(113c) Refinery-Wide Air Loss and Minor Fire Near FCC Air Blower

On March 29, 2020, ENEOS Wakayama Refinery experienced a minor fire incident in an FCC unit following a refinery wide air failure. Mechanism that led to the FCC fire from loss of air and countermeasures taken are introduced in this presentation.

Utilities outage

In compressed air unit GA, which supplies air to the entire refinery, piping failure occurred in downstream of the air compressor, and the supply of air to the refinery was completely stopped. Due to loss of the instrumentation air, boilers stopped, and supply of steam as well as air stopped altogether. Both air and steam, which are essential for continuous operation of the refinery, were lost, and all units, including the FCC, came to an emergency shutdown.

Fire near FCC air blower

The Wakayama Refinery's FCC (Exxon Model IV) has a capacity of 38 kBD and was built in 1956. Immediately after the loss of air and steam, flames and smoke rose from near air blower area that supplies combustion air to the FCC regenerator. The fire was localized and lasted several minutes before extinguishing itself. Although no one was injured, equipment such as blowers, piping, analyzers, instrumentation equipment, etc. were damaged, and it took about four months to restore and start operation.

Site observation after fire

FCC catalyst fine was observed in addition to oil in the flamed areas. From condition of the damaged equipment, it was presumed that the flame originated from vicinity of the air blower atmospheric vent. On the other hand, it was confirmed that vacuum gas oil (VGO), which is feed to the FCC, flows into the steam supply piping in the FCC unit, and that VGO and the FCC catalyst flow into a part of the combustion air piping to the FCC regenerator.

Mechanism leading to fire

The FCC unit is designed so that when the instrumentation air supply is cut off, an emergency sequence is activated to open and close specified air-operated valves and the unit stops safely. The sequence includes isolating the reactor from the regenerator, stopping the feed oil supply to the reactor, cut off the fuel gas to heating furnace, and introducing sparger steam to prevent combustibles and catalyst from leaking out of the unit.

Upon checking opening/closing status of the air-operated valves related to the sequence, it was found that a valve for stopping the feed oil supply to the reactor had not closed, and was manually closed by an operator using handwheel about 20 minutes after the sequence actuated. During this period, it is believed that the feed oil continued to be supplied to the reactor.

On the other hand, steam that should have been introduced to the reactor and regenerator was not introduced due to the loss of steam supply following the outage of instrument air.

The feed oil, which had been continuously supplied to the reactor, flowed into the steam supply line that lost pressure, then reached the combustion air piping via the steam introduction line to the regenerator. On the regenerator side, steam that should have been introduced via the combustion air piping was not introduced, and the hot FCC catalyst from the regenerator flowed back into the combustion air piping.

As a result, VGO, which is the feed oil to the FCC unit, came into contact with the hot FCC catalyst in the combustion air piping, and part of the VGO was decomposed to produce naphtha-class light oil. Due to the loss of instrumentation air, the air blower vent valve had been opened. Then, it is presumed that these mixtures in the combustion air piping were released from the air vent, exposing the feed VGO & light oil (combustible) and high-temperature catalyst (ignition source) were exposed to surrounding air, leading to ignition and the fire.

Countermeasures to prevent recurrence

Following countermeasures have been taken to prevent recurrence of this trouble (some of which are planned).

1) Stopping feed oil pumps

Feed pump was designed to continue running and circulating feed oil to preheat furnace and recycle back to feed surge drum to mitigating tube coking as emergency shutdown action from original design. The system was modified so that the feed pump is stopped at the same time when the emergency sequence is activated.

2) Change of management of valve in feed line

No abnormalities were found in the subsequent overhaul inspection of the feed air-operated valve that was not closed during FCC emergency shutdown. In addition to conventional yearly onstream open/close activation test, management procedure was changed so that the activation test is also conducted one month after start of operation. Furthermore, in event of an emergency shutdown of the FCC unit, the procedure was changed so that visual check of closure of this valve at the field would be prioritized.

3) Replacing the check valve of the regeneration air-supply line (planned)

One of causes of the fire was that the check valve in the combustion air piping did not work properly. Replace the existing check valve with a more reliable powered check valve.