Control
valves are the final control elements in an HVAC system.
In chilled water systems, their function is to regulate
the chilled water flow through the terminal units
(air handling units or fan-coil). The control link
to the valve is known as actuator. This device converts
the controller’s output, such as an electric
or pneumatic signal, into rotary or linear action
required to move the valve stem through its operating
range. Both, control valve and actuators must be properly
sized and selected for the particular application.
Control Valves It may be considered as a variable
orifice positioned by an electric actuator in response
to impulses, or signals from the controller. It may
have a throttling plug or V-port specially designed
to provide a desired flow characteristics. They are
made of materials best suited to the media handled,
which in this case is water, and for dealing with
the operating temperature and pressure. Therefore,
internal parts such as seat ring, throttling plug
or V-port skirt, disc holder, and stem, are sometimes
made of stainless steel or other hard and corrosion-resistant
metal for use in severe service. To correctly apply
the different types of control valves used y chilled
water systems, it is necessary to understand the internal
construction of each valve.
Valves Types:
Two-way single-seated:
It is the most common type used and design for tight
shutoff(Fig
101). Whether its action
is modulating or two-position, it must be installed
with the flow direction opposing the closing action
of the valve. This prevents the differential pressure
build-up from slamming the valve seat closed with
consequent noise and water hammer.
Two-way doubled-seated:
It is recommended for applications with high differential
pressures. This design creates a balanced thrust condition
which enables the valve to close off smoothly without
water hammer regardless of the differential pressure
which may exists across the valve (Fig.
102). However, it can
not be used where a tight shut-off is demanded because
the expansion of the stem portion between the seats
may force one seat open.
Three-way mixing:
It has two inlet and one outlet ports and a doubled-faced
disc operating between two seats (Fig.
103). It mixes the water
entering through the inlet ports and leaving through
the common outlet, according to the valve stem position.
Fig. Nº101.
Two-way single-seated
valve
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image to enlarge
Fig. Nº102.
Two-way
double-seated valve
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image to enlarge
Fig. Nº103.
Three-way mixing valve
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image to enlarge
Three-way diverting: It
has one inlet and two outlet ports
and two separate discs and seats
(Fig 104).
Its function is to divert the flow
to either of the outlets or to proportion
the flow to both outlets.
Fig.
Nº104.
Three-way diverting valve
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on image to enlarge
Butterfly valve:
Are the most used large-diameter
control valve in chilled water plants.
It consists of a heavy ring enclosing
a disc that rotates on an axis at
or near its center
(Fig 105).
They are excellent isolation valves
because of their almost full pipe
bore when open, simple and compact
design, and low pressure drop.
Fig.
Nº105.
Butterfly valve
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on image to enlarge
Two butterfly valve appropriately cross-linked
could be used in applications where it is
not possible to use a standard three-way mixing
or by-pass valve because of size limitations
or space constrains.
However their low pressure drop and thus low
valve authority usually make them inappropriate
for use in two-way modulating duty at cooling
coils in variable flow systems.
Butterfly valves have different flow characteristics
from standard seat and disc-type valves, so
they may be used only where their flow characteristics
fit the application requirements.
Control Valves Flow Characteristics:
The control valve performance is expressed in
terms of its flow characteristics as it operates
through its stroke, provided that a constant pressure
drop exists across the control valve.
Based on the geometry of the control valve plug,
three distinct flow conditions can be obtained
(Fig. 106).
Quick opening:
A considerable amount of flow to pass for small
stem travel.
As the stem moves toward the open position, the
increasing flow rate per movement of the stem
is reduced in a nonlinear manner.
This characteristic is used in two-position or
on-off applications.
Fig. Nº106.
Control Valve Flow Characteristics
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to enlarge
Linear:
Stem travel and flow are related in direct proportion. This
characteristic is used in the bypass port of three-way valves.
Fig. Nº107.
output characteristic
and stem travel characteristic of equal percentage
valve.
Click on image
to enlarge
Equal
percentage: Exponential flow increase with
the stem travel. This characteristic is recommended
for control on chilled water terminals.
In order to obtain a stable and efficient modulating
control, the designer should combine the valve flow
characteristics with coil performance curves. Consider
a valve with linear characteristic (the water flow
is proportional to the valve lift) that controls
the chilled water flow through a coil. Due to the
non-linear characteristic of the coil, opening the
control valve slightly can significantly increase
the emission at small and medium loads unnecessarily.
The control loop may therefore be
unstable at small loads. Equal percentage characteristic
compensates the non-linearity and thus the coil output is
proportional to the stem travel of the valve (Fig.
107).
