Control Room Equipment

4

4.1 ANNUNCIATORS AND ALARMS

580

Introduction 581 History 582 Principles of Operation 583 Operating Sequences 583 Annunciator Types 586 Integral Annunciator 586 Remote Annunciator 587 Semigraphic Annunciator 588 Recording Annunciators 589 Vocal Annunciators 590 Relay-Type Annunciators 590 Solid-State Annunciators 593 Annunciator Cabinets 595 Hazardous Area Designs 595 Intrinsically Safe Designs 595 Pneumatic Annunciators 596 Bibliography 597

Traditional Front Panel Layouts 602 Large-Case Instruments 602 Miniature Instruments 602 High-Density Instruments 603 Graphic Panels 603 Back-of-Panel Layout 607 Panel Materials of Construction 607 Panel Specifications 608 Human Engineering 608 Panel Tubing and Wiring 610 Tubing 610 Fittings 611 Panel Wiring 611 Control Center Inspection 615 Panel Shipment 616 Conclusions 616 Reference 616 Bibliography 616 4.3 CONTROL CENTER UPGRADING

4.2 CONTROL CENTERS AND PANELS— TRADITIONAL 598 Introduction 599 Traditional Control Rooms 599 Control Rooms for DCS Systems Traditional Control Panels 601 Flat Panels 601 Breakfront Panels 601 Consoles 601

600

618

Baseline Evaluation 619 Original Design Perspective 620 Information Zones and Task Analysis 621 Building a New Control Room 622 Technology Advances 623 Computerized Procedure Systems 626 Phased Approach 627 Planning Ahead 630 HMI Standards 630 Maintainability 631 Bibliography 631 575

© 2006 by Béla Lipták

576

Control Room Equipment

4.4 CONTROLLERS—ELECTRONIC ANALOG AND DIGITAL 633

Conclusions 674 References 676

Introduction 634 Analog vs. Digital Controllers 634 The Controller’s Function 635 Feature Checklist 635 On/Off and Direct-Connected Controllers On/Off Relay Outputs 636 Direct-Connected Controllers 636 Analog Controllers 637 Input Variations 638 Control Modes 639 Nonlinear Controllers 640 Special Features 641 Displays 642 Balancing Methods 643 Mounting 644 Servicing 645 Digital Electronic Controllers 645 Advantages and Disadvantages 645 Hardware Components 646 Software Capability 647 Faceplates and Programmers 648 Bibliography 649

4.7 DCS: CONTROL AND SIMULATION ADVANCES 677

4.5 CRT DISPLAYS

650

Introduction 651 Display Options 651 The Total System 653 Data Display Options 654 Keyboard 656 Display Capabilities 657 Refresh Memory 657 Character and Format Control 658 Vector Generator 658 Display Initiation 659 Propagation and Termination 661 Conclusions 662 Bibliography 662 4.6 DCS: BASIC TRENDS AND ADVANCES Introduction 666 Connectivity and Integration 666 Organizing the Project 667 The Future 669 Control Advances 669 Basic PID Algorithms 669 Auto-Tuning 671 Model-Based Control Algorithms DCS Bid Package 673 Costs 674

© 2006 by Béla Lipták

636

Introduction 677 Performance Monitoring 678 Controller Tuning 678 On-Demand Tuning 679 Adaptive Tuning 679 Fuzzy Logic Control 679 Model Predictive Control 681 Neural Network Applications 682 Process and Control Simulation 684 Conclusion 686 Reference 686 Bibliography 686 4.8 DCS: INSTALLATION AND COMMISSIONING Introduction 687 Installation 688 Power and Grounding 688 System Assembly 688 HVAC and Heat Tracing 689 Field Wiring and Checkout 689 Bus Installation 689 Installing HART Networks 690 Commissioning 690 Training and Preliminary Checkout 690 Fieldbus Testing 691 Process Startup 691 Commissioning of Control Loops 692 Advanced Control 692 Conclusion 692 References 692 Bibliography 692 4.9 DCS: INTEGRATION WITH BUSES AND NETWORKS 693

663

671

Introduction 693 Bus Integration 693 DCS Fieldbus Support 694 Field Networks 694 Fieldbus Devices 695 Function Blocks 696 Network Integration 697 Recent Integration Trends 698 Conclusions 698 References 698 Bibliography 699

687

Contents of Chapter 4

4.10 DCS: INTEGRATION WITH OTHER SYSTEMS 700

4.13 DCS: OPERATOR’S GRAPHICS

Introduction 700 Existing Systems 700 MODBUS Interface 700 OPC Interface 702 Motor Controls 702 Fieldbus Interface 703 Safety Systems 703 Conclusions 705 Reference 705 Bibliography 705

4.11 DCS: MANAGEMENT OF ABNORMAL CONDITIONS 706 Introduction 706 Abnormal Condition Management 706 Types of Control 707 Need for Operator Intervention 707 Psychological Basis for Intervention 707 Detect Phase 708 Sort/Select and Monitor Phases 709 Plan/Act Phase 709 Response Time 709 Planning the Intervention 709 Types of Operations 709 Managing Abnormal Conditions 710 Control Room Design 711 Operator Training 712 Alarm System Design 712 Graphical User Interface 716 Conclusions 718 Acknowledgments 718 References 718 Bibliography 718

4.12 DCS: MODERN CONTROL GRAPHICS

720

Introduction 720 Function Block Representation 720 Standard for Process Control 721 Function Block Modes 721 Function Block Types 722 Sequential Function Chart (SFC) 723 Ladder Diagrams 724 Batch S88 725 Safety Logic 725 Conclusions 725 Bibliography 726

© 2006 by Béla Lipták

727

Introduction 727 Operator Console Equipment 727 Video Display 728 Keyboards 728 Peripheral Devices 729 Remote and Web-Based Stations 729 Remote Clients 729 Web Pages 730 Operator Graphics 730 Types of Displays 731 Overview Graphic Displays 731 Graphic Displays 731 Faceplate with Detailed Display 732 Trend Displays 732 Static Graphic Components 732 Dynamic Elements 732 Dynamos 733 Aliases 734 Display Access 735 Process Performance Monitoring 736 Process Graphic Data Interfaces 736 Conclusion 738 Bibliography 738 4.14 DCS: SYSTEM ARCHITECTURE

739

Introduction 740 Analog Control 740 Direct Digital Control 741 Distributed Control System 741 Functional Components 742 DCS Control Network 742 Operator Console 744 Core Architectural Components 745 International Fieldbus Standards 749 Data Highway Designs 749 Control Network 749 Ethernet Configuration 749 Alarm Management 750 Alert Processing 750 DCS Attributes 752 Reliability 753 Mean Time between Failure 753 Pricing 754 Bibliography 755 4.15 DIGITAL READOUTS AND GRAPHIC DISPLAYS Introduction 758 Human Factors 759 Size and Contrast 759 Application Notes 760

757

577

578

Control Room Equipment

Mechanical and Electrical Counters 760 Gas Discharge Displays 761 Cathode Ray Tube Displays 762 Rear Projection Displays 763 Light-Emitting Diode Displays 763 Liquid Crystal Displays 764 Passive Matrix Liquid Crystal Displays 765 Active Matrix Liquid Crystal Displays 765 Vacuum Fluorescent Displays 767 New Trends in Graphic Displays 768 Plasma Displays 768 Field Emission Displays 768 Electroluminescent Displays 769 References 769 Bibliography 769

4.16 FIELDBUSES AND NETWORK PROTOCOLS 770 Introduction 770 Communications Hierarchy 770 Field Level 771 Control Level 771 Operations Level 771 Enterprise Level 772 Data Models 772 Network Basics 772 OSI Reference Model 772 Physical Layer 773 Data Link Layer 775 Network Layer 777 Transport Layer 777 Application Layer 777 Fieldbus Protocols 778 AS-i 778 HART 780 PROFIBUS DP/PROFIBUS PA 781 FOUNDATION Fieldbus 782 MODBUS 784 ControlNet 785 Industrial Ethernet 785 Netwide Data Exchange 786 OPC Servers 786 XML 787 FDT/DTM 787 Conclusion 788 References 788 Bibliography 789 Acronyms 789

© 2006 by Béla Lipták

4.17 HUMAN–MACHINE INTERFACE EVOLUTION

790

Introduction 790 Functions of the Control System HMI 790 Visualization and Control 791 Process Alarming 792 Trending 792 DCS Consoles 793 DCS Console Graphic Standards 793 DCS HMI Redundancy 794 DCS HMI Chronological Evolution 795 The Open HMI 795 Open HMI Display Standards 796 Documentation 796 Open HMI Evolution Chronology 796 Evolution of HMI Architecture 797 Evolution of Plant Networking 799 Evolution of Control Rooms 800 Evolution of the Process Operator 801 2005 and Beyond 802 Distributed and Mobile Control 802 Remote Operation of the Plant 803 The Future 803 References 804 4.18 INDICATORS, ANALOG DISPLAYS

