Presentation on theme: "Pneumatic Instruments"— Presentation transcript:
1 Pneumatic Instruments ACADs (08-006) CoveredKeywordsFlapper, pneumatic relay, pressure switch, force balance pneumatic instrument, current to pressure converter, pneumatic recorder, temperature to pressure converter, pneumatic square root extractor, motion balance pneumatic instrument.DescriptionSupporting Material
2 Pneumatic Instruments Terminal Objective: Given the appropriate equipment and procedures, the I&C Technician will calibrate and maintain pneumatic instruments. Mastery will demonstrated by successful completion of a Lab Performance Exercise and written Exam.
3 Describe the operation of a flapper/nozzle detector Describe the operation of a pressure switchDescribe the operation of a pneumatic relayDescribe the operation of a link-lever assemblyDescribe the characteristics of a Force Balance pneumatic instrumentDescribe the operation of a Current to Pressure converter
4 Describe the operation of a Pressure to Pressure converter Describe the theory of operation of a Pneumatic RecorderDescribe the theory of operation of a Temperature to Pressure converterDescribe the theory of operation of a Pneumatic Square Root ExtractorDescribe the characteristics of a Motion Balance pneumatic instrument
5 Calibrate a pressure switch in accordance with lab standards Calibrate a Current to Pressure converter in accordance with lab standardsCalibrate a Pressure to Pressure converter in accordance with lab standardsCalibrate a Pneumatic Recorder in accordance with lab standardsCalibrate a Temperature to Pressure converter in accordance with lab standardsCalibrate a Pneumatic Square Root Extractor in accordance with lab standards
6 Restriction orifice smaller than nozzle Flapper NozzleConsider a nozzle with a regulated 20 PSI air supply. The regulator is supplying all the air it can, yet backpressure in the nozzle is zero.Cover the nozzle and backpressure will rise to supply pressure. But there is no in-between. Backpressure is either zero or supply pressure.20 PSI regulated airPSIRestriction orifice smaller than nozzleAdd a restriction, and now flapper position, and not regulator setting determines nozzle backpressure.
7 Nozzle opening is usually less than 1/100 inch Flapper NozzleNozzle opening is usually less than 1/100 inchPSI20 PSI regulated airRestriction orifice smaller than nozzleFlapper motion is even smaller, usually less than ¼ the diameter of the nozzle
8 3 – 15 PSI converts to about 20 – 100 KPa This is the most linear part of the range of flapper nozzle clearance
9 The air supply, usually about 20psi The air supply, usually about 20psi. represents clean, dry air that is maintained under constant pressure. This air is allowed to expand through a restriction which has an orifice opening less than that of the nozzle. If the flapper, which is positioned by some sensing device such as a Bourdon tube. is moved slightly away from the nozzle, air will bleed through the nozzle at a faster rate than it can pass through the restriction so the output pressure will decrease to zero. As the flapper is moved closer to the nozzle, air flow through the nozzle is reduced and the output will begin to increase. When the flapper covers the nozzle completely, the output will increase to supply pressure. A movement of the flapper of only mm. produced by a very slight change in the process variable, will cause maximum change in output. A transmitter of this type would have limited use. It could only be used to operate an electrical switch or some other device that requires the output to be at maximum or minimum (on/off). There is no feedback device to rebalance the flapper nozzle to a new position.Only a limited volume of air can pass through the restriction, thus we need a way to boost the volume in order to drive a signal any distance.
