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DR. BRUCE POSTLETHWAITE

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

2

Essential Process Control for Chemical Engineers 1st edition © 2017 Dr. Bruce Postlethwaite & bookboon.com ISBN 978-87-403-1655-1 Peer reviewed by Dr. Iain Burns, Senior Lecturer, Director of Teaching, University of Strathclyde

3

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Contents

CONTENTS Foreword

8

1 Introduction

9



Main Learning points

9

1.1

Why do we need control?

9

2 Instrumentation

12



Main learning points

12

2.1

What is an instrument?

12

2.2

Factors to be considered in selecting an instrument

13

2.3

Instruments for temperature measurement

17

2.4

Pressure measurement

20

2.5

Flow measurement

23

2.6

Level measurement

27

2.7

Chemical composition

30

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ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Contents

3

Communication signals

32



Main learning points

32

3.1

Types of communication signal

32

4

Final control elements

39



Main Learning points

39

4.1

Control valves

39

4.2

Control valve sizing

41

5 Diagrams for process control systems

48



Main learning points

48

5.1

Process flow diagrams (PFDs)

48

5.2

Piping and instrumentation diagrams (P&IDs)

49

6 Inputs and outputs in control systems

55



Main learning points

55

6.1

Process inputs

55

6.2

Process outputs

56

6.3

Processes in control engineering

57

6.4

An example of variables and processes

58

7 Introduction to feedback control

59



Main learning points

59

7.1

Feedback control and block diagrams

59

7.2

Positive and negative feedback

61

7.3

Control loop problems

61

7.4

Direction of control action

64

7.5

Controller hardware

66

8 Introduction to steady-state and dynamic response

70



Main learning points

70

8.1

Steady-state gain

70

8.2

Dynamic response

73

5

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Contents

9

Dynamic modelling

87



Main learning points

87

9.1

Laplace transforms

88

9.2

Derivation of basic transforms

88

9.3 Solution of differential equations using Laplace transforms

91

9.4

Transfer functions

93

9.5

Block Diagrams

94

9.6

Block diagram algebra

95

9.7

Solutions of responses for high-order systems

95

9.8

Forming dynamic models

100

10 Analytical solution of real world models

106



Main learning points

106

10.1

Types of non-linearity

106

10.2

Linearisation of non-linear equations

107

10.3

Simplifying expressions through deviation variables

110

10.4 Procedure for simplifying and solving a non-linear model

112

10.5 Putting it all together – a reactant balance for a CSTR

112

11

PID Controller algorithm

117



Main learning points

117

11.1

Really simple feedback controller – on-off

118

11.2

Proportional-integral-derivative (PID) control

119

11.3

Proportional only control

121

11.4

Integral only control

126

11.5

Derivative action

127

11.6

Proportional-Intergral (PI) control

130

11.7

PID control response

131

11.8

Other forms of PID algorithm

133

12

Control system analysis

137



Main learning points

137

12.1

Analysis of a typical feedback control system

137

12.2

The PID algorithm as a transfer function

139

12.3 Analysis of proportional control of a first-order process

140

12.4 Example of a first order process under proportional control

142

12.5 Example of a second-order process under proportional control

145

12.6

148

Analysis of integral control of a first-order process

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ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Contents

13

Controller tuning

149



Main learning points

149

13.1

What needs to be done to tune a PID Controller?

