Control valves are valves used to control conditions such as flow, pressure, temperature, and liquid level by fully or partially opening or closing in response to
signals received from controllers that compare a "setpoint" to a
"process variable" whose value is provided by sensors that monitor changes in such conditions.[1] Control Valve is also termed as the Final Control Element.
A
control valve consists of three main parts in which each part exist in several
types and designs:
Valve's
actuator- The opening
or closing of control valves is usually done automatically by electrical, hydraulic or pneumatic actuators.
Valve's
positioner- Positioners
are used to control the opening or closing of the actuator based on electric,
or pneumatic signals. These control signals, traditionally based on 3-15psi
(0.2-1.0bar), more common now are 4-20mA signals for industry, 0-10V for HVAC systems, and the
introduction of "Smart" systems, HART, Fieldbus Foundation, and Profibus being the more common protocols.
Valve's body - valve bodies are available to
achieve specific flow regulation behavior of process fludes.
Globe Valve: The name "globe" refers to the external shape of
the valve, not the internal flow area. A typical globe valve
has a stem that is adjusted linearly (up and down) to change the position of the
plug. As the plug changes, the area for flow between the plug and seat
(opening) changes. Many different seat and plug designs are
available to achieve desired relationships between the stem position and flow
rate; see the discussion on valve characteristic below. The standard
plug must oppose the pressure drop across the valve, which is acceptable for
small pressure drops. For large pressure drops, a balanced globe
valve is used to enable a valve with small force to open and close the plug.
Ball: The restriction for this body is a
solid ball which has some part of the ball removed to provide an adjustable
area for flow. The ball is rotated to influence the amount of
flow. The example ball valve displayed through the link below has a
tunnel through the ball, and the ball is rotated to adjust the fraction of the
tunnel opening available for flow. Other types of ball valves have
different sections removed from the ball to give desired properties.
Butterfly: The butterfly valve provides
a damper that is rotated to adjust the resistance to
flow. This valve provides a small pressure drop for gas flows.
Diaphragm: The diaphragm valve has one
surface which is deformed by the force from the valve stem to vary the
resistance to flow.
Gate: These valves have a flat barrier
that is adjusted to influence the area for flow. These bodies are
used primary for hand-operated valves and valves automated for emergency
shutoff.
Summary of Control Valve Bodies
|
Valve
Body
|
Advantages
|
Disadvantages
|
|
globe
(unbalanced)
|
-large range
-good shutoff
|
-unbalanced forces
-high pressure loss
|
|
globe
(balanced)
|
-high capacity
-large range
-balanced forces
|
-poor shutoff
-high pressure loss
|
|
ball
|
-high capacity
-tight shutoff
|
-moderate pressure drop
applications
-tends to plug (except segmented
ball)
|
|
butterfly
|
-high capacity
-low pressure loss
-slurry applications
|
-high torque
-large deadband
-affects flow through limited
range (i.e.
0-60%)
-tight shutoff requires special
seat material
|
|
diaphragm
|
-slurry applications
-corrosion resistant materials
|
-short diaphragm life
-limited pressure and temperature
-small range
|
|
gate
|
-tight shutoff
|
-used only with clean fluids
|
