Control Valve

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Control Valve
Instrument Training Material
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Control Valve

• Final Control Element
• Most widely used
• Other types of final control elements are
• Dampers or louvers
• Variable pitch fan blades

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Final Control Element

• Considered as the muscle of automatic control.
• They furnish the necessary power application
  between the low energy levels of controllers and
  the high energy level needed to perform their
  function in control of the process.

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Final Control Element
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Control Valve

• A power operated device that manipulates the
  fluid flow rate, temperature, level in a process
  control system.
• It consists of a valve connected to an actuator
  mechanism that is capable of changing the
  position of a flow-controlling element within the
  valve in response to a signal from the
  controlling system.

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Control Valves

• Functions as a variable resistance, creating a
  variable pressure drop in a pipeline
• This pressure-drop process is often called
  throttling
• It is consists of three major sub-assemblies
• The valve body assembly
• The internal trim parts
• An actuator assembly
• Variety of additional valve accessories

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Control Valve
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Actuator

• The power to drive the valve.
• Power source could be
• Manual.
• lever or gear box.
• Pneumatic.
• Hydraulic.
• Electrical driven.

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Valve Positioner

• Control element determine the opening or closing
  position, action and characteristic of the valve.
• It receive signal from controller.
• It converts the signal to the appropriate opening
  or closing setting of the valve.
• It provides the accurate control of the valve.

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Valve Body

• The part of a valve that is the main pressure
  boundary,
• providing pipe connecting ends,
• the fluid flow passage way,
• in some cases, the part that supports the seating
  surfaces and closure member.

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Control Valve
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Control Valves Components - Although there are so
many types and makes of control valves, this
drawing can be used to illustrate the location
and description of the different components of
control valve. The main difference mainly will be
the type of actuator used by different
manufacturers.
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Body Construction

• In order to fulfill the control valve function,
  the valve body must
• Withstand the maximum operating pressure.
• Have adequate capacity in respect of flow area.
• Have correct flow characteristic.
• Be able to resist erosion and corrosion.

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Valve Body

• All wetted parts of the control valve, including
  external bolts/studs and nuts must be designed to
  the same rating or equivalent piping class.
• On the PEFSs you will find the following
  information

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Piping Class

• The piping classes are divided in different
  sections and the International Standard ANSI
  B.16.34 are applicable with the exception of the
  Classes 125, 250 and 400
• The following Classes are used and their
  abbreviations
• 1 Class 150
• 3 Class 300
• 6 Class 600
• 9 Class 900
• 15 Class 1500
• 25 Class 2500

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Body Size

• The control valve body size may have the same
  size as the calculated trim size, but oversized
  bodies, up to the size of the adjacent piping,
  may be required to reduce the outlet velocity of
  the fluid
• The control valve body size shall be selected
  from the following series
• Metric Sizes DN 25, 40, 50, 80, 100, 150, 200,
  300, 400, 450, 500, 600, and larger
• Imperial Sizes 1, 1-1/2, 2, 3, 4, 6, 8,
  10, 12, 16, 20, and larger

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Valve Trim
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End Connections

• The following are the possible connections for
  control valves. Normally, all control valves
  should have flanged ends.
• Some types of control valves, such as Camflex
  Valves and Butterfly Valves do have flangeless
  valve bodies, which are less expensive.

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End Connections
Threaded connections are not permitted for
process fluids. Allowed only for instrument air
regulators if flange type cannot be obtained
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Globe Valves

• is the most widely used control valve in process
  control
• provides tight shut off
• withstand high differential pressure
• changeable flow characteristic
• by using different plug style offers a wide range
  of sizes, pressure ratings, and flow
  characteristics

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Globe Valves

• Single Seated Globe Valves
• This is the most preferred type for all
  applications with the exception of very high flow
  rates in combination with low pressure drops

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Globe Valves

• Double Seated Globe Valves - provides large
  capacities and ideal for high pressure
  applications with dirty fluids
• Disadvantages
• high leakage rate
• very expensive

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Globe Valves

• Angle-Body Globe Valve -
• This type of valve is designed specifically for
  erosive liquid and gas applications. Normally
  installed with the flow entering at the side and
  leaving at the bottom

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Globe Valves

• 3-Way Globe Valves
• For combining or diverting flow applications
  provide large flow with low-pressure recoveries.

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Flow Characteristics

• Control valve Flow characteristics is the
  relationship between flow rate through the valve
  and valve travel as the travel is varied from 0
  to 100
• CV The VALVE FLOW COEFFICIENT is the number of
  U.S. gallons per minute of 60 degree F water that
  will flow through a valve at a specified opening
  with a pressure drop of 1 psi across the valve. 
• Inherent flow characteristic refers to the
  characteristic observed with at constant
  pressure drop across the valve

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Flow Characteristics

• The three most common types of flow
  characteristics are
• Equal percentage - The inherent flow
  characteristic, which, for equal increments of
  rated travel, will ideally give equal percentage
  changes of the existing flow Cv at constant Delta
  P.
• Linear - equal increments of travel provide equal
  increments of flow Cv at constant pressure drop.

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Flow Characteristics

• Cont.
• Quick opening Provides for maximum change in
  flow rate at low valve travels with a nearly
  linear relationship. Additional increases in
  valve travel give sharply reduced changes in flow
  rate. Quick open plugs are used for on-off
  applications designed to produce maximum flow
  quickly.

