explosion in sw tanks.pdf
TRANSCRIPT
EXPLOSION IN SOUR WATER DAY TANK FB-42601A OF Ph#2 SWS-UNIT Sandesh J. Cutinho Mangalore Refinery and Petrochemicals
Introduction:
Mangalore Refinery and Petrochemicals Limited have two trains Phase1 and
Phase2. Each of these has a Sour water stripping system. Sour water from
low-pressure systems is processed in refinery sour water stripper and sour
water from high-pressure system like Hydro cracker and GOHDS is
processed in two stage Hydrocracker sour water strippers. The storage and
transfer system of high pressure sour water striping system comprise of a
surge drum for oil separation and Day tanks (2 numbers). As the strippers
require oil free sour water, oil is floated and removed from surge drum using
baffle plates and from day tanks using skimmers.
In the recent shutdown when one of this day tanks FB42601 A was being
prepared to be handed over for maintenance and inspection, there was an
explosion followed by a fire.
A detailed study on the possible causes of this explosion was done and
recommendations were given to prevent such incidents in the future.
Process description:
Sour water from Hydrocracker / Gas Oil Hydro DeSulphuriser (GOHDS) is
routed to sour water surge drum (FA42602). This vessel is provided with an
underflow-overflow baffle arrangement to facilitate separation and removal of
oil. The sour water then routed to day tanks FB42601 A/B through Level
Control Valve LV1301. In case of Level High High in feed surge drum
SDV1305 opens to let out excess sour water to the tank. The sour water
tanks provide sufficient residence time for separation of any liquid
hydrocarbon, which has not separated in the surge drum. A floating oil
skimmer is provided in each tank to skim off separated hydrocarbon. The tank
is provided with nitrogen blanketing, maintained through a pressure control
valve PV1302 / 1303. Pressure Vacuum Relief Valves PVRV – 1304 / 1305
are provided on tanks to take care of in breathing and out breathing. In
addition, mechanical dead weight PSVs 1311 / 1312 are provided on the
tanks.
The tank is 11.3M diameter, 9 M height (700 CuM Stored capacity) and is
provided with epoxy lining on the inside up to safe filling height (7 M). Epoxy
painting is provided beyond safe filling height. As per calculations of
Engineering Department, the tank roof does not meet the criteria for
“frangibility” – i.e. tank to roof weld is not designed to be the weakest joint.
Sour water from Hydrocracker typically contains about 30000 ppmw of H2S
and 12400 ppmw of Ammonia. GOHDS sour water contains around 11000
ppmw of H2S and 3500 ppmw of Ammonia.
The tanks feed sour water to Sour water strippers for further processing.
Schematic is as below;
Incident:
As planned, entire Phase-2 units of the refinery were shut down for M&I, catalyst
replacement in various units and various statutory inspections. As a part of this,
Sour Water Stripper Unit Phase-2 was shut down and activities of preparing and
handing over equipment for maintenance were in progress.
SWS day tank FB42601A was drained, shell manway and cone roof manway
covers were opened and air eductor was fixed during late night shift. Early
morning, an explosion in the tank blew off the roof, which landed a few meters
To Incinerator
HCU SW Surge drum
Nitrogen
To HCU sour water strippers
HCU sour waterpumps A/B
HCU SW storage tank-A HCU SW storage tank-B
PG PG
S
LSLL
LT
LX 1301
LIC
PVRV
PVRV
away on the road north west of the tank. The roof to shell welding appeared to
have failed all along the welded area. The roof trusses collapsed into the tank.
The piping connected to the tank was damaged. The tank itself lifted up from its
foundation due to the explosion. The subsequent fire was confined to tank.
There were no reportable injuries or damages to other equipment. The fire was
put off in about 10 – 15 minutes
Thick cake like hydrocarbon was found spilled from the tank in the surrounding
place. The fire fighting operation had also displaced this hydrocarbon
Photographs of the tank after the explosion have been attached as Annexure-1 Failure Analysis:
No activities or hot work was in progress in the tank or in the vicinity at the time of
the incident. Hence it is likely that the explosion / fire could be most likely due to
pyrophoric iron Sulphide known to be present in such process conditions.
A. Presence of Iron Sulphide in the tank The Sour Water storage tank is epoxy-lined up to HLL. Epoxy painting is
provided on the surface above the liner. The painting was found to be in
reasonable condition on the inside surface of the roof. Deposits removed from
underneath the tank roof were analyzed and found to almost entirely iron oxide.
Iron Sulphide could not be detected in the roof deposits, which might have been
already oxidized in the fire.
The tank contains sour water from Hydrocracker and Gas Oil Hydro-
desulphuriser. Both processes can create Iron Sulphide by the nature of the
reactions. The vessels in both units have been given liners. However, the piping is
not lined. Iron Sulphide will be created in the process units (corrosion reaction
with piping) and can get carried into Sour Water Stripper Unit. Therefore, even
though the Sour Water Stripper Units vessels are epoxy coated, iron Sulphide will
be present in the Sour Water Storage tank as carry over from HCU / GOHDS.
B. Analysis of Hydrocarbon in the tank
The analysis of hydrocarbon collected from the spillage is given below.
