Api has not established rules for rounding off itac

API has not established rules for “rounding off”.

ITAC API 653 Summary, 2007 Page 1- 11
Minimum Thickness Calculation for
Welded Tank Shell
(API 653 Section 4.3.3.1)
t
min
= 2.6 (H-1)DG
SE
t
min
= the minimum acceptable thickness, in inches, for each course as
calculated from the above formula; however, t
min
shall not be less
than 0.1 inch for any tank course.
D = nominal diameter of tank, in feet.
H = height from the bottom of the shell course under consideration to the
maximum liquid level when evaluating an entire shell course, in ft; or
= height, from the bottom of the length L (see 4.3.2.1) from the lowest point
of the bottom of L of the locally thinned area to the maximum liquid level,
in ft; or
= height from the lowest point within any location of interest to the
maximum liquid level, in ft.
G = Highest specific gravity of the contents.
S = Maximum allowable stress in pounds per square inch; use the
smaller of 0.80Y or 0.429T for bottom and second course; use the
smaller of 0.88Y or 0.472T for all other courses. Allowable shell stresses
are shown in Table 4-1 for materials listed in the current and previous
editions of API Std. 12C and Std. 650. Note: For reconstructed tanks,
S shall be per the current applicable standard.
Y = Specified minimum yield strength of the plate; use 30,000 psi if not
known.
T = The smaller of the specified minimum tensile strength of the plate
or 80,000 psi; use 55,000 psi if not known.
E = Original joint efficiency for the tank. Use Table 4-2 if original E is
unknown. E=1.0 when evaluating the retirement thickness in a
corroded plate, when away from welds or joints by at least the
greater of one inch or twice the plate thickness.

ITAC API 653 Summary, 2007 Page 1- 12
FOR UNKNOWN MATERIAL
Maximum Allowable Stress (in PSI) Use the smaller of:
NOTE:
The Third Edition of API 653 has added Table 4-1, Maximum Allowable
Shell Stresses (not for use for reconstructed tanks). This will make stress
calculations much easier.
Sample problem for minimum thickness of a welded tank shell.
An inspection of a welded, 138 foot diameter tank, 50 feet tall, 48 feet fill height shows
some scattered pitting in the first course. What is the minimum shell thickness required
for this tank, if the specific gravity of the product is 0.9?
t
min
= 2.6 (H-1) DG
SE
t
min
= ?
D = 138'
H = 48'
G = .9
S = 23,600 (from Table 4-1)
E = 1
t
min
= 2.6 (48-1)((138)(.9)
23,600(1)
15,177.24
23,600
t
min
= .643"
First or Second Course
(Yield)
.80Y = .80 X 30,000 = 24,000
or
(Tensile)
.429T = .429 X 55,000 = 23,595
Other Courses
(Yield)
.88Y = .88 X 30,000 = 26,400
or
(Tensile)
.472T = .472 X 55,000 = 25,960

ITAC API 653 Summary, 2007 Page 1- 13
Practice Problem
t
min
= 2.6 (H-1) DG
SE
A 190' diameter tank has a pit that measures 5/16" deep in the first course, what is the
min t, if the fill is 42 feet and the specific gravity is 0.6? (The pit is not in a weld seam or
HAZ.) The material is unknown.
S = Table 4-1
E = Original joint efficiency for the tank. Use Table 4-2 if original E is unknown.