Device that can be installed in the drillstring that acts as a check valve, allowing drilling fluid to be circulated down the string but preventing back flow.
Source: API STD 53
Note: A device that is installed in the drillstring to allow fluid to flow in one direction only. This is classed as a tested barrier in API 53 table C. Generally it is installed on top of the DPSV after the well has been made secure in preparation for stripping into the well.