Control Valve
Selection for Commercial HVAC
In selecting the right control valve for a given
application, first know the application requirements then pick the valve
that meets those requirements. Following are very general questions that will help guide in
selecting the right valve. Please contact Kele for more detail on any of the following.
Is the control system electric or pneumatic?
- If electric, what is the available supply
voltage? 24 VAC? 120 VAC?
Valve actuators
are typically 24 VAC/VDC or 120 VAC. The customer may need a 120-to-24 VAC
transformer if the available power is 120 and the valve actuator is only
available in 24 VAC. A power supply (such as the Kele DCP-1.5-W) will be needed
for 24 VDC actuators.
- If pneumatic, is it a low pressure system (~20
- 25 psig) or a high pressure system (~80 - 100 psig)?
Kele sells valves that are controlled
with low pressure actuators such as zone, globe, and butterfly valves; as well as valves controlled with high pressure actuators. For a
butterfly valve application, if high pressure is available, a high pressure
actuator is often the best selection because it delivers more power and is more
compact. A customer can turn high pressure into low pressure, if necessary with
a Filter/Pressure Reducing Valve assembly such as the K-332.
Is the valve for water or steam?
- If steam, is it low pressure (~10 - 15 psi) or
high pressure (>20 psi)?
Many brass/bronze water valves that
have a maximum media temperature rating of ~250 Deg. F can also be used for low 15 psi steam. Verify the valve ratings!
High-pressure steam applications
require valves with stainless trim (internals). Kele sells globe valves with
stainless steel trim (Honeywell V5011N2xxx and JCI VG7243xx for example) that
can handle up to about 100 psig (350 Deg. F) steam.
If steam, don’t use an elastomer
seat butterfly valve. There are high performance butterfly valves for steam on/off control. Contact Kele for information on these valves.
What are the maximum and minimum temperature and pressure requirements?
Most commercial HVAC control valves handle typical chilled or hot
water systems, between about 35 and 200 Deg. F., and up to ~100 psi system pressure.
Always check the customer’s temperature and pressure requirements to verify
that the valve will handle the application.
What is the body pattern - 2-way or 3-way?
- If 3-way, is it being installed in a mixing pattern (2 inlets, 1 outlet) or a diverting pattern (1 inlet, 2 outlets)?
It is important to know what the application calls for as most
3-way valve bodies cannot do both. Some manufacturers offer both, but diverting
valves are less common than mixing valves. To control flow through a coil, a mixing valve is installed in the coil return; less common is installing a diverting valve in the supply line.
Two-way valves (1 inlet, 1 outlet) are
more common than 3-way and cost less. Valves used to control steam are always
2-way.
What is the required relationship between flow and pressure drop for the valve (Cv)?
This specification is critical to proper control performance. Please refer to Jordan Colville's article for an in-depth discussion of the importance of specifying the appropriate Cv for a control valve.
If a valve is to be used for modulating service, the Cv
(flow coefficient) must be calculated in order to select a valve that
will provide proper capacity control. An improperly sized valve causes all types of problems such as control instability, insufficient capacity to heat or
cool, noise, excessive valve wear, etc. By far, most improperly sized valves
are oversized which results in poor control.
A properly sized modulating valve will typically be 1 or
2 pipe sizes smaller than the system piping.
A two-position valve is typically pipe size, and Cv does
not typically need to be calculated.
Piping systems 2” and smaller will usually use threaded
valves, 2 ½” and larger will typically be flanged.
What is the close-off pressure requirement?
Be sure the close off rating of a valve/actuator
assembly is equal to or greater than the application close-off requirement. The
job documentation should contain this information.
If a valve/actuator assembly cannot completely close off
against the pressure in the pipe, the media will leak through, causing noise,
premature valve wear, and system inefficiencies.
Close-off ratings depend on actuator power in standard
globe valves because the actuator power directly opposes the pressure forces
inside the piping. The stronger the actuator, the higher close-off rating the
valve/actuator will have.
Close off does not depend on actuator power in ball
valves or cage valves because the actuator power is not directly opposing the
pressure forces in the piping. The actuator just has to overcome the friction
in rotating the ball or opening/closing the cage.
What type of control is required?
- 2-position (On/Off) control?
Two-position
is either full-open or full-closed, no intermediate position. On/Off is another
term used for this type of control.
An electric spring-return 2-position
actuator is typically powered by either 24 VAC or 120 VAC. When a 2-wire
2-position spring-return actuator is powered, it is driven fully in one
direction. When it is not powered, the spring drives it fully to the opposite
position.
For non-spring return 2-position
control, a Floating actuator is used and a 3-wire circuit is used to drive the
actuator to one end of the stroke or the other.
