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Failure of the transfer line in ammonia Plant and remedial MeasuresV.APPA RAO
The failure took place with release of reformed highly explosive mixture of gases, soon after ballooning of transferline.
This paper exemplifies reasons for failure of transferline, later on rehabilitation activities like fabrication/inspection, remedial measures taken by selecting improved lining material for transferline. And during plant routine inspection, what are the precautions required to be taken to avoid re-occurrence of similar type of failure.
The excessively high temperature process gas (950°C to 1000°C) available at the exit of Secondary Reformer is cooled down in steps through waste heat recovery boilers, before proceeding to the next process step of shift conversion where carbon monoxide gets converted into carbon dioxide.
This presentation describes the failure of process gas transferline located at the exit of Secondary Reformer and connecting to one of the down stream primary waste heat boilers.
The schematic arrangement of Secondary Reformer, Transferlines and Primary Waste Heat Boilers are shown in Figure-1.
The process gas from outlet of Secondary Reformer splits in two parallel paths to enter Waste Heat Boilers 101-CA & CB. The gas is at a temperature of 950° C pressure 33 Kg/cm2 and consists of H2, N2, CO2, CO, CH4, H2O gases.
The pressure Shell has 915 mm Æ, 32 mm thickness of ASTM A-516 Gr. 70 (CS) material. The inner side has bubble alumina refractory lining (Greencast 97 L). A liner of SS 310 material is provided to protect the refractory from erosion due to flow of process gas. The liner is welded to a gas distributor at the inlet and bottom of waste heat boiler.
The pressure shell has water jacketing on the outer side. The shell cooling arrangement is shown in Figure-2. The cooling media is DM water/condensate from surface condenser. The jacket pipe is constructed of 6 mm thick CS material. The cooling system works at approx. 1 kg/cm2.
It is imperative that water jackets be maintained water filled when heat is applied to the primary or secondary reformers. Loss of water flow from the jackets could also cause undue stress in the transfer line piping to the secondary reformer and rupturing of the jacketing by uneven expansion. In the event of failure of the jacketing, it is recommended that the reformer be taken out of service until water jackets are again serviceable and water levels can be maintained.
Adjacent refractory was partly damaged. The old left over refractory on the shell of waste heat boiler was removed by striking with hammer. Now refractory was cast in position. The transfer line (SS-310) between secondary reformer outlet (103D) to waste heat boiler (101CB) was partly crumbled(Ref.Figure-3).We did not have access to know the condition of bubble alumina refractory (Green cast 97L) which was between SS-310 liner and pressure shell (ASTM-A.516 Gr. 70).
As it takes a lengthy time to replace the SS-310 liner and presuming the condition of refractory was good, a decision was taken to replace the liner in the forthcoming Annual Turn Around. Accordingly clearance was given to Production Department. The condition of other transfer line between 103D and 101CA was satisfactory.
After plant start up, (i.e. on 17/04/99), CO2 supply was started on 23/04/99 at 01.37 Hrs. Some leaking sound was heard from 101CA/CB platform. On seeing the area, it was found that the 103D to 101CB line was blowing heavily, the gases accompanied with fire.
The intensity of sound had increased tremendously and cloud of gas coming out at 101-CA/CB platform was observed (Figure-4).
|Fig 4:||Fig 5:||Fig 5a:||Fig 6: The construction of transfer line is depicted in Figure-6|
It was also seen that the 103D transfer line to 101CB had become Red Hot with Red Hot Zone predominantly towards 101CB. The outermost water jacketed carbon steel shell became ballooned and water/steam gas gushed vertically upwards. The plant was put to shutdown. The damaged portion of the liner is shown on Figure-5. Also, the crack initiation of pressure shell is clearly depicted in Figure-5a.
|Fig 7:||Fig 7a: SS-310 Material with severe Sensitization|
It needs to be understood that for pressure shell failure to take place the refractory fracture between the pressure shell and liner has to occur first so that pressure shell will be exposed to high temperature beyond 343.3°C which is the designed metal temperature of pressure shell.
The area, where pressure shell rupture occurred, had no access for inspection of refractory. The voids or cracks in refractory in the region might have gone unnoticed.
The metallographic report on samples collected from failure zone revealed some changes in microstructure and this was the reason for failure. Failure due to hydrogen attack requires 10,000 hrs. as per NELSON CURVE (Ref. Figure-8) which as creep failure is time dependent on temperature.
The changes in microstructure are distinctly different due to above two reasons of failure. However, it is most important to note that refractory failure must have occurred first and reasons for the same could be thermal cycling of transfer line over a period of time or mechanical shock caused to the refractory during repairs in nearby area of 101CB bottom.
|Fig 8:Nelson Curve for Temperature Effect On time for Hydrogen Attack of Carbon Steel||Fig 9:MICROSTRUCTURE OF INCONEL 601|
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