API 653 Exam Chapter 9 – Tank Repairs and Alterations
Overall, a lot of the storage tanks in the world are in a bit of a mess. Here are the reasons why:
Poor maintenance. Tanks are easily forgotten when allocating maintenance budgets to higher priority parts of a plant. They are often seen as being less processcritical. Access is also difficult – external inspection/ maintenance requires scaffolding or mobile cranes.
- Long lifetimes. It is not unusual for tanks to be 50 or more years old.
- Multiproduct use. Changing process conditions leads to unpredictable (frequently unknown) corrosion rates.
- Construction standards. Although tank construction codes (particularly recent ones) are technically consistent in themselves, tanks are hardly high technology items. Most are made from corrosion-prone low carbon steel. They are simple utilitarian fabrications, rather than cutting-edge engineering structures, which is reflected in their low price.
- Environmental conditions. A lot of tanks are situated in dirty or marine environments. Once the external painting starts to break down, corrosion occurs quickly. It is worse if the external surfaces are lagged.
The result of all this is that most storage tanks end up needing a lot of repairs during their lifetime. These can range from small patches or replacement plate inserts through to replacing complete tank bottoms, shell courses (or ‘strakes’) or roofs. Some of these do little for the cosmetic appearance of the tank, but are perfectly technically viable – and cheaper than buying a new tank.
9.1 Repairs or alterations?
API in-service inspection codes are well known for their differentiation between repairs and alterations. They contain the definitive opinion that repairs and alteration are fundamentally different things. The rationale behind this is:
Alterations involve some kind of ‘new’ design aspect (for the tank in question) that needs to be considered. Repairs do not.
Because of the ‘new design’ aspect of alterations, technical details need to be approved and the work authorized to proceed by someone with the necessary knowledge. API certified inspectors are not always expected to have this knowledge. Some higher technical authority (a qualified ‘engineer’) has to have the final word.
Fortunately API 653 is more logical than some other codes as to the difference between repair and alterations. This is then qualified by dividing each into ‘major’ or ‘non-major’ categories, i.e. major repairs and major alterations. Note that the code does not actually classify the opposite of major repair or alteration as minor or ordinary. It just infers that they are ‘not major’. Wonderful.
Here are the key points:
A repair (non-major) is an activity used to fix some corrosion or similar problem to return a tank to a safe operating condition. Nothing is added and there is no significant design implication (API 653 definition 3.2.4).
A major repair is a repair (as above), but of a type specifically stated in API 653 definition 3.18, i.e.: .
- Removing and then replacing a shell plate of longest dimension more than 12 in situated below liquid level.
- Removing and then replacing annular ring material with the longest dimension more than 12 in.
- Removing and then replacing more than 12 in of a vertical shell plate weld or annular plate ring radial weld located anywhere in the tank.
- Removing and replacing a significant amount (>50% of weld) of the shell-to-bottom/ring weld.
- Jacking up the tank shell to renew the complete tank bottom.
An alteration (non-major) is some change or addition that changes the tank’s physical dimension or configuration (definition 3.1).
A major alteration is an alteration (as above) that specifically involves:
- Installing a new shell penetration >NPS 12 below the design liquid level (NPS is nominal pipe size).
- Installing a new bottom penetration within 12 in of the shell.
Figure 9.1 shows the situation in pictorial form. Once you have accepted the principle of this repair versus alteration differentiation then the major versus non-major split fits in fairly naturally with the integrity implications of the activity. Repairs or alterations designed as major have a higher risk of leak or failure consequences if not done correctly. Figure 9.2 shows the responsibilities for approvals and the hydrotest requirements.
9.2 Hydrotest requirements
API 653’s view is that ‘non-major’ repairs or alterations are not important enough to warrant a mandatory hydrotest. Owners/users can of course perform one if they wish but it is not a code requirement. For major repairs or alteration then a hydrotest is required by section 12.3.1b of API 653. Even this, however, can be overridden by either:
- An FFS assessment (API 653 section 220.127.116.11) – the exact type of which remains undefined.
- Complying with the relevant exemption clauses chosen from API 653 sections 18.104.22.168 to 22.214.171.124. This is a rather long list but, in summary, simply says that the major repairs/alteration meet the requirements of:
- Correct materials (with sufficient toughness)
- Qualified weld procedures . Low hoop stress (≤7 ksi)
- Good weld quality
Putting aside the minutiae of code clauses for the moment, you can see that API 653 does not actually impose a mandatory hydrotest for any repairs or alterations at all. It is
mandatory for dismantled and reconstructed tanks (see API 653 section 12.3.1a) because this is similar to new construction activity. It therefore has to comply with the construction code API 650 as if it were a new tank, required to prove its integrity before use.
