API 653 Exam Chapter 11- Hydrostatic Testing and Brittle Fracture
API inspection codes in general have a strange love–hate relationship with hydrotesting. Storage tanks are no exception – on one hand API 653 explains how and when it should be done, but this is followed up by a long list of valid scenarios for exemption. Unlike pressure equipment items, the underlying message is that (excluding new or fully reconstructed tanks) a hydrotest on a storage tank is something that you choose to do, rather then being forced to do so by API 653.
11.1 What is the subject about?
Code coverage and API exam questions are in surprising agreement on their coverage of hydrotesting. They are concerned with:
- WHY you would want to do a hydrotest.
- WHEN it is necessary (and when you do not need to bother).
- HOW the test is done, once you have decided to do it.
Hydrotesting fits well into the API exam question mix because of the way that coverage is spread around several of the codes in the API 653 body of knowledge (BOK). It is covered in API 653 (12.3), API 650 (7.3.6) and also has links to the brittle fracture section of API 571. The following list of hydrotest-related subjects can be shoe-horned into exam question format:
Allowable stress (S) levels. A tank scheduled for hydrotest must have a higher allowable S value than one which is not, to accommodate the increase in hoop stress caused by the water compared to its normal oil product (see Fig. 6.5 of this book).
Responsibilities, i.e. who decides if a hydrotest is required in various repair/alteration scenarios. .
Exemptions, and when they are allowed. This fits together with the question of responsibilities and who decides what.
There are enough basic principles, specific code clauses and data tables here to feed both open- and closed-book-style exam questions. That is why they regularly appear.
11.2 Why? The objectives of a hydrotest
Neither API 653 nor API 650 go into much detail but, simplistically, a hydrotest is:
- A code test for leaks and .
- A strength test (of sorts) plus .
- A chance for brittle fracture and excessive foundation settlement to happen, if they are going to.
It can never be a full test of fitness for service, because in reality there are lots of defects and problems that a hydrotest will not discover. Water is not good at finding its way through very fine cracks, for example, and welds in some locations can contain huge internal defects without the remotest chance that they will result in structural collapse of the tank during a static-head-only hydrotest. This is well understood, which is why API 653 section 12 lists hydrotest as only one type of NDE, among all the other (ASME V) techniques used.
Perhaps the most agreed objective of hydrotesting a tank is to check for excessive foundation settlement on tanks laid directly on a soil, sand or rubble base, i.e. without a full concrete base, or at least a concrete ringwall installed under the shell.
11.3 When is a hydrotest required?
This is straightforward.
A hydrotest is only mandatory for:
- A newbuild tank: API 650 (7.3.6)
- A reconstructed tank: API 653 (12.3.1)
A hydrotest is required for a tank that has undergone major repairs or major alterations (API 653 definition 3.18) but it can be exempted if:
Firstly it has been agreed by a tank design engineer and the owner/user (18.104.22.168a and b). and then, either
- It passes an FFP (fitness-for-purpose) assessment. or
- It meets a quite long list (nearly two pages of API 653 (22.214.171.124) to (126.96.36.199)) of criteria relating to material, type, thickness, stress and weld location.
For non-major repairs or alterations, a hydrotest is not required. It actually says this in API 653 (188.8.131.52).
Excluding new-build and reconstructed tanks then, where a hydrotest is mandatory, the answer to whether a hydrotest is required after a repair or alteration simply hinges on whether the repair or alteration is major or not, according to API 653 definition 3.18. If it is major then it requires a test unless it can qualify for any of the exemption routes above.
11.3.1 Why would you want to find an excuse not to hydrotest?
Because it requires a large amount of treated water, which then has to be legally disposed of, meeting environmental standards, etc. It takes a lot of time, is expensive and the tank may have to be cleaned and dried out afterwards. Coupled with this it is only a partial test of fitness for service of a tank and other NDE techniques are better at finding fine defects. Hydrostatic Testing and Brittle Fracture 199 This goes a long way to explain why API 653 allows exemption in many cases.
11.4 Avoiding brittle fracture API 653 does not set out to be a textbook on brittle fracture – it does not even warrant a separate mention in the definitions section 3. That is fine, because the mechanisms of brittle fracture are more than adequately explained in API 571 along with its appearance, prevention and mitigation options. What API 653 (section 5) does do is set out a well-rehearsed methodology of preventing the specific occurrence of brittle fracture on storage tanks. In many cases this is set against a scenario of the tank or its operating condition having changed in some way with the potential of increasing the risk of brittle fracture, causing wholesale fracture of the shell, with the release of all the contents.
