API 653 Inspection Standards: Safeguarding Your Storage Tanks Against Corrosion

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Aboveground storage tanks (ASTs) are critical components of the production and processing facilities in industries including oil and gas, chemical manufacturing and power generation. These tanks convey large volumes of fluids such as crude oil, chemicals, and water and their failure is a recipe for disasters, mishaps and a drain on the companies’ financial coffers. For storage tanks, one of the main causes of early removal and replacement or even failure is corrosion—a type of natural deterioration of metals. In an attempt to deal with risks of corrosion the American Petroleum Institute API has provided API 653 which is a code that covers the inspection, repair, alteration and reconstruction of aboveground tanks. This standard gives procedures for evaluating the structural integrity of storage tanks and guarantees that repair and inspection are appropriately accomplished so that the tank can be safe, non-leaking, serviceably useful for an extended period. In this blog, we will go deep and dive through API 653 standard and how it helps in protecting storage tanks from corrosion. In this article we will discuss in detail about the various critical aspects of API 653, the variety of corrosion that affect storage tanks and how to prevent them by frequent and regular inspections and repairs.

Understanding API 653: A Brief Overview

API 653 was especially created to set out standard procedures that must be followed during inspection and maintenance of aboveground storage tanks. The standard is applicable to tanks fabricated from materials that complies with API 650 or other design codes, and is particularly relevant to the storage of petroleum and chemicals. API 653 covers a wide range of topics, including:

  • Dimensions of sampling, coverage, frequency, and type of inspection for assessing corrosion and other types of damage.
  • Repair work and alterations for corrosion and structure related problems.
  • Reconstruction of tanks for further safe operation and maintaining their effective performance.

The rules concerning the assessment of corrosion related fitness-for-service of a tank in consideration of the corrosion rates and general state of the tank.

The API 653 standard helps make sure that tanks, regardless of their age, are inspected adequately and either repaired or replaced as needed to avoid failure that might cause a spill, pollute the environment or pose risks to people. It is an important element of any corrosion management program in a facility especially for tanks that have been in service for several decades.

The Threat of Corrosion to Aboveground Storage Tanks

Corrosion is an electrochemical process experienced by metals especially through contact with moisture, oxygen and other elements in the environment. On storage tanks, corrosion can be internal (areas where the contents of the tanks make contact with the tank material) and external (where factors such as rain and humidity speed up the corrosion process). The impact of corrosion can be devastating and is causing:

  • Tank wall thinning: During corrosion, the metal thins down and as a result the tank becomes vulnerable to leakage and structural failure.
  • Pitting: When anodic product formation has caused a pit or hole on the body of the tank materials then it must be this type. These pits may reach right up to the walls of the tank, as a result, developing into points that leak.
  • Undetected leaks: Demands for a more thorough penetration and inspection of the material make corrosion a more frequent phenomenon that manifests itself as the formation of a protective oxide layer within the material. Sadly, in most cases when one wakes up to the fact that they have a leak, a lot of damage has already occurred.

The most common types of corrosion that affect storage tanks include:

  • Uniform corrosion: This form of corrosion develops uniformly over the external surface of the tank and progressively thin the walls of the tank.
  • Localized corrosion: Pitting results in formation of deep holes in localized regions of the tank and is only noticeable by detailed inspection.
  • Microbiologically Influenced Corrosion (MIC): MIC is characterized by the action of bacteria that are found in water or oil and mainly affect the bottom of storage tanks due to the sludge and water that accumulate at the bottom.
  • Galvanic corrosion: The importance of correct material selection arises when two metals different in the series are exposed to an electrolyte, and galvanic corrosion occurs, in which the more reactive metal will degrade more rapidly than in a non-electrolyte environment.

Key Components of API 653 Inspections

1. Regular Inspection Intervals

In addition to requiring proper foundation, API 653 takes into account the condition of the tank as well as its service history in order to determine its interval for inspection in order to safeguard the tank. API 653 outlines different types of inspections:

  • External inspections: This is done externally in a bid to check for weaknesses on the surface area of the tank, such as corrosion, leakage, or if the tank has developed some settlement problems. The inspections which are done outside should be scheduled once a year.
  • Internal inspections: Aboveground inspections are somewhat more comprehensive than below-ground ones and include, after the removal of the tank contents and its cleaning, the visual interior inspection of the tank walls, floor, and roof. These inspections should ideally be carried out at intervals of 10 years, however shorter intervals may be needed in the case of tanks with a high corrosion potential or as determined by any assessment of risk.
  • Ultrasonic thickness testing (UTT): This is so because the methodology used in ultrasonic thickness examination is to determine the thickness of the tank walls and identify locations where they have been thinned due to corrosion. Consequently, UTT should preferably be conducted together with other tests to check the status of the tank.

This is especially important because Le Mag’s engineers need to act quickly before the corrosion becomes a leak or causes structural weakness.