9.3 Repair and alterations – practical requirements
Section 9: Tank Repair and Alterations is one of the longest sections of API 653. From an API exam perspective also, the subject is important; repair and alteration-related topics can form up to 30–35% of the total haul of exam questions. Most of these have a practical engineering aspect to them involving design, welding, testing or responsibilities rather than any deeply theoretical considerations.
Figure 9.3 shows the breakdown of the section. Note how it divides logically into activities involving shell plates,
penetrations (i.e. nozzles), bottoms and roofs. Detailed technical requirements cover all these areas – related to either repairs or alterations, as applicable. Throughout the sections, some common principles apply:
- Minimum and maximum repair sizes and thickness
- Allowable repair location
- Weld locations, types and sizes
- Methods of avoiding local hardness, leading to cracking and brittle fracture
We will look at these in turn, concentrating on those areas that feature heavily in the API exam questions.
9.3.1 Are these repairs temporary or permanent?
They are all permanent. Unlike pressure vessels, where some types of repairs have to be considered as temporary, storage tanks can be permanently repaired using fillet-welded ‘lap’ patches. They may not look particularly attractive, but they provide perfectly adequate strength and integrity against leaks. Non-welded repairs such as epoxy filler, wraps, clamps, etc., do not feature significantly in API 653 – there are a few references in API 575 but little technical detail. Welded repairs are clearly preferred, where possible.
9.3.2 The basics of code compliance
Fundamentally, tank repairs and alterations have to comply with the tank construction code API 650. Practically, however, API 650 does not cover most repair configurations, so API 653 section 9 takes over with the required technical detail. The principles are similar, although API 653 allows extra leeway in some areas, to allow for the realities of site fabrication work.
9.4 Repair of shell plates
Repair of shell plates comprises two types:
- Replacement plates, where plates are cut out, normally because of corrosion, and a new replacement ‘insert’ plate is butt-welded in its place. These are covered in API 653 section 9.2.
- Lap-welded patch plate (API 653 section 9.3). Here, a plate is fillet (lap)-welded over the top of a corroded (not cracked) area to restore the thickness and strength of the tank shell.
Both of these types are considered permanent repairs and the code clauses specify limitations on plate size and shape, allowable weld locations and design features related to them.
Engineering details of replacement shell plates are shown in API 653 Fig. 9-1. The main details of this are reproduced in Fig. 9.4 here – note the additional annotations taken from the subsections of API 653 (9.2).
API 653 section 9.3 covers similar restrictions for lap welded patch plates. There is no code figure for this – all the requirements are listed in the multiple subsections of 9.3. Figure 9.5 below summarizes the main requirements – note the limits on minimum and maximum overlap and the similar size restrictions to those for butt-welded replacement (insert) patches. Lamination checks of the parent plate before welding are important – UT checks for parent plate laminations are therefore required by API 653 (9.3.1a).
9.4.1 Repairing shell plate defects (9.6)
It is much easier to repair defective welds on shell plates in situ than to replace them or apply a lap-welded patch plate. As long as the minimum required plate thickness is maintained, corroded areas can be blended by grinding or blended and weld-repaired as need be. The requirements are fairly commonsense:
- Cracks, lack of fusion and slag must be ground out completely before rewelding.
- Excessive undercut in excess of API 650 limits needs to be removed by blending and rewelding if required (for plate ≤13 mm it is maximum 0.4 mm on vertical welds and 0.8 mm on horizontal welds).
- Weld arc strikes must be removed as they cause stress concentrations.
9.5 Shell penetrations
There are three main things you can do with tank shell penetrations (nozzles or access manholes) .
- Repair them (9.7)
- Replace or add new penetrations (9.8)
- Alter existing penetrations (9.9)
9.5.1 Repair of penetrations (9.7)
This is generally about adding of reinforcing (compensation) plates to existing nozzles. The reasons for needing to do this would be:
- An increase in tank maximum fill height or product specific gravity, meaning that existing nozzles near the bottom of the shell require additional compensation.
- The shell around existing nozzles is corroded (on either the inside or outside of the tank), so shell strength in that region needs to be restored.
Figure 9.6 shows the main technical requirements of adding these reinforcing plates to existing nozzles. Note the ‘tombstone’ plate fitted when the nozzle or manway is near the bottom of the tank (which they usually are). As you can see from the figure, the main issue is the minimum fillet weld (leg) size between the bottom of the tombstone plate and the bottom annular ring. Note how the nozzle-to-reinforcing plate weld is the same for the tombstone plate as for a reinforcing plate that does not extend all the way to the floor. The plate-to-floor weld is smaller.
Watch out for open-book exam questions about this section of the code – particularly about the optional horizontally split reinforcing plate and the positioning of the vent/tell-tale holes.
