SINGAPORE: Royal Dutch Shell Plc unexpectedly shut a gasoline-making fluid catalytic cracker (FCC) at its 500,000 barrels-per-day (bpd)Singaporerefinery, and is currently restarting the unit, trading sources said on Monday.

The restart process of the 33,000 bpd FCC unit was expected to be completed soon, one of the sources said.

The reason for the outage was not clear, but a power failure could likely have caused it, leading to the flaring of gases at the unit, a second source said. Black smoke was seen emerging from the site, traders added.

Shell was not immediately available for comment.

The Singapore Civil Defence Force (SCDF) said it received a call at10 a.m.local time (0200 GMT) about a fire alarm on Pulau Bukom, where the Shell refinery is located.

"SCDF resources were dispatched. However our services were not required as the incident was already resolved prior to our arrival," the SCDF said in an e-mailed statement.

Shell operates three crude distillation units, a condensate splitter, the fluid catalytic cracker, a hydrocracker and an ethylene cracker at the Bukom site.

The entire refinery shut down in September last year after a fire struck the complex.