Exxon says Baytown refinery units back after upsets

(Recasts with Exxon statement, adds details, background)

HOUSTON, April 26 (Reuters) - Exxon Mobil Corp (XOM.N) said on Sunday a hydrocracker and a flexicoker were back in operation at its giant 567,000 barrel-per-day (bpd) Baytown, Texas, refinery following Friday night malfunctions triggered by heavy rain.

The heavy rain triggered a drop in steam pressure, causing the units to malfunction, according to a notice filed by Exxon with the Texas Commission on Environmental Quality.

"Those units are back," said Exxon spokeswoman Neely Nelson on Sunday. "There was no impact on supply."

More than 5 inches (12.7 centimeters) of rain fell on the refinery, located 28 miles (45 km) east of downtown Houston, Exxon said in the notice. Most of the rain, 3.4 inches (8.6 cm), fell in the first hour of the storm.

Heavy rain can rapidly and drastically cool refinery units and piping, which operate at very high temperatures, affecting production.

A hydrocracker uses hydrogen under high heat and pressure to increase the amount of motor fuel refined from a barrel of oil. A coker increases the amount of material that can be refined into motor fuel. Both increase a refinery's ability to process cheaper, high-sulfur crude oils.

Exxon's Baytown refinery is the largest in the United States. (Reporting by Erwin Seba; Editing by Diane Craft)