API 510- Chapter 2 An Introduction to API 510 (Sections 1–4)

API 510 Chapter 2 An Introduction to API 510 (Sections 1–4)

2.1 Introduction

This chapter is about learning to become familiar with the layout and contents of API 510. It forms a vital preliminary stage that will ultimately help you understand not only the content of API 510 but also its cross-references to the other relevant API and ASME codes.

API 510 is divided into nine sections (sections 1 to 9), five appendices (appendices A to E), one figure and two tables. Even when taken together, these are not sufficient to specify fully a methodology for the inspection, repair and re-rating of pressure vessels. To accomplish this, further information and guidance has to be drawn from other codes.

So that we can start to build up your familiarity with API 510, we are going to look at some of the definitions that form its basis. We can start to identify these by looking at the API 510 contents/index page. This is laid out broadly as shown in Fig. 2.1

2.2 Section 1: scope

This is a very short (one-page) part of the code. The main point is in section 1.1.1, which states that all refining and chemical process vessels are included in the scope of API 510 except those vessels that are specifically excluded from the coverage of API 510. Note that this list (look at section 1.2.2) links together with a longer list in appendix A (look near the back of the document). Essentially, vessels that are excluded from the coverage of API 510 are: 

  • Mobile plant .
  • Anything designed to other parts of ASME .
  • Fired heaters

An Introduction to API 510 | Figure 2.1 API 510 contents/index


  • 1.1 General application
  • 1.2 Specific applications
  • 1.3 Recognized technical concepts




  • 4.1 General
  • 4.2 Owner-User organization responsibilities


  • 5.1 inspection plans
  • 5.2 RBI
  • 5.3 Preparation for inspection
  • 5.4 Inspection for damage mechanisms
  • 5.5 General inspection and surveillance
  • 5.6 Condition monitoring locations
  • 5.7 Condition monitoring methods
  • 5.8 Pressure testing
  • 5.9 Material verification and traceability
  • 5.10 Inspection of in-service welds and pints
  • 5.11 Inspection of flanged joints


  • 6,1 General
  • 6.2 Inspection during installation and service changes
  • 6.3 RBI
  • 6.4 External inspection
  • 6.5 Internal and on-stream inspection
  • 6.6 PRVs


  • 7.1 Corrosion rate determination
  • 7.2 Remaining life calculations
  • 7.3 MAWP
  • 7.4 FFS analysis of corroded regions
  • 7.5 API RP 579 FFS evaluations
  • 7.6 Required thickness determination
  • 7.7 Evaluation of equipment with minimal documentation
  • 7.8 Reports and records


  • 8.1 Repairs and alterations
  • 8 2 Rerating


  • Not in the API 510 exam syllabus



Machinery, i.e. pumps, compressors, etc.

Pipes and fittings

There are also some specific exemptions on size. Read the list in appendix A and relate them to Figs 2.2 and 2.3 below.

Appendix A (b6) gives an overall pressure temperature

Figure 2.2 API 510 exemption: water under pressure
Figure 2.2 API 510 exemption: water under pressure
Figure 2.3 API 510 pressure–volume exemptions (appendix A (d))
Figure 2.3 API 510 pressure–volume exemptions (appendix A (d))

exemption for vessels that contain water (or water with air provided as a ‘cushion’ only, i.e. accumulators).

Appendix A (b7) covers hot water storage tanks.

Appendix A (b8) gives a more general exemption based on minimum pressures and diameters.

Finally: Appendix A (d) covers a further general exemption based on pressure and volume.

Remember, section 1.2.2 at the front of API 510 only gives you half the story about exemptions. You have to look at the detail given in API 510 appendix A to get a fuller picture.

2.3 Section 3: definitions

Section 3.2: alteration

An alteration is defined as a change that takes a pressure vessel or component outside of its documented design criteria envelope. What this really means is moving it outside the design parameters of its design code (ASME VIII).

Note also how adding some types of nozzle connections may not be classed as an alteration. It depends on the size and whether it has nozzle reinforcement (in practice, you would need to check this in ASME VIII).

Section 3.6: authorized inspection agency

This can be a bit confusing. The four definitions (a to d) shown in API 510 relate to the situation in the USA, where the authorized inspection agency has some kind of legal jurisdiction, although the situation varies between states. Note this term jurisdiction used throughout API codes and remember that it was written with the various states of the USA in mind.

