Methodology Step 10

Corrosive Deadlegs

Confidence is not applicable.

Corrosive Injection and Mixing Points

Confidence is not applicable.

Pressure Relief Devices 

Confidence is evaluated in the StF section.

Tanks

Addition 4th question: Has Corrosion assessment been carried out?

Thermal Injection and Mixing Points

• NAR Confidence assessment.
• Additional guidance to answer the key questions based on: Type of TMP; if operating conditions are steady; ΔT variation; and mass FRR variation.
• If applies to an injection or mixing point the DM Thermal Fatigue should be assigned to the Component.
• Operating data can assist with the assessment. 

Stress Corrosion Cracking (SCC)

• Additional guidance to answer the key questions based on inspection results – damage not found / found / found & StF to be reviewed. 
• Use Lookup table to adjust Confidence score based on required Inspection Effectiveness and StF. 

Amine Stress Corrosion Cracking

Additional guidance to answer the key questions.

Corrosion Under Insolation (CUI)

No Confidence assessment.

High Temperature Hydrogen Attack (HTHA)

Additional guidance to answer the key questions. 

Hydrogen Induced Cracking HIC/SOHIC

Additional guidance to answer the key questions. 

Lean Amine Corrosion

Additional guidance to answer the key questions.

Rich Amine Corrosion

Additional guidance to answer the key questions. 

DM is stable and can be properly controlled.

Corrosion Engineer

A corrosion mechanism is considered stable when it meets two conditions:

• It remains constant over time. For example, general CR will remain more or less constant over long periods of time, if there are no significant changes in operating conditions. Pitting corrosion rates, on the other hand, can increase exponentially with exposure time, even if operating conditions remain constant.
• It does not change drastically with relatively small changes in process conditions. For example, Amine CRs increase more and less proportionally with Acid gas loading, temperatures, Heat stable Amine salts (e.g., Amine degradation products) and velocity/ turbulence. On the other hand, pitting corrosion of stainless steels can be below a threshold temperature, which depends on the composition of the steel and on the presence of corrosive species in the environment. However, if the temperature exceeds this threshold, pitting rates can be much higher. This means that pitting corrosion may show a sharp temperature discontinuity.

A corrosion mechanism is considered properly controlled if the process conditions are such that CRs remain low, or if adequate methods are applied to reduce the CRs to tolerable levels. For example, acid corrosion rates in a hydro-processing reactor effluent stream can be controlled if the system remains dry (no water condensation), or if wash water is used to dilute the acids and maintain a high pH.

In most cases the DM is both stable and can be properly controlled. In undefined situations, and the corrosion engineer needs to make a judgement as to which of the two parts of the question is more important. For example, Amine corrosion of carbon steel will proceed at an almost constant rate over time, which means it is stable but can be difficult to control. In that case, the use of corrosion inhibitors or a change in the amine composition (reduction in heat stable salts) can be used to control the CRs.

Multiple reliable inspections have been carried out.

Inspection Engineer

Have multiple inspection been carried out?

Multiple means 3 or more. An intermediate rating can be given for 2 to 3 inspections. Single or no inspections receive a rating of “No”.

Were those inspections reliable?

• Reliable means that the inspection method used, is appropriate and adequate for detecting the DM that is being evaluated. For example, ultrasonic inspections are reliable for detecting general wall loss corrosion but are not reliable for detecting pitting corrosion. Therefore, if pitting corrosion is the expected DM, multiple ultrasonic inspections are not considered reliable.
• Reliable also means that the inspection locations were appropriate and adequate. For example, if corrosion is expected in low points where water can accumulate, inspections of high velocity areas such as bends are not reliable for the expected DM.
• The inspection engineer should assess whether the size of the inspected area, or the number of inspection points are sufficient to represent the whole of the Equipment or piping that is being evaluated.
• Additionally, it is possible that the interval between the successive inspections and the accuracy of measurements may affect the judgement and consequently the confidence rating. For example, two successive inspections at a short interval may not provide sufficient confidence. Similarly, the uncertainty of the accuracy of measurements may not provide sufficient confidence.

