Valve Selection - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

VALVE SELECTION AND SIZING
Table 1. Corrosive Elements in Hydronic Systems.
Brass or Bronze Component
Corrosive Substance
Chloride
Ammonia
Carbonates
Magnesium or Calcium
Oxides
Sulphide (Hydrogen)
Iron
Iron or Steel Component
Corrosive Substance
Magnesium or Calcium
Iron

VALVE SELECTION

Proper valve selection matches a valve to the control and
hydronic system physical requirements. First consider the
application requirements and then consider the valve
characteristics necessary to meet those requirements. The
following questions provide a guide to correct valve selection.
— What is the piping arrangement and size?
The piping arrangement indicates whether a two-way
or three-way mixing or diverting valve is needed. The
piping size gives some indication of whether the valve
requires a screwed end or a flanged end connection.
— Does the application require two-position control or
proportional control? Does the application require a
normally open or normally closed valve? Should the
actuator be direct acting or reverse acting?
In its state of rest, the valve is normally open or closed
depending on the load being controlled, the fluid being
controlled, and the system configuration.
For chilled water coils, it is usually preferable to close
the valve on fan shutdown to prevent excessive
condensation around the duct and coil, and to save
pumping energy. This may be accomplished with either
normally closed valves or a variety of other control
schemes. Lower cost and more powerful normally open
valve assemblies may be used with the close-on-
shutdown feature and allow, in the case of pneumatic
systems, the capability to provide heating or cooling in
the event of air compressor failure.
Converter control valves should be normally closed and
outdoor air preheat valves should be normally open.
Corrosion Color
Light Blue-Green
Blue or Dark Blue
Dark Blue-Green
White
Black (water)
Black (Gas)
Rust
Corrosion Color
White
Rust
Glycol solutions may be used to prevent hydronic systems
freezing. Glycol solutions should be formulated for HVAC
systems. Some available glycol solutions formulated for other
uses contain additives that are injurious to some system seals.
In addition, hydronic seals react differently to water and glycol
such that when a new system is started up with water or glycol
the seals are effective. The hydronic seals are likely to leak if
the system is later restarted with media changed from to water
to glycol or glycol to water. To prevent leakage part of the
process of media changeover should include replacing seals
such as, pump and valve packing.
— Is tight shut-off necessary? What differential pressure
does the valve have to close against? How much actuator
close-off force is required?
Single-seated valves provide tight shut-off, while double-
seated valves do not. Double seated valves are acceptable
for use in pressure bypass or in-line throttling
applications.
The design and flow capacity of a valve determine who
much actuator force is required for a given close-off.
Therefore, the valve must first be sized, then, the valve
and actuator selected to provide the required close-off.
— What type of medium is being controlled? What are the
temperature and pressure ranges of the medium?
Valves must be compatible with system media
composition, maximum and minimum temperature, and
maximum pressure. The temperature and pressure of the
medium being controlled should not exceed the
maximum temperature and pressure ratings of the valve.
For applications such as chlorinated water or brine, select
valve materials to avoid corrosion.
— What is the pressure drop across the valve? Is the pressure
drop high enough?
The full open pressure drop across the valve must be
high enough to allow the valve to exercise control over
its portion of the hydronic system. However, the full open
pressure drop must not exceed the valves rating for quiet
service and normal life. Closed pressure drop must not
exceed valve and actuator close-off rating.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
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