149

13.2 How do you decide what is a good controller performance?

150

13.3

Some methods of controller tuning

154

13.4

Control loop health monitoring

161

13.5

Control loop diagnostics

162

14 More advanced single-loop control arrangements

163



Main learning points

163

14.1

Cascade control

163

14.2

Selective or autioneering control

167

14.3

Override control

169

14.4

Ratio control

172

14.5

Feedforward control

173

15

Design of control systems

181



Main learning points

181

15.1

Control envelope

181

15.2

Multivariable processes

184

15.3

How to determine the number of controlled variables

185

15.4

Plantwide mass balance control

191

16 Control system architecture

194



194

Main learning points

16.1 The effect of technology on process plant control rooms

194

16.2

Human factors in control room displays

197

16.3

Distributed control systems

200

16.4

Safety Instrumented Systems

201

17 Bibliography

202

Acknowledgements

203



Appendix

204



The use of software for teaching process control at Strathclyde University

204

7

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Foreword

FOREWORD This book is based on the course notes from the introductory process control class at Strathclyde University in Glasgow, Scotland. This course is itself based on the IChemE model-curriculum for chemical engineers and covers the material that ALL chemical engineers are supposed know. The IChemE curriculum was drawn up by a team of industrialists and academics, led by Professor Jon Love, in response to a recognised need for chemical engineers to be taught a more industrially relevant course. This book isn’t a traditional academic textbook in that there are no references anywhere in the text. The main reason for this is that the material has been gathered from many different sources after a working lifetime of teaching in the area and trying to identify an original source is impossible. I have included a bibliography for readers who wish to look further into the subject. I hope students and teachers find this book useful. A major new part of the course at Strathclyde University (where I teach) has been the introduction of new process control learning software called PISim, and this is described in the appendix. PISim will be commercially released in late Autum 2017.

8

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Introduction

1 INTRODUCTION MAIN LEARNING POINTS • Why process control is necessary Process control is concerned with making sure that processes do what they are supposed to in a safe and economical way. This isn’t an easy task as most processes are subject to many inputs called disturbances that constantly cause the controlled variables to move away from their desired values (or setpoints). To prevent this other process inputs called manipulations have to be moved to restore the process to the desired state. Process control is concerned with the overall system. A control engineer has to know about the instruments used to measure process quantities, the valves and other final control elements that allow control systems to adjust the process, communications to transmit information around, the control algorithms that decide how to respond to the information coming from the process, and finally the control engineer needs to understand how the process itself behaves: not just its steady-state behaviour but more importantly its dynamic response. Control engineering is now an area which offers big career opportunities for chemical engineers. The area used to be dominated by electrical/electronic engineers as the major challenges were in the hardware. This has changed. Sophisticated modern control systems allow much more complicated, process related, control schemes and now a major requirement for a control engineer is that they have a good understanding of the process.

1.1 WHY DO WE NEED CONTROL?

Figure 1 – a pressure trace from a SCADA system

9

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Introduction

• In real chemical plants, steady-state doesn’t exist. Things are always changing. Temperatures move up and down, levels get lower and higher, etc (see figure 1). • All processes are subject to disturbances. These are inputs to the process that change in a way that is beyond the reach of the local control system. A rainstorm on the outside of a distillation column will cool the column and require action to be taken to increase the heat input. Raw material variations are another common disturbance. Actions of other control systems can also cause disturbances to the process of interest – if a control system upstream or downstream of a process reduces a flowrate its effects will cascade throughout the rest of the process. • The control system needs to actively regulate against the effects of these disturbances. It does this by either measuring the disturbances directly (where this is possible and economic) or by measuring their effects on the controlled variables of the process. It then makes adjustments to other inputs to the process called manipulated variables to try to reduce or eliminate the effects of the disturbances. When controllers are holding controlled variables at fixed setpoints they are said to be in regulator or disturbance rejection mode. • Process don’t suddenly start at their flowsheet conditions, they don’t shut down on their own and don’t change production rate, etc without active intervention from control systems. When these major changes are being made to a process, the controllers will be acting in a setpoint tracking or servo mode. In servo mode, a controller will be trying to make the controlled variable track a moving setpoint. • Control systems also have a major part to play in process safety. The basic control system will usually ensure that the process stays within acceptable limits and will be equipped with alarms to warn operators of any problems. Interlocks may also be present in the basic system. These are used to lock particular inputs when other conditions are in existence. For example, the access doors to a kiln may be locked by a control system if the internal temperature is dangerously high. In extreme circumstances, special control systems (called safety instrumented systems or SIS ) that are separate from the normal process control system may come into play. These may be local to a particular piece of equipment, for example a high-temperature trip on a pump motor; or may have a process or plant-wide focus, for example an emergency shutdown system.

