Basic Documentation

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Absolute Pressure — Absolute pressure is referenced to a
theoretical perfect vacuum. At standard atmospheric pressure,
absolute pressure may be calculated by adding 14.7 psi to the
observed gauge pressure.
Ambient Temperature Rating – Ambient temperature refers to the
temperature of the air surrounding the device.
Angled Body — A two way valve body that has connection points
at right angles to each other.
Buttery Valve — A valve utilizing a disk rotating on a shaft to
provide control and close off. Alternately, a check valve utilizing two
semi-circular hinged plates to permit ow in one direction only.
Booster Pump — A pump used in secondary loops of hydronic
systems to provide additional ow for that section of the system.
Cavitation — The forming and imploding of cavities in a liquid
due to rapid pressure changes, producing shock waves and cyclic
stresses that can lead to undesirable noise and/or surface fatigue
damage.
Close-off Rating — The maximum differential pressure, inlet to
outlet, that a valve will close off against while uid is owing to a
given leakage rate (tightness) criteria. In a stroke valve, the primary
determinants are the force available from the actuator, the diameter
of the plug, and the valve design. In a rotary valve, such as a ball
valve, the primary determinant is typically the seal design as the
torque of the actuator has little effect.
Close-off Rating of Three Way Valves — The maximum pressure
difference between either of the two inlet ports and the outlet port
for mixing valves, or the pressure difference between the inlet port
and either of the two outlet ports for diverting valves.
Contoured Plug — In a globe valve, a contoured plug uses its
peripheral shape to affect a desired ow characteristic. This is
typically linear, equal percentage, or a modication of these. These
are differentiated from V-plugs, basket plugs, cage plugs, and
the like by the fact that the media ows around the plug and not
through it.
Controlled Medium — The controlled medium is the material that
is being conveyed and controlled though the device. In typical HVAC
systems this includes air, water, and/or steam. It may also include
fuel oil, natural gas, refrigerants, etc.
Critical Pressure Drop — The maximum pressure drop across
a valve at which gasses and vapors will follow standard ow
calculations. Pressure drops greater than this produced what is
known as “choked ow” and sizing criteria will no longer accurately
predict the volumetric ow.
Design Conditions — The assumed environmental variables that
dene the performance limits required of a HVAC system. This may
include maximum and minimum outside air temperatures, expected
solar and other thermal loads, occupancy levels, etc.
Direction of Flow — The ow of a controlled uid through the
valve is usually represented by an arrow on the valve body. If the
ow of the uid goes against the indicated direction, the disk can
slam into the seat as it approaches the closed position. The result
is excessive wear, hammering, and oscillations. Additionally the
actuator must work harder to reopen the closed valve since it must
overcome the pressure exerted by the uid on top of the disc, rather
than have the uid assist in opening the valve by exerting pressure
under the disc.
Diverting Valve — A three way valve that has one inlet and two
outlets. Water entering the inlet port is diverted to either of the two
outlet ports in any proportion desired by moving the valve stem.
These valves are not commonly use in modern control loops.
End Fitting — The part of the valve body that connects to the
piping. Union, screwed, ared, sweat and anged are typical
examples of end ttings.
Equalinear Flow — Valve Cv vs travel position is approximately
mid-way between that of linear and equal percentage.
Equal Percentage Flow Characteristic — An equal percentage
ow characteristic is one in which a ow rate change is proportional
to the ow rate just prior to the change in valve position. Equal
increments of valve travel result in equal percentage changes to the
existing ow rate. Flow capacity increases exponentially with valve
stem travel.
Flanged End Connections — A valve that connects to a pipe
by bolting a ange on the valve to a ange on the pipe. Flanged
connections are often used on larger valves, typically over 2”.
Flashing — In the context of control valves, ashing is related to
cavitation, but the mechanics are slightly different. Flashing occurs
when a liquid’s environment causes a rapid phase change from
liquid to gaseous phases. With ashing, the volume of vapor is
much greater than the volume of liquid, and rapidly accelerates the
remaining liquid droplets, which forcefully impact the mechanical
components of the valve and pipes, causing damage. This situation
can be calculated by knowing the pressures and temperatures
involved, as well as the vapor pressure of the liquid at those
temperatures. Cavitation often occurs in environments that have
not yet reached the point of ashing, due to uid ow dynamics
and velocities.
Terminology