Note: The API definition of Inspection Effectiveness can also be used. Two Usually Effective inspections equals one Highly Effective inspection; Two Fairly Effective inspections equals one Usually Effective inspection; etc. (The required Inspection Effectiveness is calculated in Step 11.)

In short: A "Yes" confirms that the controls in place work to control or predict the behavior of the DM.

Relevant process parameters are reliably monitored.

Operator / Process Engineer and the Corrosion Engineer

The corrosion engineer needs to determine which process parameters are relevant to the DM. This may include temperature, flow velocity, concentration of corrosive species, pH, etc.

The operator/process engineer needs to evaluate if those parameters are reliably and consistently monitored, for example whether the right Equipment, sampling, and data analysis tools are employed or not.

With reliable and consistent monitoring, it is assumed that:

• Process variables, relevant to the DM, are part of the unit’s proactive monitoring plan, which is regularly evaluated.
• The IOW variables are clearly defined at CL level and owned by the corrosion engineer (field instrumentation tags and LIMS samples should be clearly identified).
• The necessary IOW variables are in an Alarm Configuration Manager (ACM) (i.e., Unit Variable Table - UVT) for operational control, and action is taken on deviations and trends.

Requirements for monitoring are generally specified in the CL where they apply, not where they are measured or where the field instrument is located.

For example, the temperature of one heat exchanger in a train of several exchangers may be deduced from the overall temperature change over the full train.

Some process parameters may be intrinsic to the process and therefore may not vary much from a given design value. For example, the hydrogen pressure and the temperature across a hydrotreaters reactor do not change significantly over the run length. Then, if the process operates efficiently, it can be safely assumed that such parameters as hydrogen pressure and temperature are close to their design values. Therefore, higher confidence may result.

Note: Variables which Operations cannot control (change) or Process Engineer/Technologist cannot influence are not part of the CL’s IOW (e.g., influence of yearly precipitation on CUI).

Degradation mechanism can be properly controlled.

Corrosion Engineer

Stability for NAR DM can only be achieved through material selection and IOW control. When either of the two is not met, they can occur rapidly or instantaneously.

The control of the NAR DMs can be affected in two ways:

• By selecting an adequate material that is resistant to that form of degradation; and
• By changing the characteristics of the environment to make it less aggressive.

For example, the chloride stress corrosion cracking (Cl-SCC) of austenitic stainless steels can occur in a chloride containing aqueous environment. The duplex stainless steels with low nickel content or alloys with greater than 42% nickel are generally immune to Cl-SCC. Alternatively, the DM can be controlled by lowering the chloride concentrations and temperature, or by increasing the pH i.e., the environment control.

In some conditions, it will not be completely clear if the DM can be fully controlled under all circumstances. E.g., Chloride SCC in a non-temperature-controlled system. In these cases, the corrosion engineer needs to make a judgement as to whether the mechanism can be controlled.

Relevant process parameters are reliably monitored.

Operator / Process Engineer and the Corrosion Engineer

Same as for AR DMs.

Control of IOW as a Barrier is generally more important for NAR DM.

Reliable inspections were carried out.

Inspection Engineer

One main difference in the treatment of the NAR DMs and the AR DMs in the context of RBI, is the emphasis on inspections for the latter and the emphasis on monitoring for the former. Reliable inspections on NAR contribute less to the overall outcome of the Confidence Assessment.

Very few NAR DMs proceed at a relatively low rate, and their progress can conceivably be monitored by inspection. Hydrogen induced cracking (HIC) falls under this category.

Other mechanisms proceed at a relatively high rate compared to AR DMs, and inspections are much less reliable to monitor their progress. For example, sulphide stress cracking of high strength materials.