10

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Introduction

• Finally, good control saves money. Plants are normally operated close to constraints (e.g. the acceptable product quality). Poor control means more variability and this means that the mean value of a controlled variable needs to be held further from the constraint than is necessary with good control. Figure 2 shows a simple example – the tops quality from a distillation column. This may have a limit on the lowest acceptable composition and it’s the responsibility of the control system to hold the composition about this limit. If the control is poor and there’s lots of variability, then it will be necessary to set an average value of composition much higher than if good control is used. This higher average composition will lead to increased reflux going down the column and hence more vapour having to be generated by the reboiler, which increases steam costs.

Figure 2 – The advantage of good control

11

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ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

2 INSTRUMENTATION MAIN LEARNING POINTS • Factors involved in selecting instrumentation • Techniques for temperature, pressure, flow and level measurements The instruments on a chemical plant are the devices used to monitor the important variables that allow the condition of the process to be determined.

2.1 WHAT IS AN INSTRUMENT? Transducers or sensors are the primary sensing elements. They are devices that convert some physical quantity that we want to measure (e.g. temperature, pressure, etc). into some sort of signal that can be processed further. For example, a thermocouple converts a temperature difference into a voltage; a piezo resistive pressure sensor converts a pressure into a change in electrical resistance.

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ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

Signal conditioning is the signal processing that is applied to the output of the transducer. Sometimes this could simply be amplification, but often more complicated things like linearisation are required (ideally, the output of a device should change linearly with changes in the quantity being measured). Modern instruments such as Coriolis flowmeters have very complicated signal processing build into them to detect the phase shifts in the motion of the sensing elements. A transmitter is a device that converts the output from signal conditioning into a signal that is compatible with the communication system being used in the plant. There are many different standards in use ranging from 4-20mA analogue signals up to digital Fieldbus systems – these will be discussed later. An instrument is a device that contains at least one but usually more, and often all of the above (transducer, signal conditioning and transmitter). An instrument is a complete measurement package that senses the quantity to be measured and presents that measurement in a form suitable for use (e.g. a simple instrument might be a Bourdon gauge for pressure measurement – the transducer, a helical metal tube, distorts with pressure and drives the gauge needle directly; a more typical instrument for modern chemical plants might be a packaged RTD (resistance temperature device) – it will include the RTD, signal conditioning, and a transmitter).

2.2 FACTORS TO BE CONSIDERED IN SELECTING AN INSTRUMENT 2.2.1 RANGE

The range of an instrument is range of the measured quantity over which the instrument will give a reliable output. The range is always the same or bigger than the span of an instrument. While an instrument with a large range might seem to be always desirable this isn’t usually the case in practice. The sensitivity (change in output vs change in measurement) of transducers drops significantly in large range devices leading to reduced accuracy. 2.2.2 SPAN

The span of an instrument is an adjustable parameter (there will be a button, screw or software link on the instrument that will allow the adjustment). The span is the distance the measured quantity has to move to drive the instrument output from its minimum value to its maximum (remember that instrument outputs match communication standards which have fixed maximum and minimum values). By adjusting the span, the instrument’s sensitivity (output change vs. input change) can be altered – large spans will lead to lower sensitivities.

13

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

2.2.3 ACCURACY AND PRECISION

All measurement instruments are subject to random error – if you take repeated measurements of a fixed quantity you will always get a scatter of values about a mean. An accurate instrument is one where the mean is centred close to the actual value of the quantity being measured. An instrument can be accurate, but still have a significant amount of error on an individual measurement – the accurate mean can only be obtained by taking many repeat measurements. A precise instrument is one which, when measuring a constant quantity, returns output values which are very close to one another – the scatter between readings is small. It is possible for an instrument to be precise (high repeatability in measurement) but not accurate (with the mean some distance away from the true value of the quantity being measured). An ideal instrument is one which is both precise and accurate.

Figure 3 – accuracy and precision

2.2.4 REPEATABILITY AND DRIFT

In most instruments repeatability and precision mean the same thing. However, some sensors suffer from hysteresis. In these sensors the measurement is affected by what the variable being measured was doing prior to the measurement. It is most prevalent in systems which involve some sort of mechanical element in their sensing. For example, bourdon pressure sensors usually exhibit hysteresis – the measurement they produce will be different if the pressure was rising or falling immediately prior to the measurement. Drift is a medium to long term effect that causes some instruments to lose mainly accuracy, but also possibly precision. For example, corrosion of a thermocouple will alter its thermoelectric properties and hence the voltage produced at a particular temperature.