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Cavitation

• A two-stage phenomenon of liquid flow
• The first stage is the formation of cavities
  (vapor bubbles) within the liquid stream
• The second stage is the collapse or implosion of
  those cavities (beyond the vena contracta) back
  into an all liquid state
• The energy released by cavitating liquids can,
  under certain circumstances, cause physical
  damage of valve or piping components

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Cavitation
Pressure P1
Pressure
P2
Velocity
PVc
Liquid Vapor pressure
Pv
Velocity
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Anti-Cavitation Valves - For liquid services in
which high pressure drops occur, the application
of a conventional trim with standard flow
direction often results in very high velocities
and cavitation. This type of application requires
anti-cavitation valve.
Low Noise Valves - Liquid flow noise, cavitation
noise, and flashing noise can be generated by the
flow of a liquid through a valve and a piping
system. Special design of control valve is
necessary to reduced the noise level to
acceptable value.
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Flashing
Pressure P1
Pressure
PVc
Velocity
Liquid Vapor pressure
Pv
P2
Velocity

• The liquid pressure drops below the vapor
  pressure and remains as vapor until the
  downstream pressure recovers.
• Flashing will cause choke flow condition to
  occur.
• In addition the vapor bubbles can also cause
  mechanical damage to the valve and piping system. 

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Cavitation Flashing
Recovery Point
Cavitation point
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Packing Box and Bellows Seal - The packing box
or stuffing box is the most commonly used stem
seal. The bellows seal is used only for highly
toxic process fluid where leakage is not
permissible. For services with fluid temperature
outside the packing material temperature limits,
an extended bonnet is used. Also for very low
temperature service, an extended bonnet is
selected to prevent ice formation on the stem.
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Miniature Valves - are used for very small
flows size DN 25 (1) and smaller.
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Eccentric Rotary Plug Valve (Camflex)

• Can be substituted for globe-body control valve.
• The seat portion of the plug has the form of a
  spherical segment which is rotated through 50
  degrees for maximum opening.
• The design eliminates high torque and rapid wear
  of seat ring.
• The tight shut off is achieved through the
  flexing action of the plug legs.

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Rotary Globe Valve (Varimax)

• Provide control of high pressure compressible
  fluids without the erosion, vibration and high
  noise levels associated with conventionally
  designed rotary valves. Because of their
  relatively large flow passages they are
  particularly well suited for applications
  involving gases.

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Butterfly Valve

• High flow rate
• Low pressure drop
• Consists of a cylindrical body with a disk or
  vane.
• Advantage - reduces the likelihood of clogging.
• Disadvantages
• Not tight shut-off
• Flow characteristic cannot be change

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Ball Valve

• Similar to a butterfly valve except that its
  flow-control element is a sphere instead of a
  disk, and thus has a greater seating area.
• Because of this, ball valves allow higher
  differential pressures and better sealing.
• Normally used for on-off service (like emergency
  shutdown application).
• The ball valve is not ideal for throttling
  service.

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Actuators
Spring-Opposed Diaphragm Actuator
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Spring-Opposed Piston Actuator
Long-Stroke Piston Actuator
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Positioner Schematic for spring-diaphragm
actuator or spring-opposed piston actuator.
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Positioner Schematic - Two Output Signal
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Control Valve Accessories

• Filter Regulators - For control valves which
  cannot handle the maximum air supply pressure of
  up to 8 Bar, they are provided with Air Filter
  Regulator.
• Filters - For actuators which can handle the
  maximum air pressure, a Air Filters is used
  instead of Air Filter Regulator.

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Control Valve Accessories

• Handwheels - For control valves which needs to
  be open manually, in most cases during plant
  start-up, require hand wheel. The hand wheel
  facility is designed such that one man can
  operate the valve.
• Limit Stops are provided to prevent the control
  valve from fully closing or fully opening.

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Control Valve Accessories

• Limit Switches are installed in control valves to
  electrically indicate open, closed, or
  intermediate position of the valve stroke
• Limit switches can either be
• micro-switches enclosed in an explosion proof box
• or proximity sensors

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Solenoid Valves are used to interrupt or cut-off
air signal to the actuator. It is used to switch
a particular control valve or ESD shut-off valve
to their fail safe position (Fail Close or Fail
Open).
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Schematic of Control Valve Basic Piping
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Current to Pneumatic Transducer
P
Pneumatic Signal 3 - 15 PsiG
Pneumatic Positioner
Valve Actuator
Output To Actuator
Regulated Air Supply
Setting dependent on Actuator operating pressure
Valve travel Feedback to Positioner
Instrument Gauges
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Schematic of Double Acting Control Valve Piping
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Example of ControlValve Piping Schematic
Drawing
Air supply
The valve remains at last position incase of Air
Failure.
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Instrument Standard

• ANSI American National Standards Institute
• API American Petroleum Institute
• ASME American Society of Mechanical Engineers
• ASTM - American Society for Testing and Materials
• BSI British Standard Institute

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Instrument Standard

• DIN The German Institute for Standardization
• ISO International Organization for
  Standardization
• NEMA National Electrical Manufacturers
  Association
• NIST National Institute of Standards and
  Technology
• CENELEC - European Committee for Electrotechnical
  Standardization

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Thank You

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