API 653 Exam Chapter 3

API 653 Exam Chapter 3 – An Introduction to API RP 575

This chapter is about learning to become familiar with the layout and contents of API RP 575: Guidelines and Methods for Inspection of Existing Atmospheric and Low-Pressure Storage Tanks. API RP 575 is a well-established document (it has only recently changed from its first edition to the current 2005 2nd edition) with its roots in earlier documents published by the American refining industry. It is more a technical guide document rather than a code, as such, but it performs a useful function in supporting the content of API 653.

Note the following five points about API RP 575.

3.1 Scope

It has a very wide scope (evidenced by its title), which specifically includes all types of atmospheric and low pressure tanks. This wide scope is evident once you start to read the content; it refers to all types of storage tanks and the design features, inspection methods and damage mechanisms that go with them.

3.1.1 Damage mechanisms

API 575 introduces various corrosion and degradation mechanisms. As expected, these are heavily biased towards the refining industry with continued emphasis on petroleumrelated corrosion mechanisms and cracking. In general, although it provides a description and discussions on corrosion, API RP 575 acts only as an introduction to these corrosion mechanisms, leaving most of the detail to be covered in API RP 571.

3.1.2 Equipment

It is downstream oil industry orientated (not surprising as it is an API document). Its main reference is to the downstream oil sector, a term commonly used to refer to the part of the 25 industry involved in the refining, selling and distribution of products derived from crude oil (gas, petrol, diesel, etc.).

The types of equipment covered by the code can therefore include oil refineries, petrochemical plants, petroleum products distributors, retail outlets and natural gas distribution companies. These can involve thousands of products such as gasoline, diesel, jet fuel, heating oil, asphalt, lubricants, synthetic rubber, plastics, fertilizers, antifreeze, pesticides, pharmaceuticals, natural gas, propane and many others.

3.1.3 Related codes

API RP 575 refers to a lot of related codes that are not in the API 653 exam BOK. Examples are the out-of-print API 12 series and multiple documents in the API 300 series, API RP 307, 315, 322, etc., and others; see API 575 section 2 on page 1 of the document. These provide technical details on specific subjects and problems. Do not worry about these referenced documents; you need to know that they exist but you do not need to study them for the API 653 examination.

 3.1.4 Exam content

API 575 is all text and technical descriptions, accompanied by explanatory photographs of a fairly general nature. It contains few calculations. The only calculations it does contain of any significance are the ones on corrosion rates and inspection periods in section 6. These are important but will be covered later in API 653. In practice, most examination questions about API 575 in the API 653 certification exam are closed book. The downside to this is that API 575 contains several thousand separate technical facts, giving a large scope for the choice of exam questions.

All this means that you need to develop a working familiarity with the technical content of API 575 treating it as essential background knowledge for the API 653 syllabus, rather than as a separate ‘standalone’ code in itself. We will look at some of the more important areas as we work through the document.

3.2 API 575 sections 1 and 2: scope and references

These sections are little more than general information on where the document sits in relation to other API publications. Note however the reference to the older (superseded) API 12A and 12C specifications. There are obviously storage tanks still around that were made to these codes, rather than API 650 and 620: the codes that replaced them.

3.3 API 575 section 3: definitions

API 575 section 3 bears a startling resemblance to the equivalent section in API 653. This is a feature of API codes in general; there is repetition between them, although hopefully no (or, at least, not much) actual contradiction. Most of the definitions in this section (alteration/repair/ reconstruction, etc.) are exactly the same as given in API 653 and therefore do not need further study. There are, however, a couple of new ones.

Definition 3.3: atmospheric pressure

This used to feature in API 653 but has recently been taken out of several parts of the code. The important point is how it defines an atmospheric tank as one that is actually designed to withstand an internal pressure of up to 2.5 psi (18 kPa) gauge pressure (i.e. above atmospheric).

Definition 3.8: examiner

This is a concept that features in API 570 and 510 for pipes and vessels but is mysteriously absent from API 653, appearing, instead, here in API 575. Do not confuse this as anything to do with the examiner who oversees the API certification exams. This is the API terminology for the NDT technician who provides the NDT results for evaluation by the API-qualified storage tank inspector. API recognizes the NDT technician as a separate entity from the API-authorized storage tank inspector.

API codes (in fact most American-based codes) refer to NDT (the European term) as NDE (non-destructive examination), so expect to see this used throughout the API 653 examination. Other countries’ codes are not actually prohibited, but are not mentioned either. Welcome to America.