Density @ 15°C /Sulphur 0.8806 /0.53 Temp, 50%
recovery 432
Distillation, IBP°C 202 Temp, 60%
recovery 447
Temp, 5% recovery 264 Temp, 70%
recovery 463
Temp, 10% recovery 299 Temp, 80%
recovery 483
Temp, 20% recovery 376 Temp, 90%
recovery 509
Temp, 30% recovery 404 Temp, 95%
recovery 531
Temp, 40% recovery 419 FBP, °C 540
Analysis of the material collected from the spillage shows it to be from HCU and
could be mixture of various Gas Oil fractions. Since the analysis was carried out
after a fire, it is possible that still lighter components could have already been
burnt off.
C. Source of Hydrocarbon
It is likely that hydrocarbon will get carried over from process units; especially
during process unit start ups and on occasions of Level Control malfunctions.
The facts that feed surge drum has been provided with overflow baffles and the
Sour Water Tanks provided with oil skimmers amply demonstrate that the
designers expected hydrocarbon to be present continuously in sour water. Hence
the black sludge noticed by the operating staff could be a mixture of iron Sulphide
and solidified hydrocarbon. Iron Sulphide could also have been present sticking
to the shell or roof of the tank.
There are two locations in SWS Unit where oil coming along with sour water can
be separated. One is at Feed Surge Drum through baffle overflows. Another one
is at Sour Water Day tanks through floating oil skimmers. This separated oil can
be drained either to CBD or to OWS. Draining of oil / Sour water to OWS can be
done only when Pollutant load in ETP is low, which is seldom the case also this is
not safe as it can have high H2S liberation from the OWS. So oil is drained to
OWS only when oil content is very high. In other cases, the oil is drained to CBD
and the CBD contents are recycled back to the tank or stripper. This could have
resulted in the accumulation of the oil in the tanks.
Two tanks are provided, so that one tank can receive sour water and the other
can feed the strippers. The system is designed in such a way that enough
residence time is given for oil separation. The separated oil is then drained
through the skimmer. But during actual operation, both the tanks were kept in line
to avoid vacuum. Due to this safety concern, the oil settling would have been
affected, which in turn could have caused oil accumulation in the tank.
Blow down from all vessels of SWS
Gases to flare Oil plus water to Refinery Sour Water header – either Phase II or Phase I
Blow down drum FA42606
Oil plus water to SWS day tanks FB42601A/B
D. Operating Procedure
After water filling and draining Tank A, and when thick sludge was noticed on the
tank bottom the operating staff preferred to manually remove the same. Hence
an eductor was fixed to improve the tank conditions. Simultaneously, Nitrogen to
tank was blinded to facilitate man entry.
During the time of the incident, the refinery was in the middle of a turnaround in
Phase II of the refinery. Hydrocracker and GOHDS were scheduled for catalyst
replacements, which are heavy consumers of Nitrogen. The refinery had kept
sufficient stock of Nitrogen for these activities. However, the unexpected heavy
demand for Nitrogen in Phase I refinery on March 29, 2006 meant that stock of
Nitrogen had to be used carefully. It is possible that the actions of the operating
staff to conserve nitrogen were influenced by this reduced availability of Nitrogen.
E. Probable Causes of the explosion
By all accounts, explosion occurred first, followed by a fire. A flammable mixture
was noticed by the area operator minutes before the explosion. The flammable
mixture could be due to
a. The pyrophoric iron must have dried up and smouldered in the
presence of air. The heat released must have warmed up the solidified
oil sufficiently to create vapours.
b. Light hydrocarbons were already present in the vapour form, which was
not displaced despite the water filling and draining operations under
nitrogen pressure OR were present trapped in the solidified
hydrocarbon, which were released as soon as some heat was provided
by smouldering of pyrophoric iron.
Conclusion:
The tank is known to contain pyrophoric iron Sulphide due to the nature of the
service. The presence of light hydrocarbon vapours or trapped light
hydrocarbons and the action of introducing air via eductor could have dried up
the pyrophoric iron, which must have resulted in an explosive condition. This is
the likely cause of explosion and fire.
Recommendations:
1. Tank pressure indication is being provided in DCS with high and low
alarms so that in the event N2 is not there alarm will appear. This will also
serve in the event of rising of tank pressure.
2. Only one tank should be feeding the strippers and the other tank should
be under feed preparation, this will give enough residence time for oil
settling.
3. Epoxy coating to the entire tank instead of only safe filling height is being
thought of, this would reduce the risk of formation of phyrophoric Iron
Sulphide.
4. An alternative line up of the oil draining facility to slop tank is being
studied.
5. Procedure being made for de-commissioning and handing over the tank to
maintenance, this mainly will involve;
a. Water filling to be to the maximum height feasible to wash away as
much pyrophoric iron as possible
b. Open sour water tanks under nitrogen atmosphere, even if the tank
appears to be empty
c. Water spray to the roof using special nozzles (water fogging).
d. Remove the contents using gully sucker under nitrogen atmosphere.
e. Do not introduce air till the tanks contents are emptied
Annexure-1
Tank Top view after explosion
Tank roof after explosion
Tank side view after explosion