By Carlos Molina
I keep studying about API 653. I have to be a re-certified API 653 Inspector before september 2015. I obtained my certification back in september 2012, with only one year experience in tank repair and, as I did with my english language skills, most of it was self-taught (I took a course, though). Now I have to regain all the knowledge again, which is really easy if you know how to learn it.
I hope you are studying well for your own exam, taking advantage of what I or others offer, and if you are not, remember it is scheduled for March 15, 2015. So you better wash out of the holidays inertia and start studying right now.
Today we are going to talk about hydrostatic testing in API 653, more specifically about hydrostatic height calculations. Some of the questions of the exam are from this subject.
WHAT IS AN HYDROTEST?
Hydrostatic testing is (or a least should be) done in every new welded tank for oil storage and it is mandatory for any tank that has been under a major alteration, according to API 653.
And as we are students, let´s remember the definition of hydrotest of API 653.
As it says, for new or repaired tanks, the purpose of hydrostatic testing is to demonstrate the tank´s fitness for service, and the better you test, the less risk you have once the tank is in operation.
If you are planning to take the exam, you should be familiar with all of the requirements of API 650 and 653 regarding hydrotesting. But right now we will concentrate in calculating hydrostatic test height.
HOW TO CALCULATE HYDROSTATIC TEST HEIGHT
For a tank that has been in operation, several things can happen that may highlight the need for an hydrostatic test
*If the tank is going to be used for a new, more sever service. That means, when the liquid that will be stored has a higher specific gravity than the current stored product.
*When there have been repairs. Maybe your repairs are perfect, but some corrosion remains in other areas of the tank.
Determination of hydrostatic test height Ht, when you have calculated a controlling thickness for an entire shell course, can be achieved solving for the following equation, where Ht is the height from the bottom of the shell course under consideration to the hydrostatic test height
Determination of hydrostatic test height Ht, when you have calculated controlling thickness for a locally thinned area (I haven´t treated the subject of controlling thicknesses here so far, my bad) can be achieved solving for the following equation, where Ht is the height from the bottom of the length, L, (see 184.108.40.206) for the most severely thinned area in each shell course to the hydrostatic test height in feet
So, hydrostatic test height depends on 4 variables, which are.
St is the smaller value between fractions of yield strenght or tensile strenght, or the maximum allowable hydrostatic test stress.
E is the joint efficiency. Look how to find joint efficiency here
Tmin is the controlling thickness
D is the diameter of the tank.
Let´s see an example of height calculation. I am assuming you have some background on the formulas of API 650 and API 653.
A tank will be subjected to hydrostatic testing after repairs. After some study, the inspector decides to run calculations for hydrostatic test height over the first shell course and over a locally thinned area close to a vertical seam, 36 inches high in the 4th shell course. Steel is A36 with Y = 36000psi and T=58000psi. Shell courses are 6ft high and the tank is 48ft diameter. First two courses were welded before 1980, and the other courses were added recently. The controlling thickness for the first shell course is 7mm and for the locally thinned area is 6,35mm. What should be the hydrostatic test height?
Well, for the equations, values are these
Case 1. Lower shell course
E=0,85. See Table 4-2
St = smaller of 0,88Y or 0,472Y, then St = 27376psi
Tmin= 7mm = 0,275in
So, hydrostatic test height will be 15,93m.
Case 2. Locally thinned area in 4th course
E =1. See Table 4-2.
St = smaller of 0,9Y or 0,519Y, then St = 30102psi
Tmin = 6,35mm=0,25in
Hydrostatic test height over the locally thinned area can be 16,7m. (A total height of 23,01m)
Then the tank has a maximum fill height is 15,93m.
HAVE IN MIND FOR YOUR EXAM
You have to be aware that variables in these and other equations of the standards can have different values depending on year of fabrication, purpose of calculation (design or hydrostatic loads), etc. In the exam, you have to be very careful to avoid mistakes, as they always trhow some confusing questions.
You can notice that the presence of different defects in tank shells implies than more than one value of Tmin or Ht should be calculated in a real tank inspection. This kind of dual example analysis is not common in other literature.
And that´s it for now.
As always, I am open to all kind of comments about the information in this site. And lastly, take a look at the following Ad.