Definition
Float Sub — A float sub is a short sub in the bottomhole assembly that houses a float (back-pressure check) valve, allowing drilling fluid to flow down the string while preventing backflow of mud and cuttings up through the bit.
Also known as: float sub, float valve sub, non-return sub
What a float sub is
A float sub carries a float valve — a one-way check valve — in the drill string, usually just above or as part of the bit sub. The valve lets drilling fluid pump freely down the bore but closes against any upward flow, so mud and cuttings cannot U-tube back up through the bit when pumps are shut off.
This backflow prevention keeps the bit nozzles from plugging, holds a barrier against formation fluid entering the string, and keeps the string full for accurate volume tracking. Float valves come in flapper (plunger) and dart (drop-in) styles.
Key points
- Check valve function — free flow down, no flow up.
- Well control — provides a string-side back-pressure barrier during connections.
- Types — spring-loaded flapper, plunger, and drop-in dart valves.
Renting float subs
Float subs and valves are standard, inspected BHA rental items sized to the collar bore. Explore BHA components and downhole tools, or contact us to include the correct float valve in your assembly.