Materials Operating Envelopes
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 a MOE study, E2G’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).
MOE’s are similar to KPRP’s (Key Process Reliability Parameters), but differ in that MOE’s contain parameters that may not be controllable, but should be measured and trended. Uncontrollable parameters are reviewed in order to provide ample warning that additional inspection, surveillance, or replacement may be needed. As an example, ammonia content in a crude tower overhead cannot be readily controlled, but it should be trended to alert the refinery if there is a major change in the potential, i.e. salt deposition temperature, for ammonium chloride fouling and pitting in the overhead.
Some parameters having predefined controllable limits include overhead drum pH (typically controlled by neutralizer); free CN content (typically controlled by water wash with polysulfide); and ammonium bisulfide concentration of sour water (typically controlled by wash water).
