process instrumentation & control

Welcome to Instrumentation, Process Control and Process Instrumentation & Diagram

By: Dr. Zin Eddine Dadach

Chemical Engineering department

ADMC 1

The first part of this course introduces the students to the basics of electrical circuit theory followed by the latest process instrumentation technology and selection criteria.

This section explains the measurement of common process variables such as temperature, pressure, level and flow and describe their corresponding sensors.

A lab experiment on calibrating a manometer.

PART I: Instrumentation

2

Define and explain the various circuit components and describe the basic of electronic theory.

L.O. #1

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SENSORS, TRANSMITTERS AND CONVERTERS ARE ELECTRICAL AND ELECTRONIC DEVICES THAT TRANSFORM PHYSICAL PROPERTIES (PRESSURE, PRESSURE DROP, DISPLACEMENT, HEAT..) INTO ELECTRICAL CURRENT

IT IS THEREFORE NECESSARY TO STUDY SOME BASIC THEORIES OF ELECTRICITY

4

THE NEED OF ELECTRONIC IN INSTRUMENTATION

CURRENT AMPERE

5

WHAT IS CURRENT?

Electrical current is the movement of charged particles in a specific direction

The charged particle could be an electron ,a positive ion or a negative ion

The charged particle is often referred to as a current carrier

In a solid, the current carrier is the electron

The symbol for current is I

6

An ammeter is a measuring instrument used to measure the flow of electric current in a circuit.

Electric currents are measured in amperes, hence the name.

The word "ammeter" is commonly misspelled or mispronounced as "ampmeter" by some.

7

AMMETER?

http://en.wikipedia.org/wiki/Measuring_instrument

http://en.wikipedia.org/wiki/Electric_current

http://en.wikipedia.org/wiki/Electric_circuit

http://en.wikipedia.org/wiki/Ampere

More modern ammeters are digital, and use an analog to digital converter to measure the voltage across the shunt resistor.

The current is read by a microcomputer that performs the calculations to display the current through the resistor.

8

MODERN AMMETERS

http://en.wikipedia.org/wiki/Analog_to_digital_converter

Direct current (DC) is the unidirectional flow of electric charge. Direct current is produced by sources such as batteries, thermocouples, solar cells, and electric machines of the dynamo type.

DC: Direct Current

9

In alternating current (AC), the movement of electric charge periodically reverses direction. While in direct current (DC), the flow of electric charge is only in one direction.

Alternating Current

10

WHAT IS A VOLTAGE?VOLT

12

Voltage is the electric pressure OR POTENTIAL that causes current to flow.

Voltage is also known as electromotive force or emf or potential difference.

If there is no potential difference (V=0), there will be no current (I=0)

13

VOLTAGE =POTENTIAL

We need a unit to indicate the potential energybetween two points such as terminals of a battery.

This unit must specify the energy available ( JOULE is unit for energy) when a charge ( COULOMB is unit for charge) is transported.

The unit of voltage is : Volt= joule/coulomb

14

UNIT OF VOLTAGE?

The moving coil galvanometer is one example of this type of voltmeter. It employs a small coil of fine wire suspended in a strong magnetic field.

When an electrical current is applied, the galvanometer's indicator rotates and compresses a small spring.

The angular rotation is proportional to the current that is flowing through the coil

15

OHMMETER OR GALVANOMETER

16

OHMETER

The opposition a material offers to electrical current is called resistance

All materials offer some resistance to current

Resistance converts electric energy into heat

The symbol for resistance is R

The unit of resistance is the Ohm (Ω)

17

WHAT IS ELECTRICAL RESISTANCE?

Conductance refers to the ability to conduct current.

It is symbolized by letter G

The base unit for conductance is the siemens or S

CONDUCTANCE IS THE EXACT OPPOSITE OF RESISTANCE

R=1/G or G=1/R

18

WHAT IS CONDUCTANCE?

Materials with a big resistance are : INSULATORS or RESISTORS

Examples of insulators : paper, wood, plastics, rubber, glass and mica

Materials with a small resistance are : CONDUCTORS

Examples of conductors : Copper, aluminum, silver

19

CLASSIFICATION OF MATERIALS

OHM’LAW: The relationship between current (I), voltage ( V) and resistance (R) was discovered by the german Georg OHM

I= V/R

20

OHM‘S LAW?

How much current ( I) flows in a circuit where the voltage is 2.8 V and there is a resistance of 1.4 Ω in the circuit?

21

CLASS WORK #1:

How much voltage is required to cause 1.6 amperes in a device that has 30 ohms of resistance?

The current flowing through a 10 kΩ resistor is 35mA. What is the potential energy difference ( voltage) across the resistor?

A lamp has a resistance of 96 ohms . How much current flows through the lamp when it is connected to 120 volts?

A manufacturer specifies that a certain lamp will allow 0.8 ampere of current when 120 volts is applied to it. What is the resistance of the lamp?

22

Home work #1

Multi-loads

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A great majority of electrical circuits operate more than one load. Circuits which contain two or more loads are called multiple-load circuits.

A multiple-load circuit can be a series circuit, a parallel circuit or a series-parallel circuit

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INTRODUCTION

A series circuit is the simplest circuit.

The conductors, control and protection devices, loads, and power source are connected with only one path to ground for current flow.

The resistance of each device can be different.

The same amount of current will flow through each.

The voltage across each will be different.

If the path is broken, no current flows and no part of the circuit works

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A SERIES CIRCUIT

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SERIES CIRCUIT

VT= V1 +V2+V3+….

IT=I1=I2=I3+……

RT= R1 +R2+R3+….

Calculations

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LISTEN..LEARN..THINK..ENJOY YOURSELF 28

EXAMPLE OF CALCULATION

A parallel circuit has more than one path for current flow.

The same voltage is applied across each branch.

If the load resistance in each branch is the same, the current in each branch will be the same.

If the load resistance in each branch is different, the current in each branch will be different.

If one branch is broken, current will continue flowing to the other branches

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PARALLEL CIRCUIT

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PARALLEL CIRCUIT COMBINATION

VT=V1=V2=V3=…

IT=I1+I2+I3+…

(1/RT)= (1/R1) +(1/R2)+ (1/R3)+…

Calculations

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1) THREE RESISTANCES ( 35, 70 AND 45 OHMS) IN SERIES WITH A VOLTAGE SOURCE OF 90V,

CALCULATE : IT,RT,VR1,VR2,VR3

2) WHAT IS THE TOTAL RESISTANCE OF A SERIE OF TWO RESISTORS 20, 30 OHMS IN PARALLEL WITH A SECOND SERIE OF RESISTORS 70, 80 OHMS?


HOME WORK

Electrical energy is undoubtedly the primary source of energy consumption in any modern household.

Most electrical energy is supplied by commercial power generation plants like Tawillah

The most common power generation plants are fueled by : Fuel gas or fuel oil

35

WHAT IS ELECTRICAL ENERGY?

When a current flows in a circuit with resistance, it does work.

Devices can be made that convert this work into heat (electric heaters), light (light bulbs and neon lamps), or motion (electric motors)

P=W/t P is the power and the unit is watt,

W is energy in joules and t time in seconds

1 Watt = 1Joule/second.

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GENERAL DEFINITION OF POWER (P)

http://en.wikipedia.org/wiki/Electrical_resistance

http://en.wikipedia.org/wiki/Heat

http://en.wikipedia.org/wiki/Electric_heater

http://en.wikipedia.org/wiki/Light

http://en.wikipedia.org/wiki/Light_bulb

http://en.wikipedia.org/wiki/Neon_lamp

http://en.wikipedia.org/wiki/Kinetic_energy

http://en.wikipedia.org/wiki/Electric_motor

Electric power, like mechanical power, is represented by the letter P in electrical equations, and is measured in units called watts (symbol W).

P = I .V

where P = power in watts I = current in amperes V = potential difference in volts

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ELECTRICAL POWER

http://en.wikipedia.org/wiki/Watt

http://en.wikipedia.org/wiki/Watt

http://en.wikipedia.org/wiki/Ampere

http://en.wikipedia.org/wiki/Volt

Joule's law can be combined with Ohm's law to produce two more equations:

P= I2.R and

P=V2/R where

R = resistance in ohms. For example:

(2 amperes)2 × 6 ohms = 24 watts and

(12 volts)2 / 6 ohms = 24 watts

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ELECTRICAL POWER: OTHER FORMULAS

http://en.wikipedia.org/wiki/Ohm's_law

What is the power input to an electrical heater that draws 3 amperes from 120 volt outlet?

Find the power used by a resistor of 10 ohms when a voltage of 1.5 v is applied

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CLASS WORK #2:

How much power is dissipated when 0.2 ampere of current flows through a 100 ohms resistor?

How much energy is taken from the battery by the resistor ( 10 ohms) if the voltage is 1.5 V and the switch is closed for 30 min?

What is the cost of operating a 100 watt lamp for 3 hours if the rate is 6 cents per kWh?

An electrical iron operates from 120 volts outlet and draws 8 amperes of current. At 9 cents per kWh , how much does it cost to operate the iron for 2 hours

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HOMEWORK #2:

Measuring resistances, currents and voltages using multi-meters.

LAB #1: Basic electricity

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INSTRUMENTATION

FIRST AND THE MOST IMPORTANT STEP OF PROCESS CONTROL

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INSTRUMENTATION USE SENSORS LIKE THERMOCOUPLES, PRESSURE AND FLOW SENSORS TO MEASURE THE DIFFERENT PARAMETERS IN THE PLANT.

INFORMATION IS SENT TO THE CONTROLLER ( IN THE CONTROL ROOM) TO TAKE APPROPRIATE ACTIONS.

.

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DEFINITION OF INSTRUMENTATION

Measurements have got to be one of the most important equipment in any processing plant.

Since successful process control requires appropriate instrumentation, engineers should understand the principles of common instruments.

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GOOD INFORMATION=GOOD CONTROL

Like human body uses nerves, Sensors are used for process monitoring and for process control.

Sensors are essential elements of safe and profitable plant operation.

This can be achieved only if the proper sensors are selected and installed in the correct locations.

While sensors differ greatly in their physical principles, their selection can be guided by the analysis of a small set of issues .

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INSTRUMENTATION USE SENSORS

TEMPERATURE

PRESSURE

LEVEL

FLOW

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THE FOUR MOST IMPORTANT VARIABLES IN ANY INDUSTRIAL PLANT

Explain theory and apply the principles of temperature measurement and select the appropriate sensor for the application and discuss their common operating and troubleshooting problems.

L.O #2

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TEMPERATURE

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The temperature is the most important variable in a chemical process. Very often, the temperature should be controlled very precisely like:

In a reactor where the reaction outcome depends on the temperature’

For safety reasons where explosions can occur

Therefore, temperature need to be measured precisely with a very accurate sensor.

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INTRODUCTION

ITS-90 (International Temperature Scale of 1990- used as a worldwide practical temperature scale in national metrology labs like NIST, NPL et al).

50

INTERNATIONAL STANDARDS FOR TEMPERATURE MEASUREMENTS

Fluids and solids are composed of atoms or molecules

These atoms or molecules vibrate, rotate and move in general, the atoms have an average energy

When is cold, they move slowly and the energy is low

when it is hot, they move fast and the energy is high

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WHAT IS TEMPERATURE?

SCALES ARE INTERNATIONAL STANDARDS USED IN ALMOST ALL THE COUNTRIES

CELSIUS SCALE OR CENTIGRADE SCALE:

FROM 00C ( melting ice) TO 1000C ( boiling water) at 1 atm.

