VISCOSITY - The resistance of fluid to flow. A high viscosity fluid will not flow as easily as a low viscosity fluid (Mud will not move as easily as water).
VALVE - a device used to control the rate of flow in a line to open or shut off a line completely, or to serve as an automatic or semiautomatic safety device. Those used extensively include the check valve, gate valve, globe valve, needle valve, plug valve, and pressure relief valve.
V-BELT - a belt with a trapezoidal cross section, made to run in sheaves, or pulleys, with grooves of corresponding shape.
V-DOOR - an opening at floor level in a side of a derrick or mast. The V-door is opposite the drawworks and is used as an entry to bring in drill pipe, casing, and other tools from the pipe rack.