API 653 Exam Chapter 5

API 653 Exam Chapter 5 – Inspection Practices and Frequency

This chapter is about code recommendations on how to inspect a tank and how often you should do it. They are of course separate subjects and the two codes that address them, API 653 and API 575, approach them from slightly different viewpoints. API 653 divides inspections into different types and addresses inspection interval (or frequency) in its section 6. It also introduces the way in which a tank inspection interval can be based around a risk-based inspection (RBI) assessment. It seems to like this idea. The baton then passes across to API 575 section 7, which confirms the method of corrosion rate calculation and then launches into a detailed methodology of how to inspect the various parts of a tank inside and outside, when it is in or out of service. This API RP 575 section is a major source of exam questions (open and closed book) and, looking objectively, gives excellent experienced-based guidance on how to inspect tanks.

We will look at these relevant sections of API 653 and API RP 575 in turn.

5.1 API 653 section 6: inspection

First the good news about section 6. A lot of information about what to inspect during a tank inspection has been separated out into API 653 Annex C – a hugely detailed inspection checklist of nearly 20 pages. Its content is excellent but the good news is that it is next to impossible to transform such checklist data into multiple choice exam questions. This goes for all checklists – you just cannot get many API-style exam questions out of them, no matter how hard you try.

The remaining content of API 653 section 6 is logically structured, if a little unbalanced. Figure 5.1 shows its breakdown. Note the way in which it divides inspection into four discrete types, routine external, full external, ultrasonic testing (UT) thickness and interval. As you can

Figure 5.1 The contents of API 653 section 6: inspection
Figure 5.1 The contents of API 653 section 6: inspection

see from Fig. 5.1, the main technical content of section 6 is about putting limits on the intervals for internal inspections. This has changed significantly since the previous edition of API 653. The main points are now:

  • There are different maximum intervals for the first internal inspection after putting into service, and subsequent ones.
  • Both sets are, loosely, RBI-based.
  • First inspection intervals range from 12 to 25 years ( This is shorter than those for subsequent intervals which extend from 20 to 30 years (

Figure 5.2 shows the idea. Note the story behind the allowable spread of these intervals. In both categories the existence of a release prevention barrier (RPB), i.e. a bund or some similar method to contain spills, allows a significantly longer interval to be used. There is nothing particularly earth-shattering about this – it simply reflects the fact that an

Figure 5.2 The API 653 tank inspection types
Figure 5.2 The API 653 tank inspection types

RPB reduces the consequences of failure (COF) if the tank leaks. Note two other key points: . Viscous substances such as tar are classed as low risk products and are exempted from both the ‘maximum’ interval tables (it says so in

Section explains how to adapt these maximum interval tables (newly introduced into API 653: 2009 edition remember) to tanks where different RBI-based intervals have been set before 2009. Figure 5.3 shows how this works. Read this in conjunction with section and you should not find it too difficult.

5.2 API 653 section 6: view of RBI

API 653 section 6 does not go into huge detail on RBI. It accepts and supports the activity, as long as it follows the good-sense guidelines mentioned in These are:

The RBI has been carried out to the guidance of API RP 580 with systematic attention given to both probability and consequences of failure (POF and COF).

It was performed by a knowledgeable group.

The assessment (and conclusions presumably) were approved by an API-authorized inspector and a storage tank engineer.

It has been reassessed after all failures or service changes, and at least every 10 years to make sure it is still relevant.

These points are fundamental to API code views on RBI, so watch out for them in exam questions – open or closed book.

5.2.1 Any there other possible exam questions on RBI?

Yes. The two new individual checklists on probability (called likelihood) factor ( and consequence ( are good, but difficult to turn into multichoice exam questions. More likely are questions that check your appreciation of which factors should be in which list, to test if you really

Figure 5.3 Dealing with historical inspection intervals
Figure 5.3 Dealing with historical inspection intervals

understand the fundamental difference between POF and COF. These are generally straightforward; just watch out for the occasional convoluted wording or questions phrased as a negative: e.g. which of the following factors does not have an effect on POF? Watch out for these awkward ‘not questions’ in all API ICP exams (you will not be disappointed).

