Basic Documentation
www.usa.siemens.com/hvac 175
Control Valve Sizing and Selection
Valve Reference
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 sufcient 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.
Selecting Globe and Ball Valves
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, specically 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 prole 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; buttery valves are
also used for these applications.