805

Terminology 805 Electrical Movements 806 Indication of Measurements 806 Fixed-Scale Indicators 807 Movable-Scale Indicators 808 Parametric Indication 809 Digital Indicators 810 Acoustic Indicators 810 Bibliography 811 4.19 LIGHTS

812

Introduction 812 Light Source Characteristics 813 Light Selection 814 Colors and Flashing 814 Lenses and Operating Environments Light Components 814 Lamp Types 815 Incandescent Lamps 815 Neon Lamps 815 Solid-State Lamps (LEDs) 816 Virtual Lights 816 Checklist 816 Conclusions 816 Bibliography 817

814

Contents of Chapter 4

4.20 RECORDERS, OSCILLOGRAPHS, LOGGERS, TAPE RECORDERS 818 Introduction 819 Sensor Mechanisms 819 Galvanometric Recorders 820 Light-Beam Recorders (Oscillographs) Potentiometric Recorders 820 Open Loop Recorders 821 Linear Array Recorders 821 Recording Methods 822 Ink-Writing Systems 822 Inkless Systems 822 Paperless Systems 822 Charts and Coordinates 822 Circular Chart Recorders 823 Strip-Chart Recorders 824 Multiple Recorders 824 X–Y Recorders 825 Event Recorders 826 Tape Recording 826 Data Loggers 827 Bibliography 828

4.21 SWITCHES, PUSHBUTTONS, KEYBOARDS Introduction 830 Switch Designs and Operation 830 Switching Action 831 Contact Arrangements 831 Switching Elements and Circuits 831 Grades of Switching Devices 833 Types of Switching Devices 833 Pushbuttons 833 Toggle Switches 836 Rotary Switches 837 Thumbwheel Switches 838 Application and Selection 839 Human Factors 839 Display Movement 841 Error Prevention 842 Mechanical Features 844 Environmental Considerations 844 Bibliography 844

4.22 TOUCH-SCREEN DISPLAYS

845

Introduction 845 Touch Technology 845 Advantages 845 Touch-Screen Designs 845 Evaluating Touch Technologies

© 2006 by Béla Lipták

848

Overall System Design 851 Mechanical Considerations 851 Physical Attributes 851 Programming Considerations 852 Bibliography 853 820 4.23 UNINTERRUPTIBLE POWER AND VOLTAGE SUPPLIES (UPS AND UVS) 854

829

Introduction 854 Uninterruptible Voltage Sources (UVS) 855 Uninterruptible Power Supply (UPS) Features 856 Networks and Buses 856 Power Failure Classifications 857 Source Failure 857 Equipment Failure (Inverter) 858 Common Bus Branch (Load) Failure 860 System Components 861 Rotating Equipment 861 Batteries 862 Static Inverters 863 Bus Transfer Switches 865 Protective Components 865 Standby Power Supply Systems 865 Multicycle Transfer System 865 Sub-Cycle Transfer System 866 No-Break Transfer System 866 System Redundancy 866 Specifications 866 Bibliography 867

4.24 WORKSTATION DESIGNS

868

Classification of Workstations 868 Hardware Architecture 868 Function 869 Hardware Components 870 Software Features 871 Selection of Correct Platform 871 Comparing Various Operating Platforms 872 Cost 872 Reliability 872 Manageability and Administration 873 Scalability 873 Security 873 Error Handling 873 Integration of Software and Hardware 873 Openness 874 Conclusions 874 Glossary 874 Bibliography 875

579

4.1

Annunciators and Alarms J. A. GUMP

(1972, 1985)

E. M. MARSZAL

(2005)

B. G. LIPTÁK

(1995, 2005)

LAL

Low level alarm

TAH

High temperature alarm

PAHL High and low pressure alarm

Flow sheet symbol

580 © 2006 by Béla Lipták

Types:

A. Audiovisual Annunciators: integral, remote, and semigraphic systems with audible and visual display and electromechanical (relay) or solid-state (semiconductor) designs B. Recording Annunciators: integral, solid-state systems with recorded printout C. Bargraphs D. Vocal Annunciators: integral, solid-state systems with audible command message

Cost per Alarm Point:

Integral cabinet costs $75 to $175; remote system, $125 to $250; semigraphic system, $125 to $250; recording annunciator or annunciator with communications will add about 30% to the cost per point. These figures are budgetary in nature (± 20%), and a number of variables can affect the price. These factors include system size, window size, number of alarm points per board, field contact voltages, type of lamp, communications options, and required certifications (e.g., Class I, Div. 2, etc.).

Partial List of Suppliers:

4B Components Ltd. (A) (www.go4b.com) Acromag Inc. (A) (www.acromag.com) Adaptive Micro Systems Inc. (A) (www.adaptivedisplays.com) Advotech Inc. (D) (www.advotechcompany.com) Ametek Power Instruments (A, B, mosaic graphic) (www.ametekpower.com) Barnett Engineering Ltd. (A) (www.barnett-engg.com) Beta Calibrators div. Hathaway Process Instrumentation Corp. (A, B) (www.the-esb.com) CAL Controls Inc. (www.cal-controls.com) CEA Instruments Inc. (www.ceainstr.com) Cole-Parmer Instrument Co. (www.coleparmer.com) CTC Parker Automation (www.ctcusa.com) Daytronic Corp. (A) (www.daytronic.com) Devar Inc. (A) (www.devarinc.com) Draeger Safety Inc. (www.draeger.com/gds) Druck Inc. (www.pressure.com) Fisher Controls International Inc. (A) (www.fisher.com) Flow Tech Inc. (www.flowtechinc.com) Fluid Components International (www.fluidcomponents.com) Foxboro Co. (A) (www.foxboro.com) GE Kaye div. General Electric Co. (A) General Monitors (www.generalmonitors.com) Graybar Electric Co. (www.graybar.com) Honeywell Industry Solutions (www.iac.honeywell.com) ImageVision Inc. (www.imagevisioninc.com) Mauell Corp. (A) (www.mauell-us.com) Matrikon Inc. (www.matrikon.com) Metrix-PCM/Beta (A) (www.metrix1.com) Moore Industries Inc. (www.miinet.com) North American Manufacturing Co. (A) (www.namfg.com) Oceana Sensor (www.oceanasensor.com) Phonetix Inc. (www.sensaphone.com) Powers Process Control, A Unit of Mark Controls Corp. (A) (www.powerscontrols.com) Precision Digital Corp. (www.predig.com)

4.1 Annunciators and Alarms

581

ProSys Inc. (www.prosysinc.com) Puleo Electronics Inc. (A) (www.annuciator.com) Raco Manufacturing and Engineering (www.racoman.com) Robicon div. High Voltage Engineering Corp. (A) (www.robicon.com) Ronan Engineering Co. (A, B, mosaic graphic) (www.ronan.com) Schneider Electric/Square D (www.squared.com) Scott Aviation div. Tyco Inc. (A) (www.tycoelectronics.com) Seekirk Inc. (D) (www.seekirk.com) Sierra Monitor (www.sierramonitor.com) Swanson Engineering & Manufacturing (A) (www.amtonline.org) Texmate Inc. (A) (www.texmate.com) Thermo Brandt Instruments (www.brandtinstruments.com) Tips Inc. (www.tipsweb.com) Transmation Inc. (D) (www.transmation.com) Trip-A-Larm (A) (www.modicon.control.com) Visi-con div. Visicomm Industries (A) (www.alarmpanels.com) Vorne Industries Inc. (A) (www.vorne.com) Western Reserve Control (www.wrcakron.com) White Electronic Designs Corp. (C) (www.motionnet.com) Wilkerson Instrument Co. (A) (www.wici.com) Zetron Inc. (A) (www.zetron.com)

In addition to this section, safety alarm systems are discussed in several other parts of this handbook, particularly in connection with DCS and CRT systems in Chapter 4 and PLCs in Chapter 5. The subject of process alarm management is separately covered in Section 1.6. This section concentrates on dedicated, conventional annunciators and other alarm devices.

INTRODUCTION The purpose of an alarm system (annunciator) is to bring attention to an abnormal or unsafe operating condition in the plant. Traditional annunciators used discrete alarm modules for this purpose. These dedicated hardware units are diminishing in numbers yet are still used in installations where simplicity is desired or where separation from the basic process control system is required for safety reasons. In some installations where traditional units have been replaced by PLC- or DCS-based annunciators, the recognition of and response to alarm conditions have deteriorated because on computer screens they are not very visible and can go unnoticed. In addition, because of the low incremental cost of adding new alarm points, excessive numbers of alarms been configured. Because of the floods of alarms, an important new component of safety system design is alarm rationalization and alarm management (Section 1.6). It is possible to connect conventional annunciators as frontend devices to DCS systems through various communication links. There is a wide variety of such links available, ranging from serial links employing MODBUS protocol to Ethernet links utilizing Object Linking and Embedding (OLE) for Process Control (OPC). This hybrid solution adds the visibility, reliability, and built-in redundancy of dedicated annunciators to the flexibility and record-keeping convenience of DCSbased systems.

