1、LOOP CHECKING:A TECHNICIANS GUIDEJeffrey05.book Page i Thursday, March 3, 2005 2:56 PMJeffrey05.book Page ii Thursday, March 3, 2005 2:56 PMLOOP CHECKING:A TECHNICIANS GUIDEHarley JefferyISA TECHNICIAN SERIESJeffrey05.book Page iii Thursday, March 3, 2005 2:56 PMCopyright 2005 by ISA Instrumentation
2、, Systems, and Automation Society67 Alexander DriveP.O. Box 12277Research Triangle Park, NC 27709All rights reserved.Printed in the United States of America.1098765432ISBN 1-55617-910-3No part of this work may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, e
3、lectronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher.NoticeThe information presented in this publication is for the general education of the reader. Because neither the author nor the publisher has any control over the use of the informat
4、ion by the reader, both the author and the publisher disclaim any and all liability of any kind arising out of such use. The reader is expected to exercise sound professional judgment in using any of the information presented in a particular application. Additionally, neither the author nor the publ
5、isher have investigated or considered the effect of any patents on the ability of the reader to use any of the information in a particular application. The reader is responsible for reviewing any possible patents that may affect any particular use of the information presented. Any references to comm
6、ercial products in the work are cited as examples only. Neither the author nor the publisher endorses any referenced commercial product. Any trademarks or tradenames referenced belong to the respective owner of the mark or name. Neither the author nor the publisher makes any representation regarding
7、 the availability of any referenced commercial product at any time. The manufacturers instructions on use of any commercial product must be followed at all times, even if in conflict with the information in this publication.Library of Congress Cataloging-in-Publication DataJeffery, Harley.Loop check
8、ing :a technicians guide / Harley Jeffery.p. cm. - (ISA technician series)Includes bibliographical references.ISBN 1-55617-910-3 (pbk.)1. Process control-Automation. I. Title. II. Series. TS156.8.J44 2005629.83-dc222005001679Jeffrey05.book Page iv Thursday, March 3, 2005 2:56 PMTo Tamra, John, Chris
9、sy, Ashton, and SarahJeffrey05.book Page v Thursday, March 3, 2005 2:56 PMJeffrey05.book Page vi Thursday, March 3, 2005 2:56 PMACKNOWLEDGMENTSDuring the past several years, I have met many people who have shared their invaluable and memorable knowledge and insights. Loop Checking: A Technicians Gui
10、de combines this wisdom with the experiences of control loop device manufacturers, control system engineers, plant I the chapter takes a closer look at loop checking with and without smart instruments. A start-up scenario provides the process control example in Chapter 3.Chapter 4 Performance Benchm
11、arking reviews this tool which checks the loop and ensures performance is at the highest level possible, and which can set the benchmark for performance that is monitored over time (Chapter 5). Although performance testing occurs after start-up in this project sequence, the procedure can also be use
12、d with any loop/unit operation as a troubleshooting guide when process control problems arise, or as a general approach to improve existing operations. Again, planning is important; items to watch for are suggested and typical forms are included, along with some recommendations on how to perform the
13、 checks. The example process is then “benchmarked” using the techniques discussed in this chapter.Chapter 5 Sustaining the Performance makes sure you can maintain the high level of performance achieved and benchmarked in Chapter 4. Maintenance strategies show where the performance monitoring fits. S
14、uggestions on starting this program are included along with some in-depth considerations of how the program will address the elements of the loop. The implementation of a performance monitoring program is dependent on the particular control system architecture (single loop controllers, PLCs, DCSs, h
15、ybrid control, digital control systems, etc.) that your plant has installed (or is planning to install). Thus, several of these hardware architectures are examined with ideas and options for implementation. Jeffrey05.book Page xiv Thursday, March 3, 2005 2:56 PM1INTRODUCTION TO LOOPCHECKINGProcess c
16、ontrol loops have a major impact on the financial performance of todays manufacturing facilities. It is also recognized that a “good foundation” of the basic regulatory control loop is essential to the success of higher-level “Advanced Process Control (APC)” program (Ref. 6). Thus, it is critical th
17、at these facilities technicians focus on loop checking and performance. For this reason, this guide defines loop checking broadly to include control loop performance rather than merely in terms of plant start-up situations as in the traditional definition. Loop checking is also an important componen
