Basic Process Measurements E-Book

Cecil L. Smith

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 501

Veröffentlichungsjahr: 2011

Beschreibung

A unique resource for process measurement Basic Process Measurements provides a unique resource explaining the industrial measuring devices that gauge such key variables as temperature, pressure, density, level, and flow. With an emphasis on the most commonly installed technologies, this guide outlines both the process variable being measured as well as how the relevant measuring instruments function. The benefits of each technology are considered in turn, along with their potential problems. Looking at both new and existing technologies, the book maintains a practical focus on properly selecting and deploying the best technology for a given process application. The coverage in Basic Process Measurements enables the practitioner to: * Resolve problems with currently installed devices * Upgrade currently installed devices to newer and better technologies * Add instruments for process variables not previously measurable * Evaluate device installations from a perspective of both normal process operating conditions and abnormal conditions * Determine the best technology for a given set of process conditions Designed for a wide range of technical professionals, Basic Process Measurements provides a balanced treatment of the concepts, background information, and specific processes and technologies making up this critical aspect of process improvement and control.

Leseprobe

Contents

Cover

Half Title page

Title page

Copyright page

Preface

Chapter 1: Basic Concepts

1.1. Continuous vs. Discrete Measurements

1.2. Continuous vs. Sampled Measurement

1.3. In-Line, On-Line, and Off-Line

1.4. Signals and Resolution

1.5. Zero, Span, and Range

1.6. Turndown Ratio and Rangeability

1.7. Accuracy

1.8. Repeatability

1.9. Measurement Uncertainty

1.10. Measurement Decision Risk

1.11. Calibration

1.12. Measurement Device Components

1.13. Current Loop

1.14. Power Supply and Wiring

1.15. Serial Communications

1.16. Smart Transmitters

1.17. Environmental Issues

1.18. Explosive Atmospheres

1.19. Measurement Device Dynamics

1.20. Filtering and Smoothing

Literature Cited

Chapter 2: Temperature

2.1. Heat and Temperature

2.2. Temperature Scales

2.3. Thermowells

2.4. Bimetallic Thermometers

2.5. Thermocouples

2.6. Resistance Temperature Detectors

2.7. Thermistors

2.8. Temperature Transmitters

2.9. Pyrometers

2.10. Others

Literature Cited

Chapter 3: Pressure

3.1. Force and Pressure

3.2. Measures of Pressure

3.3. Pressure-Sensing Elements

3.4. Indicators and Switches

3.5. Pressure Sensors

3.6. Strain Gauge Pressure Sensors

3.7. Capacitance Pressure Sensors

3.8. Resonant Frequency

3.9. Installation

3.10. Differential Pressure

Literature Cited

Chapter 4: Level and Density

4.1. Level, Volume, and Weight

4.2. Pressure Transmitter

4.3. Differential Pressure Transmitter

4.4. Capacitance and Radio Frequency

4.5. Ultrasonic

4.6. Noncontact Radar

4.7. Guided Wave Radar

4.8. Nuclear

4.9. A Few Others

4.10. Level Switches

4.11. Interface

4.12. Density

Literature Cited

Chapter 5: Flow

5.1. Mass Flow, Volumetric Flow, and Velocity

5.2. Static pressure and Fluid Velocity

5.3. Flashing and Cavitation

5.4. Fluid Dynamics

5.5. Flow Meter Application Data

5.6. Orifice Meter

5.7. Head Meters

5.8. Coriolis Meters

5.9. Magnetic Flow Meter

5.10. Vortex-Shedding Meter

5.11. Transit-Time Ultrasonic Flow Meter

5.12. Doppler Ultrasonic Flow Meter

5.13. Thermal Flow Meters

5.14. Turbine Meter

5.15. Other Flow Meters

5.16. Flow Switches

Literature Cited

Index

BASIC PROCESS MEASUREMENTS

Copyright © 2009 by John Wiley & Sons, Inc. All rights reserved

Published by John Wiley & Sons, Inc., Hoboken, New JerseyPublished simultaneously in Canada

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ISBN: 978-0-470-38024-6

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PREFACE

My business is exclusively process control. And like many other activities, process control relies on measurement devices—every control loop contains at least one measurement device that provides a very critical function. Most activities, and especially process control, are subject to the garbage-in, garbage-out characterization. The reason is simple—decisions based on bad data are likely to be bad decisions. The Iraq War is a prime example.

This book is intended for anyone involved in the application of measurement devices in an industrial environment. The target audience includes chemical engineers, mechanical engineers, electrical engineers, and industrial chemists, but anyone with a technical background will find this book helpful for identifying appropriate measurement devices for an application. For the benefit of those with a limited process background, the relevant basic concepts are explained at the beginning of each chapter. For example, the Reynolds number is explained at the beginning of Chapter 5, which covers flow measurement. Chemical engineers and mechanical engineers are certainly familiar with the Reynolds number, but most electrical engineers and industrial chemists are far less familiar, if at all.

This book concentrates on measurement devices for the basic variables: temperature, pressure, level, density, and flow. This book does not attempt to cover every possible approach to measuring these variables but instead focuses on technologies that are most commonly installed in industrial facilities. Equal emphasis is given to the attributes that make each attractive and to the factors that lead to problems. One must understand both the process and the principles on which the measurement device relies. Remember, those who know what they are doing get what they pay for; those that do not get what they deserve!

