TATA PROJECTS LIMITED
Control Valves and
An activity can be termed as an accomplishment only when the purpose is
fulfilled. The accomplishment of any activity involves a continuous unflinching
effort, motivation and support from its mentor. I would like to extend my
heartfelt gratitude to my guide and mentor Mr. Neeladri Roy for having
constant faith in me throughout and directing me and supporting me in every
possible way at each step.
Process plants consist of hundreds, or even thousands, of control loops all
networked together to produce a product to be offered for sale. Each of these
control loops is designed to keep some important process variable such as
pressure, flow, level, temperature, etc. within a required operating range to
ensure the quality of the end product. Each of these loops receives and
internally creates disturbances that detrimentally affect the process variable,
and interaction from other loops in the network provides disturbances that
influence the process variable.
To reduce the effect of these load disturbances, sensors and transmitters
collect information about the process variable and its relationship to some
desired set point. A controller then processes this information and decides
what must be done to get the process variable back to where it should be after
a load disturbance occurs.
When all the measuring, comparing, and calculating are done, some type of
final control element must implement the strategy selected by the controller.
The most common final control element in the process control industries
is the control valve. The control valve manipulates a flowing fluid, such as gas,
steam, water, or chemical compounds, to compensate for the load disturbance
and keep the regulated process variable as close as possible to the desired set
When we talk about control valves or valves , we are referring to a control
valve assembly. The control valve assembly typically consists of the valve body,
the internal trim parts, an actuator to provide the motive power to operate the
valve, and a variety of additional valve accessories, which can include
positioners , transducers, supply pressure regulators, manual operators,
snubbers , or limit switches.
The control valve regulates the rate of fluid flow as the position of the valve
plug or disk is changed by force from the actuator. To do this, the valve
Contain the fluid without external leakage.
Have adequate capacity for the intended service.
Be capable of withstanding the erosive, corrosive, and temperature
influences of the process.
Incorporate appropriate end connections to mate with adjacent
pipelines and actuator attachment means to permit transmission of
actuator thrust to the valve plug stem or rotary shaft.
Many styles of control valve bodies have been developed through the years.
Some have found wide application, others meet specific service conditions
and are used less frequently. The following summary describes some popular
control valve body styles.
The control valves can be broadly classified into two types
1. Linear Motion Valves
2. Rotary Motion Valves
These are further classified into the different valves which perform the
respective type of fluid motion. The classification can be seen in the following
Classification Of Different types of Control Valves
Linear Motion Valves
The linear motion valves are characterised by the following features :
i. TORTUOUS FLOW PATH
ii. LOW RECOVERY OF PRESSURE
iii. CAN THROTTLE SMALL FLOW RATES
iv. OFFERS VARIETY OF SPECIAL TRIM DESIGNS
v. SUITED TO HIGH-PRESSURE APPLICATIONS
vi. USUALLY FLANGED OR THREADED
vii. SEPARABLE BONNET
Types Of Closures of Linear Motion Valves are :
a. Single Seated
b. Double Seated
c. Three way
Double Seated Globe Valve
Fig 1. here shows a double-seat globe control valve having two seats 2 and 3 in
a body 1 and two disks 5 and 6 on a stem 4. This control valve can regulate a
large volume of fluid. Because the force acting on the stem 4 depends on the
difference between the projecting areas of the upper and the lower disks on
the corresponding seats, the force for driving the stem 4 can be remarkably
reduced by substantially equalizing the projected areas of the two disks.
Therefore this type of valve is used in large diameter applications. When
closed, however, it cannot shut fluid completely off because the disks 5 and 6
have to close the seats 2 and 3 simultaneously. Fluid leakage is much greater
than single-seat type. In addition, its intricate interior makes disk motion
unstable, which in turn induces erosion and cavitations . Consequently the
body and other parts wear out rapidly or break down and noise becomes
Fig 1. Fig 2.
The following figure depicts a reverse-acting double seat globe style
1. Used for high flow and high pressure services.
Single Seat Globe Valve
Fig 3. Shows a single-seat globe control valve having a seat 8 in a body 7 and a
disk 10 on a body 9. Contrary to the above described double-seat globe control
valve, this type has a good shut-off and dynamic characteristics. However, the
force acting on the 9 depends on the product of the projected area of disk 10
on the seat 8 and the difference between the two pressures at the inlet and
the outlet of the valve. Accordingly it requires more force than the double –
seat type. When the output of the valve actuating system or a operating
machine is limited , the allowable differential pressure of the fluid becomes
small. Consequently, this type of valve is available only for small diameter
Fig 3. Fig 4.