© 2006 by Béla Lipták

More sophisticated annunciator designs can incorporate bargraph-type displays, color computer graphics, and eventrecording or data-logging systems. Much of the new development in annunciator system designs involves enhanced methods of communication and reporting. As a consequence, annunciator status can be logged and used for tasks such as alarm management and abnormal event analysis. Graphic displays can be dynamic, where flow in pipes is shown by actual movement, and CRT displays can concentrate large amounts of information into a single display. Figure 4.1a illustrates such a display, where the CRT displays

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 A B C D E F G H I J K L Area 3-H of plant layout is flashing in coded color

Color code: 1-Combustibles 2-Smoke 3-Fire 4-High pressure

5-CO 6-SO2 7-Vinyl chloride

FIG. 4.1a The overall safety status of the plant can be displayed on a single CRT.

582

Control Room Equipment

the plot plan of the plant as the background. Such a plot plan can be separated into small square segments, so that if an unsafe condition is detected in a particular segment, the corresponding square can start flashing in the color that corresponds to the type of safety problem detected. This type of annunciator display is easily and quickly comprehended and can provide a summary report on a large number of safety conditions in an efficient manner.

HISTORY The term “drop” was initially applied to individual annunciator points, from which we may infer that annunciator systems developed from paging systems of the type used in hospitals and from call systems used in business establishments to summon individuals when their services are needed. These systems consisted of solenoid-operated nameplates that dropped when deenergized. The drops were grouped at a central location and were energized by pressing an electrical pushbutton in the location requiring service. The system also included an audible signal to sound the alert. Similar systems were used for fire and burglar alarms. The drops were operated either by manual switches or by trouble contacts that monitored thermal and security conditions in various building locations. The use of these systems in the chemical processing industry was a logical development when alarm switches became available. This development, however, was preceded by explosionproof, single-station annunciators that were designed to operate in the petroleum and organic chemical process plants constructed immediately before, during, and after World War II. They were usually installed on control panels located either outdoors near the process unit or in local control houses. A drop-type system could not be used in these locations because they were electrically hazardous. By the late 1940s, centralized control rooms were introduced. Drop-type annunciators were suited for these

Supplier

Optional pressure loss alarm switch or electrical power control unit

general-purpose central control rooms. However, more compact, reliable, and flexible annunciators were subsequently introduced. In the early 1950s, the plug-in relay annunciator was developed. Instead of utilizing solenoid-operated drops, it used electrical annunciator circuits with small telephone-type relays to operate alarm lights and to sound a horn when abnormal conditions occurred. The alarm lights installed in the front of the annunciator cabinets were either the bull’s-eye type or backlighted nameplate designs. The annunciators were compact, reliable, and because of the hermetically sealed relay logic modules, they could also be mounted in certain hazardous areas in addition to the general-purpose control rooms. In order to be mounted in Class 1 explosion-proof areas, they required purging (Figure 4.1b). Miniaturization of instruments and the use of graphic control panels initiated the development of remote annunciator systems, consisting of a remotely mounted relay cabinet connected to alarm lights installed at appropriate points in the graphic or semigraphic diagram. Solid-state annunciator systems with semiconductor logic modules were developed in the late 1950s. These permitted additional miniaturization and lowered both the operating power requirements and the amount of heat generated. The semigraphic annunciator was introduced in the late 1960s and fully utilized the high-density capabilities of solid-state logic. It has permitted the designs of very compact and flexible semigraphic displays in control centers. With the spread of digital communication networks and microprocessor-based smart instruments, the number of alarm points increased, their management as a function of importance (Section 1.6) became a separate field, and their displays were further miniaturized. These days, with the greater availability and reliability of integrated circuit logic components, alarms can be displayed on handheld tools in the field, on computer screens at workstations, or on CRTs in DCS systems.

Enclosure pressure indicator

Required enclosure protection vent

Installer Venturi orifice

Reference out Rapid exchange pressure gauge

Rapid exchange control valve

Purging gas supply

Enclosure warning nameplate Supply in

Rapid exchange pressure control filter regulator

Enclosure pressure control valve

Protected enclosure

FIG. 4.1b Local annunciators were available in explosion-proof designs or were mounted in air-purged enclosures when mounted in Class 1 areas. (Courtesy of Bebco Industries.)

© 2006 by Béla Lipták

4.1 Annunciators and Alarms

PRINCIPLES OF OPERATION The annunciator system consists of multiple alarm points. Each alarm circuit includes a trouble contact (alarm switch), a logic module, and a visual indicator (Figure 4.1c). The individual alarm points are operated from a common power supply and share a number of annunciator system components, including an audible signal generator (horn), a flasher, and acknowledge and test pushbuttons. In normal operation the annunciator system and individual alarm points are quiescent. The trouble contact is an alarm switch that monitors a particular process variable and is actuated when the variable exceeds preset limits. In electrical annunciator systems it is normally a switch contact that closes (makes) or opens (breaks) the electrical circuit to the logic module and thereby initiates the alarm condition. In the alert state, the annunciator turns on the visual indicator of the particular alarm point, the audible signal, and the flasher for the system. The visual indicator is usually a backlighted nameplate engraved with an inscription to identify the variable and the abnormal condition, but it can also be a bull’s-eye light with a nameplate. The audible signal can be a horn, a buzzer, or a bell. The flasher is common to all individual alarm points and interrupts the circuit to the visual indicator as that point goes into the alert condition. This causes the light to continue to flash intermittently until either the abnormal condition returns to normal or is acknowledged by the operator. The horn acknowledgment pushbutton is provided with a momentary contact: when it is operated, it changes the logic module circuit to silence the audible signal, stop the flasher, and turn the visual indicator on “steady.” When the abnormal condition is corrected, the trouble contact returns to normal, and the visual indicator is automatically turned off. The lamp test pushbutton with its momentary contact tests for burned-out lamps in the visual indicators. When activated,

Common system components

Power supply Audible Lamp signal test generator (horn) Flasher

Individual alarm points

Horn acknowledge

Trouble Logic contacts modules (process alarm switches)

Visual indicators

FIG. 4.1c The main components of a traditional annunciator system.

© 2006 by Béla Lipták

583

the pushbutton closes a common circuit (bus) to each visual indicator in the annunciator system, turning on those lamps that are not already on as result of an abnormal operating condition. Operating Sequences A wide variety of sequences are available to define the operation of an individual alarm point in the normal, alert, acknowledged, and return-to-normal stages in the annunciator sequence. The five most commonly used annunciator sequences are shown in Table 4.1d, identified by the original code designation of the Instrumentation, Systems, and Automation Society (ISA). These sequences were specified by the ISA-recommended practice RP-18.1, which has since been revised and updated into standard ISA 18.1. Because the old sequence designations are still used in some plants, some of their more common versions are listed in Table 4.1d and also described below. The sequence designations of the present standard ISA 18.1 will also be discussed below. The Old ISA Sequence Designations ISA Sequence 1B, also referred to as flashing sequence A, is the one most frequently used. The alert condition of an alarm point results in a flashing visual indication and an audible signal. The visual indication turns off automatically when the monitored process variable returns to normal. ISA Sequence 1D (often referred to as a dim sequence) is identical to Sequence 1B except that ordinarily the visual indicator is dim rather than off. A dimmer unit, common to the system, is required. Because all visual indicators are always turned on—for dim (normal), flashing (alert), or steady (acknowledged)—the feature for detecting lamp failure is unnecessary. ISA Sequence 2A (commonly referred to as a ring-back sequence) differs from Sequence 1B in that following acknowledgment the return-to-normal condition produces a dim flashing and an audible signal. An additional momentary contact reset pushbutton is required for this sequence. Pushing the reset button after the monitored variable has returned to normal turns off the dim flashing light and silences the audible signal. This sequence is applied when the operator must know if normal operating conditions have been restored. ISA Sequence 2C is like Sequence 1B except that the system must be reset manually after operation has returned to normal in order to turn off the visual indicator. This sequence is also referred to as a manual reset sequence and, like Sequence 2A, requires an additional momentary contact reset pushbutton. Sequence 2C is used when it is desirable to keep the visual indicator on (after the horn has been silenced by the acknowledgment pushbutton) even though the trouble contact has returned to normal. ISA Sequence 4A, also known as the first-out sequence, is designed to identify the first of a number of interrelated

