You want to learn how to rank your equipment based on criticality then this chapter from the "Rules of Thumb for Maintenance and Reliability Engineers Handbook.
This document will describe the structured evaluation methodology used to “Identify Critical Equipment”. Criticality Analysis identifies the assets which contribute the most asset reliability, throughput, safety, etc. Without an effective criticality analysis an organization lacks focus on what assets contribute the most to their business.
If you have questions about asset criticality analysis send an email to Ricky Smith at firstname.lastname@example.org.