API 653 Exam Chapter 8

API 653 Exam Chapter 8 – Tank Non-destructive Examination

Non-destructive examination (NDE) of storage tanks is a subject of direct relevance to tank inspections. Although an API inspector would rarely perform NDE themselves (the NDE technician or API-termed ‘examiner’ does that) it is the role of the inspector to specify the scope, check the technique, and evaluate the results. In the context of the API 653 ICP, NDE questions form a sizeable chunk of the body of knowledge (BOK) and appear, fairly predictably, as exam questions of several sorts.

Let us start with these points about the NDE coverage of API 653:

The NDE content of API 653 nearly all relates to tank repairs.

The requirements supplement the fuller coverage in the tank construction code API 650. They are necessary to fill in the gaps, as API 650 is about new construction, so does not cover in-service repairs.

There are two, almost completely separate, parts to storage tank NDE. The techniques and technical details (covered in ASME V), and scope or extent of NDE that is required, which is found in API 653 itself.

The NDE content of API 653 is separated into two parts. More than 90% is found in section 12: Examination and Testing, catalogued by area of the tank. This is then summarized by techniques (VT, PT, UT, etc.) in Annex F: NDE Requirements Summary near the back of the code.

Figure 8.1 below summarizes the situation. Note how strictly (as clearly stated in API 653 (12.1.1.1)) the source code for NDE compliance is actually the new construction code API 650. In practice, this tends mainly to influence tanks that are being reconstructed (cut up and reassembled somewhere else), as these are effectively treated as if they are new-build

Figure 8.1 The tank NDE scope of API 653 and 650
Figure 8.1 The tank NDE scope of API 653 and 650

tanks. In most practical in-service inspection/repair/alteration scenarios the relevant NDE requirements are not fully covered in API 650, and API 653 steps in to take over (and take priority).

8.1 The ideas behind API 653 section 12: examination and testing

Although API 653 section 12.1 is set out as a long list of uninspiring ‘100% NDE’ text clauses (with no diagrams) the overall technical ideas behind it are quite straightforward.

Visual inspection. Almost all welding activity requires visual testing (VT) at stages before, during and after the welding.

Crack detection. Completed fillet welds (patches, reinforcing plates, etc.) are checked for cracks by PT/MT, as well as the usual VT.

Butt welds. Used for shell insert patches, plate replacement and similar, these require volumetric NDE to check for cracks and other defects inside the weld.

Parent material. This will be welded and must be free of defects. The main scenarios are:

  • After removal of attachments: visual/PT/MT for surface cracks (12.1.4.1, 12.1.2.2, 12.1.8.2, etc.) 
  • Before welding penetrations: shell requires UT for laminations (12.1.2.1, 12.1.8.1).
  • After grinding out of defects and backgouging the cavities: need visual/PT/MT for surface cracks (12.1.3.1).

These fairly general points actually consume quite a lot of the text clauses of API 653 section 12.1. It is probably easier to remember their principles (have a look at Figs 8.2 and 8.3) than the fragmented way they are set out in section 12.1. Remember that they are also repeated in API 653 Annex F.

8.2 Weld leak testing

API 653 section 12 explains four methods of leak testing in repaired/new welds. The purpose of these is to make sure there are no leak paths existing through the welds that would cause a leak when the tank is put into service. Note two important points about these tests:

They are in addition to the usual VT/PT/MT surface crack detection used on (mainly fillet) welds.

They are designed to be more searching than the hydrostatic test. Water is notoriously poor at finding its way through tight or staggered cracks, and will often not

Figure 8.2 API 653: NDE content
Figure 8.2 API 653: NDE content

show leaks that are easily detectable using more searching methods.

API 653 section 12 is quite careful in the way in which it recommends each of the techniques for the application to which it is best suited. The way in which this is presented in the code text does not exactly make it jump out at you, but it

Figure 8.3 Some general principles of tank repair/NDE
Figure 8.3 Some general principles of tank repair/NDE

is there. Figure 8.4 shows the idea and Fig. 8.5 shows the techniques in more detail.

