Valve Style; Valve Body Materials; Valve Trim; Valve Packing - Honeywell AUTOMATIC CONTROL Engineering Manual

CHILLER, BOILER, AND DISTRIBUTION SYSTEM CONTROL APPLICATIONS

VALVE STYLE

Single seated valves are recommended because of their tight
shut-off and availability with equal percentage and reduced
capacity (Cv) trim.
Double seated valves are not recommended because they do
not provide tight shut off. High velocity leakage past the plug
of a double-seated valve causes greater erosion to the seats than
a single-seated valve.
Three-way valves are not recommended because they are not
available with equal percentage flow characteristics and
therefore do not provide the required control stability. They
also are not available with reduced Cv trim for accurate sizing.
Valve bodies should be flanged although threaded bodies can
be used for sizes of 1-1/4 inch or less. Weeping and flashing at
threads is likely but usually not visible except for deposits left
behind. If threaded valves are used in the HTW lines, use
adjacent bolted flanges not union fittings.

VALVE BODY MATERIALS

Cast steel bodies with 300 psi or 600 psi body ratings are
recommended. These have temperature ratings above 750F, the
limit for packing materials. Manufacturers literature lists actual
pressure and temperature ratings.
Bronze bodies are not recommended because the maximum
temperature limit is 400F. In addition, since HTW piping is
steel, galvanic action between bronze and steel can be serious
at the elevated temperatures encountered.
Cast iron bodies are not recommended because maximum
operating conditions are limited to 406F at 250 psig, which are
too close to operating conditions.

VALVE TRIM

Use stainless steel valve stems and trim. Standard trim for
industrial valves is usually 316 stainless steel. Composition discs
leak and corrode at the seat and disc and are not used.

VALVE PACKING

Shredded Teflon, Teflon asbestos, graphite asbestos, and
spring loaded Teflon V-rings, are available in industrial valves
and acceptable to 450F. A valve with a deep packing gland is
normally required.
ENGINEERING MANUAL OF AUTOMATIC CONTROL

VALVE FLOW CHARACTERISTICS

The flow characteristics of a valve is the relationship between
fluid flow expressed in percent of flow and valve stem travel
expressed in percent of travel. Always use equal percentage
characteristic valves for HTW control. Equal percentage valves
compensate for the nonlinear heat output of HW heat
exchangers so percent of heat output change is equal to percent
of valve travel. These valves provide the best control at low
flows. An industrial valve with an equal percentage plug has
the required rangeability of 50 to 1.

ACTUATORS

Valve actuators must close the valve on loss of power or
control air.
If the close-off rating is less than twice the expected pressure
difference across the closed valve, positioners on pneumatic
valves are used only, otherwise, sequencing of multiple valves
is required. Electro-pneumatic positioners or separate electro-
pneumatic transducers can be used to receive an electric signal
(4 to 20 mA) and operate the pneumatic valve.
Electric actuators are not satisfactory for HTW valves because
they are not available with a safe ambient temperature rating
and the operating speed is too slow.

VALVE SIZE

Accurate valve sizing is critical for satisfactory HTW control
since an oversized valve controls over only a small fraction of
its total stem travel and loses sensitivity. Safety factors should
be avoided but if used should be very small. See the Valve
Selection and Sizing section.
The pressure drop across a control valve should be between
50 and 80 percent of the drop across that part of the piping
system in which flow will vary as a result of valve action.
Always use a 5 psig or greater pressure drop across the valve.

VALVE LOCATION

Always locate a HTW valve on the return side of the
converter, coil, steam generator, or other equipment because it
allows the valve to operate at lower pressures and temperatures.
HTW is less likely to flash when the valve is located on the
return side of the equipment.
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