Integrity Operating Windows

Defining Operating Limits

Integrity Operating Windows (IOWs) are a set of operating limits and operating parameters (in this case referred to as ‘operating windows’) assigned to process variables that can affect the integrity and reliability of a process unit. They are a framework of limits that are established in order to prevent potential breaches of containment that might otherwise occur as a result of operation outside the recommended limits.

Operation outside the IOW limits can result in accelerated damage to equipment from any one or more of the damage mechanisms covered in API RP 571 including general or localized corrosion, mechanical or metallurgical damage, high temperature corrosion and environmentally assisted cracking. Conversely, operation within the recommended limits should result in predictable and or reasonably low rates of degradation of equipment.

Format of IOWs

API is in the process of developing a recommended practice API RP 584 for the development and use of IOWs that will address the content and methodology for establishing IOWs. E2G is a member of that API Task group. Although the final document has not yet been balloted or approved, the API recommended practice will not prescribe a specific template for establishing operating limits. The format is left to the end user as long as its content meets the intent of the IOW practice.

Various IOW formats have been established over recent years and are often referred to by different acronyms. Examples of some of these documents include Material Operating Envelopes (MOEs); Critical Process Variables (CPVs); Corrosion Manuals; Corrosion Control Documents (CCDs) and many others. They vary in scope, detail and complexity but comply with the overall intent and spirit of the API IOW methodology.

Tasks for IOW projects typically include:

• Create a list of pre-visit questions and information/data needed
• Highlight process variables that impact corrosion for the actual materials present in the unit
• Provide targets and limits where they can be defined
• Highlight issues associated with corrosion control measures

For efficiency reasons, the process meetings for the IOW should be done in conjunction with the process overview meetings where RBI implementation is in effect.

MOEs

RBI studies are based on assumptions regarding past and future operating conditions, and are focused more on inspection planning than on controlling current operations and monitoring activities. A MOE study, which provides knowledge and control of a unit’s present operating envelope, complements RBI and helps a RBI plan to succeed. Together, these two technologies provide an improved chance for reliability and safety. MOEs can be created prior to or after RBI is implemented, but a standalone study is also possible.

Refineries typically conduct Management of Change (MOC) reviews when process changes are intentionally made. Often changes are made inadvertently. In many cases, the people involved either do not have the tools or experience to evaluate the effects of minor process creep or knowledge of the boundaries for materials degradation. Ideally, boundaries are established to permit the process to safely operate within set limits without major concerns for the health of the equipment.

In an MOE study, Equity’s experienced materials engineers define the limits for each part of a unit for operating parameters, such as feed contaminant content, pH, flow rate, temperatures, chemical or water injection rates, and acceptable levels of corrosive constituents. If limits are not exceeded, degradation should be predictable and reasonably low. If limits are exceeded, excessive equipment degradation due to corrosion, stress corrosion cracking, metallurgical embrittlement, or hydrogen effects, such as high temperature hydrogen attack, could occur. Control action limits can be identified and graded (long term, intermediate, or immediate action required).