KELVIN SCALE :

0 K = -2730C

T (K)= T(0C) + 273

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SCALES FOR TEMPERATURE

AMERICAN SCALE:

RELATIONSHIP BETWEEN FAHRENHEIT AND CELSIUS SCALES :

320F = 00C

2120F= 1000C

T(0F)= 1.8xT(0C) +32

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FAHRENHEIT SCALE

RANKINE SCALE :

T(0R) = T(0F) + 460

T(0R) = 1.8 x T(K)

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RANKINE SCALE

Convert 1000C into : K, 0F,0R

Convert 50 K into:

0C, 0F,0R Convert -750F into:

0C, K, 0R Convert 0 0R into:

0C,0F, K

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HOME WORK

T = temperature

TI = Temperature Indicator ( in plant)

TT= Temperature Transmitter

TC= Temperature Controller

TRC= Temperature Recorder & Controller

TCV= Temperature Control Valve

TAG DESCRIPTORS FOR TEMPERATURE

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RTD= RESISTANCE TEMPERATURE DETECTOR

THERMISTOR= THERMAL RESISTORS

THERMOCOUPLES

Radiation pyrometers

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TEMPERATURE SENSORS USED FOR PROCESS CONTROL SYSTEMS

RESISTANCE TEMPERATURE DETECTORS

RTD

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A Resistance Temperature Detector (RTD) is a device with a significant temperature coefficient (that is, its resistance varies with temperature).

It is used as a temperature measurement device, usually by passing a low-level current through it and measuring the voltage drop.

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DEFINITION OF A RESISTANCE TEMPERATURE DETECTOR

The relationship between the resistance of a RTD and the temperature of the medium is the temperature coefficient α of the RTD .

coefficient α is also the sensitivity of the RTD

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TEMPERATURE COEFFICIENT α OF A RTD

α IS A LINEAR APPROXIMATION BETWEEN RTD RESISTANCE AND THE TEMPERATURE :

R(T)= R(TO) { 1+ α.ΔT} R(T)= approximation resistance at Temperature T R(T0)= resistance of RTD at T0

ΔT = T-T0

α depends on R(T0) and α> 0 because Metal resistance increases with temperature

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TEMPERATURE COEFFICIENT α OF A RTD

Platinum is very repeatable, quite sensitive and very expensive For Platinum, coefficient α is around 0.004/0C Example: for PRTD of 100 Ω, if the temperature increases by 10C, R(T)

changes by 0.4 Ω

Nickel is not quite as repeatable, more sensitive and less expensive

For Nickel, coefficient α is around 0.005/0C Example: For RTD of 100 Ω, if the temperature increases by 10C, R(T)

changes by 0.5 Ω

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SENSITIVITY α OF DIFFERENT METALS

RTD's are the best choice for repeatability, and are the most stable and accurate. However they have a slow response time and because they require a current source they do have a low amount of self heating.

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ADVANTAGES & DISATVANTAGES OF RTDs

RTDs work in a relatively small temperature domain, compared to thermocouples, typically from about

-200 °C to a practical maximum of about 650 to 700 °C.

Some makers claim wider ranges and some construction designs are limited to only a small portion of the usual range.

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RANGE OF TEMPERATURES FOR RTD

A special set of RTD’s are called PRT’s because they use platinum are a material

A special set of PRTs, called SPRTs, are used to perform the interpolation in such labs over the ranges 13.8033 K (Triple point of Equilibrium Hydrogen) to the Freezing point of Silver, 971.78 °C.

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RANGE OF TEMPERATURE FOR PRT ( PLATINUM RESISTANCE TEMPERATURE)

THERMal resisISTORSTHERMISTORS

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Thermistors are temperature sensors that use semiconductor materials not metals like RTD’s

R(T) = R(T0) {1+ α (T-T0)}

Semiconductors for temperature sensing have Negative

Temperature Coefficient (NTC) OR α< 0 Semiconductor becomes a better conductor of current.

Resistance decreases when the temperature increases.

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DEFINITION OF THERMAL RESISTORS

The characteristics of these devices are very different from those of RTD’s

Thermistors are the most sensitive and fastest temperature measurement devices.

Thermistors can be used for small range of temperatures

Thermistors are non-linear .

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PROPERTIES OF THERMISTORS

Because the resistance become too high at low temperature, the low limit is -1000C

Because the semiconductor can melt or be deteriorated at high temperatures, the high limit is 3000C

In most cases, the thermistor is encapsulated in plastic , epoxy, Teflon or some other material to protect the thermistor from the environment

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THERMISTOR’ S LIMITATIONS

Thermistors have a fast output and are relatively inexpensive but are fragile and have a limited range. They also require a current source and do experience more self heating than an RTD and are nonlinear.

ADVANTAGES & DISADVANTAGES OF THERMISTORS

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THERMOCOUPLES

71

When a pair of dissimilar metals are joined together for the purpose of measuring temperature, the device formed is called a thermocouple.

Thermocouples for instrumentation use metals of high purity for an accurate temperature/voltage relationship (as linear and as predictable as possible).

Thermocouples cover a range of temperatures from

-2620C to 27600C

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DEFINITION OF THERMOCOUPLES

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THERMOCOUPLE

Thermocouples suffer from 2 major problems that cause errors when using them

1) Small voltage generated EX: 10C temperature change on a platinum

thermocouple results of an output change of 5.8 μV

2) the non-linearity that requires polynomial conversion

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PROBLEMS OF THERMOCOUPLES

The voltage (emf) produced by a heated junction of two wires is directly proportional to the temperature.

This fairly linear relationship is called SEEBECK EFFECT

Thus, the Seebeck effect provides for us an electric method of temperature measurement

RTD’S AND THERMISTORS USE RESISTANCES FOR MEASUREMENT BUT THERMOCOUPLES USE VOLTAGE

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SEEBECK EFFECT

ε = α. ( T2-T1)WHERE:

ε= THE EMF

TYPES OF THERMOCOUPLES

α = SEEBECK COEFFICIENT

T2 ,T1= JUNCTION TEMPERATURE IN K

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SEEBECK COEFFICIENT

K = Chromel-alumel

Temperatures : -190 to 13710C

Seebeck Coefficient= 40 μV/0C

J = Iron-constantan


Seebeck Coefficient= 50 μV/0C

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T = Copper-constantan

Temperatures: -190 to 7600C

Seebeck coefficient : 50 μV/0C

E = Chromel-constantan


Seebeck coefficient: 60 μV/0C

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S= Platinum- 10% Rhodium/Pt

Temperatures: 0 to 17600C

Seebeck Coefficient: 10 μV/0C

R = Platinum-13%Rhodium/Pt

Temperatures: 0 to 16700C

Seebeck coefficient : 11 μV/0C

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Thermocouples are inexpensive, rugged, and have a fast response time but are less accurate and the least stable and sensitive. Thermocouples also read only relative temperature difference between the tip and the leads while RTD's and thermistors read absolute temperature.

ADVANTAGES AND DISDVANTAGES OF THERMOCOUPLES

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Temperature Measurement Comparison Chart

Criteria Thermocouple RTD Thermistor

Temp Range -267°C to 2316°C -240°C to 649°C -100°C to 500°C

Accuracy Good Best Good

Linearity Better Best Good

Sensitivity Good Better Best

Cost Best Good Better

COMPARISON BETWEEN THE DIFFERENT TEMPERATURE SENSORS

Temperature Measurement Comparison Chart

81

Find the seebeck emf (ε) for a thermocouple J with α. = 50 μV/0C if the junction temperatures are 20 and 1000C

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CLASS WORK

Objective of the lab: I) During the experiment: Reading of the temperature of the water being

heated and the corresponding values for the three temperature sensors.

II) After the lab, draw the three different calibration curves and find the sensitivity factor α for each sensor using the corresponding formula.

RTD = Resistance vs. Temperature Thermistors: Resistance vs. temperature Thermocouples = Voltage vs. Temperature

III) Write a lab report

LAB #2TEMPERATURE SENSORS

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EX: CALIBRATION CURVE OF THERMOCOUPLE

84

Explain theory and apply the principles of pressure measurement and select the appropriate sensor for the application and discuss technical issues including calibration.

L.O #3

85

PRESSURE MEASUREMENTCONTROL & SAFETY

86

Pressure is the second most important measurement in process control

Pressure is controlled for process reason but also for safety reason.

The most familiar device are manometers and gauges but they require a manual operator

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IMPORTANCE OF PRESSURE

DEFINITION OF PRESSURE

PRESSURE IS THE AMOUNT OF FORCE EXERTED ON A UNIT AREA OF A SUBSTANCE:

A

FP

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P= Pressure

PI= Pressure Indicator

PT= Pressure Transmitter

PC= Pressure controller

PRC= Pressure Recorder & Controller

PCV= Pressure Control Valve

PSV= Pressure Safety Valve

PRV= Pressure Relief Valve.

TAG DESCRIPTORS FOR PRESSURE

SI UNITS:1Pa = 1N/M2=1KG/S2.M1ATM (ATMOSPHERIC PRESSURE)= 1.01x105 Pa1 ATM= 101 kN/M2

1ATM= 760 MM. HG

US UNITS:1PSIA = 1LBF/IN2

1PSIA = 6894.7 Pa1ATM= 14.696 PSIA

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UNITS OF PRESSURE

STATIC PRESSURE IS FOR A FLUID WITH IS NOT IN MOTION

EX: FLUID IN A TANK

DYNAMIC PRESSURE IS FOR A FLUID IN MOTION IN PIPES

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STATIC VS DYNAMIC PRESSURE

P= F/S

F= m.g

P= mg/S = (mgxh)/ (Sxh)

=( mgh/V)

m/V= ρ

P= ρ.g.h

92

Hydrostatic or Static pressure

THE PRESSURE OF A FLUID IN A PIPE IS MEASURED BY A PRESSURE GAUGE.

FLOW CALCULATED BY BERNOUILLI EQUATION

93

DYNAMIC PRESSURE

IT IS EXTREMILY IMPORTANT TO MAKE THE DIFFERENCE BETWEEN THE ABSOLUTE AND RELATIVE PRESSURE

THE ABSOLUTE PRESSURE IS THE REAL PRESSURE OF THE FLUID WHERE THE RELATIVE PRESSURE IS THE PRESSURE WE READ IN A PRESSURE INDICATOR WITH REFERENCE THE ATMOSPHERIC PRESSURE

94

ABSOLUTE AND GAUGE PRESSURE

PA = PG + 1 ATM

EXAMPLE #1 :EXPRESS A PRESSURE GAUGE OF 155 KPa TO ABSOLUTE PRESSURE WHEN THE ATMOSPHERIC PRESSURE IS 98 Kpa

EXAMPLE #2: WHICH PRESSURE DO YOU READ IN A GAUGE MANOMETER FOR A PRESSURE OF 225 KPa ( ABSOLUTE ) WHEN ATMOSPHERIC PRESSURE IS 101 KPa

95

RELATIONSHIP BETWEEN PA AND PG

CLASS WORK

In many cases, gauge pressure is more important than the absolute pressure because we read gauge pressure in manometers.

Pg= Pabs- Patm96

GAUGE PRESSURE

PRESSURE INDICATORS

97

PRESSURE IS USUALLY MEASURED FOR INDICATION ONLY BY READING:

GAUGES

U TUBES

98

PRESSURE INDICATORS

A hard metal tube ( bronze or brass) is flattened and one end is closed. Under pressure, the tube is bent into a curve or arc.

The open end is attached to a header by which the pressure can ne introduced inside the tube

99

MANOMETER= GAUGE OR BOURDON TUBE

100

PRESSURE INDICATORS IN PLANTS

101

U-TUBES

I) A tank open to atmosphere holds water with a depth of 7 m. Density of water = 1000 kg/m3

a) What is the pressure in a gauge at the bottom of the tank in Pa ?

b) Draw the figure showing the manometers readings

102

CLASS WORK

in a closed tank under vacuum, the bottom pressure of an unknown liquid at 1.2 m depth is 12.55 kPa (absolute).

