Failure of Sulfur condensers

We have faced some failures in the sulfur condensers of Sulfur recovery unit, in one of the gas field development plants. There were 4 condensers, which were installed in series, with carbon steel shells and tubes. The inlet channel of the condensers were refractory lined up to bottom tube row. The failures were mostly located at tube to tube sheet welds. Also the refractory lining was degraded. The failures occurred in less than 2 years from start of the operation. There was no drawback of any problem, until a decrease in temperature was recorded through thermowells at the 4th condenser outlet line, a few days before the blockage of the tubes by sulfur solidification and unit shutdown.  

Inlet and outlet temperature of each condenser is specified in the table:

The 1st condenser was operating with inlet temperature of 285°C. No major corrosion could be seen on the 1st condenser. But the 2nd condenser with 299°C inlet operating temperature, showed minor corrosion which caused some failures in tube to tube sheet welds. The most dramatic corrosion was seen in the 4th condenser with inlet operating temperature of 203°C.

To investigate the corrosion, although sulfidation is not normally a dominant failure mechanism in condensers of Sulfur recovery units, but based on API 939-C and API 571, sulfidation is possible in the first 2 condensers for carbon steel and also the refractory lining. Such phenomena could be occurred at temperatures above 260°C in absence of hydrogen and 230°C in presence of hydrogen. With a specific sulfur content, the higher the temperature the higher the corrosion rate. Due to this fact the more noticeable corrosion on the 2nd condenser (with higher temperature) is reasonable. The corrosion rate could be predicted using modified McConomy curves.

On the other hand, there was also, some amount of hydrogen available in the fluid which promotes the high temperature H2/H2S corrosion, for which the threshold temperature is 230°C. Such corrosion mechanism can be predicted using Couper-Gorman curves, but such curves are provided for gas oil and naphtha fluids in the literature. Also they are not provided for low pressure services as could be seen in SRU. Other related curves are also provided in API 939-C, according to which the corrosion rate of 0.6 mm/y is expected for carbon steel at such process condition, which is extremely high.

In order to analyze the case, failure of tube to tube sheet could be occurred due to sulfidation in the 2nd condenser which caused the cooling medium ingress into the main acid gas stream which would convert into vapor at such temperature. The other possibility of tube to tube sheet failure could be the poor welding quality, which caused mechanical failure in operation. The same failure could be occurred in other downstream condensers. Failure of tube to tube sheet welds and water ingress into acid gas, could be the source of temperature reduction in the fluid, since thermowells records, had shown decrease in temperature form 167°C to 120°C at the 4th condenser outlet. 

Such decrease in temperature was the key point in revealing the corrosion mechanism of the 4th condenser (which was heavily corroded) since sulfuric acid dew point is normally 138°C and reduction of the fluid temperature to 120°C might caused condensation of sulfuric acid which was resulted in drastic corrosion and failure of the 4th condenser which caused sulfur choking in the tubes and finally resulted in unit shut down.