INSTRUMENTATION AND CONTROL ENGINEERING

1) Define Viscosity. It is a measure of fluidity of the system. Many fluids undergo continuous deformation with the application of shearing stress. 2) Define Newtonian fluids If the force flow relation is linear then the fluid is Newtonian . 3) Define Non Newtonian fluids If the force flow relation is non linear then the fluid is Newtonian . 4) Define Kinematic Viscosity. Ratio of absolute viscosity to the density of the fluid. 5) Define Specific Viscosity. Ratio of absolute viscosity of the fluid to the absolute viscosity of a standard fluid at the same temperature. 6) Define Relative Viscosity. Ratio of absolute viscosity of the fluid at a given temperature to the absolute viscosity of a standard fluid at 20°c. 7) Define Viscosity index It is an empirical number that indicates the effect of change of temperature on viscosity if a fluid. 8) Define fluidity. It is the reciprocal of viscosity. It is unit is 1/ poise. 9) Define Humidity. It is basically moisture cont...

1. List any four objectives of process control. Suppressing the influence of external disturbances, Optimizing the performance, Increasing the productivity, Cost effective. 2. Define process Any system comprised of dynamic variables usually involved in manufacturing and production operations. It is defines as a series of operations during which some materials are placed in more useful state. 3. What is manipulated variable It is a variable which is altered by the automatic control equipment so as to change the variable under and make it conform with the desired value. 4. Define Controlled variable It is the quantity of control system which is directly measured and controlled. 5. What do you mean by self regulation? The output will move from one steady state to another for the sustained change in input. This means that for change in some input variable the output variable will rise until it reaches a steady state (inflow = outflow). It is the tendency of the process to adopt a ...

Admittance (Y) :- An admittance of a linear constant-parameter system is the ratio of the phasor equivalent of the steady-state sine-wave current or current-like quantity (response) to the phasor equivalent of the corresponding voltage or voltage-like quantity (driving force). Capacitance (C) :- Capacitance is that property of a system of conductors and dielectrics which permits the storage of electrically separated charges when potential differences exist between the conductors. Its value is expressed as the ratio of an electric charge to a potential difference. Coupling Coefficient (k) :- Coefficient of coupling (used only in the case of resistive, capacitive, and inductive coupling) is the ratio of the mutual impedance of the coupling to the square root of the product of the self-impedance of similar elements in the two circuit loops considered. Conductance (G):- 1. The conductance of an element, device, branch, network, or system is the factor by which the mean-square volt...

Foundation Fieldbus is an all-digital, serial, two-way communications system that serves as a Local Area Network (LAN) for factory/plant instrumentation and control devices. The Fieldbus environment is the base level group of the digital networks in the hierarchy of plant networks. Foundation Fieldbus is used in both process and manufacturing automation applications and has a built-in capability to distribute the control application across the network. Unlike proprietary network protocols, Foundation Fieldbus is neither owned by any individual company, nor regulated by a single nation or standards body. The Foundation Fieldbus, a not-for-profit organization consisting of more than 100 of the world's leading controls and instrumentation suppliers and end users, controls the technology. While Foundation Fieldbus retains many of the desirable features of the 4-20 mA analogue system, such as a standardized physical interface to the wire, bus-powered devices on a single wire, and intr...

HART stands for 'Highway Addressable Remote Transducer' and is a standard originally developed as a communications protocol for control field devices operating on a 4-20 mA control signal. The HART protocol uses 1200 baud Frequency Shift Keying (FSK) based on the Bell 202 standard to superimpose digital information on the conventional 4-20 mA analogue signal. Maintained by an independent organisation, the HART Communication Foundation, the HART protocol is an industry standard developed to define the communications protocol between intelligent field devices and a control system. HART is probably the most widely used digital communication protocol in the process industries, and: Is supported by all of the major suppliers of process field instruments. Preserves existing control strategies by allowing 4-20 mA signals to co-exist with digital communication on existing 2-wire loops. Is compatible with analogue devices. Provides important information for installation and mainte...