1) Draw a figure showing the manometer readings 2) What is the density of the fluid?

A crude oil, in a tank at 60 kPa top absolute pressure, has a specific gravity of 0.89 and a pressure of the bottom of 345 kPa ( gauge).

1) Draw a figure showing the manometer readings 2) What is the level of the oil in the tank ?

A fluid in a tank has a specific gravity of 0.76 and a absolute pressure at the top 150 kPa and a gauge pressure at the bottom of 140 kPa.

1) Draw a figure showing the manometer readings 2) What is the level of liquid in the tank?

HOMEWORK

103

CALIBRATION OF A MANOMETER BY MEASURING THE PRESSURE OF A GIVEN WEIGHT USING A HYDRAULIC OIL

USE DIFFERENT WEIGHTS

READ THE PRESSURES IN THE MANOMETER

APPLY THE FORMULA (P=m.g/S)

COMPARE the reading with the calculated PRESSURE and calculate the error

104

LAB #3 : calibration of manometers

LEVEL MEASUREMENT

105

Explain theory and apply the principles of level measurement and select the appropriate sensor for the application instruments and discuss technical problems including calibration.

L.O #4

106

In any chemical plant, you will find tanks, reservoirs, vessels and drums where liquids are stored. These could be for:The feed of the plantIntermediate between sectionsThe products before selling themLiquid capacities are also found in distillation

columns and reactors

107

LIQUID CAPACITIES IN A CHEMICAL PLANT

Level of liquid in a vessel should be maintained above the exit pipe because if the vessel empties the exit flow will become zero, a situation that could damage PUMPS.

A minimum level of liquid is then necessary to avoid cavitation of the pump

This minimum should be known (measured) and respected during the production

108

MINIMUM LEVEL

The level should also have a maximum value to:

not overflow an open vessel (safety for workers)

should not exit through a vapor line of a closed vessel, which could disturb a process designed for vapor ( safety for COMPRESSOR , TURBINES)

109

MAXIMUM LEVEL

L= Level

LI= Level Indicator

LT= Level Transmitter

LC= Level controller

LRC= Level Recorder & Controller

LCV= Level Control Valve

LLA and VLLA: Low level Alarm and Very…

HLA and VHLA: High Level Alarm and Very..

TAG DESCRIPTORS FOR LEVEL

110

Level measurement sensors are divided into two categories:

point level switches for ALARMS

continuous level gauges for CONTROL

111

LEVEL MEASUREMENT SENSORS

Point level is used mostly for SAFETY.

Will operate when the liquid is above or below a certain point.

Switches devices indicate when a vessel is full, empty or at intermediate level

You will have LLA ( low level Alarm) and HLA ( high level Alarm)

112

POINT LEVEL SWITCHES

Continuous level gauges provide information about material level at all points in the vessel

Continuous level gauges are used for control purpose

113

CONTINUOUS LEVEL GAUGES

Pressure ( hydrostatic)

Float

Nuclear

ultrasonic

114

SENSORS FOR CONTINUOUS LEVEL MEASUREMENT

Float

Capacitance

Conductive level probes

Thermal & light beam

115

SENSORS FOR POINT LEVEL MEASUREMENT

116

ULTRASONIC SENSOR (NO-CONTACT)

The differential pressure is the most commonly used for continuous level measurement of liquids.

a membrane is used where the value

H(Level)= ΔP/ρ.g

117

LEVEL MEASUREMENT BY HYDROSTATIC PRESSURE

118

TANK OPEN TO ATMOSPHERE

119

TANK UNDER PRESSURE OR VACUUM

A tank open to atmosphere holds water. The pressure at the bottom is 200 kPa ( absolute)

1) Draw the figure showing the tank and the differential pressure ’s reading

2) What is the level in the tank ?( density of water = 1000 kg/m3)

In a closed tank under vacuum and containing crude oil ( ρ= 780 kg/m3) , the bottom pressure is 12.55 kPa (absolute). 1) Draw a figure showing the tank and the differential

pressure ’s reading.2) What is the level in the tank?

120

Class Work

A crude oil, in a tank at 120 kPa top absolute pressure, has a specific gravity of 0.80 and a gauge pressure of the bottom of 345 kPa .1) Draw a figure showing the tank and the differential pressure s reading.2) What is the level in the tank?

A fluid in a tank has a specific gravity of 0.65 and a gauge pressure at the top 150 kPa and a absolute pressure at the bottom of 140 kPa.1) Draw a figure showing the tank and the differential pressure

’s reading.2) What is the level of liquid?

HOME WORK

121

FLOW MEASUREMENT

122

Explain theory and apply the principles of flow measurement and select the appropriate sensor for the application and discuss technical problems including calibration.

L.O #5

123

Quantity of fluid flowing in a system by unit time.

This quantity can be expressed in three ways:

Volume Flow rate ( Q) :Bring a flask and a stop watch to measure volumetric flow

Mass Flow rate ( M)

Weight Flow rate ( W)

124

WHAT IS FLOW?

F= Flow

FI= Flow Indicator

FT= Flow Transmitter

FC= Flow controller

FRC= Flow Recorder & Controller

FCV= Flow Control Valve

TAG DESCRIPTORS FOR LEVEL

125

If we know the volume flow rate Q, we can calculate the mass flow rate by : M=ρ.Q

If we know the volume flow rate Q, we can calculate the weight flow by : W=γ.Q

126

RELATIONSHIP BETWEEN FLOWS

The volume flow rate is the volume of fluid flowing past a section per unit time

In a pipe, we can have the relation: Q=A .v

(where v is the average velocity of flow)

Units used:

SI : EX: v (m /s) Q (m3/s)

US : EX: v (ft /s) Q(ft3/s)

127

Volume flow rate Q

An average flow rate of water produced by a plant is 11600 m3 /hr. Find the equivalent flow rate in m3/s, mass flow rate in kg/s ( density of water = 1000 kg/m3) and the weight flow rate ( Weight= Mass x gravity) and gravity = 9.8 m/s2

128

CLASS WORK (units)

A) MATERIAL BALANCE OF A PLANT: VERY VERY IMPORTANT Measure flow of feedsMeasure flow of productsWe should have : IN=OUT in mass ( Otherwise

we have leaks in the plant) B) FLOW IS A IMPORTANT VARIABLE FOR THE

SYSTEM ( EX:REACTOR) WHEN YOU HAVE A RATIO CONTROL SYSTEM

129

WHY WE NEED TO MEASURE FLOWS

In the instrumentation market, we find two types of flow-meters:

Energy-extractive Flow meters

Energy additive Flow meters

130

FLOW MEASUREMENT TECHNIQUES

Several sensors rely on the pressure drop or headoccurring as a fluid flows by a resistance.

131

THE PRINCIPLE OF FLOW SENSORS

ORIFICE

VENTURI TUBE

FLOW NOZZLE

ELBOW METER

PITOT TUBE

TURBINE

132

MOST IMPORTANT FLOW SENSORS

Bernouilli Equation

Old system : use low measurement devices that reduce the energy of the system. The differential pressure is used to measure flow using Bernoulli equation:

Applying Continuity equation: QA=QB ( assuming constant density). Find the relationship between flow ( You want to estimate) and ΔP ( your readings).

this relationship is used in Energy extractive flow meters as a conversion factor

22

2

1

2

1BB

BAA

A vg

zp

vg

zp

133

From Bernouilli Equation:

𝑄 = ∆𝑃.2( 𝐴1.

2 𝐴22 )

𝜌(𝐴12 −𝐴2

2)

Pressure drop in PaArea in m2

Density in kg/m3

Q in m3/s

Calculating volumetric flow rate Q

134

In a pipe of 0.3 diameter, water is flowing at 600C. We use a venturi tube to measure the flow rate. The venturi tube has a diameter of 0.2 m and we observe a pressure drop of 50 pa

What is the volume flow rate and the conversion factor?

What is the mass flow rate?

135

CLASS WORK

PROCESS CONTROL

136

Define the terms used in chemical process control and discuss the role and importance of process control systems in industrial plants.

Define P, PI and PID controllers

Explain feedback control and the dynamic behavior of this controller.

Apply the principles of feed-forward and show how this type of control can be applied.

Describe how the principles of cascade control, ratio, the selective control and split - range control are used in processes control.

Define the principles of computer control and distinguish between direct digital control and supervisory control.

Do experiments and write laboratory reports in a professional manner.

PART II: PROCESS CONTROL

137

L.O #1

Define the terms used in chemical process control and discuss the role and importance of process control systems in industrial plants.

138

139

THE SEVEN OBJECTIVES OF A CONTROL SYSTEM

• 1. Safety

• 2. Environmental Protection

• 3. Equipment protection

• 4. Smooth Operation and production rate

• 5. Product Quality

• 6. Profit

• 7. Monitoring and Diagnosis

Example

Heating up the temperature in the tank is a process that has the specific, desired outcome to reach and maintain a design value for the temperature (e.g. 80°C), kept constant over time.

The desired temperature (80°C) is the set point. The controller will manipulate the valve of hot water to maintain the room temperature at 800C.

140

http://en.wikipedia.org/wiki/Temperature

141EXAMPLE OF CONTROL SYSTEM

142WHAT ARE THE DESIGN VALUES?

THE DESIGN ENGINEERS CALCULATE THE VALUES OF SOME VERY IMPORTANT VARIABLES OF THE PROCESS THAT SHOULD BE MAINTAINED CONSTANT IN ORDER TO GIVE MAXIMUM PROFITABILITY BY RESPECTING SAFETY AND ENVIRONMENT ( OPTIMIZATION)

THESE CALCULATED VALUES ARE THEN INTRODUCED AS SET POINTS ( VALUES TO BE RESPECTED) IN THE CONTROLLER ONCE THE PLANT IS BUILT .

143

HOW ARE THE VALUES OF THE IMPORTANT VARIABLES ( SET POINTS) MADE CONSTANT?

ACTING ON SOME OTHER LESS IMPORTANTVARIABLES OF THE PROCESS IN ORDER TO SUPPRESS THE EFFECTS OF EXTERNAL DISTURBANCES ON THE IMPORTANTVARIABLES

Examples for Process Automation

144

Overview of Process Automation

The process is “that portion of an automation operation which use energy measurable by some quality such as pressure, temperature, level, flow, (and many others) to produce changes in quality or quantity of some material or energy.”

PROCESSSome Quality or Quantity

of theMaterial or Energy

Input Energy

or Material

DesiredResult

Example of a Temperature Process

Heating Element

Water BathTemperature

The objective of this process is to maintain a constant water bath temperature.

Temperature Process Terminology

Heating Element

Water BathTemperature

This is a Temperature Process

The measuring means is the thermometer. (Temperature Indicator- TI)

The process temperature is maintained at a desired point (Set Point – SP)

Steam (Control Agent) is used to vary the temperature by opening and closing the control valve (Final Control Element)

Level Process

Oil Stock

Level Indicator

Oil Feed to next process

The control objective is to maintain a constant liquid level of oil inside the tank (e.g. 100 gallons +/- 20 gallons). The hand valve is opened and closed as required to maintain the desired tank level.

Terminology used to describe the process

PROCESS: Level

CONTROLLED VARIABLE: Level by Head pressure at bottom of tank

CONTROL POINT: The level of oil in the tank (Set Point = 100 gallons)

MEASURING MEANS: Level Indicator (Head Pressure)

MANIPULATED AGENT: Volume of oil stock

MANIPULATED VARIABLE: Flow rate of oil (gpm)

Oil Stock

Level Indicator

Oil Feed to next process

Basic Model of a Process

The process is maintained at the desired point (SP) by changing the FCE based on the value of the PV

Manipulated Variable

DesiredResult

Control Agent

PROCESS

(Temperature, pressure, level, flow)

FINAL CONTROL ELELMENT

(valve)

Measuring Means

(transmitter)

Process Variable (PV)

Controlled Variable

Actuating Input

pH, conductivity, humidity, density, consistency, etc.

