API 653 Exam Chapter 4

API 653 Exam Chapter 4 -Reasons for Inspection: DamageMechanisms

First of all, what exactly is the point of tank inspections? Granted, some leak or catch fire, and there are no doubt a small number of major failures, but the everyday world is not exactly full of catastrophic tank disasters.

On the face of it, API codes are quite clear on the subject – their objective is to achieve tank integrity (it says so in API 653 section 1). Integrity must surely mean structural integrity, i.e. the avoidance of catastrophic failure or major collapse leading to total loss of the tank contents.

All right. What about leaks? Clearly, leaks are undesirable and published codes have quite a bit to say about avoiding them. API 575 starts the ball rolling in its section 5: Reasons for inspection and causes of deterioration. It mentions the objective of avoiding holes in all areas of a tank, to avoid the risk of hazards from flammable leaks or environmental pollution. The situation elsewhere in the codes is not quite so straightforward – it is a long-standing principle of API codes that fairly deep isolated pits are unlikely to lead to structural failure. For tanks, this is balanced by API 653’s approach to repairs; patch plates, flush insert repairs or hot taps are really no problem as far as API 653 is concerned, so leaks from isolated pitting can be repaired if or when they occur.

4.1 The approach to damage mechanisms (DMs) API 653 and its associated codes have a well thought-out approach to DMs. It is better organized than equivalent pipework and vessel codes; a little less fragmented and hence easier to understand. In essence, most of the information on DMs has been relegated out of API 653 into API 571 Damage Mechanisms. Awkwardly, quite a lot of the introductory information remains in API 575. These relatively short sections are a frequent source of exam questions, many taken word-for-word from the rather dense narrative paragraphs. It is fair to say that the key points don’t exactly jump out of the pages at you. Let’s see what they are.

4.2 API 575 section 5: reasons for inspection

API 575 takes a logical approach to this:

Section 5.2 identifies corrosion as the prime cause of deterioration of tanks. This is then subdivided into section 5.2.1 external corrosion and Section 5.2.2 internal corrosion.

For external corrosion, the emphasis is heavily on corrosion of the tank bottom being the main problem. This fits with the overall approach of API 653. External shell corrosion is not covered in detail in API 575 section 5.2.1. API 653 deals with that in detail later.

For internal corrosion, API 575 section 5.2.2 simply reinforces the idea that it is the tank product that generally causes the corrosion. It also mentions bottom sediment and water (BS&W) – a common examination question.

Section 5.4 is about leaks, cracks and mechanical deterioration, i.e. just about anything that is not classed as corrosion. Note these API value judgement points that appear in this section: 

  • The most critical place for crack-like flaws is at the shell-to-bottom weld, owing to the high stresses. 
  • Settlement underneath the tank bottom caused by freezing and thawing can trap water and cause bottom corrosion.

Section 5.5 is about auxiliary equipment such as tank vents, drains, structural steelwork and ladders. It mainly cross-references annex C in API 653 – a long checklist of tank inspection items. This is a very comprehensive checklist, but thankfully next to impossible to use as the subject of multichoice examination questions.

4.2.1 Similar service

Once you start to consider the reasons for inspecting (or conversely, not inspecting) a tank, the issue of similar service raises its head. There is no great difficulty about the principle – it simply means that instead of going to all the trouble of measuring the corrosion rate of a tank, you just assume it is the same as that already determined from another tank in similar service. This is a great idea, as long as you actually believe it.

This idea of similar service is growing in acceptance in the API codes – API 653: 2009 now contains a complete new Annex H about it, and there is also a simpler introduction in API 575 Annex B. We will look at this in detail later; just remember for the moment that it can be used as the justification for not inspecting a tank, rather than the justification for inspecting it.

4.2.2 Reasons for inspection

Both API 575 and 653 infer that the condition of a tank bottom is mainly what drives the need for inspection. They don’t exclude the shell and roof but, on balance, it is the corrosion of bottoms that causes the problems. API RP 651: Cathodic Protection takes a similar view, concentrating mainly on the soil side with its differential aeration and resulting corrosion currents.

