API 510 Chapter 6 API 572 Inspection of Pressure Vessels
6.1 API 572 introduction
This chapter is about learning to become familiar with the layout and contents of API 572: Inspection of Pressure Vessels (Towers, Drums, Reactors, Heat Exchangers and Condensers). API 572 is a well-established document (it is still on its 2001 edition) with its roots in earlier documents published by the American Refining Industry (IRE). It is more a technical guide document rather than a code, as such, but it does perform a useful function in supporting the content of API 510.
Note the following five points about API 572:
Point 1. It has a very wide scope as evidenced by its title, which specifically mentions the types of vessels that it covers: Towers, Drums, Reactors, Heat Exchangers and Condensers. This wide scope is evident once you start to read the content; it refers to all these types of equipment and the materials, design features and corrosion mechanisms that go with them.
Point 2. API 572 introduces various corrosion and degradation mechanisms. As you would expect, these are heavily biased towards the refining industry, with continued emphasis on sulphur/H2S-related corrosion mechanisms and cracking. In general, although it provides description and discussions on corrosion, API 572 acts only as an introduction to these corrosion mechanisms, leaving most of the detail to be covered in API 571.
Point 3. It is downstream oil industry orientated (not surprising as it is an API document). Its main reference is to the downstream oil sector. Downstream is a term commonly used to refer to the part of the industry involved in the 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 and propane, etc.
Note that the upstream oil sector (i.e. exploration and production (E&P) equipment) is not overtly covered in API 572. E&P vessels are specifically covered in API 510 section 9 but are excluded from the API 510 exam syllabus.
Point 4. This refers to quite a few related codes that are not in the API 510 exam syllabus (API 660/661 for heat exchangers, API RP 938/939/941, etc., and others; see API 572 section 2 on page 1 of the code). These provide technical details on specific subjects and problems but don’t worry about them. You need to know that they exist but you do not need to study them for the API 510 examination.
And finally, the most important point. API 572 is all text and technical descriptions, accompanied by explanatory photographs of a fairly general nature. It contains no calculations. This means that most examination questions about API 572 in the API 510 certification exam will inevitably be closed book. The downside to this is that API 572 contains several thousands of 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 572, treating it as essential background knowledge for the API 510 syllabus, rather than as a separate ‘stand-alone’ code in itself. We will look at some of the more important areas as we work through the code.
6.2 API 572 section 4: types of pressure vessels
API 572 section 4 is little more than four pages of general engineering knowledge. It provides basic information about types of pressure vessels and their methods of construction. Note the points below.
Section 4.1: types of pressure vessels A pressure vessel is defined as a container designed to withstand internal or external pressure and is designed to ASME VIII or other recognized code (section 4.1). Note the ‘cut-off point’ at 15 psi gauge pressure. This fits in with the 15 psi minimum pressure limit we saw previously in API 510 appendix A.
A vessel is most commonly a cylinder with heads of various shapes, such as:
We will look at the calculations associated with some of these shapes later in this book.
Cylindrical vessels can be both vertical and horizontal and may be supported by:
- Columns (legs)
- Cylindrical skirts
- Plate lugs attached to the shell
Spherical vessels may be similarly supported by:
- Columns (legs)
- A skirt
- Resting on the ground (either partially or completely)
Section 4.2: methods of construction Most vessels are of fully welded construction. There may be some old riveted vessels remaining but they are not very common.
The cylindrical shell courses (or rings) are made by rolling plate material and then welding the longitudinal joint. The courses are then assembled by circumferentially welding them together to give the required length of vessel. Hot forging of cylinders can be used, as this produces a seamless ring. Multilayer methods in which a cylinder is made up of a number of concentric rings can be used for heavy-wall vessels and items subject to high pressure. This is a very expensive method of fabrication.
Heads are made by forging or pressing, either hot or cold, from a single piece of material or built up of separate ‘petal’ plates.
6.3 API 572 Section 4.3: materials of construction
Section 4.3 summarizes the types of materials commonly used for pressure vessels. Treat this as general information only; materials are described in much more detail in ASME VIII and API 577 covered in other chapters of this book. The material categories are:
1. Carbon and low alloy steels
2. Stainless steels
- Ferritic (13% Cr)
- Austentic (18% Cr 8% Ni)
- Duplex (25% Cr 5% Ni)
3. Non ferrous
- Nickel alloys
4. Lined vessels (a low-cost carbon steel base material with corrosion-resistant lining)
- Roll bonded
- Explosion bonded
- Welded sheets (API 572 Fig. 4)
- Weld overlay
5. Non-metallic liners (used for corrosion resistance or insulation) .
Section 4.4: internal equipment
Some vessels have no internal parts while others can have the following:
- Distribution trays
- Mesh grids
- Packed beds
- Internal support beams
- Spray nozzles
Section 4.5: uses of pressure vessels
- These are the uses indicated in API 572 section 4.5:
- To contain the process stream
- Reactors (thermal or catalytic)
- Fractionators (to separate gases or chemicals)
- Surge drums
- Chemical treatment vessels
- Separator vessels
This is well short of being an exhaustive list, so treat it as ‘general knowledge’ information only.
6.4 API 572 sections 5, 6 and 7
Taken together, these comprise less than one full page of text. Read through them and note the following:
- The cross-references to the vessel construction code ASME VIII divisions 1 and 2 (make sure you understand what each division covers, although the API 510 syllabus is concerned with division 1 vessels only).
- The cross-references to TEMA (Tubular Exchangers Manufacturers Association) and API 660/661, the construction codes used for heat exchangers and condensers.
- The references to OSHA (Occupational Safety and Health Administration) and NB-23 (The National Board vessel inspection code, in some states of the USA used as an alternative to API 510). You don’t need to study these documents, just recognize that they exist.
