What device is installed in an oil supply line to prevent seepage after shutdown?

The check valve is a crucial component installed in an oil supply line to prevent the backflow of oil, which can lead to seepage after the system is shut down. It allows oil to flow in one direction while preventing any reverse flow that could occur when the pressure drops, as it often does after shutdown. This mechanism is essential for maintaining system integrity, ensuring that oil does not leak out of the system when not in operation.

In contrast to this function, other devices listed serve different purposes. A pressure relief valve is used to maintain safe pressure levels within a system by venting excess pressure, and a flow meter is designed to measure the quantity of oil passing through the line, while an oil filter's role is to remove contaminants from the oil to ensure the cleanliness of the oil being circulated. Thus, the specific intention of preventing seepage when the gas turbine engine is off is best served by the check valve.