8.2.1 Vacuum box testing (F5)

This is done using a right-angled box surrounded by rubber seals. It is placed over repair welds and a vacuum drawn inside the box. Any air leaking in through weld cracks soon shows up as an increase in pressure. See API 650 (8.6.1) for procedure details.

Figure 8.4 Weld testing methods
Figure 8.4 Weld testing methods

8.2.2 Diesel oil ‘wicking’ test (F7)

API 650 sees this as an alternative to a vacuum box test. Section 12.1.6 is a good example – it specifies either a vacuum box or diesel oil test on a completed shell-to-bottom fillet weld. This works by simply painting diesel oil on to one side of a weld and seeing if, over a minimum period of 4 hours, it creeps through to the other side by capillary ‘wicking’ action though penetrating cracks or other defects. Spreading chalk or similar powder on the weld helps show when the diesel oil is creeping through.

Figure 8.5 Weld leak testing techniques
Figure 8.5 Weld leak testing techniques

8.2.3 Tracer gas testing (F6)

This is an extra-sensitive technique (again, an alternative to vacuum box testing) which can be used on new tank bottom welds. The gas (usually helium) is inserted under the tank bottom and sniffer detectors used to detect any leakage through to the top surface. Owing to the difficulty of constraining the gas to one side of the weld, this technique is normally only used for tank bottoms. It is such a sensitive technique that it can be very difficult to get welds totally leak free

8.2.4 Air leak testing (F8)

The main purpose of this test is for testing new nozzles/shell penetrations when they are fitted with reinforcing (compensation) pads. Air is introduced between the two plates, hence testing whether there are any leak paths in the fillet weld(s). Threaded air connection/bleed nipple holes are installed for this purpose.

As a special case the air test can be used as an alternative to diesel oil testing of the first pass of a new (or repaired) shell-to-bottom fillet weld. Figure 8.4 shows the details – note how weld ‘blockages’ need to be installed to isolate the area under test and how the air entry point and pressure sensing points must be at opposite ends of the part of the annulus being tested.

8.2.5 Weld radiography

API and ASME codes have long preferred RT to other volumetric methods such as shear-wave (angle probe) UT. This is probably partly because of the fact that RT gives a permanent record and partly for historical reasons, rather then necessarily its effectiveness in finding defects. In reality, doing RT on a plant where repairs are being carried out can be awkward owing to health and safety requirements so many operators prefer UT. In recent years, API codes have become more open to replacement of RT with UT, although their equivalent (ASME and API) construction codes have been slower to follow.

Fundamentally, the RT required by API 653 section 12 starts with that specified in API 650. Note where it says this in API 653 (12.2.1). This is then supplemented (added to) by the clauses of API 653 (12.2) – catering for repair and replacement activities that are not covered by API 650 (which is only about new construction). While the code clauses are not particularly memorable, the principle makes sense – site repair welds are rarely done under optimum conditions, so extra RT is a good idea to find any defects that may be caused as a result.

8.3 How much RT does API 650 require?

It all depends on the thickness of the shell. API 650 section 8.1.2: Number and Location of Radiographs sets out three scenarios: for shell plate material up to 10 mm, 10 mm to 25 mm and above 25 mm thick. It differentiates between vertical, horizontal and intersections and shows the requirements fairly clearly in the code Fig 8-1. For shells up to 10 mm thick in particular, the requirements are not that obvious, i.e.: .

For vertical welds: one spot RT in the first 10 ft and in each 100 ft thereafter, 25 % of which need to be at intersections. and .

For horizontal welds: one spot RT in the first 10 ft and in each 200 ft thereafter.

Look at API 650 Fig 8-1 for the full details.

8.4 How much RT does API 653 require?

You can be excused for being a little confused about this. While straightforward in concept, the clauses API 653 (12.2.1.1) to (12.2.1.6) look a bit daunting. To simplify it, think of shell plate welds as being of the following types:

  • ‘New construction’ type welds: i.e. new welds between new plates. This is exactly the same as new construction, so just follow API 650 (8.1.2) as API 653 has no additional requirements.
  • ‘Repair’ welds. These may be between either new-toexisting plates or existing-to-existing plates. Treat these as repair welds (with a higher risk), so they need some additional RT.