584

Control Room Equipment

TABLE 4.1d The Old ISA Designations of Annunciator Sequences ISA Code for the Sequence IB

ID

2A

2C

4A

Annunciator Condition Normal

Process Variable Condition (Trouble Contact)

Visual Indicator

Normal

Off

Audible Signal

Use Frequency

Off

55%

Alert

Abnormal

Flashing

On

Acknowledged

Abnormal

On

Off

Normal again

Normal

Off

Off

Test

Normal

On

Off

Normal

Normal

Dim

Off

Alert

Abnormal

Flashing

On

Acknowledged

Abnormal

On

Off

Normal again

Normal

Dim

Off

Normal

Normal

Off

Off

Alert

Abnormal

Flashing

On

Acknowledged

Abnormal

On

Off

Return to normal

Normal

Dim flashing

On

Reset

Normal

Off

Off

Test

Normal

On

Off

Normal

Normal

Off

Off

Alert

Abnormal

Flashing

On

Acknowledged

Abnormal

On

Off

Return to normal

Normal

On

Off

Reset

Normal

Off

Off

Test

Normal

On

Off

Normal

Normal

Off

Off

Alert

Abnormal

Initial

Flashing

On

Subsequent

On

Off

On

Off

Acknowledged

4%

5%

28%

Abnormal

Initial Subsequent

On

Off

Normal again

Normal

Off

Off

Test

Normal

On

Off

All others

variables that have exceeded normal operating limits. An off-normal condition in any one of a group of process variables will cause some or all of the remaining conditions in the group to become abnormal. The first alarm causes flashing, and all subsequent points in the group turn on the steady light only. This sequence monitors interrelated variables. The visual indication is turned off automatically when conditions return to normal after acknowledgment. The New ISA Sequence Designations In the updated annunciator standard ISA 18.1, the sequence designations are different, as shown in Table 4.1e. The most widely used sequence, the basic flashing sequence, is now designated as

© 2006 by Béla Lipták

1%

7%

sequence A. The sequence designations in ISA 18.1 use the following letter codes: A M R F

= Automatic Reset = Manual Reset = Ringback = First-out

Therefore, using the ISA 18.1 sequence designations A-13 means that the annunciator has automatic reset and is provided with Option 13, which suggests the presence of a dim lamp monitor. For definitions of less frequently used sequences, refer to ISA 18.1.

4.1 Annunciators and Alarms

585

TABLE 4.1e The New ISA Designations of Annunciator Sequences as Defined by ISA Standard ISA 18.1

Normal

Alert

Condition-sensing Returns to Normal Before Acknowledge

Visual

Off

Flash

Flash

On

Off

—

Audible

Off

On

On

Off

Off

—

Visual

Off

On

On

On

Off

—

Audible

Off

On

On

Off

Off

—

Visual

Off

Flash

Off

On

Off

—

Audible

Off

On

Off

Off

Off

—

Visual

Off

On

Off

On

Off

—

Audible

Off

On

Off

Off

Off

—

Visual

Dim

Flash

Flash

On

Dim

—

Memory—flasher

Audible

Off

On

On

Off

Off

—

Continuous lamp test

Signal Device

Sequence A

A-5

A-4

A-4-5

A-13

Optional Operating Features Annunciator sequences may be initiated by alarm switch trouble contacts that are either open or closed during normal operations. These are referred to as normally open (NO) and normally closed (NC) sequences, respectively, and the ability to use the same logic module for either type of trouble contact is called an NO-NC option. It is important because some alarm switches are available with either an NO or an NC contact but not with both, and therefore without the NO-NC option in the logic module two types of logic modules would be required. The logic module is converted for use with either form of contact by a switch or wire jumper connection. The relationship between the NO and NC sequences required in the logic module to match the various trouble contacts and analog measurement signal actions is shown in Figure 4.1f. A high alarm in a normally closed annunciator

High

Off-normal limit NC

Annunciator sequence Sensor action

Direct

Trouble contact-“on shelf” NC High alarm

Reverse Direct Reverse NO

Off-normal limit

NO

NC Direct

NO Trouble contact-“on shelf” Low alarm

NO

Reverse Direct Reverse NC

NC

FIG. 4.1f Logic trees for the NO and NC annunciator sequences.

© 2006 by Béla Lipták

NC

Low

Annunciator sequence Sensor action

NO

NO

Acknowledge

Condition-sensing Returns to Normal

Return to Normal Reset

Remarks Flasher memory

Memory

Flasher

system requires a normally closed trouble contact operated by a direct-acting analog input. If an increase in the measured variable results in an increased output signal, the detector is direct acting; if the output signal is reduced, it is a reverse-acting sensor. If the trouble contacts in all alarm switches in the plant are standardized such that normal operating conditions will cause all trouble contacts to be NC (or NO), the required annunciator sequence is also NC (or NO), and Figure 3.1f need not be consulted. Annunciator systems are fail safe or self-policing if they initiate an alarm when the logic module fails because of relay coil burnout. The feature is standard for most NO and NC annunciator sequences; annunciators using NC trouble contacts are also fail safe against failures in the trouble contact circuit. The lock-in option locks in the alert condition initiated by a momentary alarm until the horn acknowledgment button is pushed, preventing loss of a transient alarm condition until the operator can identify it. The logic module is usually changed from lock-in to non-lock-in operation by either addition of a wire jumper or operation of a switch. The lock-in feature is useful for monitoring unstable or fluctuating process variables. Test and Repeater Features The test feature in a standard annunciator serves only to test for burned-out lamps in the visual indicators. The operational test feature provides a test of the complete annunciator system, including logic modules, lamps, flasher, audible signal, and acknowledgment circuits. The operational test circuit usually requires an additional momentary contact pushbutton, which can replace the regular lamp test pushbutton. The logic module of relay-type annunciators may have spare (electrically isolated) auxiliary contacts that can operate shutdown and interlock systems when alarm conditions occur. The auxiliary contacts are wired to terminal blocks in the annunciator cabinet for connection to external circuitry.

586

Control Room Equipment

Repeater lights may be located away from the common logic module and serve to alert operators in other areas. Annunciator cabinet terminals for connecting these repeater lights in parallel with the annunciator visual indicator are available. It may also be desirable to actuate a horn in more than one location. The electrical load of multiple audible signals requires an interposing relay, called a horn-isolating relay, operated by the logic modules. This relay has contacts of adequate capacity to operate multiple audible signals. Hornisolating relays may be installed either in the annunciator cabinet or in a separate assembly. Annunciator systems can be used for several operational sequences without changing system wiring, and many logic modules can supply more than one operational sequence. This multiple sequence capability is sometimes useful when the sequence has not yet been determined.

ANNUNCIATOR TYPES The audiovisual annunciator can be packaged as an integral, remote, or semigraphic annunciator.

Integral Annunciator The integral annunciator, a cabinet containing a group of individual annunciator points wired to terminal blocks for connection to external trouble contacts, power supply, horn and acknowledge and test pushbuttons, is the most economical of the various packaging methods available in terms of cost per point. It is also the simplest and cheapest to install. Two methods of packaging integral annunciators are illustrated in Figure 4.1g. In the nonmodular type, plug-in logic modules are installed inside the cabinet and connected to alarm windows on the cabinet door through an interconnecting wiring harness; in the modular type, individual plug-in alarm point assemblies of logic module and visual indicator are grouped together. The nonmodular and modular cabinet styles are both designed for flush panel mounting with the logic modules and visual indicators accessible from the front. Electrical terminals for the external circuitry are located in the rear of the cabinet and are accessible from the back. Integral annunciators are used on nongraphic and on semigraphic control panels in which physical association of the visual indicators with a specific location in the graphic

Back lighted name plates Pushbutton stations

Relay type logic modules

Front view-door closed Solid state alarm module Lamp module

Window bezel

Front view

Alarm point terminals

Mounting clamp

Front view with door open

Visual indicators

(1) Nonmodular Rear view of terminal enclosure

Filter module

(2) Modular type

Power and system function terminals

Rear view with cover removed

FIG. 4.1g Integral annunciator cabinets are available in modular (right) and nonmodular (left) construction.

© 2006 by Béla Lipták

4.1 Annunciators and Alarms

587

(5) (1)

(3)

(6)

(2)

(4)

(7)

FIG. 4.1h Integral annunciator window configurations. 1. Modular single-point annunciator. 2. Modular double-point annunciator. 3. Modular triplepoint annunciator. 4. Modular quadruple-point annunciator. 5. Nonmodular single-point. 6. Nonmodular triple-point. 7. Nonmodular singlepoint with small nameplate.

process flow diagram is not required. Integral annunciator cabinets occupy more front but less rear panel space than the equivalent remote designs. The electrical terminals are in a general-purpose enclosure at the rear of the cabinet, and trouble contacts can be wired directly to them, thus eliminating the need for and resultant costs of intermediate terminal blocks for trouble contact wiring. An advantage of the modular-type cabinet is that it can be expanded by enlargement of the panel cutout and by the addition of modular alarm point assemblies. Nonmodular cabinets cannot be expanded, and new cabinets must be installed to house additional alarm points. Consequently, one should include more spare points when specifying the cabinet size for a nonmodular system. The modular cabinet is also more compact, takes up less panel space, and has a greater visual display area per point than the nonmodular design. Figure 4.1h illustrates various configurations of visual indicators that can be supplied with integral annunciator cabinets. Many of these groupings are also available in singleunit assemblies for remote annunciator systems.

