The main 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In some installations where traditional units have been replaced by PLC- or DCS-based annunciators, the recognition of and response to alarm conditions have deteriorated because on computer screens they are not very visible and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.
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 . 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.
Working principle of annunciators
The annunciator system consists of multiple alarm points. Each alarm circuit includes a trouble contact (alarm switch), a logic module, and a visual indicator . 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 push buttons.
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 there by 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 back lighted 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.
Annunciator application guide
Annunciators are normally used to call attention to abnormal process conditions. Annunciators may also be used to show normal process status. Annunciators usually include individual illuminated visual displays that are labeled to identify the particular monitored variable that is abnormal and audible devices. Annunciators may also call attention to the return to normal of the process conditions.
Visual displays usually flash to indicate abnormal process conditions. Manual operation of push buttons is usually required to silence audible devices and acknowledge new alarms. Visual displays usually change from flashing to on when alarms are acknowledged. Figure illustrates a typical annunciator sequence.
Additional types of flashing can indicate that process conditions have returned to normal or which of a group of alarm points operated first. Additional push buttons can be used to acknowledge alarms that return to normal, to reset first out indications, and to test annunciator lamps and circuits.
Annunciators are available in an almost infinite variety of physical arrangements, operating sequences, and special features. In some cases, the annunciation function is performed by computer systems using electronic screen displays or recording annunciators.
Integral logic annunciators :-
Integral logic annunciators include visual displays and sequence logic circuits in one assembly . Plug-in alarm modules contain the sequence logic circuits. The visual display lamps and windows may or may not be a part of the plug-in alarm modules. Terminal blocks are provided for the field contact, push button, audible device, and power source wiring.
When large annunciator windows are subdivided to form two, three, or four smaller windows, a single plug-in alarm module may serve each group of alarm points.Common flashers, audible device drivers, and required power supplies are usually located within integral logic annunciators. Small pushbuttons and audible devices can be mounted in window positions when preferred and when annunciators are easily accessible.
The Old ISA Designations of Annunciator Sequences
SA 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.