Api 620 reinforcement pad calculation 1

[shakila21]

reinforcement pad is mentioned in Api620 5.16.there is a part which I do not understand.
in figure 5-7 there is a formula for pad length: greater value of (d+2c)/2+ (tn-c)-(tw-c) and (d+2c) but 5.16.3.2 says
a) A distance equal to the diameter of the opening after corrosion; in the case of noncircular openings, a distance
equal to the corresponding clear dimension is substituted for the diameter of the opening.
b) A distance equal to the radius of the opening after corrosion plus the thickness of the nozzle wall plus the thickness of the tank wall, all taken in the corroded condition; in the case of noncircular openings, a distance equal to the corresponding half chord is substituted for the radius of the opening.

these 2 parts doe not match. in figure the term (tw-c) is subtracted but in 5.16.3.2b they offer to add (tw-n).in figure the first term is (d+2c)/2 but 5.16.3.2b says "A distance equal to the radius of the opening after corrosion" which means (d/2)-C

on the other hand 5.16.3.2a is also does not match with (d+2c)

can any one please clarify this for me?

I am confused and don`t know how to calculate reinforcement pad dimensionbased on Api 620.

 

[Geoff13]

In the future corroded condition your shell opening will have an inside diameter of (d+2c). Thus the radius to the outside of the permitted reinforcing area is (d+2c). This matches the lower dimension in Figure 5-7 and the clause 5.16.3.2.a rules. This is typically the greater dimension for "large" openings, which will be from about 3" or 4" and above.

For clause 5.16.3.2.b we start with the nozzle corroded inside radius (d+2c)/2 and add on the corroded neck thickness (tn-c) and the corroded wall thickness (tw-c). I agree with you that Figure 5-7 is incorrect using a minus sign for the (tw-c) term. I guess I've always used the words in 5.16.3.2 and never looked at Figure 5-7 closely enough. This dimension will typically be greater only for "small" nozzles (2" or 3") using couplings instead of pipe necks.

The words in 5.16.3.2.b make sense (adding tw-c) whereas Figure 5-7 (subtracting tw-c) doesn't. I wouldn't hesitate to follow the words in the 5.16.3.2.b and ignore the figure.

I suggest you forward what you've noticed to the Code Committee in accordance with Annex A.5, unless someone jumps in this discussion and explains how Figure 5-7 is correct.

 

[shakila21]

Thank you Geoff for your complete description. I will go to let them know.