14

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

2.2.5 DELAY

Some instruments can take time to develop a measurement after the process is ‘sampled’. An example of these are chemical analysers – some of these are, in effect, automated analytical laboratories that can take several minutes to develop an output. Any delay in measurements for control can cause problems to a control system – the controller is looking at the process as it was some time in the past rather than where it is at present. In addition to delay, the instrument’s speed of response needs to be taken into account. A large resistance thermometer sensor will take much longer to respond to temperature changes than a tiny thermistor (this is simply because of the relative thermal capacities). 2.2.6 LINEARITY

Almost all commercial control systems are based around linear control algorithms and work best when controlling linear or nearly linear systems (a linear system is one where doubling a change in the input will produce double the effect on the output). Some transducers (e.g. orifice plates (see figure 4) and thermistors) produce outputs that are not linearly related to the quantity being measured. In modern instruments these signals will be linearised internally in the instrument, but high transducer non-linearity will reduce sensitivity in some areas.

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ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

Figure 4 – pressure drop/flowrate relationship for an orifice plate

2.2.7 CALIBRATION

Calibration involves matching the instrument’s span and zero to the quantity being measured. This will involve two or more standard measurements being made (for example, a pH meter would be calibrated using several standard buffer solutions) and the instrument adjusted. Calibration is a time consuming and sometimes difficult process. Many packaged instruments, such as RTDs and pressure instruments, are factory calibrated and can be installed directly into the process. Others, like orifice plates, need to be calibrated in the field after installation. Some instruments, like pH meters, suffer from regular drift and need to be recalibrated on a routine basis. 2.2.8 NOISE

Noise is a random variation in the measurement signal. Noise can be generated by the process itself (e.g. by bubbles in the flow being measured by an orifice plate) or it can be generated in the measuring instrument or communication lines. If instrument generated noise is significant, then it’s better to pick the instrument that generates the least (see accuracy and precision above). 2.2.9 RELIABILITY

Instrumentation in a chemical plant needs to be highly reliable. The measurements are used for control and ensure the safe operation of the process. Reliability specifications are usually provided on instrument data sheets and are often quoted as the mean time between failures (MTBF). This data can be incorporated into a hazard analysis to decide whether instrument redundancy is required.

16

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

2.2.10 SUITABILITY FOR PLANT

The instrument must be capable of tolerating the physical conditions (e.g. temperature, pressure, vibration, etc.) that it will be exposed to and also be resistant to any chemical it is likely to come into contact with. The instrument should also be compatible in an engineering sense with the rest of the plant – it should use similar thread sizes, flanges, etc, to other plant fittings and also have similar service (e.g. power supplies or instrument air) requirements to other plant devices. Finally, many companies tend to limit the number of suppliers they purchase from. This allows them to negotiate better deals and also allows more extensive stocks of spare parts to be kept. 2.2.11 COST

A cheap purchase price is not usually the best indicator of a good instrument choice. The overall lifetime cost needs to be considered – more expensive instruments might last longer and require less frequent calibration. 2.2.12 BELLS AND WHISTLES

Since cheap microprocessors have come on the scene many instruments have a host of added features, such as averaging and storage of previous values. These are only useful if you actually plan to use them, but otherwise should be ignored when selecting an instrument.

2.3 INSTRUMENTS FOR TEMPERATURE MEASUREMENT Temperature measurement is one of the four most common measurements in a chemical plant (the others are pressure, flow, and level). 2.3.1 THERMOCOUPLES

A thermocouple is a sensor made from two wires with dissimilar thermo-electric properties (i.e. heat liberates electrons to different extents). The wires are joined at each end and a small voltage is generated (by the Seebeck Effect) which is proportional to the difference between the temperature at the two ends of the device

Figure 5 – A thermocouple

17

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

Originally, the cold end of a thermocouple was kept at a constant temperature by sticking it in an ice and water bath. Nowadays, the cold junction is simulated by an electronic chip containing a thermistor and some fancy electronics. The electronics also amplify the mV output of the thermocouple to something which is more usable in the rest of the instrument. It’s still possible to buy thermocouple wire (with two strands of dissimilar metals) for special applications, but the majority of industrial thermocouples are sold as packaged units with a transmitter attached (for example see figure 6). Different types of thermocouple are available that cover different ranges and have different applications – see table 1.