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The following figure depicts a single-seat globe control valve :
To control large range of process parameters, specially for oil and gas
production, power generation, chemical, petrochemical, fertilizer,
pharmaceutical, processed food and other process industries.
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Three Way Globe Valve
Three-way globe valves can be used for either mixing or diverting service
depending upon the plug and seat arrangement inside the valve. Here, the
actuator pushes a disc or pair of valve plugs between two seats (Fig 5.),
increasing or decreasing the flow through ports A and B in a corresponding
The three way globe control valves have a high flow capacity and a high
rangeability. The leakage capability of the three way globe control valves are
consistent with the normal single seat globe control valve types.
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The following figure depicts a three-way globe control type valve :
Used for high temperature fluid and particularly for the textile induatries, to
mix or divert fluids.
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Angle valves are nearly always single ported. They are commonly used in boiler
feed water and heater drain service and in piping schemes where space is at a
premium and the valve can also serve as an elbow. The angle valve gives a
normal flow, have a high flow capacity and a tight shut off.
Used for very high pressure drop and also used where the fluids contain solid
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Diaphragm valves constitute the third major type of linear motion valves. The
stem of the valve is used to push down a flexible diaphragm, which in turn
blocks the path of the fluid. There are two different classifications of
diaphragm valve based on the geometry of the valve body:
Weir type - A weir is cast into the body, and when closed, the diaphragm
rests on the weir, restricting the flow.
Straight-through type - The bore runs laterally through the body and a
wedge shaped diaphragm is used to make the closure.
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Figure below depicts a Diaphragm Valve :
The main advantage of a diaphragm valve is the fact that the diaphragm
isolates the moving parts of the valve from the process fluid. They are
therefore suitable for handling aggressive fluids and for those containing
suspended solids. In addition, as the bonnet assembly is not exposed to the
fluid, it can be made from inexpensive materials such as cast iron, thereby
reducing the overall cost. The development of new diaphragm materials
enables diaphragms to be used on most fluids. Their application is however
limited by the temperature that the diaphragm can withstand - typically less
than 175°C. Diaphragm valves are generally used on process fluid applications.
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Pinch valves used for fluids usually employ a device that directly contacts
process tubing. Forcing the tubing together will create a seal that is equivalent
to the tubing's permeability. Major components of a pinch valve consists of
body and a sleeve. The sleeve will contain the flow media and isolate it from
the environment hence reducing contamination to the environment.
Generally used for slurries or processes with entrained solids, because the
flexible sleeve allows the valve to close droptight around solids—solids that
would typically be trapped by the seat or stuck in crevices in globe, diaphragm,
butterfly, gate, or ball valves. The sleeve material can be selected upon the
corrosiveness and abrasiveness of the flow media, a suitable synthetic polymer
can be chosen. A pinch valve may be the best type of valve for flow control
application if the operation temperature is within the limit of the polymer.
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Rotary Motion Valves
The linear motion valves are characterised by the following features :
i. STREAMLINED FLOW PATH
ii. HIGH RECOVERY
iii. MORE CAPACITY
iv. LESS PACKING WEAR
v. CAN HANDLE SLURRY AND ABRASIVES
vii. INTEGRAL BONNET
viii. HIGH RANGEABILITY
Types Of Closures of Linear Motion Valves are :
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Fig 6. shows a ball valve consisting of a spherical ball located between two
sealing rings in a simple body form. The ball has a hole allowing fluid to pass
through. When aligned with the pipe ends, this gives either full bore or nearly
full bore flow with very little pressure drop. Rotating the ball through 90°
opens and closes the flow passage. Ball valves designed specifically for control
purposes will have characterized balls or seats, to give a predictable flow
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Ball valves are an economic means of providing control with tight shut-off for
many fluids including steam at temperatures up to 250°C (38 bar g, saturated
steam). Above this temperature, special seat materials or metal-to-metal
seatings are necessary, which can be expensive. Ball valves are easily actuated
and often used for remote isolation and control. For critical control
applications, segmented balls and balls with specially shaped holes are
available to provide different flow characteristics.