In API 575, discussion of the reasons for inspection soon give way to the actual techniques of inspection, and their frequency. Sections 7.2 and 7.3 cover external inspection and section 7.4 internal inspections. API 653 explains the reasons for inspection in various fragments of sections 4 and 6 but is fairly resolute at not going into specific technical details of damage mechanisms. This makes sense – API 571 deals exclusively with DMs (corrosion-based and other types) and is a much better way of presenting them than a piecemeal coverage in API 653. As it develops, API 653 maintains its reputation as a set of good practical engineering guidelines rather than a corrosion handbook.

4.2.3 The link with API 571 damage mechanisms

API exam question setters seem to like API 571. Divided into neat packages of damage mechanisms, each package contains a fairly logical structure of description, appearance, critical factors, affected equipment, inspection and mitigation for each DM in turn. This stuff is just made for exam questions; either closed-book questions on subjects requiring a bit of reasoning or open-book questions based on little more than the verbatim wording with the odd paraphrase thrown in for good measure.

In terms of extent, the API 653 BOK contains only a few of the full range of DMs contained in API 571. This reflects the fact that storage tanks are less likely to see such a wide variety of DMs as refinery vessels or pipework, which can come into contact with high temperatures, aggressive catalysts, sour (H2S) fluids and similar.

Remember the background to API 653 – it exists to anticipate, monitor and ultimately repair the effects of the DMs that attack tanks, so API 571 will always remain a key part of the BOK, and a well-used source of exam questions.

4.3 API 571: introduction

API 571 was added some years ago to the BOK for the API examinations and replaces what used to be included in an old group of documents dating from the 1960s entitled IRE (Inspection of Refinery Equipment). The first point to note is that the API 571 sections covered in the API 653 ICP exam syllabus are only an extract of ten DMs from the full version of API 571.

4.3.1 The ten damage mechanisms

Your API 653 exam copy of API 571 contains (among other things) descriptions of ten damage mechanisms. Here they are in Fig. 4.1.

Remember that these are all DMs that are found in the petrochemical/refining industry (because that is what API 571 is about), so they may or may not be found in other

Figure 4.1 The ten tank damage mechanisms from API 571
Figure 4.1 The ten tank damage mechanisms from API 571

industries. Some, such as brittle fracture and fatigue, are commonly found in non-refinery plant whereas others, such as sulphuric acid corrosion and microbiological-induced corrosion (MIC), are more common in tanks containing petroleum products. In reality, storage tank farms are rarely just limited to refinery products so the boundaries are less well defined than for pressure vessels and pipework.

4.3.2 Are these DMs in some kind of precise logical order?

Yes, more or less. The list contains a mixture of corrosion and non-corrosion DMs, some of which affect plain carbon steels more than alloy or stainless steels and vice versa. There are also various subdivisions and a bit of repetition thrown in for good measure. None of this is worth worrying about, as the order in which they appear is not important.

In order to make the DMs easier to remember you can think of them as being separated into three groups. There is no code significance in this rearrangement at all; it is simply to make them easier to remember. Figure 4.2 shows the revised order.

One important feature of API 571 is that it describes each DM in some detail, with the text for each one subdivided into

Figure 4.2 Tank damage mechanisms: the revised order
Figure 4.2 Tank damage mechanisms: the revised order

six subsections. Figure 4.3 shows the subsections and the order in which they appear.

These six subsections are important as they form the subject matter from which the API examination questions are taken. As there are no calculations in API 571 and only a few tables of detailed information, you can expect most of the API examination questions to be closed book, i.e. a test of your understanding and short-term memory of the DMs. The questions could come from any of the six subsections as shown in Fig. 4.3.

4.4 The first group of DMs

Figures 4.4 and 4.5 relate to the first two DMs extracted from API 571: brittle fracture and mechanical fatigue. Note that these are not corrosion mechanisms but damage mechanisms, with a mechanical basis. When looking through these figures, try to cross-reference them to the content of the relevant sections of API 571.

