API 510 Chapter 3 Inspection Practices (Section 5)
3.1 Introduction to API 510 section 5: inspection practices
Section 5 of API 510 contains many of the important principles on which the syllabus (and examination) is based. It is not really a stand-alone chapter; it relies on additional information included in sections 6 and 7 to give the full picture of what API considers is involved in the inspection of pressure vessels. Figure 3.1 shows the situation. This section has changed emphasis significantly since the previous API 510 edition; its main emphasis is now the existence and use of an inspection plan (written scheme of examination) linked with the application of risk-based inspection (RBI) techniques to help decide inspection scope and frequency. It also includes information on pressure testing, to link in with the requirements of ASME VIII.
3.2 Inspection types and planning
Section 5.1: inspection plans Have a quick look through this. It is mainly commonsense about what should go in a vessel inspection plan. There is nothing in here that should be new to engineers who have worked with written schemes of examination (WSEs). It is, however, a good area for closed-book exam questions.
Section 5.2: risk-based inspection This is a heavily expanded section compared to previous editions of API 510. Mention RBI in the world of inspection and it seems you just can’t go wrong. Notice the two fundamental points:
Inspection scope and frequency can be decided by
considering the risk that individual pressure vessels represent. . Risk is determined by considering both probability of failure (POF) and consequences of failure (COF).
The content of this section 5.2 is taken from the document API RP 580: Risk-Based Inspection. This document is not in the API 510 syllabus (it forms a supplementary examination and certificate in itself), but you are expected to know the summary of it that has been transplanted into section 5.2. Notice the breakdown:
- POF assessment
- COF assessment
- RBI assessment frequency
Section 5.3: preparatory work
Section 5.3 is mainly about good health and safety practice and commonsense. There is nothing in here that should be new to engineers who have worked on industrial sites. It is, however, a good area for an occasional closed-book exam question.
Section 5.4: Modes of deterioration and failure
This section is an introduction (only) to the types of failure and damage mechanisms (DMs) that can affect pressure vessels. As with so many of the API code clauses, it is a mixture of general descriptions and a few specifics. Note the general DM categories that are given in the list:
- General/local metal loss
- Surface-connected (breaking) cracking
- Subsurface cracking
- Microfissuring/microvoid formation
- Metallurgical changes
- Dimensional changes
- Material properties change
Most of these are covered in much more detail in API 571:Deterioration Mechanisms (this is part of the API 510 syllabus and we will be looking at it later in this book).
Section 5.5: general types of inspection and surveillance
This fairly general section introduces the different types of inspection that are commonly used for pressure vessels. In reality, there is very little information in here; most technical details come later in section 6. One clear requirement, however, is the need to assess the condition of linings and
claddings, to guide the inspector’s decision as to whether they need to be removed to inspect underneath.
Section 5.5.4: external inspection
This introduces the general requirements for external visual examination of pressure vessels. Note how it gives various different areas that should be assessed, including those for buried vessels. These are largely commonsense.
Section 18.104.22.168: CUI inspection
API codes like to warn against CUI (corrosion under insulation) and there are normally questions on it in the API 510 exam. They have recently revised the ‘at-risk’ temperatures for CUI to:
- Low carbon/alloy steels: 10 °F to 350 °F
- Austenitic stainless steels: 140 °F to 400 °F
Note that these are for systems that operate at a constant temperature. By inference, all systems that operate intermittently may be at risk from CUI whatever their temperature range.
Section 22.214.171.124: insulation removal
For vessels with external insulation, provided the insulation and cladding is intact and appears to be in good condition, API’s view is that it is not necessary to remove the coating. It is, however, often good practice to remove a small section to assess the condition of the metal underneath.
Note the list of aspects to take into account when considering insulation removal:
- Visual condition and age of the insulation
- Evidence of fluid leakage
This section introduces the principle that shell thicknesses in areas of CUI susceptibility (corrosion of the external surface) may be checked from the inside of a vessel during an internal inspection.
3.3 Condition monitoring locations (CMLs)
API 510 makes a huge fuss about CMLs (until recently referred to as thickness measurement locations (TMLs)). The change was made to recognize the fact that, in many systems, wall thinning alone is not the dominant damage mechanism. Other service-specific mechanisms such as stress chloride corrosion cracking (SCC) and high-temperature hydrogen attack (HTHA) are likely to be equally or more important.
