API 653 Exam Chapter 10 – Tank Reconstruction
Strangely at odds with the sensible idea that a storage tank can be heavily repaired throughout its life lies the subject of tank reconstruction. This is the wholesale dismantling of a tank into a large number of transportable pieces and then reassembling them somewhere else, on a completely different site. Opinions differ on whether this is all worth the bother. It is a labour intensive exercise and many countries with high labour rates would not find it economic – it would be cheaper to build a new one.
Technical practicality plays a part in the decision – it can be physically awkward getting the disassembled parts to fit, particularly for larger tanks. The state of corrosion can also cause problems – additional corrosion is often discovered during disassembly, requiring repairs before reassembly of the parts. Old, large tanks are the biggest risk.
Notwithstanding the practical and cost uncertainties, tank reconstruction is clearly a viable option in those countries that do it, so it is a long-standing part of API 653, with specific technical principles. It is also a valid topic for API 653 exam questions.
10.1 Code requirements for tank reconstruction
Unlike repairs, tank reconstruction is considered a ‘build as new’ activity. In most cases this turns into a ‘build better than new’ activity as one of the fundamental requirements is that reconstruction activities must be done to the ‘current applicable standard’, i.e. the code edition relevant to when it is reconstructed, not the old one to which it was originally built. This applies only to all new material, components, welding and NDE used during the reconstruction. Existing materials, components and welds that are not reworked can remain as they are, to the as-built standard. Figure 10.1 summarizes the situation.
The main code requirements for reconstruction are found in three places:
Section 8 of API 653 covers design consideration for reconstruction tanks. This is a one-page section setting out the relevant code-compliance requirements as described above.
API 653 section 10: Dismantling and Reconstruction is a longer section covering the procedures of dismantling the tank and putting it back together again. The dismantling bits (10.3) are unique to API 653 because they are not covered in the construction code. The reconstruction requirements (10.4) are practical additions to the construction code (API 650) – mainly about welding acceptance criteria and broad dimensional tolerances for the rebuilt tank.
API 650, the construction code itself, contains the most detailed information. This drives the technical detail of the reconstruction, reinforcing the principle that reconstruction is a new-build activity rather than a repair or alteration one. There are other non-API construction codes, which is why API 653 is careful to refer to the construction codes in generic terms rather than exclusively to API 650. Such niceties do not extend to the API exam – construction code questions will all be about API 650.
10.2 Reconstruction responsibilities
API 653 does not make such a well-defined split between the meaning of the words approval and authorization, as do API 510 or API 570 relating to pressure vessels and pipework. The inherent meanings of the words (as decided by API), however, are the same, as follows:
Approval means approval of a design procedure method or way of doing something, independent of whether the activity has started or not. It can also apply to accepting some work or activity that has finished (i.e. agreeing it has been done properly).
Authorization means authorizing an activity to start.
You can see these words in action in API 653 (10.1.4), although they are not defined for you. One long-standing controversial area of API ICP exams is the large number of questions about personnel responsibilities and their relevance to inspection practice outside the USA. Like them or not, Fig. 10.2 shows what they are for tank reconstruction. Note
how the API inspector alone can approve not only hold points and the extent of the completed tank document package but also authorize reconstruction work to start and approve its completion. The tank engineer therefore has no mandatory role in reconstruction. That is all there is about reconstruction responsibilities in API 653.
10.3 API 653 section 10: structure
Section 10 is split logically into dismantling (10.3) and reconstruction (10.4) activities. These subsections provide parallel coverage for the main components of the tank: bottom, shell and roof. The main issue of both dismantling and reconstruction is the objective of avoiding cracking in the reconstructed tank welds. The first part of achieving this is to ensure that parts of old existing welds are not left in place to interfere with new reconstruction welds causing local carbide concentrations and the risk of cracking. Much of section 10.3 is centred around this objective. Note particularly how:
Bottom plates must be cut a minimum of 2 in away from existing bottom seam welds (10.3.2.1) and 1/2 in away from the shell-to-bottom fillet weld (API 653 Fig. 10-1).
If the entire tank bottom is to be reused it must be cut a minimum of 12 in from the shell-to-floor junction, leaving the shell with the remaining part of the bottom still intact (API 653 10.3.2.2b and Fig. 10-1).
Shell plates must be cut a minimum of 6 in away from existing shell seam welds (10.3.3.1c) and 1 2 in up from the shell-to-bottom fillet weld (Fig. 10-1 again).
Roof plates must be cut a minimum of 2 in away from existing roof seam welds.
Figure 10.3 summarizes the content of API 653 Fig. 10-1 and some of its referenced sections. This is a common source of open-book exam questions. It is difficult to write an awkward question on this – so they are normally straightforward, just requiring you to read various minimum dimensions from the code Fig. 10-1. Easy.
10.4 Reconstruction (10.4 and 10.5)
This is simply a case of welding the cut parts back together again in a way that avoids brittle fracture, cracks or other integrity-threatening defects (10.4). In addition the completed tank has to meet a set of dimensional tolerances (10.5) on inclination (plumbness), roundness, peaking and banding in order that it does not suffer from excessive ‘out of design’ stresses in use.
Given that tank reconstruction is essentially a new construction activity, these sections of API 653 section 10 are based on the straight requirements of API 650, with a few
additional practical aspects thrown in for good measure. Salient points are:
- Weld spacings have to be maintained with a minimum 5t stagger on shell plate vertical joints (10.4.2.1).
