a self-draining property. On gas services, the transmitter is mounted above the orifice plate because gases self-vent and to avoid condensate formation. On steam service, the transmitter is mounted below the orifice plate with condensate pots at the same level.

Que.12: Draw and explain any flow control loop?

Ans.:

[Diagram explanation not provided]

Que.13: An operator tells you that flow indication is more? How would you start checking?

Ans.:

1. Flush the transmitter and both impulse lines. Adjust the zero by equalizing if necessary.

Check the L.P. side for a choke. If clean, then
Check for leaks on the L.P. side. If not,
Calibrate the transmitter.

Que.14: How would you perform a zero check on a D.P. transmitter?

Ans.: Shut off either the H.P. or L.P. valve and open the equalizing valve. The output should register zero.

Que.15: What is the process for glycol filling or filling seal liquids in seal pots? Illustrate and explain.

Ans.: The glycol filling procedure involves the following steps:

Close the primary isolation valves.
Open the vent on the seal pots.
Drain any used glycol present.
Use a hand pump on the L.P. side while filling the H.P. side with glycol.
Keep the equalizer valve open.
Close the L.P. side valve.
Start pumping and fill with glycol.
Repeat the process for the L.P. side by connecting the pump to the H.P. side, keeping the equalizer open, and the H.P. side isolation valve closed.
Close the seal pot vent valves.
Close the equalizer valve.
Open both primary isolation valves.

Que.16: How can you calculate a new factor from the new range using the old factor and old range?

Ans.: New Factor = (Old Factor / √Old Range) × √New Range. Flow = K × √(Factor / Range).

Que.17: What is the procedure for venting air in the D.P. cell? What if seal pots are utilized?

Ans.:

Vent air by opening the vent plugs on a liquid service transmitter.
On services using seal pots, isolate the primary isolation valves and open the vent valves. Fill the line from the transmitter drain plug using a pump.

Que.18: Why is flow measured in square root?

Ans.: Flow varies directly as the square root of the differential pressure (F = K√ΔP). The pen does not directly indicate flow because of this square root relationship. The actual flow can be determined by taking the square root of the pen reading. For instance, if the pen reads 50% of the chart, the actual flow is the square root of 50%.

DEFINITIONS:

ACCURACY: A numerical value defining the limit of error under reference conditions.

ATTENUATION: A reduction in signal magnitude between two points or frequencies.

DEAD TIME: The time interval between initiating a change or stimulus and the resulting response.

DRIFT: An undesired output change over time, unrelated to input, operating conditions, or load.

ERROR: The difference between the indication and the true value of the measured signal.

SPAN ERROR: The difference between the actual span and the specified span, expressed as a percentage of the specified span.

ZERO ERROR: The error when the device operates under specified conditions with the input at the lower range value.

STATIC GAIN: The ratio of output change to input change after reaching a steady state.

HYSTERESIS: The maximum difference between upscale and downscale indications during a full-range traverse for the same input.

INTERFERENCE: Spurious voltage or current from external sources in a device's circuits.

COMMON MODE INTERFERENCE: Interference between the measuring circuit terminals and ground.

NORMAL MODE INTERFERENCE: Interference between measuring circuit terminals.

LINEARITY:

The degree to which a curve approximates a straight line.

RANGE:

The limits within which a quantity is measured, received, or transmitted, expressed by stating lower and upper range values.

REPEATABILITY:

Agreement among consecutive measurements of the output for the same value of the measured signal under the same operating conditions.

REPRODUCIBILITY:

Agreement among repeated measurements of the output for the same input value under the same operating conditions.

RESPONSE:

The overall behavior of a device's output as a function of input, considering time.

SIGNAL TO NOISE RATIO:

The ratio of signal amplitude to noise.

TIME CONSTANT:

The time needed 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 CONVERSIONS:

1 psi = 27.74 " H2O

1 Kg/cm2 = 14.223 psi

1 Bar = 14.504 psi

1 Kpa = 0.145 psi

1 Kg/cm2 = 10.000 mm of H2O

1 Bar = 1.0197 Kg/cm2

1 Kg/cm2 = 098 Bar

1 Torr = 1mm of Hg.

Que.19: What defines absolute pressure?

Ans.: Absolute pressure encapsulates the overall pressure within a system, encompassing both Gauge pressure and Atmospheric pressure.

Que.20: Can you explain absolute zero pressure?

Ans.: Absolute zero pressure equates to 760 mm Hg in a vacuum setting.

Que.21: And what about the maximum Vacuum?

Ans.: The pinnacle of vacuum pressure aligns with 760 mm Hg as well.

Que.22: How would you characterize Vacuum?

