By Mike Sparago, Principal Engineer II, and Chris Massengale, Consultant Engineer II at The Equity Engineering Group. This article appears in the January/February 2021 issue of Inspectioneering Journal.
CorrSolutions provides Corrosion and Inspection services to the Refining and Petrochemical Industries.
Our areas of specialty include:
We are sole developers of the patented Selective Corrosion Rate Analysis technique for advanced statistical validation, analysis and trend identification of equipment thickness data. CorrSolutions offers a unique combination of process and mechanical expertise to assess the specific effects of operating variables on equipment damage mechanisms.
Our next generation thickness analysis tool is Advanced Degradation Analysis (U.S. Patent Pending), which combines historical thickness data with related inspection program information plus inspector experience to provide better (more predictive) results than using data alone. This method results in more effective inspections, while reducing unnecessary efforts.
CorrSolutions delivers innovative, practical solutions by placing advanced tools into the hands of plant inspectors and engineers.
The Selective Corrosion Rate Analysis Pat. Pending technique utilizes a unique combination of data validation checks, corrosion models, statistical techniques and advanced trend identification tools to highlight specific corrosion mechanisms and provide accurate remaining life projections from UT thickness data. The analysis is completely data-driven and requires only a single analysis parameter – the system’s required reliability.
Selective Corrosion Rate Analysis reports are designed to enable rapid review by Inspectors and Engineers. Re-inspection schedules are optimized to provide a practical minimum level of inspection to achieve the required reliability.
Since Selective Corrosion Rate Analysis utilizes state-of-the art statistical principles and does not require the use of multiple, subjective analysis parameters, the results are completely defensible from a regulatory standpoint.
At the heart of the Selective Corrosion Rate Analysis Report is the Corrosion Rate Reliability Plot. This plot shows the corrosion rates for all ‘active’ TML’s in the system (horizontal axis) vs. the cumulative occurrence of the corrosion rate (vertical axis). The vertical axis on a Corrosion Rate Reliability Plot is labeled “Reliability”, since the corrosion rate distribution is projected to the system’s required reliability for determining remaining life.
Similar plots have been used for many years by Rotating Equipment Engineers for predicting equipment failures. Only recently have these plots been used to analyze the reliability of fixed equipment. For Corrosion Rate Reliability Plots, the upper (right) end of the plot provides the value used for determining remaining life.
Selective Corrosion Rate Analysis finds the best TML/corrosion rate relationship and statistical distribution in order make accurate remaining life projections and to identify trends. This is achieved by obtaining the best statistical “fit” for the corrosion rate data. A visual indication of the fit may be made by observing how well the corrosion rate data clusters around the best straight line on the Corrosion Rate Reliability Plot. Statistical indicators are used to accurately measure this “goodness-of-fit”.
Several factors may cause an unacceptable fit on a Corrosion Rate Reliability Plot. There may be more than one corrosion mechanism (i.e. localized corrosion) in the system or an incorrect statistical distribution may have been selected. If corrosion rate data follows a Weibull distribution, for example, a plot based on normally distributed data may yield an unacceptable fit. There may also be significant thickness variations within components, or perhaps components were replaced with those having a different nominal thickness (e.g. a different pipe schedule for piping systems) between inspections. Selective Corrosion Rate Analysis correctly identifies and corrects for these real-life situations.
In the Corrosion Rate Reliability Plot (illustrated), the measured long-term corrosion rates range from about 3 mils/year to just over 20 mils/year. The data clusters nicely around the best straight (solid) line for the (log normal) statistical distribution. In this example, Selective Corrosion Rate Analysis identified the best distribution and found a single corrosion mechanism, as evidenced by the single solid line. Projecting the line to 95% reliability (typical for API 570 Class 2 systems) yields a corrosion rate near the maximum measured rate of 20 mils/year. The dotted line represents the short-term corrosion rate. The change in slope for the short-term data indicates a possible recent change in the system, since the corrosion has become less uniform (occurs over a wider range).
Provide TML identification data, historical thickness, and related drawings from 25 representative pieces of equipment.
Identify minimum recommended piping TMLs
A: Selective Corrosion Rate Analysis (SCRA) is a patented, statistically-based analysis process developed by CorrSolutions for verifying data integrity, identifying localized corrosion, estimating remaining life and optimizing inspection schedules for fixed equipment based on historical wall thickness data.
SCRA is offered as an internet-based analysis tool. Users do not purchase any software; rather they access reports through a web browser interface as part of an overall, data management service
CorrSolutions is committed to SCRA’s continuing development in order to provide our customers with measurable reliability improvements and reduced inspection costs.