They are used with full and semigraphic control panels and in nongraphic applications in which an integral annunciator cabinet may require too much front panel space. Figure 4.1i

(1) Chassis with general purpose enclosure

Remote Annunciator The remote annunciator differs from the integral annunciator in that the visual indicators are remote from the cabinet or chassis containing the logic modules. Remote annunciators were developed to allow the visual indicators to be placed in their actual process location in the graphic flow diagram.

© 2006 by Béla Lipták

(2) Chassis

FIG. 4.1i General purpose, remote annunciator cabinets.

588

Control Room Equipment

illustrates a remote annunciator chassis with optional cabinet enclosure. The chassis contains spare positions for plug-in logic modules and a system flasher. Auxiliary system modules, such as horn-isolating relays, may also be plugged into the logic module chassis positions. The chassis and cabinet enclosure are designed for wall or surface mounting behind the control panel. Each chassis position has terminal points for connecting the visual indicator and trouble contact. In addition, the chassis has a system terminal block for connecting electrical power, horn, flasher, and acknowledge and test pushbuttons. The disadvantages of remote annunciators include higher equipment and installation costs and an increased requirement for back panel space. In addition, the wiring connections from field trouble contacts must be made to intermediate

Replaceable translucent drawing sheet

terminal blocks rather than directly to the cabinet terminals, as with the integral annunciator. These terminal blocks, the terminal enclosure, and the required wiring result in higher installation costs and extra space requirements. Finally, the remote annunciator is difficult to change, and modification costs of remote systems are substantially higher than those of the integral type, partially because spare visual indicators cannot be installed initially. Semigraphic Annunciator The semigraphic annunciator developed in the late 1960s combines some of the advantages of the integral annunciator with the flexibility to locate visual indicators at appropriate points in a graphic flow diagram. Figure 4.1j illustrates a semigraphic annunciator.

Removable transparent plastic protective panel

Lamp nesting panel

Aluminum housing Main bus terminals (Accessory)

Knockouts

(+) HV CN (−) LC L2 L1

FC FC FC F2 F1 FR R K T C

(+) HV CN (−) LC L2 L1

FC FC FC F2 F1 FR R K T C

(+) HV CN (−) LC L2 L1

1-2 1-1

1-3

1-4

1-5

1-6

1-8 1-7

1-9

1-10

1-11

1-12

1-13

1-14

FC FC FC F2 F1 FR R K T C

G15 G14 G13 G12 G11 G10 G9 G8 G7 G6 G5 G4 G3 G2 G1 FC FC FC

Annunciator logic-rack, plug-in modules and power supplies

2-2 2-1

2-3

2-4

2-5

2-6

2-8 2-7

2-9

2-10

2-11

2-12

2-13

2-14

POS.

S15 S14 S13 S12 S11 S10 S9 S8 S7 S6 S5 S4 S3 S2 S1 FC FC FC 2-15

Terminals located under rear cover

1-15

POS.

S15 S14 S13 S12 S11 S10 S9 S8 S7 S6 S5 S4 S3 S2 S1 FC FC FC

Main bus terminals (Power)

G15 G14 G13 G12 G11 G10 G9 G8 G7 G6 G5 G4 G3 G2 G1 FC FC FC

Lamps

Window position numbers (4 high by 15 wide)

3-2 3-1

3-3

3-4

3-5

3-6

3-8 3-7

3-9

3-10

3-11

3-12

3-13

3-14

3-15

POS.

S15 S14 S13 S12 S11 S10 S9 S8 S7 S6 S5 S4 S3 S2 S1 FC FC FC

G15 G14 G13 G12 G11 G10 G9 G8 G7 G6 G5 G4 G3 G2 G1 FC FC FC 12 10

8

12 10

8 12 10 8 12 10 8 12 10 8

Pin numbers of socket

4-2 4-1

4-3

4-4

4-5

4-6

4-8 4-7

4-9

4-10

4-11

4-12

4-13

4-14

4-15

Detailed view of the grid (lamp nesting panel)

POS.

A15 A14 A13 A12 A11 A10 A9 A8 A7 A6 A5 A4 A3 A2 A1

B15 B14 B13 B12 B11 B10 B9 B8 B7 B6 B5 B4 B3 B2 B1 11

8

7

11

9

7

To signal terminals of row 4 wired for transformers

11 9 7 11 9 7 11 9 7

Pin numbers of socket Rear view showing terminal block arrangement

FIG. 4.1j Construction of a typical semigraphic annunciator, showing the plug-in modules,the lamp nesting panel, and the arrangement of the terminal blocks on the rear of the unit.

© 2006 by Béla Lipták

4.1 Annunciators and Alarms

A

589

T

A A

T

T

Integral

Remote

Semigraphic

FIG. 4.1k Control panels with integral, remote, and semigraphic annunciators.

It consists of a cabinet containing annunciator logic modules wired to visual indicators placed in a 3/4-inch (18.75-mm) lamp insertion matrix grid forming the cabinet front. The semigraphic display is placed between the lamp grid and a transparent protective cover plate, and the visual indicators are positioned to backlight alarm name plates located in the graphic display. The protective cover and lamp grid are either hinged or removable to provide access to the logic module and lamp assemblies. The lamp assemblies are connected to intermediate terminals located behind the lamp grid, and the terminals in turn are connected to the logic modules. Terminal points for trouble contact wiring are in the back of the cabinet. The semigraphic annunciator is flexible, and changes in the annunciator system, graphic display, and related panel modifications can be made easily and cheaply. It is practical to prepare the graphic displays in the drafting room or model shop, thus protecting proprietary process information of a confidential nature. The graphic display has little or no effect on completing either the annunciator or the control panel because it can be installed on site or at any time. The semigraphic annunciator has a high density of 40 alarm points per linear foot (0.3 m) and a solid-state rather than relay-type logic design. Power supplies are self-contained in the semigraphic annunciator cabinet. Front panel layouts illustrating integral, remote, and semigraphic annunciators are shown in Figure 4.1k. Integral systems similar to the one shown at the left in the figure are normally specified on nongraphic control panels. The graphic panel in the center contains a remote annunciator

© 2006 by Béla Lipták

with backlighted nameplates (shaded rectangles) and pilot lights (shaded circles) for visual indication. The remote system may also be used with miniature lamps in a semigraphic display similar to the one shown at the right. Recording Annunciators Solid-state, high-speed, recording annunciators are available to amend or substitute a printed record of abnormal events for only visual and/or audible alarms. These systems print out a record of the events and identify the variable, the time at which the alarm occurs, and the time at which the system returns to normal. They can also discriminate among a number of almost simultaneous events and print them out in the time sequence in which they occurred. A number of optional features, including secondary printers at remote locations, supplementary visual indication, and computer interfacing, are also available. The typical unit consists of logic, control, and printer sections. The input status is continuously scanned. If a change in the trouble contacts has occurred since the preceding scan cycle, the central control places the exact time, the alarm point identification, and the new status of the trouble contact (normal or abnormal) into the memory and initiates the operation of the output control unit (Figure 4.1l). The output control unit accepts the stored information and transfers it to the printer, which logs the event. Following this, the memory is automatically cleared of the data and is ready to accept new information. In addition to or in place of the printer (if a permanent record is not required), a CRT display can serve as the event readout. Trouble contacts are

590

Control Room Equipment

Armature

d nte ou C m N ld r Fie O ole N ub s trontact co ut Inp us t a t s c i log

Coil

Spring

an Sc ol tr con

Form B Form A

Spacer

A

Coil Armature

B

Permanent magnet

al ntr Ce trol con

NS

Coil

Coil

ry mo Me al g it Di k & c clo ndar e c al t tpu Ou trol con

Pr

r te in

FIG. 4.1l Functional block diagram of a digital, recording annunciator with its printer, which can be replaced by a CRT.