11.4.1 What causes increased risk of brittle fracture?
- Brittle material. Many cheaply produced materials lack toughness (tough is the opposite of brittle). This is actually the result of low ductility (because ductility and toughness are not precisely the same thing).
- Thickness. The thicker a material section is, the more brittle (less tough) it is. Strange but true. Lots of textbooks will explain this to you if you need to know why.
- Temperature. Simple steels get more brittle as they get colder. Effects vary with the specific material but 7–10 8C is used as a rough benchmark for a temperature below which brittle fracture (rather than ductile failure) may become a risk.
- Stress. All failures need to be caused by stress of some sort. Brittle fracture is more likely at higher stresses. Simplistically, dynamic impact-induced stresses are the worst although more statically induced membrane stresses can also cause it, particularly during fast product filling or unexpected wind or seismic loadings.
These four factors can act singly or in combination (see Fig. 11.1) so measures need to be in place to restrict them all, to keep the chance of brittle fracture low.
11.4.2 When does API 653 section 5 worry about brittle fracture?
API 653’s views owe more to accumulated experience and intuition than advanced metallurgical theory – proven or otherwise. Clause 5.2.2 sets out four scenarios where it may happen and then the rest of section 5 goes on to describe a methodology to assess which you have. It is best to accept API’s view of this, whether you think it is oversimplified or not, because it forms the basis of their exam questions. API 653 (5.2.2) says that you should be worried about the risk of brittle fracture if a tank is being:
- Hydrotested for the first time.
- Filled for the first time in cold weather.
- Changed to a service (product, site environment or both) where it will see a lower temperature than previously.
- Repaired, altered, or reconstructed.
Conversely, if a tank is not in one of these four scenarios it is reasonable to expect that the risk of brittle fracture is minimal.
11.4.3 If there is a risk, what do you do?
Easy, do a hydrotest. If the tank passes without fracture, then its ‘brittle fracture integrity’ is proven and the problem has effectively gone away, even if the remainder of the scenario conditions are still there. Now you can see the main purpose of API 653 section 5 – to provide a methodology to decide whether or not you need to do an ‘enforced’ hydrotest, to prove a tank’s resistance to brittle fracture.
11.5 Is a hydrotest needed? API 653 flowchart (Fig. 5-1)
In common with many other API documents, API 653 expresses decision-making activities in the form a flowchart (see API 653 Fig. 5-1). You either like flowcharts or you do not. This one is very easy – it just looks complicated on first viewing – see Fig. 11.2 here which is a simplified version. The full API 653 Fig. 5-1 version has 11 steps, each of which is actually explained in the text of section 5.3.
Starting on the left of the flowchart, the objective is to spend as little time on the flowchart as possible by taking the first possible exemption from doing a hydrotest that you can.
Failure is indicated by being unable to escape from the flowchart before arriving in disgrace at the right-hand end, where the only real practical option is to do the hydrotest, to prove that brittle fracture will not occur. The exemptions available in the flowchart are all based on API code experience – and support the idea that there is no advantage to be gained from doing a hydrotest if you really do not need to.
If you prefer things to be presented another way, then the list below shows all the exemptions. So there is no need to do a hydrotest if you can meet any one (not all) of the following criteria:
- The tank was built to API 650 7th edition or later, so the material will be sufficiently tough (non-brittle). or .
- It has already been hydrotested, and did not fail then. or .
- The shell plate is 1/2 inch thick or less. or
- It will not see temperatures lower than 60 ℉. or
- It will not see hoop stresses greater than 7000 psi. or
The material is shown as exempt (by API 653 Fig. 5-2 if the grade is unknown or API 650 Fig. 4-1 if the grade is known). If you look at this figure in API 650 (see chapter 7) you will see how some grades of steel can keep their tough (non-brittle) characteristics down to beyond -30 ℉, but you need to be able to confirm that the material grade is the correct one. or
The tank has already been proven safe, when full at the lowest one day mean average temperature (LODMAT) for the geographical area where it is located (see API 650 Fig. 4-2).
If a tank cannot meet any one of the above criteria, you need to do a hydrotest. Figure 11.3 shows some important technical and procedural points.
11.5.1 Exam questions?
Yes. The API 653 exam regularly contain questions on both the generic API 571 aspects of brittle fracture and the tank specific aspects from API 653 and 650 explained above. Expect both open- and closed-book variants. The split may appear a bit random, but the questions themselves are normally fairly straightforward.
Now try these practice questions.
11.6 API 653: hydrotesting: practice questions
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