Process equilibrium (balance) is when the input energy maintains the output at a constant “desired” point.

Basic Model of a Process

The measuring means provides the standardized signal that represents the condition of the process, i.e. is the process at the desired point?


DesiredResult

Control Agent

PROCESS



(valve)

Measuring Means

(transmitter)


Controlled Variable

Actuating Input


Review of Measuring Means

Pressure

Level

Flow

Temperature

ThermocouplesRTDs / ThermistorsFilled SystemsBi-metallic

Strain gaugePiezo-electricCapacitanceBourdon Tube

Head meters (orifice, venturi)Coriolis, velocity, Mass,

Mechanical FloatsGuided WaveWeight (load cell)UltrasonicDifferential Pressure

Transmitters

Pressure Transmitter

Level Transmitter

Differential Pressure Cell

Flow Transmitter

Temperature Transmitter

Pneumatic3-15 PSI

Electrical

Current4 – 20 mA0 – 20 mA10 – 50 mA

Voltage

0 – 5 V

1 – 5 V

0 – 10 V

Digital

ON/OFF

Field Bus

ModBus

ProfiBus

HART

Manual ControlOpen loop (or manual control) is used when very

little change occurs in the Process Variable (PV)


DesiredResult

Control Agent

PROCESS



(valve)

Measuring Means

(transmitter)


Controlled Variable

Actuating Input


Corrective action is provided by manual feedback

THE FOUR BASIC STEPS OF A PROCESS CONTROL SYSTEM

HOW DOES IT WORK?

154

155

THE FIRST STEP: TAKING THE INFORMATION

IF WE DO NOT KNOW WHAT IS WRONG, HOW CAN WE CONTROL ?

TAKING INFORMATION OF THE IMPORTANT VARIABLES ( Design Values) OF THE PROCESS.

156IN OUR CASE:

Temperature of the tank has to be controlled.

Temperature SHOULD FIRST BE MEASURED.

THE EQUIPMENT FOR temperature MEASUREMENT IS : thermocouple

157

THE SECOND STEP OF A PROCESS CONTROL SYSTEM: TRANSMISSION OF THE INFORMATION

LINK BETWEEN THE PLANT AND THE CONTROL ROOM)

THE MEASUREMENT OF THE CONTROLLED VARIABLE IS SENT TO THE CONTROLLER IN THE CONTROL ROOM.

THE EQUIPMENT FOR TRANSMISSION IS THE TRANSMITTER

Thermocouple is also a transmitter

IN OUR CASE:

THE ANALOG SIGNAL OF THE VALUE OF FB ( MEASURED VARIABLE) IS TRANSMITTED TO A/D CONVERTER

THE RESULTING DIGITAL SIGNAL IS SENT TO THE CONTROLLER (digital or computer software)

WHY A/D CONVERTER?

158

159

THE THIRD STEP :THE CONTROLLER MAKE DECISION

THE THIRD STEP IS THE CONTROLLER IN THE CONTROL ROOM THE CONTROLLER:

1) RECEIVE THE INFORMATION FROM THE PLANT2) COMPARE IT WITH THE SET POINT

3) CALCULATE THE DIFFERENCE ε BETWEEN THE SET POINT AND THE INFORMATION. 4) MAKE A DECISION FOR ACTION TO BE TAKEN IN THE PLANT.

IN OUR CASE:

THE CONTROLLER WILL FIRST COMPARE T ( MEASURED VARIABLE) TO ITS SET POINT TSP.

THE CONTROLLER WILL THEN CALCULATE THEIR DIFFERENCE ε =( TSP-T)

THIS DIFFERENCE ε IS MULTIPLIED BY A FACTOR K DEPENDING ON THE TYPE OF CONTROLLER ( P,PI OR PID TO BE STUDIED LATER)

160

161

THE FOURTH STEP: ACTION ON A CONTROL VALVE OR MOTOR IN THE PLANT

A SIGNAL FROM THE CONTROLLER, RELATED TO THE DIFFERENCE ε IS SENT TO THE VALVE TO MANIPULATE THE FLOWRATE OF STEAM WHICH IS A LESS IMPORTANT VARIABLE

THE VALVE IS THE FOURTH AND LAST EQUIPMENT OF THE PROCESS CONTROL SYSTEM

THE FLOW OF STEAM IS THE MANIPULATED VARIABLE.

162IN OUR CASE TO ELIMINATE THE EFFECTS OF THE SURRONDINGS (

DISTURBANCES) ON THE IMPORTANT VARIABLE TEMPERATURE WHICH IS MEASURED

TO BRING T AS CLOSE AS POSSIBLE TO ITS SET POINT VALUE TSP THE CONTROLLER ACT ON ANOTHER VARIABLE FA CALLED MANIPULATED VARIABLE

163BLOCK DIAGRAM OF A PROCESS CONTROL SYSTEM

CLASS WORK

We want to produce ammonia from nitrogen and hydrogen in a reactor where the temperature should be maintained constant by a coolant in a jacket around the reactor.

Draw the process

Draw the process control system

Show the FOUR steps of the control loop

164

THE DIFFERENT CONTROL ACTIONS

165

EXAMPLE OF OPEN LOOP SYSTEM : SYSTEM WITH NO CONTROL

level

LISTEN..LEARN..THINK..ENJOY YOURSELF

166

166

http://www.atp.ruhr-uni-bochum.de/DynLAB/dynlabmodules/Examples/WhatIsControl/WaterLevel1.html

LEVEL WITH SET POINT BUT NO CONTROL

LEVEL2


167

167


A SYSTEM WITH CONTROLCLOSED LOOP

168

169A) MANUAL CONTROL DURING START UP AND SHUT DOWN: OPERATOR CONTROL THE

PLANT OPERATIONS

LEVEL3


170AUTOMATIC CONTROL

DURING OPERATING CONDITIONS: THE CONTROLLER TAKES ACTIONS

ON AND OFF CONTROLLER:

CONTROLLER TAKES ACTION ONLY WHEN THE MINIMUM AND THE MAXIMUM OF THE LEVEL ARE REACHED

NOT USED VERY OFTEN ONLY IN SIMPLE SITUATIONS WHEN SAFETY AND PRODUCTIVITY ARE NOT AFFECTED

LEVEL4


171CONTINUOUS AUTOMATIC CONTROL:

THE MOST USED CONTROLLERS: PROPORTIONAL ( P)

PROPORTIONAL- INTEGRAL ( PI)

PROPORTIONAL-INTEGRAL-DERIVATIVE ( PID)

172CONTROL SYSTEM: P,PI,PID CHANGE THE SET POINT OF THE LEVEL AND OBSERVE THE

BEHAVIOR OF THE PROCESS

LEVEL5


THE DIFFERENT FUNCTIONS OF A PROCESS CONTROL LOOP

Between the measuring device and the final control element, we have different steps and each step has its own function

THE SENSOR : the output ym(t) of the sensor is related to the real value in the controlled variable y (t) by a transfer function

THE TRANSMITTER : The value yt (t) entering the controller is related to ym(t) by a transfer function ( we have delay in the information)


173

173

DIFFERENT FUNCTIONS

THE CONTROLLER : after comparing to the set point ySP , the input to the controller is then ε (t) = ySP- ym(t). The output c(t) is related to

ε (t) by a transfer function of the controller (P,PI,PID)

The way c(t) and ε (t) are related depends on the type of controller ( TO BE STUDIED LATER)

THE VALVE: The output signal of the valve is related to c(t) by a transfer function depending on the type of the valve


174

174

Lab #4:Demonstration lab

Demonstration lab for the pressure controller including:

1) The four steps

2) Converters P/I , I/P for electronic Controllers

3) A/D and D/A converters for digital controllers

175

VIDEO FOR BASIC STEPS

176

Closed Loop ControlClosed loop or feedback control provides a corrective

action based on the deviation between the PV and the SP

Automatic

Controller Output

(3-15 psi, 4-20mA etc)

CONTROLLINGMEANS


DesiredResult

Control Agent

PROCESS



(valve)

Measuring Means

(transmitter)

Controller Input (PV)

(3-15psi, 4-20mA etc)

Controlled Variable


Manual

SP

Controlling Means

Controllers provide the required control action to position the FCE at a point necessary to maintain the PV at the desired SP.

PID (single loop feedback controller)

DCS (distributed controllers)

PLC (programmable logic controllers)

Single Loop Feedback Control

1. Measuring Means

2. Controlling Means

3. Final Control Element

4. Temperature Process

Temperature Controller and Recorder

SensingBulb

Temperature Transmitter

Pneumatic Control Valve

Heat Exchanger

Steam

2

34

1

The TT provides the signal (PV) that represents the condition of the process being controlled. The TIC compares the PV to the SP and opens and closes the FCE to maintain the process at equilibrium.

Summary

Process automation makes use of instrumentation to maintain the process at some desired condition.

Common instrumentation used in a process loop are the measuring means (usually transmitters), the controlling means (usually a PID controller), and the Final Control Element (usually some type of valve)

The measuring means provides the feedback signal (PV) used in the process loop. The controlling means operates the FCE based on the difference between the PV and the SP.

Process equilibrium is maintained when the difference between the PV and SP is zero or constant (offset?)

NEXT?

What are transmitters?

What is PID? What are P&ID symbols?

What types of FCE are

there?

What am I doing here?

How do I measure?

Pressure Level

Temperature Flow

How do I tune a loop?

What is Integral action?

What is a?

FIC

TT

LRC

PRV

Should I use a 3-15 psi or 4-20 mA valve?

Check out

TRANSMITTERS, TRANSDUCERS AND CONVERTERS

182

In the context of industrial process control, a "transmitter" is a device that converts sensor measured units into an electrical signal then directs this data (via cabling or wirelessly) to be received by a display or instrumentation control device within the system.

183

Transmitters

Analog transmitters are the most commonly used type in most industrial sectors. The transmitter is connected to the rest of the system via 2 wires which create something know as the 'current loop.'

The two wires can be used for both powering the unit and for transmitting signals typically at a range of 4 mA to 20 mA

184

Analog Transmitter

In an increasing number of industrial situations wireless sensors are an appropriate upgrade to classic industrial transmitters. This is because current of generation sensors offer flexible system solutions which are ideal for temporary installations and in processes with moving parts/objects.

Such wireless sensor networks can be comprised of hundreds or thousands of intelligent sensors. This allows for complex network mapping that can provide advanced solutions to today's processing environments.

185

Wireless transmitters

If the measuring device is pneumatic and the controller is electronic: A P/I transducer is needed to transform a physical movement into electrical current.

The I/P transducer does the opposite direction but not very used because most controllers are now electronic or digital.

186

Transducers: P/I and I/P

If the controller is digital and the measuring device is pneumatic, we need:

1) convert pneumatic into electrical by P/I transducer 2) convert electrical to digital using A/D converter.

At the exit of the digital controller we need:

1) D/A is the valve is electrical 2) D/A + I/P is the valve is pneumatic

187

Converters: A/D and D/A

Define P, PI and PID controllers

L.O #2: CONTROLLERS

188

CONTROLLERS

THE HEART OF PROCESS CONTROL LOOP

189

P CONTROLLER IS PROPORTIONAL CONTROLLER

PI CONTROLLER IS PROPORTIONAL CONTROLLER WITH INTREGRAL ACTION

PID CONTROLLER IS PROPORTIONAL CONTROLLER WITH INTEGRAL ACTION AND DERIVATIVE ACTION.