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ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

Thermocouple Type

Materials

Range

Comments

Type K

Chromel/Alumel

0OC to 1100OC

Very commonly used

Type T

Copper/Constantan

-185OC to 300OC

Suitable for cryogenic applications

Type J

Iron/Constantan

0OC to 750OC

Higher sensitivity than type K

Types B, R and S

Platinum alloy/ Platinum

0 C to ~1600 C

Very stable but expensive – used only for high temperatures

O

O

Table 1

Thermocouples used to be the most common temperature measurement device in chemical plant, but are now being overtaken by RTDs.

Figure 6 – a temperature probe assembly

19

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

2.3.2 RESISTANCE TEMPERATURE DETECTORS (RTDS)

An RTD, or resistance thermometer, is a device that uses a sensor whose resistance changes with temperature. The most common type of process measurement RTD is a device with a Platinum or Platinum alloy sensor (for stability) whose resistance increases with temperature (as it is a conductor). Thermistors are semi-conductor based temperature sensors whose resistance decreases as the temperature increases. The change is much less linear than that achieved by the Plantinum RTDs and the overall accuracy of temperature measurement is also much less. However, thermistors can be made to be very small, and hence very fast, and are often used in applications where very fast temperature measurement is required (e.g. they are used to detect vortices in some vortex shedding flowmeters). The sensor elements in Platinum RTDs only generate a change in electrical resistance, and this needs to be measured by a bridge circuit. It’s very difficult to buy a platinum sensor on its own – most RTDs are complete packages consisting of sensor, signal conditioning and transmitter (figure 6). The sensing element in an Pt RTD is bigger than that in a thermocouple, and so RTDs are a bit slower. They are also more expensive. However, Pt RTDs are more accurate and considerably more stable than thermocouples and are now the usual choice for temperature measurement between -200oC and 500oC.

2.4 PRESSURE MEASUREMENT Pressure measurement is very important in chemical plant both as a fundamental measurement, and also as an implied measurement of flowrate and level. Pressure measurement can be classed into four main categories: 1. Gauge pressure. The pressure above the local atmospheric pressure (which changes according to altitude and weather conditions). If the instrument isn’t connected to a pressure source, it will read zero. Pressures below the local atmospheric may be registered as negative pressures, if the instrument is configured to do this. 2. Absolute pressure. The instrument measures the absolute pressure – it will always generate some reading (except in a complete vacuum). Pressures below atmospheric will be registered as positive absolute pressures. 3. Vacuum. Sometimes used in vacuum systems. The pressure below local atmospheric pressure is measured. (e.g. if local atmospheric pressure is 14.7 psia, an absolute pressure of 10.7psia will be measured as -4 psig on a gauge pressure device, and 4psi vacuum on a vacuum measuring device. 4. Differential pressure. The pressure instrument has two measurement ports and measure the pressure difference between them. DP devices can be used as a part of flow and level measurement systems.

20

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

2.4.1 BOURDON TUBES

These were the basis of the pressure gauges we’re used to seeing on old movies with steam engines, etc, and used to be common on chemical plant. The sensing element is simply a coiled metal tube – pressure increases inside the tube cause an ‘unrolling’ force and this is used to drive the pointer on the gauge. Although cheap and still used on portable devices (e.g. car tire inflators), they are not popular in chemical plant applications as it’s awkward to get a signal suitable for transmission to a remote location (the control room).

21

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

Figure 7 – a locomotive pressure gauge based on a bourdon tube

2.4.2 MODERN PRESSURE SENSORS

Figure 8 – Pressure transmitters

Nowadays pressure sensors are normally bought ‘packaged’ in a small instrument that includes the sensor, signal conditioning and a transmitter (e.g. 4-20mA). Several different types of sensor are available, the two most common are: • Piezoresistive. The sensing element is a ‘diaphragm’ of semi-conductor material. When the element is subjected to pressure, the diaphragm deforms and the movement is detected as a resistance change in the material.