10 Pneumatic RelaysInput pressure, acting upon the effective area of the top diaphragm, produces a force that is balanced by the force produced by the output pressure applied over the effective area of the lower diaphragm.Any imbalance in these opposing forces will operate the plunger, increasing or decreasing air supply to the output chamber. (The amplifying or reducing ratio is fixed by the ratio of input-to-output diaphragm areas.)An increase in input opens the pilot valve to admit supply air directly to the output. A decrease in input opens the exhaust port to exhaust air from the output.Frequently volume booster relays are integral to pneumatic instruments. Example: 43AP, E69 I2P
11 Considering the transmitter shown in Fig Considering the transmitter shown in Fig. 5, an increase in value of the process variable will move the link and the bottom end of the flapper to the right. Less air will bleed through the nozzle, causing the back pressure in the nozzle and the capsule to increase. Upward movement of the capsule and the pilot valve will reduce the exhaust opening and increase the opening of the air supply port to increase the output. This causes the feedback bellows to expand and draw the top end of the flapper slightly away from the nozzle to produce a proportional output much faster than the transmitter in Fig 4.Use of an air relay produces a larger output volume, also. the nozzle pressure is amplified and air loss through the nozzle is reduced. For example, a variation in nozzle pressure from 5 to 20 kPa will cause a 20 to 100 kPa change in output.Fig. 6 shows an industrial differential pressure transmitter that may be used for level or flow measurement.Volume booster relay
12 Which Can Move More, a Diaphragm or a Bellows? Usually a bellowsBut an overrange will ‘set’ a bellowsThus mechanical stops are often set
13 Rule of thumb when calibrating pneumatics: The zero adjustment refers to the flapper nozzle gap. The span adjustment refers to the amount of feedback.Rule of thumb #2 for pneumatics: Since you are dealing with mechanical devices that move, zero and span adjustments interact with each other more than in electronic devices. So be very suspicious of making gross adjustments.PressureDiaphragmThis transmitter responds quite slowly to a change in the process variable, as considerable time is required to fill the bellows and a quarter-inch copper tube of substantial length. To speed up response and reduce air losses through the nozzle, a pneumatic amplifier, often called a pneumatic relay, has been added as shown in Fig. 5.Feedback in pneumatic circuits is usually by a bellows. Feedback improves response, repeatability and stability by re-balancing flapper nozzle clearance.
21 Links, Levers, pivots, actual vs Links, Levers, pivots, actual vs. effective levers, lever configurations, angularity error,First class leverLevers can be:1:1MultiplyDivideChange directionSecond class leverThird class lever
22 Pivot PointZ configurationAs one lever moves, the other lever moves in the opposite rotationGain is adjusted by differing the relative lengths while maintaining as close to the 90 degree relationshipPivot Point
23 When linkages are square, force is transmitted in the most linear way possible. U configurationAs one lever rotates, the other rotates in the same direction.No gain adjustment assuming a 90 degree relationship is maintainedPivot Point
24 When linkages are not square, force is transmitted as a function of the sine of the angle. The secondary element does not move the same amount as the primary element.This is referred to as Angularity Error. The output lever travels through different angles than the input lever.This is minimized by placing the input lever (usually the valve) at 50% of travel and adjusting linkage for 90 degrees.Pivot Point
26 Lever and Linkage Systems Rocker ArmHas an input and output arm which rock around a fulcrum pointActs as a link which transmits force or motion between moving partsOperates usually as a first class leverChanges linear input motion to an opposed linear motion
27 Lever and Linkage Systems Bell CrankA bent first class lever that pivots at the bent point or elbowUsed to change the direction of force or motion 90 degrees or lessUsed to convert rotary motion to straight line or reciprocating motion
28 Bell Crank Reciprocating motion Top pin A moves slider to right Lever and Linkage SystemsReciprocating motionToppin A moves slider to rightpin B moves bell crank which moves slider to leftBottompins move crank; spring returns to start position
29 Double Bell Crank Push/Pull Type Push requires stiff connecting rods Lever and Linkage SystemsPush/Pull TypePushrequires stiff connecting rodsPullcan use flexible wires,cables, ropes
30 Differential LinkageUsed to combine several motion inputs into a resulting outputThree pivot points, none of which are fixed but are free to float within limitsAny pivot can function as an inputA motion control mechanism rather than a means of transmitting force or powerLever and Linkage Systems