Ans.: Vacuum denotes any pressure below atmospheric pressure. Atmospheric pressure, standing at 760 mm Hg, serves as the reference point, with zero-gauge pressure signifying zero absolute pressure.

Que.23: Could you shed light on the primary elements for measuring pressure?

Ans.: Primary pressure measurement elements encompass Bourdon tube, Diaphragm, Capsule, Bellows, and Pressure Springs—collectively referred to as elastic deformation pressure elements. Bourdon tubes come in types like 'C' type, Spiral, and Helix.

Que.24: Explain the underlying principle of a pressure gauge.

Ans.: A pressure gauge operates on Hooke's law, essentially gauging the strain within an elastic medium.

Que.25: Walk us through the calibration process for an absolute pressure transmitter using a vacuum manometer with a range of 0-400 mm abs.

Ans.: Calibration steps:

Connect an air supply to the transmitter.
Attach a test gauge (0-1.4 Kg/cm2) to the output.
Connect a vacuum pump to the manometer via a tee-off.
Apply 760 mm Vacuum (or the nearest value) to adjust the zero.
Apply 360 mm Vacuum to adjust the span (760 - 360 = 400 mm abs.).

Que.26: Given a mercury manometer with a range of 0-760 mm, a vacuum gauge indicating 60 mm vacuum, and a test manometer reading 50 mm vacuum—can you determine which is correct?

Ans.: The transmitter is accurate in this scenario because 760 - 50 = 710 mm abs.

Que.27: Provide a concise overview of the different methods for measuring level.

Ans.: Level measurement methods fall into two categories: Direct and Indirect.

Direct Level Measurement:

BOB AND TOP.
SIGHT GLASS: Utilizes a graduated glass tube mounted on the vessel's side.

Indirect Liquid Level Measurement:

PRESSURE GAUGE: Measures pressure at the liquid's zero level.
PURGE SYSTEM: Involves a vertically installed pipe open at the zero level, connected to a regulated air supply and a pressure gauge.
DIFF.PRESSURE METHOD: Applies a differential pressure (D.P.) meter with connections at the vessel top and bottom, effectively balancing the vessel pressure. The D.P. meter discerns changes in liquid level based on pressure differentials.

Displacer Type Level Measurement:

The Leveltrol is a widely used instrument for measuring levels in closed tanks, operating on Archimedes' principle. The displacer is immersed in the liquid, resulting in a loss of weight based on the liquid's specific gravity. This displacer hangs freely on a knife, transmitting its motion to the pneumatic or electronic counterpart.

Que.28: Explain how you measure level with a differential pressure (D.P.) transmitter.

Ans.: Connect the vessel's bottom to the high-pressure side of the transmitter. The differential pressure (D.P.) is calculated as H x D, where this difference pressure is applied to the high-pressure side of the calibrated transmitter.

Que.29: How is a D.P. transmitter applied to a closed tank?

Ans.: For a closed tank, connect the tank's bottom to the high-pressure side and the top to the low-pressure side. This configuration balances the vessel pressure.

Que.30: How is a D.P. transmitter applied to open tank?

Ans.: In an open tank, vent the low-pressure side to the atmosphere. The pressure on the high-pressure side, which corresponds to the level, is measured.

Que.31: What is a purge level system?

Ans.: The purge level system, also known as the bubbler method, involves a vertical pipe with its open end at the zero level. It's connected to a regulated air supply and a pressure gauge or a D.P. transmitter. By adjusting the air supply, bubbles leave the open end, and the pressure needed to overcome the liquid pressure is measured. This method is suitable for corrosive liquids.

Que.32: Explain the working of a level control.

Ans.: The level control measures liquid levels in a closed vessel based on Archimedes' principle. It consists of a displacer, relay, reversing arc, proportional unit, and a control setting unit. The displacer's buoyancy change with the liquid level varies the net weight, triggering proportional motion.

Que.32: How does an electronic level control work?

Ans.: Changes in buoyancy due to the liquid level alteration cause a proportional change in the displacer's net weight. This movement affects the rotary variable differential transformer (RVDT), converting it to a proportional DC current.

Que.33: What is interface level, and how do you calculate it?

Ans.: Interface level occurs when a vessel contains two liquids with different specific gravities. The formula is DP = H × (D - d), considering the difference in specific gravities.

Que.34: How do you calibrate a level control in the field?

Ans.: Calibrate by closing isolation valves, draining the chamber, adjusting zero, and filling a transparent PVC tube for span adjustment. Check linearity at different fill levels.

Que.35: How about calibrating an interface level control?

