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BALANCING VALVES
Two approaches are available for balancing hydronic systems:
(1) a manual valve with integral pressure taps and a calibrated
port, which permits field proportional balancing to the design
flow conditions;
(2) or an automatic flow-limiting valve selected to limit the
circuit’s maximum flow to the design flow.
Manual Balancing Valves
Manual balancing valves can be provided with the following features:
• Manually adjustable stems for valve port opening or a
combination of a venturi or orifice and an adjustable valve
• Stem indicator and/or scale to indicate the relative amount
of valve opening • Pressure taps to provide a readout of
the pressure difference across the valve port or the venturi/orifice
• Capability to be used as a shutoff for future service
of the heat transfer terminal
• Locking device for field setting the maximum opening
of a valve
• Body tapped for attaching drain hose
Manual balancing valves may have rotary, rising, or nonrising
stems for port adjustment
Meters with various scale ranges, a field carrying case, attachment
hoses, and fittings for connecting to the manual balancing valve
should be used to determine its flow by reading the differential
pressure. Some meters employ analog measuring elements with direct-reading
mechanical dual-element Bourdon tubes. Other meters are electronic
differential pressure transducers with a digital data display.
Manual Balancing Valve
Many manufacturers of balancing valves produce circular slide
rules to calculate circuit flow based on pressure difference readout
across the balancing valve, its stem position, and/or the valve’s
flow
coefficient. This calculator can also be used for selecting the
size
and setting of the valve when the terminal design flow conditions
are known.
Automatic Flow-Limiting Valves
A differential pressure-actuated flow control valve, also called
an automatic flow-limiting valve, regulates the flow of fluid
to a preset value when the differential pressure across it is
varied. This regulation (1) helps prevent an overflow condition
in the circuit where it is installed and (2) aids the overall
system balance when other components are changing (modulating
valves,
pump staging, etc.).
Typically, the valve body contains a moving element containing
an orifice, which adjusts itself based on pressure forces so that
the
flow passage area varies.
The area of an orifice can be changed by either (1) a piston
or cup
moving across a shear plate or (2) increased pressure drop to
squeeze the rubber orifice in rubber grommet valves.
The flow rate for the valve is set. The flow curve is divided
into
three ranges of differential pressure: the start-up range, the
control
range, and the above-control range.
Balancing Valve Selection
The balancing valve is a flow control device that is selected
for
a lower pressure drop than an automatic control valve (5 to 10%
of
the available system pressure). Selection of any control valve
is
based on the pressure drop at maximum (design) flow to ensure
that
the valve provides control at all flow rates. A properly selected
balancing valve can proportionally balance flow to its terminal
with
flow to the adjacent terminal in the same distribution zone
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