18、t in any plants continuous improvement planning program insofar as it helps define and reduce the variability of key process parameters on an ongoing basis.The chapters of Loop Checking: A Technicians Guide are arranged to follow a typical automation project from design checkout at the factory accep
19、tance test (FAT) through to an ongoing sustaining loop performance program. The steps of such projects is as follows: loop checking basics the factory acceptance test (FAT)start-up performance benchmarking sustaining the performanceThis guide is intended to discuss general methods and practices that
20、 can be applied across many processes or industries. The technician will encounter different plans and programs in his or her own company for addressing loop performance. These will, of course, affect how loop checking is defined and accomplished for the technicians specific environment. However, th
21、e instrument technician typically has the best overall knowledge and skills for checking and maintaining control loop performance.Jeffrey05.book Page 1 Thursday, March 3, 2005 2:56 PM2 Introduction to Loop Checking1.1 THE OPPORTUNITYIn todays intensely competitive markets, manufacturers are striving
22、 to continually improve manufacturing performance to meet their business needs and goals. Typical business drivers are as follows: increased throughput increased yield increased quality minimized waste and off-specAs we noted, the control loop (and the continual checking of performance) plays a vita
23、l role in the plants financial performance. However, it has been observed that up to 80 percent of all loops are not performing their intended function of reducing the variability that results from the problems caused by the factors shown in Figure 1-1. Such issues as measurement placement and the d
24、ead time or process mixing it causes, undersized headers and valves, loop tuning, and control strategy, all affect the loops ability to accomplish the desired objectives.FIGURE 1-1Control Loop Performance IssuesIn addition, plant performance studies (such as those summarized in Figure 1-2) have show
25、n that the largest opportunity for reducing costs (1.5%) is provided by field device performance and loop tuning, where loop checking methods can be applied. Adequate Tuning-Reduce Variability20%Loop Tuning 30%Valve and Instrument Maintenance30%Control Strategy15%Process Design5%Jeffrey05.book Page
26、2 Thursday, March 3, 2005 2:56 PMLoop Checking 3FIGURE 1-2Process Improvement OpportunitiesThe Control System Technician (CST) can become involved in the performance of the plants control loops, from the control implementation stage through to the checkout phase and then continuing through start-up,
27、 commissioning, and ongoing operations.1.2 LOOP CHECKING: INTRODUCTIONThe following section reviews the components of the control loop and the scope of loop checking.Defining the LoopThe purpose of control loops has been defined in various ways: to force the process to perform in a predetermined, de
28、sirable manner. The process may be a flow, pressure, temperature, level, or some other variable in the manufacturing plant (Ref. 3). to adapt automatic regulatory procedures to the more efficient manufacture of products or processing of material (Ref. 4). to ensure safety, environmental regulation,
29、and profit (Ref. 5).The average chemical company could reduce COGS by 6% by using the best practices in process controlpCategories of Control00.20.40.60.811.21.41.6 Field DevicePerformance and LoopTuningUnit Operation ControlAdvanced regulatoryControlProduction ManagementControlAdvanced Multivariabl
30、eControlGlobal On-LineOptimizationAdvanced AdvisorySystemsProcess Data AccessManufacturing DataIntegrationSource: Dow ChemicalJeffrey05.book Page 3 Thursday, March 3, 2005 2:56 PM4 Introduction to Loop CheckingBasic to any discussion of control loops is “feedback” control. In this control, the loop
31、starts by measuring the process variable (PV). It then compares the PV to the desired value, that is, the set point (SP), and acts on the difference between SP and PV (error) using a control algorithm (typically PID). The loop then outputs to the final control element. The diagrams below indicate th
32、at the main elements of the loop are: transmitter/sensor (for measuring the PV) process controller (with an operator-entered SP and control algorithm) final control element (valve/actuator and accessories)Control system engineers use the block diagram in Figure 1-3 to show the relationships of the c
33、ontrol loop elements.FIGURE 1-3Feedback Loop Block DiagramIn a more practical view, the block diagram looks like Figure 1-4 below when depicted with hardware for measurement, controller and final control element functions.DisturbanceGd(s)ProcessGp(s)ValveActuatorGf(s)ControllerGc(s)SensorTransmitter
