Types of Alarms
These alarms may be to do with efficiency of the process or indicate defects in the equipment.
This type of alarm is normally incorporated into the plant control system (typically a DCS) and shares the same sensors as the control system
Machinery or Equipment Alarms
These alarms assist with detection of problems with equipment and do not directly affect the operation of the process.
Safety Related Alarms
These alarms are used to alert operators to a condition that may be potentially dangerous or damaging for the plant.
Such alarms should normally have a high priority and where they are involved in protecting against mal-operation by the control system they should be independent of the devices they are monitoring. In many cases these alarms are generated by the safety shutdown system itself.
This type of alarm tells the operator that an automatic shutdown event has been reached and has been initiated by the SIS.
However, it must be remembered that not all alarm systems will result in shutdowns. There are cases whereby such alarms are the result of automatic start-up.
For example, in the case of power failure, the system will cause the stand-by generator to power-up. Another example is the auto-start of the stand-by pump when the regular unit goes down. As shown in the Figure, an on-off pump is provided for a sour water K.O. drum. The pump is started when the water level in the drum is high, and stopped when the water level is low. Alarms are provided to indicate high or low liquid level. A pump running light is also provided to show the pump status, i.e. “ON” or “OFF”.
Other Alarm Systems: In addition to critical process safety alarms, the separate hardwire annunciator system could incorporate the following alarms
Fire (e.g. Satellite Instrument House)
Spill or toxic release
Power system fault
Electrical room or Substation smoke detector