By Carlos Molina.
Today we will have a glimpse on reconstructed tanks, as needed to pass the api653 exam. This is the first post of a 2 article series.
First of all, let´s see the definition of a reconstructed tank according to API 653
Any work necessary to reassemble a tank that has been dismantled and relocated to a new site. In short, a reconstructed tank is a tank that has been dismantled and its pieces used together to make a new tank. But this should be made carefully.
WHY IT IS IMPORTANT
I have a confession to make: I have never seen one reconstructed tank in my professional exercise. Maybe one of my readers will give us more tips about it. At this stage, I seriously doubt that I would ever know one reconstructed tank, given that the subject of brittle fracture has prevented a lot of owners from using old tanks for new ones.
But the subject of reconstructed tanks should be important to all of us inspectors, and I know about one case in which use of a reconstructed tank went really wrong. Please look the following excerpt about the Ashland Oil Spill:
On January 2, 1988, a four-million gallon tank was used for the first time after being dismantled (from an Ohio location) and rebuilt in Pennsylvania. It was this tank, holding approximately 3.5 million gallons of diesel oil, that failed and collapsed, dumping nearly 1 million gallons of the oil into a storm sewer that leading to the Monongahela River. At 4:58pm, a worker checked the tank levels and verified the tank was almost full. At 5:02, when the worker was walking away from the tank, he heard a loud boom and turned to find the roof of the tank collapsed. Ashland Co. later took full responsibility for the incident, accepting that they did violate industry standards when reconstructing the tank
(According to this revision of wikipedia)
The failure in this case was initiated in an welded TEE area as a result of carburization as a consequence of cutting or welding operations to the plates. One million people were affected. The fuel contaminated river ecosystems, killing thousands of animals, such as waterfowl and fish. Ashland paid U$8M to the people directly affected by the spill. And although only a U$2.25 million were fined to the company (U$5M in 2015) , the cleanup effort really took U$23million in 2015 dollars. (The amount of fines was lower then. Compare to the U$43billion fined to BP for the Deepwater Horizon oil spill)
Here we again see the devastating effects of brittle fracture, this time in a reconstructed tank. In fact, the Ashland Spill Oil is credited as the triggering event for the creation of the API 653 and other integrity standards. With that in mind, now let´s go on to what you should study for yout API 653 exam.
WHAT TO STUDY
The book of knowledge says this about reconstructed tanks:
The inspector should be able to determine the minimum thickness of the shell of a reconstructed tank. The inspector should be able to:
a) Determine “Sd”, allowable stress for design condition (API-650, table 5-2, API-653, 8.4.2)
b) Determine “St”, allowable stress for hydrostatic test condition (API-650, Table 5-2, API-653, 8.4.3)
c) Calculate “td”, design shell thickness (API-650, 220.127.116.11, for tanks of 200 foot diameter and smaller)
d) Calculate “tt”, hydrostatic test shell thickness (API-650, 18.104.22.168)
RECONSTRUCTED TANKS SHELL DESIGN IS FOUND IN SECTION 8.4 OF API 653
The following are the numerals dealing with allowable stresses is reconstructed tanks
8.4.2 The maximum design liquid level for product shall be determined by calculating the maximum design liquid level for each shell course based on the specific gravity of the product, the actual thickness measured for each shell course, the allowable stress for the material in each course, and the design method to be used. The allowable stress for the material shall be determined using API 650, Table 5-2. For material not listed in Table 5-2, an allowable stress value of the lesser of 2/3 yield strength or 2/5 tensile strength shall be used.
8.4.3 The maximum liquid level for hydrostatic test shall be determined by using the actual thickness measured for each shell course, the allowable stress for the material in each course, and the design method to be used. The allowable stress for the material shall be determined using API 650, Table 5-2. For material not listed in Table 5-2, an allowable stress value of the lesser of 3/4 yield strength or 3/7 tensile strength shall be used.
Knowing this 2 numerals, let´s go on to the determination of allowable stresses.
DETERMINE ALLOWABLE STRESSES
First we will see how to determine allowable stresses for reconstructed tanks. If you were going to study by yourself, it will be easy to get confused and use for reconstructed tanks the table 4.1 of API 653 in search of allowable stresses, but that is a mistake. You should use table 5-2 of API 650 instead
The following are 2 questions of the kind that would appear in the open book section of the exam.
DETERMINE ALLOWABLE STRESSES USING TABLE 5-2 OF API 650
QUESTION: For plates of A283 Gr C steel used in a reconstructed tank, determine Sd (allowable stress for design condition).
ANSWER: You simply go to Table 5.2B of API 650 and read from the sixth column that Sd is 20,000psi.
QUESTION: For plates of A516 Gr 60 steel used in a reconstructed tank, determine St (allowable stress for hydrostatic test condition)
ANSWER: Reading the seventh column, we get a value for St of 24,000psi.
And now let´s take a look at some examples of questions that can be made in the exam.
DETERMINE ALLOWABLE STRESSES USING THE FRACTION VALUES OF SECTION 8.4 OF API 653
QUESTION: For a material not listed in Table 5.2, having Y = 36,000psi and T = 62,000psi, which is the allowable stress for the design condition?
ANSWER: The lesser of 2/3*36,000 = 24,000psi or 2/5*62,000 = 24,800psi, then choose 24,000psi
QUESTION: For a material not listed in Table 5.2, having Y = 30,000psi and T = 55,000psi, which is the allowable stress for the hydrostatic condition?
ANSWER: The lesser of 3/4*30,000 = 22,500psi or 3/7*55,000 = 23,570psi, then choose 22,500psi
CALCULATION OF DESIGN AND HYDROSTATIC SHELL THICKNESS
Calculation of minimum thicknesses for design and hydrostatic conditions in reconstructed tanks follow the same rules for new tanks. Let´s see a summary.
- Joint efficiency E is 1, as in new tanks. That´s why the E variable won´t show up in the formulas. Hey, note that this is for tanks that have been completely cutted apart.
- Values for Sd and St are the same as in new tanks. This has to do with the fact that a new and a reconstructed tank haven´t been subjected to an hydrostatic and hasn´t proved itself against operational conditions. Values of Sd and St for new and reconstructed are LOWER than for existing tanks, for the same reason.
- Compare API 653 8.4.2 to API 650 22.214.171.124 and API 653 8.4.3 to API 650 126.96.36.199. The values of the fractions for Sd an St are the same.
- The values of Sd and St for a reconstructed tank are the same for all shell courses, as opposed to API 653, in which this values vary according to shell height for existing tanks. (See table 4.1 of API 653)
The formulas for thickness calculation for reconstructed tanks are found in 5.6.3 of API 650. That will be the subject of my next post.
HAVE IN MIND FOR THE EXAM
The formulas and tables to use in reconstructed tanks are found in API 650, as they are treated as new tanks.
Thanks a lot for your attention, and see you next time