INSTRUMENTATION AND CONTROL ENGINEERING

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.

Instrument engineer usually finds term ‘accuracy’ in some catalog and ‘inaccuracy’ in some others. To know whether there is a difference between both of them, try to understand the meaning of each. The accuracy of an instrument is a measure of how close the output reading of the instrument to the correct value. Inaccuracy does mean similar i.e. the degree to which a measurement might be error from the correct value. Let’s take a measurement device with 0-100 psig range and 2% of span accuracy. It does not mean the error will be 98 psig, instead the error will not be more than 2 psig. Although in this condition the term inaccuracy could be more suitable, yet most industry practice express the error reading from correct value as accuracy. The term accuracy and inaccuracy is often used interchangeably.

Instrumentation and control engineers works with the goal of improving, -Productivity -Optimization -Stability -Reliability -Safety -Continuity These engineers design, develop, and maintain and manage the instruments and the instrumentation systems.

An instrumentation and control engineer is required to do, - Design and develop control systems - Maintain the existing control systems - Manage the control systems - Collaborate with design engineers, purchasers and other staff members involved in the production processes - Manage projects within the given restraints including cost and time - Troubleshooting - Ensure that the instruments comply with health and safety regulations - Ensure that quality standards are maintained - Provide consultancy support

Instrumentation is the use of measuring instruments to monitor and control a process. It is the art and science of measurement and control of process variables within a production, laboratory, or manufacturing area. An instrument is a device that measures a physical quantity such as flow, temperature, level, distance, angle, or pressure.