CR and Insp Interval (Chap 1-3 / A-1.3.11.13)

Question 1

What is the definition of metal loss?

Answer 1

tprevious-tactual

Question 2

How to calculate CR?

Answer 2

tprevious-tactual/time
time = short = last previous thickness measurement
time = long = before the previous thickness measurement

Question 3

How to calculated remaining life?

Answer 3

tactual - tmin/CR

Question 4

How to calculated remaining CA?

Answer 4

tactual - tmin = RCA

Question 5

External Inspection Interval

Answer 5

-5 years or RCA/4N, whichever is less

N=Corrosion rate

Question 6

External UT interval

Answer 6

-15 years or RCA/2N, whichever less

if CR is not known, than 5 years

Question 7

Internal Inspection, and max ?

Answer 7

Shall not exceed 10 years unless otherwise in table 6.1, max is 20 year w/o RPB, 30 years w/ RPB..

Question 8

What is table 6.1 in API653 (tank safeguards)

Answer 8

1. fiberglass reinforce lining of the product side of tank = 5 years
2. installation of internal thin-film = 2 years
3. CP of the soil side = 5 years
4. release prevention barrier = 10 years
5. bottom CA greater than 0.15” = (actual CA / 15 mpy)
6. Bottom is SS = 10 years

Question 9

How to determine if the shell corroded area is acceptable for continued service?

Answer 9

(i) t1 > tmin
(ii) t2 > tmin (.6)
fit for service (need to add CA to tmin for FFS until next inspection)

Question 10

What is the formula for criteria length?

Answer 10

L=3.7 (Dt2)^.5
D=ft
t2= lowest thickness point of a corroded area
L = cannot exceed 40 inches if it is need to repick the area.
-need at least 5 equally space points on L

Question 11

How to calculated tmin (portion of a shell courses)

Answer 11

tmin=2.6(HDG)/SE 
where 
D=ft
H=ft (height of point taken to liquid level)
S= stresses found in table 4.1.

Question 12

How to calculated tmin (entire shell courses)

Answer 12

tmin=2.6(H-1)DG)/SE 
where 
D=ft
H=ft (height of point taken to liquid level)
S= stresses found in table 4.1.

Question 13

how to get t2?

Answer 13

lowest thickness point in a corroded area

Question 14

how to get t1?

Answer 14

avg thickness of the area in a vertical plane.

Question 15

what if your tank is not FFS?

Answer 15

• corroded or damage area shall be repaired
• reduce the allowable liquid
• retire tank
• reduce the inspection cycle

Question 16

How to calculate the height allowed?

Answer 16

H=SEtmin/2.6 D

Uses lowest thickness value that does not meet criteria -CR to obtain tmin.

Question 17

What is the min bottom plate thickness ? (3)

Answer 17

1. 0.10” - if no tank bottom/foundation design with no mean of detection /containment
2. 0.05” if there is a means detection/containment
3. 0.05” if the reinforce lining > 0.05” thickness

Question 18

What is the min bottom plate thickness formula?

Answer 18

MRT= (min RTbc-RTip) - Or (StPr +UPr)
RTbc = min remaining thickness of bottom side
RTip = min remaining thickness of top side
UPr=CR of soil/bottom side , 0 if CP
StPr=CR of top side, 0 if liner in good condition
Or=in service interval of operation in years

Question 19

What is min of critical zone thickness?

Answer 19

smaller of

1. 1/2 the original bottom plant thickness (exclude CA)
2. 50% of tmin of the lower shell course (not less than 0.1)

Question 20

Where is the critical zone of a plate?

Answer 20

3”

Question 21

annular plate min thickness

Answer 21

if sg <1.0, follow table 4.5 + CA

Question 22

How to know if the tank can last till next cycle of 10 years?

Answer 22

use formula MRT, and calculated for Or. MRT will based on tmin calculated(based on critical zone, annular plate, or min thickness table)

Question 23

What can use RBI for?

Answer 23

to establish the interval for subsequent inspection interval

Question 24

What is stress analysis for?

Answer 24

assess the annular palates or the critical zone of the sketch plates?