Corrosion Control Documents and Integrity Operating Windows
Corrosion control documents (CCD) are essential inputs into effective mechanical integrity programs and risk-based inspection programs. They have been referred to by different acronyms, including Material Operating Envelopes (MOEs); Critical Process Variables (CPVs); Corrosion Manuals; Corrosion Control Documents, Corrosion Circuits, Corrosion Reports, Corrosion Loops, etc. They can be combined in a risk-based inspection program implementation or can be standalone documents. They are particularly useful for capturing in one document the essential elements of a chemical or refining process that influence corrosion, i.e. materials of construction, process stream chemistry, operating conditions and damage mechanisms.
Semi-quantitative, risk-based inspection assessments and the various RBI software are very useful for capturing and presenting large amounts of equipment data, for facilitating the structured assignment of risk to equipment and for setting inspection intervals. However, RBI programs and the various software tools are not particularly useful for identifying the damage mechanisms in a unit and do not address upset or startup/shutdown damage mechanisms at all. In fact, one could argue that a typical RBI assessment program is particularly ineffective in reducing risk and largely a poor use of personnel and money in the absence of expert input into a unit’s damage mechanism and where those mechanisms are likely to exist.
Download the CCD presentation from the 2011 API Inspection Summit and Expo (PDF).
Corrosion Control Document Content
A corrosion control document summarizes:
- Unit process description
- Rationale for materials of construction
- Defines corrosion circuits
- Defines damage mechanisms in each circuit:
- Includes critical locations
- Includes integrity operating windows
- Start up, shut down and upset operation influences
- Includes predicted (or actual) corrosion rates and environmental cracking tendencies
Uses of Corrosion Control Documents
CCD’s have several uses including:
- A training tool for new materials and corrosion engineers and unit inspectors
- TML and CML placement tool
- Input into RBI inspection programs
- Inspection Planning
- Optimizing inspection techniques
- Establish integrity operating windows
The essential information for an individual corrosion circuit is typically summarized in a tabular, easy to understand format (PDF).
Integrity operating windows (IOW’s)
As an essential component of a CCD, Integrity operating windows (IOW’s) are gaining recognition as a valuable input into a corrosion control document. What are IOWs? The American Petroleum Institute (API) has a new recommended practice in the ballot stage (API RP 584) that discuss the need to establish operating limits that address the controls necessary for process variables that might affect the integrity or reliability of the process unit. An example of an IOW is the establishment of furnace tube temperature limits to avoid premature rupture. At some established limit, say 800F, a furnace tube designed for 775 F would have a shortened service life, so operators would understand the need to maintain control of furnace firing to remain below the 775F limit. That limit of 775F would be an IOW limit for the furnace tubes.
The Hendrix Group has developed corrosion control/IOW reports for the following processes:
- Diesel hydrotreater units (DHT)
- Naphtha hydrotreater unit (NHT)
- Saturation gas unit (SAT)
- Gasoline stabilization unit
- Natural gas plants
- Fluid catalytic converter (FCCU) units
- Gas concentration unit (GCU)
- Merox Unit
- Sulfur recovery units (SRU)
- Sour water stripper units (SWS)
- Hydrogen generation units
- CO boilers
- Amine units
- Methanol Unit
- Acetic acid/Anhydride Unit
- Vinyl acetate monomer Unit
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