5.2.2 Reports and recommendations (section 6.9)

Owners/users have an ongoing battle getting tank inspectors to write sufficiently detailed inspection reports. Many are short on detail, carefully indecisive in their conclusions and diplomatically vacant about what should be done next. API 653 section 6.9.2 (another newly added section) is an attempt to improve this. Look at what it requires:

Inspection reports must recommend repairs and/or future monitoring, and give reasons why (

They must give the maximum next inspection interval and show where it comes from (by corrosion rate calculation or reference to API 653 clauses).

However, note that the precise repair scope and timing is the responsibility of the owner/user. Figure 5.4 shows this in a visual way.

5.3 API 575 section 6: inspection frequency and scheduling

Do not expect anything new on inspection frequency in API RP 575; it just cross-refers to API 653, as you would expect. It does, however, add the requirement for the monthly routine inspection. Note also the suggestion made in API 575 (6.1) that external inspection be carried out after unusual events such as:

  • Obvious settlement
  • High winds, rain or lightning
  • Seismic movements

Of these, seismic events are perhaps the most important, particularly on floating roof tanks. Any ground movement at all can cause the tank shell to distort, particularly near the top, causing a floating roof to stick.

5.3.1 Corrosion rate

All API in-service inspection codes place great importance on determining a corrosion rate. This is covered in section 6.2 of

Figure 5.4 Inspection reports and recommendations
Figure 5.4 Inspection reports and recommendations

API RP 575. It is not repeated in API 653, but it is needed to be able to work to API 653, as it governs the life of a tank. Determining an accurate corrosion rate is not easy – tank components normally corrode at different rates and there can be big differences within a component. In tank floors, areas around heating coils, drain sumps and the shell-to-floor weld can have dramatically different (normally higher) corrosion rates than areas only 100–200 mm away. Shells, similarly, are likely to corrode more quickly around the product/air interface, or at the top if vapours are corrosive, or at the bottom if the product is such to encourage a corrosive water layer here.

With externally lagged tanks the situation is worse because CUI almost never happens uniformly. You can expect to find more seriously corroded areas around water traps such as wind-girders or, more randomly, wherever the cladding allows rain water in, ensuring that the lagging material remains wet.

Practicalities apart, API RP 575 (6.2) takes a fairly simplistic view of the corrosion rate, assuming it is real, predictable and conveniently uniform and linear, for any particular product service. Figure 5.5 shows the situation – the calculations are fairly simple. Remember that for exam questions these can be done in either USCS or SI units, but you would be well advised to use USCS (inches) units if you want to avoid conversion and rounding errors.

5.3.2 Changing corrosion rate

API exam questions like to check whether you appreciate that corrosion rates do not necessarily remain constant through time. This is particularly important for multiproduct storage tank farms, which may contain many different types of product during their life. The idea of a changing corrosion rate is shown in API RP 575 Fig. 32 with the long-winded title, Hypothetical Corrosion Rate Curve for the Top Course of a Storage Tank. There is nothing difficult about this figure; it just has its axes oriented in a rather bizarre way. To understand it, simply turn the page 90 degrees anticlockwise and it will all make sense – with the years on the horizontal axis and shell thickness on the vertical axis. Remember that the idea is all a little hypothetical, but you can expect real exam questions about it. Figure 5.6 shows a better interpretation of this idea.

Figure 5.5 Corrosion rate: a simple concept
Figure 5.5 Corrosion rate: a simple concept

5.4 API RP 575: inspection practices

Whereas API 653 is a little thin on actual tank inspection practice, API RP 575 is full of information. Packed into dense narrative passages (not the easiest to read) are hundreds of valid points and instructions on how to inspect a tank. There is some very good information here, built up by an army of unknown inspection warriors over many years. Firstly, look at the structure. Figure 5.7 shows how the

Figure 5.6 Changing corrosion rate
Figure 5.6 Changing corrosion rate

chapter breaks down into the three main inspection types: (7.2) external (tank in-service), (7.3) external (tank out-of service) and (7.4) internal (empty and out-of-service). To save repetition a lot of the information relevant to all three types is included in the first section 7.1. You can expect about 10% of the API exam questions to come from this section (some

Figure 5.7 Methods of inspection: what is in API RP 575 (section 7)
Figure 5.7 Methods of inspection: what is in API RP 575 (section 7)

picked out of the text word-for-word) so we will look at each of these sections in turn.

