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G-2
Globe and Ball Valves
The control valve is the most important single element in
any uid handling system, because it regulates the ow
of uid to the process. To properly select a control valve,
a general knowledge of the process and components is
usually necessary. This reference section can help you
select and size the control valve that most closely matches
the process requirements.
The sizing of a valve is very important if it is to render
good service. If it is undersized, it will not have sufcient
capacity. If it is oversized, the controlled variable may
cycle, and the seat, and plug will be subject to wire
drawing because of the restricted opening.
Systems are designed for the most adverse conditions
expected (i.e., coldest weather, greatest load, etc.). In
addition, system components (boiler, chiller, pumps, coils,
etc.) are limited to sizes available and frequently have
a greater capacity than system requirements. Correct
sizing of the control valve for actual expected conditions is
considered essential for good control.
A basic rule of control valve sizing is:
The higher the percentage of drop across the wide open valve
in relation to the percentage of pressure drop through the line
and process coil, the better the control.
Technical Comparison Between
Globe and Ball Valves
Technically, the globe valve has a stem and plug,
which strokes linearly, commonly referred to as “stroke”
valves. The ball valve has a stem and ball, which turns
horizontally, commonly referred to as “rotational” valves.
Early ball valves used a full port opening, allowing large
amounts of water to pass through the valve. This gave
HVAC controls contractors the ability to select a ball valve
two to three pipe sizes smaller than the piping line size.
Compared to traditional globe valves that would be only
one pipe size smaller than the line size, this was often a
more cost-effective device-level solution. In addition, the
ball valve could be actuated by a damper actuator, rather
than expensive box-style “Mod” motors.
Pricing Comparison
Today, with equivalent pricing between ball and globe
valves, the full port ball valve is falling out of favor for
most HVAC control applications. This is also due to
its poor installed ow characteristic that leads to its
inability to maintain proper control. New “ow optimized”
or characterized ball valves, specically designed
for modulating applications, have been developed.
Characterized ball valves are sized the same way as
globe valves. They provide an equal percentage ow
characteristic, enabling stable control of uids. Additionally,
there are more cost-effective valve actuators now available
for globe valves. Better control and more-competitive
pricing now puts globe valves on the same playing eld as
characterized ball valves.
Selection Guidelines
Globe Valve
• High differential pressure across valve
• Rebuilding of the valve is desired
• Better control performance
• Better low ow (partial load) performance
• Use for steam, water or water/glycol media
• Smaller physical prole than a comparable ball valve
Characterized Ball Valve
• Tight shutoff or high close offs of around 100 psi* are
required
• Isolation or two position control**
• Use for water or water/glycol solution only
* This equates to a pump head pressure of approximately 230 ft. Not very common HVAC applications.
** Valve can be line sized to minimize pressure losses; buttery valves are also used for these applications.
Selecting Valves: Globe vs. Ball