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VALVE SELECTION AND SIZING
INTRODUCTION
This section provides information on valve selection and
sizing. Valves must be selected for ability to meet temperature,
pressure, flow control characteristic, and piping connection
requirements of the hydronic system. Valve sizing is critical to
DEFINITIONS
VALVE COMPONENTS
Actuator: The part of an automatic control valve that moves the
stem based on an electric, electronic, or pneumatic signal
from a controller. The actuator and valve can be two
separate devices or together they can be one device.
Body: The valve casting through which the controlled fluid
flows (Fig. 1).
STEM
BONNET
DISC
HOLDER
DISC
SEAT
IN
BODY
PLUG
Fig. 1. Globe Valve Components.
Bonnet: The part that screws to the top of the valve body and
contains the packing that seals and guides the valve stem.
Disc: The part of the valve assembly that contacts the valve
seat to close off flow of the controlled fluid. Some
valve assemblies are built so the disc is replaceable.
Replaceable discs are usually made of a composition
material softer than metal.
Plug: The part that varies the opening for the fluid to flow
through the valve body. The following describes the
three most common types of plugs:
— A contoured plug has a shaped end that is usually
end-guided at the top or bottom (or both) of the
valve body. The shaped end controls fluid flow
through the valve with respect to stem travel.
ensure support for heating and cooling loads with adequate
valve capacity, yet able to control system flow to provide stable
building conditions efficiently.
— A V-port plug has a cylinder, called a skirt, that
— A quick-opening plug is flat and is either end-
Port: The opening in the valve seat.
Seat: The stationary part of the valve body that has a raised lip
Stem: The shaft that runs through the valve bonnet and connects
Trim: All parts of the valve that contact the controlled fluid.
OUT
VALVE FLOW CHARACTERISTICS
M12225
Direction of flow: The correct flow of the controlled fluid through
Equal percentage: A valve which changes flow by an equal
Linear: A valve which provides a flow-to-lift relationship that
428
rides up and down in the valve seat ring. The skirt
guides the plug and varies the flow area with
respect to stem travel via its shaped openings.
guided or guided by wings riding in the valve
seat ring. The flat plug provides maximum flow
soon after it lifts from the valve seat.
to contact the valve disc when closing off flow of the
controlled fluid.
an actuator to the valve plug.
Trim includes the stem, packing, plug, disc, and seat;
it does not include the valve body.
the valve is usually indicated on the valve body. If the
fluid flow through the valve is incorrect, the disc can
slam into the seat as it approaches the closed position.
The result is poor control, excessive valve wear, and
noisy operation. In addition, the actuator must work
harder to reopen the closed valve since it must overcome
the pressure exerted by the fluid on top of the disc rather
than have the fluid assist in opening the valve by exerting
pressure under the disc.
percentage (regardless of flow rate) for similar
movements in stem travel (at any point in the flow range).
is directly proportional. It provides equal flow changes
for equal lift changes, regardless of percentage of valve
opening.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
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