In Part 4 of our series on guidelines for providing process conditions for Risk Based Inspection (RBI), we move on to discuss High Temperature Damage Mechanisms and offer some guidance on how to select process conditions that will be effective with respect to your RBI probability and consequence calculations.
High temperature damage mechanisms occur at temperatures higher than 400°F and are characterized by the degradation of the base metal induced either by a reaction between a corrosion precursor and the base metal, or as a result of loss of the mechanical properties.
Representative fluid, initial fluid phase, operating pressure, operating temperature, toxic, and toxic percent are the basic process points required by most RBI software to determine the consequence and probability of loss of containment. In general, this basic process information is sufficient in calculating the consequence of loss of containment. However, in some cases, such as high temperature damage mechanisms, this data is not always enough to perform the probability calculation.
It is important to mention that the probability of failure in RBI is determined by the potential for occurrence of the applicable damage mechanisms and by the quality and quantity of inspections performed for early detection of problems. Therefore, when assessing the probability of loss of containment in RBI, a key factor is the determination of the potential for occurrence of the damage mechanism (also named susceptibility) that, in case of high temperature damage, depends on different process factors as shown below.
API 571 establishes 400°F as the temperature limit for High Temperature Corrosion and includes the following seven damage mechanisms in this category:
- Metal Dusting
- Fuel Ash Corrosion
This group of damage mechanisms have in common that the degradation of the material is typically induced by a corrosion precursor that reacts or diffuses in the base metal to produce thinning and, in some of the damage mechanisms, also induces loss of the mechanical properties of the metal.
Key process factors to consider when assessing the potential for occurrence of damage are:
- amounts of the corrosion precursors present in the process stream (i.e. total Sulphur, H2S, TAN, N2)
- type of atmosphere prevalent in the combustion chambers of heaters or boilers (carburizing, oxidizing or reducing)
- type of fuel used in heaters or boilers and level of contaminants
Since the potential for occurrence and severity of these damage mechanisms depends on temperature (see Table 1), the operating temperature provided should be as accurate as possible (recommended accuracy within 10%).
|Table 1. Temperature Thresholds (per API 571) for Different High Temperature Damage Mechanisms|
|Damage Mechanism||Temperature Threshold|
(Per API 571)
|Oxidation||1000°F (Carbon Steel)|
1500°F (Series 300 SS)
|Oxidation of carbon steel begins to become significant above about 1000°F.|
|Sulfidation||500°F||Sulfidation of iron-based alloys usually begins at metal temperatures above 500°F.|
|Carburization||1100°F||Temperature high enough to allow diffusion of carbon into the metal is typically above 1100°F.|
|Decarburization||400°F (Carbon Steel)||Decarburization occurs during exposure to high temperatures, during heat treatment, from exposure to fires, or from high temperature service in a gaseous environment (e.g. Hydrogen attack).|
|Metal Dusting||900°F||Usually occurs in the operating temperature range of 900°F to 1500°F.|
|Fuel Ash Corrosion||700°F and higher||Corrosion typically occurs when using fuel or coal containing sulfur, sodium, potassium and/or vanadium. For boilers waterwall corrosion, liquid species can have melting points as low as 700°F.|
|Nitriding||600°F||Nitriding begins above 600°F and becomes severe above 900°F.|
Loss of Mechanical Properties
There is another group of High Temperature damage mechanisms which are characterized by inducing mainly a loss of mechanical properties of the material. These damage mechanisms include the following, among others:
- Sigma Phase
- Temper Embrittlement
- 885 Embrittlement
- Dissimilar Metal Weld Cracking (DMW)
- Thermal Fatigue
- Stress Relaxation Cracking
Damage potential for this second group of mechanisms depends on time, temperature, metallurgy, stress level, temperature swings and number of cycles, and number of shutdowns and startups of the equipment. For some of the damage mechanisms (e.g. creep), exceeding the design operating temperature in a relatively small range can reduce drastically the design lifetime of the components (increase of about 25°F can cut the remaining life in half or more, depending on the alloy). In some cases, operating procedures are used to avoid equipment damage (e.g. use of minimum pressurization temperature procedure to avoid temper embrittlement in heavy wall reactors). For this second group of damage mechanisms, key process factors to consider include operating temperature (consider operating temperature breaches), temperature swings and number of cycles, number of shutdowns/startups per year and availability of operational procedures to avoid equipment damage.
In summary, the basic process information required by most RBI software is, in some cases, not enough to calculate the probability (damage potential) of loss of containment for most of the High Temperature damage mechanisms. It is recommended to provide the temperature data as accurate as possible (recommended accuracy within 10%), considering operating temperature breaches as well. Since additional process data will be required in some instances, the corrosion engineer/specialist should request the process engineer to provide any additional process data needed to determine the damage potential.
Stay tuned for the next entry in this eight-part series covering guidelines on assigning process conditions for RBI efforts:
- Guidelines for Providing Process Conditions for Risk Based Inspection (RBI) Implementation and Revalidation (Introduction)
- Corrosion Under Insulation (CUI) and How it Relates to Risk Based Inspection (this article)
- Process Fluids and Consequence Models
- High Temperature Damage Mechanisms (this article)
- Low Temperature Damage Mechanisms
- High Temperature Hydrogen Attack
- Environmental Cracking Damage Mechanisms
- Concluding Remarks
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