Section 5.6 contains a page or so of commentary on good practice for selecting CMLs. Most NDE techniques are mentioned as being suitable, as long as their application is carefully chosen. This is followed by section 5.7.1, a welldefined list of NDE techniques and the type of defect they are best at finding. This is an important list for exam questions; the content also appears in other parts of the syllabus such as API 577 and ASME V.
Section 5.7.2: thickness measurement methods Simple compression-probe ultrasonic testing (UT) is generally explained to be the most accurate method of obtaining thickness measurements. Profile radiography may be used as an alternative and in reality is often more useful. Note the requirements of section 126.96.36.199, which requires compensation for measurement inaccuracies when taking thickness measurements at temperatures above 65 °C (150 °F). This is covered in more detail in ASME V article 23.
3.4 Section 5.8: pressure testing
The requirement for doing a pressure test is often misunderstood, not least because of the fact that the mandatory requirement for it has been softened over the past 20 years or so. The situation in the current 9th edition of 510 is fairly clear, as follows:
- A pressure test is normally required after an alteration.
- The inspector decides if a pressure test is required after a repair.
- A pressure test is not normally required as part of a routine inspection.
API 510 gives no new requirements for test pressure; referring directly back to ASME VIII-I UG-98/99 requirements. Whereas in earlier editions (pre-1999 addendum) ASME VIII has used 1.5 MAWP as the standard multiplier for hydraulic test pressures, this was amended (1999 addendum and later) to the following:
Test pressure (hydraulic) = 1.3 MAWP ratio of material stress values .
Ratio of material= Allowable stress at test temperature
Stress Values Allowable stress at design temperature
Remember that this test pressure is measured at the highest point of the vessel. The allowable stress values are given in ASME II(d). Note that where a vessel is constructed of different materials that have different allowable stress values, the lowest ratio of stress values is used. You will see this used later in ASME VIII worked examples.
Section 5.8.6: test temperature and brittle fracture US codes are showing an increasing awareness of the need to avoid brittle fracture when pressure testing of vessels. API 510 therefore now introduces the concept of transition temperature. To minimize the risk of brittle fracture, the test temperature should be at least 30 °F (17 °C) above the minimum design temperature (MDMT). There is no need to go above 120 °F (48 °C) as, above this, the risk of brittle fracture is minimal. The temperature limitation is to avoid the safety risks that arise from brittle fracture of a vessel under pressure. Even when a hydrostatic (rather than pneumatic) test is performed, there is still sufficient stored energy to cause ‘missile damage’ if the material fails by brittle fracture. Note also the requirement for temperature equalization; if the test temperature exceeds 120 8F, the test should be delayed until the test medium reaches the same temperature as the vessel itself (i.e. the temperatures have equalized out).
The hydrostatic test procedure ASME VIII UG-99 (g) gives requirements for the test procedure itself. This is a fertile area for closed-book examination questions. An important safety point is the requirement to fit vents at all high points to remove any air pockets. This avoids turning a hydrostatic test into a pneumatic test, with its dangers of stored energy.
Another key safety point is that a visual inspection of the vessel under pressure is not carried out at the test pressure. It must be reduced back to MAWP (actually defined in UG-99 (g) as test pressure/1.3) before approaching the vessel for inspection. If it was a high-temperature test (> 120 °F, 48 °C), the temperature must also be allowed to reduce to this, before approaching the vessel.
Once the pressure has been reduced, all joints and connections should be visually inspected. Note how this may be waived provided:
- A leak test is carried out using a suitable gas.
- Agreement is reached between the inspector and manufacturer to carry out some other form of leak test.
- Welds that cannot be visually inspected on completion of the vessel were given visual examination prior to assembly (may be the case with some kinds of internal welds).
- The contents of the vessel are not lethal.
In practice, use of these ‘inspection waiver points’ is not very common. Most vessels are tested and visually inspected fully, as per the first sentences of UG-99 (g).
A footnote to UG-99 (h) suggests that a PRV set to 133 % test pressure is used to limit any unintentional overpressure due to temperature increases. Surprisingly, no PRV set to test pressure is required by the ASME code. You just have to be careful not to exceed the calculated test pressure during the test.
Now try these familiarization questions.
3.5 API 510 section 5 familiarization questions
Free Study Guide of API 510 Exam