Debasis 1. Answering to you first qeustion Basically, mechanical designer to API code of a tank calculates at the minimum thickness for the particular application. Immaterial of the calculated thickness arrived for t min for a particular material and for a particular service duty (design pressure and design temperature), designer provides a minimum thickness from structural rigidity, welding strength, wind load and seismic load considerations. Immaterial of any thickness, mechanical and process design calls for to arrive at calculated thickness, designer looks at the nearest thickness and size of plate available from sheet and plate material manufacturers. Plate manufacturers mostly make only standard or commonly used thickness and plate /sheet size. No tank, even if it is used for atmospheric operation, should be made less than 6 mm thickness for the first shell course - taking care of C.A., wind load and seismic load and foundation anchorage load + weight load from other shell courses and roof plate and roof structure loads added to the above. 2. Answering to your second question You have to add C.A after arriving at the t min and then get the nearest plate thickness range in which manufacturers supply plates. Here also you cannot go below 5 or 6 mm thickness for the first shell course and for tank bottom plates for tank constuction to API 650 0r API 653 codes. Trust this answers your querry C.V.Srinivasan Nishi Engineers Pvt Ltd Chennai India June22 E-mail:nishi@vsnl.com >I have question pertaining to API 653 interpretations : > ><1> Para 4.3.3.1 of API 653 for calculation of tmin (minimum >acceptable thickness)is limited to tanks with diameters 200 >ft or less. This also states that tmin for any shell course >shall not be less than 0.1 inch. I was calculating tmin for >a small dia, small ht. tank and always I find tmin to be >.008in which indicates that tmin formula for my case is not >applicable. >The question is what is the limiting ht. and dia below which >tmin formula is not applicable? > ><2> For interpretation of para 4.3.2.1 (e) whether should I >first add corrosion allowance with tmin & 60%tmin to >evaluate t1 & t2? >regards and awaiting for expert interpretations.