Ans.: For liquids with different specific gravities, calibrate at zero and 100% levels using water, adjusting zero and span accordingly. Check linearity.

Que.36: What happens if the displacer falls down while in line?

Ans.: The output will be maximum (100%).

Que.37: What if the displacer has a hole in it while in line?

Ans.: The output will be maximum.

Que.38: What is the purpose of suppression and elevation?

Ans.: Suppression and elevation are used in level applications where transmitters are not directly mounted or in the presence of condensable vapors.

Que.39: What are the limitations of level control?

Ans.: The limitations include a maximum length of 72 inches.

Que.40: How do you commission a D.P. transmitter in a pressurized vessel?

Ans.: Close both isolation valves, vent the high-pressure side, fill with sealing liquid, open the low-pressure side vent valve, adjust zero with the suppression spring, and then open both isolation valves.

Que.41: How do you check the zero of a level D.P. transmitter while in line?

Ans.: Close both isolation valves, open the vent valve on the low-pressure leg, and drain the high-pressure leg. Check and adjust zero if necessary.

Que.42: Explain the working of an Enraf level gauge.

Ans.: Enraf level gauges use a servo-powered null balance technique. A displacer, suspended from a measuring drum, undergoes tension changes proportional to its immersion in the liquid. A servo motor and capacitive balance system control the measuring wire, with a receiver motor indicating level variations.

Temperature

Que.43: What are the different methods of temperature measurement?

Ans.: Mechanical and electrical methods.

Mechanical Methods:

Mercury in glass thermometers.
Bimetallic thermometers.
Pressure spring thermometers (liquid-filled, vapor pressure, gas-filled, mercury-filled).
Compensated thermometer systems.

Que.44: Could you explain a compensated thermometer system?

Ans.: Compensation in liquid-filled expansion thermal systems involves a second tubing and helical element filled with liquid. The compensating helical and the measuring helical are matched in volume to nullify the effects of temperature changes at the instrument case.

Gas-filled Thermometers: Gas-filled thermometers operate based on the increase in pressure of a confined gas (constant volume) due to a temperature increase. The relationship between temperature and pressure in this system follows Charles' law. Nitrogen is commonly used in such systems due to its chemical inertness and favorable coefficient of thermal expansion.

Vapor-Pressure Thermometers: Vapor-pressure thermometers rely on the vapor pressure of a liquid that only partially fills the system. The vapor pressure change is small at low temperatures and much greater at higher temperatures.

Electrical Methods of Temperature Measurement:

Thermocouples: Thermocouples are simple devices consisting of dissimilar metal wires joined at their ends. When one end of each wire is connected to a measuring instrument, thermocouples become accurate and sensitive temperature measuring devices. Different types of thermocouples include Iron-Constantan, Chromel-Alumel, Platinum-Platinum 10% Rhodium, Platinum-Rhodium 13%, Chromel-Constantan, and Copper-Constantan.
Resistance-Temperature Detectors (RTD): RTDs are used for precise temperature measurement. They consist of a wire wrapped around an insulator and enclosed in a metal sheath. The resistance increases as temperature increases, following the formula Rt = Ro(1 + αt), where Rt is the resistance at the measured temperature, Ro is the resistance at zero temperature, α is the coefficient of thermal expansion, and t is the temperature to be measured.

Calibration of Pt100: For Pt100, Ro = 100, and with the coefficient of thermal expansion for platinum (α) as 0.00385 /°C, the resistance at 100°C (R100) is calculated as R100 = 100[1 + (38.5 x 10^-4 x 100)] = 138.5.

Questions and Answers:

Que.45: What does PT100 mean?

Ans.: PT100 means 100 OHMS at 0°C for a platinum resistance bulb.

Que.46: What are two-wire and three-wire RTD systems?

Ans.: Two-wire RTD systems are used for short distances, like a compressor field local panel. Three-wire systems are used for long distances, such as field control.

Que.47: Draw a potentiometric temperature measuring circuit and explain its operation.

Ans.: The potentiometric temperature measuring circuit subtracts the input signal from a known constant voltage in a potentiometric measuring circuit, producing an error signal. This error signal is then used to drive a servo balancing motor to adjust the circuit until the difference between the feedback voltage and the input voltage is balanced out.

Que.48: What is the constant voltage unit?

Ans.: The constant voltage circuit consists of a rectifier, CR, a filter capacitor C1, and two stages of zener regulation. It provides a relatively constant current to a zener (CR4) to maintain line voltage regulation.

Que.49: Explain the working of a balancing motor.

Ans.: The balancing motor is an induction motor that creates a rotating magnetic field in the stator. The rotor turns by following this field, which is generated by two windings in the stator. The motor adjusts the slide wire until the feedback voltage and the input voltage are balanced.

