CHAPTER 1.
PNEUMATIC COMPONENTS
TOPICS COVERED
1.FLAPPER NOZZLE
2.BELLOWS
3.PNEUMATIC RELAYS
4.SAFETY RELIEF VALVE’S ANDPILOT VALVES
5.PNEUMATIC OPERATED VALVES
6.BOOSTERS
7.PNEUMATIC CYLINDER MOTORS
8.FLIP FLOPS
Flapper nozzle:
- This is an important component of many pneumatic systems
- The power source of this device is the supply of air at constant pressure
·
Pressurized air is fed through the orifice & is ejected
from thee nozzle to wards the flapper
·
The flapper is positioned against the nozzle open and the
nozzle back pressure is controlled by the nozzle flapper distance “e”
·
As the flapper approaches the nozzle, the resistance to
the flow of air through the nozzle increases with the result that the nozzle back pressure increases
·
If the nozzle is completely closed by the flapper, the nozzle
back pressure becomes equal to the supply pressure
·
If the flapper is moved away from the nozzle, so that nozzle
flapper distance is large, then there is practically no restriction to flow & the nozzle back pressure takes on a minimum
value close to the ambient pressure
·
Thus the flapper valve converts small changes in the position
of flapper in to large changes in back pressure
Bellows:
- It consists of a hallow chamber with thin metallic wall
- The side walls of bellows are corrugated, while the input & out put surfaces are flat
- The action of bellows is similar to that of a spring
- An increase in the pressure between the bellows results in an
Increase in the separation
between the in put & out put surfaces
·
If the flapper is moved away from the nozzle so that the
nozzle flapper distance is large
Pneumatic relays:
·
An pneumatic amplifier is generally known as pneumatic relay
·
In this bellow and flapper nozzle are cascaded
·
A ball is attached to the lower bellow surface
- When the ball rests on it’s upper seat, the atmospheric opening is closed & the out put pressure becomes
equal to supply pressure
- When the ball rests on its lower seat it shuts off the air supply & the out pressure drops to ambient pressure
- The out put pressure thus can be made to vary from ambient to full supply pressure
- The movement of the flapper away from the nozzle causes the nozzle back pressure to decrease, thus the bellows
contract, moving the ball up wards
- The atmospheric opening is partially closed & the out put pressure increases
- When the flapper moves towards the nozzle, the opposite action takes place
- It is thus seen that an increase in separation between nozzle & flapper
results in a decrease in the out pressure in the case
of the flapper valve & an increase in out pressure when the flapper valve is used with the pneumatic relay
Safety relief valves &pilot valves:
·
Valves basically consists of three parts;
1.stem, 2. Plug, 3. Seat
·
The different types of pneumatic valves are
1. Plug valve
2. Ball valve
3. Butterfly
valve
Plug valve:
Fig:
Main characteristics:
Characteristic curves
-
Quick open characteristics
-
Linear characteristics
- Equal % characteristics
When there is a lift in stem there will be equal amount of lift in
the valve
It is considered in %. Percentage
lift in the stem is equal to the open in the valve
- A control valve basically consists of 3 part’s, stem, plug, seat
- The valve in order to position itself in the pneumatic system should balance with the other active forces, such
as, inertial force’s, static frictional forces and thrust forces due to air pressure and suspended weights
- Cv = coefficient of capacitive rating
-
The capacitive rating coefficient
is defined as “The number of US gallons of water flowing through the valve at 60 degree Fahrenheit /min, with specified
opening of the valve and specified pressure drop across the valve, which is usually PSI (pressure per square inch)
Pilot valve:
·
When large capacity pneumatic valves are considered the
operating force required to move the valve is too large for direct operation of a piston and so a secondary smaller valve
known as pilot valve is added to allow main valve to be operated by the system pressure.
Butterfly valve:
- A butterfly valve is one of the oldest types of valves still in use.
- These valves are practically applicable to low pressure ON OFF services, usually encountered in water
works applications
- Today modern butterfly valves design are suitable for a wide variety of fluid applications, including
those with high pressure drop, light shut off, and corrosive characteristics
Operation:
·
Butterfly valve operation is basically simple, since it
involves only rotating the vane, disc, louver, or flapper by means of shaft to which it is fastened
·
This may be done manually by a lever handle on small size
or by a hand wheel and rotary gear box on large sizes
·
Automatic operation may be accomplished by pneumatic, hydraulic
(or) electrical motor drives attached to the shaft by various methods.
·
As the disc moves through 90 degrees rotation, the valve
moves from fully closed to fully open
·
The free area developed by the disc as it moves towards
the full open position provides the throttling operation
For good pressure flow
Throttling position |
q=60°
Free flow or open position
|
Conventional type butterfly valve |
q=180°
Cambridge characterized
Butterfly valve
|
Fish tail conventional type butterfly valve
|
Ball valves:
- Ball valves contains a spherical plug that control’s flow of fluid through the valve body
It is of three types:
- Conventional ball valve
- Cage ball valve
- Characterized ball valve
- The quarter turning (90°) requires to fully cover or uncover the
opening in the valve body can be imparted by the ball. This valve is not only used for control but for light shut of then
acting as a block valve.
- The spherical plug lends it self not only for precise control of flow through the valve body but also to tight
shut off. Thus the ball valves may assume the double role of control & block valve.
- The ball valves are so designed that lubrication is unnecessary & the torque required to turn the ball is negligible.
Conventional ball valves:
1.
Cage ball
valve:
characterized ball valve
3.
2.
