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Engineering
G-23
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.
PICV — A Pressure Independent Control Valve is a control
valve and automatic differential pressure regulator
in a single device. The differential pressure regulator
automatically adjusts to changes in differential pressure in
the system to maintain a consistent ow corresponding to
the given position of the control valve portion of
the device.
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.
Presetting — Presetting is the part of the adjustable ow
limiter in Siemens Pressure Independent Control Valves
used to set the maximum ow of the valve. It can also refer
to the setting of the ow limiter that the valve was set to at
the factory.
Pressure Drop — The difference in pressure between
the inlet and outlet ports of the control valve, commonly
refered to as ΔP (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.
Resolution — Resolution applies to the valve actuator.
The resolution of an actuator denes the smallest discrete
increment the actuator can position to relative to the total
control signal range. For example, with a modulating
actuator that controls to a tenth of a volt, and has a 0 to
10 Volt control signal, can control to within 1/100th of the
entire control range, therefore a resolution of 100:1.
Saturated Steam — Steam which is at its lowest possible
temperature at a given pressure without a phase change
to liquid.
Screwed- end connection — A valve body with a threaded
pipe connection, usually female NPT threads, in valve
bodies through 2”.
Seat — The stationary portion of the valve which seals the
valve, thus prevents ow, when in full contact with the
movable ball, plug or disc.
Static Pressure rating — The maximum pressure that
the valve body will tolerate per a dened standard. The
standards may dene the pressure at temperatures other
than that observed, so one must understand the standard
to understand the actual pressure rating for the given
application. Common pressure standards for HVAC valves
in North America include ANSI (125, 250) and WOG
(300, 600), but others such as CWP are sometimes used.
Stem — The cylindrical shaft of a control valve moved by
an actuator, to which the throttling plug, ball or wafer disc
is attached.
Stroke — The total distance that a linear valve stem travels
or moves. It is also known as lift.
Superheated Steam — Steam at a temperature higher
than saturation temperature at the given pressure.
System Pressure Drop — The sum of all pressure drops in
a Hydronic system.
Three Way Valve — A valve body with one inlet and two
outlets or two inlets and one outlet.
Tight Shut-off — A valve body with no ow or leakage in
a closed position. This is relative to the dened tightness of
the seal, usually dened by a measurement standard. The
most common standard is ANSI/FCI 70 -2, which classies
“tightness” from Class I to Class VI. Class I is non-dened
leakage, Class II through Class IV are descriptive based on
leakage as a percent of total capacity, and Class V and Class
VI are descriptive based on leakage as a nite rate per inch
of orice diameter. Since the criteria and testing method
for Class II – IV are signicantly different than Class V – VI,
these groups cannot be directly compared.
Control Valves
Selection and Sizing