Spirochete Piping code design takes : variation in pipe manufacturing tolerances ( i.e., 12.5% variation in plus or minus tolerances in thickness for the piping material that is being used for the dia and rated schedule used in the plant) + process designer takes Corrosion allowance for the material (velocity, flow, process fluid, temp and pressure conditions). While taking UT thickness for the first time, since you will not be sure whether it is real loss or this measured value had come within piping code allowed 12.5% variation (especially when measured value comes lower or close to the 12.5% piping allowed tolerances for negative tolerance of 12.5% of thickness), you have to re-monitor depending on the frequency of inspection schedule fixed in the plant. Then you have to establish by comparing the first measured thickness with the second time measured thickness values at the same location to determine if the first measured thickness had exceeded the pipe code allowed tolerance limits. After this evaluation done twice at the same location, take the process allowed C.A factor and then calculate the discard thickness taking into consideration from pressure, temp, and allowable stresses permitted as per piping code for the material used. Based on this system of monitoring you can work out the corrosion rate in the piping and then re-calculate the discard or revised retirement or replacement schedule for the piping used in a particular process cycle. Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India Jan 18, 2009 E-mail: nishi@vsnl.com >I been doing UT thickness gauging of process line in a >petrochemical company, and have been asked a question by a >colleague I cant answer. I understand that on piping the >ASME code allows a 12.5% reduction of wall thickness and >still remain acceptable. I understand this to be true for >new fabrication. > >But wahat about pipes that has been in service for several >years? > >At what point would a pipe be considered to be worn beyond >its service life due to corrosion are required to replace? >Is this a judgment call by the owner/consultant?