Note the sentence hidden away in API 572 section 7.3 saying that periods of repair/replacement (and inspection) are based on corrosion rates and remaining corrosion allowances. This statement summarizes the entire philosophy of the API inspection codes and appears in various guises in all the codes included in the API 510 exam syllabus.
6.5 API 572 section 8: corrosion mechanisms
This is one of the most significant sections of API 572 but in reality there is very little in API 572 section 8 that is not covered in as much, or more, detail in API 571. Section 8 provides a few more photographs and adds explanations of a few additional degradation mechanisms (DMs) but there is little that is fundamentally new.
Three new DMs that are introduced are given in section 8.3.3: dealloying. They are:
- Dezincification (affects brass, which is a Cu/Zn alloy)
- Dealuminization (affects aluminium brasses or bronzes)
- Denickelification (affects cupronickel heat exchanger tubes or Monel metals)
These affect copper alloys and are all related to the leaching out of alloy constituents (the alloys are obvious as they appear in the DM title).
API 572 section 8.3.7 introduces another new DM: hydriding of titanium alloys. This is a type of embrittlement caused by absorption of hydrogen. Embrittlement is an important theme of API 571/572 and is evidence of the emphasis placed on refinery-type DMs.
Apart from these, all the other DMs in API 572 appear in API 571.They also appear in some other API codes (e.g. API 570: Inspection of Pipework).
The final ‘new’ part of Section 8 is 8.5: faulty fabrication. This is an unusual subject to cover in a chapter entitled ‘Corrosion Mechanisms’, but it only covers half a page and contains some useful points on vessel problems that have their root in the manufacturing stages. The causes are divided into (see section 8.5 on page 16 of API 572):
- Poor welding
- Incorrect heat treatment
- Wrong dimensions
- Incorrect installation
- Incorrect fit (assembly)
- Incorrect materials
Note the last one, incorrect materials (API 572 section 8.5.7). This short section mentions the advantages of positive material identification (PMI), carried out using a ‘Metascope’ or X-ray hand-held analyser. There is actually a dedicated API code for PMI techniques (API 578) but this is not part of the API 510 syllabus. It is, incidentally, part of the API 570 in-service inspection of pipework syllabus, presumably because someone has decided that the use of incorrect materials is more common in pipework components than in vessels
6.6 API 572 section 9: frequency and time of inspection
This is a short section (less than two pages). It provides fairly general explanations of the principles of inspection frequency and reintroduces (once again) the half-life concept of API 510. Note the following two key points hidden away in the text:
Section 9.2 contains the general statement of principle that ‘on-stream inspections can be used to detect defects and measure wall thickness’ (it is in 9.2 (d)). This is reinforcement of the general principle of API 510 that internal ‘shutdown’ examinations of vessels are not absolutely essential and may be replaced, where applicable, with a good-quality on-stream inspection using equipment able to detect defects and measure the wall thickness.
Section 9.4: alternative rules for exploration and production (E&P) vessels is little more than an acknowledgement that E&P vessels may need a more risk-based approach. The text is much the same as that in API 510, and is not in the exam syllabus.
6.7 API 572 section 10: inspection methods and limitations
Here are some key points about API 572 section 10.
Section 10.2: safety precautions and preparatory work While not particularly technically orientated, this short subsection is a fertile source of closed-book exam questions. Safety questions are always popular in examination papers so it is worth looking at this section specifically in terms of identifying content that could form the subject of an exam question. Note how continued emphasis is placed on safety aspects such as vessel isolations, draining, purging and gas testing.
Section 10.3: external inspections This is a long section containing a lot of good-quality technical information on the external inspection of vessels. It contains 13 subsections that work through the physical features of vessels, providing guidance on what to inspect. The section complements section 6 of API 510, but goes into much more technical detail.
Remember that the emphasis of section 10.3 is on external inspections; don’t confuse this with the more detailed on stream inspection that, as a principle of API 510, can be used, where suitable, to replace an internal examination. In practice, this will involve advanced NDT techniques (corrosion mapping, eddy current, profile radiography, etc.) or similar. Strictly, this is not what section 10.3 is about; it restricts itself to more straightforward visual inspections.
Section 10.4: internal inspections Section 10.4 goes into much more detail than previous sections of API 572. Spread over 7 to 8 pages, it provides a comprehensive technical commentary on the techniques for internal inspection of vessels. Once again, it is all qualitative information (there are no calculations) restricting the content to mainly closed-book examination questions.
From an API 510 examination viewpoint, the difficulty with section 10.4 is its wide scope. It covers subjects relating to general pressure vessels but intersperses these with techniques and corrosion mechanisms relating to specific refining industry applications (fractionating towers containing trays, low chromium alloy hydroprocessing units and similar).
Before we look at some simple questions, note the overall structure of section 10.4. It addresses things in the following order:
This is the main content
- 10.4.1 General
- 10.4.2 Surface preparation
- 10.4.3 Preliminary visual inspection
Specific require types of lined vessels
- 10.4.4 Detailed inspection (a long section)
- 10.4.5 Inspection of metallic linings
- 10.4.6 Inspection of non-metallic ments for various linings
There is a logic (of sorts) in the way this is set out. It attempts to be a chronological checklist of the way that a vessel is inspected. Don’t forget the overall context of API 572 however; it is a technical support document for API 510 and so does not have to be absolutely complete in itself.
Overall, there is a lot of technical wisdom contained in API 572. The difficulty from an examination viewpoint is that it contains thousands of technical facts (and many opinions also) that can, theoretically, be chosen for exam questions. On the positive side, many API 572-related questions can be answered from general engineering inspection experience.
You can improve your chances, however, by working through the code highlighting key points that may be you would not have anticipated from your experience.
Now try these familiarization questions.