Presentation on theme: "ACADs (08-006) Covered Keywords Flapper, pneumatic relay, pressure switch, force balance pneumatic instrument, current to pressure converter, pneumatic."— Presentation transcript:
ACADs (08-006) Covered Keywords Flapper, 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. Description Supporting Material
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.
Describe the operation of a flapper/nozzle detector Describe the operation of a pressure switch Describe the operation of a pneumatic relay Describe the operation of a link-lever assembly Describe the characteristics of a Force Balance pneumatic instrument Describe the operation of a Current to Pressure converter
Describe the operation of a Pressure to Pressure converter Describe the theory of operation of a Pneumatic Recorder Describe the theory of operation of a Temperature to Pressure converter Describe the theory of operation of a Pneumatic Square Root Extractor Describe the characteristics of a Motion Balance pneumatic instrument
Calibrate a pressure switch in accordance with lab standards Calibrate a Current to Pressure converter in accordance with lab standards Calibrate a Pressure to Pressure converter in accordance with lab standards Calibrate a Pneumatic Recorder in accordance with lab standards Calibrate a Temperature to Pressure converter in accordance with lab standards Calibrate a Pneumatic Square Root Extractor in accordance with lab standards
20 PSI regulated air PSI Restriction orifice smaller than nozzle Flapper Nozzle Consider 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. Add a restriction, and now flapper position, and not regulator setting determines nozzle backpressure.
20 PSI regulated air PSI Restriction orifice smaller than nozzle Flapper Nozzle Nozzle opening is usually less than 1/100 inch Flapper motion is even smaller, usually less than ¼ the diameter of the nozzle
3 – 15 PSI converts to about 20 – 100 KPa This is the most linear part of the range of flapper nozzle clearance
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.
Pneumatic Relays Input 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
Volume booster relay
Which Can Move More, a Diaphragm or a Bellows? Usually a bellows But an overrange will ‘set’ a bellows Thus mechanical stops are often set
Pressure Diaphragm 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. Feedback in pneumatic circuits is usually by a bellows. Feedback improves response, repeatability and stability by re-balancing flapper nozzle clearance.
Diaphragm Pressure Switches
Another Diaphragm Pressure Switch
Links, Levers, pivots, actual vs. effective levers, lever configurations, angularity error, First class lever Second class lever Third class lever Levers can be: 1:1 Multiply Divide Change direction
Z configuration As one lever moves, the other lever moves in the opposite rotation Gain is adjusted by differing the relative lengths while maintaining as close to the 90 degree relationship Pivot Point
When linkages are square, force is transmitted in the most linear way possible. U configuration As one lever rotates, the other rotates in the same direction. No gain adjustment assuming a 90 degree relationship is maintained
Pivot Point 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.
Effective vs. Actual Levers
Rocker Arm Has an input and output arm which rock around a fulcrum point Acts as a link which transmits force or motion between moving parts Operates usually as a first class lever Changes linear input motion to an opposed linear motion Lever and Linkage Systems
Bell Crank A bent first class lever that pivots at the bent point or elbow Used to change the direction of force or motion 90 degrees or less Used to convert rotary motion to straight line or reciprocating motion Lever and Linkage Systems
Bell Crank Reciprocating motion Top – pin A moves slider to right – pin B moves bell crank which moves slider to left Bottom – pins move crank; spring returns to start position Lever and Linkage Systems
Double Bell Crank Push/Pull Type Push – requires stiff connecting rods Pull – can use flexible wires,cables, ropes Lever and Linkage Systems
Differential Linkage Used to combine several motion inputs into a resulting output Three pivot points, none of which are fixed but are free to float within limits Any pivot can function as an input A motion control mechanism rather than a means of transmitting force or power Lever and Linkage Systems
Why Square up a Linkage?
Geneva Drive or Maltese Cross Rack and Pinion
Force Balance Uses feedback of the output signal to balance the primary input signal from the measuring element. The balanced output signal is proportional to the measured variable All forces on the instrument should balance such that there is virtually no motion Since there need be no motion, they tend to be more maintenance free than are motion-balance devices Examples: I2P, Pneumatic pressure transmitters (P2P)
Types of Force Balance Moment Balance vs. True Force Balance Moment Balance – uses a balance beam True Force Balance – uses a force rod
Motion Balance Uses 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 Balance Example: Valve positioners, pneumatic instruments with 2 or more pivots
E69R or E69F I/P Transducer Input: 4-20 mA, dc Output: 3-15 psi Supply: 20 psi 4-20ma in moves flapper
E69F I/P Transducer Input: 4-20 mA, dc Output: 3-15 psi Supply: 20 psi Nozzle moves because of pneumatic feedback Both flapper and nozzle move
Pressure to Pressure Converter
Moore Series 5329M Pneumatic Recorder 120VAC in to drive chart paper 3-15PSI in on two channels to drive pneumatic servos which drive pens 20PSI supply air required We still have some of these in the Radwaste Control Room They contain lots of 1/8” flexible tubing that tends to get brittle and leak
Moore Series 5329M Pneumatic Recorder
Moore Nullmatic Temperature to Pressure Converter Helium Filled True Force Balance 3-15PSI out proportional to temperature We use these on the boric acid concentrator
Moore 65 Pneumatic Square Root Extractor
Used to linearize signals from differential pressure flow transmitters Uses the cosine function of a small angle to give square root conversions
Operating Events SOER Instrument Air Failures – Dew point – Particulates – Oil content Fixes – Alarms – Dryer maintenance – Low point blow downs – PMs