instrumentation & control

1. P R THUMAR

2. SELF BALANCING INSTRUMENT
• A specimen of unknown mass
is placed in one pan of the
scale, and precise weights are
placed in the other pan until
the scale achieves a condition
of balance. When balance is
achieved, the mass of the
specimen is known to be equal
to the sum total of mass in the
other pan. It has a single mark
indicating a condition of
balance.
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3. CONTINUE….
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4. CONTINUE….
• When mass is added to the left-hand pan, the pointer
(baffle) will move ever so slightly toward the nozzle
until enough backpressure builds up behind the nozzle
to make the bellows exert the proper amount of
balancing force and bring the pointer back (very close)
to its original balanced condition. This balancing action
is entirely automatic.
• The output of the system (nozzle backpressure)
continuously adjusts to match and balance the input
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5. FLAPPER NOZZLE SYSTEM
• It converts very small
displacement signal (in order
of microns) to variation of air
pressure.
• Constant air pressure is
supplied to one end of the
pipeline. There is an orifice
at this end. At the other end
of the pipe, there is a nozzle
and a flapper
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6. CONTINUE….
• The gap between the nozzle and the flapper is set by
the input signal. As the flapper moves closer to the
nozzle, there will be less airflow through the nozzle and
the air pressure inside the pipe will increase.
• if the flapper moves further away from the nozzle, the
air pressure decreases. At the extreme, if the nozzle is
open (flapper is far off), the output pressure will be
equal to the atmospheric pressure. If the nozzle is
blocked, the output pressure will be equal to the
supply pressure.
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7. P R THUMAR

8. PNEUMATIC RELAYS
Pneumatic
relay
Direct
acting
Bleed type
Non bleed
type
Reverse
acting
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9. • In direct acting relays, the input is directly proportional
to the output. So when the input increases , the output
also increases. And when the input decreases, the
output also decreases.
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10. NON-BLEED TYPE RELAY
• The non-bleed relay is a type of
direct acting relay, it consists
of two bellows connected to the
force beam
• It also consists of a rod, and
plugs are connected to the both
ends of the rod.
• The spring is connected to plug
at the downward side.
• The air supply is given from the
bottom side of the non-bleed
type of relay.
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11. NON-BLEED TYPE RELAY
• WORKING
• When the nozzle back pressure increases, there is a movement of bellows.
The bellows move towards the downward direction.
• the nozzle back pressure increases. Hence the output also increases.
• The air bleed stops when equilibrium condition is obtained, no loss of
pressurized air at steady state position.
• When the nozzle back pressure decreases , the bellows starts moving to
upward direction. The air supply is given to the spring from the downward
direction, hence the spring moves in upward direction. There is no
restriction to the air, because the nozzle back pressure decreases.
• Hence the output also decreases.
• The air bleed stops when equilibrium condition is obtained, no loss of
pressurized air at steady state position.P R THUMAR

12. BLEED TYPE OF RELAY
• It consists of a main diaphragm on
which the nozzle back pressure acts.
• The diaphragm is connected to the
metal rod.
• At the both ends of metal rods the
plugs are connected.
• The plugs are connected to the spring.
• The air supply is given to the spring
from the bottom side of the relay.
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13. WORKING
• The bleed type of relay is a type of direct acting relay.
In this relay, the output is directly proportional to the
input. Means if the input increases ,the output also
increases. And if the input decreases, the output also
decreases.
• In all position of valve excepts the position of shut off
the air supply, air continues to bleed in atmosphere
even after equilibrium condition is obtained between
nozzle back pressure & control pressure.
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14. REVERSE ACTING RELAY
• CONSTRUCTION
1) The reverse acting relay is mainly
consists of a metal diaphragm.
2) The diaphragm is connected to the
rod.
3) The rod is connected to the ball.
4) The air supply is given to the ball
from the downward side of the relay.
P R THUMAR

