What is an Emergency Shutdown System?

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What an “Emergency Shutdown” does which an “Emergency Stop” does not, is to be able to detect a potentially hazardous condition and react to it by shutting the system down to protect personnel, facilities, and even the environment.

Let's consider an Emergency Shutdown in a real-world environment, in the Oil and Gas industry.

In the Oil and Gas industry, an Emergency Shutdown is a safety system that is designed to minimize the consequences of an emergency situation, such as a failure, to reduce the potential of flooding, escape of hazardous materials, or outbreak of fire.

This is normally done by monitoring the state of field mounted sensors, valves, trip relays and inputs to a control system as alarms.

The control system is able to determine a cause and effect type analysis which are determined to protect the facility.

The Emergency Shutdown does not need to completely shut down the entire plant. This can sometimes be more dangerous.

What the system will do is to minimize the effects. It could be to reduce the number of plant items available or shut down part of the systems.

In the event of a fire, a Fire Damper control system may override existing controls to open or close vents as needed, and close fire doors.

There are so many examples of what an Emergency Shutdown system might be used for.

In a system that isolates hydrocarbon inventories, it is of great importance that an Emergency Shutdown system is effective so that nothing is released into the atmosphere.

Another important use of an Emergency Shutdown is in an Emergency Ventilation system.

When a problem is detected which requires rapid venting, it is crucial that a safety system can detect and react to the problem, or even detect it before it becomes one.

An example of an Emergency Ventilation system could be a smoke detection system.

If a fire is detected, the system would likely shut down all plant equipment to contain the fire, not allowing oxygen in to allow it to burn, but if smoke was detected they may want to vent the smoke out, therefore switching on plant equipment to do so.

Emergency Shutdown systems usually have their own logic controller, one that reacts to failures much faster than a normal PLC system. Milliseconds count and can be the difference between a problem and a catastrophic failure.

Safety systems use a classification based on risk and probability. This is called Safety Integrity Level or SIL for short.

There are 4 levels to SIL.

SIL Level 1 represents the integrity required to avoid relatively minor incidents and is likely to be satisfied by a certain degree of fault-tolerant design using guidelines that follow good practice.

SIL Level 2 represents the integrity to avoid more serious, but limited, incidents some of which may result in serious injury or death to one or more persons.

SIL Level 3 represents the integrity required to avoid serious incidents involving a number of fatalities and/or serious injuries.

SIL Level 4 represents the integrity level required to avoid disastrous accidents.

When designing the safety system, the required level, and associated risk is taken into account by using a safety matrix.

This will look at each of the risks, and attach a probability and consequence to them, to get to the required safety integrity level required for the safety system.

Different to an Emergency Stop pushbutton on a panel, or inside a machine cell, the Emergency Shutdown system can detect potential failures based upon field sensors, valves, and trip relays and react quicker than us humans can to stop the escalation of a small problem becoming a catastrophe!

Every alarm on a system is assigned a rating based upon its probability to occur and the consequence if it does.

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