Corrosion and Remnant Life - Theory

Age-Related Degradation Mechanisms are mostly Corrosion mechanisms, with two exceptions:

• Creep (e.g., fired heater tubes), and
• Low-Cycle Fatigue or Cyclic Loading (e.g., pressure-swing absorber vessel).

Remnant Life

For Corrosion, the time to failure, i.e., the Remnant Life (RL), can be determined. This is important to know, since it will, together with the Interval Factor (IF) (based on Consequence and Confidence of the DM), influence the Maximum Inspection Interval (MII=RLxIF) and thus the Next Inspection Date (NID=LID+MII). RL is based on Corrosion Rate (CR) and Remaining Corrosion Allowance (RCA), and is calculated as follows:

RL = RCA / CR

Where:
RL = Remnant Life [year]
RCA = Remaining Corrosion Allowance (at last inspection) [mm] or [mils]
CR= Corrosion Rate [mm/y] or [mpy]

Remaining Corrosion Allowance (RCA)

Initially, the Remaining Corrosion Allowance (RCA) is the difference between Nominal thickness of, e.g., a pipe, and the Renewal thickness. This is also called the Design Corrosion Allowance (DCA). However, over time, as Corrosion causes the Wall Thickness (WT) to decrease, the RCA will decrease (see the figure below). 

Initially RCA is the difference between Nominal thickness and Renewal Thickness. However, it decreases over time.

In the IMS context, the Renewal thickness is the Minimum Allowable WT (see the figure below). When the DCA is unknown, RCA can be calculated as the difference between the Remaining WT and the Minimum Allowable WT.

When additional Wall Thickness is added, the RCA increases.

 

Corrosion Rate (CR) (and Circuits)

Corrosion Rate (CR) is Wall Loss over times. It can be calculated from measured Wall Thickness (WT) data.

In IMS a Circuit is defined for each section of an Equipment where the same Corrosion Rate is Expected. The purpose of a Circuit is to enable Wall Thickness inspections and Corrosion calculations. For each Circuit, Condition Monitoring Locations (CMLs) and associated Measurement Points (MPs) are defined. These specify where the measurements need to take place. Measurements are then taken, based on the Circuit’s Corrosion Inspection Schedule, and from the inputted WT values, IMS calculates the CR. The relationship between Circuits, CMLs and MPs are explained in PEI Hierarchy - Overview.

Both Short Term, Long Term and Linear regression calculations are used (see the figure below). There is also an Extreme Value Analysis (EVA) method that can be used for Heat Exchangers to extrapolate the maximum Wall Loss over the entire tube space. Furthermore, the S-IDAP module, which forms part of IMS, can be used to process, analyze, and visualize the WT data further. This helps with detecting anomalies, flagging readings to be ignored, etc.

Corrosion Rate is determined form Wall Thickness measurements over time.

When CR is determined from measured data it is called trended CR. When a theoretical model is available the theoretical CR can also be determined. When the CR is based on experience it is called historical CR. From these three, the CR for the RL calculation, should be selected conservatively.

For more information on Corrosion Calculations in IMS see: PEI Corrosion Calculations and Calc Summary.

Liner or Coating extends the Remnant Life

Coating: Coating is a liquid or mastic compound applied to the external surfaces of a tank/pipe that, after curing, forms a solid, uniform thin layer and serves as a protective film against corrosion. Examples are the epoxy coating systems applied to the external tanks surfaces.

Lining: A liner is a solid, usually thin, non-metallic material applied to the internal surfaces of a tank/pipe for corrosion protection or product purity. Examples are the rubber liners in demineralized water tanks or the brick liners in slop or off-spec tanks.

Lining or Coating will extend the RL of a Component. Therefore, in the S-RBI Analysis one can take credit for Lining/Coating (see Step 6 Estimate Remaining Functional Life). The Component’s Next Inspection Date (NID) will then be extended by the Lining/Coating RL, adjusted by an Interval Factor (IF). This IF_{Liining/Coating} is determined by the Lining/Coating’s quality of installation.

NID = LID + MII_steel + MII_{Liining/Coating}

MII_steel = RL_steel x IF

MII_{Liining/Coating} = RL_{Lining/Coating} x IF_{Liining/Coating}

The graphs show the effect of Lining/Coating on CR.

Determining RL for S-RBI

To determine the RL for the S-RBI Analysis see Step 6 Estimate Remaining Functional Life (AR DMs and CUI).