Defining Systems is one of the first and most important steps in an RCM analysis. The way you group Equipment into Systems directly affects how you analyze Functions, Functional Failures, Failure Modes, and maintenance strategies.
Well-defined Systems help ensure that analyses remain manageable and accurately reflect how assets operate within the process.
The goal is to create logical groupings of Equipment that perform a common operational function. When defining Systems, follow these rules:
A FLOC or Equipment item can belong to only one System.
Each System should have clearly defined Functions and Functional Failures.
System boundaries should reflect process functionality rather than equipment type.
When to Create Multiple Systems
A Unit may contain many Functions, Functional Failures, and Equipment items. To make the analysis manageable, divide large or complex Units into multiple Systems.
If a Unit is large and complex, you can create as many Systems as needed.
Consider creating additional Systems when:
The Unit performs multiple distinct process functions.
The number of Functional Failures becomes difficult to manage.
The FMEA becomes too large or complex.
Different process areas require separate analysis.
Note
In IMS, a Unit needs to comprise at least one System to perform an RCM analysis.
Performing Failure Mode and Effect Analyses
A useful way to determine System boundaries is to consider the intended scope of the Failure Mode and Effects Analysis (FMEA). Although FMEA is not a part of the RCM analysis, you can use it to prioritize FLOCs and Equipment in the System and to identify Equipment Failure Modes that can cause Functional Failures.
Effective FMEA requires both:
Equipment knowledge to identify credible Failure Modes.
Process knowledge to understand the impact on System Functions.
For this reason, FMEA workshops should include a multidisciplinary team, such as process engineers, operations personnel, discipline engineers and maintenance engineers.
Defining System Boundaries
Always verify System boundaries using plant documentation, such as:
Piping and Instrumentation Diagrams (P&IDs)
Electrical diagrams
Safeguarding diagrams
Package-unit or skid documentation
P&IDs are typically the preferred source because they clearly show process relationships and equipment interfaces.
You can determine System boundaries based on:
Changes in product specifications or quality requirements.
Production process inputs and outputs, covering the main product stream, recycle streams, utility streams, and energy flows.
Documenting System boundaries directly on marked-up P&IDs provides a clear and auditable record of the analysis.
Example
A production Unit may consist of multiple process and utility subsystems represented across several P&IDs.
In the above example, there are 6 P&IDs.
System 1 (P&IDs 1.1 and 1.2) and System 2 (P&IDs 1.4 and 1.6) cover the main production process (represented by the red line).
System 3 (P&ID 1.3) is a compressor lubrication oil system, also called a sub-unit.
System 4 (P&ID 1.5) is a Furnace burner control system, also called sub-unit.
Separating these areas into individual Systems enables more focused and effective reliability analysis.
Note
Avoid creating Systems based solely on equipment categories such as pumps, motors, or valves. These groupings are often difficult to relate to Functional Failures of the production process.