A drill pipe float valve, also known as a non-return valve (NRV) or float sub, is a downhole tool installed in the drill string, typically in the drill pipe just above the bit or in a special sub. Its primary purpose is to prevent backflow of fluids from the wellbore into the drill string.

Main Functions of a Float Valve:

1. Prevents backflow of drilling mud or formation fluids into the drill string.
2. Maintains pressure control during connections and bit trips.
3. Improves well control by reducing the risk of kick or influx when pumps are off.
4. Minimizes mud spillage during pipe disconnection at surface.
5. Helps during float drilling, reducing the hydrostatic pressure at the bit.

Placement in the Drill String:
-Installed in a float sub just above the bit or in the bit itself if compatible.
-It can be retrievable or non-retrievable depending on the design and drilling program.

Operational Considerations:
-Pressure Drop: Causes a slight pressure drop in the circulation system.
-Stuck Valve: Valves may stick open or closed due to solids or debris.
-Maintenance: Requires regular inspection and cleaning.
-Compatibility: Must be compatible with downhole tools like MWD/LWD if present.