However to obtain this compensation
the differential pressure across the control valve must
be constant. In actual conditions, the pressure drop across
the valve varies between a maximum, when it is modulating,
and a minimum when the valve is near full open. The ratio
of these two pressure drops is known as “authority”.
The lower the valve authority, the bigger the distortion
of the theoretical valve characteristic. The Fig.
108 shows the distortion of
a linear (left) and an equal percentage (right) valve characteristic
as a function of its authority. For on-off controllers,
the authority concept is meaningless since the control valve
is either opened or closed. Its characteristic is therefore
not very important.
Another issue that must be checked in modulating
control applications is the valves close-off rating.
For ball and butterfly valves it is not a concern
because the fluid pressure does not affect the
closing force, but fluid pressure is a factor
with globe valves. Usually, valves have two close-off
ratings, one for on-off (two-position) duty and
another for modulating duty that is commonly known
as “dynamic” close-off rating. The dynamic rating, which
is always lower than the two-position rating,
is the maximum differential pressure allowed for
smooth modulation of the valve, particularly near
shut-off.
Fig. Nº108.
Distortion of the control valve characteristic
as a function of the valve authority
Click on image
to enlarge
Above this differential pressure
the valve performance will be affected.
A common practice in variable flow (two-way valves) hydronic
systems is selecting valves with close-off ratings just
above the pump shut-off head plus a safety factor (commonly
25% to 50%). This is indeed a conservative strategy for
systems with variable-speed driven pumps, but it is advisable
since the pumps must be operated at its rated speed in case
of variable frequency drive (VFD) failure. The valve actuators
should also be sized to close against at least 50% above
pump head in order to insure good valve positioning.
Actuators The actuator is the element that
links the control and the valve. This device uses compressed
air, electricity or hydraulic fluid to power the motion
of the valve stem through its operating range. The most
common actuator types are: Pneumatic actuators:
Consists of a spring-opposed flexible diaphragm clamped
between an upper and a lower chamber (Fig. 109). Increasing
air pressure on the diaphragm pushes the valve stem down
and overcomes the force of the load spring to close the
valve. Springs of various pressure ranges, can sequence
the operation of two or more devices, if properly selected
or adjusted. The manufacturer’s close-off rating tables
need to be consulted to determine if the actuator is of
an adequate size or if a larger actuator is available.
Electric actuators:Consist
of a double-wound electric motor coupled to a gear train
and an output shaft connected to the valve stem with a
cam or rack-and-pinion gear linkage (Fig. 110). The motor
shaft typically drives through 160° of rotation. Gear
trains are coupled internally to the electric actuators
to provide a timed movement of valve stroke to increase
operating torque and to reduce overshooting of valve movement.
They can also be fitted with limit switches, auxiliary
potentiometers, etc., to provide position indication and
feedback for additional system control functions. Electric
actuators operate with two-position, floating, proportional
electric, and electronic control systems. Actuators usually
operate with a 24 VAC low voltage control circuit. The
rotation time ranges typically from 30 sec to 4 min, with
60 sec being the most common.
Electrohydraulic actuators:Consist of a sealed housing containing
an uncompressible fluid, a pump, and some type of metering
or control device to provide pressure control across a
piston or piston/diaphragm. The pressure control device
is activated by a coil controlled by a low to medium DC
voltage.
Solenoids:Is
an electromechanical element that opens or closes a valve
when a solenoid coil is energized (Fig. 111). Solenoid
coils are used in control valves ranging from 1/8 to 2
in. pipe size. Solenoid actuators are themselves two position
control devices and are available for operation in a wide
range of alternating current voltage as well direct current
Fig. Nº109.
Two-way control valve
with a pneumatic actuator.
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to enlarge
Fig. Nº110. Two-way
control valve with electric actuator
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to enlarge
Fig. Nº111. Two-way
direct-acting solenoid valve
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to enlarge
Selection of Valves
Valve Sizing The control valve size should be
selected by calculating the valve flow coefficient Cv required
to provide the design flow at an assumed pressure drop ?p.
The Cv coefficient is defined as the number of gallons per
minute of fluid (water in chilled water systems) that will
flow through a wide-open valve at a pressure drop of one
psi. Where Q: volumetric flow, gpm
DelthaP: pressure drop, psi
The pressure drop used must be
a substantial fraction of the overall system pressure
drop in order to obtain a valve authority that makes control
as stable as possible. However, higher pressure drops
mean higher pumping costs and also higher energy costs.
These two considerations must be balanced when making
valve selections.
An old rule-of-thumb states that a pressure drop of 25
to 50% of the available pressure between the supply and
return riser (pump head) should be selected for the control
valve. This pressure drop gives the best flow characteristic
as described above in Control Valve Flow Characteristics.