API codes only really recognize US NDE qualifications, hence the reference to SNT-TC-1A, etc., qualifications for NDE ‘examiners’ (that term again).

Definition 3.13: reconstruction

The term reconstruction means dismantling a tank and then reassembling it on a new site. Visualize a large tank on the back of a 48-wheel trailer holding you up on the motorway and you get the picture. Do not confuse this with tank repair (definition 3.15), which does not involve moving the tank to a new site.

All the other definitions are the same (more or less) as in API 653 and are nothing to get excited about, unless you have a particularly low excitement threshold.

3.4 API 575 section 4: types of storage tanks

Although it looks fairly complicated, API 575 section 4 only covers two generic types of storage tank: atmospheric tanks and low pressure tanks. The definitions are simple:

Atmospheric tanks (API 650) are designed to a gauge pressure of ≤ 2 1/2 psi.

Low pressure tanks (API 620) are designed to a gauge pressure of 2 1/2 -15 psi.

Most of the API examination questions will be about the atmospheric type of tanks since API 620 is (strictly) not in the BOK.

Note in API 575 section (4.2.2) the definition of vapour pressure. This is defined as the pressure existing on the surface of a confined liquid caused by the vapours given off by the liquid. It is the value of this vapour pressure that determines whether a fluid can be stored in an atmospheric tank or requires a (low) pressurized tank.

Section 4.2.3: types of atmospheric storage tank roofs

Atmospheric tank designs are differentiated mainly by their type of roof. There are several different designs of shell (flanged, ribbed, spiral-wound, etc.) but they are not covered in the scope of API 575. The two main generic types are fixed roof and floating roof. Floating roofs are used when there is a need to avoid the accumulation of flammable/explosive vapours in the space above the fluid.

Look at section 4.2.3 and highlight the main points of the descriptions of the following roof types:

  • Fixed cone roof
  • (Fixed) umbrella roof
  • (Fixed) dome roof
  • Pan-type (floating) roof
  • Pontoon (floating) roof
  • Double-deck (floating) roof

Have a look also at the type that has a floating roof, but with an additional fixed roof above it (Fig. 13 of API 575). This is to isolate the floating roof from the weather and/or to ensure that particularly hazardous vapours are not released into the atmosphere. Two other (fairly rare) atmospheric types are the breather type and balloon type (Fig. 15)

Section 4 also shows various types of seals that are used on floating roof tanks (see the figures in section 4). These are fairly straightforward and do not give great scope for exam questions.

Section 4.3: low pressure storage tanks

Low pressure tanks do not form a major part of the API 653 syllabus and they are not covered in much detail in API 575. Note the few significant points about them, however, in section 4.3.2:

They are constructed to API 620, not API 650.

They are necessary when the vapour pressure of the stored fluid exceeds the 2 1/2 psi limit of API 650-type atmospheric tanks. Volatile products such as light crude, some petrol products and liquid oxygen/nitrogen fall into this category.

There are various designs, some of them quite elaborate domed shapes, but many are now made spherical, as this is the best shape for retaining pressure and they are not too difficult to make.

Have a look at the various figures in API 575, highlighting the design features of storage tanks.

Now try these questions on types of tanks and their construction features.

3.5 API RP 575: practice questions


Q1. API 575: construction standards
Which of the following old specifications covered riveted atmospheric storage tanks?



Q2. API 575: use of atmospheric tanks
Which of the following fluids would not be suitable for being held in an atmospheric tank?



Q3. API 575: atmospheric storage tank roofs
What type of fixed roof does not need supporting rafters/columns, etc.?



Q4. API 575: atmospheric storage tank roofs
Which of these is not a floating roof design?



Q5. API 575: atmospheric storage tank roofs
Why do some floating-roof tanks have an additional dome roof on the top?



Q6. API 575: atmospheric storage tank roofs
What kind of floating-roof seal may be filled with foam?



Q7. API 575: atmospheric storage tank roofs
What is the purpose of using a breather-type roof?



Q8. API 575: low-pressure storage tanks
Which code covers the venting of low-pressure tanks?



Q9. API 575: storage tank design pressure
What is the maximum design gauge pressure of an API 650 tank?



Q10. API 575: storage tank design pressure
What is the maximum design gauge pressure of an API 620 tank?


Click Here To Read Next API 653 Exam Chapter 4 -Reasons for Inspection: DamageMechanisms

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