Valves In Industry
(Part 2)
VALVE

1)
2)
Can operate:
Manually (Human)
Automatically (with the addition of
actuators)

Hydraulics
 Air (pneumatic)
 Electrical/ Magnetic
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GLOBE VALVE
3
Globe Valve
4
Globe Valve

A type of valve for regulating flow in a pipeline, consisting of a movable
disk-type element and a stationary ring seat in a generally spherical
body.

Named for their spherical body shape with the two halves of the body being
separated by an internal baffle. This has an opening that forms a seat onto
which a movable plug (also called a disc) can be screwed in to close the
valve.
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Globe Valve

The disc is connected to a stem which is operated by screw action in
manual valves. Typically, automated valves use sliding stems.

Automated globe valves have a smooth stem rather than threaded and are
opened and closed by an actuator assembly.
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
The bonnet assembly is the part of
the valve body through which the
valve plug stem moves.

The valve stem extends through the
bonnet to permit positioning of
the valve plug, and therefore
provide a variable restriction to the
fluid flow.
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Globe Valve



Globe valve bodies can be classified as :
direct acting valve body - a downward movement of the
valve plug stem results in the valve closing.
reverse acting valve body - a downward movement of
the valve plug stem results in the valve opening.
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Globe Valve

A globe valve can contain
either one or two plugs. When
there is only one plug/disc,
the valve is called single
ported globe valve.

When there are two plugs, it
is called a double-ported
globe valve.
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Globe Valve

The double-ported valve arrangement
produces almost no unbalanced
force on the valve stem.

The fluid flows through the valve ports
in opposite directions and therefore
generates forces that offset each
other.

As a result, only a relatively small
actuator force is needed for
positioning the valve plugs.

This makes the double-ported globe
valve suitable for high pressure
applications.
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Globe Valve

Globe valves are used for applications requiring
throttling and frequent operation.

Globe valves are the most frequently
encountered control valves in process plants.
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Globe Valve
12
Globe Valve
http://www.youtube.com/watch?v=bOGY8hDJfKA&list=LPPuBqkm27yyk&i
ndex=2&feature=plcp
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Gate Valve

Used when a straight-line flow of fluid and
minimum restriction is desired.

The diameter of the opening through which the fluid
passes is nearly the same as that of the pipe and
the direction of flow does no change.

Therefore, there is small pressure drop or flow
restriction through the valve.

Usually left fully open or fully closed.

Gate valves are not suitable for throttling
purposes since the control of flow would be difficult
due to valve design and since the flow of fluid
slapping against a partially open gate can cause
extensive damage to the valve.
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GATE VALVE
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Gate Valve can be classified as:
1) rising stem valves
 The gate and stem rise and
lower together as the valve is
operated.
 The threaded part of the stem
is above the valve body.
 Less corrosion problems
2) non-rising stem valves
 As the hand wheel on the stem is
rotated, the gate travels up or down
the stem on the threads, while the
stem remains vertically stationary.
 The stem does not come out of the
valve body.
 More chance of corrosion (valve stem
is in contact with the liquid flowing in
the valve).
 Used underground or where vertical
space is limited.
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Non-rising stem valves
http://www.youtube.com/watch?v=lVgkHm0Q9vA&feature=related
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Ball Valve

Stop valves that use a ball to stop or start the flow of fluid.

The ball rotates to a point where the hole through the ball is in line with the
valve body inlet and outlet.

When the valve is shut, which requires only a 90-degree rotation of the
hand wheel for most valves, the ball is rotated so the hole is
perpendicular to the flow openings of the valve body, and flow is
stopped.
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BALL VALVE
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Ball Valve
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http://www.youtube.com/watch?v=txDANNh4_ME
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Ball Valve

Most ball valves are of the quick-acting type (requiring only a 90-degree
turn to operate the valve either completely open or closed), but many are
planetary gear operated.

This type of gearing allows the use of a relatively small hand wheel and
operating force to operate a fairly large valve. The gearing does, however,
increase the operating time for the valve.
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Ball Valve


Some ball valves contain a swing check located
within the ball to give the valve a check valve
feature.
Ball valves are normally found in the following
systems aboard ship: seawater, sanitary, trim and
drain, air, hydraulic, and oil transfer.
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Assignment 2

Make a group of 4-5 persons.

Each group has to prepare slides presentation to discuss the following valves:
- Check valve
- Butterfly valve
- Plug valve
- Pressure relief valve
- Diaphragm valve
- Needle valve
- Solenoid valve

The contents of the slide presentation:
- Description of the valves
- Function
- Application
- Figure
- Video
- References

Please submit the assignment on Friday (4th October 2013) before 5 p.m.
- hard copy & soft copy (cd)
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Valve Actuators

A control valve actuator is a device which is
used to drive the valve plug stem and
therefore sets the position of the plug with
respect to the valve seat.

The most common valve actuator is the
pneumatic diaphragm actuator. It is simple in
construction and very reliable. It operates by the
injection of a single, low pressure air signal into
the diaphragm housing.
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Valve Actuators

The diaphragm housing is made up
of two sections separated by a
flexible diaphragm.

The air pressure applied on the
diaphragm develops a working
force. This force is transmitted to
the actuator stem via the
diaphragm plate, which is a
supportive metal disk attached to
the diaphragm.

