corrosion allowance for skim tank shell plate (oily water)

[Engmarwan]

Dear gentelmen,

i am doing a safety sheet for an existed tank (20000bbl). i already have the drawing and all design data, however shell plate (courses) corrosion allowance is not there. i was trying to calculate it according to API650 but i get (negative number).

the formula which is fit to our case is;

td = 4.9*D(H-03)SG/Sd + CA

FOR the H , should i use the total tank liquid level highet or only each course highet. ??

can you please give a general advice about what could be wrong that result in a negative CA. ??

YOU HELP IS HIGHLY RESPECTED.

 

[JStephen]

H would be the depth from the bottom of that particular shell course to the design liquid level. The design liquid level could be the top of the shell, could be limited by overflow slots, could be limited by floating roof travel, or could be arbitrarily specified at some height. Make sure you have the right specific gravity and right steel grade. Some shells may be a little thinner if designed by the variable design point method.

 

[mariog123]

About "H", please see 5.6.3.2 of APi 650

H is the design liquid level, in m;
= height from the bottom of the course under consideration to the top of the shell including the top angle, if
any; to the bottom of any overflow that limits the tank filling height; or to any other level specified by the
Purchaser, restricted by an internal floating roof, or controlled to allow for seismic wave action

I would define H as hydraulic head at the bottom course (hydrostatic head+ [gas] pressure head); in case there is a floating roof, you should include a term due to the floating roof weight.

 

[Engmarwan]

thank you so much gentelmen.

i could understand from what you said that i will have differant corrosion allowance for each course. is this right??

 

[mariog123]

Engmarwan,

What I understood from your words was you tried to obtain clarifications on the "H" that API 650 considers in calculation of the minimum thickness of shell plates.
The subject is covered by API 650/ 5.6.3.2 "The required minimum thickness of shell plates shall be the greater of the values computed..." and gives the formulas for calculation. Of course the Manufacturer can consider shell plates of thickness greater than minimum, and in this case the "extra" thickness can be considered as added to the CA. However the problem is not so simple because the actual (increased) thickness may be a result of other criteria (for example seismic calculation).

API 650 requires in 5.3.2.1 "The Purchaser, after giving consideration to the total effect of the liquid stored, the vapor above the liquid, and the atmospheric environment, shall specify in the Data Sheet, Table 1 and Table 2, any corrosion allowances to be provided for all components, including each shell course, for the bottom, for the roof, for nozzles and manholes, and for structural members."
API 650 does not consider forensic calculations for existing tanks- but you can do such calculations, following all API 650 criteria (and here it is other discussion, which edition you would follow).

 

[mariog123]

Please see also API 653/ 4.3 Tank Shell Evaluation procedure.

Please consider guidances in 4.3.3.5:

The thickness determinations of 4.3.3.1, 4.3.3.2, and 4.3.3.3 consider liquid loading only. All other loads shall also be evaluated according to the original standard of construction; and engineering judgment shall be used to evaluate different conditions or new information. As applicable, the following loadings shall be taken into account:
a) wind-induced buckling;
b) seismic loads;
c) operation at temperatures over 200 °F;
d) vacuum-induced external pressure;
e) external loads caused by piping, tank-mounted equipment, hold down lugs, etc.;
f) wind-induced overturning;
g) loads due to settlement.