API 653 Exam Chapter 7- API 650: Tank Design
7.1 Reminder: the API 653 body of knowledge (BOK)
Strictly, the API 653 BOK and examination are about the in service life of storage tanks, i.e. that period after construction when relevant approvals and certification requirements have been completed. This is the same philosophy as the other two major ICPs: API 510 (vessels) and API 570 (pipework). The practical situation is a bit different. Whereas API 653 provides good coverage of inspection, repair, alteration and reconstruction (its title) it only contains a limited amount of technical data on shell and nozzle design. This is often needed for alteration and (particularly) reconstruction; hence there is a need for the BOK to fall back on the new tank construction code: API 650.
7.1.1 How much of API 650 is in the API 653 BOK?
This depends on how you look at it. API 650 is a good code, full of technical details. Its 400+ pages divide roughly 50:50 between the body of the code (sections 1 to 10) and a long procession of appendices (A to X) (see Fig. 7.1). It would be next to impossible to learn all of this volume of material. The API published BOK is not particularly useful in helping you decide which bits of API 650 you need to learn. The necessary information is there, but it does not exactly jump out of the page at you. To confuse the matter, part of it lists what is not in the BOK and then lists exclusions from these exclusions. Fortunately, the situation is simpler than it appears. Look at these guidance points:
Almost all of the exam questions sourced from API 650 appear in the open-book section of the exam. They have to, if you think about it, or candidates would need to learn all
of API 650, 400+ pages, most of which is fairly peripheral to the API 653 BOK.
Exam questions tend to be chosen from a few selected areas of API 650. They are surprisingly predictable – most are about:
- Toughness requirements of materials
- Allowable stresses in shell material and the resulting shell thickness
- Arrangement and dimensions of shell nozzles/connections (particularly welds), roof fittings or foundations
- A few specific bits in the appendices about elevated temperature tanks, stainless steel tanks or floating roofs.
Figures 7.2 and 7.3 below summarize the situation. Taken together these two figures show almost all of the exam question subjects that are sourced from API 650. Remember:
- They are likely to be open-book questions.
- The questions will either be about reconstructed tanks (API 653 section 10) or specifically mention new tanks.
- There will be little technical interpretation involved – it is simply a case of picking bits of information from arrangement drawings or tables.
To help you anticipate the questions that can appear in the exam, we will look at the technical aspects in turn. Remember, again, that there is no great technical depth to the exam question content. Getting the correct answer is simply a matter of knowing where to look in the main sections or few relevant appendices of API 650.
7.2 API 650: material allowable stresses
We first saw this idea back in Chapter 6 of this book. All feasible shell materials are allocated an allowable stress value S, which is then used in a simple equation to calculate the minimum required thickness. There are two possible options
to choose from for each material – a slightly higher S value being used if the tank is to be hydrotested.
For calculating tmin for corroded existing tanks the table of S values in API 653 Table 4-1 is used. This makes sense as the tank is already built and no new shell components are being added. For new or reconstructed tanks, however (reconstruction is treated the same as a new build), Table 5-2 (a or b
of API 650 is used. This contains much the same range of material in a similar format to Table 4-1 of API 653 but the values are different.
7.2.1 Why do API 653 and 650 have different allowable stress (S) values?
Simply because they are different scenarios. API 650 is for new build, when the objective is to specify an S value that allows for a certain ‘factor of safety’. API 653 then takes over for the assessment of corroded tanks where the objective is to keep the tank in operation. It does this by using up some of the margin that was previously provided by the API 650 S value. Hence the S values in API 653 Table 4-1 are higher, by about 10–20 %, depending on the material.
Here it is again, so you do not get confused:
API 650 Table 5-2 contains S values to be used for new build or reconstructed tank calculations. The figures come from the percentages of yield (Y) and tensile (T) values specified by API 650 (220.127.116.11), i.e. 66 % Y or 40% T, whichever is less. They are the same for all shell courses.
API 653 Table 4-1 contains S values to be used for the assessment of existing corroded tanks. The figures come from the higher percentages of yield (Y) and tensile (T) values specified by API 653 (18.104.22.168). They are the smaller of 80 % Y or 42.9 % T for the lower two shell courses and the smaller of 88 % Y or 47.2% T for all the other courses.
Look back at Fig. 6.5 of this book and you can see this comparison expressed in a table.
Figure 7.4 shows how both API 650 and 653 present this S data. Most API 653 exams contain questions that require you to pick out S values from one of these tables, so just use this guideline:
If the question refers to the evaluation of existing corroded tanks, use the S values from API 653 (Table 4-1) and note which shell courses are referred to but .
If the question mentions reconstructed or newbuild tanks, use the S values from API 650 (Table 5-2) and always .
Watch for whether the question mentions a hydrotest or not, as that will affect the values to use.
