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High — A major release of a toxic or flammable material caused by a machinery failure that impacts the community.

Medium — A significant release of a toxic or flammable material caused by a machinery failure could harm plant personnel.

Finally, we must consider economic consequences of machinery damage. In general, we know that larger process machines are more expensive to repair if they fail catastrophically than smaller process machines. For this reason, it is common to rank the economic consequences related to catastrophic failure in terms of a machine’s rated horsepower. Perhaps, a machine rated below 200 hp can be considered to represent a low economic consequence if it fails catastrophically, a machine rated between 200 hp and 2000 hp can be considered to represent a medium economic consequence if it fails catastrophically, and a machine rated above 2000 hp can be considered to represent a high economic consequence if it fails catastrophically.

Consequences are additive, which means we can tally up all the various consequence to arrive at a cumulative consequence level. Let’s continue with the example compressor at the start of this section and list all the consequences:

A 2500 hp, multistage centrifugal hydrogen recycle compressor is unspared. If the compressor goes down, then there is an immediate upset of the process leading quickly to a unit shut down. There are three categories of consequences related to a failure:

Economic consequences related to process losses:

Multiday Outage (<1 week) = High Consequence

Extended Outage (>>1 week) = Extremely High Consequence

Safety consequences:

A significant release of a toxic or flammable material caused by a machinery failure could harm plant personnel = Medium Economic consequences of machinery damage

Table 1.2 Summary of potential failure consequences of interest.

Type of consequence	Low consequence	Medium consequence	High consequence	Extremely high consequence
Process Loss			X	X
Environmental		X
Safety
Machine Damage			X

Since the compressor is rated at over 2000 hp, the economic consequences of a catastrophic failure are high.

We can now summarize our risk assessment, as seen in Table 1.2, and conclude the highest consequences are associated with process losses. Our summary only contains significant consequences of interest.

Machines associated with potential high consequence events are considered critical machines, while those associated with low consequence events are considered non-critical machines. In our example, the extreme consequences are related to extended outages and catastrophic machine damage. It makes sense to label this machine a high criticality machine in your listing of plant assets. When you are done with your site assessment, you should have a listing of all your machines along with their criticality ratings. If you force-rank the list, the machines at the top of the list should be your highest priority in terms of potential impact on the plant.