connected to terminal points in the logic cabinet, and a cable connects the cabinet and the printer. A recording annunciator can perform more sophisticated monitoring than an audiovisual annunciator and is correspondingly more expensive on a per-point basis. System cost per point decreases as the system size increases. Higher equipment cost, however, is offset in part by savings in control panel space and in installation costs. Recording annunciators are frequently used by the electrical power generating industry but may be applied to advantage in any industrial process that must monitor large numbers of operating variables and analyze abnormal events efficiently. Vocal Annunciators Vocal annunciators are unique in the type of abnormal audible message they produce. The audible output is a verbal message identifying and describing the abnormal condition when it occurs and repeating the message until the operator acknowledges the difficulty. The system continuously scans the trouble contacts, and when an abnormality is found, it turns on a flashing visual indicator and selects the optional proper verbal message for broadcast. The visual indicator is turned off by the system when the point returns to normal. The control unit also arranges the messages to be broadcast in the order in which the difficulties occur. In the event of multiple alarms, the second message is played only after the first has been acknowledged. The flashing visual indicator for each point, however, turns on

© 2006 by Béla Lipták

Pivot C

D

Glass capsule

FIG. 4.1m Standard electro-mechanical relay structures include: A) the clappertype, B) the phone-type, C) the balanced-force, and D) the Reed relay.

when the point becomes abnormal. The verbal message may be broadcast simultaneously in the control room and related operating areas, thus permitting personnel at the operating unit to correct the problem immediately. Relay-Type Annunciators The basic element of this annunciator is an electrical relay wired to provide the logic functions required to operate a particular sequence. Figure 4.1m illustrates some of the basic relay designs. At least two relays per logic module are necessary for most sequences. The relays are installed and wired in a plugin assembly, which is the logic module for a single alarm point. The plug-in module assembly is usually hermetically sealed in an inert atmosphere to prolong the life of the relay contact. The sealing also makes the logic module acceptable in certain hazardous electrical areas. Figure 4.1n is a semischematic electrical circuit for a remote system with sequence operation according to old ISA Sequence 1B (new Sequence A): Two logic modules are shown; one is in normally closed operation and the other is in normally open operation. The remote visual indicators for each alarm point, the horn, the flasher, and the acknowledge and test pushbuttons common to the system are also shown. Each logic module has two relays, A and B, shown in their deenergized state according to normal electrical convention. The operation of these circuits during the various stages of sequence operation is as follows: Normal The trouble contact of the NC alarm point is wired in series with the A relay coil at point terminals H and NC. In the normal condition, the trouble contact is closed, relays

4.1 Annunciators and Alarms

591

Operational sequence Signal lamps

Audible signal

Normal Abnormal Abnormal

Off Flashing Steady−on

Normal Normal

Off Steady−on

Trouble contact

Condition Normal Alert Audible silenced (acknowledged) Normal again Lamp test

Flasher

F2 Internal wiring of plug-in

F1

9 6 4 7

“B” relay

Off On Off

Energized Deenergized Deenergized

Energized Energized Deenergized

Off Off

Energized Energized

Energized Energized

Logic module

Logic module

A5

A5

B5

A

B

A1 M

“A” relay

A7

B1

A6

8 10 11 9 2 1

5 7

A B3 B4

A1

B2 A2 A3 A4

B1

B6

4

3

B5

6

B A7

B3 B4

B6 A6

B2 A2 A3 A4

8 10 11 9 2 1 5 7 4

3

6

Resistor R

Internal chassis wiring

R

K N F T R C H

K N F T R C H

K N F T R C H

Main bus terminals

SL H NC NO L B

A

SL H NC NO L B

No auxiliary contact terminals

Lock-in jumper NC Trouble contact

L

Remote lamp

Lock-in jumper

Field wiring T Lamp test

R

C Audible signal

N H

Audible silence (acknowledge)

A

Typical set of point terminals

No trouble contact L “H” wire common to all signal lamps

Power supply

FIG. 4.1n Semischematic diagram of a relay-type annunciator designed for ISA new sequence A (old sequence 1B), which sequences are described in Tables 4.1d and 4.1e.

A and B are energized, and all A and B relay contacts are in the state opposite that shown. Relay A is energized from power source H through the closed trouble contract, relay coil A, resistor R, terminal K,

© 2006 by Béla Lipták

and jumper to the neutral side of the line N. Relay B is also energized from H through the normally closed acknowledge pushbutton to terminal C, closed contact A2, and relay coil B, to N. Relay B is locked in by its own contact B2, which

592

Control Room Equipment

closes when relay B is energized. The visual indicator is turned off by open contact A5. The audible signal and the flasher motor are turned off by open contacts A3 and B4 in the same circuit. Alert The trouble contact opens, deenergizing relay A and returning all A relay contacts to the state shown in Figure 4.1n. The visual indicator is turned on, flashing through circuit H, lamp filament, terminal L, closed contacts A5 and B6, bus F, and flasher contact F1, to N. The flasher motor is driven through circuit C, closed contacts A3 and B3, R bus, and the flasher motor to N, and the audible signal is turned on by the same circuit. Acknowledged Relay B is deenergized by operating (opening) the momentary contact horn acknowledgment pushbutton and is locked out by open contact A2. All B relay contacts are returned to the state shown in Figure 4.1n. The visual indicator is turned on steady through closed contact B5 to N and is disconnected from flasher contact F1 by open contact B6. The flasher motor and audible signal are turned off by the horn acknowledgment pushbutton and remain off as a result of open contact B3. Normal Again When the variable condition returns to normal, the trouble contact closes to energize relay A, the visual indicator is turned off by open contact A5, relay B is energized by closed contact A2, and all circuits are again in the state described under normal. Lamp Test The lamp test circuit operates the visual indicators of only those alarm points that are in the normal condition. The circuit is completed through power source H, lamp filament, terminal L, closed contact A6, bus T, and normally open momentary contact lamp test pushbutton, to N. Closing the lamp test pushbutton to N completes the circuit and lights the visual indicators. Alarm points that are in the off-normal condition (either alert or acknowledged) do not operate because their A relays are deenergized and the A6 contact is open. The visual indicators of these abnormal alarm points are already turned on (either flashing or steady) through the operation of the alarm sequence. Lock-In The lock-in feature operates to prevent an annunciator alert condition (caused by a momentary alarm) from returning to normal until the horn acknowledgment button is pushed. Point terminals H and SL are jumpered to provide the lock-in feature (see Figure 4.1n). When the trouble contact opens, the A relay is deenergized, and power source H is applied to the N side of the relay through closed contacts A1 and B1. The power is dissipated through resistor R, terminal K, and jumper to N. If the trouble contact returns to normal, relay A will remain deenergized because potential H is on both sides of the coil. If the acknowledgment button is pushed before the trouble contact closes again, relay B will

© 2006 by Béla Lipták

be deenergized, opening contact B1 and the lock-in circuit, thus permitting the system to return automatically to normal when the trouble contact closes. If the acknowledgment button is pushed after the trouble contact has reclosed, contact B1 opens momentarily, allowing the A relay to reenergize. Contact A1 opens, and the circuits are reestablished in their normal operating state. Operational Test Full operational test is incorporated in the annunciator sequence shown by replacement of the jumper connection between main bus terminals K and N with a normally closed momentary contact pushbutton, which when pushed opens all annunciator circuits, thus initiating the alert condition of all alarm points in the normal condition. Auxiliary Contacts Normally closed contact A7 connected to point terminals A and B is available for auxiliary control functions. Relay Fail-Safe Feature Two parallel circuits, one consisting of closed contact A3 and open contact B3 and the other of open contact A4 and closed contact B4, operate an alert signal when there is a failure of either the A or the B relay coil. A failure of the former initiates a normal alert in the same way as the trouble contact. A failure of the B relay turns on the audible signal through closed contacts A4 and B4. Normally Open Trouble Contacts The annunciator sequence and features described for NC trouble contacts operate in essentially the same way when NO contacts are used. In the NO system, however, the trouble contact is wired in parallel with the A relay coil to point terminals H and NO, and a wire jumper is installed between point terminals H and NC. Normally, the trouble contact is open and the A relay is energized from terminals H and jumper to terminal NC. In the alert condition, the trouble contact closes to deenergize the A relay by applying power source H to the N side of the relay. Electromechanical relays are available for use with a variety of AC and DC voltages, but 120 AC, 50 to 60 Hz, and 125 DC are the most popular. Power consumption of the logic modules is normally less than 10 voltamperes (AC) and 10 watts (DC). Special low-drain and no-drain logic modules are available; these consume no power during normal operation. Visual indicators consume different amounts of power, depending on the type. Small bull’s-eye lights and backlighted nameplates use approximately 3 watts, whereas large units require 6 to 12 watts, depending on whether one or two lamps are used. Electromechanical annunciator systems are reliable and may be used at normal atmospheric pressures and ambient temperatures in the 0 to 110°F (–17.8 to 43.3°C) range. They are not position sensitive. They will generate a substantial amount of heat during plant shutdown when a large number of points are askew, and therefore power should be disconnected during these periods. The principal disadvantages of

