The Valve wheel is a key item found in the level Trajan's markets in Tomb Raider Chronicles. To find the chamber where it's located Lara has to swim in the underwater tunnels in the sewers after defeating the Laser Head.
Lara has to pry the wheel off from a machine by using her crowbar and take it to the room where the Laser Head was. There's a small side room with reddish lightning and a machine which is missing a part. The Valve Wheel must be placed to it and then turned to switch off some Water Fans in the sewers.