V-G meter. See: direct-indicating viscometer.
VAC. Alternating current voltage.
Valence. A number representing the combining power of an atom, that is, the number of electrons lost, gained, or shared by an atom in a compound. It is also a measure of the number of hydrogen atoms with which an atom will combine or replace, for example, an oxygen atom combines with two hydrogens, hence has a valence of 2. Thus, there are mono-, di-, tri-, etc., valent ions.
Valence effect. In general, the higher the valence of an ion, the greater
the loss of stability to emulsions, colloidal suspensions, etc., these polyvalent ions will impart.
Velocity. Time rate of motion in a given direction and sense. It is used as a measure of fluid flow and may be expressed in terms of linear. velocity, mass velocity, volumetric velocity, etc. Velocity is one of the factors that contribute to the carrying capacity of a drilling fluid.
Velocity head. Head (relating to pressure when divided by the density of the fluid) created by movement of a fluid, equal to an equivalent height
of static fluid.
Venturi. Streamlining up to given pipe size following a restriction (as in a jet in a mud hopper) to minimize turbulence and pressure drop.
Vibrating screen. A screen with motion induced as an aid to solids
separation. See: shale shaker.
Vibration isolaters. Elastomers ranging from solid to air-pressured or springs that allow the shale shaker screens to vibrate but do not transmit the vibratory motion to the rest of the machine.
Vibrators. Weights rotated about an axis that does not pass through the
center of mass.
Viscometer. An apparatus to determine the viscosity of a fluid or
suspension. Viscometers vary considerably in design and methods of
testing.
Viscosifiers. Material added to a drilling fluid to increase the low-shearrate
viscosity.
Viscosity. The ratio of shear stress to shear rate in a fluid. If the shear
stress is measured in dynes/cm2 and the shear rate in reciprocal
seconds, the ratio is the viscosity, in Poise. Viscosity may be viewed as
the internal resistance offered by a fluid to flow. This phenomenon is
attributable to the attractions between molecules of a liquid and is
a measure of the combined effects of adhesion and cohesion to
the effects of suspended particles and to the liquid environment. The
greater this resistance, the greater the viscosity. (2) A characteristic
property of a fluid, liquid, or slurry crudely defined as resistance
to flow (by accurate definition the ratio of shear stress to shear rate).
See: apparent viscosity, plastic viscosity, API RP 13B.
Viscosity gravity (V-G) meter. The name more commonly used for the
direct-indicating viscometer. See: viscometer.
Viscous flow. See: laminar flow.
Volatile matter. Normally gaseous products given off by a substance,
such as gas breaking out of live crude oil that has been added to a
drilling fluid. In distillation of drilling fluids, the volatile matter is the
water, oil, gas, etc., that are vaporized, leaving behind the total solids,
which can consist of both dissolved and suspended solids.
Volt. The unit of electrical pressure or electromotive force. One volt
produces a current flow of 1 ampere through a resistance of 1 ohm.
Glossary 647
Volume percentage. Volume percentage is the number of volumetric
parts of the total volume. Volume percentage is the most common
method of reporting solids, oil, and water contents of drilling fluids.
See: weight percentage, ppm.
Vortex. A cylindrical or conical-shaped core of air or vapor lying along
the central axis of the rotating slurry inside a hydrocyclone.
Vortex finder. A cylinder extending into the upper end of a hydrocyclone
to cause drilling fluid to move in a circular spiral direction within the
cone and prevent the entering fluid from short circuiting directly to the
hydrocyclone overflow.
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