DIFFERENT KINDS OF CONTROLLERS

190

PROPORTIONAL CONTROLLER

The proportional CONTROLER means that the controller output c(t) is linearly related to the error ε (t)

The proportional controller has a gain Kc or Proportional Band (PB) related by the formula (Kc= 100/PB)


191

191

http://www.20sim.com/webhelp4/library/signal/Control/PID_Control/Setpoint.htm

Chapter 15 - Process Control Methods 192

Proportional Band

Proportional band is defined as the percentage change in the controlled variable that causes the final correcting element to go through 100 percent of its range

PB = Controlled Variable % Change

Final Correcting Element % Change

PROPORTIONAL ACTION

The proportional action means that the controller output c(t) is linearly related to the error between set point (SP) and measurement of process output ym (t) :

c(t) = Kc .ε(t) = Kc (SP – ym(t) )

The proportional gain Kc of a analog controller can be adjusted by knob in the controller.

Direct or reverse actions ?


193

http://www.20sim.com/webhelp4/library/signal/Control/PID_Control/Setpoint.htm

SIGN OF THE GAIN KC

If he controller is direct acting the gain K is positive.

When the controller is reverse actingthe gain K is negative


194

PROPORTIONAL BAND

Proportional controllers are defined by their Proportional Band (PB) or the proportional gain (Kc)with PB =100/Kc

For pneumatic valves, we define Kcp which is the output from the controller to the valve. The range of the instrumentation pressure for pneumatic valves is 3 -15 psia.

For electrical valves, we define Kce which is the output from the controller to the valve. The range of the instrumentation current for electrical valves is 4-20 mA.


195

DIFFERENT SITUATIONS:

A) IF A FULL CHANGE IN THE CONTROLLED VARIABLE IS ALSO A FULL RANGE FOR THE VALVE , WE WILL HAVE:

PB= 100%/100%= 1= 100% ,KC=1

IF WE ARE CONTROLLING TEMPERATURE FOR A RANGE OF 60-100, WE WILL HAVE : Kcp = 0.3 PSIA/ DEGRE


196

B) IF A 10% CHANGE IN THE CONTROLLED VARIABLE GIVES A FULL RANGE IN THE 100% IN THE VALVE, WE WILL HAVE PB= 10%/100% = 10%

IF WE CONTROL TEMPERATURE FOR THE SAME TOTAL RANGE, 10% WILL BECOME 4 DEGRE AND WE WILL HAVE Kcp= 3PSIA/DEGRE

THE CONTROLLER IS MORE SENSITIVE


197

C) IF A 100% CHANGE IN THE CONTROLLED VARIABLE GIVES A 20% RANGE IN THE VALVE, WE WILL HAVE PB= 100%/20% = 500%, KC=0.2

IF WE CONTROL THE SAME TEMPERATURE , WE WILL HAVE Kcp= 0.06 PSIA/DEGRE

THE CONTROLLER IS LESS SENSITIVE


198

EXAMPLE #1

Let’s consider a control system for a temperature in a process where the output of the controller is a pressure signal to the final element or valve.

The controller is used to control temperature within the range of 600F to 1000F.

The controller is adjusted so that the output signal varies from 3 psi (valve fully open) to 15 psi (valve fully closed) as the controlled temperature (measured) varies from 710F to 750F.

FpsiFF

psipsipKcp

0

00/3

)7175(

)315(

%10100.)60100(

)7175(00

00

FF

FFPB


199

EXAMPLE #2

Now, if we consider a PB of 75% for the same range of 600F to 1000F, what will be the Gain Kc?

From the PB formula, we find ΔT ( the change of the measured variable)

From the Gain formula:

FFrangePBT 00 30)40.(75.0.

FpsiF

psipsiKcp

0

0/4.0

30

)315(


200

OFFSET OF PROPORTIONAL CONTROLLER

An important characteristic of a proportional controller is the OFFSET

In a proportional controller, there is always a residual error of the controlled variable.

It can be minimized by a large Kc which also reduce the PB

See figure 9-10 page 198


201

EXAMPLES OF USES OF A PROPORTIONAL CONTROLLER

Proportional controllers are mostly used for level controlwhere variations of the controlled variables carry no economical and where others control modes can easily destabilize the loop

It is actually recommended for controlling the level of a surge tank when manipulating the flow of the feed to a critical downstream process.


202

CHARACTERISTICS OF PROPORTIONAL CONTROLLER

Relationship between the output c(t) and error ε (t) is: c(t) = Kc .ε(t) = Kc .ε(t)

Proportional Controller gives always an Off-Set, which is a difference between the controlled variable and set point.

A proportional controller will have the effect of reducing the rise time but never eliminate THE OFF SET

Increasing the gain or decreasing the PB will eliminate decrease off set but gives fluctuations

203

We can reduce the off set by increasing the gain BUT if the gain is too high, the controller become too sensitive and we will experience fluctuations and instability.

GAIN AND OFF SET

204

Selecting the Right Proportional band or PB

That bit was the “hard part” to understand...

But it is not so difficult to understand if we take a look at what it does in the actual application... 205

PB too small

C°

(t)

SV

PB correct

C°

(t)

SV

PB too large

C°

(t)

SV

A Proportional Band that is too narrow causes hunting! The TC will than behave like an ON/OFF controller!

A correctly sized P-Band results in an Overshoot, followed by an Undershoot and than Stabilization, with a small offset near the Set Point.

With a (far) too large P-band the Setpoint temperature

will never be reached! (As the heater capacity will be reduced too much).

This will create a large offset from the Set Point!

P-Action.The right setting of PB is very important !

206

Lets have a look now what will happen if we add the PI controller

That explains the P-Action so far...

The “Integral Action”

207


The need of an Integral Action

Because of the introduction of offset in a control process, proportional control alone is often used in conjunction with Integral control.

Offset is the difference between set point and the measured value after corrective action has taken place


Integral or Reset Action

Integral control is also referred to as reset control as the set point is continuously reset as long as an error is present

Integral adjustments that affect the output are labeled 3 ways:

Gain - expressed as a whole number

Reset - Expressed in repeats per minute

Integral Time - Expressed in minutes per reset

PI controller is a Proportional controller in which integral action is added. It has then two constants:

A) PB

B) Integral time

An integral control will have the effect of eliminating the OFF SET , but it may make the response more oscillatory and needs longer to settle.

PI OR PROPORTIONAL INTEGRAL CONTROLLER

210

The output of the controller is related to the error ε (t) by the relationship:

c(t) = Kc { 1+ (1/τi.s) }. ε(t); τi is the integral time.

Integral action eliminates the off set but the response becomes more oscillatory and needs longer to settle down.

CHARACTERISTICS OF PI

211

As explained: The I-Action eliminates the Offset, but influences the whole process from the start ( fluctuations).

Making the Integral time shorter will give you more intense control with a quicker response to eliminate the offset. But a too short Integral time would result in “oscillation” (=hunting) !

Making the Integral time too long will reduce the possibility of hunting but will slow down your overall Process response. So the RIGHT setting is very important.

The right Integral Time

212

The setting of the right I-Time is very important !

0

20

40

60

80

100

120

140

°C

SP

PV @I=80s

PV @I=38s

PV @I=20s

SV:

100oC

The best way is to explain with a real control graph :

A too long I-Time slows down the whole Process

The RIGHT I-Time will enable the TC to reach the Setpoint quickly and to eliminate the Offset correctly.

Making the I-Time too short creates a (large) overshoot. Also takes a long time to correct:

Example of behaviour after a disturbance

213

Lets have a look now at the PID controller

Well.. That explains the “P+I Action”...

The “Differential Action”

214


Derivative Action

For rapid load changes, the derivative mode is typically used to prevent oscillation in a process system

The derivative mode responds to the rate of change of the error signal rather than its amplitude

Derivative mode is never used by itself, but in combination with other modes

Derivative action cannot remove offset

PID or Proportional Integral Derivative Controller

PID controller is a PI controller in which the derivative action is added. It has then three constants:

A) PB B) Integral time : τi C) Derivative : τd

A derivative action will have the effect of increasing the stability of the system, reducing the overshoot, and improving the transient response.

216

The relationship between the output of the controler and the error ε(t) is c(t) = Kc { 1+ (1/τi.s) + τd.s }. ε(t); τi is the integral time and τd is the derivative time

All design specifications can be reached.

CHARACTERISTICS OF PID

217

A too long D-Time leads to “excessive” response!Than we will Over- and Undershoot the setpoint.(Far too long D-time will create oscillation, like ON/OFF Controller)

A correctly sized D-Time results in a fast return to the Set Point. Could be followed by a small overshoot and than return rapidly to the Setpoint.

With a too short D-time the Process will behave like a PI (only) controller, so will have a (too) slow response to disturbances.Note: With a setting of D-Time of 0 sec, we will have a PI Controller!

The right setting of the D-Action is also very important !oC

oC

oC

The value of the D-Time is usually around ¼ of the I-Time. (For example: if the I-Time is 180sec., than the D-Time will be 45sec.)

218

CONCLUSION:These 3 actions combined:

* The “P-Action” * The “I-Action” * The “D-Action”

= PID controller.

That was a “tough part” to combine these 3 actions....

219

CL RESPONSE

RISE TIME-First Time to

reach set point

OVERSHOOT-Highest

value/set point value

SETTLING TIME-Time to be inside 5%

of set pointOFF SET

Kp Decrease Increase Small Change Decrease

τi Decrease Increase Increase Eliminate

τd Small Change Decrease Decrease Small Change

EFFECTS OF PB, INTEGRAL TIME AND DERIVATIVE TIME ON THE PROCESS

220


Control Mode Summary

The following additional explanation can also help to

understand the actions of the PID-controller:

• The “P-Action” deals with the “present”Depending on the deviation from the Setpoint:

more or less Output capacity will be given.

• The “I-Action” deals with the “past”

If we have been below setpoint: the Output will be increased.

If we have been above setpoint: the Output will be decreased.

• The “D-Action” deals with the “future”

If the controlled variable is going down: the Output will be increased.

If the controlled variable is going up: the Output will be decreased.

This “combination”, of “Present + Past + Future”,

makes it possible to control the application very well.222

TUNING THE CONTROLLER

The task of controller tuning is usually left to an

instrument technician with experience in the cause and

effect of process reaction and controller adjustments.

223

224

Usefulness of PID Controls

Most useful when a mathematical model of the plant is not available

Many different PID tuning rules available

Sources

K. Ogata, Modern Control Engineering, Fourth Edition, Prentice Hall, 2002, Chapter 10

IEEE Control Systems Magazine, Feb. 2006, Special issue on PID control

Proportional-integral-derivative (PID) control framework is a method to control uncertain systems

225

Type A PID Control

Transfer function of PID controller

The three term control signal

sT

sTK

sE

sUsG d

i

pPID

11

ssEKsEs

KsEKsU dip 1

226

PID-Controlled System

PID controller in forward path


Control Mode Summary


Tuning the Controller

Fine-tuning is the process to optimize the controller operation by adjusting the following settings:

Gain setting (proportional mode)

Reset rate (integral mode)

Rate (derivative mode)

Three steps are taken when tuning a systems

Study the control loop

Obtain clearance for tuning procedures

Confirm the correction operation of the system components

229

PID Tuning

Controller tuning---the process of selecting the controller parameters to meet given performance specifications

PID tuning rules---selecting controller parameter values based on experimental step responses of the controlled plant

The first PID tuning rules proposed by Ziegler and Nichols in 1942

Other resource: K. Ogata, Modern Control Engineering, Prentice Hall, Fourth Edition, 2002, Chapter 10


Trial-and-Error Tuning

Does not use mathematical methods, instead a chart recorder is used and several bump tests are made in the proportional and integral modes

Trial-and-error tuning is very time consuming and requires considerable experience on the part of the technician or operator

231

Ziegler-Nichols Tuning Methods

Two formal procedures for tuning control loops:

Step response of plant

Continuous cycling method

232

PID Tuning---First Method

Start with obtaining the step response

233

The S-shaped Step Response

Parameters of the S-shaped step response

234

The S-Shaped Step Response

The S-shaped curve may be characterized by two parameters: delay time L and time constant T

The transfer function of such a plant may be approximated by a first-order system with a transport delay

1

Ts

Ke

sU

sC Ls

235

PID Tuning---First Method

236

Transfer Function of PID Controller Tuned Using the First Method

237

Ziegler-Nichols PID Tuning---Second Method

Use the proportional controller to force sustained oscillations


Continuous Cycling Method

The continuous cycling method analyzes the process by forcing the controlled variable to oscillate in even, continuous cycles

The time duration of one cycle is called an ultimate period. The proportional setting that causes the cycling is called the ultimate proportional value

These two values are then used in mathematical formulas to calculate the controller settings

For a set point change : set the proportional band to high value and reduce this value to the point where the system becomes unstable

The proportional band that required causing continuous oscillation is the ultimate value PBu.