22

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

• Capacitance. In these instruments, one side of an electrical capacitor is formed by a diaphragm that is exposed to the pressure to be measured. As the diaphragm deforms, the pressure is measured by the change in capacitance Modern sensors are available for all pressure measurement applications. Those used for differential pressure measurement are usually called DP Cells.

2.5 FLOW MEASUREMENT Flow measurement is an area where the effect of changes in technology are most noticeable. Up until a few years ago almost all flow measurement was carried out using differential pressure devices (mainly orifice plates) but now a much larger ranges of flowmeters are commonly used. 2.5.1 DIFFERENTIAL PRESSURE DEVICES (ORIFICE PLATES, NOZZLES AND VENTURI METERS

Figure 9 – Orifice plate (from 1871!)

Orifice plates are essentially a plate with a hole in the centre which is fitted between the flanges at a pipe junction. The flow through the pipe can be estimated by measuring the differential pressure over the orifice. There is a considerable subtlety in the design of the orifice plate itself (e.g. the ratio of the hole to the pipe, the entrance and exit shape of the hole), the positioning of pressure trappings (particularly the downstream trapping which should be as near to the vena contracta – the place where the diameter of the streamlines is least – as possible) and finally the position of the orifice plate itself (it needs to be a reasonable distance from things that cause significant disruption to flow patterns – bends, pumps, valves, etc.). There is a British Standard (BS EN ISO 5167) for the design of orifice plate (and nozzle and venturi meters) installations.

23

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

Orifice plates used to be the standard method of measuring flowrates (both gases and liquids) in process plants and probably still represent the biggest number of installed flowmeters. However, they are no longer the first choice for new installations as alternatives which avoid the problems with orifice plates are now available at reasonable cost. The main problems with orifice plates are: • The orifice plate needs to be sited carefully – flow disturbances need to be avoided. • Measurements are subject to significant noise when the process stream contains bubbles or solid particles. • The orifice will be eroded causing measurement drift over time, particularly if the stream contains abrasive particles. • The orifice can become clogged, seriously affecting the measurement. • The orifice plate causes a significant, unrecoverable, pressure drop – wasted energy. • Each orifice plate needs to be carefully calibrated in situ for accurate measurement (although often correlations are used in place of calibration, but this produces only approximate measurements of flow). • The turndown ratio (the maximum reliable flow measurement divided by the minimum), or rangeability, is poor compared to more modern types of flowmeter.

24

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

Nozzle and venturi meters are Differential Pressure (DP) meters like the orifice plate, although they have lower unrecoverable pressure drops. They are however, very rarely used in chemical plant applications. 2.5.2 VORTEX METERS

Figure 10 – vortex meter

Vortex meters work by inserting a shaped body into the flowpath of the fluid. When this is done alternating, swirling, vortices are shed from the back of the body. The frequency at which these vortices are shed is directly related to the fluid velocity. In the vortex meter the vortices are detected by temperature or pressure sensors built into the device. Vortex meters have a good turndown ratio (typically 20:1), low pressure drop and high reliability. They also have a stable long term accuracy and repeatability. Disadvantages are that they are not suitable for low flow velocities and care needs to be taken in their placement to avoid flow stream disturbances (just like orifice plates).

25

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

2.5.3 CORIOLIS METERS

The Coriolis effect was first discovered by the 19th century mathematician Gustav-Gaspard Coriolis. He was concerned with the forces on a body moving from the equator towards the poles of the Earth. At the equator, the rotational speed of the Earth is just over 1,000 miles per hour in a West to East direction and everything at the equator that is ‘stationary’ with respect to the surface has this rotational speed too. At the poles the rotational speed drops to zero. This means that bodies moving North from the equator will tend to deflect to the East and those moving South to the West. To hold the body on a directly Northward (or Southward) path, a force (the Coriolis force) is required to decelerate the body to the local rotational speed. The Coriolis Effect plays an important part in weather systems and is popularly claimed (wrongly) to determine the direction water goes down a plughole in the Northern and Southern hemispheres!