Used in paper, pulp and textile industries.
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Fig 7. is a simple schematic diagram of a butterfly valve, which consists of a disc
rotating in trunnion bearings. In the open position the disc is parallel to the pipe wall,
allowing full flow through the valve. In the closed position it is rotated against a seat,
and perpendicular to the pipe wall. Traditionally, butterfly valves were limited to low
pressures and temperatures, due to the inherent limitations of the soft seats used.
Currently, valves with higher temperature seats or high quality and specially
machined metal-to-metal seats are available to overcome these drawbacks. Standard
butterfly valves are now used in simple control applications, particularly in larger
sizes and where limited turndown is required.
A fluid flowing through a butterfly valve creates a low pressure drop, in that the valve
presents little resistance to flow when open. In general however, their differential
pressure limits are lower than those for globe valves. Ball valves are similar except
that, due to their different sealing arrangements, they can operate against higher
differential pressures than equivalent butterfly valves.
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The figure bellow depicts a butterfly valve :
Steel, Sugar and textile industries and specially for fluid containing slurry
particles. Used for on-off as well as control applications.
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Eccentric Plug Valve
Fig 8. shows a typical eccentric plug valve. These valves are normally installed
with the plug spindle horizontal as shown, and the attached actuator situated
alongside the valve.
Plug valves may include linkages between the plug and actuator to improve the
leverage and closing force, and special positioners that modify the inherent
valve characteristic to a more useful equal percentage characteristics.
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The operation of a control valve involves positioning its movable part (the plug,
ball or vane) relative to the stationary seat of the valve. The purpose of the
valve actuator is to accurately locate the valve plug in a position dictated by
the control signal.
The actuator accepts a signal from the control system and, in response, moves
the valve to a fully-open or fully-closed position, or a more open or a more
closed position (depending on whether 'on / off' or 'continuous' control action
There are several ways of providing this actuation. This Tutorial will
concentrate on the two major ones:
a) Piston actuators
b) Diaphragm actuators
a) VMD ( Valve Motor Drive )
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Piston actuators are generally used where the stroke of a diaphragm actuator
would be too short or the thrust is too small. The compressed air is applied to a
solid piston contained within a solid cylinder. Piston actuators can be single
acting or double acting, can withstand higher input pressures and can offer
smaller cylinder volumes, which can act at high speed.
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Diaphragm actuators have compressed air applied to a flexible membrane
called the diaphragm. Figure 6.6.2 shows a rolling diaphragm where the
effective diaphragm area is virtually constant throughout the actuator stroke.
These types of actuators are single acting, in that air is only supplied to one
side of the diaphragm, and they can be either direct acting (spring-to-retract)
or reverse acting (spring-to-extend).
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VMD ( Valve Motor Drive )
This basic version of the electric actuator has three states:
1. Driving the valve open.
2. Driving the valve closed.
3. No movement.
Figure below shows the VMD system where the forward and reverse travel of
the actuator is controlled directly from any external 3-position or two 2-
position switch units. The switches are rated at the actuator voltage and may
be replaced by suitable relays.
Limiting devices are fitted within the VMD actuators to protect the motors
from over-travel damage. These devices are based on either the maximum
motor torque or physical position limit switches. Both devices stop the motor
driving by interrupting the motor power supply.
Position limit switches have the advantage that they can be adjusted to
limit valve strokes in oversized valves.
Torque switches have the advantage of giving a defined closing force on
the valve seat, protecting the actuator in the case of valve stem seizure.
If only position limit switches are used, they may be combined with a
spring-loaded coupling to ensure tight valve shut-off.
A VMD actuator may be used for on/off actuation or for modulating control.
The controller positions the valve by driving the valve open or closed for a
certain time, to ensure that it reaches the desired position. Valve position
feedback may be used with some controllers.
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In order to position the control valve in response to the system requirements a
modulating actuator can be used. These units may have higher rated motors
(typically 1 200 starts/hour) and may have built-in electronics.