When it comes to process operations, measurement devices have become our eyes. There was a time when process operators could rely on their own senses to make decisions. In the paper industry, they could reach into a stock tank, grab a handful of the fiber, squeeze out the water, use the sole of their shoe to form a crude mat or sheet, and then tell you what kind of paper it would make. Perhaps the most impressive was a person one who could chew a polymer sample for a few minutes and tell you more than the QC lab could tell you hours later! Did his manager know he did this? Sometimes he did, and sometimes he did not.

But the way to reduce emissions is to replace open tanks with closed tanks and otherwise button up a process. We are correctly more sensitive to employee exposure to industrial chemicals, so things that were tolerated back in the 1960s when I got into this business are appropriately taboo today. I have also witnessed the adjustments, occasionally painful, that occurred when production operators were forced to rely increasingly more, and in some cases exclusively, on the information provided by the measurement devices.

For a commercial measurement device to be successful in process installations, two requirements must be satisfied:

The measurement device must rely on a sound basic principle. In many cases, this basic principle establishes boundaries in which the measurement device can be used successfully. For example, the vortex shedding flow meter cannot be used to measure flows in the laminar region. The better one understands the principles behind a measurement device, the better one can recognize viable applications for the measurement device and avoid misapplications and the ensuing consequences, which occasionally extend beyond no return for the money and time invested.The measurement device must be constructed in a manner consistent with the process conditions to which it will be exposed. Designing a measurement device for industrial service is definitely a specialty. Unfortunately, some of the best in the business occasionally stumble. In the early efforts to use thin film technology in pressure measurement devices, the strain gauges were bonded to the pressure-sensitive diaphragm. In essence, bonded means “glued”; the use of this technology in industrial measurement devices has been largely unsuccessful.

In modern measurement devices, the basic principle usually involves the translation (by a sensor or transducer) of the process variable of interest (temperature, flow, etc.) to an electrical property (voltage, resistance, capacitance, etc.) that can be sensed. These are generally well understood, and most can be expressed mathematically and analyzed. But rarely is this the case for the manner in which the measurement device is constructed. One learns this from experience—that is, install a few of a given model and see how well they perform. Maintenance issues usually take the front seat, and the potential problems may not surface for several years.

Some industries have special requirements, such as sanitary conditions or food approval. But in the end, the requirements of the various segments of the process industries are more alike than different. The manufacturers respond by offering their products in different models to suit a variety of special requirements. For example, the filling fluid in a capillary seal system can potentially leak into the process. When you install such devices in plants that operate 24/7 for many years, anything that can happen will happen. The consequences of the filling fluid leaking into the process must be tolerable.

As noted previously, I get into process measurement issues primarily through my process control activities. When troubleshooting process control problems, you always have to include the possibility that a measurement device is lying to you, perhaps only under certain situations. With the incorporation of microprocessor technology into the measurement devices, this is becoming less frequent, but has not and probably never will entirely disappear. Incorporating the microprocessor within the measurement device improves the signal processing but also enhances the capability to detect when something has gone awry. As we replace current loop interfaces with digital communications, such situations can be more effectively reported.

In process control endeavors (and in others as well), it is imperative that we take advantage of new technologies and earnestly pursue continuing process improvement by:

Resolving problems with currently installed measurement devices.Installing measurement devices whose performance is superior to those currently installed.Installing measurement devices for process variables that were not previously measurable.

Both of these have demonstrated the capability to produce recognizable improvements in process operations (improved product quality, better economic returns, etc.).

In process installations, we usually get it right for the normal process operating conditions. But every process inevitably operates, at least for short periods, under conditions very different from the normal process operating conditions. This is appropriately a consideration during the hazards analysis—that is, what will these operating conditions be? and Will all measurement devices perform properly under these conditions?

A major concern is a multiple failure accident, with one of the failures being that a measurement device that is lying to us. Unfortunately, presenting a bad piece of information during an abnormal event will likely compound the consequences. When you have only one problem, you usually recognize it quickly. But when you have two or more at the same time, it takes a little longer.

I am alarmed by the extent to which new developments are now coming from outside the United States. The DIN standard for the 100-Ω RTD came from Germany. Rosemount pioneered the capacitance cell for pressure transmitters, but Yamatake commercialized the piezoelectric technology and Yokogawa, the resonant frequency technology. Micro Motion pioneered the coriolis flow meter, but Khrone was first with a straight tube version. The Europeans clearly led the transition to intrinsically safe installations, using barriers from MTL (England) and R. Stahl (Germany). I am alarmed, but not surprised. With the emphasis on the financial sector, investing in technology and manufacturing has not been in vogue in the United States for some time.

In the latter stages of preparing this book, I chose to remove all reproductions of commercial products. These quickly become out of date (a couple of them were already out of date before I removed them). Today, there is a very easy way to obtain information on the latest models of commercial products for measurement devices: the Internet and a search engine! For example, if you want the latest on magnetic flow meters, just do a search. For the same reason, I have included the temperature-voltage relationship only for the type J thermocouple, and not even all of the available data. You can easily download the complete table for any type of thermocouple from the National Institutes of Science and Technology (NIST) website. Including such tables in a book makes no sense.

The process control business has been very good to me. As a consultant, I get to work on both diverse and interesting problems. However, I thoroughly enjoy teaching professional development courses, and I spend about a third of my life doing that. All are in some way related to process control, but included in my offerings is a course on process measurements.

Finally, a special thanks to my wife, Charlotte. She endures my staring into a computer screen for hours at a time. But fortunately I can now do this in places like Taos and Key West.

CECIL L. SMITHBaton Rouge, LouisianaNovember 2, 2008