Figure 11 – operation of a coriolis meter

In a Coriolis flowmeter the effect is used by passing the fluid whose flow is to be measured through a tube that is oscillated in a limited rotary axis. As the fluid moves through the tube it goes from zero to maximum rotational velocity in one half of the tube and then from maximum to zero rotational velocity in the other half. This produces two Coriolis forces which produce a twisting force on the tube. This twisting gives a measure of the mass flowrate of the liquid in the tube. Coriolis flowmeters measure mass flowrate rather than velocity or volumetric flowrate. However, they can also be made to measure fluid density (by finding the resonant frequency of the tube), allowing volumetric flowrate to be estimated too. They have a very high turndown ratio (typically 100:1) and have high accuracy and repeatability. Although they normally have a low pressure drop, they can have problems with very viscous fluids. They also rely on moving parts (the tube) and electromagnets to drive the motion, so there may be questions about reliability (although there seems to be no data on this at present).

26

ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

2.5.4 OTHER FLOWMETERS

There are many other types of flowmeter in use including: Doppler; transit time (ultrasonic); and magnetic.

2.6 LEVEL MEASUREMENT The measurement and control of liquid levels within a chemical plant is one of the most important functions of the instrumentation. Liquid level controllers maintain the overall process mass balance and also maintain liquid seals (to prevent vapour going where it shouldn’t).

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Instrumentation

2.6.1 DIFFERENTIAL PRESSURE MEASUREMENT

Figure 12 – DP cell for level measurement

A very common way of measuring liquid levels is to use a differential pressure instrument. One end (the ‘high’ pressure connection) is connected to a tapping near the bottom of the vessel, and the other to a tapping near the top. It’s important that the top tapping is in the vapour space above the liquid. The differential pressure is a measure of the hydrostatic head in the vessel. If the density of the liquid is known, then the level can be easily obtained. One problem with this setup, if the fluid contains solids or is likely to form solids, is that the pressure tapping can become clogged with solid material. If the clog is partial the instrument response will become slower and slower, and if it clogs completely no measurement will be possible. 2.6.2 CAPACITANCE MEASUREMENT

This type of level measurement uses a long probe which is inserted into the tank as one side of an electrical capacitor, and another conductor (usually the metal wall of the vessel) as the other. As the liquid level in the vessel rises, the measured capacitance will change and this can be calibrated to provide a measurement of the liquid level.

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Instrumentation

Instrument

Figure 13 – capacitance level meter

Capacitance measurement is very subject to errors caused by changes in the electrical properties of the liquid or coating of the vessel or probe. It is no longer very popular in the process industries. Another new, but similar, technique called Radio-Frequency (RF) admittance is however growing in popularity. 2.6.3 ULTRASONIC AND RADAR MEASUREMENT

Figure 14 – ultrasonic/radar level measurement

Ultrasonic and microwave measurement devices use a transmitter element which sends either an ultrasound or radar signal into the vessel from its top. The signal bounces off the liquid surface and is detected by a receiver. The distance between the unit and the liquid surface is obtained from the time lag between transmission and receipt of the return signal. Both types of device are independent of liquid density. Ultrasonic devices can be fooled by dense foam layers that deaden the sound signal. Both types of device are expensive compared to other level measurement methods, but radar devices are more expensive than ultrasound.

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2.7 CHEMICAL COMPOSITION The measurement of composition is one of the most desired, but also most difficult and expensive, measurements that have to be made in chemical and process plants. Composition is not just important in reactors, but also in separations like distillation columns, flash chambers, membrane units, etc. 2.7.1 PH METERS

The pH of a solution is the negative logarithm to the base ten of the Hydronium ion concentration:

[

pH = − log10 H 3O +

]

The most common use of pH measurement is probably in waste treatment where it is important to ensure that waste water is close to neutral (pH 7) prior to discharge. pH is also important inside many process units to encourage particular reactions or to prevent (or encourage) precipitation of particular materials.

