Luis TEMA Class "R" designed exchangers - gas to gas or gas to liquid ( explosion prone liquid or gas service) where tight leak sealing is needed - a double groove expansion followed by strength welding is essential. In some cases, stress relief may be needed to prevent S.C.C or crack development due to left over weld residual stress. TEMA Class "C" designed exchangers - gas to gas or gas to liquid - where crevice corrosion is likely at the expansed tube to tube sheet or from crevices - S.C.C possibility - chloride or HIC types - double groove expansion and seal welding is specified. TEMA Class "C" designed exchangers where cladding is done for the tube sheet for high temperature gas or liquid impingement - double groove expansion and strength welding is needed. If tight double groove expansion is tolerable for a service where leak may be a nuisance but will not pose a process hazard or process leak likely to lead catastrophic fire or explosion and you do not anticipate failure probability due to S.C.C or gas leak, double groove expansion may be OK. Answering to your other question on: Tube Intet and Tube Outlet Box - Differential temperature point raised Please amplify on the points raised by you related to Inlet box and Out let box - service duty, temperature differential expected at the inlet and outlet boxes, material used before any comment is given by the forum. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd India Nov 8, 2005 E-mail: nishi@vsnl.com >API 661 “ Air –cooled heat exchangers for general refinery >services”-1997, states in paragraph 11.2.3: All tubes not >strength welded to the tube sheet shall be expanded and seal >welded. >Does any one know why? >Is it acceptable to have the tubes just expanded? Or the >seal weld is mandatory? >When there is a big temperature differential between tube >inlet and tube outlet should the inlet header box be >independent from the outlet header box? >An answer from the forum would be appreciated, >Regards >Luis Marques

Hi C.V.Srinivasan/Shabbar Thank you very much for your sharing. All our existing air cooler designs in Hydrocarbon+H2S high-pressure services are tube expanded into header tube sheet. My understanding is that according to “API 661 “ Air –cooled heat exchangers for general refinery services”-tubes should be strength welded to tube sheet only if specified by the purchaser. I don’t think for the need of strength-welded tubes to tube sheet in our Vacuum Hydrobon high-pressure service. I suppose the welding failure in our tubes was also powered by the fact that the common header box for tube inlets and outlets didn’t accommodate temperature differential. Thermal expansion was not properly done and some tube welds have been submitted to overstress leading to premature failure. If tubes have been only expanded and header box was split in a separate inlet and an outlet header, for sure we hadn’t had the failure. Thanks Regards Luis Marques