I want to know if there are any standards for "Identification and Maintenance of CRITICAL Instrumentation & Control (I&C) equipment in Power plants/process industry"? And if I can download them from somewhere.
The standards should include the criteria for assessing criticality of I&C assets in a power plant such as redundancy, maintainability, capacity loss in case of instrument failure, safety & environmental impact, cost of spares etc.
No Standards that I'm familiar with, but there are standard practices that involve a design/safety reviews.
At one extreme they can be intensive, involving process, operations, mechanical, instrument, and maintenance, but that is generally reserved for processes that require traceability. At the other extreme, minimal review of the basics, under the proviso of single event failures, not combined failures.
There is no such standard for identification and maintenance of CRITICAL Instrumentation & Control equipment. However, it can have best practices. Senior instrument Engineer or technician can help you identifying the critical equipment.
For reliability, there are number of Reliability Engineering metrics are used like reliability, unreliability, availability, unavailability, mean time to failure (MTTF) etc. You also need to define Failure Modes of the equipment under discussion as over all safety instrument function evaluation.
As a matter of fact, you may go through ANSI/ISA-84 series of standards to get more useful information.