2. n. [Drilling]
The vertical height of a fluid column, regardless of the length or other dimensions of that fluid column. For example, a deviated wellbore has a longer length than vertical depth. The hydrostatic head at any point in that wellbore is not a function of its measured depth (MD) along the wellbore axis, but rather its vertical distance or true vertical depth (TVD) to the surface. The term "head" or "hydrostatic head" is also commonly used as a measure of the output of centrifugal pumps, usually expressed in "feet of head" or psi. Since this type of pump is a centrifugal (or "velocity") device, the capability of the pump as expressed in feet of head is independent of the density of the fluid being pumped. For example, if a pump is rated as producing "sixty feet of head," it will pump a column of fluid up an open-ended vertical pipe until the top of the liquid is 60 ft [18 m] above the discharge of the pump, regardless of the density of the liquid being pumped.