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Pneumatic instrumentation

instrumentation & control

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Pneumatic instrumentation

  1. 1. P R THUMAR
  2. 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. P R THUMAR
  3. 3. CONTINUE…. P R THUMAR
  4. 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 P R THUMAR
  5. 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 P R THUMAR
  6. 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. P R THUMAR
  7. 7. P R THUMAR
  8. 8. PNEUMATIC RELAYS Pneumatic relay Direct acting Bleed type Non bleed type Reverse acting P R THUMAR
  9. 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. P R THUMAR
  10. 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. P R THUMAR
  11. 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. 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. P R THUMAR
  13. 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. P R THUMAR
  14. 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. 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. P R THUMAR
  16. 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. P R THUMAR
  17. 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. P R THUMAR
  18. 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. P R THUMAR
  19. 19. FORCE BALANCE PRINCIPLE P R THUMAR
  20. 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. P R THUMAR
  21. 21. MOTION BALANCE PRINCIPLE P R THUMAR
  22. 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. • P R THUMAR
  23. 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. P R THUMAR
  24. 24. MOMENT BALANCE PRINCIPLE P R THUMAR
  25. 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 P R THUMAR
  26. 26. PNEUMATIC PROPORTIONAL CONTROLLER P R THUMAR
  27. 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. P R THUMAR
  28. 28. PROPORTIONAL DERIVATIVE CONTROOLER P R THUMAR
  29. 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. P R THUMAR
  30. 30. PROPORTIONAL-INTEGRAL (PI) CONTROLLER P R THUMAR
  31. 31. PROPORTIONAL-INTEGRAL (PI) CONTROLLER P R THUMAR
  32. 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. P R THUMAR
  33. 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) P R THUMAR
  34. 34. PROPORTIONAL-INTEGRAL-DERIVATIVE (PID) CONTROLLER P R THUMAR
  35. 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 P R THUMAR
  36. 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 P R THUMAR
  37. 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 P R THUMAR

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