22nd AFA Int’l. Fertilizers Technology Conference & Exhibition June 30th - July 2nd, 2009 Marrakech, Morocco
Process Air Vent Line Rupture
Iftikhar H. Turi Head Of Section Ammonia-2 QAFCO
Qatar
PROCESS AIR VENT LINE RUPTURE Iftikhar H. Turi QATAR FERTIOLIZER CORPORATION, MESAIEED - QATAR
Abstract On August 06, 2006 leak was detected at ammonia refrigeration condensers (E0901) located at ammonia -4 synthesis section. The leak was conformed by analyzing condenser outlet sea water. To attend the condenser leakage, ammonia -4 was brought down partially, keeping front end in service. At the time of incident two field operators were in the process of diverting process air cooling steam to silencer though the steam vent line, when the vent line got ruptured producing big noise. The plant front end was in operation and both the air and process gas temperature were in the range of 8000C and 5300C respectively. Shift staff on duty tried to locate the exact location of the leak but due to excessive heat in the surrounding they were not able to come closer to the suspected area to locate the leak. It was decided to conduct a complete shutdown of the plant. After total shutdown, upon inspection it was found that 6” vent line had a Fish Mouth opening (Fig 02 A/B).
Introduction Located in Qatar (Middle East) Qatar Fertilizer Corporation (QAFCO) is one of the leading producers of urea and ammonia in the world. At present, QAFCO operates four ammonia and four urea plants located at the same site. The present annually production is 2.1 MMT of ammonia and 2.9MMT of Urea. QAFCO is in expansion process to increase its ammonia and urea production by installing two new ammonia (2200 MTPD) each and one urea (3850 MTPD) plant. The installation and commissioning of these three plants is expected in mid 2011. After completion of these projects QAFCO will be the world’s largest single site Urea and Ammonia producer. Ammonia – 4 is UDHE technology with nameplate capacity of 2000 MTPD. It was commissioned and stared in 2004. The plant is operating with average production rate of about 2300 TPD without any major issues.
1 of 9
Ammonia - 4 process air system In ammonia - 4 hot gases exit primary reformer (R0301) combustion chamber at 1023 0C and enter the convection zone for heat recovery before released to the environment. Process air from process air compressor (K0201) is pressurized up to 4350kpa and preheated up to 540 0C in heating coils (E0317) and (E0313) located in primary reformer convection zone before introducing to secondary reformer (R0301). The max allowable temperature for the air pre heaters is about 600 0C. During plant startup and planned shutdown steam vent (X0303) is used for process air after passing through heating coils. During plant startup and rundown either process air or medium pressure steam (5000kpa) is used as a cooling medium to protect process air coils from over heating. (Fig 01)
Sequence of events A;
On August 06, 2006 at about 08:30 hrs (E0901) ammonia condenser tube started leaking producing abnormal sound. At that time (V0901) ammonia receiver level started to drop gradually. The leak was also conformed by analyzing condenser outlet cooling sea water for ammonia presence.
B.
Decision was made to shutdown and depressurize ammonia 4 synthesis section to attend condenser (E0901) leakage.
C.
Plant run down started at 11:30 hrs by reducing front end load and NG compressor was bypassed while keeping the primary reformer (B0301) in service.
D.
At about 16:24 hrs operation staff was in a process to cutoff process air to secondary reformer by slowly opening vent valve (VF 03315).
E.
At the time of incident 27336.9 m3/hr of process air and 16.1 tons/hr SY steam was passing through the process air coils.
F.
The reforming gas and steam was passing through the secondary reformer (R0301) at about 796.6 0C.
G.
The process air discharge pressure was 4013 Kpa with the 27336.9 m3/hr flow.
H.
Secondary reformer outlet temperature was 796.620C. The reformed gas was
2 of 9
a mixture of 110.8 tons/hr steam and 39838.8 m3/hr natural gas and the exit temperature was 800.90C (design 8230C).
I.
At about 16: 31 hrs filed operator reported a heavy noise from the air line while he was planning to close (MOV 03090) secondary reformer air inlet valve by hand jack. He tried to locate the leak but because of excessive noise and heat in the surrounding, he was unable to stay there and locate the leak.
J.
At 16:40 hrs trip C was initiated from DCS to locate and attend the leakage.
K.
After complete shutdown it was observed that 6” vent line has ruptured with fish mouth opening.
Inspection finding After removing the insulation from process air vent line, visual inspection was conducted to check the integrity of the vent line. A.
No deviation observed in circumference / thickness measurement.
B.
Line found sagging between the supports and one sliding support moved by 4 inches.
C.
Inspection results showed that the hardness values of the 6” process Air vent line had been reduced at the failed location and somewhat reduced downstream of the failure combined with visual distortion of the line and the “ Fish Mouth Opening”.
Probable cause of failure A.
At the time of incident (16:31hrs) the process air flow increased from 28144 m3/hr to 37831 m3/hr.
B.
No change to SY steam to the coils as it was on manual control at that time.
C.
The temperature of the Air/Steam mixture passing through the vent line exceeded the design temperature as the reformed gas temperature was 800.90C.
D.
NRV 03020 was checked and found seat passing by pressurizing downstream of R0301 with nitrogen. Fig 04
3 of 9
E.
Decrease in pipe material hardness and rupture indicates a short time overheating.
F.
Reduction in gas flow to the flare B 0601 was observed between 16:24 hrs and 16:31hrs, when the 6� manual vent valve opening started.
G.
During 2006 warranty shut down, the internals of the Silencer (X0303) were removed due to some defects causing decrease in back pressure.
H.
Upon investigation it was observed that the shutdown sequence was not followed properly.
Conclusion During plant run down, process air coils cooling vent line valve (VF 03315) was open prior to initiating trip C. NRV (03020) in the process air line to secondary reformer did not prevent the reverse flow of reformed gas into the Air line as it was passing through seat, causing unusual high flow through the vent line. Reformed gas mixed with process air and steam reacted and heated the process air coils cooling vent line beyond the capacity of P91 material to withstand the internal pressure resulting in failure of the line.
Acknowledgement Ammonia - 4 HOS, plant engineer, operation staffs and inspection team helped to compile this paper.
4 of 9
PI
X0303 VF03388 VF03315 MV03090 M
E0313
VC03020
E0322
FV02001
XV02002
R0301
SY STEAM E0301
FV03004 E0317 L0204
FV02003
XV02001 B0301
K020 FV03009 E0308
SY STEAM
BFW
BFW XV03007
Fig # 01 Ammonia - 4 process air system
5 of 9
XV03031
Fig # 02A
6 of 9
Fig # 02B
7 of 9
Fig # 03
8 of 9
Fig # 04
9 of 9