Brochure

Engineering
www.usa.siemens.com/hvac
G-22
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.
Flow Characteristic — The relation between volumetric
ow and valve position.
Flow Coefcient — The ow coefcient is the constant
that relates volumetric ow, differential pressure, and
specic gravity of a uid through a metering device.
Cv is the ow coefcient in imperial units. For liquids
through a standard orice it is calculated to be equal
to the volumetric ow in gallons per minute times the
square root of the specic gravity divided by the square
root of the differential pressure in psi. For water systems
the specic gravity can be assumed to be 1, therefore it is
often simplied to GPM divided by the square root of ΔP.
For HVAC applications, a control valve closely follows this
orice model.
Flow Limiter — A ow limiter is a device used for limiting
the maximum ow. This can be accomplished using a
manual balancing valve or with the eld adjustable ow
limiter integrated in the Siemens Pressure Independent
Control Valves (PICV).
Flow Rate — The volume of media conveyed per unit of
time. Typical US units are gallons per minute (GPM) for
water and pounds per hour (#/hr) for steam.
FPM — Feet per minute.
Full Port — Maximum ow capacity possible for a
particular ball valve orice. In a ball valve, this typically
refers to a valve with no ow characterizer or restrictor.
Gauge Pressure — Pounds per square inch (PSI) as read
on a gauge face. This differs from Absolute Pressure in
that it is relative to the current ambient pressure, not a
xed reference such as absolute vacuum. Gauge pressure,
therefore, uses the local ambient pressure as its zero point
(14.7 psia at sea level and standard conditions).
GPM — Gallons per minute.
Incompressible — Description of liquids, because their
change in volume due to pressure is negligible.
Laminar Flow — Also known as viscous or streamlined
ow. A non-turbulent ow regime in which the stream
laments glide along the pipe axially with essentially no
transverse mixing. This is usually associated with viscous
liquids. The area inside a valve is typically turbulent —
the opposite of laminar.
Linear Flow Characteristic — A ow characteristic in
which the percentage of maximum ow is equal to the
percentage of maximum stroke of the valve. For example,
50% stroke would provide 50% of the maximum ow of
the valve. In other words, “Linear valves produce equal ow
increments per equal stem travel throughout the travel
range of the stem.” (2012 ASHRAE Handbook, 13.14)
Load — A demand on the mechanical equipment in an
HVAC system.
Load Change — A change in the building cooling or
heating requirements as a result of air temperature
variations, caused by wind, occupants, lights, machinery,
solar effect, etc.
Mixing Valve — A three way valve having two inlets and
one outlet. The proportion of uid entering each of the
two inlets can be varied by moving the valve stem. These
valves are typically not suitable for diverting applications.
Normally Closed (N.C.) — Condition of the valve upon
loss of power or control signal to the actuator. Also as
relates to a stroke valve body that has been manufactured
as a N.C. valve body. In stroke valves, this is typically the
valve’s state when the stem is in the “up” position.
Normally Open (N.O.) — Condition of the valve upon loss
of power or control signal to the actuator. Also as relates to
a stroke valve body that has been manufactured as a N.O.
valve body. In stroke valves, this is typically the valve’s state
when the stem is in the “up” position.
Terminology