Corrosion Loop Theory

During the Threat and Barrier identification, Equipment and piping within a site’s unit or plant, with similar Degradation Mechanisms (DMs), should be grouped into manageable sections that can be monitored. In IMS these groups are called Corrosion Loops (CLs). To define a Corrosion Loop in IMS see PEI - Corrosion Loops.

Corrosion Loop Definition

A CL is defined as a section of the unit or plant, where the construction materials and process conditions, under which the loop operates, are similar, resulting in the same Degradation Mechanisms to be active in this section of the unit or plant. Additionally, the section should have the same Integrity Operating Windows  and it should be one processing step (e.g., cooling/heating, condensation, reaction, etc.).

In practice similar construction materials means that in one CL:

• Piping is made of similar piping classes;
• Equipment is made of comparable materials; and
• The trim class of equipment is similar to the piping classes.

 Similar process conditions in this context, means that for one CL:

• The corrosive species (or precursors) are the same throughout the CL, although reactions may occur and/or condensation may take place;
• Temperatures are such that no stepwise changes in corrosion phenomena occur; and
• An unambiguous Integrity Operating Window (IOW) can be defined.

Corrosion Management Framework (CMF)

The CMF covers part of the 3 Degradation Management phases. Corrosion Management Framework (CMF) is a structured framework that identifies: 1) the Threats, i.e., Degradation Mechanisms (DMs), that are likely to affect the process Equipment in each section of a unit or facility and 2) the controls and limits necessary to maintain integrity (Barriers). This information can be documented on Corrosion Loop level, in the Degradation Management section. The CMF helps to improve inspection strategies and the management of the mechanical integrity of the Equipment and Piping, i.e., inspection planning as well as plant safety is supported. See Degradation Management - Work Process in IMS.

All applicable DMs (Threats), as well as the Barriers to manage the degradation, are defined at the CL level. The image below illustrates the process of creating a CL, identifying the DMs (Threats) and then identifying the Barriers. Additionally, for each Barrier, depending on the type, either Components should be identified for inspection (and risk assessed), or variables should be selected to be monitored (via the IOW) and added to a proactive monitoring plan for the asset. The IOW is also defined at the CL level. This way Indicators can be identified to determine the Barrier Statuses.

Degradation Mechanism and Barrier Identification takes place at the Corrosion Loop Level.

By identifying the Threats and Barriers, an overview table is created in the CL Degradation Management section. Additional information (Indicators, Actions Planned, and Barrier Status) will automatically be added to the overview as they become available (see PEI - Corrosion Loops). This overview can be used for Barrier verification (see Reviewing Barrier Statuses). Barrier Verification is an ongoing process, which should be performed regularly.

The CMF steps:

• Define Corrosion Loop - CLs are defined on Plant or Unit level.
• Degradation Identification - Once the CL extent is defined, applicable Degradation Mechanisms (DMs) are defined.
• Barrier Identification - For each applicable DM, specific Barriers that limit the effect of the DM needs to be identified.
• Barrier Assessment - For each Barrier, depending on the type, either applicable Components should be inspected, or variables should be monitored, to determine the Barrier status.
• Design Barrier Verification and Maintenance - For the variables to be monitored, an Integrity Operating Window (IOW) should be defined and added to a monitoring plan. For Components to inspect, RBI risk assessment should be carried out to determine the Next Inspection Date (NID).

Corrosion Loop Boundaries

Usually CL Boundaries are placed at one of the following 3 places (the corresponding numbers show in the figure below:

1. Where the construction material of the containment envelope changes;
2. Where the process conditions change; or
3. Where the process unit limits (“battery limit”) are.

Example of CL Boundaries.

 Take Note:

• Boundaries should be “natural”, e.g., nozzle flanges, draw-off trays, block valves, non-return valves, etc.
• Relatively small “boundary” Equipment should be put in the CL with the most severe (potential) DMs, e.g., the Shell side of a heat exchanger, where an inert vapour condenses to an acidic condensate, is put in the acid condensate loop.
• Large Equipment, e.g., distillation columns, may be split over more than one CL, or Exchangers (Shell and Tube sides belong to different CLs)
• The boundaries of CL can change (slightly) over time, as new info becomes available. For example, it may be concluded that boundary Equipment behaves more like the adjacent loop.
• The number of CLs per process unit or asset should be limited. Detail (resolution) and overview should be balanced. Typically, the number of CLs ranges from 20 (average size process unit) to 50 (large process unit). For this reason, small CLs with similar DMs are sometimes merged or large CLs split.

Defining CLs in IMS

To create and define a CL in IMS see PEI - Corrosion Loops.