Brochure

www.usa.siemens.com/hvac
Engineering
G-21
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.
Booster Pump — A pump used in secondary loops of
hydronic systems to provide additional ow for that section
of the system.
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.
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.
Differential Pressure Regulator — A differential pressure
regulator is a device used to maintain consistent ow
regardless of differential pressure changes. A differential
pressure regulator can be an independent device, but
is part of a Pressure Independent Control Valve (PICV)
resulting in consistent ow corresponding to the given
position of the control valve portion of the device and a
pressure independent maximum ow corresponding to the
ow limiter setting of the device.
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.
Terminology
Control Valves
Selection and Sizing