2. Corrosion Rate Determination

Through API 653 inspection, thickness measurement is carried out to determine the corrosion rate with which the tank metal is corroding. Corrosion rate is applied for approximate estimation of the further service life of the tank, thus defining the moments when repair or, on the contrary, replacement of the tank may be required. Calculating corrosion rate allows the facility operators to determine how long the tank will be suitable for use before it will need to be replaced.

API 653 has general information on acceptable minimum thickness of tanks. Whenever the thickness described above is below the acceptable standards, some action has to be taken to redo the tank or decrease the pressure level in use.

3. Fitness-for-Service Evaluations

API 653 inspection procedure mandates a fitness-for-service evaluation that considers re-coiling probability as a major factor when evaluating older tanks that might have experienced considerable corrosion. This assessment determines effectiveness in evaluating a tank concerning its safety for functioning at the moment. Parameters such as metal thickness, corrosion rate, operation conditions, and the type of the product, which is stored in the tank, are considered.

Tank defects that may result in a decision of tank unfitness lead to a tank being removed from service, waiting for repairs to be made or requiring total decommissioning or reconstruction.

Best Practices for Safeguarding Tanks Against Corrosion

While API 653 provides guidelines for inspecting and repairing corrosion-related issues, there are additional best practices that can help facility operators prevent corrosion from occurring in the first place.

1. Implementing Cathodic Protection Systems

Cathodic protection (CP) is also one of the most efficient means of corrosion protection for the aboveground storage tanks. CP systems operate on the principle of interfering a small current through the tank and counteracts the chemical reaction that induces corrosion. There are two main types of cathodic protection systems:

  • Sacrificial anode systems: These systems employ an anode of a still more reactive metal (such as magnesium or zinc) that corrodes rather than the tank material.
  • Impressed current systems: Such systems avail an external source of electricity that constantly flows into the tank, thus preventing corrosion.

Cathodic protection is especially effective with regard to the bottoms of tanks, to which water and sludge are commonly subjected, which provides best conditions for corrosion.

2. Coating and Lining Systems

Another seminal protection system is the use of coatings and linings. Coatings are also used both internal and external in the surfaces of the tank to avoid the corrosive substances affecting the metal. Coatings should be applied to the code of practices and also frequently checked so that they may provide the expected results.

3. Regular Maintenance and Cleaning

Among the easiest strategies for stopping corrosion is through frequent tank maintenance and washing. The contents of the tanks should also be periodically washed if there has been sludge, water etc. building up, which is bad for corrosion particularly in the bottom of the tank. Also, any breached coatings or linings must be restored as early as possible to avoid penetration to the metallic surface by moisture or chemicals.

The Role of Technology in Corrosion Detection

Technological developments have on the other hand presented methods for detecting and observing corrosion in storage tanks. Some of the cutting-edge technologies being used in tank inspections include:

  • Magnetic Flux Leakage (MFL): It is an effective method of defining corrosion and pitting on the tank bottom by taking known measurements in terms of the magnetic field going through the metal thickness. MFL is most valuable for identifying corrosion that may be invisible during the simplest kinds of inspection.
  • Drone Inspections: With cameras and sensors, modern UAVs are widely applied for external examinations of storage tanks, especially in such areas as a tank roof. Drone inspection allows early detection of corrosion or damage without having to attach scaffolding or any time taking method.
  • Acoustic Emission Testing: This method identifies the acoustic vibrations created by the expansion of flaws or corrosion in the tank material. The acoustic emission testing can give on-going data about the situation in the tank and indicates possible problem zones before they develop into serious problems.

Overcoming Common Challenges in API 653 Inspections

Even though API 653 inspections are important for assessing, recognizing and quantifying corrosion in storage tanks as well as ascertaining their soundness, various factors can make the inspection process challenging. These challenges include:

  • Accessing the tank interior: For internal inspections, the tank must be removed from service, depressurized and, often, drained and cleaned, both of which can be costly and time consuming. For much of the impact to be avoided, there should be planning of inspections to be undertaken at regular maintenance intervals.
  • Dealing with hazardous materials: More risks are associated with tanks that contain dangerous compounds as well as flammable liquids when conducting inspections and repairs of the tanks. Appropriate measures of protection should be taken to ensure that the people affected are protected and the environment is also protected.
  • Interpreting inspection data: Interpretation of results of corrosion tests and fitness-for-service assessments needs some level of understanding. Using knowledge from certified API 653 inspectors and engineers can also be used effectively in making the right inspection and making sure that whatever repair is required, it is done.

Conclusion

API 653 is an important standard in ensuring that ASNT aboveground storage tanks are protected with regard to corrosion. Corrosion is one of the biggest risks on storage tanks and with Inspections, Corrosion management plans and Controls plus following the API 653 guidelines, operators are able to keep their tanks safe, dependable and legal.

Incorporation of proven strategies like cathodic protection, protective coatings, and schedule cleaning coupled by utilization of new technologies in inspecting the equipment, a company can minimize risks of corrosion driven failures. In the long run, it is better to take time and provide regular maintenance and inspection that will help prevent leakage and possibly the harm to the environment more than is necessary and most importantly ensure the sustainability of the storage business.

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