The ultimate periodic time is Pu.

From these two values the optimum setting can be calculated.

239

ULTIMATE PROPORTIONAL BAND


Continuous Cycle Calculations

Proportional only controller

Proportional Gain

Kc = Gu x 0.5

KC = proportional gain,

Gu= ultimate gain

Proportional Band

PB = Pbu x 2

PB = proportional band

PBu = ultimate proportional band

The frequency of continuous oscillation is the cross over

frequency ωco

Pu= 2Π/ωco

241

Pu = Ultimate period of sustained cycle

242

Graphic method to Find Pu or Pcr

Measure the period of sustained oscillation

243

PID Tuning

244

Transfer Function of PID Controller Tuned Using the Second Method


Ziegler-Nichols Reaction Curve Tuning Method

This method avoids the forced oscillations that are found in the continuous cycle tuning method

Cycling should be avoided if the process is hazardous or critical

This method uses step changes and the rate at which the process reacts is recorded

The graph produces three different values used in mathematical calculations to determine the proper controller settings


Reaction Curve Tuning Formulas

CONTROL VALVES

FINAL CONTROL ELEMENT

247

Final Control Elements

These are some devices the controller operates:

Pneumatic valves, solenoid valves, rotary valves, motors, switches, relays, variable frequency drives.

Control valves are valves used to control conditions such as flow, pressure, temperature, and liquid level by fully or partially opening or closing in response to signals received from controllers that compare a "set-point" to a "process control variable" whose value is provided by sensors that monitor changes in such conditions

249

Definition

The opening or closing of control valves is usually done automatically by electrical, hydraulic or pneumatic actuators.

Positioners are used to control the opening or closing of the actuator based on electric, or pneumatic signals.

These control signals, traditionally based on 3-15psi (Pneumatic Valves), more common now are 4-20mA ( Electrical Valves) for industry, 0-10V for HVAC systems.

The introduction of "Smart" systems, HART, Fieldbus Foundation, and Profibus being the more common protocols.

250

Types of Control Valves

251

Actuator & positioner of a control valve

Control valves are used by automated systems to adjust flow rates.

The adjustments are dependent on the controlling system's setup. They can be automated based on sensor data and presets or manually controlled by an operator at a remote workstation.

For pneumatic valves, an actuator changes the current from the controller into pressure.

The relationship of current and pressure is calculated based on the process specifications and the equipment used.

This system will be designed by control vendors or in-house engineers in most cases.

252

ELECTRICAL OR PNEUMATIC CONTROL VALVES?

When an issue develops in a manufacturing process, the control valve will be designed to move into an open or closed position.

The safer option is dictated based on the process and the process stream involved.

For this reason, valves that require energy to be open, are called: Air or electricity to open Fail-close Reverse Acting

The valves that require energy to be closed, are called: Air or electricity to close Fail-open Direct Acting

253

Fail-Open and Fail-Close Valves

Control valves

Reverse acting: Fail-close or Air to Open

Direct Acting: Fail-Open or Air to close

254

Fail-open valves will open and continue to allow flow when the control valve loses energy in a failure situation.

For example, a valve might fail open to avoid allowing pressure of non-harmful gas to build up.

Cooling system control valves will usually fail open, since in most cases overcooling a system will not harm the equipment.

When a failure causes energy to be lost, fail-close valves will close to keep streams contained until they can be checked and fixed.

Toxic streams will almost always fail closed to prevent contamination.

Reactor heating streams usually fail closed in order to avoid feeding energy to runaway reactions.

255

Examples for Fail –open & Fail-close Valves

256

Flow Characteristics of the Control Valve

The relationship between control valve capacity and valve stem travel is known as the Flow Characteristic of the Control Valve.

Trim design of the valve affects how the control valve capacity changes as the valve moves through its complete travel.

Because of the variation in trim design, many valves are not linear in nature. Valve trims are instead designed, or characterized, in order to meet the large variety of control application needs.

Many control loops have inherent non linearity's, which may be possible to compensate selecting the control valve trim.

The most common characteristics are shown in the next figure.

The percent of flow through the valve is plotted against valve stem position. The curves shown are typical of those available from valve manufacturers.

These curves are based on constant pressure drop across the valve and are called inherent flow characteristics.

257

Flow Characteristics

258

Inherent Flow characteristics Curves

When valves are installed with pumps, piping and fittings, and other process equipment, the pressure drop across the valve will vary as the plug moves through its travel.

When the actual flow in a system is plotted against valve opening, the curve is called the Installed Flow Characteristic.

259

Installed Flow Characteristics

In most applications, when the valve opens, and the resistance due to fluids flow decreases the pressure drop across the valve. This moves the inherent characteristic:

•A linear inherent curve will in general resemble a quick opening characteristic

•An equal percentage curve will in general resemble a linear curve

260

Installed flow Characteristics

VALVES

261

Valve Types

Ball Valve

Butterfly Valve

Gate Valve

Globe Valve

Check Valve

262

Ball ValveSphere with a port in a housing, rotate to expose channel.

Applications: Flow control, pressure control, shutoff, corrosive fluids, liquids, gases, high temp.

Advantages – low pressure drop, low leakage, small, rapid opening

Disadvantages – seat can wear if used for throttling, quick open may cause hammer263

Ball Valve

264

Gate Valve

Sliding disk, perpendicular to flow

Applications: Stop valves, (not throttling), high

pressure and temp, not for slurries, viscous

fluids

Advantages – low pressure drop when fully

open, tight seal when closed, free of

contamination buildup

Disadvantages – vibration when partially open,

slow response and large actuating force265

Gate Valve

266

Butterfly Valve

rotating disk on a shaft, in a housing

Low pressure, large diameter lines where leakage is unimportant

Advantages – low pressure drop, small and light weight

Disadvantages – high leakage, high actuation forces so limited to low pressures 267

Butterfly Valve

268

Check Valves

allows flow in only one direction

Swing valve similar to butterfly except hinged along one edge rather than rotate about the diameter, used primarily for check valves.

269

Check valve

270

Rupture Disk (not a valve – ruptures at a set

pressure)

271

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http://www.bsbsystems.com/products/forward_acting_disks/forward_acting_disks.html

http://www.bsbsystems.com/products/graphite_disks.html

http://www.bsbsystems.com/products/graphite_disks.html

PROCESS CONTROL SYSTEMS

272

Servo & Regulator Problems

Two major problems could happen in any plant:

1) REGULATOR: The most common situation is when a disturbance appears in the plant. The controller will make correction to bring the controlled variable to set point.

2) SERVO: Very often, operators in the control room will have to change the set point of some controlled variable. How the controller will bring the controlled variable to the new set point.

Both situations will be investigated in the labs 5 & 6.

273

LABS #5 & 6Controlling pressure in a tank using

digital P, PI and PID digital controllers Tuning of a P, PI and PID controller to maintain

the pressure in a water tank constant during servo or regulator situations:

Lab #5: The main objective of the lab is to analyze and compare the graphs of the P,PI and PID controllers.

Lab #6: Study constants of controllers to avoid instability in the plant.

274

L.O #3

Explain feedback control and the dynamic behavior of this controller.

275

276

Driving your car

SenseVehicle Speed

ComputeControl “Law”

ActuateGas Pedal

Goals

Stability: system maintains desired operating point (hold steady speed)

Performance: system responds rapidly to changes (accelerate to 65 mph)

Robustness: system tolerates perturbations in dynamics (mass, drag, etc)

Basic Feed back Control

House is too coldFurnace

Thermostat Controllerrecognized the house is too cold

sends signal to the furnace to turn on and heat the house

furnace turns onheats the housenatural

gas

house temperaturemeasured

is temperature below setpoint?

Set-point = 200C

Controlled variable: temperature (desired output)Input variable: temperature (measured by thermometer in thermostat)Set-point: user-defined desired setting (temperature)Manipulated variable: natural gas valve to furnace (subject to control)

277

Output of the system y(t) is fed back to the set-pint r(t) through measurement of a sensor

Controller senses the difference between the set point and the output and determines the error ε(t)

Controller changes the manipulated variable u to Process to eliminate the error.

Feedback Control is a Single Loop

278

Example #2 for Feedback Control

Examples: Room temperature control Automatic cruise control Steering an automobile Supply and demand of chemical engineers

Controller

Transmitter

Set point

stream

Temp

sensorHeat loss

condensate

Feedback Control-block diagram

Terminology: Set point Manipulated variable (MV) Controlled variable (CV) Disturbance or load (DV) Process controller

Σ Controller process

Sensor +

transmitter

+

-Set point

Measured value

error

Manipulated

variable

Controlled variable

disturbance

281

THE ELEMENTS OF A FFEDBACK PROCESS CONTROL SYSTEM

LEVEL6:


282

A modern Feedback Control System

Feedback control is not predictive: Controlled variable has to be affected before controller takes action

Requires management or operators to change set points to optimize system:

- Changes can bring instability into system

- Optimization of many input and output variables almost impossible

Limitations of Feedback Control

283

Apply the principles of feed-forward and show how this type of control can be applied.

L.O #4

284

285

FEED-FORWARD CONTROL

The feedback control can never achieve perfect control of a chemical process

Why? Because the feedback control reacts only when it has detected a deviation of the CONTROLLED VARIABLE from the desired set point.

However, the feed-forward control measures the disturbance directly and takes control action to eliminate its impact on the CONTROLLED VARIABLE

Therefore Feed-forward controllers have the theoretical potential to achieve perfect control

Feedforward Control

Window is openFurnace

FeedforwardRecognize window is open and house will get cold in the future:

Someone reacts and changes controller setpoint to turn on the furnace preemptively.

furnace turns onheats the housenatural

gas

house temperatureis currently OK

turn on furnace

Decreasesetpoint to turnfurnace on

Pre-emptive moveto prevent house from getting cold

286

Feed-forward control avoids slowness of feedback control

Disturbances are measured and accounted for before they have time to affect the system In the house example, a feed-forward system measured the fact

that the window is opened

As a result, automatically turn on the heater before the house can get too cold

Difficulty with feed-forward control: effects of disturbances must be perfectly predicted

There must not be any surprise effects of disturbances

Feed-forward is a single loop

287

288

THE FEEDBACK AND FEED FORWARD CONTROL

Both control involve a single loop with :

One measurement

One manipulated variable.

However:

In a feedback control, we measure the controlled variable

In a feed-forward control, we measure the disturbance

L.O #5

Describe how the principles of cascade control, ratio, the selective control and split - range control are used in processes control.

