Filter
Top Products
244
Appendix A: Valve Selection and Sizing
Appendix A: 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
ensure support for heating and cooling loads with adequate
valve capacity, yet able to control system flow to provide stable
building conditions efficiently.
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).
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. "Metal trim" valves use
precisely-machined metal plugs and seats operated by
high force actuators instead of a disk.
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.
— A V-port plug has a cylinder, called a skirt, that
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.
— A quick-opening plug is flat and is either end-
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.
Port: The opening in the valve seat.
Seat: The stationary part of the valve body that has a raised
lip to contact the valve disc when closing off flow of the
controlled fluid.
Stem: The shaft that runs through the valve bonnet and
connects an actuator to the valve plug.
Trim: All parts of the valve that contact the controlled fluid.
Trim includes the stem, packing, plug, disc, and seat; it
does not include the valve body.
Valve Flow Characteristics
Direction of Flow: The correct flow of the controlled fluid
through 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.
Gate and butterfly valves may offer bi-directional flow.
Equal percentage: A valve which changes flow by an equal
percentage (regardless of flow rate) for similar
movements in stem travel (at any point in the flow
range).
Linear: A valve which provides a flow-to-lift relationship that is
directly proportional. It provides equal flow changes for
equal lift changes, regardless of percentage of valve
opening.
Quick-opening: A valve which provides maximum possible
flow as soon as the stem lifts the disc from the valve
seat.
Valve flow characteristic: The relationship between the stem
travel of a valve, expressed in percent of travel, and
the fluid flow through the valve, expressed in percent
of full flow.
M12225
STEM
BONNET
SEAT
DISC
BODY
PLUG
DISC
HOLDER
OUT
IN