Vessel Thickness allowance

[vatk]

Hello.
Could you please advice me which is the allowable deviation in Vessel Thickness in a new vessel?
Corrosion allowance as far as i know prescribes the deviation allowed in used vessel (in most occasions 3mm), but what is the allowance for a newly fabricated and installed in the field vessel?
Thank you for your answer!

[faizol]

Hello.
Could you please advice me which is the allowable deviation in Vessel Thickness in a new vessel?
Corrosion allowance as far as i know prescribes the deviation allowed in used vessel (in most occasions 3mm), but what is the allowance for a newly fabricated and installed in the field vessel?
Thank you for your answer!

Corrosion Allowance is the corrosion added to compensate for the corrosion expected for the lifetime of the vessel.

Example: If a corrosion rate of 0.2mm/year is expected and lifetime of this item is 15 years. So, corrosion allowance at the end 15 years lifetime is 3mm.

usually, 3mm corrosion allowance is applied for Carbon Steel Vessel and 0mm for Stainless Steel or Duplex Vessel.

[vatk]

Thank you for your answer, but let's say that we have a new vessel, in which a pore in the shell is spotted.
After grinding locally, which is the allowable thickness?
How can we measure such a modification?

[faizol]

Thank you for your answer, but let's say that we have a new vessel, in which a pore in the shell is spotted.
After grinding locally, which is the allowable thickness?
How can we measure such a modification?

Generally, thickness of vessel are calculated based on pressure. According to ASME BPVC Section VIII, Div.1 para UG-27, minimum required thickness of shell shall not be less than that computed by the following formula:
Min. thickness (excluded corrosion allowance), t=PR/SE-0.6P, where; P = Internal Design Pressure
R = Inside Radius under Consideration
S = Maximum Allowable Stress
E = Joint Efficiency
Example: Let say required minimum thickness is 2.437mm and corrosion allowance applied for carbon steel is 3mm. Thus, required thickness is 5.437mm and you have stock plate 10mm with pore spotted or pitting on shell face. You ask your people to grind it and after gringding, you got 8.75mm at thinner area. So, plate can be used.
*Thickness at effected (grinding) area can be measure by thickness measuring equipment.

Hope this explaination can be answer your question.
Thank you.

[duazo2009]

Did you grind the entire shell surface area? You're very much concern about corrosion allowance that never restrict any grinding works.