API 653 Exam Chapter 12 – Tank Linings: API RP 652
This chapter is about learning to become familiar with the layout and contents of API RP 652: Lining of Above-ground Storage Tanks. Similar to API RP 651, API 652 is a wellestablished document (recently issued in its 3rd edition: 2005). As a short code, it supplies supplementary technical information suitable mainly for open-book examination questions. Similar to the other short code in the syllabus, it is more a technical guide document than a true code, but it performs a function in supporting the content of API 653. It contains useful technical information on the lining of tank bottoms to minimize the effects of corrosion. API 652 is one of those API documents that, in each revision, seems to grow in technical detail, rather than keeping the scope constant and just making a few paragraph amendments, as many codes do.
Note the following five points about API 652:
Point 1.Tank bottoms can be lined on the inside or outside (i.e. the bottom surface). This is mentioned in a couple of places in the document but perhaps not made particularly clear.
Point 2. API 652 covers linings that are either applied to new tanks or retrofitted to old ones, normally to try to stop future corrosion where significant corrosion has already been found. It introduces both thin- and thick-film linings, but does not go into a lot of technical detail about the chemical and mechanical properties of the linings themselves. This is because lining materials are incredibly varied (there are hundreds of different proprietary ones, each specified by its own manufacturer’s data sheet) so it would be next to impossible to produce any meaningful generic information that would apply to all of them.
Point 3. Although very relevant to the API 653 body of knowledge, API 652 contains little information on inspection itself. It concentrates more on describing the need for tank bottom linings, their properties and how they can fail. Within this scope, however, lies a lot of technical information points suitable for use as closed-book exam questions.
Point 4. In the few areas that API 652 does contain information related to the inspection of linings (and, more importantly, the preparation of the surface before lining), it cross-references various related US codes. The main ones are from:
- SSPC (The Steel Structures Painting Council)
- NACE (The National Association of Corrosion Engineers)
While most oil/gas/petrochemical inspectors have heard of NACE codes, the SSPC ones are less familiar outside the US. These SSPC codes deal with metal surface cleaning, shot blast and surface finish grades. Most European countries use the Swedish standard SIS 055966 instead (the source of the SA shot blast surface finish grades).
Point 5. Like API 651, API 652 discusses corrosion mechanisms, specifically of the tank floors, so you can expect some crossover with the coverage of API 571 damage mechanisms. Note, however, that the corrosion discussed in API 652 is mainly related to the internal surfaces of the tank bottom, rather than the external galvanic corrosion described in more detail in API 571.
Finally, API 651 is mainly text and technical descriptions, accompanied by a few tables of a fairly general nature. It contains no calculations relevant to exam questions but does contain a few quantitative facts and figures that are worth remembering. API examinations have an annoying habit of using questions that test candidates’ short-term memory of facts and figures that appear in the codes. Watch out for these as you read through this code.
12.2 Linings and their problems, problems, problems
Inspectors soon become familiar with the application and inspection of linings and the problems that they bring. If you have never inspected linings, or been involved with them at all, you can give yourself a head start by reading the following commentary.
12.2.1 What are they?
The linings we are talking about here are mainly of the epoxy or rubber-based types, not loose steel linings, welded ‘buttered’ linings or anything like that. They are used on new tanks to try and stop corrosion, and on old tanks that are badly corroded, either because they were not lined in the first place or because the original lining has fallen off.
12.2.2 So what’s the problem?
The problem is that linings love to fall off. Overall, perhaps almost 50 % of new linings fall off and do not reach their design life, and that includes those that were reasonably well applied and inspected. If you consider those that were not well chosen, applied, inspected or whatever, then most of them fall off. All of each lining does not fall off of course, only bits of it annoyingly small bits clinging to (or supposed to be clinging to) rough welds, bits of spatter, sharp corners, etc. This leaves small unprotected areas of parent material.
Small unprotected areas are good news for the corrosion, which sees each one as an opportunity to produce a pinhole, the rate of corrosion being greatly accelerated by the small area. At the same time, it nibbles away at the not-very-well bonded lining on either side of the pinhole and starts to peel this back; the corrosion creeps underneath and the hole gets bigger. The incredibly unsurprising end result is that the parent metal corrodes through and everyone gets together in a big meeting to discuss why the lining failed. Works every time.