5.4.1 Preparation for inspection (7.1)

There are two aspects to this: safety aspects and tools for inspection. The safety aspects of storage tank entry are of course based on statutory codes and rules applicable to the USA. Requirements will be different in other countries although the principles will be much the same, being based

Figure 5.8 Tank inspection: safety precautions: API RP 575 (7.1)
Figure 5.8 Tank inspection: safety precautions: API RP 575 (7.1)

on experience and common sense. Figure 5.8 summarizes the major points that pop up as exam questions.

Inspection tools is a strange subject for exam questions, but there are a few common exam questions built around them. Their mention is also scattered about other subsections of section 7.1. Figure 5.9 picks out the main points. Note that there is a separate API individual certificate programme

Figure 5.9a Tank inspection tools
Figure 5.9a Tank inspection tools

(ICP) for tank entry supervisors (TES). This is entirely based on US legislation and so has limited applicability in other countries.

5.4.2 External inspection of an in-service tank (7.2)

Section 7.2 provides a fairly well-structured list of items to check during an external inspection of a tank that is still in use.

Figure 5.9b Tank inspection tools (continued)
Figure 5.9b Tank inspection tools (continued)

5.4.3 Ladders, platforms, walkway inspection (7.2.1– 7.2.2)

Figure 5.10 shows points to check on ladders, platforms and walkways. On tanks situated in dirty or corrosive environments (steelworks for example) these components present a bigger personal safety risk than the tank itself, so they are important inspection items. Do not expect a lot of exam questions on them though – the answers would be too obvious to make a good exam question.

5.4.4 Foundation inspection (7.2.3)

Note the two important points here:

  • Foundation settlement causes floor settlement – a subject
Figure 5.10 External inspections: ladders, platforms and walkways
Figure 5.10 External inspections: ladders, platforms and walkways

covered not in API RP 575 but in API 653 Annex B. We will look at this in detail later.

Check the mastic seal between the bottom of a tank and its foundation. If the joint is unsealed, water will get in and corrode the bottom.

5.4.5 Anchor bolts and earth connections (7.2.4– 7.2.5)

There are a few straightforward points here that occasionally appear as rather uninspiring exam questions:

  • A simple hammer test on anchor bolts can show if they are loose or heavily corroded (if they break).
  • Tank earthing (‘grounding’) connections should be visually checked for corrosion where they enter the soil. Watch for the exam question that asks what the maximum earth resistance should be (the answer is 25 ohms).

5.4.6 Tank shell inspection (7.2.8)

The main content of API RP 575 section 7.2 is in here. A lot of effort goes into inspecting tanks shells as they are easily accessible, even though it is still often the tank bottom that governs the tank life. Both this and later sections 7.3 and 7.4 contain information about tank inspection, much of which is related to finding shell plate thickness using some types of UT measurement. Figure 5.11 shows the key points.

5.4.7 Shell UT thickness checks

The key point about shell UT thickness readings is that there must be sufficient accurate readings available to feed the evaluation calculation that we will look at later, in API 653 section 6. Five points per corroded (vertical) plane are required as an absolute minimum for any averaging calculation. For light or fairly regular corrosion this can often be achieved by individual UT (pulse-echo) thickness gauge readings. If the corrosion is more widespread, or serious, then full scanning is normally required, of individual shell plates or the entire shell. It all depends on the severity.

Figure 5.11 Tank shell inspection: general points from API RP 575 (7.2, 7.3, 7.6)
Figure 5.11 Tank shell inspection: general points from API RP 575 (7.2, 7.3, 7.6)

API RP 575 seems to favour full scanning (also called ‘corrosion mapping’). Scanning techniques themselves are not part of the API 653 BOK; you just need to know that it exists and what it does.

The recommended periodicity of shell UT thickness checks is an interesting one. API 653 sees UT thickness checking as being a totally separate activity from a tank external inspection, even though, in reality, it will often be carried out at the same time. It sets a maximum periodicity of:

Periodicity of shell UT thickness checking = half the remaining corrosion life (to a maximum of 15 years).

This is a popular examination question. Expect it to come up almost every time.

5.4.8 Tank bottom inspection (7.4)

Unless you have access to robotic equipment, most tank floor inspections are carried out during the internal inspection programme after the tank has been emptied and cleaned. The bottom is normally inspected last, for safety reasons, after the condition of the roof and shell have been confirmed. Tank bottom problems divide squarely into two: soil-side and product-side. In older tanks laid on a soil or rubble base, soil-side problems are often the worst. Soil-side galvanic and/ or crevice corrosion is notoriously unpredictable and localized, and often remains hidden until it is serious enough to cause leakage or serious structural problems.

