heat exchanger corrosion in ammonia plant

[tarojm]

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.
tarojm

 

[metengr]

Could you please describe the appearance of the surface corrosion on the T22 tubing? Be as detailed as you can.

Also, I am not sure if I followed your post correctly, what is the gas on the tube side (ID surface) and the shell side of the heat exchanger.

Third question - what has been the operating history, base load operation or have you had to isolate the heat exchanger for short duration over the last 2 years?

 

[tarojm]

Thanks for your interest to my question please find below the history and 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 of new exchanger 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 ).

Meanwhile any comments regarding the chance of carbonic acid formation and effect of it on the failure of tubes would be appreciated.
masoud

 

[metengr]

masoud;
You really need to have several of the failed tubes analyzed by a competent metallurgical lab to determine if the damage mechanism is corrosion or erosion/corrosion based on the shell side inlet temperature conditions and mass flow rate of water and gas.

Did you replace the heat exchanger or re-tubed in place? When you changed from T1 (carbon-moly) to T22, it would seem to me that this would have been an appropriate upgrade with the increase in chromium and moly.

Considering that this damage mechanism is confined to the upper row of tubes strikes me as a design problem with the heat exchanger, and it could be several possible scenarios like the one you described with conditions favoring carbonic acid formation. Maybe all that is needed is to install a diffuser or re-direct wet gas flow on the shell side of the heat exchanger to avoid impingement on the exterior of the tubes.