- Preheat of weld joints is necessary in cold climates. The preheat temperature depends on thickness (10.4.2.3):
- Below 0 ºF, no welding is allowed.
- 0 ºF to 32 ºF on thickness > 1 in, preheat to 140 ºF.
- Thickness > 1 1/2 in, preheat to 200 ºF (10.4.4.3).
- Weld maximum undercut (as per API 650) and reinforce
ment limits (API 653 Table 10-1) apply. This is to avoid stress concentrations and crack initiation points.
Figure 10.4 above summarizes some of these important points. Note how they are mainly about the shell plates.
10.4.1 Use of low hydrogen welding rods
Most API codes are in agreement that whenever welding is carried out under any type of non-optimum conditions (e.g. on site), it is best to use low hydrogen welding rods. This minimizes the risk of cracking, caused by hydrogen molecules expanding in small discontinuities (such as grain boundaries) and progressively weakening the structure. Couple this with heating and cooling and you get cracks. Both API 571 and 577 (both in the API 653 body of knowledge) address hydrogen cracking and its prevention, so the subject almost always appears as (several) exam questions in one form or another.
Figure 10.5 shows the major issues relating to avoiding hydrogen cracking. Note how these are taken not only from API 653 section 10: Reconstruction but also from section 11: Welding.
10.4.2 Reconstructed tank dimensional tolerances in API 653 (10.5)
In engineering terms, atmospheric storage tanks are large, rather floppy structures that do not take well to dimensional inaccuracies. The activity of reconstructing accurately an old tank after dismantling it can be difficult – it may be a slightly different size due to cutting, rejection of old weld seams, etc., and the old plates are frequently distorted, damaged during transport, or weakened by corrosion. The main problems usually occur with the shell. Excessive shell distortion has two effects:
Sticking roofs. Shells usually distort more near the top of the tank. This causes floating roofs to either stick or leak past the seals, letting vapour out and rain in.
Excessive stress. Tank shells are designed to the simple hoop membrane stress equation, which works on the assumption that the shell is round and of uniform (plane) section over its height, within certain tolerances. Any distortion that takes a shell outside this idealized shape causes stress to rise, indeterminately. This gives an increased risk of failure at almost any area of stress concentration, such as weld undercut or excessive weld cap convexity. API 650 (10.5) sets dimensional tolerances to try and avoid these problems.
10.4.3 Foundation tolerances
Uneven foundations will cause distortion in an otherwise accurate shell. Tolerances are specified by API 653 (18.104.22.168), depending on whether the foundations include a concrete ringwall (an annular concrete foundation ring on which the shell sits).
10.4.4 Shell tolerances
There are four of these: plumbness, roundness, peaking and banding. All can cause problems if excessive, and things get worse if they act together.
Plumbness (10.5.2) is simply inclination or out-of-verticality. The limit is 1 in 100 to a maximum of 5 inches (see 10.5.2.1). . Out-of-roundness (10.5.3) is the measured deviation of the shell shape from a true circle. Simplistically it is the difference between the minimum and maximum measured diameter. It causes bending stresses, which are not allowed for a simple hoop (membrane) stress design theory. As out-of-roundness (OOR) generally gets worse higher up the tank, it is measured in two places: 1 foot above the bottom as a reference and then higher up as required.
Peaking. This is distortion along the longitudinal (vertical) weld in the shell. It is mainly initiated by material rolling and/or welding stresses during new construction. Occasionally it can get worse in service.
Banding. Banding is similar to peaking but involves distortion around the circumferential (horizontal) shell welds rather than the vertical ones. It is less common and almost always the result of construction (or repair/ alteration/reconstruction) inaccuracies.
Figure 10.6 shows the tolerances allowed for these features in reconstructed tanks. Being large-dimension related, they require specialized equipment to measure accurately. The usual method is via laser measuring equipment, although API 653 refers to the older, but still effective, method of using plumblines and long ‘sweepboards’ made out of light wooden planks or aluminium strips.
The dimensional tolerances above are checked before the hydrostatic test on a reconstructed tank (10.5.1.2). Remember that tank reconstruction is the only situation after new construction where a hydrostatic test is truly mandatory to comply with API 653 – look back to API 653 section 12 and you can see it specified in 12.3.1a. Settlement measurements are also taken as per API 653 section 12.5 if there is any doubt about the strength or rigidity of the tank foundations.
10.4.5 Tank reconstruction: API 653 versus API 650
Fundamentally, tank reconstruction is considered a new build activity. Remember that all new welding activity is treated as exactly that, as if it were being done on a completely new tank – with the requirement to follow the construction code API 650. In its role as an in-service code, however, API 653 is allowed several overrides to cater for site practicalities, while still providing a tank of sufficient structural integrity.
10.4.6 Reconstruction and other related codes
For the purposes of the API 653 examination (and practically as well), other codes also play their part in reconstruction activities. NDE is covered by ASME V and welding qualifications by ASME IX. Exam questions are rarely detailed enough to examine these links in any great depth – they are more likely to be either an API 653-specific question about reconstruction design or procedures, or very generic questions (unrelated to specific new-build/repair/alteration/ reconstruction activities) picked verbatim out of the wording of ASME V or IX, not both.
Now try these practice questions.
10.5 API 653 section 10: dismantling and reconstruction: practice questions
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