The extent of floor inspection tends to be decided by the level of test equipment available. Simple pulse-echo UT meters can check for soil-side thinning in selected areas but is a hit-and-miss affair at best. Floor scanning equipment using ultrasonic or magnetic floor leakage (MFL) gives much more reliable coverage, and more detailed UT can be done on areas of concern that have been found.

From an API examination question perspective, there are a few technical points that are worth remembering:

Statistical analysis of scanning results can be done on data samples of 0.2–10 % of the bottom area. Its anyone’s guess where the 0.2% figure came from.

Statistical analysis is less reliable once significant corrosion (usually soil-side) has actually been detected. If this happens 100% scanning is the best.

Bottom corrosion is made worse by bottom settlement. API 653 Annex B covers settlement in detail. We will look at this later in chapter 6 of this book.

Figure 5.12 Tank floor inspections: API RP 575 (7.4.4)
Figure 5.12 Tank floor inspections: API RP 575 (7.4.4)

Figure 5.12 shows some other key points for floor inspection. They are all good exam question material.

5.4.9 Tank roof inspection (7.4.8)

The inspection of a tank roof can be split into those activities applicable to fixed and floating roof tanks. Before inspecting either, note the general safety points outlined in Fig. 5.13.

Figure 5.13 Tank roof inspection: API RP 575 (7.2.5)
Figure 5.13 Tank roof inspection: API RP 575 (7.2.5)

5.4.10 Fixed roof inspection

Figure 5.13 shows some areas of inspection on a fixed roof tank. Most are to do with thickness checking, normally just by a simple pulse-echo UT meter, pit gauge or calipers. The following points regularly appear as exam questions:

  • Structural members, rafters and columns can corrode from both sides, resulting in a high (double) corrosion rate.
  • Corrosion is normally more serious in dents or depressions where rainwater has been standing.
  • Evaluation of roof corrosion is not covered in API RP

575. It is covered by API 653 section 6 so we will look at it later in chapter 6 of this book.

5.4.11 Floating roof inspection

Floating roofs have more problems than fixed roofs. They have a more involved structure (so corrode more) and are complicated by seals and articulated drain arrangements. These suffer from mechanical problems (sticking, distortion or broken parts) as well as corrosion. Figure 5.13 shows some points; see also the detailed checklists in API 653 Annex C.

5.4.12 Inspection of auxiliary equipment (7.10)

Auxiliary equipment fitted to tanks (connected pipework, valves, drains, flame arrestors, level gauges and similar) are responsible for their fair share of problems and inspection findings. They do not fit particularly well into the API exam questions about inspection practices however. Surprisingly, they appear more often as questions linked to the design aspects of API 650: the tank construction code. The questions are not particularly difficult and relate mainly to a few specific drawings of design features.

Now try these practice questions.

5.5 API 653: inspection intervals: practice questions


Q1. API 653: tank inspection intervals
Fundamentally, under the API methodology, how are the inspection intervals (both internal and external) for storage tanks decided?



Q2. API 653: tank external inspection intervals
What is the maximum period that API 653 specifies for an external inspection by an API-authorized tank inspector?



Q3. API 653: shell UT inspection intervals
What is the maximum period that API 653 specifies for UT thickness checking of the shell if the corrosion rate is not known?



Q4. API 653: shell UT inspection intervals
What is the maximum period that API 653 specifies for UT thickness checking of the shell when the corrosion rate is known and has been validated within the past 5 years?



Q5. API 653: tank internal inspection intervals
Which part of the tank normally ‘controls’ the internal inspection interval?



Q6. API 653: initial internal inspection intervals
What is the maximum period that API 653 specifies for the first internal inspection interval if a formally documented RBI evaluation has not been carried out?



Q7. API 653: following inspection intervals
What is the absolute maximum internal inspection interval for a tank containing oil (following the initial one) under any scenario?



Q8. API 653: interval exemptions
Tanks containing which of these products are exempt from API 653 maximum internal inspection intervals?



Q9. API 653: repair recommendations
Whose job is it to provide repair recommendations for tanks?



Q10. API 653: NDE qualifications
Examiners doing NDE of tanks must be qualified according to the requirements of:


Click Here To Read Next API 653 Exam Chapter 6 – Evaluation of Corroded Tanks

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