289

MULTI LOOPS PROCESS CONTROL SYSTEM

290

291

CONTROL SYSTEMS WITH MULTIPLE LOOPS

Other simples configurations which may use:

* More than one measurable variable and one manipulated variable

* One measurable variable and more than one manipulated variable

CASCADE CONTROL

In this configuration, we have :

More than one measurement

One manipulated variable

292

293

CASCADE CONTROL LOOPS

Cascade control is two control loops using two different measurements :

1) One measurement for the controlled variable 2) One measurement for the disturbance 3) One manipulated variable

The loop that measures the controlled variable is the dominant or primary or master control loop

The loop that measures the disturbance is the secondary or slave loop

CASCADE FOR HEAT EXCHANGER

294

Cascade for jacketed CSTR

TRC

FC

Tc

T, Ca

W

Set Point

Wc

2A B

Ratio Control is a special type of feed-forward control

Two disturbances are measured and held in a constant ratio

It is mostly used to control the ratio of flow-rates of two streams

RATIO CONTROL :

296

EX: RATIO CONTROLLER IN A BURNER

297

RATIO CONTROL LOOP

298

We measure both flow-rates and take their ratio

The ratio is compared to the desired ratio

The error is sent to the ratio controller

Strategy of ratio control:

299

300

SELECTIVE CONTROL

In this kind of control, we One manipulated variable Several controlled output

Since with one manipulated variable, we can control only one output, The selective control systems transfer control action from one controlled output to another according to needwe will discuss

* Override Control* Auctioneering control

301

SAFETY OF EQUIPMENTS: OVERRIDE CONTROL

During the normal operation of a plant or during its startup or shutdown , it is possible that a dangerous situation may arise and may lead to destruction of equipment.

In such cases, it is necessary to change from production control to safety control in order to prevent a process variable from exceeding an allowable upper or lower limit

This can be achieved by the use of switches: The switch is used to select between the production controller and the safety controller.

The HSS ( high selector switch) is used whenever a variable should not exceed an upper limit

The LSS ( low selector switch) is used whenever a variable should not exceed a lower limit.

EXAMPLE OF OVERRIDE

The steam header must be maintained above a minimum pressure (PC FOR SAFETY). Steam from the header is used to heat water in a heat exchanger.

The temperature of the hot water is controlled by TIC-101(PRODUCTION CONTROLLER)

SAFETY FIRST: t is more important that the header pressure be above its minimum than that the water temperature be at its set-point.

302

304

SAFETY OF EQUIPMENTS: AUCTIONEERING CONTROL

In this control system, among several similar measurements, the one with the highest value will feed the controller

This is a selective control between several measured variables.

EX: TUBULAR REACTOR

305

The split range control has

One measurement only ( Controlled variable)

More than one manipulated variable ( control valve)

If the valves are pneumatic: The instrumentation pressure range ( 3-15 psia) is divided.

If the valves are electrical: The instrumentation current ( 4-20 mA) is divided.

Ex: If we have two pneumatic valves:

Valve #1 will operate between 3- 9 psia and Valve #2 will operate between 9 -15 psia.

SPLIT RANGE CONTROL

306

Split Range Flow Control

In certain applications, a single flow control loop cannot provide accurate flow metering over the full range of operation.

Split range flow control uses two flow controllers (one with a small control valve and one with a large control valve) in parallel.

At low flow rates, the large valve is closed and the small valve provides accurate flow control.

At large flow rates, both valve are open.

307

EX: Split Range Temperature Control

TT

Cooling

Water

Steam

Split-Range

Temperature

Controller

TT TC

RSP

308

L.O #6: DIGITAL CONTROL SYSTEM

THE CONTROLLER IS A SOFTWARE

IN COMPUTER

309

DIGITAL CONTROLLER

Digital control is a branch of control theory that uses digital computers to act as system controllers.

Depending on the requirements, a digital control system can take the form of a microcontroller to an ASIC to a standard desktop computer.

Since a digital computer is a discrete system, the Laplace

310

PLC: Programmable Logic Controller

CPU

System

User LadderDiagram

Working memoryregisters

Input

Flag

Output

Input Module

OutputModule

311

DIGITAL CONTROLLER

Typically, a digital controller requires:

A/D conversion to convert analog inputs to machine readable (digital) format

D/A conversion to convert digital outputs to a form that can be input to a plant (analog)

A program that relates the outputs to the inputs

312

313

Block diagram of a digital control system

control:

difference

equations

D/A and

hold

sensor

1

r(t) u(kT) u(t)e(kT)

+-

r(kT) plant

G(s)

y(t)

clock

A/DT

T

y(kT)

digital controller

voltage → bit

bit → voltage

An Large Size PLC

The main module measures

19” x 20” x 14.5”.

have upto 10,000 I/O points

supports all functions

expansion slots to

accommodate PC and other

communication devices.

Allen-Bradley PLC-3

314

A Small Size PLC

Measures 4.72”x 3.15” x

1.57”.

32 I/O points

Standard RS 232 serial

communication port

Allen-Bradley MicroLogix 1000

315

PLC ARCHITECTUREProgrammable controllers replace most of the

relay panel wiring by software programming.

ProcessorI/O Modules

MemoryPower Supply

Program Loader

Printer

Cassette Loader

EPROM Loader

Switches

Machines

Peripherals External Devices

PC

A typical PLC316

PLC COMPONENTS

1. Processor Microprocessor based, may allow arithmetic operations, logic operators, block memory moves, computer interface, local area network, functions, etc.

2. Memory Measured in words.

ROM (Read Only Memory),

RAM (Random Access Memory),

PROM (Programmable Read Only Memory),

EEPROM (Electric Erasable Programmable ROM),

EPROM (Erasable Programmable Read Only Memory),

EAPROM (Electronically Alterable Programmable

Read Only Memory), and

Bubble Memory. 317

PLC COMPONENTS3. I/O Modular plug-in periphery

AC voltage input and output,

DC voltage input and output,

Low level analog input,

High level analog input and output,

Special purpose modules, e.g., high speed timers,

Stepping motor controllers, etc. PID, Motion

4. Power supply AC power

5. Peripheral hand-held programmer (HHP)

CRT programmer

operator console

printer

simulator

EPROM loader

graphics processor

network communication interface

modular PC

318

Distributed Control Systems

319

Distributed Control Systems

Collection of hardware and instrumentation necessary for implementing control systems

Provide the infrastructure (platform) for implementing advanced control algorithms

History of Control Hardware

Pneumatic Implementation:

Transmission: the signals transmitted pneumatically are slow responding and susceptible to interference.

Calculation: Mechanical computation devices must be relatively simple and tend to wear out quickly.

History (cont.)

Electron analog implementation:

Transmission: analog signals are susceptible to noise, and signal quality degrades over long transmission line.

Calculation: the type of computations possible with electronic analog devices is still limited.

History (cont.)

Digital Implementation:

Transmission: Digital signals are far less sensitive to noise.

Calculation: The computational devices are digital computers.

Advantages of Digital System

Digital computers are more flexible because they are programmable and no limitation to the complexity of the computations it can carry out.

Digital systems are more precise.

Digital system cost less to install and maintain

Digital data in electronic files can be printed out, displayed on color terminals, stored in highly compressed form.

Computer Control Networks

1. PC Control:

Good for small processes such as laboratory prototype or pilot plants, where the number of control loops is relatively small

PROCESS

Final

control

element

Data

acquisition

Main

Computer

Display

Computer Control Networks

2. Programmable Logic Controllers:

specialized for non-continuous systems such as batch processes.

It can be used when interlocks are required; e.g., a flow control loop cannot be actuated unless a pump has been turned on.

During startup or shutdown of continuous processes.

DCS: Computer Control Networks

Operator

Control

Panel

Main

Control

Computer

Operator

Control

Panel

Archival

Data

Storage

Supervisory (host)

Computer

PROCESS

Local

Computer

Local

Computer

Local

Computer

Local Display Local Display

Data highwayTo other Processes To other Processes

Local data acquisition and

control computers

3. DCS

•Most comprehensive

DCS Elements-1

Local Control Unit: This unit can handle 8 to 16 individual PID loops.

Data Acquisition Unit: Digital (discrete) and analog I/O can be handle.

Batch Sequencing Unit: This unit controls a timing counters, arbitrary function generators, and internal logic.

Local Display: This device provides analog display stations, and video display for readout.

Bulk Memory Unit: This unit is used to store and recall process data.

DCS Elements-2

General Purpose Computer : This unit is programmed by a customer or third party to perform optimization, advance control, expert system, etc

Central Operator Display: This unit typically contain several consoles for operator communication with the system, and multiple video color graphics display units

Data Highway : A serial digital data transmission link connecting all other components in the system. It allow for redundant data highway to reduce the risk of data loss

Local area Network (LAN)

Advantages of DCS

Access a large amount of current information from the data highway.

Monitoring trends of past process conditions.

Readily install new on-line measurements together with local computers.

Alternate quickly among standard control strategies and readjust controller parameters in software.

A sight full engineer can use the flexibility of the framework to implement his latest controller design ideas on the host computer.

Modes of Computer control

signals from digital

computer

Local PID

controller

Supervisory Control mode

Direct digital Control mode

valve setting

from computer

Flow measurement

to computer

1.Manual

2.Automatic

• PID with local set point

3.Supervisory

• PID with remote set

point (supervisory)

4.Advanced

Additional Advantage

Digital DCS systems are more flexible. Control algorithms can be changed and control configuration can be modified without having rewiring the system.

Categories of process information

ExampleType

Relay, Switch

Solenoid valve

Motor drive

1. Digital

Alphanumerical displays2. Generalized digital

Turbine flow meter

Stepping motor

3. Pulse

Thermocouple or strain gauge (mill volt)

Process instrumentation (4-20 am)

Other sensors (0-5 Volt)

4. Analog

A/D and D/A converters or transducers are the Interface between digital computer and analog

instruments

(A/D) Transducers convert analog signals to digital signals.

(Sensor Computer)

(D/A) Transducers convert digital signals to analog signals.

(Computer Valve)

Data resolution due to digitization

Accuracy depends on resolution.

Resolution depends on number of bits:

Resolution = signal range × 1/(2m -1)

m = number of bits used by the digitizer (A/D) to represent the analog data

Data Resolution

Signal = 0 - 1 Volt, 3 bit digitizer:

Analog range

covered

Analog

equivalent

Digital

Equivalent

Binary

representation

0 to 1/14

1/14 to 3/14

3/14 to 5/14

5/14 to 7/14

7/14 to 9/14

9/14 to 11/14

11/14 to 13/14

13/14 to

14/14

0

1/7

2/7

3/7

4/7

5/7

6/7

1

0

1

2

3

4

5

6

7

0 0 0

0 0 1

0 1 0

0 1 1

1 0 0

1 0 1

1 1 0

1 1 1

Data Resolution

0 1/7 2/7 3/7 4/7 5/7 6/7 1

0

1

2

3

4

5

6

7

Analog data

Dig

ital data

Utilization of DCS

DCS vendor job:

installation

Control Engineer Job:

Configuration

Built-in PID control:

How to Tune the PID control?

Utilization of DCS

Implementation of advanced control:

Developed software for control algorithms, DMC, Aspen, etc.

Control-oriented programming language supplied by the DCS vendors.

Self-developed programs using high-level programming languages (Fortran, C++)

Advanced control topology

DCS Vendors

Honeywell

Fisher-Rosemont

Baily

Foxboro

Yokogawa

Siemen

PI&D AND PFD

342

Process flow diagrams (PFDs) are used in chemical and process engineering. These diagrams show the flow of chemicals and the equipment involved in the process.

Generally, a Process Flow Diagram shows only the major equipment and doesn't show details. PFDs are used for visitor information and new employee training.

