Alarm Only (Level 1)
The system that provides the lowest level of automatic physical protection is usually the alarm-only system. At this level, process conditions are monitored and impending problems are brought to the attention of the operator. The alarm-only level is used where the operator has time to take corrective action and remedy the
problem. It is also used to indicate minor failures or abnormal operation of equipment
or monitoring devices.
Individual Equipment Shutdown (Level 2)
The second lowest level system shuts down individual units of equipment such as a pump or a compressor, or, on a production platform, a test separator. An example from a refinery is the automatic shutdown of a compressor due to mechanical failure.
Process Train Shutdown (Level 3)
The next level of protection is a process train shutdown. At this level all of the process systems affected by a malfunction are shut down. An example is a production separator, one of several feeding gas to a compressor, which feeds a glycol contactor. Liquid from the separator is fed to a dedicated heater-treater and then to pumps. A malfunction in the separator shuts down the inlet to the separator, the heater-treater, and the pumps. It does not shut down the compressor and glycol systems. In a refinery, parallel trains in a sulfur removal facility is an example in which one process train could be shut down while the other continues to operate.
Complete Process System Shutdown (Level 4)
The second highest level of shutdown is a complete process system shutdown, commonly in large gas processing plants. Upon detection of a possibly dangerous operating condition, such as high pressure in the inlet separator, the plant is isolated from all incoming and outgoing streams, and process equipment is shut down.
Emergency Shutdown (Level 5)
The highest level of shutdown is usually referred to as an emergency shutdown (ESD). At this level, all process systems are shut down, as in a complete process shutdown. Additionally, the entire plant is usually depressured to the extent practical. This shutdown level usually results from either the automatic detection of catastrophic conditions such as fire or major gas leaks, or from operator decision and action to initiate the ESD.
Not every protective system includes all levels of shutdown. For example, very few refinery facilities have complete emergency shutdown systems as defined above. Refineries are staffed 24 hours a day, and the operators are trained to respond to emergencies. On the other hand, manned offshore platforms are required by law to have complete emergency shutdown systems, even though they are staffed 24 hours a day by trained operators.
In the event of a major release or fire, it is essential that personnel be in a position to quickly respond to the emergency. The highest level of protective system, emergency shutdown, enables the operators to concentrate on emergency response and life safety while the facility automatically shuts down.
Generally, refinery protective systems go up to Level 2, with a few process train shutdown systems (sulfur plants, hydrotreaters). Most onshore gas processing plants have Level 4 shutdowns, and many have complete emergency Level 5 shutdown systems. Some of these locations operate unattended.