Vandals bent on opening fire hydrants will have an increasingly tougher time as the city steps up installation of a new generation of locking devices, according to Chicago Water Management Department officials.

Under a $3.65 million contract awarded recently to a Texas firm, as many as 10,000 so-called Custodian nut-locking systems will go on hydrants during the next five years, joining 12,000 already in service.

Placement of the devices will be determined by analyzing 311 calls of unauthorized hydrant openings, officials said.

The new model represents the eighth generation of locking devices as designers seek to stay a step ahead of people who try to misuse hydrants, said Tom LaPorte, a Water Management Department spokesman.

“You never are going to have a completely tamperproof lock, but this has been very effective in discouraging open hydrants,” he said. “In the old days, you might have seen 3,000 or 4,000 being opened on a given day during a bad heat wave. Now we are down into the low hundreds.”

The new devices have saved labor hours, LaPorte said. In the past, the city “might have spent a couple of hundred thousand dollars” a year in overtime for crews dispatched to turn off open hydrants. In the last two years “we haven’t spent a dime,” he said.

Chicago has more than 45,000 hydrants. Those not equipped with the Custodian models have less sophisticated locking systems.

Firefighters and Water Management crews will have keys to unlock the new bullet-shaped caps, manufactured by Hydra-Shield Manufacturing Inc. of Irving, Texas.Open hydrants can result in a variety of problems, including reduced water pressure, officials said.

Perhaps the biggest concern is children playing in the streets under sprays of water that obscure the vision of drivers, who tend to speed up when they go through, LaPorte said.

Basements of neighboring buildings also can be flooded. And if a water main is old and brittle, it can rupture if a hydrant is opened too quickly, LaPorte said.