Guardian valves are installed on main propulsion turbines primarily to:

Guardian valves are specifically designed to prevent steam from leaking into the astern element while the turbine is operating at full sea speed. This is crucial because if steam were to leak into areas not intended for it, it could create inefficiencies, potential damage to the turbine system, and disruptions in propulsion. By effectively sealing off these sections when not in use, guardian valves ensure that the system operates safely and efficiently, allowing for proper performance of the main propulsion turbines.

The other options relate to different functions within steam systems but do not accurately describe the primary purpose of guardian valves in the context of main propulsion turbines.