Figure 1300-2 shows, in addition to a shutdown system, a typical alarm-only system, consisting of a sensing device, an annunciator, and supporting instrumentation, including wiring, conduit, and valves for isolating the sensor from the process for testing. This schematic represents the minimum design sufficient to achieve a testable alarm system.
The purpose of the annunciator is to alert the operator to an abnormal condition through the use of lights or horns. Many recently installed protective systems utilize electronic annunciators with solid-state circuitry and backlighted windows. Pneumatic annunciators are available for applications where there is no electric power or where electronic equipment may be unsuitable.
Most facilities contain multiple, interconnected processes, with an upset to one often causing an upset to another. It is frequently difficult to determine which upset occurred first. Annunciators are available with first out indication, which allows the operator to distinguish which alarm occurred first, and to take appropriate action. Many new annunciation systems for larger facilities are using cathode ray tube (CRT) monitor screens and printers to communicate process information, alarms, and shutdowns to the operator. First out alarm indication may be specified.
When sensing devices are connected to a shutdown system, the following should be considered:
• Alarms should be provided for each sensor.
• All components of a protective system (alarms, trips, interlocks, etc.) that are associated with a sensor should actuate at the same time as the sensor unless a time delay sequence is required.
• Pre-shutdown alarms can be provided at manned facilities to warn that a trip is impending. Pre-shutdown alarms enable the operator to take corrective action before the protective system is tripped. For some processes, the time taken to reach the trip condition after the operation of the pre-trip alarm might be too short for successful manual intervention. In these cases the pre-shutdown alarms may be omitted.
• Bypass switches are commonly used to isolate sensors for testing and repair. Bypass switches should be installed for individual sensors so that the entire protective system is not deactivated during testing and repair work. Switches in bypass should be indicated locally or alarmed remotely so that operators do not accidentally operate without the full protection afforded by the system. Where practical, bypass timers or limit check timers are provided which allow testing of the protective system without shutting the process down. After a preset time the protective system will automatically go back in service. Programmable controllers allow the addition of limit check timers easily at no additional cost.