This is a vertical Alloy 2205 shell and tube exchanger. We have corrosion of the bottom tubesheet and failures in that tube-tubesheet joint from the shell side which contains calcuim chloride brine. There is also wear at some baffle locations on the tubes and shell at contact points. Previously, 316L stainless tubes and carbon steel shell were utilized in this service, but the shell was failing due to corrosion at the baffle contact points. Evidently, we have some type of crevice corrosion and possibly a vibration problem. We have had very little to zero corrosion on tubes. I welcome any advice, especially on what is occurring and any NDE methods for locating and quantifying the damage.

Alloy 2205 Tybe - Tube-Sheet Corrosion Problem of bottom tube to tube sheet corrosion seen at the weld areas or at the tube -tube sheet joint areas may be due to: 1. Velocity increase of fluid flow at the bottom more. 2. Higher temperature at the bottom tube to tube sheet joint. 3. Concentration of Calcium Chloride depostis at the bottom tube sheet more likely resulting in failure if the bottom tube sheet temperature is more than top tube sheet. Alloy 2205 a semi-austenitic with 22 Cr / 5 Ni, 3 Mo, 0.15 Nitrogen is a better material with proven strength and also pitting and crevice resistance . I guess the basic cause could be fluid flow and high temperature effect to allow concentration of calcium chloride deposit to cocnentrate and result in this failure. If the tube (Carbon Steel) and the tube sheet joint had not been properly welded (either strength or seal welded) you could expect failure due to concentration of calcium chloride ions due to higher temperature at the bottom tube sheet . Also you have not explaiend what is the temperature at the inlet and outlet on shell side and at the bottom and top of this vertical exchanger. Crevice corrosion in a baffle location is unlikely in avertical exchanger. . If vibration had been experienced (noise pattern during operation) then the exchanger baffle hole drilling and the clearances between the tube and baffle plate had not been made as per TEMA Class C stanadards. Due to excessive clearance vibration (rattling in baffle plates) could be experienced. If fluid flow is more at the bottom tube sheet ezpecially close to inlet entry of bottom tube sheet and the failure pattern indicated that more failures had taken place at portions or areas close to nozzle inlet area, then possibly flow induced erosion -corrosion pattern would have been witnessed at the bottom tube sheet areas. For this, you could think of a deflector or catch pot type where liquid entry at the bottom tube sheet is regulated. This would avoid splashing of liquid at the bottom tube sheet (This could be done if there is enough gap between the tube sheet and the bottom cover plate to install a cat pot or deflector assembly. NDE: You could think of Eddy Current Inspection of all tubes. This will also give information of possible damage on the external side of the tubes at the baffle locations if vibration pattern had been seen due to excess clearance - i.e., more than specified as per TEMA Class C - between the baffle plate and the tubes. Trust this is of help to you for your problem C.V.Srinivsan Tech Director Nishi Engineers Pvt Ltd Chennai 20 India April 01, 2004 E-mail: nishi@vsnl.com