Basic Documentation

www.usa.siemens.com/hvac 197
Valve Reference
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 ∆P. For HVAC applications, a control
valve closely follows this orice model.
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.
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.
NPT — A pipe thread standard describing tapered pipe threads,
common in North America (National Pipe thread – Tapered).
Packing — Seals used around the valve stem so that the controlled
medium will not leak outside the valve.
Port — Opening (inlet or outlet) that allows ow through a
valve body.
Positive Positioner — A device that eliminates the actuator shaft
positioning error due to load on the valve body. This device is closed
loop, and applies the necessary force required to positively position
the valve stem to a referenced (commanded) position.
Pressure Drop — The difference in pressure between the inlet and
outlet ports of the control valve, commonly refered to as ∆P (delta P).
PSI — Pounds per square inch.
PSIA — Pounds per square inch absolute.
(Also see Absolute Pressure.)
PSIG — Pounds per square inch gauge.
(Also see Gauge Pressure.)
Rangeability — The ratio of the maximum controllable ow to
the minimum controllable ow. As an example, a valve with a
rangeability of 50 to 1 having a total ow capacity of 100 GPM, fully
open, will be able to control ow accurately down to 2 GPM.
Reduced port — A smaller ow capacity that is possible for the
particular end tting.
Reducer — A pipe tting that is used to couple a pipe of one size to
a pipe of a different size. An increaser may be used when the pipe
sizes are reversed.
Saturated Steam — Steam which is at its lowest possible
temperature at a given pressure without a phase change to liquid.