15. WORKING
• In reverse acting relay the output is indirectly proportional to the
input.
• When the nozzle back pressure increases above the set point value,
the metal diaphragm moves towards the downward side.
• As the diaphragm is connected to the ball, the ball also moves towards
the down side.
• The air supply is given to the ball from the bottom side of the relay.
• Therefore the air is restricted by the nozzle back pressure.
• Due to this action the air moves to the atmosphere. So the output
pressure is decreases.
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16. WORKING
• When the nozzle back pressure is decreases, the metal
diaphragm moves towards the upper side.
• The ball which is connected to the diaphragm is also
moves towards the upper side.
• The air supply is given to the ball from the bottom side
of the relay.
• Because of decreases in pressure the air does not
restricted and the output pressure increases.
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17. PRESSURE REGULATOR
• A single stage regulator contains a
single diaphragm and valve.
• High pressure gas enters the regulator
through the inlet into the high pressure
chamber or valve chamber.
• The pressure is indicated by the inlet
pressure guage fitted to the regulator.
• The gas fills the high pressure chamber
completely.
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18. PRESSURE REGULATOR
• As the valve remains closed without any external interference , the high pressure gas
remains contained in the valve chamber.
• When the adjusting knob is turned clockwise, it compresses the range spring and
exert a downward force on the diaphragm which in turn pushes the valve stem open.
• This releases gas into low pressure chamber exerting an opposing force on diaphragm.
• An equilibrium is reached when the range spring force downwards on the diaphragm
is equal to the combined upward forces of the gas in the low pressure ,the upward
force exerted by the valve spring and the upward force of the high pressure gas in
valve chamber acting on the valve.
• While the regulator is in use the initial high pressure starts to drop at the source, as
the cylinder empties.
• Once the cylinder is empty of the inlet gas flow shuts off , the outlet pressure drops to
zero.
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19. FORCE BALANCE PRINCIPLE
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20. FORCE BALANCE PRINCIPLE
• Some input element produces force due to input pressure and nozzle
back pressure is produced. The output of nozzle back pressure is
proportional to the applied input pressure.
• The input signal is applied to the input bellow connected at the left
side of the beam.
• The balancing bellow is connected at right side of the beam.
• When pressure is applied to the input bellow, beam goes upward
direction at the left side and goes downward at right side.
• The distance between flapper and nozzle reduced so back pressure is
increases. This back pressure is applied to the balancing bellow which
balance the beam.
• The balancing pressure is proportional to the applied input pressure.
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21. MOTION BALANCE PRINCIPLE
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22. MOTION BALANCE PRINCIPLE
• Some input element produce a motion rather than a change in force. In this
mechanism motion produced by the beam is balance with the nozzle back
pressure.
• When pressure at bourdon tube is zero, there is no motion created
• By the bourdon tube.
• The motion of the beam is goes to left side and clearance between the flapper
and nozzle is increased, so output of the back pressure is decreased.
• If the input pressure is applied to the bourdon tube so that end of the bourdon
tube create a motion at the beam and the beam goes up at the left side and
distance between flapper and nozzle is decreased. So the nozzle backpressure
is increased.
• Back pressure balance the beam position. The output f the nozzle back
pressure is proportional to the applied input motion.
•
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23. MOMENT BALANCE PRINCIPLE
• The turning effect of a force is known as the moment. It is the product
of the force multiplied by the perpendicular distance from the line of
action of the force to the pivot or point where the object will turn.
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24. MOMENT BALANCE PRINCIPLE
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25. MOMENT BALANCE PRINCIPLE
• Input pressure is applied at the left side of the beam through a bellows and the
right side pressure is balanced through a bellows.
When the input pressure is zero
• The beam goes downward due to spring tension
• Output pressure is almost zero
• Flapper nozzle distance is more
• Nozzle back pressure is almost zero or we can set it 3 psi through zero adjust
spring.
If some pressure is applied then
• The beam goes upper direction
• Flapper nozzle distance is decreases
• Nozzle back pressure is increases
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26. PNEUMATIC PROPORTIONAL CONTROLLER
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27. PNEUMATIC PROPORTIONAL CONTROLLER
• An increase in process variable signal (pressure) results in an
increase in output signal (pressure).
• Increasing process variable (PV) pressure attempts to push
the right-hand end of the beam up, causing the baffle to
approach the nozzle.
• This blockage of the nozzle causes the nozzle’s pneumatic
backpressure to increase
• Thus increasing the amount of force applied by the output
feedback bellows on the left-hand end of the beam and
returning the flapper (very nearly) to its original position.
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28. PROPORTIONAL DERIVATIVE CONTROOLER
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29. PROPORTIONAL DERIVATIVE CONTROOLER
• .To add derivative control action to a P-only controller, all we
need to place a restrictor valve between the nozzle tube and the
output feedback bellows, causing the bellows to delay filling or
emptying its air pressure over time.
• If any sudden change occurs in PV or SP, the output pressure
will saturate before the output bellows has the opportunity to
equalize in pressure with the output signal tube.
• Thus, the output pressure “spikes” with any sudden “step
change” in input: exactly what we would expect with derivative
control action.
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30. PROPORTIONAL-INTEGRAL (PI) CONTROLLER
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31. PROPORTIONAL-INTEGRAL (PI) CONTROLLER
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32. PROPORTIONAL-INTEGRAL (PI) CONTROLLER
• opening up the reset valve just a little bit, so that the output air pressure of 3
PSI begins to slowly fill the reset bellows.
• As the reset bellows fills with pressurized air, it begins to push down on the left-
hand end of the force beam.
• This forces the baffle closer to the nozzle, causing the output pressure to rise.
The regular output bellows has no restrictor valve to impede its filling, and so
it immediately applies more upward force on the beam with the rising output
pressure.
• With this greater output pressure, the reset bellows has an even greater “final”
pressure to achieve, and so its rate of filling continues.
• The result of these two bellows’ opposing forces (one instantaneous, one time-
delayed) is that the lower bellows must always stay 3 PSI ahead of the upper
bellows in order to maintain a force-balanced condition with the two input
bellows whose pressures differ by 3 PSI.
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33. PROPORTIONAL-INTEGRAL (PI) CONTROLLER
• The greater the difference in pressures between PV and SP
(i.e. the greater the error), the more pressure drop will
develop across the reset restriction valve, causing the reset
bellows to fill (or empty, depending on the sign of the error)
with compressed air at a faster rate2, causing the output
pressure to change at a faster rate.
• Thus, we see in this mechanism the defining nature of
integral control action: that the magnitude of the error
determines the velocity of the output signal (its rate of
change over time)
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34. PROPORTIONAL-INTEGRAL-DERIVATIVE (PID) CONTROLLER
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35. PROPORTIONAL-INTEGRAL-DERIVATIVE (PID) CONTROLLER
• Three term pneumatic control can be achieved using a
P-I-D controller. Here the action of the feedback
bellows is delayed. The output is given by,
• The terms gain K, derivative time Td, integral
time Ti which can be set by beam pivot point and two
bleed valves
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36. Advantages of pneumatic controllers
• Simplicity of the components and no complex structure
• Easy maintainability
• Safe and can be used in hazardous atmospheres
• Low cost of installation
• Good reliability and reproducibility
• Speed of response is relatively slow but steady
• Limited power capacity for large mass transfer
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37. Limitations of pneumatic controllers
• Slow response
• Difficult to operate in sub-normal temperatures
• Pipe-couplings can give rise to leaks in certain ambient
conditions
• Moving parts - more maintenance
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