DEFINITION OF PFD

343

A Process and Instrument Drawing (P&ID) includes more details than a PFD. It includes major and minor flows, control loops and instrumentation.

P&ID is sometimes referred to as a Piping and Instrumentation Drawing. These diagrams are also called flow-sheets.

P&IDs are used by process technicians and instrument and electrical, mechanical, safety, and engineering personnel.

DEFINITION OF PI&D

344

PFD & PI&D

In both diagrams arrows show the flow of material and symbols show tanks, valves, and other equipment. The symbols used vary somewhat from organization to organization. So you may see several different symbols that all represent a motor. 345

EXAMPLE OF PFD

346

EQUIVALENT PI&D

347

P& ID SYMBOLS

ISA Symbols and Loop Diagrams

348

Piping and Instrumentation Diagrams or simply P&IDs are the “schematics” used in the field of instrumentation and control (Automation)

The P&ID is used to by field techs, engineers, and operators to better understand the process and how the instrumentation is inter connected.

INTRODUCTION

349

Most industries have standardized the symbols according to the ISA Standard S5.1 Instrumentation Symbol Specification.

Piping & Instrumentation Drawing (original)

Process & Instrumentation Diagram (also used)

Process Flow Diagram – PFD (simplified version of the P&ID)

ISA Standard S5.1 Instrumentation Symbol Specification

350

Building the P&ID using examples like pressure or temperature control.

L.O #1

351

From the typical example, define Tag Numbers and Tag Descriptors.

L.O #2

354

Learn the ISA S5.1 Identification Letters like (TI, PC, LR, TRC, …)

L.O #3

357

Class work: identify the following

359

Determine the instrumentation location

L.O #4

360

Learn about Shared Displays/Shared Control and draw a summary of instrument type & location

L.O #5

362

PIPING & CONNECTIONS SYMBOLS

L.O #6 : Identify Piping, Connection and valves Symbols

365

VALVE SYMBOLS

367

Different industrial examples will be studied.

L.O #7

369

CLASS WORK

371

SOLUTION

372

PFD: PROCESS FLOW DIAGRAM

373

PFD Drawing Symbols

375

Other Symbols for PFD

Table 1.2 : Conventions Used for Identifying Process Equipment

Process Equipment General Format XX-YZZ A/B

XX are the identification letters for the equipment classification

C - Compressor or Turbine

E - Heat Exchanger

H - Fired Heater

P - Pump

R - Reactor

T - Tower

TK - Storage Tank

V - Vessel

Y designates an area within the plant

ZZ are the number designation for each item in an equipment class

A/B identifies parallel units or backup units not shown on a PFD

Supplemental Information

Additional description of equipment given on top of PFD376

Equipment Numbering

XX-YZZ A/B/…

XX represents a 1- or 2-letter designation for the equipment (P = pump)

Y is the 1 or 2 digit unit number (1-99)

ZZ designates the equipment number for the unit (1-99)

A/B/… represents the presence of spare equipment

377

Examples

T-905 is the 5th tower in unit nine hundred

P-301 A/B is the 1st Pump in unit three hundred plus a spare

378

Equipment Information

Equipment are identified by number and a label (name) positioned above the equipment on the PFD

Basic data such as size and key data are included in a separate table (Equipment Summary Table).

379

Streams identification

380

Process and Utility Streams

381

Number of streams and information

Stream Number 1 2 3 4 5 6 7 8 9 10

Temperature (°C) 25 59 25 225 41 600 41 38 654 90

Pressure (bar) 1.90 25.8 25.5 25.2 25.5 25.0 25.5 23.9 24.0 2.6

Vapor Fraction 0.0 0.0 1.00 1.0 1.0 1.0 1.0 1.0 1.0 0.0

Mass Flow (tonne/h) 10.0 13.3 0.82 20.5 6.41 20.5 0.36 9.2 20.9 11.6

Mole Flow (kmol/h) 108.7 144.2 301.0 1204.4 758.8 1204.4 42.6 1100.8 1247.0 142.2

Component Mole Flow

(kmol/h)

Hydrogen 0.0 0.0 286.0 735.4 449.4 735.4 25.2 651.9 652.6 0.02

Methane 0.0 0.0 15.0 317.3 302.2 317.3 16.95 438.3 442.3 0.88

Benzene 0.0 1.0 0.0 7.6 6.6 7.6 0.37 9.55 116.0 106.3

Toluene 108.7 143.2 0.0 144.0 0.7 144.0 0.04 1.05 36.0 35.0

A Portion of Table 1.5

382

Class work: identify all the information

383

CASE STUDY FOR THE COURSE

384

SEPARATION PROCESS: THE DIFFERENT SECTIONS OF THE

PROCESS

385

386

THE SECTIONS OF THE UNIT

The natural gas liquefaction ( NGL) is divided into 4 sections:

- The feed preparation Section

- The expansion and separation Section

- The recovery Section

- The propane refrigerant Section

387

THE FEED PREPARATION SECTION

The feed preparation section contains:

- Feed Gas Scrubber D-405

- Feed Gas separator D-401

- The feed gas/ gas off exchanger E-401

- The gas and liquid dehydrators

Purpose of the section: To remove any liquid from the inlet gas and to prepare the inlet feed gas for cooling and separation

388

EXPANSION AND SEPARATION SECTION

The expansion and separation section contains:- High Level Gas Chiller E-402- Low Level Gas Chiller E-404- Chiller Separator D-403- Expander Feed Separator D-404- Expander KT-100- Cold gas/ off gas exchanger E-405- Intermediate gas/off gas exchanger E-403

Purpose of the section: Cool the inlet feed gas in a series of heat exchangers and to provide several feeds at various temperatures to the demethanizer D-402

389THE SEPARATION SECTION

The separation section contains:

- Demethanizer Column D-402

- Demethanizer Reboiler E-406

- Compression Section K-100 and KT-100

- Two product booster pumps P-401A/B

390

REFRIGERATION SECTION

Propane refrigerant is used

Cooling takes place in the high Level Chiller E-402 and Low Level Chiller E-404

GROUP WORK: STUDY THE PROCESS CONTROL SYSTEM

391

392

CONTROL OF THE FEED PREPARATION SECTION ( SCRUBBER)

TI-350: indicates inlet feed gas to gas Scrubber D-405 ( 0-650C)

HC-350: This is a manual valve to control the flow of inlet gas to the feed gas scrubber D-405

HS-400: This hand switch initiates the plant emergency shut down by closing the inlet feed valve HC-350

393

FI-405: Indicates the inlet feed gas vapor from the top of the scrubber D-405 ( 0 -1600 kNm3/d

FI-406: Indicates the liquid flow from the bottom of D-405 ( 0-50 m3/h)

LC-350: this controls the level of liquid in the scrubber D-405 by regulating the flow of methane to the D-402. Flow shown by FI-406

394

CONTROL OF THE FEED GAS SEPARATOR D-401

LC-411: controls the liquid level in the feed gas separator D-401 by regulating the flow of liquid from the bottom of D-401 to tray 20 of D-402

TI-400: Indicates the vapor temperature exiting the top of D-401

395

CONTROL OF HIGH LEVEL GAS CHILLER

LC-401:This controls the propane refrigerant level in the shell side of the High level gas chiller E-402

LAH-401: this alarm will start when the level of the liquid in the shell side of E-402 rises above 80% as read in LC-401

CONTROL OF HIGH LEVEL GAS CHILLER

PC-405: from 0 to 7 Barg , this controls the pressure in E-402 by regulating the flow of propane vapor from the chiller to the inlet of second stage of compressor ( not simulated)

PAH-405: This alarm when the pressure in the gas chiller E-402 rises above 2.94 as read in PC-405

396

397

CONTROL OF THE LOW LEVEL GAS CHILLER

LC-402:This controls the liquid propane in the shell side of E-404 by regulating the flow of propane from the propane tube side of E-409

LAHL-402: Alarm when the propane refrigerant level in shell side of E-404 rises above 80% or falls below 20%

PC-406 : from 0 to 3.5 barg, this controls the pressure in E-404 by regulating the flow of propane vapor from the chiller to first stage of the compressor ( not simulated)

CONTROL OF THE LOW LEVEL GAS CHILLER

PAH-406: Alarm when the pressure in E-404 rises above 0.62 Barg as read in PC-406

Ti-404: indicates the temperature of the effluent from the tube shell of E-404. Indicates also the temperature of the feed entering D-403

LC -403: Controls the level of D-403 by regulating the flow from the bottom of the separator to tray 14 of D-402

398

399

CONTROL OF THE EXPANSION SECTION

PC-401:from 0 to 50 Barg, this controls the pressure of the expander feed separator through a split-range control. This is also the pressure at the inlet to the expander section.

PAHL- 401: alarm when the pressure in D-404 rises above 48.0 Barg or falls below 44.13 Barg as read at PC-401

TI-405: Indicates the temperature of the feed gas as it enters D-404. this is also the temperature of the feed gas from the tube side of E-405

400


LC-404:this controls the liquid level in the chiller separator D-404 by regulating the flow of liquid from the bottom of separator to tray 8 of D-402

LAH-404: Alarm when the liquid level in D-404 rises above 80% as read at LC-404

HC-401: Manual control adjusts the position of the inlet louver vanes in expander4 section

HS-401:switch that determines witch controller HC-401 or PC-401 will regulate the position of the expander inlet vanes

401


FC-403: from 0 to 1600 kNm3/d, this regulates the inlet flow to the compressor section

FAL-403: Alarm when compressor recycle ( anti-surge) flow falls below 750.KNm3/d as read in FC-403

HS 100: this switch operates the expander section KT-100. When the switch in ON , the expander is in operation

402

DEMETHANIZER CONTROL SYSTEM

AAH-401 : Alarm when the concentration of methane in the bottom of the demethanizer D-402, rises above 2% as read in AC-401

AC-401: Controls the methane composition in the bottom of the demethanizer D-402 from 0 to 10% by sending a remote set-point signal to the demethanizer reboiler effluent temperature controller TC-403

403


FI-408: this instrument indicates the flow of the methane product from the bottom of the demethanizer D-402 to the product pipeline . This flow is regulated by the Demethanizer level controller LC-400.

HS-401A-B : this are the product booster pumps. These pumps draw methane product from the bottom of the demethanizer D-402 and send it to the product line

404


LAHL-400: This alarm fires when the level in the bottom of the demethanizer D-402 rises above 80% or below 20% as read at LC-400

LC-400 : this controls the level in the bottom of the demethanizer by regulating the flow of methane product from the bottom of D-402 as read in FI-408

405


PAHL-402 : Alarm when the pressure in the top of D-402 rises above 13.48 Barg or below 10.69 Barg as read in PC-402

PC-402: Controls the pressure in the top of D-402 by regulating the flow of vapor from the discharge of the compression section K-100

406


TAH-401 D-402 TRAY-1 Temperature:

This alarm is on when the temperature of TRAY-1 of D-402, rises above -950C as read in TC-401

TC-401( -1250C to -200C):

This controls the temperature of the feed of tray-1 of D-402 by regulating the amount of gas bypassing KT-100 and flows directly from the top of D-403

407


TC-403: ( from -200C to 1000C):

This controls the effluent temperature from the D-402 re-boiler E-406 by regulating the flow of heating medium to the re-boiler. This controller can receive a signal control from the D-402 methane composition controller AC-401

408


TI-410: Indicator top of D-402

-1250C to -500C

TI-411: Indicator tray 24 of D-402

-1000C to 100C

TI-412 : Indicator of bottom D-402

-500C to 100C

process instrumentation & control

Education

electrical current

current i

current carrier

flow of electric current

current ampere

direct current dc

alternating current

unit of voltage