9.5.2 Alteration of existing shell penetrations (9.9)
This is about the effects of installing an additional tank bottom on top of an existing corroded one. The additional bottom is normally added either directly on or an inch or two above the existing one, with a layer of cushioning material such as sand or crushed stone in between the two. This causes the new bottom-to-shell weld to be raised up nearer the lowest shell nozzles reinforcing plate, frequently reducing the weld spacing to below the minimum distance required. Section 9.9 gives three solutions to this:
- Trim the bottom of the existing nozzle reinforcing plate, as long as there is still adequate reinforcement available (as per the API 650 calculation, section 126.96.36.199).
- Remove the existing reinforcing plate and replace with a new one (188.8.131.52). Sometimes it is possible to just replace the bottom half, leaving the top half in place.
- Move the offending nozzle and its reinforcing plate upwards, increasing the spacing to the shell-to-bottom weld.
Figure 9.7 summarizes these three methods. As most of the technical detail is about weld spacing and sizes, these are normally only suitable for open-book exam questions. It is difficult to construct sensible closed-book exam questions from this topic.
9.6 Adding an additional bottom through an existing tombstone plate (9.9.4)
API 653 (9.9.4) is, arguably, a special case, where an additional bottom is added through an existing tombstone reinforcing plate. This is a rather complex new code section –a bit too involved for more than the occasional open-book exam question. The basic idea (see Fig. 9-6 of API 653) is that the lower edge of the tombstone plate is cut and bevelled to allow it to be welded into the new fillet weld added between the existing shell plate and the new bottom. This is then ‘backed up’ with an additional fillet weld on that.
9.7 Repair of tank bottoms
Tank bottoms can corrode from either the product side or soil side, so bottom repairs are a common occurrence, particularly on old, multiproduct tanks without cathodic protection. Similar to shells, bottoms can be permanently repaired using individual fillet-welded lap patch plates. Alternatively, if corrosion is very widespread, a complete new bottom can be fitted, usually directly on top of the existing one, unless there is some pressing reason for removing it.
9.7.1 Patch plate bottom repairs
API 653 Fig. 9-9 contains all the necessary information about what you can and can not do when doing bottom patch plate repairs. Figure 9.8 below shows some of the major points. Note how:
- Minimum patch plate dimension is either 6 in or 12 in, depending on whether it overlaps an existing seam (184.108.40.206b).
- Patch plates may be almost any shape or maximum size.
- Minimum spacing distances have to be met to avoid HAZ interaction causing local hardening and cracking problems.
- Special restrictions apply in the critical zone (the annular area extending 3 in in from the shell)
9.7.2 Repairs in the critical zone
The critical zone (API 653 definition 3.10) is the annular area of the tank bottom extending 3 in in from the shell. For tanks fitted with an annular ring, the critical zone is part of the
annular ring, but generally not all of it. The reason for separately identifying the critical zone is that it sees high bending stresses if foundation washout or distortion causes edge settlement. This bending would have a tendency to tear apart the shell-to-bottom fillet welds, as they have little strength. Stresses increase with small radial lengths and large vertical deflections, so repair restrictions are put in place to limit this.
Figure 9.9 shows typical restrictions on patch repairs.
9.7.3 Repairing pitting in the critical zone (220.127.116.11)
Pitting can be repaired by overlay welding as long as the parent material underneath is not less than 0.1 in thick. The usual 2 in in 8 in cumulative maximum applies (the definition of isolated pitting), but in this case the plane in which this is calculated is on an arc parallel to the shell. This is the plane on which shear stress acts on the bottom plate if there is any foundation washout around the tank or circumference.
9.7.4 Replacement of tank bottom (9.10.2)
Full bottom replacement is a major exercise, used when the tank bottom is so severely corroded that wholesale replacement is the most practical option. This section really refers to replacing all the bottom plates, but while leaving the annular ring (with its critical zone) in place. Figure 9.10 shows the idea.
Note how Fig. 9.10 shows the definition of this activity as a repair, not a major repair or an alteration. In a fit of logic, API 653 definition 3.18(f) says:
Replacement of a tank bottom is not classed as ‘replacement
of a tank bottom’ if the annular ring remains unaffected. You may need to read that again.
This means that it is classed as a straightforward repair without the requirement for a post-repair hydrotest.
Replacement of the tank bottom is predominately an exercise in complying with the construction code API 650. There is also a list of API 653 requirements (in 9.6.2) that override this, as they specifically refer to repairs. They are:
- Sand/gravel cushion material is required between the existing and new floors (18.104.22.168.1). .
- Penetrations may need to be raised to maintain minimum spacings above the bottom-to-shell weld (22.214.171.124.4 and 126.96.36.199).
For convenience, the activity of ‘replacing a tank bottom’ is normally done while leaving the existing bottom in place. API 653 sections 188.8.131.52.1 to 184.108.40.206.5 specifically cover this scenario, as the most popular option.