The actuator spring provides a
restoring force which positions
and returns the actuator stem.
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Valve Actuators
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Valve Actuators

The travel indicator (a pointer
attached near the stem
connector) indicates the valve
travel on the indicator scale.

The actuator is supported rigidly
on the valve bonnet assembly by
the yoke.

The actuator stem is connected
by the stem connector to the
valve plug stem.
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Valve Actuators

The diameter of the diaphragm plate
determines the force that will be applied
to the actuator stem.

For example, if the maximum input signal
pressure is 100 kPa and the plate
diameter is 30 cm, then:
Force applied to stem = Pressure x Plate Area
= 100 kPa x 3.14 x (0.15)2 m2
= 7.07 KN
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Valve Actuators



Although the signal pressure of 100 kPa seems
to be fairly low, a substantial force can still be
generated if the diaphragm diameter is large.
A larger valve and/or a higher differential
pressure to work against, needed more force to
obtain full valve movement.
To create more force, a larger diaphragm area
or higher spring range is needed.
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Valve Actuators

Diaphragm actuators can be classified as :

direct acting actuator - cause the actuator stem to be pushed downwards
as a result of applying signal air to the top of the diaphragm.

reverse acting actuator - push the actuator stem upwards as signal air is
applied to the bottom of the diaphragm.
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Valve Actuators
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Valve Actuators
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Valve Actuators

The choice between direct acting and reverse
acting pneumatic controls depends on what
position the valve should revert to in the event of
failure of the compressed air supply.
i.e. it depends upon the nature of the application
and safety requirements.

It makes sense for steam valves to close on air
failure (fail-close), and cooling valves to open on
air failure (fail-open).
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Valve Actuators
Actuator
Direct
Reverse
Reverse
Direct
Valve body
Direct
Reverse
Direct
Reverse
Valve
action
Air-to-close
Air-to-close
Air-to-open
Air-to-open
Failure
mode
Fail-open
Fail-open
Fail-close
Fail-close
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Valve Actuators



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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Valve Positioners

Pneumatic valve positioners
are the most commonly used
valve accessories.

A valve positioner is a device
which will accurately
position a control valve in
accordance with the
pneumatic control signal.

The control signal is routed to
the positioner where
comparison of the valve
position (actual) to the
control signal (desired) is
used to develop an output
pneumatic signal which
operates the valve actuator.
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Valve Positioners

The positioner compares the control signal (the requested valve position)
with the actual valve position through the mechanical feedback linkage.

If the valve position is incorrect, the positioner will either load or exhaust air
from the valve actuator until the correct valve position is obtained.

A positioner requires both a control signal and an instrument supply air
for normal operation.

Most positioners come equipped with three gauges to indicate supply air
pressure, control signal pressure and actuator diaphragm signal (output)
air pressure.
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Valve Positioners
39
Valve Positioners
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Valve Positioners

1)
Advantages of the valve positioner include:
Minimizing the effect of friction, hysteresis and
deadband on the valve stem. With a high pressure
system, tighter valve stem packing is needed to prevent
leakage and a high frictional force is generated. With a
positioner valve stem movements of as little as 25 µm
are possible.
2) Enables signal range change. A positioner can
amplify the incoming control signal when a greater
actuating force is needed. A 20-100 kPa control signal
can be amplified to 40-200 kPa before being applied to
the actuator.
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Valve Positioners
3) Allows signal reversal. A positioner can operate in
either direct or reverse acting mode. For example, in
reverse acting mode, an increase in control signal
pressure causes a decrease in positioner output air
pressure. For example, in reverse mode, a 100 - 20 kPa
actuator signal would correspond to a 20 - 100 kPa
control signal from the I/P transducer.
4) Increases the speed of response of the actuator. The
speed of response of the valve actuator depends on:
(a) the actuator volume, and
(b) the flow rate of the control signal air.
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Valve Positioners


Allows valve flow characteristic to be
changed. Most valve positioners employ a
rotating cam in the feedback system. This cam
can be changed to simulate different valve flow
characteristics. A linear globe valve can be used
to respond in an equal percentage manner.
Allows split range operation. In a split range
control loop, one controller is used to drive two
control valves.
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Valve Positioners


A positioner ensures that there is a linear
relationship between the signal input pressure from
the control system and the position of the control
valve.
This means that for a given input signal, the valve
will always attempt to maintain the same position
regardless of changes in valve differential pressure,
stem friction, diaphragm hysteresis and so on.
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Valve Positioners


A positioner may be used as a signal amplifier or
booster.
It accepts a low pressure air control signal and,
by using its own higher pressure input, multiplies
this to provide a higher pressure output air
signal to the actuator diaphragm, if required, to
ensure that the valve reaches the desired
position.
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Valve Positioners

Some positioners incorporate an electropneumatic
converter so that an electrical input (typically 4 - 20
mA) can be used to control a pneumatic valve.
Pneumatic control signal, typically 3 – 15 psi.

Some positioners can also act as basic
controllers, accepting input from sensors.
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Selecting a pneumatic valve and actuator
In summary, the following is a list of the major factors that must be considered
when selecting a pneumatic valve and actuator:





Select a valve using the application data.
Determine the valve action required in the event of power failure, fail-open
or fail-closed.
Select the valve actuator and spring combination required to ensure that the
valve will open or close against the differential pressure.
Determine if a positioner is required.
Determine if a pneumatic or electric control signal is to be provided. This will
determine whether an I to P converter or, alternatively a combined I to P
converter/positioner, is required.
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