Q. What is a control system? A. A control system consists of subsystems and processes (or plants) assembled for the purpose of obtaining a desired output with desired performance, given a specified input. Q. When is a control system said to be robust? A. control system is said to be robust when 1. It has low sensitivities 2. It is stable over a wide range of parameter variations; and 3. The performance stays within prescribed (but practical) limit bounds in presence of changes in the parameters of the controlled system and disturbance input. Q. What is the main difference between open loop and closed loop systems? A. Open-loop systems, do not correct for disturbances and are simply commanded by the input. The closed-loop system compensates for disturbances by measuring the output response, feeding that measurement back through a feedback path, and comparing that response to the input at the summing junction. If there is any difference between the two responses, the system driv...

■ TRIP : The term trip refers to an action that is initiated by the control system and which forces a device or devices to a pre-determined state. Example of Trip Signals: Close Valve, Open Valve, Stop motor, etc. The Safety Instrument System (SIS) or a Hardwired systems normally initiate trips, however the PLCs or DCS may also initiate trips provided the necessary independence and SIL ratings are met.Once a device or devices have been forced to a pre-determined state by the action of a Trip they will remain in that state until the Trip is manually reset by a conscious operator action. For example: High level in a vessel initiates a trip system which stops the pump feeding that vessel, the pump will remain stopped even if the level in the vessel falls to a safe level. The Trip must be ‘reset’ by the operator before the pump can be re-started. The Trip can only be ‘reset’ if the level in the tank has fallen to a safe level. Resetting the Trip will no...

1.What are different types of orifice plates? State their uses. Different orifice plates are: 1. Concentric 2. Segmental 3. Eccentric – These plates are used for ideal liquid as well as gases and steam service. Concentric holes are present in these plates, thats why it is known as concentric orifice. Concentric: – This plate has hole in the form of segment of the circle. This plate is used for colloidal and sherry flow measurement. Segmental: – This plate has the eccentric holes. This plate is used in viscous and sherry flow measurement. Eccentric: Field instruments interview questions and answers 2.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. 3.Why is the orifice tab provided? Following reasons justify for providing orifice tab: 1. Indication of orifice plate in a line 2. The orifice diameter is marked on it. 3. The material of the ...

Instrument range shall be selected based on operating range of process to be measured. In common practice, it is recommended to select instrument with full scale range that can give operating process reading between 30% and 70% especially for pressure gauge. The following is an example of selecting pressure gauge range Given Process data: Minimum operating pressure: 5 barg Maximum operating pressure: 9.5 barg Maximum Design pressure: 25 barg Available standard Pressure Gauge range: 0 – 10 barg, 0 – 16 barg, 0 – 25 barg [X] Selecting 0-10 barg will give 95% reading during maximum operating condition. Instrument has only narrow margin above maximum operating pressure. [√] Selecting 0-16 barg range will give best reading during operating condition. However, the maximum overpressure of pressure gauge is commonly 1.3x of the maximum scale which is only 16×1.3 = 20.8 barg. In such case, pressure gauge shall be equipped with overrange protector to withstand ...

Control valve can not be without positioner. The purpose of the positioner is to control the control valve stroke so as to keep the valve in desired position. The positioner receives signal from the controller, and send the boosted signal to the actuator to reach the desired position as and when the valve reaches the desired position the positioner cuts the boosted signal to the actuator and keeps the position.

A ratio control system is characterized by the fact that variations in the secondary variable don’t reflect back on the primary variable. A ratio control system is the system where secondary flow is hold in some proportion to a primary uncontrollable flow. If we assume that the output of a primary transmitter is A. and the output of the secondary transmitter is B, and that the 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 off the relay.

1. Emitter +ve of meter and base -ve output =Low resistance 2. Emitter -ve of meter and base +ve output =High resistance 3. Collector +ve and base -ve output =Low 4. Collector -ve and base +ve output =Low Emitter: Collector = High resistance PNP: Opposite Results

A controller which generates an output signal by motion of its parts. The increase in the baffle is to move towards the nozzle. The nozzle back pressure will increase. This increase in the back pressure acting on the balancing bellows, will expand the bellows. The nozzle is moved upward due to this. The nozzle will move until motion almost equals the input baffle motion.

