08:32 Aug042004 Re: corrosion rate If it will be helpful for you accept the transcription bellow, only as a guideline. Regards Luis Marques Determination of MAWT Minimum Acceptable Wall Thickness (MAWT) shall be calculated in accordance with the ASME VIII equations. Corrosion allowance should not be considered when using these equations. Example: For a cylindrical shell (longitudinal joint): MAWT = PR/SE0.6P (1) P = Design pressure (psig) R = Inside radius of vessel (inches) S = Stress value (psi) from table UCS23 (ASME VIII) E = Joint efficiency factor from table UW12 (ASME VIII) Determination of Corrosion Rate From API 510, the following equations shall be used to determine long term (CRLT) and shortterm (CRst) corrosion rates. Corrosion rates are expressed in mils (thousandths of an inch) per year (MPY). CRLt = 1000(tItL)/ time (years) between initial and latest inspection) (2) CRLt = 1000(tPtL)/ time (years) between previous and latest inspection) (3) Where: TL = Thickness of wall at most recent survey(inches) TP = Thickness of wall recorded in previous survey (inches) TI = Thickness of wall recorded in baseline survey (inches) Determination of MAWPCURRENT The current Maximum Allowable Working Pressure (MAWP) shall be calculated using the following ASME VIII equations. Exemple: For a cylindrical shell (long seam): MAWPCURRENT =SEt/ R+0.6t (4) S = Stress value (psi) from table UCS23 (ASME VIII) E = Joint efficiency factor from table UW12 (ASME VIII) R = Inside radius of vessel (inches) Forecast Inspection Interval It is possible to make a decision as to whether inspection frequency may be increased or decreased, or whether the vessel should be repaired, by considering the worst case corrosion rate and estimating the wall thickness at the date of next inspection. The worstcase corrosion rate may be either CRLT or CRST. Estimated wall thickness loss at date of next inspection may be calculated in accordance with API 510 using equation (5) below. tEST = (CRwc/1000) X (Years to next inspection) (5) Where: tEST = estimated thickness loss at the date of next inspection (inches) CRWC = worst case corrosion rate (MPY) Determination of Remaining Life Remaining life in years shall be calculated for each vessel using the equation below, REMAINING LIFE (YEARS) = 1000(TL MAWT)/CRwc (6) If remaining life < time to next inspection then one of the following actions must be taken: 1) Inspection frequency must be increased to closely monitor deterioration prior to failure 2) The vessel must be repaired or retired. Luis Marques
