Materials of Construction
Washing
Fabrication
Erection

The image illustrates the corrosion of a typical carbon steel tube sheet of a gas-to-gas heat exchanger.

The image shows the corrosion of a tube-to-tubesheet weld of a carbon steel SO3 Cooler.  The cause of the corrosion was probably a poor weld which was susceptible to acid attack.

Sulphate deposits in heat exchangers are typical in Cold, Cold Interpass and SO3 Cooler duties.  Sulphates are formed by the corrosion of the carbon steel by   sulphuric acid that has condensed from the gas or carried over from a strong acid tower.  Sulphates will fould both the shell and tube side of the exchanger leading to higer pressure drops and reduced thermal performance.  Tube side depsoits are easily removed but shell side deposits are more difficult to remove. (See Washing Heat Exchangers)

Heat exchangers should be inspected during each shutdown for buildup of sulphates and the exchangers cleaned if deposits are excessive.

Sulphates can be reduced by eliminating the carry-over of acid from the towers or prevent the condensation of acid in the exchangers.  The problem of carry-over is addressed by improving mist elimination or minimizing droplet and mist formation.  Condensation is a more difficult problem to resolve.  Operating temperatures in Cold and Cold Interpass exchangers are generally fixed and cannot be changed.  Cooling air to SO3 Coolers can be preheated to a certain degree to increase the tube wall temperature but this will have an affect on the thermal performance of the unit.

Changing the material of construction to stainless steel is another method of preventing the formation of sulphates.

SO3 Coolers cool process gas using ambient air or air that has been preheated.   Preheating the air before it enters the cooler inceases the tube wall temperature to minimize condensation of acid from the process gas.

The two photos opposite are of the lower tubesheet of an SO3 Cooler.  Process gas flows down the tubes and cooling air flows up the shell side.   The portion of the tubesheet that is plugged with sulphates is the region of the tube bundle where the preheated ambient air enters on the shell side.  It is apparent that even though the air is preheated, the tube wall temperature is still low enough for acid condensation to occur.  As the air cross the tube bundle it is heated further and the temperature of the tube wall increases.  At the side opposite the air inlet no acid condensation occurs and the tubes are sulphate free.