1. Introduction

Reliability –Based Inspection is an assessment technique utilized to identify equipment most likely to fail first. This helps to prioritize equipment inspections during a turnaround.

In modern maintenance strategies, not every piece of equipment is opened for inspection during a turnaround. Instead, Reliability –Based Inspection uses existing inspection records, fit for service assessments and evaluations by operations staff to rank various pieces of equipment. Equipment that ranks low is not opened during a turnaround. This provides significant saving of resources.

Determining the remaining life of a component is not always straightforward, especially in situations where there is no data on corrosion rate or reliable crack growth rate. Sometimes there isn’t available corrosion rate data, or reliable crack rate date. In such cases, monitoring or remediation is employed to manage the uncertainty. When a component has little to no remaining life, repair becomes the necessary course of action.

2. Remediation

Remediation is required in situations where;

• A flaw in a component is deemed unacceptable in its current state
• When the remaining life of the component is minimal, uncertain,
• When current assessment methods are inadequate to predict the behaviour of the flaw.

Various remediation methods can be employed to address these issues to ensure the continued safe operation of the equipment. Some common remediation methods include:

• Grinding out cracks on weld overlay: This involves removing cracks from the welded area of a component by grinding. It eliminates the weakened or damaged parts of the weld.
• Welding on Sleeves or Pads: This method involves adding additional material, such as sleeves or pads, to the affected area to reinforce it and restore its integrity.
• Monitoring the condition of the component: This to assess the progression of damage and inform future maintenance actions. These include:
  • Corrosion Probes: Devices used to measure the rate and extent of corrosion in a component, helping to predict its remaining life and identify when intervention is needed.
  • Hydrogen Probes: Used specifically to detect and measure hydrogen ingress in materials, which is a common cause of cracking.
  • Coupons and Physical Probes: These are pieces of material placed in the operating environment of the equipment. Over time, they undergo the same conditions as the equipment, and their examination provides insights into the rate and type of degradation occurring.
  • UT (Ultrasonic Testing) Measurements and Scanning: A non-destructive testing method used to detect internal flaws, measure the thickness of materials, and monitor changes over time to assess the condition of the component.

3. Damage Mechanisms

Before conducting a Fit for Service (FFS) assessment, it’s important to first identify the damage mechanism. A lack of damage mechanism can lead to incorrect conclusions about the component’s stress state and potentially result in failure. The identification process involves using the appropriate Non-Destructive Evaluation (NDE) method, estimating future damage rates to determine remaining life, and applying suitable monitoring and mitigation methods.

Comprehensive information about the materials in question is essential for damage assessment. This includes data on heat treatment, chemical composition, and strength level. These factors are crucial in understanding how a material will react under various conditions and how susceptible it is to different types of damage.

• Material Toughness and Heat Treatment: Material toughness is significantly influenced by the grain size, which in turn is determined by the heat treatment process. Different heat treatment methods can alter the material’s properties, affecting its resistance to damage.
• Chemistry-Dependent Damage Mechanisms: Most damage mechanisms are influenced by the chemical makeup of the material. For instance, certain alloys may be more prone to specific types of corrosion or stress cracking depending on their chemical composition.
• Corrosion Rate: A general guideline for tolerable corrosion is a rate of 5 mils per year. Exceeding this rate may necessitate the use of a higher-grade alloy that is more resistant to corrosion.
• Corrosion During Operational Excursions: Corrosion often accelerates during operational anomalies or upsets. It’s not a binary (on-off) phenomenon but varies in intensity and rate under different conditions.
• Service Exposure and Operational Factors: General and specific service exposures, both under normal and upset conditions, can introduce trace amounts of corrosives. Operational cycles, leaking valves, and human factors also significantly influence corrosion rates.
• Carryover of Corrosives: Not all chemicals are completely removed during cleaning processes, meaning corrosives can be carried over to other operational units. This is particularly problematic with substances like caustic, which are hard to decontaminate and can be transported via steam and other mediums. Caustic substances, while relatively harmless under ambient conditions, become highly corrosive at high temperatures, especially in units not designed to handle them.

4. Categories of Damages

Damage mechanisms in materials and equipment can be broadly categorized into two main types: pre-service flaws and in-service flaws:

4.1 Pre-service Flaws

• Material Flaws from Production: These include issues like laminations, voids, shrinks, and cracks that occur during the material manufacturing process.
• Welding-Induced Flaws: Problems such as weld undercutting due to inadequate penetration and fusion, weld porosity, and hydrogen-induced cracking fall under this category.
• Fabrication Fit-Up Flaws: These refer to issues like out-of-roundness and lamellar tearing that occur during the assembly or fabrication of components.
• Heat Treatment Flaws Resulting in Embrittlement: These flaws can be induced by reheat cracking, sigma phase embrittlement, 885°F embrittlement, and sensitization. Sensitization specifically occurs in austenitic stainless steels when carbides precipitate at grain boundaries during heat treatment.

4.2 In-Service Flaws

1. General Corrosion: This leads to uniform thinning across the exposed surface without significant localization.
2. Localized Corrosion: A corrosive attack confined to a small, specific area.
3. Galvanic Corrosion: Occurs when a metal is in contact with a more noble metal or conducting non-metal in the same electrolyte.
4. Environmental Cracking: A brittle fracture of a normally ductile material due to environmental factors causing embrittlement, such as stress corrosion cracking.
5. Erosion-Corrosion, Cavitation, and Fretting: Caused by the relative movement between corrosive fluid and the metal surface.
6. Inter-granular Corrosion: Preferential attack at or near grain boundaries, often due to sensitization or polythionic acid attack.
7. De-alloying: Selective removal of one constituent of an alloy, altering its structure.
8. High-Temperature Corrosion or Scaling: Formation of thick corrosion layers on metal surfaces at high temperatures.
9. Internal Attack: Alteration of metal properties by environmental constituents at high temperatures, such as carburization (carbon entry) or hydrogen attack (hydrogen infusion).

5. Mitigation of Damages

Mitigation strategies for preventing or minimizing damage mechanisms in materials are diverse and can be tailored to the specific type of damage anticipated. These strategies can be particularly effective in preventing issues like polythionic acid attacks, which result in inter-granular corrosion. Here are some key mitigation techniques:

• Avoiding Polythionic Acid Attack: Keeping water above the aqueous dew point and neutralizing it with an alkaline solution helps prevent the conditions conducive to polythionic acid attack. Also using materials that are less susceptible to sensitization, such as non-austenitic steels or properly treated austenitic steels, can be effective.
• Physical Process Modifications: Altering process temperatures and fluid velocities can reduce the likelihood of certain corrosion mechanisms. Regularly removing fractions that contribute to corrosion can be a preventive measure.

Using organic coatings or metallic linings can provide a barrier between the material and corrosive elements. Applying additional material over the existing surface can increase resistance to corrosion and wear.

Also Techniques like shot peening can improve the surface’s resistance to cracking and fatigue.

• Chemical Process Modifications: Adjusting the chemical composition of the environment or the materials in contact can help mitigate corrosion and other forms of damage.
• Stress State Alteration: Processes like annealing can reduce internal stresses that contribute to damage mechanisms like stress corrosion cracking.