The UK situation is completely different, as the Pressure Systems Safety Regulations (PSSRs) are the statutory requirement. The nearest match to the ‘authorized inspection agency’ in the UK is probably ‘The Competent Person’ (organization) as defined in the PSSRs. This can be an independent inspection body or the plant owner/user themselves

For API 510 exam purposes, assume that ‘The Competent person’ (organization) is taking the role of the authorized inspection agency mentioned in API 510 section 3.6.

Section 3.7: authorized pressure vessel inspector

This refers to the USA situation where, in many states, vessel inspectors have to be certified to API 510. There is no such legal requirement in the UK. Assume, however, that the authorized vessel inspector is someone who has passed the API 510 certification exam and can therefore perform competently the vessel inspection duties covered by API 510.

Section 3.9: condition monitoring locations (CMLs)

These are simply locations on a vessel where parameters such as wall thickness are measured. They used to be called thickness measurement locations (TMLs) but have now been renamed CMLs. CMLs pop up like spring flowers in a few places in API 510 and 572, with emphasis being placed on how many you need and where they should be.

Section 3.19: engineer

In previous editions of API 510, reference was made to the ‘pressure vessel engineer’ as someone to be consulted by the API inspector for detailed advice on vessel design. This person has now been renamed ‘The Engineer’. There’s progress for you.

Section 3.20: examiner

Don’t confuse this as anything to do with the examiner that 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 pressure vessel inspector. API recognizes the NDT technician as a separate entity from the API authorized pressure vessel 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 510 training programme and examination.

Section 3.37: MAWP

US pressure equipment codes mainly refer to MAWP (maximum allowable working pressure). It is, effectively, the maximum pressure that a component is designed for. European codes are more likely to call it design pressure. For the purpose of the API exam, they mean almost the same, so you can consider the terms interchangeable.

Note how API 510 section 3.37 defines two key things about MAWP:

  • It is the maximum gauge pressure permitted at the top of a vessel as it is installed (for a designated temperature). This means that at the bottom of a vessel the pressure will be slightly higher owing to the self-weight of the fluid (hydrostatic head). The difference is normally pretty small, but it makes for a good exam question.
  • MAWP is based on calculations using the minimum thickness, excluding the amount of the actual thickness designated as corrosion allowance.

A significant amount of the exam content (closed-book and open-book questions) involves either the calculation of MAWP for vessels with a given amount of corrosion or the calculation of the minimum allowable corroded thickness for a given MAWP.

Section 3.53: repair

This is a revised definition added in the latest edition of API 510. It is mainly concerned with making a corroded vessel suitable for a specified design condition. If an activity does not qualify as an alteration then, by default, it is classed as a repair.

Section 3.54: repair organizations

API 510 has specific ideas on who is allowed to carry out repairs to pressure vessels. Look how definition 3.54 specifies four possible types of organization, starting with an organization that holds an ASME ‘code stamp’ (certificate of authorization). This links in with the general philosophy of ASME VIII, requiring formal certification of companies who want to manufacture/repair ASME-stamped vessels.

Section 3.56: re-rating

The word re-rating appears frequently throughout API codes. Re-rating of the MAWP or MDMT (minimum design metal temperature) of pressure vessels is perfectly allowable under the requirements of API 510, as long as code compliance is maintained. In the USA, the API authorized
inspector is responsible for re-rating a pressure vessel, once happy with the results of thickness checks, change of process conditions, etc. In the European way of working, this is unlikely to be carried out by a single person (although, in theory, the API 510 qualification should qualify a vessel inspector to do it). Re-rating may be needed owing to any combination of four main reasons – we will look at this in detail in Chapter 5.

Section 3.62: transition temperature
API codes are showing increasing acceptance of the problem of brittle fracture of pressure equipment materials. The new API 510 9th edition introduces the well-established idea of transition temperature, the temperature at which a material changes from predominantly ductile to predominantly brittle. As a principle, it is not advisable to use a material at an MDMT below this transition temperature (although we will see that there are possible ‘get-outs’ in the ASME VIII part of the syllabus) .

