3.plant instrumentation and control theory

PLANT INSTRUMENTATION AND CONTROL THEORY POWER PLANT FUNDAMENTAL

• Control system is an important mechanism which alters the state of the system based on the input to the system.

• Control system is to provide fine control over the desired range.

INTRODUCTION


Basics of control systemThe control system consists of three components

• Input•Logic operation•Output or decision device


THE INSTRUMENTATION Instrumentation is measuring devices for • Observation/ Monitoring /ControlAn instrument is a device that measures a physical quantity such as • Flow, • Temperature• Level,• Pressure.


TEMPERATURE INSTRUMENTS

Thermowell is a closed end metal tube that sticks into the chamber or fluid, and the thermometer sits in this well, making contact with its sides

Temperature categories: 1. Thermometers2. Probes 3. Non-contact


TEMPERATURE INSTRUMENTSTHERMOCOUPLE

• An electrical device consisting of two different conductors forming electrical junctions at differing temperatures.

• produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature.


TEMPERATURE INSTRUMENTSTHERMISTOR

Thermistor is an element with an electrical resistance that changes in response to temperature.Thermistors are semiconductor, meaning they have greater resistance than conducting materials, but lower resistance than insulating materials.


TEMPERATURE INSTRUMENTSRESISTANCE TEMPERATURE DETECTORS

RTDs - or Resistance Temperature Detectors - are temperature sensors RTDs work on a basic correlation between metals and temperature. As the temperature of a metal increases, the metal's resistance to the flow of electricity increases. Similarly, as the temperature of the RTD resistance element increases, the electrical resistance, measured in ohms (Ω), increases..


PRESSURE MEASURING DEVICES

The 3 methods are : • absolute • gauge • differential


PRESSURE INSTRUMENTSABSOLUTE PRESSURE

The absolute pressure is measured relative to the absolute zero pressure - the pressure that would occur at absolute vacuumPa (Pascal’s absolute) or psia (pounds per square inch absolute)


PRESSURE INSTRUMENTSGAUGE PRESSURE

A gauge is often used to measure the pressure difference between a system and the surrounding atmosphere. This pressure is often called the gauge pressure(Pascal’s gauge) or psig (pounds per square inch gauge) are used to describe gauge pressure.


PRESSURE INSTRUMENTSDIFFERENTIAL PRESSURE

• the reference point is another pressure point in the system rather than the ambient atmospheric pressure.

• relative pressure between two vessels such as a compressor tank and an associated feed line.

• Pad (Pascal’s differential) or PSID (pounds per square inch differential)


PRESSURE INSTRUMENTS

MANOMETER BOURDON GAUGE


LEVEL MEASURING DEVICES• Level measurement devices can detect, indicate or control

liquid or solid levels.• Level measurement sensors fall into two main types. 1. Point level measurement sensors

• functions as a high alarm, signaling an overfill condition, or as a marker for a low alarm condition.

2. Continuous level sensors • monitoring of an entire system. They measure fluid level within a

range, rather than at a one point, • To create a level management system, the output signal is linked

to a process control loop and to a visual indicator.


LEVEL INSTRUMENTSLEVEL MEASURING DEVICES

Level measurement determines the position of the level relative to the top or bottom of the process fluid storage vessel. A variety of technologies can be used, determined by the characteristics of the fluid and its process conditions.


FLOW MEASURING DEVICES• Flow measurement is the quantification of bulk fluid

movement. The most common principals for fluid flow metering are:

• Differential Pressure Flowmeters• Velocity Flowmeters• Positive Displacement Flowmeters• Mass Flowmeters• Open Channel Flowmeters


FLOW INSTRUMENTSFLOW MEASURING DEVICESOrifice PlateWith an orifice plate, the fluid flow is measured through the difference in pressure from the upstream side to the downstream side of a partially obstructed pipe. The plate obstructing the flow offers a precisely measured obstruction that narrows the pipe and forces the flowing fluid to constrict.


FLOW INSTRUMENTSVENTURI TUBE

The Venturi effect is the reduction in fluid pressure that results when a fluid flows through a choke of a pipe.


FLOW INSTRUMENTSFLOW NOZZLES

Flow nozzles are often used as measuring elements for air and gas flow in industrial applications.

3.plant instrumentation and control theory

Engineering