Figure 4.3 API 571 coverage of DMs
Figure 4.3 API 571 coverage of DMs
Figure 4.4 Brittle fracture
Figure 4.4 Brittle fracture
Figure 4.5 Mechanical fatigue
Figure 4.5 Mechanical fatigue

Now try this first set of self-test questions covering the first two DMs.

4.5 API 571 practice questions (set 1)


Q1. API 571: brittle fracture
Which of these is a description of brittle fracture?



Q2. API 571: brittle fracture: affected materials
Which of these storage tank construction materials are particularly susceptible to brittle fracture?



Q3. API 571: brittle fracture: critical factors
At what temperature is a brittle fracture of a storage tank most likely to occur?



Q4. API 571: brittle fracture
Which of these activities is unlikely to result in a high risk of brittle fracture of a storage tank?



Q5. API 571: brittle fracture: prevention/mitigation
What type of material change will reduce the risk of brittle fracture?



Q6. API 571: brittle fracture: appearance
Cracks resulting from brittle fracture will most likely be predominantly:



Q7. API 571: mechanical fatigue: description
What is mechanical fatigue?



Q8. API 571: mechanical fatigue: critical factors
As a practical rule, resistance to mechanical fatigue is mainly determined by what aspect of a piece of equipment?



Q9. API 571: mechanical fatigue: appearance
What will a fracture face of a component that has failed by mechanical fatigue most likely exhibit?



Q10. API 571: prevention/mitigation
Mechanical fatigue cracking is best avoided by:


4.6 The second group of DMs Figures 4.6 to 4.8 relate to the second group of DMs: atmospheric corrosion, CUI and soil corrosion. Note how these DMs tend to be related to the environment outside the

Figure 4.6 Atmospheric corrosion
Figure 4.6 Atmospheric corrosion
Figure 4.7 Corrosion under insulation (CUI)
Figure 4.7 Corrosion under insulation (CUI)
Figure 4.8 Soil corrosion
Figure 4.8 Soil corrosion

storage tank. Remember to identify the six separate subsections in the text for each DM. Try these practice questions.

4.7 API 571 practice questions (set 2)

Please go to API 653 Exam Chapter 4 to view the test

4.8 The third group of API 571 DMs

Now look through Figs 4.9 to 4.12 covering the final group of five DMs: MIC, caustic corrosion, chloride SCC, caustic SCC and sulphuric acid corrosion. These relate predominantly to corrosive conditions on the product side (i.e. inside) of a tank. Again, remember to identify the six separate subsections in the text for each DM, trying to anticipate the type of examination questions that could result from the content.

Figure 4.9 Microbial-induced corrosion (MIC)
Figure 4.9 Microbial-induced corrosion (MIC)

4.8.1 Finally: specific DMs of API 652

For historical reasons the ten DMs extracted into the BOK from API 571 exclude one of the most important ones, corrosion caused by sulphate reducing bacteria (SRB). Discussion of this is hidden away in API RP 652: Lining of Above Ground Storage Tank Bottoms. All of this code is included in the BOK so SRBs need to be added to the DMs considered from API 571. API 652 section 4.5 provides a detailed, if slightly contradictory, explanation of the effect of SRBs. They are basically a facilitation mechanism for concentration cell pitting rather than a separate DM by themselves.

Figure 4.13 summarizes the situation. Read this figure as a clue to the content of API exam questions rather than a detailed technical treatise on the subject. Look carefully at

Figure 4.10 Chloride stress corrosion cracking (SCC)
Figure 4.10 Chloride stress corrosion cracking (SCC)

the form of words used – and do not be surprised if they pop up as exam questions.

Now attempt the final set of self-test questions covering these DMs (start on p. 53).

Figure 4.11 Caustic stress corrosion cracking (SCC)
Figure 4.11 Caustic stress corrosion cracking (SCC)
Figure 4.12 Sulphuric acid corrosion (SCC)
Figure 4.12 Sulphuric acid corrosion (SCC)
Figure 4.13 Sulphate reducing bacteria (SRB): some key points
Figure 4.13 Sulphate reducing bacteria (SRB): some key points
Please go to API 653 Exam Chapter 4 to view the test

Click Here To Read Next API 653 Exam Chapter 5 – Inspection Practices and Frequency

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