Explosion in SW Tanks.pdf

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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

Transcript of Explosion in SW Tanks.pdf

Page 1: Explosion in SW Tanks.pdf

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

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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;

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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

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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.

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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

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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

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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

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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

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Annexure-1

Tank Top view after explosion

Tank roof after explosion

Tank side view after explosion