Alarm thresholds define the gas concentration levels at which different alarm levels are triggered, initiating appropriate response actions. Proper threshold selection is critical for effective safety system operation.
Alarm Range 1 (Low Alarm) is typically set at 20% LEL for flammable gases. This activates countermeasures like ventilation, provides early warning level, and allows time for preventive action. This level should be reached occasionally in normal operations, triggering preventive measures.
Alarm Range 2 (High Alarm) is typically set at 40% LEL for flammable gases. This activates emergency measures like shutdown, represents critical level requiring immediate action, and should rarely be reached in properly designed systems. If this level is reached frequently, system design should be reviewed.
Alarm response is clearly defined: Alarm Range 1 triggers countermeasures activated to reduce concentration, while Alarm Range 2 triggers emergency measures activated (shutdown, evacuation). Properly designed systems rarely reach Alarm Range 2, indicating effective preventive measures.
Threshold selection is based on gas properties (LEL, toxicity), considered response time requirements, accounts for system response delays, and must allow time for safe response. Safety considerations include thresholds must be set below dangerous levels, accounting for sensor response time, considering environmental conditions, and regular review and adjustment may be needed.