Que.50: What is burnout feature?

Ans.: Burnout provides a warning feature by driving the indicator to the end of the scale if the input circuit opens. It uses a burnout resistor to develop a voltage drop between the measuring circuit and the amplifier.

Que.51: Explain the block diagram of an amplifier in a temperature recorder.

Ans.: The block diagram consists of an input, a converting stage, a voltage amplifier, a power amplifier, a balancing motor, and a feedback loop. The input signal is converted into an AC voltage, amplified, and used to control the power amplifier, which, in turn, drives the balancing motor.

Que.52: Why is a converter used in a temperature recorder?

Ans.: A converter is used to convert DC input voltage into an AC input voltage proportional in amplitude to the input.

Que.53: Why are thermowells used?

Ans.: Thermowells are used to protect temperature sensors from damage, corrosion, erosion, abrasion, and high-pressure processes. They also shield sensors from physical damage during handling and normal operation.

Que.54: How will you calibrate a temperature recorder using a potentiometer?

Ans.: Connect the potentiometer output to the input of the temperature recorder. Adjust the potentiometer for the correct ambient temperature and subtract ambient temperature millivolts from the corresponding temperature millivolts. Make necessary adjustments.

Que.55: What type of sensing element would you use to measure very low temperatures?

Ans.: The sensing element used for measuring very low temperatures is an RTD (Resistance Temperature Detector).

Que.56: What are skin temperature thermocouples?

Ans.: Skin temperature thermocouples are directly connected to the process without any thermowell. They are used for measuring the skin temperature of heaters, furnaces, flue gas, etc.

Que.57: What is the specialty of thermocouple lead wires?

Ans.: Thermocouple lead wires should be of the same material as the thermocouple.

Que.58: What is the difference between a Wheatstone bridge and a potentiometer?

Ans.: The difference between a potentiometer and a Wheatstone bridge is that a potentiometer is a voltage measuring instrument, while a Wheatstone bridge is a current measuring instrument.

Que.59: Explain the continuous balance potentiometer system using RTDs.

Ans.: In a continuous balance Wheatstone bridge resistance thermometer, a resistance bulb is connected to one branch of a DC bridge circuit. A calibrated slide wire represents the variable resistance in another branch. The self-balancing Wheatstone bridge recognizes unbalance conditions and adjusts the slide wire position to rebalance the bridge and indicate the temperature on the scale.

Que.60: How is automatic reference junction compensation carried out in temperature recorders?

Ans.: Automatic reference junction compensation is achieved using a variable nickel resistor. This resistor, located at the temperature of the reference junction, compensates for changes in temperature by adjusting its resistance.

Control Systems:

Que.61: What is an automatic controller?

Ans.: An automatic controller is a device that measures the value of a variable quantity or condition and operates to correct or adjust it based on the deviation of this measured value from a selected reference.

Que.62: What is an automatic control system?

Ans.: An automatic control system is any operable arrangement of one or more automatic controllers in closed loops with one or more processes.

Que.63: What is a self-operated controller?

Ans.: A self-operated controller is one in which all the energy needed to operate the final control element is derived from the controlled medium through the primary element.

Que.64: What is a relay-operated controller?

Ans.: A relay-operated controller is one in which the energy transmitted through the primary element is supplemented or amplified by employing energy from another source.

Que.65: What is a process in control systems?

Ans.: A process in control systems comprises the collective functions performed by the equipment in which a variable is to be controlled.

Que.66: What is self-regulation?

Ans.: Self-regulation is an inherent characteristic of the process that limits the deviation of the controlled variable.

Que.67: What is a controlled variable?

Ans.: The controlled variable is the quantity or condition that is measured and controlled in a control system.

Que.68: What is a controlled medium?

Ans.: The controlled medium is the process energy or material in which a variable is controlled. It is the medium in which the controlled variable is a condition or characteristic.

Que.69: What is a manipulated variable?

Ans.: The manipulated variable is the quantity or condition varied by the automatic controller to affect the value of the controlled variable.

Que.70: What is a control agent?

Ans.: The control agent is the process energy or material of which the manipulated variable is a condition or characteristic.

Que.71: What is an actuating signal?

Ans.: The actuating signal is the difference at any time between the reference input and a signal related to the controlled variable. It is also known as the error signal.

Que.72: What is deviation in control systems?

Ans.: Deviation is the difference between the actual value of the controlled variable and the value of the controlled variable corresponding to the set point.

Que.73: What is offset in control systems?

Ans.: Offset is the steady-state difference between the control point and the value of the controlled variable corresponding to the setpoint.