9.8 Repair of tank roofs
The repair of tank roofs is a fairly straightforward exercise, as long as you comply with the construction code requirements of API 650. API 653 does not have many preferences or overrides. This is probably more to do with the fact that tank roofs are little more than a simple plate structure under little stress, than any great technical philosophy. Looking at API 650 section 5.10: Roofs and appendix F you can see that they contain three types of information:
- Calculation equations (which are not in the API 653 BOK).
- Cross-references to other sections of API 650 (which are also not included in the BOK).
- Limiting physical dimensions (minimum thickness, etc.) of roof plates and supporting components.
This self-limits the topics that appear as API 653 exam questions. A few questions appear about minimum thickness of roof plates or the roof-to-shell junctions, but they rarely extend further than that. You can expect these to be open book questions – easily picked out from section 10 of API 650. You can see some examples at the end of this chapter and Fig. 9.11 shows some corresponding points.
9.9 Hot tapping: API 653 (9.14)
Hot tapping’ is the term given to cutting a new nozzle penetration into a tank when it still contains the product at its storage temperature (it does not need to be ‘hot’). This is a much quicker and less troublesome method than emptying and cleaning the tank in order to add the new penetration. It is a common procedure, normally performed without any mishaps, and also in pipelines and vessels, as well as storage tanks.
This API 653 section is a common source of exam questions. The hot tapping activity has specific requirements in nozzle reinforcement, weld size and testing in order to ensure that the design is strong enough and is completed without leaks or weld cracking. Note some key points about hot tapping a storage tank shell:
- Hot tapping is always an alteration rather than a repair, as it changes the physical configuration of the tanks (API 653 definition 3.1).
- If the new penetration is larger than NPS 12 then it becomes a major alteration, as hot taps are always installed below the liquid level (definition 3.18c).
- Materials and stresses must be chosen to avoid brittle fracture.
- The main issue during the installation is to avoid weld cracking, leading to leaks or fracture. This requires limitations to be placed on weld electrode type, weld spacings and weld joint type and size.
Figure 9.12 below shows how the hot tapping procedure is done. Note the steps of the operation:
1 The new flanged nozzle is welded to the tank shell, followed by the reinforcing (compensation) pad. 2 A valve is bolted to the flange and the tapping machine mounted on the other side of the flange (so the machine is isolated from the tank by the valve). The tapping machine
is fitted with glands, completely sealing the cutter and its driveshaft inside the fluid boundaries.
3 The valve is opened and the cutting head traverses through the valve, cutting the opening in the tank. A pilot drill and ‘catch wire’ arrangement holds on to the cut coupon, preventing it falling into the tank.
4 When the cut is complete the cutting head is retracted back through the valve, which is then closed, isolating the liquid so the cutting machine can be removed.
9.9.1 API RP 2201
API Recommended Practice RP 2201: Procedures for Welding or Hot Tapping on Equipment in Service is a detailed document covering the subject of hot tapping. It is very comprehensive, but the good news is that its content is not in the API 653 exam body of knowledge, so you do not need to study it. Knowledge of its existence, however, is an exam topic, as it is mentioned in the reference section of several of the API 653 subject codes.
9.9.2 API 653 (9.14) hot tapping requirements
This code section provides a good example of what API codes do best. Instead of bothering with too much technical detail of procedure, it just gets straight to those points that will have an influence on the integrity of the hot tapping penetration. This section 9.14 is valid examination question material. Most are open-book topics, but there are also several points of technical principle that make valid closed book questions
Figure 9.13 summarizes some key points. A lot of it centres around the requirement that hot-tapped nozzles require a reinforcement plate, which must be made from sufficiently tough material and then meet well-defined sizing and weld requirements. Penetration position is defined by the fact that it must be a minimum 3 feet below the liquid level (220.127.116.11) but not see sufficient static head pressure to cause a hoop stress of more than 7000 psi (18.104.22.168(4)).
Note how Fig. 9.13 summarizes key points of API 653 Fig. 9-10. This is a good concise figure summarizing a lot of technical detail, and a well-established source of API 653 examination questions.
9.10 Tank repair and alteration – other requirements
Remember that API 653 section 9 does not cover all the requirements of tank repair and alteration. Repair and alteration are fundamentally API 650 construction code
activities with the requirements of API 653 added to them, to cover the practical aspects of site work. This should become clearer when we look at the subject of tank reconstruction in Chapter 10. This is almost a purely API 650-based activity, treating the reconstruction as the same as building a new tank from scratch.
For the procedural aspects of welding and NDE of tank repair and alteration, ASME V, IX and API 577 provide more detail than API 653 itself. These therefore provide the source of exam questions of a more generic nature, e.g. related to other types of repair/alteration as well as hot tapping. We will cover them in separate chapters of this book. Exam questions tend to be fairly polarized, however, concentrating on one or the other, because that is how the questions are compiled.
Now try these practice questions.
9.11 Repair and alterations: practice questions
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