All is not finished yet we can reline it (they say).
Yes, you can reline tank or vessel surfaces successfully, either by piecemeal repairs or complete relining. You can also line vessels that were not lined originally and have already suffered, corrosion. The problem is that more than 50% of the linings (relinings that is) fail due to either poor preparation or for a new reason. Here it is:
To retrospectively line a corroded item, you have to get the surface totally clean of soluble salts and similar contaminants. It is no good just shotblasting the surface; it has to be chemically clean as well. This is always difficult, as, by definition, any surface that is corroded has probably been exposed to salts in some form.
12.2.3 The conclusion
The conclusion is simply that getting linings to perform well on tank or vessel surfaces is difficult. Although the linings themselves, like paints, are the results of a lot of expensive development and testing, and are generally of high quality, the practice of their pre-preparation and application is not always of such a high standard. The result is that many fail well before their design life and some components require continual relining throughout their life to keep corrosion to a manageable level.
12.3 So where does API 652 fit in?
If you were to put together a document to try to address the problems described above you would have to:
- List the problems that cause linings to fail.
- Decide ways to minimize them, particularly relating to pre-cleaning and application.
- Specify some basic guidance on lining choice and types.
- Ensure QA procedures and record systems were put in place to check the job was done properly.
- Recommend regular inspections.
Eventually, after consulting various people and bodies (and listening to shiny presentations from lining technical sales teams), you would end up with something that looks surprisingly like API 652.
12.3.1 The contents of API 652
Figure 12.1 shows the contents list of API 652. It is a short document of 15–16 pages providing a fairly generic introduction to the subject of tank linings. It contains mainly text and tables with a few figures. There are no calculations. The chapter headings are in a logical order, starting with explaining why bottom linings are required and then progressing through lining selection, surface preparation, application, inspection and repair.
12.3.2 API 652 section 3: definitions
There are a few definitions here that are specific to US and API codes in particular. Watch out for them in exam questions:
An anchor pattern (API 652 definition 3.4) is the strange name given to a surface profile (or roughness) before the lining is applied. It is achieved by shotblasting and looks absolutely nothing like an anchor.
A thick-film lining (definition 3.33) is one with a dry film thickness (dft) of 20 mils (0.020 in) or 0.51 mm or greater. These are frequently epoxy-based, have multiple coats or are reinforced with glass-fibre-reinforced polymer (GRP) fibres.
A thin-film lining (definition 3.33) is one with a dft of less than 0.020 in (0.51 mm).
A lot of chemical-based names appear in the definition list: adduct, amine, bisphenol-A-polyester, copolymer, iso phthalic polyester, polyamide, phenolic and a few others. Do not worry too much about these – their definitions say what they are, and they generally only appear in open book questions (and then not very often).
12.3.3 API 652 section 4: corrosion mechanism
For historical reasons, API 652 contains descriptions of a few of the corrosion mechanisms that can give rise to the need for a bottom lining in the first place. These supplement the more detailed descriptions provided in the main damage mechanism code API 571. There is nothing that contradicts API 571, just one or two additional points that are expressed in a slightly different way.
- Chemical corrosion
- Concentration cell (crevice) corrosion under deposits or mill scale
- Galvanic cell corrosion
- Sulphate-reducing bacteria (SRB) corrosion
- Fretting (rubbing) corrosion
Of these, SRB corrosion contains the most detail (API 652 (4.5)). It is caused by bacteria colonies depositing on the steel, resulting in concentration cell pitting or chemical attack caused by the reduction of sulphate to sulphide. This is a common mechanism found on petroleum tank bottoms and the lower shell area.
12.3.4 API 652 (section 6): tank bottom lining selection
This is a long section for API 652, containing a lot of generic information about the suitability, advantages and disadvantages of the three main types of lining: thin-film, thick-film unreinforced and thick-film reinforced. There is information contained in here suitable for both open- and closed-book exam questions. Figure 12.2 shows a summary of some of the more useful points, expressed in the form of a table. Remember that all this is mainly with reference to linings applied to the inside (product-side) of the tank bottom. Shells can be lined also, but API 652 is predominately about bottom linings, as its title suggests.
Remember that API 652 only provides generic information. There are several hundred proprietary coating system products available. These differ a lot in chemical content, longevity and price. Manufacturer’s datasheets for these products are freely available and contain excellent technical detail.