4.1 Annunciators and Alarms

the relay-type annunciator are size, power consumption, and heat generation. Solid-State Annunciators A solid-state logic module consists of transistors, diodes, resistors, and capacitors soldered to the copper conductor network of a printed circuit board supplying the required annunciator logic functions. The modules terminate in a plug-in printed circuit connector for insertion into an annunciator chassis; they may also contain mechanical switching or patching devices to provide lock-in and NO-NC options. Figure 4.1o is a semischematic electrical circuit for a remote system with ISA Sequence 1B. The logic module shown is in normally closed operation. Remote lamps for two points and a flasher-audible module, speaker, and acknowledge and test pushbuttons common to a system are also included. Switch S1 is the NO-NC option switch and is shown in the NC operating position. Switch S2 is the lock-in option switch and is shown in the lock-in position. The following description uses negative logic, i.e., a high equals a negative voltage, whereas a low is approximately 0 volts. Normal The trouble contact of the NC alarm point is connected to an input filter circuit consisting of resistors R13 and R50 and capacitor C1. This provides transient signal suppression as well as voltage dropping. The slide switch S1 connects the trouble contact and filter network to resistorR14. In this state transistor T1 is conducting, causing the full negative voltage to be dropped across resistor R17, resulting in a low voltage at the bottom end of resistor R20. Transistors T2 and T3 are the active elements of the input memory and are roughly equivalent to the A relay of an electromechanical module (see Figure 4.1n). The base of T2 has four inputs, including resistor R20, either directly from the trouble contact in NO operation or from the collector of T1 in NC operation; resistor R19 with a locking signal from the alarm memory transistor T5; resistor R28 and capacitor C2, which form a regenerative feedback from T3; and resistor R15 from the test circuit. The base of T3 has one input, resistor R29 from the collector of T2. In normal operation, all four inputs to the base of T2 are low, T2 is not conducting, and T3 is conducting. Conversely, when a high signal is present at any one of the four inputs to the base of T2, T2 conducts and T3 turns off. Transistors T4 and T5 are the active elements of the alarm memory and are approximately equivalent to the B relay of an electromechanical module. T4 and T5 together with bias resistors R30 and R33 and cross-coupling resistors R31 and R32 form a bi-stable (flip-flop). In normal operation T4 is off and T5 conducts. The upper end of capacitor C4 is connected to the collector T2. When T2 is off, its collector is at a high and capacitor C4 will change from top to bottom, minus to plus. Transistor T7 is a high-capacity lamp amplifier and T6 is its preamplifier. In normal operation the base of T6 is high and T6 is on, T7 is off, and the visual indicator is off.

© 2006 by Béla Lipták

593

Before completion of the description of the normal condition, the operation of the flasher-audible module in Figure 4.1o will be described. The module has two oscillators. The first is a 3-Hz unit generating a signal that is amplified and supplied to the logic modules through the Fl bus. The second is a 700-Hz oscillator generating a signal that is amplified and supplied to the audible signal through the R bus. The audible signal is a permanent magnet-type transducer (speaker) that converts the electrical energy into sound. Initiated by an audio oscillator, the active elements of which are transistors T1 and T2, these transistors together with passive components (capacitors C1 and C2 and diodes D3 and D4) form an unstable multivibrator when an input is present on the FR bus. In the normal conditions there is no FR signal, the voltage necessary to turn on transistor T1 is missing, and the oscillator will not operate. This is the normal, or quiescent, condition. Alert The trouble contact opens and the base of T1 becomes low, turning off T1. This action produces a high on the base of T2 through R20, which turns on T2 and turns off T3. The negative end of C4 is clamped to common through T2, causing a positive pulse at the base of T5 through diode D12, turning T5 off and T4 on. With T2 conducting, the base of T6 becomes low through resistor R6, and with T5 off the clamp on the flasher signal is removed through diode D4 at the junction of R1 and R2. The flasher source provides an alternating high and low voltage at the Fl bus, which turns T6 on and off, which in turn turns T7 and the light off and on. The flasher signal is generated by transistors T8 and T9, which are the active elements of an unstable multivibrator used as an on/off signal to the output driver stage. Resistor R24 and capacitor C6 decouple the oscillator from the power lines so that its frequency is not affected by that of the other oscillators. The output driver stage consists of transistors T10 and T11, a switching inverter, and an emitter follower stage, which produces an alternating high and low voltage of F1 bus through R23. Transistor T11 is a high-current transistor capable of driving a multiple lamp load. The audible signal is initiated by a high on the FR bus, which turns on an audio oscillator, the elements of which are transistors T1 and T2. The audio oscillator output is amplified in an audio amplifier stage composed of transistors T3, T4, T5, and T6 connected in two pairs—one T3 and T5; the other T4 and T6. The input components to the stage from the audio oscillator are opposite each other, i.e., whenever one is high (negative voltage) the other is low (near zero), causing only one pair of transistors to conduct at a time. When T5 is off, T6 is on and the capacitor is discharged. This alternating action causes an alternating current to flow in the speaker coil, giving an audible signal. Acknowledged A negative voltage is applied to the base of T5 through resistor R40 by closing the acknowledge pushbutton. This turns T5 on and T4 off, and the FR bus becomes

594

R18

R17

R19

R24

R20

R22

R23

R21

R7 R6

R

3

9

D6

11

C5

5

8 12

S G

Κ C (+) F2

Main bus terminals

No trouble contact L

2

Capacitors (1) filter network required per chassis

T8

Typical set of point terminals

G

NC trouble contact Remote lamps

R8

L1 S

R9

FR F1 (−)

R5

1

Τ

R3

7 10 9 8

S1 Shown for NC field contacts S2 Shown for ‘lock-in’ on momentary alarms

T10

T9

R1

* *

D1

R2

5

*

C (−)

C3 C4

T1 T2 R11

3

R12

R14

2

R10

D2 R33

1

T5

R40

12

T4

T4

D4 D3 C1

T6

R30

6 11

R23

R43

D3

T3

T K

C2

D14

R34

R35

C1

R21

R16

R13

R50

R9

R1

R15

D1

R29

R48

D2

T2

T11

(+) Resistors

T3

T5

T6 T1

R4

R15

R32

R31

R19

R25

R28

S1

**

S2

R41 D12 C4

C2

R17

*

R14

R6

R2

T7

R24

R20

R10

D4

R13

D5

To additional signal lamps

L

LC T

R

C

Capacitor

Operational sequence Nameplate Trouble Signal lamps Contact Normal Off Abnormal Flashing Abnormal Steady-on

Condition Normal Alert Audible silenced (Acknowledged) Normal again Functional test

Off Flashing

Normal Normal

Audible Signal Off On Off Off On

T1 NO Off Off Off Off Off

T1 NC On Off Off On On

R

Functional test CN

Audible silence (acknowledge)

Symbols: Collector

−

Resistor

Diode

+

Capacitors

Pad no.

Base Emitter

PNP

T3

T4

T5

T6

T7

Off On On Off On

On Off Off On Off

Off On Off Off On

On Off On On Off

On On/Off Off On On/Off

Off Off/On On Off Off/On

NPN Transistors

FIG. 4.1o Semischematic diagram of a solid-state annunciator designed for ISA new sequence A (old sequence 1B), which sequences are described in Tables 4.1d and 4.1e. © 2006 by Béla Lipták

G F2 CN (−)

T2

Speaker Volume control

S

Control Room Equipment

Flasher−audible module

Logic module

T K FR C F1 (+)

Main bus terminals

4.1 Annunciators and Alarms

low, thus silencing the audible signal. When the point is acknowledged, T5 conducts; this restores the clamp at R1 and R2, which removes the flash source voltage. T6 is off all the time, T7 is on all the time, and the light is on steady. Normal Again When the variable condition returns to normal, the trouble contact closes and the base of T1 becomes high, turning T1 on. This produces a low on the base of T2, which turns T2 off and T3 on. All circuits are again in the state described under the normal condition.

595

The per-point cost of solid-state systems is slightly higher than that of their relay-type equivalent, due to the cost of power supplies and interfacing accessories that may be required with solid-state systems. The cost of the logic modules, visual indicators, and cabinets themselves is not excessive. Integrated circuit components using recently developed microcircuits will most likely reduce size, power consumption, and heat dissipation of annunciator systems.

ANNUNCIATOR CABINETS Lamp Test No separate lamp test is normally provided. One initiates a full operational test by pushing the test button, which applies an alternate input signal through resistor R15 to the base of transistor T2. This turns T2 on, initiating a full operational test of the system as already described. Lock-In The lock-in feature is provided by a switch S2. If the switch is in the lock-in position (see Figure 4.1o), when T5 turns off (on the alarm condition), a high at the collector of T5 is coupled to the base of T2 through R19, which keeps T2 turned on even if the trouble contact returns to normal. Transistor T5 will remain off and keep T2 turned on until the acknowledgment button is pushed. If the switch is in the non-lock-in position, the circuit between the collector of T5 and the base of T2 is open; therefore, T2 will turn off if the trouble contact returns to normal before acknowledgment and return all circuits to normal. Operational Test

See description under lamp test.

