INDUSTRIAL

INSTRUMENTATION

AND

PROCESS CONTROL 1

This training system closes the gap between learning the fundamentals and applying them in companies

PI DIAGRAM

INTRODUCTION

Process Control & Instrumentation 1

The branch of control engineering that involves the regulation of variables, which affect products as they are being manufactured, is known as Process control. These variables include pressure,temperature, flow, Levels and the product composition of gases, liquids, and solids

During the early years of industrial manufacturing, the

process variable were controlled manually. Today, the

same operation are performed by automated system that

require only minimum human intervention.

The hardware used in an automation system is called

instrumentation equipment.

DEFINITION

Instrumentation is the art of measuring the value of some plant parameter, pressure, flow, level or temperature to name a few and supplying a signal that is proportional to the measured parameter. The output signals are standard signal and can then be processed by other equipment to provide indication, alarms or automatic control.

An instrument used in process control directly performs one or more of the following three functions:

Measurement: Measuring instrument are the eyes of the automated system function (SENSORS)

Control : The control section is the brain of automated system(PLC, CONTROLLERS, INVERTERS)

Manipulation: The final control element are the actuator, or muscle of an automated system (VALVES, MOTORS)

A basic instrument system consists of three elements:

i SENSOR or INPUT DEVICEii SIGNAL PROCESSORiii RECEIVER or OUTPUT DEVICE

Things that we commonly measure are:

Temperature

Pressure

Speed

Flow rate

Force, Movement

Velocity and Acceleration

Stress and Strain, Level or Depth

Mass or Weight, Density

Size or Volume, Acidity/Alkalinity

Sensors may operate simple on/off switches to detect the following:

Objects (Proximity switch) Empty or full (level switch)

Hot or cold (thermostat) Pressure high or low (pressure switch)

The block diagram of a sensor is shown below.

TEMPERATURE TRANSDUCERS

Thermal transducers sense and monitor changes in temperature.There are two primary types of thermal transducers.

The first type Measures internal resistance change due to temperature variation;RTDs, termistors.

The second type Measures voltage differentials as a result of temperature variation;thermocouples

THERMOCOUPLES

Thermocouple Advantages and Disadvantages

Advantages:

• Thermocouples are used on most transformers. The hot junction isinside the transformer oil and the cold junction at the meter mounted on the outside. With this simple & rugged installation, the meter directly reads the temperature rise of oil above the ambient temperature of the location.

• In general, thermocouples are used exclusively around the turbinehall because of their rugged construction and low cost.

• A thermocouple is capable of measuring a wider temperature rangethan an RTD.

Disadvantages:

• If the thermocouple is located some distance away from themeasuring device, expensive extension grade thermocouple wires orcompensating cables have to be used.

• Thermocouples are not used in areas where high radiation fields arepresent (for example, in the reactor vault). Radioactive radiation(e.g., Beta radiation from neutron activation), will induce a voltagein the thermocouple wires. Since the signal from thermocouple isalso a voltage, the induced voltage will cause an error in thetemperature transmitter output.

• Thermocouples are slower in response than RTDs

• If the control logic is remotely located and temperature transmitters(milli-volt to milli- amp transducers) are used, a power supplyfailure will of course cause faulty readings.

Failure Modes:

An open circuit in the thermocouple detector means that there is no path for current flow, thus it will cause a low (off-scale) temperature reading.

A short circuit in the thermocouple detector will also cause a lowtemperature reading because it creates a leakage current path to the ground and a smaller measured voltage.

RTD using a Wheatstone Bridge

Three-Wired RTD

RTD Advantages and Disadvantages

Advantages:• The response time compared to thermocouples is very fast . in the

order of fractions of a second.

• An RTD will not experience drift problems because it is not self powered.

• Within its range it is more accurate and has higher sensitivity than athermocouple.

• In an installation where long leads are required, the RTD does notrequire special extension cable.

• Unlike thermocouples, radioactive radiation (beta, gamma andneutrons) has minimal effect on RTDs since the parameter measuredis resistance, not voltage.

RTD Advantages and Disadvantages

Disadvantages:

• Because the metal used for a RTD must be in its purest form, theyare much more expensive than thermocouples.

• In general, an RTD is not capable of measuring as wide atemperature range as a thermocouple.

• A power supply failure can cause erroneous readings

• Small changes in resistance are being measured, thus all connectionsmust be tight and free of corrosion, which will create errors.

• Among the many uses in a nuclear station, RTDs can be found in thereactor area temperature measurement and fuel channel coolanttemperature.

