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Failure of the transfer line in ammonia Plant and remedial Measures V.APPA RAO Senior Manager(Mech.) Contact

ABSTRACT:

Fig :

Krishak Bharati Cooperative Limited is a large Nitrogenous Fertiliser Complex situated in Western India. This plant is in operation since 1985. In the year 1999 after few days of plant start-up, one of the transfer lines connected between secondary reformer and waste heat boiler had a failure.

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.

AA) KRIBHCO

Fig :

KRIBHCO operates two Ammonia plants and four streams of urea plants whose installed capacity is 2700 MT/Day ammonia and 4400 MT/Day urea. Besides above, KRIBHCO also operates 3 Nos. boilers 275T/Hr. @ 105 Kg/CM² and 510°C; 2Nos.15MW Power plants. Ammonia Plants are designed by M/s. M.W. Kellogg, USA, Urea plants are designed by M/s. Snamprogetti, Italy, Power Plants are designed by M/s. Foster Wheeler, U.K. Since 1985 till date all these plants are operating in an excellent way.

BB) THE PROCESS

In any conventional Ammonia plant the process stream consisting of hydrocarbon and Steam mixture is processed in two steps namely Primary Reforming and Secondary Reforming in order to generate a product gaseous stream consisting of a mixture of gases like hydrogen, nitrogen, carbon monoxide, carbon dioxide etc.

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.

Fig 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, CH_4, 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.

Fig 2:

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.

CC) INCIDENCE:

During Annual turn around (March/April-1999), inspection was carried out to know the condition of GAS distribution in 101CA/CB (Waste Heat Boilers). Flexible Borescope was installed through bottom of 101CA/101CB. Gas distributor in 101CB observed in crumbled condition. Then the vertical tube bundle was lifted to have an entry to the gas distributor. The crumbled SS-310 gas distributor was removed and replaced with Inconel - 601 material.

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).

Fig 3:

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.

DD) INSPECTION/OBSERVATIONS

It was observed that Hydrogen attack caused fracture of pressure shell (ASTM-A-516 Gr. 70) due to pressure and temperature followed by Blister formation on the Inner diameter surface of the Pressure (Ref. Figure-7). The SS-310 inner liner shroud had shown
• Severe Sensitization(Ref. Figure-7a)
• Ditch structure; which is indicative of Hydrogen -embrittlement.

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

EE) REPAIRS:

The entire transfer line was fabricated with improved liner material Inconel-601, whose metallic structure is depicted in Figure-9. Stress Relieving operations carried out on pressure shell (ASTM-A516-Gr.-70). Refractory (Greencast-97L) was poured in between liner and pressure shell. New Carbon Steel water jacket over the pressure shell was also fabricated and released for Production Department. Thereafter the plant performance is in an excellent condition.

RECOMMENDATION

• During every ATR, visual examination of the liner be carried out.
• In-situ Metallography examination at various locations of the Inconel-601 liner shall be carried out.
• In-situ Refractory removal at the bottom of 101CA/101CB shall be without Mechanical shocks.
• Inspection of refractory between liner and pressure shall be carried out at regular intervals of time

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