8.4.1 Key point – the difference between new plates and replacement plates

Do not get confused by this one:

  • 12.2.1.1 to 12.2.1.4 are about the situation where new shell plates are installed in a shell, to replace corroded ones. Figure 8.6 covers this scenario.
  • 12.2.1.6 and its subsection. This is about when plates are cut out to provide access doors (to allow work to be carried out inside the tank) and then rewelded up afterwards.

Figure 8.7 below shows this situation. Note how one less vertical weld RT shot is required than when fitting a new shell plate (i.e. 12.2.1.1b).

For thicker materials > 1 in the additional chance of defects means that full RT is required on vertical seams. In some cases, it is easier to cut out circular access doors than rectangular ones. This only requires 1 RT shot (see 12.2.1.6.1) unless, again, it is more than 1 in thick, when full RT is required.

Figure 8.8 shows what to do about reconstructed tanks.

8.4.2 What about exam questions?

It is easy to get overly excited about exam questions on RT. Surprisingly there are generally not that many about the required scope of RT for the multiple possible permutations of vertical, horizontal, intersection, new and repaired welds. One or two pop up in the open-book question section, typically one from API 650 and one from API 653. More common are questions from ASME V article 2 and API 577. These are of a more generic nature and often less technically rigorous than you might expect.

8.4.3 NDE procedures and qualifications

API codes (and their ICP exams) are littered with stuff about who needs to be qualified to what level and which procedures they should be working to. NDE activities are no exception.

Figure 8.6 RT of shell plate repairs
Figure 8.6 RT of shell plate repairs

These statements reflect a fairly consistent API view of the inspection world, but there is absolutely no reason why they should fit your situation.

API 653 Annex F provides the best summary of this information. Note how all NDE scopes from other chapters

Figure 8.7 RT of access door sheets
Figure 8.7 RT of access door sheets

are compiled together at the end of each subsection. Figure 8.9 is another way of presenting this.

Figure 8.8 RT of reconstructed tanks
Figure 8.8 RT of reconstructed tanks
Figure 8.9 NDE procedure requirements
Figure 8.9 NDE procedure requirements

Now try these practice questions.

8.5 Tank NDE: practice questions

1.

Q1. API 653: shell penetrations
What NDE must always be carried out before adding a hot tap connection to a shell plate?

 
 
 
 

2.

Q2. API 653: shell penetrations
A hot tap connection to a shell plate has been examined by PT. Under what circumstances should the use of UT or fluorescent MT also be considered?

 
 
 
 

3.

Q3. API 653: shell penetrations
What NDE is required on the completed welds of stress relieved assemblies?

 
 
 
 

4.

Q4. API 653: repaired weld flaws
What is the minimum level of NDE required on a completed butt weld repair?

 
 
 
 

5.

Q5. API 653: temporary and permanent shell attachments
How must completed permanent attachment welds be examined?

 
 
 
 

6.

Q6. API 653: shell to bottom weld
A welded-on patch plate is to be placed over a shell to the bottom weld. If the plate will cover 16 inches of the bottom weld, what length of the shell to bottom plate weld needs to be inspected before the patch is applied?

 
 
 
 

7.

Q7. API 653: bottoms
In addition to PT or MT, what testing is required for areas of bottom plates repaired by welding?

 
 
 
 

8.

Q8. API 653: number and location of radiographs
A new shell plate has been welded into an existing tank. The RT requirement is in accordance with API 650. What additional radiography will be required?

 
 
 
 

9.

Q9. API 653: acceptance criteria for existing shell plate welds
An intersection between a new and old weld contains defects unacceptable to the new standard. The defects are, however, acceptable to the original construction standard. Must they be repaired?

 
 
 
 

10.

Q10. API 653: marking and identification of radiographs
What does the letter R mean on a weld radiograph?

 
 
 
 

Click Here To Read Next API 653 Exam Chapter 9 –Tank Repairs and Alterations

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