7.3 API 650: material toughness requirements
As well as strength, API codes are always concerned with material toughness. It is toughness (not ductility), that provides the resistance to brittle fracture either during hydrotest or in cold conditions. API 653 section 5 has its own crude assessment of the risk of brittle fracture during hydrotest but API 650 covers it in more detail for use at the newbuild or reconstruction stage.
In common with most other API (and ASME) codes, API 650 uses a straightforward routine for assessing toughness. It is based on the premise that some combinations of material, thickness and minimum design temperature do not require impact testing because previous experience dictates that there will be no problem with brittle fracture; i.e. toughness is adequate. Conversely, if the material/thickness/temperature combination does not meet the necessary threshold levels, then impact testing is required to test whether the material has sufficient toughness or not. Impact (Charpy) specimens are tested in groups of three specimens and the results compared with minimum single and average reading requirements given in a table in the code.
The API 650 method (see Fig 7.5 below) is set out in section 4 of the code and is as follows:
Step 1. For a given material, identify its group number (I to VI) from API 650 Table 4-4(a or b). Be careful to note if the material is ‘killed’ and/or normalized as this can affect its group. Watch out for information given in the notes at the bottom of the table also.
Step 2. For a given location in the USA determine the lowest one day mean (average) temperature (LODMAT) from the map in API 650 Fig. 4-2.
Step 3. Go to the graphs in API 650 Fig. 4-1 and plot the LODMAT temperature against the thickness of the material in question, then compare it with the line on the graph for the current material group. .
Step 4. If the plot point on the API 650 Fig. 4-1 graph falls above the line for the material group, impact tests are not needed as brittle fracture is not considered likely.
Step 5. If the plot point on the Fig. 4-1 graph falls below the line for the material group then the material must be impact tested to see whether it has adequate toughness or not. The acceptance values are shown in Table 4-5 of API 650 and range from 20 to 68 J (15–50 ft-lb) depending on material group, thickness and the orientation of the
specimen taken from the parent metal plate. All test results are calculated from the average of three test specimens
7.4 Tank component arrangement and sizes
The second API 650 topic that is included in the API 653 BOK covers acceptable material/weld sizes and arrangements for some of the major tank components. As a construction code, API 650 contains a lot of detailed technical requirements about how to attach nozzles and other fittings to the shell, roof and bottom. There are several thousand of these requirements, compressed into a family of full-page arrangement drawings, many further qualified by referenced tables of data. These tables mainly cover material thickness, pipe schedules, minimum weld sizes and such like. Fortunately, not all are in the API 653 BOK. The main ones that are included are:
- Shell manholes API 650 Fig. 5-7 and Tables 5-3 to 5-5
- Shell nozzles API 650 Fig. 5-8 and Tables 5-6 to 5-9
- Shell nozzles near weld seams API 650 Fig. 5-14
- Minimum spacing of welds API 650 Fig. 5-9
- Shell connections flush with the bottom API 650 Fig. 5-14
- Draw-off sumps API 650 Fig. 5-21 and Table 5-16
- Clean-out fittings API 650 Fig. 5-12 and Table 5-9
- Roof manholes API 650 Fig. 5-6 and Table 5-13
- Roof rectangular hatches API 650 Fig. 5-17
- Flanged roof nozzles API 650 Fig. 5-19
- Threaded roof nozzles API 650 Fig 5-20
7.5 Some tips on exam questions
The API 653 examination frequently contains questions taken from the long list of figures and tables above. There are normally only a handful, however, and the exam questions that can be asked are constrained by:
- The exam questions do not reproduce drawings from API 650, so they have to rely on text or data tables to get their questions.
- It is surprisingly difficult to ask a question, in text form, about something on a drawing unless it is kept very simple. Questions are commonly therefore about material thickness, weld size or some angle or other that can be picked out of one of the API 650 figures without too much chance of misinterpretation.
7.6 Finally: bits and pieces from the API 650 appendices
Very little of the content of all the API 650 appendices ever appears in the API 653 exam. There is far too much of it, most of which is of little practical necessity to API 653 storage tank inspection. Picking through the published API body of knowledge (BOK) reveals the few bits of the appendices that are included:
Appendix A: Optional design small tank
- B: Tank foundations
- C: External floating roofs
- H: Internal floating roofs
- N: Unidentified materials
- U: UT in lieu of RT
- X: Duplex stainless steel tanks
Appendix G1 (only): Geodesic dome roof
- I1, I2 (only): Underfloor leak protection
- M1 (only): Elevated temperature tanks
- O1, O2 (only): Under-bottom connections
- S1 (only): Stainless steel tanks
It is difficult to see any great underlying pattern in this selection. The good news is that they are not a major source of exam questions – perhaps one or two from the 50 open book questions in the exam. They are nearly always very straightforward – as long as you can find the correct appendix, you should be able to pick out the answer. Figure 7.6 shows some points from these appendices that make good subjects for exam questions.
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