Control Valve Terminology
Authority
The ratio of the pressure drop across the valve when it is fully open, to the pressure drop across the whole branch
circuit the valve is controlling.
Ball valve
A ball valve has a precision ball between two seats within a body. Ball valves can have several port sizes for a given body size and go from open to closed with a 90-degree turn of the stem.
Bonnet
The upper part of a valve body that is usually separable and contains the valve stem seals and guides.
BSP (a.k.a. BPT)
British standard pipe (a.k.a. British pipe thread) – the most common thread pattern in Europe.
Butterfly valve
A valve consisting of a cylindrical body with a rotating vane to control flow. Tight shutoff is obtained by an elastomer
body liner into which the vane seats. Advantages of butterfly valves are low flow resistance in open position,
compact overall size, and relatively low cost due to simple design.
Cage globe valve
A type of double-seated valve that uses a cage style plug with upper and lower seat/seals. Cage valves usually have
a lower leakage rating than a basic double-seated valve.
Cavitation
When the pressure of a flowing liquid falls below the liquid’s vapor pressure, vaporization occurs and bubbles form.
When the pressure then rises above the vapor pressure the bubbles implode and can damage valve and piping
surfaces.
Close-off
The maximum differential pressure (psid) across a valve at which point the actuator can still keep the valve tightly
closed.
Cv
The valve flow coefficient or valve flow capacity. Cv is the water flow rate, in gallons per minute, that causes a one
psi pressure drop across a fully open valve.
Diverting valve
A three-way valve that allows water to flow into one opening (inlet) and divert to one of two openings (outlets). Most
often used for two-position control but can be used in a modulating application.
Double-seated
globe valve
A valve with two seats and plugs, suitable for higher close-off requirements because of the “pressure-balanced”
design.
Equal percentage
A flow characteristic which controls flow exponentially (non-linear) compared to the actuator stoke. This design is
used to inversely match the natural BTU output characteristic of a water or steam coil, for a resultant of somewhat
linear system response.
Fail-safe actuator
An actuator that automatically goes to a predictable safe position when actuator power (electric or pneumatic) is not
present.
Floating
Also called tri-state or three-position, it is a type of modulating control based on timing, stroke length, and stroke
speed of an actuator. An electric motor is driven either clockwise or counterclockwise for a specific amount of time,
to reach a given position within two stroke limits.
Globe valve
A valve that controls flow by moving a circular disk or plug in and out of a rigid seat. The stem/plug movement is
linear, as opposed to rotary. The early body castings were globe-shaped, thus the name.
GPM
Gallons per minute.
Head pressure
The maximum pressure a pump can exert in a piping system, usually indicated in feet of water units.
The significance is that the valve furthest from the pump must close off against the full force of the pump head
pressure. Ft-H2O = psi x 2.309
Kv
The metric flow coefficient or valve flow capacity. Cv = Kv/0.855
Linear flow
A flow characteristic that controls flow linearly to the actuator stroke. This is commonly used for two-position control
and some quick opening applications.
Linear valve
A term used (mostly by Europeans) to describe a globe style valve because the stem moves in a linear motion, as
opposed to a rotary motion, to control the flow.
Mixing valve
A three-way valve that allows water to flow into two of the openings (inlets), mix, and out of the third opening (outlet).
Most often used for modulating control to adjust water temperature.
Modulating
A control signal term (and actuator type) which refers to positioning anywhere between fully open and fully closed.
It can be accomplished using tri-state (floating), or proportional signal types.
N.C.
Normally Closed refers to a valve/actuator assembly that is fully closed, allowing no flow, when actuator power
(electric or pneumatic) is not present. This term is associated with fail-safe return or spring return actuation.
N.O.
Normally Open refers to a valve/actuator assembly that is fully open, to allow full flow, when actuator power
(electric or pneumatic) is not present. This term is associated with fail-safe return or spring return actuation.
Non-spring return
An actuator without a spring. Most often used for non-fail-safe applications; it stays in its current position when
power is not present.
NPT connection
National Pipe Tapered – the most common thread pattern in the U.S.
Pneumatic actuator
Any actuator that uses air to actuate the valve. It can be low pressure (15-30 psi) or high pressure (30-150 psi).
Before the development of modern electronics, these were the most popular way to operate a valve.
Proportional
A type of modulation where the positioning of the controlled device is in response to an analog signal (example 4-20
mA, or 2-10 VDC).
PSIA
Pounds per square inch of pressure absolute. Pressure inside the pipe compared to an absolute vacuum.
Same as psig + 14.7 = psia at sea level.
PSID
Pounds per square inch of pressure differential. Pressure difference from one side of a device to the other side.
Also called pressure drop across a valve or coil.
PSIG
Pounds per square inch of pressure gauge. Pressure inside the pipe compared to the outside of the pipe.
Rangeability
The Ratio of maximum valve flow capacity to minimum controllable flow capacity.
Rotary valve
Any control valve which controls flow by rotation of the valve stem. Ball valves and butterfly valves are rotary.
Spring return
An actuator that relies on a mechanical spring to drive it fully closed or fully open when power is not present.
Sweat (a.k.a. solder)
connection
Copper tubing and valve are connected with solder. Generally used on smaller valve sizes.
1/2" valves use 5/8" OD copper tubing
3/4" valves use 7/8" OD copper tubing
1" valves use 1 1/8" OD copper tubing
Three-way valve
A valve that has three openings, a combination of inlets and outlets. Most are designed specifically for mixing or
diverting applications.
Trim
The parts of a valve that are in contact with the flowing medium but are not part of the valve shell or casting. Thus,
plugs, seats, discs, stems, packing rings, etc. are all trim components.
Turndown ratio
The ratio between maximum usable flow and the minimum controllable flow; usually less than the rangeability. In
comparing rangeability and turndown, rangeability is a measure of the predicted stability of the control valve, and
turndown is a measure of the actual installed stability of the valve.
Two-position
A control signal term (and actuator type) that refers to fully open or fully closed, no in between position. Referring to
the actuator/valve this either allows full flow or no flow (on or off).
Two-way valve
A valve that has two openings, one inlet and one outlet. Control can be two-position or modulating.
Union connection
A male thread tailpiece held to the valve body with a nut, this connection type allows removal of a valve without
cutting or twisting the piping.
Yoke
The area of a globe valve bonnet, or upper body, where the actuator is attached to the valve body.
Zone valve
A general term used for valves usually smaller than 1" in size and designed for use in unitary HVAC equipment that
serves a specific area or zone of a building. Also called unitary valves.