Analysis of the reasons for internal leakage in the main combustion furnace’s waste pot

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This post was last edited by liaifeng on 2018-8-5 at 10:35. Analysis of the reasons for internal leakage in the waste pot of the main combustion furnace for sulfur recovery. I. Equipment details: The steam generator (E-1801) is a Class II pressure vessel. Its primary function is to recover the residual heat from the main combustion furnace to generate S05 steam; it also cools the process gas, converting gaseous sulfur into liquid form, which is then transferred to the liquid sulfur tank via a flow lockhopper. The specific parameters of the equipment are as follows:
Shell side/Tube side: Operating pressure (MPa) 0.5/0.052; Design pressure (MPa) 0.8/0.3; Hydrostatic test pressure (MPa) 1.0/0.5.
Operating temperature: Inlet/outlet (°C) 137/164, 964/178; Design temperature (°C) 180, 340/220.
Medium: Boiler feed water, process gas.
Material of main pressure-bearing components: Q245R, 20.

II. Existing problems: On November 21, 2015, the pressure difference at the main combustion burner for sulfur recovery was high, and liquid sulfur contained water, indicating an internal leak in steam generator E-1801. After shutting down the unit to locate the leak, it was found that one of the tubes had an internal leak. The leak was sealed by welding, after which the unit operated normally again ； On April 11, 2016, water was detected in the liquid sulfur; upon inspection, an internal leak was found in the tube sheet of the E-1801 steam generator. After sealing the leak through welding, normal operation was restored ； On September 30, 2016, it was found that no sulfur was being discharged from Liquid Sulfur Hopper No. 1. Upon inspection after opening the hatch, steam was detected behind the liquid flow ball valve; a shutdown followed for further inspection, which revealed an internal leak in the tube sheet. The leak was sealed through welding, and operations resumed normally. On March 9, 2017, it was again found that no sulfur was being discharged from Liquid Sulfur Hopper No. 1. Inspection after opening the hatch showed steam behind the liquid flow ball valve, and a shutdown led to the discovery of an internal leak in the tube sheet, which was sealed using welding. III. Cause analysis: Since the start of sulfur recovery operations, there has been 1 internal leakage in the tubes of the E-1801 steam generator and 3 internal leaks in the tube sheet, for a total of 4 internal leaks. The possible reasons for these frequent leaks are as follows: 1. The medium flowing inside the tubes of the E-1801 steam generator is highly corrosive gas such as hydrogen sulfide, sulfur dioxide, carbon disulfide, and sulfur vapor; prolonged operation has led to severe corrosion of the tube sheet, reducing its strength. Moreover, once internal leakage occurs and steam enters the process gas system, it will accelerate the corrosion rate. According to the principles of corrosion, hydrogen diffuses into the steel, while sulfur penetrates into the inner surface of the steel mainly in the form of iron sulfide. During welding, hydrogen will cause hydrogen embrittlement and white spots in the weld, leading to the formation of pores and cold cracks ； During welding, sulfur easily causes hot cracks, reducing the plasticity and toughness of the weld metal and thereby lowering the overall mechanical properties of the weld. 2. The tube sheet of the E-1801 steam generator is made of Q245R, while the tubes are made of 20# steel; these materials have different coefficients of thermal expansion. If the unit operates unstably with frequent start-ups and shut-downs, uneven thermal expansion can lead to cracks in the welds, resulting in internal leaks. 3. Under normal conditions, welds, especially the fusion line area, are most susceptible to corrosion due to the high residual stresses in that region and the uneven distribution of the microstructure. As a result, the welds and their heat-affected zones become weak points for hydrogen sulfide stress corrosion; therefore, such tube sheets must undergo heat treatment after welding. However, due to the large area of the tube sheet, overall on-site heat treatment is difficult, which leads to internal leakage.