VGuarino98
Materials
- Nov 8, 2021
- 1
Hello all,
I was wondering if anyone would be able to help explain the MRT calc in API 653, under section 4.4.5.
According to the equation:
"StPr is the maximum rate of corrosion not repaired on the top side"
"UPr is the maximum rate of corrosion on the bottom side. To calculate the corrosion rate, use the minimum
remaining thickness after repairs."
My confusion stems from the fact that the two corrosion rate variables (StPr and UPr) tell you you calculate a corrosion rate after your floor has been repaired.
Example
You are performing an inspection on a tank floor and find a pit that is .05 inches deep (only product side corrosion, no soil side so Upr=0). Given the fact that the tank has means to provide detection and containment of a bottom leak, the minimum bottom plate thickness is permitted to be .05 (per Table 4.4). If the tank floor was originally .25 inches thick, that would mean that you have .2 inches of floor remaining (.15" until you reach your MRT value), and your MRT would be .05 inches (given the fact the tank has a leak detection system). The tank has been in service for 10 years.
Option A
You do not weld fill the pit.
MRT= .05"
UPr = 0"
RTbc = .25" - 0" = .25"
RTip = .25" - .05" = .20"
StPr= .05"/10 years = .005 inches/year
Option B
You do weld fill the pit.
MRT= .05"
UPr = 0"
RTbc = .25" - 0" = .25"
RTip = .25"
StPr= .0"
Is option B correct? It would make sense that RTip goes back to .25", but I am confused on StPr. Would you now say you don't have a corrosion rate since your only defect was repaired? I would assume that when you are calculating your allowable in-service interval(Or) you would want to use the corrosion rate from before the floor was repaired. If you theoretically fixed all of your thin spots and returned your floor to nominal thickness, the code makes it sound like you would have no corrosion rate since you repaired your defect (since the verbiage is "maximum rate of corrosion not repaired on the top side").
I was wondering if anyone would be able to help explain the MRT calc in API 653, under section 4.4.5.
According to the equation:
"StPr is the maximum rate of corrosion not repaired on the top side"
"UPr is the maximum rate of corrosion on the bottom side. To calculate the corrosion rate, use the minimum
remaining thickness after repairs."
My confusion stems from the fact that the two corrosion rate variables (StPr and UPr) tell you you calculate a corrosion rate after your floor has been repaired.
Example
You are performing an inspection on a tank floor and find a pit that is .05 inches deep (only product side corrosion, no soil side so Upr=0). Given the fact that the tank has means to provide detection and containment of a bottom leak, the minimum bottom plate thickness is permitted to be .05 (per Table 4.4). If the tank floor was originally .25 inches thick, that would mean that you have .2 inches of floor remaining (.15" until you reach your MRT value), and your MRT would be .05 inches (given the fact the tank has a leak detection system). The tank has been in service for 10 years.
Option A
You do not weld fill the pit.
MRT= .05"
UPr = 0"
RTbc = .25" - 0" = .25"
RTip = .25" - .05" = .20"
StPr= .05"/10 years = .005 inches/year
Option B
You do weld fill the pit.
MRT= .05"
UPr = 0"
RTbc = .25" - 0" = .25"
RTip = .25"
StPr= .0"
Is option B correct? It would make sense that RTip goes back to .25", but I am confused on StPr. Would you now say you don't have a corrosion rate since your only defect was repaired? I would assume that when you are calculating your allowable in-service interval(Or) you would want to use the corrosion rate from before the floor was repaired. If you theoretically fixed all of your thin spots and returned your floor to nominal thickness, the code makes it sound like you would have no corrosion rate since you repaired your defect (since the verbiage is "maximum rate of corrosion not repaired on the top side").