Que.74: What is corrective action in control systems?

Ans.: Corrective action is the variation of the manipulated variable produced by the controlling means. It operates the final control element to vary the manipulated variable.

Que.75: What is a reference input?

Ans.: The reference input is the reference signal in an automatic controller. It represents the desired value to which the controlled variable should be brought.

Que.76: What is a set point?

Ans.: The set point is the position to which the control point setting mechanism is set. It represents the desired value of the controlled variable.

Que.77: What is a control point?

Ans.: The control point is the value of the controlled variable at which, under any fixed set of conditions, the automatic controller operates to maintain.

Que.78: What is primary feedback in control systems?

Ans.: Primary feedback is the signal related to the controlled variable that is compared with the reference input to obtain the actuating signal. It is the actual measurement of the controlled variable.

Que.79: What is positioning action in control systems?

Ans.: Positioning action is the control action in which there is a predetermined relation between the value of the controlled variable and the position of the final control element.

Que.80: What is proportional action in control systems?

Ans.: Proportional action is the control action in which there is a continuous linear relationship between the value of the actual measurement of the controlled variable and the value of the position.

Que.81: What is floating action in control systems?

Ans.: Floating action is the control action in which there is a predetermined relation between the deviation and speed of the final control element.

Que.82: What is derivative action in control systems?

Ans.: Derivative action is the control action in which there is a predetermined relation between the time derivative of the controlled variable and the position of the final control element.

Que.83: What is reset action in control systems?

Ans.: Reset action is the control action in which there is a value movement at a speed proportional to the magnitude of deviation.

Que.84: What is rate action in control systems?

Ans.: Rate action is the control action in which there is a continuous linear relationship between the rate of change of the controlled variable and the position of the final control element.

Que.85: What is proportional band in control systems?

Ans.: Proportional band is the range of values of the controlled variable that correspond to the full operating range of the final control element.

Que.86: What is reset rate in control systems?

Ans.: Reset rate is the number of times per minute that the effect of proportional position action upon the final control element is repeated by proportional speed floating action.

Que.87: How is reset action expressed?

Ans.: Reset action can be expressed in two ways: reset rate and reset time.

Reset Rate: It is expressed as a number of "repeats" per minute, determined by the travel of the final control element in one minute resulting from the effect of proportional speed floating action, divided by the travel as a result of the effect of proportional position action with the same deviation in both cases.
Reset Time: It is the time interval by which the rate is expressed in minutes, determined by subtracting the time required for a selected motion of the final control element resulting from the combined effect of proportional position plus rate action from 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 the controlled variable in both cases.

Que.88: Explain the application of proportional, integral, and derivative actions.

Ans.: - Proportional Control Only: This control action attempts to return a measurement to the set point after a load upset but cannot eliminate offset. It is often used for level controls.

Proportional + Reset Control: Reset action is introduced to eliminate offset, integrating any difference between measurement and setpoint. It is commonly used in industrial process control where predominant dead times occur.
Proportional + Reset + Derivative: Derivative or rate action helps overcome system inertia and results in faster, more precise control. It is used in temperature controls.

Que.89: What is differential gap control?

Ans.: Differential gap control is similar to on-off control, but a band or gap exists around the control point. It is often used in non-critical level control applications where the goal is to prevent a tank from flooding or drying.

Que.90: Where is on/off control used?

Ans.: On/off control is used when:

Precise control is not needed.
Processes have sufficient capacity for the operator to keep up with the measurement cycle.
It is mainly used in refrigeration and air conditioning systems.

Que.91: Why does reset cause wind up in controllers?

Ans.: When reset action is applied in controllers, where the measurement is away from the set point for long periods, the reset may drive the output to its maximum, resulting in reset wind up. This can lead to large overshoots when the process starts again. Antireset wind up circuits can be included to eliminate this problem.

Que.92: Why is reset called integral and rate derivative?

Ans.: RESET is called integral because of its mathematical relationship to the output. RATE is called derivative because the equation for output includes a time derivative term.

Que.93: Explain the tuning of controllers.

Ans.: Tuning controllers involves adjusting proportional, integral, and derivative parameters to achieve good control. The settings depend on the gain, time constants, and dead times around the loop. Tuning methods include closed-loop methods like the Ultimate Gain Method and open-loop methods like the process reaction curve.

Ultimate Gain Method: The ultimate gain method involves determining the ultimate gain (sensitivity) and ultimate period. The ultimate sensitivity (Ku) is the maximum allowable value of gain for which the system is stable. The ultimate period is the period of the response with the gain set at its ultimate value.