Force balance principle: A controller which generates an output signal by opposing torque. The input force is applied on the input bellows which moves the beam. This crackles nozzle back pressure. The nozzle back pressure is sensed by the balancing bellows which brings the beam to balance. The baffle movement is very less about 0.002 for full scale output. Advantages: a. Moving parts are fewer. b. Baffle movement is negligible c. Frictional losses are less

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 the air. 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 vapour in air mixture at the most ignited concentration.

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 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 a very slight negative pressure of the top of furnace.

Primary elements of measuring pressure are: a. Bourdon Tube b. Diaphragm c. Capsule d. Bellows e. Pressure springs These elements are known as elastic deformation pressure elements.

Valve positioner can be used for following reasons: a. Quick action b. Valve hysterisis c. Viscous liquids d. Split range. e. Line pressure changes on valve f. Bench set not standard g. Reverse valve operations

The most common range for differential range for liquid measurement is 0-100. This range is high enough to minimize the errors caused by unequal heads in the seal chambers. It is also dependent on the differences in the temperature of the load lines. The 100 range permits an increased in capacity up to 400. While decrease down up to 20 by merely changing the range tubes or range adjustments.

Turbine meters consist of straight flow tube within which a turbine or fan is free to rotate about it s axis which is fixed along g the centre line of the tube. Mostly, a magnetic pick up system senses the rotation of the rotor through the tube walls. The turbine meter is a flow rate device, since the rotor speed is directly proportional to the flow rate. The output is usually in the form of electric pulses from the magnetic pick up with a frequency proportional to the flow rate.

In automatic reference junction compensation, variable nickel resistor is used. As the temperature changes, so does its resistance. This reference junction compensator is located, so that it will be at the temperature of the reference junction. The reference junction is at the poset where the dissimilar wire of the thermocouple is rejoined. This joint is invariably at the terminal strip of the instrument.

In numerous applications, it is neither desirable nor practical to expose a temperature sensor directly to a material. Wells are therefore used to protect against damage corresion, arosion, aborsion and high pressure processes. A thermo well is also useful in protecting a sensor from physical damage during handling and normal operations. Materials used in thermo wells: Stainless steel, Inconel, Monel, Alloy Steel, Hastelloy

Flow varies directly as the square root of pressure. Thus, F=K of square root of applied pressure. Since this flow varies as the square root of differential pressure. The pressure pen does not directly indicate flow. Thus flow can be determined by taking the square root of the pen. Assume the pen reads 50% of the chart. So, flow can be calculated using the pen measure in the chart.

Bernoulli’s theorem states that, "Total energy of a liquid flowing from one point to another remains constant." It is applicable for non-compressible liquids. For different types of liquid flow Bernoulli’s equation changes. There is direct proportion between speed of fluid and its dynamic pressure and its kinetic energy. It can be used in various real life situations like measuring pressure on aircraft wing and calibrating the airspeed indicator. It can also be used to low pressure in the venturi tubes present in carburetor.

Following reasons justify for providing orifice tab: 1. Indication of orifice plate in a line 2. The orifice diameter is marked on it. 3. The material of the orifice plate. 4. The tag number of the orifice plate. 5. To mark the inlet of an orifice.

An orifice tab is welded on the orifice plate which extends out of the line giving an indication of the orifice plate.

Different orifice plates are: 1. Concentric 2. Segmental 3. Eccentric - Concentric: These plates are used for ideal liquid as well as gases and steam service. Concentric holes are present in these plates, thats why it is known as concentric orifice. - Segmental: This plate has hole in the form of segment of the circle. This plate is used for colloidal and sherry flow measurement. - Eccentric: This plate has the eccentric holes. This plate is used in viscous and sherry flow measurement.

Two most common temperature instruments used in process industry are RTD and thermocouple. To determine which one to use, the following should be considered: RTD is preferred if one of the following aspects becomes a concern in measurement: Accuracy, Stability, Sensitivity and Linearity. Thermocouple is preferred in application for high temperature measurement (more than 400 degC) or when exposed to shock or vibration. This is the reason why projects usually specify RTD for most process temperature measurement, while thermocouple is applied in heater, flare or for vibration monitoring of compressor or pump. Cost? It depends on the installation. Some panel or I/O card does not accept RTD directly so it requires a transmitter. On the other hand, extension wire would be an additional cost of thermocouple.