34、Gm(s)+-SetpointJeffrey05.book Page 4 Thursday, March 3, 2005 2:56 PMLoop Checking 5FIGURE 1-4Control Loop For the purposes of this guide, well focus on the single input, single output control loop as depicted in Figures 1-3 and 1-4.Elements of the LoopLets discuss each element in the loop. Although
35、several chapters could be dedicated to each element of the loop (a good resource is Reference 1), we will try to keep the discussion brief and highlight important features for our expanded definition of loop checking, which includes performance. Sensor / TransmitterThe loop starts here and cannot do
36、 a good job unless the measurement is accurate, reproducible (reproducibility is the closeness of agreement of an output for and input approaching from either direction at the same operating conditions over a period of time and is a better number for control and measurement evaluations see Reference
37、 6). Total Probable Error is another important performance specification that you may use for comparison purposes. Measurement resolution of the signal within the control system is usually not an issue with todays control Jeffrey05.book Page 5 Thursday, March 3, 2005 2:56 PM6 Introduction to Loop Ch
38、eckingsystems I/O design, but if you configure the loop for large spans (watch for temperature loops), then small changes can go undetected. Of course, proper selection and installation of the sensor and transmitter is critical based on service conditions, accuracy, reproducibility, stability, relia
39、bility, and other plant standards. Deadtime and noise introduced by measurement installation location can really hurt the loop performance (the typical controller, proportional, integral and derivative PID does not handle deadtime very well). For example, mounting a consistency transmitter so that i
40、t is convenient to work on versus placing it near the dilution source can introduce unwanted deadtime, while a sensor installed near the valve outlet instead of upstream of the valve will have excessive noise. ControllerThe controller compares the transmitter measurement (PV) to the operator-entered
41、 set point (SP), calculates the difference (error), acts on the error with a PID algorithm and outputs a signal to the valve. Todays control systems all have very capable controllers but you need to be aware of the type PID algorithm that your plants control system manufacturer has implemented. The
42、two (2) common types are called “classic” and “non-interacting” (Ref. 5). Others have called them “series” and “parallel”. There is a difference in how you tune the loop with these two types of controllers. If your plant has just one control system, then your plant standard tuning methods can be use
43、d without worrying about this difference. However, as plant purchases come from different vendors/OEMs, different control system types are employed (e.g., programmable logic controller PLC vs. distributed control system DCS). You need to pay attention when tuning the different controllers to make su
44、re the right tuning methods are applied. The microprocessor-based systems have also introduced us to configurable loop scan (execution) times, which can also be critical to loop performance. Youll want to make sure your controller is executing fast enough for the process dynamics. Table 1-1 suggests
45、 a starting point for some typical measurement types.Final Control ElementThe final control element takes the signal from the controller and attempts to position the flow controlling mechanism to this signal. There are various types of final control elements and some have better performance in terms
46、 of “positioning” the device. Final control elements can be variable speed drives for pumps or fans, dampers/louvers, heaterJeffrey05.book Page 6 Thursday, March 3, 2005 2:56 PMLoop Checking 7TABLE 1-1.Typical Scan Timescontrols, but the most common is the control valve. The valve receives the most
47、attention in the loop check because it receives an electrical signal from the controller (i.e., 420 mA current or digital value on a bus), converts the electronic signal to a pneumatic signal that must then drive an actuating device to a precise location. Well talk more about valve and loop performa
48、nce in later chapters but youll be hard pressed to beat a sliding stem valve with spring-and-diaphragm actuator and a two-stage positioner for performance. In addition to the controller performance enemies of deadtime and noise mentioned previously, the valve could also introduce non-linearities and
49、 deadband into the loop neither of which is good for the PID controller. In receiving an electronic signal and converting it to a valve plug/ball/disk position in the pipe, various sources of non-linearity and deadband can build up. Friction from seals and packing, backlash of mechanical parts, relay dead zones, shaft windup can keep the valve from maintaining the signal required by the control system. Proper valve sizing and selection of valve characteristic can help linearize the flow response to controller output changes a