PROCERS REACTION CURVE: To determine the process reaction curve, it is recommended to follow these steps:

Allow the system to reach a steady state at the normal load level.
Place the controller in manual mode.
Manually set the controller output to the value it had in automatic mode.
Allow the system to reach a steady state.
With the controller in manual mode, introduce a step change in the controller output.
Record the response of the controlled variable.
Return the controller output to its previous value and switch the controller back to auto operation.

Que.94: Explain the operation of an electronic P.I.D. controller.

Ans.: The electronic P.I.D. controller operates by comparing input from the measurement transmitter with the set point voltage to generate a deviation signal. This deviation signal, combined with a characterized feedback signal, serves as the input for the function generator amplifier. The amplifier's output is directed to both the feedback network and the final output, which is a 10-50m.a. dc signal for actuating final operators.

Que.95: What is an analogue integrator and an analogue differentiator?

ANS.: ANALOGUE INTEGRATOR:

Explanation needed.

ANALOGUE DIFFERENTIATER:

Explanation needed.

Que.96: What is anti-reset wind up?

Ans.: Anti-reset wind-up occurs when the limit acts in the feedback section of the control amplifier's integral circuit. In this case, the controller output immediately starts moving in the opposite direction as soon as the process signal crosses the set point. This phenomenon is referred to as anti-reset wind-up.

Que.97: What are desaturators?

Ans.: Desaturators are used in processes where long transient responses lead to sustained deviations. In such cases, integral action of the controller continuously drives the output to minimum or maximum values. This phenomenon is known as "integral saturation of the control unit," and desaturators are implemented to address this condition.

Que.98: Explain the operation of a Rotameter.

Ans.: Variable area meters, such as Rotameters, are a special type of head meters where the area of the flow restrictor varies to maintain constant differential pressure. The Rotameter consists of a vertical tapered tube through which the fluid flows. A float, denser than the fluid, creates an annular passage. As flow varies, the float adjusts, varying the passage area, and the float's position is a measure of the flow rate.

Que.99: Explain the operation of a magnetic meter.

Ans.: Electric potential is generated when a conductor moves across a magnetic field. In magnetic meters, a cylindrical, electrically insulated tube with electrodes detects this potential in electrically conductive liquids. The resulting AC voltage is proportional to the volume flow rate and magnetic field strength, making it suitable for measuring slurries and dirty fluids.

Que.100: Explain the operation of a turbine meter.

Ans.: Turbine meters consist of a flow tube with a rotating turbine or fan, whose speed is directly proportional to the flow rate. A magnetic pick-up system senses the rotor's rotation, generating electrical pulses proportional to the flow rate. Turbine meters are primarily used for measuring clean and non-corrosive hydrocarbons.

Que.101: Explain the operation of a Pilot tube.

Ans.: The Pitot tube measures fluid velocity at a point in the conduit. For quantity rate measurement, it requires calculating the ratio of average velocity to the velocity at the measurement point. The principle involves fluid impinging on the open end of the tube, creating a pressure difference, measured to determine the stream's velocity.

Que.102: Where is the integral orifice used?

Ans.: The integral orifice is used to measure small flow rates and is directly mounted on the secondary device. The diameter of the integral orifice typically ranges from 0.020 to 0.250 inches. It finds applications in laboratory and pilot plants.

Que.103: Explain the operation of a target meter.

Ans.: The target meter combines a primary element and a force balance flow rate transmitter in a single unit. A circular disc (or target) within the pipe's annular orifice configuration produces a force proportional to the square of the flow rate. This force is transmitted out of the pipe for measurement, making it suitable for sticky or dirty materials.

Que.104: Where is a quadrant orifice used?

Ans.: A quadrant orifice is preferred in situations where the fluid is viscous, and the operating Reynolds number is low.

Que.105: What are the types of taps used for orifices?

Flange Taps:

Most commonly used for pipe sizes of 2 inches or larger, located in the orifice flange.

Corner Taps:

Used on pipe sizes less than 2 inches, directly located at the face of the orifice plate.

Vena Contracta and Radius Taps:

Vena contracta taps located at 1 pipe diameter upstream and at the point of minimum pressure downstream.
Radius taps located 1 pipe diameter upstream and 1/2 pipe diameter downstream for the inlet face of the orifice.

Face Flow Taps:

Located at 2 1/2 pipe diameter upstream and 1/2 pipe diameter downstream, similar to vena contracta or radius taps.

Que.106: What is Reynolds number?

Ans.: Reynolds number is a dimensionless ratio indicating dynamic similarity of fluid forces. It plays a crucial role in distinguishing between laminar and turbulent flow, providing information about the velocity profile shape. Reynolds number effects are deviations from theoretical equations due to varying flow characteristics.

