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Engineering
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G-24
Trim — All parts of the valve which are in contact with
the owing media, but are not part of the valve shell or
casting. Ball, stem, disc, plug, and seat are all considered
trim components.
Turndown — Ratio between the maximum usable ow
and the minimum controllable ow. Turndown is usually
less than Rangeability, and cannot be applied to a valve
exclusive of the specic application it is placed in, It is a
function of the valve, actuator, piping, coil, and all other
system parameters that determine the maximum usable
ow. Since the valve only has reasonable control over one
part of the ratio, the minimum controllable ow, this is not
a good criteria for evaluating
valve quality.
Two-way Valve — A valve body with a single ow path —
one inlet and one outlet.
Valve — A control device which will vary the rate of ow
of a medium such as water or steam.
Valve Actuator — A device that uses a source of power
to position or operate a valve, sometimes also called a
valve operator. The source of power may be anything,
examples include manual (via a hand wheel), pneumatic,
or electronic.
Valve Authority — Valve authority is measured as the
percentage of the differential pressure across the valve
divided by the differential pressure of the entire loop or
branch controlled by the valve, multiplied by 100. As a rule
of thumb, valve authority should be between 25% and 50%
for good control of the loop/branch. Alternatively, from the
2012 ASHRAE Handbook, “Using ow coefcient analysis,
however, results in a slightly modied denition for
authority, comparing the ow coefcient of the valve (Cv)
to the coefcient of the remaining system components
(Cs).” (2012 ASHRAE Handbook, 13.14) Valve authority
using this denition would ideally have the ow coefcient
of the valve matching the ow coefcient of the rest of
the system, resulting in an ideal value of 1. It is therefore
important when discussing valve authority to be clear on
which denition is being used.
Valve Body — The portion of the valve casting through
which a controlled medium ows.
Valve Disc — The movable part of a buttery valve which
makes contact with the seat when the valve is closed.
Valve Flow Characteristic — The relationship between the
stem travel, expressed in percent of travel, and the ow
of the uid through the valve, expressed in percent of full
ow or gallons per minute.
Valve Guide — The part of a globe valve throttling plug
that keeps the disc aligned with the valve seat.
Velocity — The rate of movement for air or water, distance
per unit time.
Viscous — Having a relatively high resistance to ow.
Volumetric Air Flow — Area x Velocity.
Wire Draw — The process where high velocity media
erodes a path across the mechanical components of a
valve. This typically occurs in a stroke valve when the valve
is operated primarily with the plug very close to the seat ,
causing very high velocities of media across the plug and
seat. The damage appears as if a wire has been drawn
across the components. This differs from the other typical
valve mechanical damage modes – cavitation and ashing
– where the surface appears to have been pulled away as
or struck by very small particles, respectively.
Terminology