A: Conventional software makes a number of assumptions about thickness and corrosion rate data in a given piece of equipment:
Research by CorrSolutions has revealed that in general, corrosion rates are not normally distributed. The corrosion process is better modeled using other life distributions such as Weibull and log normal. SCRA selects the best-fitting distribution for each piece of equipment based on historical data.Better modeling produces improved remaining life estimates and more realistic re-inspection schedules.
SCRA runs a series of Corrosion Models for each piece of equipment in order to identify the presence of typical localized/accelerated corrosion mechanisms. If a piece of equipment experiences higher corrosion rates on elbows than straight pipe, for example, then the elbows are analyzed separately as they belong to a different corrosion rate distribution. Corrosion Models also examine the data for different corrosion rate distributions between equipment features such as sizes, orientations and particular ‘zones’. If localized/accelerated corrosion is identified through of these models, the trend is reported so that appropriate action can be taken.
SCRA is fundamentally different in that the most likely (as opposed to most conservative) corrosion rate case is used for remaining life estimates and re-inspection schedules. The combination of the best-fitting distribution plus identification of localized/accelerated corrosion provides significantly improved remaining life estimates over conventional software packages. Therefore, SCRA allows inspection organizations to better focus their resources on real problem areas.
SCRA includes advanced data validation technology to highlight ‘suspect’ thickness data that can lead to erroneous remaining life predictions. A possible cause and suggested action for each suspect reading is also provided.
A: Analysis results are posted on a secure web site and made available to authorized users via a standard web browser. The Inspector simply logs on and accesses the drill-down type web reports for remaining life information, re-inspection schedules and equipment analysis details. Overview web pages for the entire plant and individual pages for each unit provide quick summary information for low remaining-life equipment.
SCRA web reports are quickly deployed from a dedicated Windows NT server with redundant connections to the Internet. All analysis, reporting and web page changes are immediately made available to authorized users through the web browser interface.
A: After the distribution of corrosion rates has been estimated based on historical data, SCRA establishes confidence bounds on that distribution. Corrosion rates falling outside of these confidence bounds indicate either accelerated corrosion or suspect data. These TMLs and thickness data are presented on the Data Validation Exceptions Report along with the possible cause and suggested action. For example, an undocumented replacement may have taken place, an incorrect nominal thickness assumed or a data entry typo may have occurred.
A: SCRA has a number of features not available in other analysis systems. In addition to selecting the best-fitting corrosion rate distribution and identifying localized/accelerated corrosion, SCRA offers other advanced features, including:
Optimized Inspection Schedules:
SCRA proposes cost-saving re-inspection opportunities if a lower total inspection cost (over a used-defined period) is projected by breaking up re-inspection into two segments.
“Interval” and “Less Than” Rates:
Certain components, such as Tees, Reducers and Socket-Welded fittings have poor thickness-measurement repeatability that can unnecessarily shorten re-inspection intervals. This effect is pronounced when the interval between surveys is short. SCRA offers a statistically based, practical approach to insure that these components are not over-inspected due to their inherent thickness variations.
Email Notification for Re-inspection Schedules and Critical Equipment:
Inspectors may receive automatic email notification for user-defined events such as upcoming inspections, low remaining life and high corrosion rates.
Risk-Based Inspection Applications:
Since SCRA provides a completely data-driven methodology for determining the likelihood of failure (LOF) due to thinning mechanisms, it is ideally suited for RBI studies. SCRA does not require the use of generic failure frequencies or equipment modification factors to estimate the LOF. When combined with consequence of failure results, SCRA yields the best estimate of risk.
These results may be used to validate existing inspection plans, identify opportunities for improvement and in some cases, provide a fully documented means for reducing the number of TMLs and/or the inspection frequency (see following item).
TML Reductions & Inspection Interval Extensions:
SCRA offers a fully documented approach for reducing the number of TMLs and extending re-inspection intervals. This unique statistical approach insures that the corrosion rate distribution does not change significantly and the risk does not exceed a defined threshold when fewer TMLs are inspected.
A: SCRA complies fully with API 510 & API 570 language for statistical analysis of thickness data, which requires that statistical predictions “accurately reflect” the actual equipment conditions. Since SCRA develops a best-fitting model for the corrosion behavior in each piece of equipment and properly analyzes localized/accelerated corrosion, the remaining life estimates and re-inspection schedules are truly representative of the equipment condition.
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