Que.107: How would you choose the differential range?

Ans.: The most common differential range for liquid measurement is 0-100" H20. This range minimizes errors caused by unequal heads in seal chambers, temperature differences, and allows flexibility with a capacity increase up to 400" and a decrease down to 20" by adjusting range tubes or range settings.

Que.108: What are positive displacement meters?

Ans.: Positive displacement meters measure liquids by using a measuring element that seals off the measuring chamber into compartments, each holding a specific volume. Examples include reciprocating piston, rotating or oscillating piston, nutating disc, fluted spiral rotor, sliding vane, rotating vane, and oval gear meters.

CONTROL VALVES

Que.109: What is a control valve?

Ans.: A control valve is the final control element that directly alters the value of the manipulated variable by changing the flow rate of the control agent. It consists of an operator and valve body, where the operator varies the position of the valve plug inside the body, affecting the flow rate.

Que.110: What are the different types of control valves?

Depending on Action:

Air to close.
Air to open.

Depending on Body:

1. Globe valves (single or double-seated).

2. Angle valves.

3. Butterfly valves.

4. Three-way valves.

Que.111: What purpose does a single-seated valve serve?

Ans.: The single-seated valve finds application in smaller sizes and in larger valves where complete shut-off is necessary. Its usage is constrained by the pressure drop across the valve in the closed or nearly closed position.

Que.112: When would you opt for a double-seated valve?

Ans.: Double-seated valves are employed in larger valves and high-pressure systems. They help equalize the upward and downward forces on the plug due to the reduction of fluid pressure. This results in the requirement of smaller actuators.

Que.113: Explain the concept of CV in a valve.

Ans.: CV, or valve capacity, is defined as the number of gallons per minute of water passing through a fully open valve at a pressure drop of 1 psi. It is proportional to the area 'A' between the plug and valve seat measured perpendicularly to the direction of flow.

Que.114: Enumerate the types of valve actuators.

Ans.: Actuators come in two primary types: diaphragm-operated and piston-operated.

Que.115: What types of bonnets are suitable for high and low-temperature applications?

Ans.: For high temperatures, bonnets equipped with radiation fins prevent damage to gland packing. For low temperatures, extended bonnets are used to protect gland packing from freezing.

Que.116: How do you work on a control valve when it is in-line?

Ans.: To work on a control valve in-line, it needs to be bypassed, and the line must be drained and depressurized.

Que.117: What role does a valve positioner play?

Ans.: A valve positioner is employed for quick action control valves, addressing valve hysteresis, managing valves used with viscous liquids, handling split range situations, responding to line pressure changes, accommodating non-standard valve bench sets, and reversing valve operations.

Que.118: Under what conditions can a bypass not be used on a positioner?

Ans.: A bypass on a positioner cannot be used during split-range operation, with a reverse-acting positioner, or when the valve bench set is not standard.

Que.119: What purpose does a link connected to the valve positioner serve?

Ans.: The link serves as feedback to the valve, detecting any valve movement. It is essential for cases where line pressure changes may alter the valve position, allowing the positioner to correct and return the valve to its original position.

Que.120: Why are booster relays used?

Ans.: Booster relays, categorized into volume boosters, ratio relays, and reversing relays, serve as air-loaded, self-contained pressure regulators. They multiply or divide the pressure of an input signal, providing flexibility in various control applications.

Que.121: In what scenarios are angle valves and butterfly valves typically utilized?

Ans.: Angle valves are deployed in situations requiring high pressure drops and severe conditions to prevent erosion damage. Butterfly valves are employed in systems where a small pressure drop across the valve is acceptable, with the butterfly fully open at a 90-degree rotation.

Que.121: What is the purpose of three-way valves?

Ans.: Three-way control valves are used in special systems where a controlled ratio of flow division or mixture is required.

Que.122: Define a cage valve and its application.

Ans.: A cage valve utilizes a piston with a piston ring seal attached to the single-seated valve "plug." It is commonly used for noise reduction in valve applications.

Que.123: What advantages do Camflex valves offer?

Ans.: Camflex valves, positioned between globe and butterfly valves, feature a rotating plug for full opening. Their advantages include requiring minimal actuator forces, extended bonnet options for versatile service conditions, variations in flow control, and lightweight construction.

Que.124: Explain the different types of plugs used in valves.

Ans.: Commonly used plugs include V-port plugs for double-seated valves and contoured plugs for single-seated valves with small trim sizes.

Que.125: Enumerate the various valve characteristics.

