Alonizing: case studies
Case Study One: ALON™ DIFFUSION COATING PROTECTS TUBES IN SOUR GAS PLANT WASTE-HEAT BOILERS
Situation: The Shell Caroline gas plant, one of the largest sour natural gas processing facilities in the world, is designed to process 8533 x 103 m3/d of raw gas and gas equivalent of liquid hydrocarbons. The overall sulphur recovery for the facility is 99.8% of the total sulphur intake, and sulphur production is approximately 4000 tonne/day. This facility began operating in late 1992.
As is the case with most western Canadian hydrocarbon sources, the gas in this plant is contaminated with a quantity of sulfur species. The concentration of H2S in the feed to the plant is approximately 35%.
During operation, the total raw gas and liquid hydrocarbons from the field are processed in the inlet feed facilities. The overhead sour gas from the feed fractionator after being stripped of heavy hydrocarbons, is sent to the gas sweetening facilities where the acid gases are removed by physical and chemical absorption. The acid gas is then sent to the Claus sulphur recovery unit.
In the Claus process, H2S in the acid gas stream is partially oxidized in the reaction furnace. This reaction is highly exothermic. Among existing gas plants, gas temperatures are reported to exit the reaction furnace at a temperature between 1400F and 2300F (760 C and 1260 C) with most plants operating in the range of 1700 F/2200 F (927 C / 1204 C). The Shell Caroline plant is designed around an exit gas temperature between 2200 / 2300 F (1204 / 1260C).
This gas plant contains two sulphur recovery units, each having two reaction furnaces and waste-heat boilers operating in parallel. The steam generating capacity of each boiler is approximately 100 t/hr at 3650 kPa. Each boiler contains 1010 tubes, 2-1/4” (57.1mm) diameter, approximately 12 meters in length. These tubes can experience high-temperature sulfidation, which can result in inefficient operation, potential for catastrophic failure and premature shut down for maintenance or replacement of tubes.
Action: All of these carbon steel tubes were Alonized™ to eliminate high-temperature sulphidation.
Alonized™ tubing is specified in many gas plants to protect the tubes in the waste-heat boilers and condensers. Additionally, a wide variety of metallurgies and configurations are Alonized for use in many other types of gas-processing plants.
Although the hot gases pass through the inside of the tube, both surfaces of the tube were Alonized™. Care was taken to eliminate the presence of aluminum alloy in the area of the tube that was welded into the tube sheets during assembly. In accordance with standard practice, the tubes were processed through Endurance’s high-temperature vapor diffusion treatment, then re-rounded by passing them through a roller straightener to bring them back to ASTM A450 tolerances. The tubes were trimmed to final length, ends dressed, and protective plugs inserted in the ends. NOTE: The heat of the Alonizing™ process may cause a slight change in tube length. For this reason, tubes or pipes with a finish length longer than about 16’ (4900 mm) are typically ordered 1% over length and then trim cut to final length after Alonizing™.
Result: After over 20 years these tubes are still in service, allowing the gas plant to operate efficiently, with minimal risk of failure, and no lost production time due to tube replacement.
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