A positioning circuit may be included in the modulating actuator, which
accepts an analogue control signal (typically 0-10 V or 4-20 mA). The actuator
then interprets this control signal, as the valve position between the limit
To achieve this, the actuator has a position sensor (usually a potentiometer),
which feeds the actual valve position back to the positioning circuit. In this way
the actuator can be positioned along its stroke in proportion to the control
signal. A schematic of the modulating actuator is shown in Figure below.
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The control valve is used in plant processes to regulate the rate of fluid flow as
the position of the valve plug or disk is changed by force from different types
of actuators available.
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Actuator*: A pneumatic, hydraulic, or electrically powered device
that supplies force and motion to open or close a valve.
Actuator Stem: The part that connects the actuator to the valve
stem and transmits motion (force) from the actuator to the valve.
Actuator Stem Force: The net force from an actuator that is available
for actual positioning of the valve plug.
Angle Valve: A valve design in which one port is co-linear with the
valve stem or actuator, and the other port is at a right angle to the
Bonnet: The portion of the valve that contains the packing box and
stem seal and can guide the stem. It provides the principal opening
to the body cavity for assembly of internal parts or it can be an
integral part of the valve body. It can also provide for the
attachment of the actuator to the valve body. Typical bonnets are
bolted, threaded, welded, pressure-seals, or integral with the body.
Closure Member: The movable part of the valve that is positioned in
the flow path to modify the rate of flow through the valve.
Capacity* (Valve): The rate of flow through a valve under stated
Controller: A device that operates automatically by use of some
established algorithm to regulate a controlled variable. The
controller input receives information about the status of the process
variable and then provides an appropriate output signal to the final
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Control Valve Assembly: Includes all components normally mounted
on the valve: the valve body assembly, actuator, positioner, air sets,
transducers, limit switches, etc.
Diaphragm: A flexible, pressure responsive element that transmits
force to the diaphragm plate and actuator stem.
Diaphragm Actuator: A fluid powered device in which the fluid acts
upon a flexible component, the diaphragm.
Disk: A valve trim element used to modulate the flow rate with
either linear or rotary motion. Can also be referred to as a valve plug
or closure member.
Fail-Safe: A characteristic of a valve and its actuator, which upon loss
of actuating energy supply, will cause a valve closure member to be
fully closed, fully open, or remain in the last position, whichever
position is defined as necessary to protect the process. Fail-safe
action can involve the use of auxiliary controls connected to the
Flangeless Valve: Valve style common to rotary-shaft control valves.
Flangeless valves are held between ANSI-class flanges by long
through-bolts (sometimes also called wafer-style valve bodies).
Globe Valve: A valve with a linear motion closure member, one or
more ports, and a body distinguished by a globular shaped cavity
around the port region. Globe valves can be further classified as:
two-way single-ported two-way double-ported
Positioner*: A position controller (servomechanism) that is
mechanically connected to a moving part of a final control element
or its actuator and that automatically adjusts its output to the
actuator to maintain a desired position in proportion to the input
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Rangeability: The ratio of the largest flow coefficient (Cv) to the
smallest flow coefficient (Cv) within which the deviation from the
specified flow characteristic does not exceed the stated limits. A
control valve that still does a good job of controlling when flow
increases to 100 times the minimum controllable flow has a
rangeability of 100 to 1. Rangeability can also be expressed as the
ratio of the maximum to minimum controllable flow rates.
Reverse Flow: Flow from the shaft side over the back of the disk,
ball, or plug. Some rotary-shaft control valves are capable of
handling flow equally well in either direction. Other rotary designs
might require modification of actuator linkage to handle reverse
Relay: A device that acts as a power amplifier. It takes an electrical,
pneumatic, or mechanical input signal and produces an output of a
large volume flow of air or hydraulic fluid to the actuator. The relay
can be an internal component of the positioner or a separate valve
Seat: The area of contact between the closure member and its
mating surface that establishes valve shut-off.
Sensor: A device that senses the value of the process variable and
provides a corresponding output signal to a transmitter. The sensor
can be an integral part of the transmitter, or it may be a separate
Transmitter: A device that senses the value of the process variable
and transmits a corresponding output signal to the controller for
comparison with the set point.
Travel*: The movement of the closure member from the closed
position to an intermediate or rated full open position.
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Trim*: The internal components of a valve that modulate the flow of
the controlled fluid.
Yoke: The structure that rigidly connects the actuator power unit to
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1. Control Valve Handbook by Fisher.
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