When selecting an alarm system, it is important to understand all the associated terms. This brief glossary can help you sort through the choices.
Process/Temperature Alarms. Control instruments can be provided with a number of process alarms. Alarm messages are displayed locally and are available on communications. The setpoints for these alarm types may have an associated hysteresis, which is the difference between the alarm setpoint ON value and OFF value. Hysteresis normally is adjustable and prevents output chatter from occurring by separating the ON and OFF points.
Equipment Alarms. These devices annunciate instrument malfunctions that can have serious process consequences. Equipment alarms generate display and communications error messages and may be connected to fixed relay alarm outputs that default to safe conditions
HI and LO Alarms. These alerts are activated when the measured value crosses the absolute high or low alarm setpoint from the safe region to the alarm region. Some instruments provide as many as four such alarms. Generally, HI and LO alarms provide annunciation only while HI-HI and LO-LO alarms provide annunciation and shutdown interlocks.
Deviation Alarms. These alarms include deviation high, deviation low and deviation band type. A deviation alarm is tied to the difference between the measured value and the process setpoint. It will follow the setpoint if it is changed. The deviation band alarm has both a high and low deviation, which may be the same as or different than the high and low bandwidths.
Sensor Break Alarm. These alarms are activated when the measure value input is broken.
Loop Break Alarm. When an output ceases to function correctly, loop break time determines the period of time that elapses before an alarm output changes state. For example, suppose the loop-break limit (Lbt) is set at 300 sec (5 min). If the process temperature is below setpoint, and the controller increases the output, but nothing happens for 300 sec, then the loop break alarm will activate. Typical loop break conditions might include a broken heat contactor, a blown fuse or a missing thermocouple.
Non-Latched and Latched Alarms. A latching alarm remains in alarm condition until it is reset. A non-latching alarm resets itself as soon as the condition that triggered the alarm ceases to exist. The alarm condition continues throughout the hysteresis value
Clues to Understanding Controls
September 1, 2008