Failure Modes:

• An open circuit in the RTD or in the wiring between the RTD and the bridge will cause a high temperature reading.

• Loss of power or a short within the RTD will cause a low temperature reading.

Thermal Wells

The process environment where temperature monitoring is required, is often not only hot, but also pressurized and possibly chemically corrosive or radioactive. To facilitate removal of the temperature sensors (RTD and TC), for examination or replacement and to provide mechanical protection, the sensors are usually mounted inside thermal wells.

RESISTANCE TYPE SENSORS

THERMISTOR.

RTDs

Platinum, nickel, copper & nickel-iron (PTC)

NTC

BIMETALLIC TYPES

Bimetallic type thermometer

Pressure measurement

P = F/A

PRESSURE SCALE

Comparison of different pressure scale

Comparison of readings by absoluteand gage measurement scales at a vacuum and at atmospheric pressure

PSI (pounds per square inch)

PRESSURE TRANSDUCERSPressure sensors either convert the pressure into mechanical movement or into an electrical output. Completegauges not only sense the pressure but indicate them on a dial or scale.

Mechanical movement is produced with the following elements.

ꞏ Bourdon Tube.ꞏ Spring and Piston.ꞏ Bellows and capsules.ꞏ Diaphragm.

BOURDON TUBE

Pressure gauge

PISTON TYPE

This type is usually used inhydraulics where the ability to withstand shock, vibration and sudden pressure changes is needed (shock proof gauge). The piston movement maybe connected to a secondary device to convert movement into an electricalsignal.

Capsules

CAPSULES AND BELLOWSA bellows is made of several capsules. These are hollow flattenedstructures made from thin metal plate. When pressurized the bellowsexpand and produce mechanical movement. They are very useful formeasuring small pressures.

DIAPHRAGMSThese are similar in principle to the capsule but the diaphragm isusually very thin and perhaps made of rubber. The movementis transmitted to a pointer on a dial through a fine mechanicallinkage.

Pressure transducers

How can you tap electrical signals

from mechanical pressure transducers

ELECTRICAL PRESSURE TRANSDUCERS

There are various ways of converting the mechanical movement of the preceding types into an electric signal. The following are types that directly produce an electric signal.

ꞏ Strain Gauge types.ꞏ Piezo electric types.ꞏ Other electric effects.

STRAIN GAUGE TYPES

PIEZO ELECTRIC TYPES

Optical type

Magnetic pick ups

TACHOMETERS

Differential Pressure Transmitters

Typical DP Transmitter Construction

Application

FLOW METERS

There are many hundreds of types of flow meters depending on the make and application. They may be classified roughly as follows.

ꞏ POSITIVE DISPLACEMENT TYPESꞏ INFERENTIAL TYPESꞏ VARIABLE AREA TYPESꞏ DIFFERENTIAL PRESSURE TYPES

POSITIVE DISPLACEMENT TYPES

These types have a mechanical element that makes the shaft of the meter rotate once for an exact known quantity of fluid.

ꞏ Rotary piston type.ꞏ Vane type.ꞏ Lobe type or meshing rotor.ꞏ Reciprocating piston typeꞏ Fluted spiral gear.

MESHING ROTOR

INFERENTIAL TYPE METERS

ꞏ Turbine rotor typesꞏ Rotary shunt typesꞏ Rotating vane typesꞏ Helical turbine types

The pictures show two industrial flow meters.

ROTATING VANE TYPE

The jet of fluid spins around the rotating vane and the speed of the rotor is measured mechanically or electronically.

Flow meter optical type

DIFFERENTIAL PRESSURE FLOW METERS

ꞏ ORIFICE METERS.

ꞏ VENTURI METERS

ꞏ NOZZLE METERS

ꞏ PITOT TUBES.

DP Transmitters

Resistive Pressure Transmitter

Orifice Plate with Flange Taps and Three Valve Manifold

Calibrated measuring assembly

Calibrated orifice plate

Orifice plate

concentric eccentric segmental

DEPTH GAUGES (measurement)230vac or 24v supply

Signal outputto controller

Capacitive probe

Hydrostatic level sensor

Open Tank Application for Bubbler System

Capacitive probe

When the tank is empty, the dielectric Is the air. As the tank fills, the nonconductiveLiquid and the air becomes the dielectric.

Conductive level probe

14-15

PANEL MOUNTED CONTROLLER

PANEL MOUNTED CONTROLLER OPERATION

0- 100% Output signal versus common dimensional ranges

Pressure reducing valveregulate pressure maintains constant desiredpressure.