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ESSENTIAL PROCESS CONTROL FOR CHEMICAL ENGINEERS

Instrumentation

The standard method of measuring pH is using a pH electrode. Those used in industry have to be much more rugged than those used in standard laboratory applications and often the electrode will have to be chosen to suit the particular process conditions. Generally, you’d buy the pH device as a self-contained transmitter – the signal from the device would match your instrumentation system (e.g. 4-20mA, digital, etc). 2.7.2 CHEMICAL ANALYSERS

Although science fiction TV shows have devices like ‘Tricorders’ that produce complete chemical analyses at the touch of a button, real analysers aren’t quite there yet. The first difference is that most analysers need to sample the material to be analysed. This involves piping, valves, pumps and control gear that carry out the sampling process. Sampling also introduces dead-time (time delay) into any measurement, and significant dead-time causes all sorts of problems for feedback controllers. Once the sample has been taken there are a variety of approaches to obtaining an analysis. However, the analyser is always designed to analyse for particular materials – they aren’t general purpose. This means that an analyser has to be designed and calibrated for a particular application which pushes the price up substantially. Some of the techniques used include ‘wet’ analysis (where the analyser is a little automated lab); chromatography; and modern methods like: NIR, MIR, UV-visible, Raman scattering, Fluorescence, NMR, Microwave, Acoustic, and Mass spectrometry techniques. 2.7.3 LABORATORY ANALYSIS AND INFERENTIAL CONTROL

Due to the cost and other problems with on-line chemical analysers, they are not very commonly used. This is beginning to change as new analyser technology is developed, but for now the most common way of dealing with control of compositions is to use periodic laboratory analysis coupled with inferential control. The idea of inferential control is that we can use other measurements from a process to infer the compositions. The simplest systems use just a single measurement (e.g. using a measurement of a tray temperature near the top of a distillation column to infer the tops composition), but others use several measurements combined using some sort of process model (e.g. using a tray temperature, a pressure measurement, and a knowledge of the equilibrium data to infer the tops composition). All inferential systems make assumptions about the process (e.g. about the ratios of the different components) and need to be corrected using periodic (once or twice a shift) laboratory analysis.

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Communication signals

3 COMMUNICATION SIGNALS MAIN LEARNING POINTS • Analogue communications • Digital communications • How to convert between signals and engineering units Process control systems are information processing systems. They gather information about the process state from the instruments, then use this to decide what to do to move the process to a desired state, and then send information to the various final control elements (usually control valves) to produce the desired changes in the process. To handle this information there needs to be a means for moving it from the instruments to the controllers and then onto the final control elements. This is where process communications come in.

3.1 TYPES OF COMMUNICATION SIGNAL There are a number of different sorts of signal that can exist in a process control communications system. Some plants may have just one type, but most have a mixture of different sorts of signal moving around the system. 3.1.1 ANALOGUE SIGNALS

Analogue signals are continuous and capable of taking any value at all within the range between their maximum and minimum values. They are called analogue signals because the level of the communication signal is directly analogous to the value that is being transmitted. 3.1.1.1  Pneumatic signals Pneumatics are the oldest form of communication signal used for remote monitoring and automatic control in chemical plant. A pneumatic communication signal is an air pressure between 3 and 15 psig (the 3 psi offset zero allows tube breaks or malfunctioning instruments to be detected). The signal is carried in small diameter (about 10mm) metal tubes. As the tube length increases, a significant dynamic lag develops as it takes time for the signal generator (instrument or controller) to add enough air to increase the tube pressure to the desired value. This limits the practical length of communication tube runs, and means that plants using this form of communication have to have a number of local control rooms near to the site of the instruments. Pneumatic communications are now obsolete, but may still be present in some very old processing plants.

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Communication signals

3.1.1.2  Current loops The most common type of analogue signal used in chemical plant is the 4-20mA current loop. The instruments/controllers using this communication system send an electrical current around a loop between the transmitter and receiver. This is a better communication signal than a voltage as it is independent of the length of the communication line. If a voltage signal was to be used there would be some voltage loss in the communication wire, and this would increase with line length. With a current loop, the transmitter simply increases the transmission voltage until the current around the loop matches the desired signal. In theory, 4-20mA signals can be carried on two wires (or even one if a common earth is used), but communication cables often include additional wires for power supplies and so three and four wire cable is common. The signal wires will be electrically shielded (with the shielding earthed) or twisted into a twisted pair to reduce the effects of electrical noise.

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3.1.1.3  Example calculation for analogue signals A temperature instrument is adjusted to give a span of 20–120oC. It can be connected to either a pneumatic, or to a current loop transmitter and the conversion is linear. What will be the output of each transmitter type when the measured temperature is 85oC? +@8G?4F