Ans.: Valve characteristics can be linear, equal percentage, or quick opening. Linear characteristics show a proportional relationship between valve opening and flow rate, equal percentage provides equal increments in flow rate for equal valve opening increments, and quick opening results in higher flow rate increments at small valve openings.

Que.126: What is a solenoid valve, and where is it used?

Ans.: A solenoid valve is an electrically operated valve consisting of a solenoid (coil) with a magnetic plunger connected to the plug. It is used for safety purposes.

Que.127: How can you change the valve characteristics with a positioner?

Ans.: Valve characteristics can be changed by selecting the appropriate cam in the positioner. The cam influences the valve opening characteristics.

Que.128: How can you change the action of a control valve?

Ans.: To change the action of a control valve:

If the control valve lacks a bottom cap, adjust the actuator stem.
If a bottom cap is present, disconnect the stem, reverse the plug, reassemble, and recalibrate the valve.

Que.129: How do you select control valve characteristics?

Ans.: Selection is based on the desired relationship between plug position and flow through the valve. It considers a linear relationship over a wide range of pressure drops and aims for minimal pressure drop across the valve.

Que.130: If an operator reports a stuck control valve, what steps would you take to check it?

Ans.: 1. Bypass the control valve from operation. 2. Check the linkage to the diaphragm.

Disconnect the actuator stem, stroke the actuator, and check for operation.
If the actuator operates, reconnect it to the plug stem and check valve movement. If it doesn't, consider removing the control valve for further inspection.

Que.131: Where is air-to-close and air-to-open control valves commonly used?

Ans.: Air-to-close control valves are used in reflux lines, cooling water lines, and safety relief services. Air-to-open control valves are used in feed lines and steam service.

Que.132: Why do control valves operate at 15 psi?

Ans.: Control valves operate at 15 psi to maintain manageable actuator sizes while still producing sufficient force for effective control.

Que.133: Explain the cascade control system and its advantages.

Ans.: Cascade control involves two controllers in series, with a master or primary controller and a secondary or slave controller. It prevents outside disturbances from affecting the process immediately, leading to improved control quality and fast recovery from load changes.

Que.134: What is a ratio control system?

Ans.: A ratio control system ensures that variations in the secondary variable do not affect the primary variable. It maintains a proportional relationship between a primary uncontrollable flow and a secondary flow.

Que.135: Describe fuel-to-air ratio control in furnaces.

Ans.: Fuel-to-air ratio control in furnaces involves adjusting the ratio of fuel to air to optimize combustion efficiency and maintain desired operating conditions.

Que.136: What is furnace draft control?

Ans.: Furnace draft control is employed in boilers with balanced draft, aiming for a slight negative pressure at the top of the furnace to prevent hot gas leakage.

Que.137: Differentiate between feedforward and feedback control.

Ans.: Feedforward control responds to disturbances by predicting and compensating for errors instantly. Feedback control, on the other hand, detects a difference between the desired and actual results before taking corrective action.

Que.138: Explain the concepts of force balance and motion balance principles.

Ans.: Force balance involves controllers generating an output signal by opposing torques. Motion balance principles, on the other hand, generate an output signal through the motion of their parts.

Que.138: What is a diode, and how does it function?

Ans.: A diode is a two-electrode semiconductor device allowing current flow in only one direction. It functions by utilizing the properties of a P-N junction.

Que.139: Define half-wave, full-wave, and bridge rectifiers.

Ans.: Half-wave rectifiers allow current flow for only one-half of the AC cycle, full-wave rectifiers conduct during both halves, and bridge rectifiers utilize four diodes for full-wave rectification.

Que.140: What is a Zener diode, and how does it contribute to voltage regulation?

Ans.: A Zener diode operates in the breakdown region, maintaining a constant voltage despite changes in current. It plays a crucial role in voltage regulation.

Que.141: What is a transistor, and what are its different types?

Ans.: A transistor is a three-legged semiconductor device with P-N junctions. Types include PNP and NPN transistors.

Que.142: Explain CB, CE, and CC configurations in transistors.

Ans.: CB (Common Base), CE (Common Emitter), and CC (Common Collector) are different configurations in which transistors can be used, each offering specific advantages.

Que.143: How would you test a transistor using a multimeter?

Ans.: Testing a transistor involves checking resistance between certain transistor terminals under different polarities to ensure proper functioning.

Que.144: What is a thyristor, and what are its uses?

Ans.: A thyristor is a semiconductor device with internal feedback for latching action. It is used to control large amounts of load power in applications such as motors, heaters, and lighting systems.

These questions and answers cover a range of topics related to valves, control systems, electronics, and semiconductor devices.

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