MASOUD Tube specification should be A 213 T 11 or T 22 for your exchanger. A 209 comes as Grade T1, T1 a, T1 b . A 209 is Carbon -Moly tubes. For your syn gas exchanger (Gas to Gas exchange) A 213 T 22 is OK especially the outlet gas temperature is 310 deg Cent. So long as there is carry over of Pot Carbonate from Absorber is there, external corrosion on the shell side will be present. S.S will be OK provided the carry over Potassium Carbonate does not contain chlorides. At this gas exit temperature S.S may fail due to Chloride SCC than from external corrosion due to carrry over Potassium Carbonate. There is no need for any change for tube material from A 213 T 22 or A 213 T 11. Both will stand the gas inlet and gas exit conditions at 30 KSC. Basic problem of carry over of hot potassium carbonate is necessary to solve this problem. Retubing with SS may create problems of SCC if carry over occurs regularly as you have described. For this temp and pressure for syn gas exchange - T 11 or T 22 is OK so long as carry over is not there. Process improvement will solve this problem of tube failure in A 213 T 22. For carry over to occur, please review your operation personnel on start up and shut down practices in CO2 removal or tray collapse problems in abosrobr resulting in hot potassium carbonate to this exchanger. If you suspect tray collapse you have to inspect absorber and set rigth the tray collapse problem - especially at the top tray sections in your CO2 Absorber. Tube replacement is not a solution for this process related upset and acarry over of hot carbonate to this exchanger. Excchanger has been adequately designed with T 22 ot T 11 material that is meant to handle gas to gas at this inlet and exit temperature of syn gas - in my opinion. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd Chennai India April 15 E0mail: nishi@vsnl.com >Due to the leakage from one of gas-gas heat exchangers in >our ammonia plant we have faced with shut down of the plant. >The exchanger has been in service only two years and the >outer surface of the leaked tubes shows sever corrosion >however the condition of inner surfaces are good.The >material of corroded tubes is A-209 Gr.T22 ( the original >material was A-209 Gr.T11) and the gas composition is as >follow: >.05% CO , .03% CO2 , 73% H2 , 20% N2 , .2% CH4 , H2O >remain >gas temp (in) : 220 C >gas temp (out) : 310 C >pressure : 30 Kg/Cm2 >During the operation due to the process problems we had also >carry over of benfild (potasium carbonat ) from absorbtion >unit to gas intering to shell of exchanger. >I would like to have your comments regarding to the kind of >corrosion and the effect of carry over of benfid to it. >Any comments with respect to material of the tubes and need >for up grading of them ( eg. to stainless steel ) will be >appreciated. >masoud

MASOUD Answering to your querry on A 213 T 22 tube failure in 2 years versus A 209 Gr T1a for 15 years. Carbon Moly tubes had served you for 15 years. A superior CR-Mo tube had failed in 2 years. This means it is not the material that is the cause . It is the carry over of carbonate to the exchanger shell - First pass and at the top portions that had caused the failure. for both the materials. For A 209 T1a you had failure at the tube to tube sheet joint which you have not seen in the ASTM A 213 T 22 material usage. For the top portion in the first row alone to be affected i guess it is simple carry over of hot potassium carbonate that had affected and caused tube leaks. Process improvement is vital to stop any carry over of hot potassium carbonate from your absorber tower. This could be tray collapse. Tray collapse if process operation had not been steady during start up or upset times of your CO2 removal section. I DO NOT THINK carbonic acid corrosion will take place in this exchanger at this temperature range you have specified for inlet and exit gas and the composition of gas given. For the traees of CO, CO2 given in your process spec, carbonic acid generated will be so weak as to cause failure only at the top row of tubes. on the straight length portions . Failure experienced is at the straight protion of tubes and in the first or top rows of inlet entry gas to shell. One way you could think of avoid of direct impingement of hot carbonate liqud along with syn gas - on the first rows in first pass is to provide a deflector plate at the inlet nozzle so that even if there is carry over of hot carbonate it will first attack the deflector plate and not the tubes directly. I suggest that you think of an inspection of the absorber when plant shut down is more than 7-10 days to solve this problem and set right the tray supports from carry over of hot carbonate through higher gas velocity. Inspect the absorber when upset shut down or process upsets in CO2 removal happens whenver upstream or downstream ammaonia operation is affected. Usually it is likely the start up conditions would have created a collapse of tray segments resulting in gas channeeling and carryover of hot potassium carbonate through higher gas velocity. 15 years is a long period for your process people to stabilize the absorber to steady loads and control upsets and prvent such carry over of hot por. carbonate along with gas. If they cannot do that and you do not inspect absorber each time, exchanger tube failure will occur as often as you describe in so many years I suggest that you look for improvements in proecess and not from materials change. Trust this is of help to you C.V.Srinivasan Nishi Engineers Pvt Ltd Chennai India April 16 E-mail: nishi@vsnl.com >Dear Srinivasan, >Thanks a lot for your useful information .Please find below >the correct process data of shell and tube of the exchanger: >Shell side: >Temperature (IN) : 68 C (OUT) 283 C >Pressure : 28.5 Kg/Cm2 >Dry Composition of gas: >%CH4 0.243 >%CO 0.458 >%CO2 0.2 >%H2 74.55 >%N2 24.1 >%Ar 0.3 >Total 57531 Kg/Hr + 1285 Kg/Hr H2O > >Tube side: >Temperature (IN) :363 C (OUT) 146 C >Pressure : 27.3 Kg/Cm2 >Dry gas Composition: >%CH4 0.95 >%CO 2 PPM >%CO2 2 PPM >%H2 74 >%N2 24.7 >%Ar 0.37 > >The original material of the tubes was SA 209 T1A but after >more than fifteen years operation due to leakage from tubes >and the weld of tube to tube sheet we changed the material >to A213 Gr T22. >After two years operation the leakage from the tubes began >again. After shutdown of the the plant we withdrowned the >leaked tubes and found the severe corrosion at the outer >surface of tubes.The leaked tubes are mainly located in >the first row of the upper part of tube sheet . The >corrosion occured in two distinct part along the lenght of >the tubes and only in half of the surface at top of the >tubes ( the exchanger have four baffle ).The photoes of the >failed tubes are available and can be sent to you by e-mail. >Mean while any comments regarding the chance of carbonic >acid formation and effect of it on the failure of tubes >would be appreciated. >masoud