33 Force BalanceUses feedback of the output signal to balance the primary input signal from the measuring element. The balanced output signal is proportional to the measured variableAll forces on the instrument should balance such that there is virtually no motionSince there need be no motion, they tend to be more maintenance free than are motion-balance devicesExamples: I2P, Pneumatic pressure transmitters (P2P)
34 Types of Force BalanceMoment Balance vs. True Force BalanceMoment Balance – uses a balance beamTrue Force Balance – uses a force rod
35 This particular instrument contains two movable arms, a force bar connected to a capsule and a range rod which is connected to the feedback bellows. Both ends of the range rod are free to rotate about the range nut that rests against the chassis. This fulcrum point can be moved up or down to change the span of the output signal. One end of this range rod is connected to a force bar by a flexure connector while the other end is rotated by the force in the feedback bellows. The fulcrum of the force bar is fixed at point D. An increase in differential pressure due to an increase in flow or level will force the capsule to the left causing the force bar to rotate slightly clockwise about point D.The flapper, mounted against the flexure connector, approaches the nozzle causing the nozzle pressure and relay output to increase. An increase in feedback pressure will cause the bellows to expand to the right and force the upper end of the range rod to move to the left to move the flapper slightly away from the nozzle. The output is adjusted so that the measuring and feedback moments are equal. Output of this transmitter varies between 20 and 100 kPa and is proportional to the differential pressure. The relay is designed slightly differently but its principle of operation is similar to the one previously explained.
36 Motion BalanceUses the motion of the measuring element against a spring to reach a balance of forces representing the magnitude of the measured variable.Requires motion over a range, or there is no change in output.Can be further categorized as Linear motion balance or Angle motion BalanceExample: Valve positioners, pneumatic instruments with 2 or more pivots
42 Moore Series 5329M Pneumatic Recorder 120VAC in to drive chart paper3-15PSI in on two channels to drive pneumatic servos which drive pens20PSI supply air requiredWe still have some of these in the Radwaste Control RoomThey contain lots of 1/8” flexible tubing that tends to get brittle and leak
48 Moore 65 Pneumatic Square Root Extractor supply air flows through arestriction, into the inner bellows, through the center stem and to atmospherethrough the nozzle at the top of the center stem. The restricted pilot air also actsupon the top of the booster diaphragm, controlling the pilot valve, either to admitsupply air or to exhaust air to atmosphere through the exhaust port. The pilot airpressure variation is 1/6 of the transmitted air pressure variation.With an increase in input pressure to the input capsule, the floating link is movedcloser to the center stem, further closing the nozzle and decreasing the flow of pilotair to atmosphere. The pressure above the booster diaphragm is increased, closingthe pilot valve exhaust and opening the supply port, which increases the transmittedpressure. This increase in transmitted pressure acts on the bottom of the exhaustdiaphragm, opening the exhaust port and throttling the supply port, thus balancingthe booster pilot system to a higher transmitted pressure. The increasedtransmitted pressure is also fed back to the chamber between the inner and outerbellows. The bellows expands against the force exerted by the output zero spring,bringing the center stem and nozzle to a higher position, thus establishingequilibrium.Supply air flows through a restriction, into the inner bellows, through the center stem and to atmosphere through the nozzle at the top of the center stem. The restricted pilot air also acts upon the top of the booster diaphragm, controlling the pilot valve, either to admit supply air or to exhaust air to atmosphere through the exhaust port. The pilot air pressure variation is 1/6 of the transmitted air pressure variation.With an increase in input pressure to the input capsule, the floating link is moved closer to the center stem, further closing the nozzle and decreasing the flow of pilot air to atmosphere. The pressure above the booster diaphragm is increased, closing the pilot valve exhaust and opening the supply port, which increases the transmitted pressure.This increase in transmitted pressure acts on the bottom of the exhaust diaphragm, opening the exhaust port and throttling the supply port, thus balancing the booster pilot system to a higher transmitted pressure. The increased transmitted pressure is also fed back to the chamber between the inner and outer bellows. The bellows expands against the force exerted by the output zero spring, bringing the center stem and nozzle to a higher position, thus establishing equilibrium.
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