What is the rule of thumb practice for heat exchangers tubes retubing? One should replace tubes when they reach 1/3 of wall thickness? Or tubes should be replaced when they reach %C2%BD of wall thickness? Thanks Luis Marques

Luis Your question is too general without specifying details. Is the tube affected uniformly to have wall thickness of 50% or 33% wall thickness reduction? Is the tube affected only partially at some locations by 50% or 33% reduction in wall thikness? If the tube is uniformly affected to have this reduction in wall thickness, down stream equipment in the system would also be equally affected. Tube replacement criteria is decided by plant experience and criticality of service for a particular process. Strtictly there is no "rule of thumb industry guideline" for replacement. If internal or external corrosion is so severe as to have 50% or 33% wall thickness reduction to cause frequent shut down of plant or process steams, then most plant users would have to weigh on the economics of tube replacement versus plant outage / plant production loss considerations. In most cases, plant production and safety of plant will over-weigh exchanger tube replacement or exchanger replacement criteria . Generally, heat exchanger frabricated to TEMA combined with ASME or BS code may / can permit even upto half of wall thickness reduction for retirement due to the factor of more allowable stresses built-in for ASME or B.S Code practices. (assuming there is uniform reduction of tube wall redution by 50% or 33% in a particular design and in a particualr process service) Generally, heat exchanger fabricated to TEMA combined with German code may not permit more than 1/3 wall thickness reduction - due to the factor of allowale stresses in German code is much stringent (assuming there is uniform reduction of tube wall redution by 50% or 33% in a particular design and in a particualr process service). Trust this will help you C.V.Srinivasan Nishi Engineers Pvt Ltd India June 29, 2006 E-mail: nishi@vsnl.com > >What is the rule of thumb practice for heat exchangers tubes >retubing? One should replace tubes when they reach 1/3 of >wall thickness? Or tubes should be replaced when they reach >%C2%BD of wall thickness? > >Thanks > >Luis Marques

Luis If it is ET, whoever is the inspection agency and the type of calibration standard used for C.S and their inspection experience, the agency should grade your tubes in a way that you do not normally resort to whole sale replacement . For ET also, tube replacement criteria is sorted out based on gradation and at 33% reduction in wall thickness is likely to be the recommended replacement criteria. I guess - 50% reduction in wall thickness is an extreme case one would recommend for a 4 year run period in refinery service for ferritc steel tubes. It is quite possible to give higher tolerance of 50% redution in wall thickness for Cu-Ni. Brass, S.S austenitic or duplex S.S or Ti type materials. With confidence levels of 95% or more in ET prediction availability - depending on the calibration testing set before survey. a well experienced ET scan agency can more or less safely predict the corrosion pattern today even for C.S tubes. You can be more or less assured of a safe run for 4 years run period with a retirement schedule at 33% wall thickness reduction and also with calculation made based on the remaining wall thickenss available and the pattern of preferential corrosion in selected tubes, you can do both theoretical and practical RBI calculation for remainiing life estimation for the next 4 year run life. It is not too difficult to make such predictions today for C.S tubes. In extreme case if ET prediction confidence limit is uncertain, you can always resort to plugging selective tubes and run your exchanger for the next 4 year cycle replcement scheduling if you keep an option to retire tubes at 33% wall thickness reduction as the general basis and not as an "thumb rule industry guideline". Trust this helps you. C.V.Srinivasan Nishi Engineers Pvt Ltd India June 29, 2006 E-mail: nishi@vsnl.com >My question is related with Eddy Current and IRIS >inspection. What are others practice? Replace tubes when >they reach 1/3 of wall thickness? Or tubes should be >replaced when they reach %C2%BD of wall thickness? > >Heat exchangers tubes thickness is based in BWG. Most of the >carbon steel tubes are 2.8mm thick. So, when you have >exchanger tubes with nominal thickness of 2.8mm you replace >them if you loss about 2.1mm of the thickness, which is a >little bit less conservative than when tubes reach 1/3 of >nominal wall thickness. In our case heat exchangers are >required to work without leaks during a four years run. Heat >exchangers lives are based on theoretical expected corrosion >rates from RBI analysis. In some services we have exchangers >working during 20 to 16 years without problems and in some >few cases we have exchangers, which don%E2%80™t withstand a four >years run. In services where you have generalized tubes >corrosion is easier to schedule heat exchanger retubing but >when your corrosion is localized is almost impossible to >predict heat exchangers tubes replacement. When those few >exchangers which don%E2%80™t withstand a four years run, a root >cause analysis will be required to determine or not a >material upgrade. > > >Thanks for your sharing > >Luis