12.3.5 Preparation and application
Surface preparation by blasting is a critical part of the lining application procedure. Figure 12.3 shows the details. The required level is SSPC SP-5 ‘white metal’ finish, with the lower ‘near-white metal’ level SSPC SP-10 being acceptable in some situations. Equally as important is cleaning to remove surface containments, typically chemicals, common on items that have been in use in a contaminated environment. This is done using a combination of chemical cleaning,
steam cleaning and rinsing with demineralized water. Note the following important points:
- Blasting should not be performed if the temperature of the steel surface is less than 37 ℉ (3 ℃) above dewpoint or if the relative humidity is greater than 80 %.
- The profile ‘anchor’ pattern required is typically 0.0015– 0.04 in (38–102 μm) and should be sharp and angular.
Following on from surface preparation, API 652 (8.2–8.5) gives guidelines for the lining application process. These are useful practical points as linings are prone to peeling off in early service if they are not applied properly. Note the following, which are valid points for either open- or closed book exam questions:
- Excess lining thickness, as well as insufficient thickness, can cause failure of linings (8.5).
- Curing times and temperatures must be in compliance with the lining manufacturer’s datasheets (8.5).
- Wet film thickness (wft) can be checked during application to standard ASTM D4414.
- After drying, dry film thickness (dft) can be checked to standard SSPC PA2.
12.3.6 Testing of existing linings (API 652 section 10)
All newly applied and existing linings require visual examination to check for pinholes and areas of obvious low or excessive coverage. The most common procedure is the spark (‘holiday’) test. Figure 12.4 shows the details. A high voltage is applied between the parent metal under the lining and a wire brush or sponge passed over the top surface of the dry lining. Any pinhole or discontinuity in the lining (known mysteriously as a ‘holiday’) causes current to flow, resulting in a visible blue spark and audible alarm.
Thin-film lining tests normally use a low voltage (67.5 V) supply and a wet sponge detection technique. Thick-film reinforced linings can use as high as 20 kV. The lining manufacturer’s datasheet will specify the correct voltage to be used. Insufficient voltage will not detect fine pinholes, whereas excessive voltage may actually break down weak areas of lining, causing additional discontinuities that were not there before.
Controversially, API 652 (10.5e) says that holiday testing is typically not recommended for linings that have been in previous service ‘since the pressure of moisture in the film can cause damage when exposed to the voltage’. Some lining manufacturers believe their lining specification have superseded this problem – but they do not write the API exam questions.
12.4 European surface preparation standards
In Europe, most industries use grades of metal shotblasting preparation based on the Swedish standard SIS 055900. This specifies the ‘SA’ shot blast grades, of which the most commonly used are:
- Grade SA-3. A pure ‘white’ finish (a near-perfect blasted finish with no tarnish or staining).
- Grade SA-2 1/2 . An ‘almost perfect’ blasted finish, but one that allows a minor tarnish or stained appearance. This grade SA-2 1/2 is the most common grade specified for preparation for painting and linings. It is much easier and cheaper to achieve than grade SA-3, which would be uneconomic for most utility steelwork, tanks and vessels.
- Grade SA-2. A ‘thorough blast finish’ that still has significant surface staining and contamination. This is considered a slightly substandard surface finish that is not suitable as a base for paints or linings. .
- Grade SA-1. A ‘light blast cleaning’ that still has significant surface staining and contamination. This is considered a poor quality surface finish that is not suitable as a base for paints or linings.
API codes do not use these grades. Instead, they use those grades specified by the US Society for Protective Coatings (formerly the Steel Structures Painting Council), known generically as the SSPC grades. These work to the same principles but instead of an SA grade, they are given as an SP grade. The conversions are broadly as follows:
SA grade (used in SSPC SP grade (used by API
Europe) codes and in the USA)
SA-3 White metal finish SP-5
SA-2 1/2 SP-10
No equivalent Power tool cleaning to bare metal
No equivalent Water-jetted finish before relining
No equivalent Solvent-cleaned finish
Section 7 of API 652 specifies the surface grades required for preparation prior to lining. Note how they compare to the grades, and their European comparisons shown above. These comparisons are shown for understanding only; API exams will only mention the SSPC SP grades, not the European SA grades.
Now try these practice questions.
12.5 Tank linings: practice questions
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