2-position valves are usually the
same body size as the piping and generally there's no need to calculate Cv.
2-position pneumatic valves usually
use a spring return actuator in combination with an electric 3-way solenoid valve.
The control then is actually electric, with pneumatic power being the driving
force for the actuator.
- Modulating (floating or proportional)?
Modulating
is a term used to describe a type of actuator or control action that can be
positioned anywhere between two stroke limits.
Proportional
is a type of modulation where the positioning of the controlled device is in
response to a proportional analog signal (example 4-20 mA, or 2-10 VDC). If an actuator has
a 4-20 mA control signal, and it receives a control signal of 12 mA, the
actuator will be positioned half way between full open and full closed.
Floating (or 3-position) control is a type of modulation based on timing, stroke length, and stroke speed of an
actuator. An electric motor is driven clockwise or counterclockwise for
a specific amount of time, to reach a given position within two stroke limits.
Determining Cv is important for
modulating control. It must be calculated unless the valve Cv is already specified
(rare). Select a valve with a Cv rating equal to or slightly greater than the
calculated Cv, and do not oversize (bigger is not better)! One rule of thumb
is: a valve Cv rating 10% lower than the calculated Cv is generally acceptable.
Is a particular flow characteristic specified?
HVAC valves have equal percentage, linear, quick-opening or other similar flow characteristics. The flow characteristic determines how much flow passes through
the valve relative to the stroke position of the valve. Just because a valve is
half way open doesn’t always mean half the flow rate is passing through the
valve.
Different automation companies and different regions of
the world have different opinions on what works best. Most common, in the USA,
“equal percentage” is recommended for hot water and chilled water and “Linear”
is recommended for steam. Quick opening is designed for 2-position service.
Is spring return (a fail position) required?
- If yes, in what position does it need to fail open or closed for 2-way body-pattern, thru-port or bypass-port for 3-way body
pattern?
A 2-way valve might be required to fail-open or fail-closed, depending on the application. If so, use a spring
return (or fail-safe) actuator. If fail-safe is not required, use a non-spring return actuator to save money.
Different combinations of spring
return actuators and valves can accomplish fail safe positions. It is common in a
heating application to fail-open (or fail to the coil) to enable heat and minimize
the potential of coil freezing in the winter in cases of power loss. For
cooling applications, spring return is usually not as critical but a
spring return cooling valve might be specified to fail closed to prevent
excessive condensation around the coil and duct.
Steam valves are always fail-closed
and therefore require a spring return actuator.
Will the valve be located indoors or outdoors?
- If outdoor, a NEMA 4 actuator or weather-shield will be needed.
Weather shield accessories are available for
some ball valves and globe valves, depending on the brand. Traditional metal or
plastic weather shields are effective and can be time consuming and expensive
to install. The fabric Kele Kover (KOV-1, -2, -3, -4) can be a cost-effective
solution for control valves using direct-mount actuators. Most butterfly valves are
available with NEMA 4 actuators.
Is the valve type specified?
For unitary equipment such as unit
heaters, fan coil units, and unit ventilators where physical space is a
constraint. Zone valve sizes are typically ½” to 1”.
Many times the lowest cost type of control
valve provides decent control for HVAC. They typically cannot be repaired or rebuilt
and are replaced after a 5 to 10 years. Ball valve sizes are typically ½” to
2” though some are larger.
Globe valves are considered by many
to be the most accurate type of control valve. Internals can be re-built if
necessary without having to remove the valve body from the piping. They can
last for many years. Globe valve sizes are typically ½” to 2” threaded and 2 ½”
to 6” flanged.
Pressure balanced valves usually
have double seats or cage type internals, for higher close-off applications.
The valve actuator does not have to overcome the media pressure to close the
valve, so lower power, thus lower cost actuation can be used. Pressure balanced
valve sizes are typically 2 ½” to 6” flanged.
Commonly used in central plants like boilers and chiller rooms for isolation and diverting water flow depending
on seasons and load on the building. They are generally acceptable for modulating control
of water and are comparatively low cost and lightweight compared to large globe valves. Butterfly valve sizes are typically 2” to 24”.
Relatively new for commercial HVAC
systems, the PI valve is a combination of a pressure reducing valve and control
valve. They control at constant flow regardless of fluctuations of pressure in
the system. They are selected based on GPM rating of the coil; sizing by
calculating Cv is not required.
Is there a brand preference?
Does the project specification call for a specific
brand, or do the project decision makers have a brand preference? Brand loyalty
can be a factor in decision making. Kele handles most popular brands for the
commercial HVAC industry, for that reason.
