1. Pneumatic Instruments
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

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 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

4. 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

5. 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

6. 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.
20 PSI regulated air
PSI
Restriction orifice smaller than nozzle
Add a restriction, and now flapper position, and not regulator setting determines nozzle backpressure.

7. Nozzle opening is usually less than 1/100 inch
Flapper Nozzle
Nozzle opening is usually less than 1/100 inch
PSI
20 PSI regulated air
Restriction orifice smaller than nozzle
Flapper 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 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

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 bellows
But an overrange will ‘set’ a bellows
Thus 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.
Pressure
Diaphragm
This 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.

15. Diaphragm Pressure Switches

16. Another Diaphragm Pressure Switch

17. Pneumatic Relays

21. Links, Levers, pivots, actual vs
Links, Levers, pivots, actual vs. effective levers, lever configurations, angularity error,
First class lever
Levers can be:
1:1
Multiply
Divide
Change direction
Second class lever
Third class lever

22. Pivot Point
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

23. 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

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

25. Effective vs. Actual Levers

26. Lever and Linkage Systems
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

27. 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

28. Bell Crank Reciprocating motion Top pin A moves slider to right
Lever and Linkage Systems
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

29. Double Bell Crank Push/Pull Type Push requires stiff connecting rods
Lever and Linkage Systems
Push/Pull Type
Push
requires stiff connecting rods
Pull
can use flexible wires,cables, ropes

30. 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

31. Why Square up a Linkage?

32. Rack and Pinion
Geneva Drive or Maltese Cross

33. 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)

34. Types of Force Balance
Moment Balance vs. True Force Balance
Moment Balance – uses a balance beam
True 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 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

39. E69R or E69F I/P Transducer
Input: 4-20 mA, dc Output: 3-15 psi Supply: 20 psi
4-20ma in moves flapper

40. 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

41. Pressure to Pressure Converter

42. 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

43. Moore Series 5329M Pneumatic Recorder

44. Moore Series 5329M Pneumatic Recorder

45. Moore Series 5329M Pneumatic Recorder

46. 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

48. Moore 65 Pneumatic Square Root Extractor
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.
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.

50. 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

51. Operating Events SOER 88-01 Instrument Air Failures Fixes Dew point
Particulates
Oil content
Fixes
Alarms
Dryer maintenance
Low point blow downs
PMs

52. The end of pneumatics
On to valves