Auxiliary Contacts Auxiliary contacts are not supplied as part of the logic module. Adapter assemblies consisting of relays operated by the semiconductor logic, however, are available. Relay Fail-Safe Feature

Not available in solid-state circuits.

Normally Open Trouble Contacts The annunciator sequence and features already explained for NC trouble contacts operate in essentially the same way for NO contacts. The NO-NC option switch bypasses inverter stage transistor T1. When the contact closes on an abnormal condition, it turns on T2 though R20 and the sequence operation proceeds exactly as described for the NC operation. Solid-state annunciators are for use with DC voltages ranging from 12 to 125 DC. Power consumption of the logic modules ordinarily is less than 5 watts. Visual indicators consume different amounts of power, depending on the type. Bull’s-eye lights use approximately 1 watt, whereas back-lighted nameplates use from 1 to 6 watts, depending on the number and wattage of lamps. Solid-state annunciators are very reliable and are not position sensitive. They offer the advantages of compactness, low power consumption, and little heat generation, factors that make them particularly useful in large integral annunciators.

© 2006 by Béla Lipták

Annunciator systems are installed in areas ranging from general purpose to hazardous. Annunciator cabinets are installed indoors and outdoors in a variety of dusty, moisture-laden, and other adverse environments. Industrial annunciator cabinets are usually designed for general-purpose, dry indoor use. Special cabinets and enclosures are used in hazardous, moist, and outdoor locations. Hazardous Area Designs The requirements of Class 1, Division 2 hazardous locations as defined in Article 500 of the National Electric Code (NEC) are satisfied by the visual indicators and logic modules (either relay or solid state) of most annunciator systems. A manually operated or door-interlocked power disconnect switch is used with annunciator cabinets in those locations to turn off power when logic modules are relamped or changed. Annunciator equipment for Class 1, Division 1 areas is installed in cast steel or aluminum housings approved for the hazardous environment. The housings are expensive to purchase and install. These annunciators are available in both integral and remote configurations. The remote type is generally wired to explosion-proof bull’s-eye lights. Annunciator power must be disconnected either manually or automatically before the enclosures are opened to prevent an accidental arc or spark when logic modules are relamped or changed. One can weatherproof annunciator cabinets installed in either general-purpose or hazardous areas (class 1, division 2) either by housing them in a suitable enclosure or by covering the exposed cabinet front with a weatherproof door. Housings that comply with class 1, division 1 requirements are also weatherproof. Figure 4.1p illustrates several weatherproof and hazardous-area enclosures. For the design of purging systems, refer to Figure 4.1b. Intrinsically Safe Designs Annunciators are classified as intrinsically safe if they are designed to keep the energy level at the trouble contact below that necessary to generate a hot arc or spark. Care must also be taken in installing the system to place wiring so as to prevent a high-energy arc or spark at the trouble contact caused by accidental short circuit or mechanical damage. Thus, general-purpose trouble contacts may be used with

596

Control Room Equipment

Front view Slotted-knurled thumbscrew Backlighted nameplate

NC

(8)

3–15 PSIG (4) (0.2 –1.0 bar) I O From LT-9 S

NC

NO

(5) I O S

(6) I O S

(7) (3)

Enclosure window Pushbuttons Audible signal

O (1) I

(2)

NO: supply (S) connected to output (O) NC: (S) blocked to (O)

80–100 PSIG(0.6–0.69 MPa) Air supply Relay (4) Relay (5) Relay (6) Horn (7) Indicator (3) Condition Open Normal Closed Closed Off Off Open Open Closed Alert On On Acknowledge Open Open Closed Off On Normal again Closed Closed Open Off Off

Annunciator cabinet in weatherproof enclosure

FIG. 4.1q The tubing configuration and components of a pneumatic annunciator circuit.

Annunciator cabinet with weatherproof door

Integral annunciator for class1 division 1 hazardous area

FIG. 4.1p Annunciator enclosures for weatherproof and hazardous areas.

intrinsically safe annunciator systems even though they are installed in a hazardous area. The annunciator logic modules and visual indicators, however, must conform to the electrical classification of the area in which they are installed. (For more on intrinsically safe designs, see Section 7.2 in Volume 1 of this handbook.)

PNEUMATIC ANNUNCIATORS Pneumatic annunciators consist of air-operated equivalents of the trouble contact, logic module, and visual indicator stages of an electrical annunciator system. A single-point system furnishing high tank level monitoring is shown in Figure 4.1q.

© 2006 by Béla Lipták

Power supply to the system is instrument air at 80 to 100 PSIG (0.6 to 0.69 MPa), which is reduced to the required operating pressure by pressure regulator (1). The operating pressure is indicated on pressure gauge (2). A 3 to 15 PSIG (0.2 to 1.0 bar), an analog input signal from a direct-acting level transmitter (LT-9) enters high-pressure limit relay (4), which is normally closed and set to open when the high level limit is exceeded. When this happens (alert condition), an input at supply pressure from (4) turns on a pneumatic visual indicator (3), and a normally open high-pressure limit relay (6) allows supply air flow to air horn (7), turning it on. Simultaneously, the air output from (4) enters normally closed high-pressure limit relay (5) and momentary contact pushbutton (8), which is a normally open acknowledgment pushbutton for the system. In the alert condition, the pneumatic indicator and horn are both on. One acknowledges the alert condition by pushing button (2), closing it, and thereby opening high-pressure limit relay (5). Supply air pressure from (5) closes high-pressure limit relay (6), which cuts off the operating air to the horn, thereby turning it off. Simultaneously, operating air pressure from (5) is fed back to the inlet of (5). The feedback pressure locks up (5) so that it will not close when the acknowledgment pushbutton (8) is released. In the acknowledged condition, the pneumatic indicator is on and the horn is off. The system returns to normal when the 3- to 15-PSIG (0.2- to 1.0-bar) analog input falls below the setpoint. This closes high-pressure limit relay (4) which turns off the pneumatic indicator (3). It also closes high-pressure limit relay (5) by venting the lock-in circuit through relay (4). Pneumatic annunciators are used when one or two alarm points are needed but electrical power is not readily available and in hazardous electrical areas where an electrical annunciator might not be practical. Pneumatic annunciators require

4.1 Annunciators and Alarms

a substantial amount of installation space and are expensive to manufacture.

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© 2006 by Béla Lipták

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“Functional Safety: Safety Instrumented Systems for the Process Industry Sector,” IEC 61511, 2002. Gangaloo, N. R., “New Annunciators,” InTech, March 1989. Gerold, J., “Managing Abnormal Conditions Pays Process Dividends,” Automation World, September 2003. Ghosh, A., Abnormal Situation Management Is Your First Line of Defense, Dedham, MA: ARC Advisory Group, 2000. Hendershot, D., and Post, R., “Inherent Safety and Reliability in Plant Design,” Proceedings of the Mary Kay O’Connor Process Safety Center Annual Symposium, College Station, TX, 2000. ISA TR91.00.02, “Criticality Classification Guideline—Enunciator Sequences and Specifications,” ANSI/ISA S18.1, 1992. Jamieson, G., and Vicente, K., “Modeling Techniques to Support Abnormal Situation Management in the Petrochemical Processing Industry,” Proceedings of the Symposium of Industrial Engineering and Management, Toronto, Ontario, Canada, 2000. Jutila, J. M., “Guide to Selecting Alarms and Annunciators,” InTech, March 1981. Mattiasson, C., “The Alarm System from the Operator’s Perspective,” Paper Presented at IEE People in Control Meeting, Bath, U.K., 1999. Mostia, B., “How to Perform Alarm Rationalization,” Control, August 2003. National Electrical Code, Articles 725 and 760, National Fire Protection Association, 1981. Nimmo, I., “Abnormal Situation Management,” Chemical Engineering Progress, September 1995. Nimmo, I., “The Importance of Alarm Management Improvement Project,” Paper Presented at ISA INTERKAMMA, 1999. Nochur, A., Vedam, H., and Koene, J., Alarm Performance Metrics, Singapore: Honeywell Singapore Laboratory, 2001. PAS (Plant Automation Services), The Cost/Benefit of Alarm Management, Houston, TX: Plant Automation Services, 2000. Poole, A. D., “Design Considerations for Discrete Alarm Systems,” InTech, September 1992. Smith, W., Howard, C., and Foord, A., Alarm Management—Priority, Floods, Tears or Gain? www.4-sightconsulting.co.uk: 4-sight Consulting, 2003.