The manager of a Shell Oil complex that spilled 175,000 gallons of crude oil, killing scores of animals and birds, says workers broke company policy by keeping open a storm-drain valve that allowed the pollution to spread.

But Shell, which has promised to pay for the $1 million cleanup in the Carquinez Strait, denied suggestions by a state official that the design of its storm drainage system may have violated federal anti-pollution regulations.The spill resulted from the rupture of a 4-inch drainage pipe intended to carry rainwater from the roof of a 300,000-barrel oil tank through the inside of the vessel and into the ground beneath a containment dike at the base of the tank, Larry Kolb of the Regional Water Quality Control Board said.

The oil instead flowed out the bottom of the storm pipe, where a shutoff valve had been left open against company rules, said Ron Banducci, manager of the Shell complex.

When the storm-drain valve was left open last week, "They did not anticipate a major mechanical failure," he said Thursday. "If the valve had been closed, it would have prevented the spill."

Banducci did not cite any individual employee and said he did not know whether supervisors condoned leaving the valve open.

Banducci said that despite a policy that such valves remain closed, they were routinely left open when it rained.

"The procedure said one thing, and the actual practice was something else," said Shell spokesman Bill Sharkey.

The leak produced one of the worst oil spills in the San Francisco Bay area in recent years, blackening a surrounding marshland and killing nearly 200 migratory birds and small mammals. Fish and game workers fired cannons at regular intervals around the clock to keep birds from landing in the blackened area.