2.4 Section 4: owner/user/inspection organizations

Figure 2.4 summarizes the situation as seen by API.
Sections 4.1–4.2: responsibilities of user/owners
These sections are quite wide-ranging in placing an eyewatering raft of organizational requirements on the user/ owner of a pressure vessel. This fits in well with the situation in other countries where the owner/user ends up being the predominant duty holder under the partially sighted eye of the law.

The idea is that the owner/user should have a maintained QA/inspection/repair management system covering . . . just about everything. There is nothing particularly new about the list of requirements of this (listed as section 4.2.1 a to s); they are much the same as would be included in an ISO 9000 audit or similar act of organizational theatre. They are also the same as those given in the API 570 Piping Inspection code. Note a couple of interesting ones, however.

Section 4.2.1(j): ensuring that all jurisdictional requirements for vessel inspection, repairs, alteration and re-rating are continuously met

Remember that the term jurisdiction relates to the legal requirements in different states of the USA. In the UK this would mean statutory regulations such as the PSSRs, HASAWA, COMAH, PUWER and suchlike.

Section 4.2.1(n): controls necessary so that only materials conforming to the applicable section of the ASME code are utilized for repairs and alterations

This is clear. It effectively says that only code-compliant material and procedures must be used for repairs and alterations if you want to comply with API 510. Note that (along with definition 3.3), it does not specify exclusively the ASME code; this is a significant change from previous API 510 editions which recognized only ASME as the ‘applicable code’. You can think of this as a way of trying to make API 510 more relevant to countries outside the US, but remember that API 510 does not actually say this. The exam paper will be about what is written in the code, not your view of how it fits into the inspection world in other countries.

Reminder: API 510 says that: only materials conforming to the applicable codes and specifications should be used for repairs and alterations.

Figure 2.4 The balance of power
Figure 2.4 The balance of power

Section 4.2.1(0): controls necessary so that only qualified nondestructive examination (NDE) personnel and procedures are utilized

This means that API 510 requires NDE technicians to be qualified, although it seems to stop short of actually excluding non-US NDE qualifications. Look at section 3.27 and see what you think.

Some plant owner/users who have not read API 510 (why should they, as they leave that to the inspector?) may need convincing that they are ultimately responsible for the long list of responsibilities in 4.2.1. However, they find out pretty quickly after a pressure-related incident.

Section 4.2.4: responsibilities of the API authorized pressure vessel inspector This section appears in many of the API codes. The overiding principle (see Fig. 2.5) is that the API-certified pressure vessel inspector is responsible to the owner/user for confirming that the requirements of API 510 have been met. You will see this

Figure 2.5 API inspector responsibilities
Figure 2.5 API inspector responsibilities

as a recurring theme throughout this book (and there will almost certainly be examination questions on it).

Section 4.2.4 places the requirements for candidates to have minimum qualifications and experience, before they are allowed to sit the API 510 exams (see appendix B where these requirements are listed).

Now, using your code, try to answer these familiarization questions.

2.5 API 510 sections 1–4 familiarization questions

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Q1. API 510 section 1.2.2 and appendix A: exclusions
Which of these vessels is excluded from the requirements of API



Q2. API 510 section 1.2.2 and appendix A: exclusions
Which of these vessels containing steam is excluded from the requirements of API 510?



Q3. API 510 section 2: references
Which API code (not in the API 510 syllabus) covers welding on equipment in service?



Q4. API 510 section 3.2: alterations
Which of these is likely to be classed as an alteration to a pressure vessel?



Q5. API 510 section 3.37: definitions of MAWP
In simple terms, MAWP means much the same as?



Q6. API 510 section 3.37: definitions of MAWP
At what position is MAWP calculated for in a vertical pressure vessel?



Q7. API 510 section 3.37: definitions of MAWP
A pitted vessel is measured at an average of 10 mm thick and has a ‘designed’ corrosion allowance of 1.6 mm. It is now installed in a corrosion-free environment. What thickness is used when calculating the MAWP?



Q8. API 510 appendix B3: inspector recertification
How often must an API 510-certified vessel inspector be ‘recertified’ by API?



Q9. API 510 section 4.4: responsibilities
During the repair of a pressure vessel, who is the API-certified vessel inspector responsible to?



Q10. API 510 section 4.2: API code compliance
During the repair of a pressure vessel, who is ultimately responsible for compliance with the requirements of API 510?


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