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Common Symbols Used in Pneumatic Systems and Instrumentations

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Pneumatic Systems are still popular in older plants and even in modern plants where their use is inevitable. Pneumatic diagrams representing pneumatic systems have defined ways they are represented. They are commonly represented with symbols. These symbols needs to be understood before you can correctly interpret pneumatic drawings and diagrams. These common symbols are shown below:

Pneumatic Directional Control Valves Symbols
Symbols of Directional Control Valves (Photo Credit: AutomationDirect.com)

Symbols of Simple Pneumatic Valves
Pneumatic Valve Symbols (Photo Credit: AutomationDirect.com)

Line Symbols Used in Pneumatic Drawings
Line Symbols in Pneumatic Symbols (Photo Credit: AutomationDirect.com)

Symbols of Common Equipment Used in Pneumatic Drawings and Systems
Symbols of Common Pneumatic Equipment. (Photo Credit: AutomationDirect.com)

Common Actuator Symbols Used in Pneumatic Drawings
Actuator Symbols (Photo Credit: AutomationDirect.com)

Troubleshooting Guide for Pneumatic Field Transmitters

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Pneumatic instruments are still a critical component in most instrumentation systems of today even though recent technology is making them to become somewhat obsolete. For plants where pneumatic instruments especially pressure transmitters are still being used, there are common problems with these instruments that are encountered in their everyday use. These problems are highlighted below so that when troubleshooting any potential problem with such instruments, we know what the culprits are.
Pressure Transmitter  Problem Possible Cause(s)
No Output
  • Bent flapper
  • No air supply; plugged restrictor (this is  very common)
  • Corroded pneumatic relay or components
  • Dirty pneumatic relay seats
  • Flapper is away from the nozzle due to freezing, improper adjustment, bent "C" flexure or transmitter has been dropped
  • Leak in the feedback bellows
  • Leak in the nozzle circuit
  • Leak in the sensor pressure circuit
  • Disconnected or broken links in a motion balance pressure transmitter
Partial Output
  • Plugged low pressure leg on a DP cell
  • Worn pneumatic relay parts
  • Partially plugged supply screen or filter
  • Burr on the flapper assembly
  • Hole in the flapper assembly
  • Damaged feedback bellows
  • Worn capsule diaphragms
  • Warped or distorted "C" or "A" flexure on a DP cell
  • Wrong range-sensing unit
  • Pin hole leaks in the control relay diaphragm
Full Output
  • Plugged nozzle
  • Ballooned capsule diaphragm
  • Loose nozzle lock nut
  • Blocked pneumatic relay vent
  • Sensing capsule impacted with process solids
  • Flapper assembly distorted or bent
Zero shift diaphragms
  • Dirty flapper assembly set point capsule problems - coating, fatigue, warped
  • Temperature changes -  either ambient or process temperatures
  • Process static pressure changes.
  • Worn zero or span adjustments
  • Flapper is depressed or hollowed on the surface
  • Pin hole leak in the flapper
  • Flashing and/or condensate on either leg of a DP cell installation
Output Oscillates
  • Liquid in the feedback bellows - water, oil etc
  • "C" flexure look nut loose
  • Close coupled pneumatic system
  • Loss of capsule fill fluid
  • Hole in the feedback bellows
  • Loose bleed/vent valves
  • Flashing due to pressure variations

The list of pneumatic field transmitters problem and their probable cause in the table above is by no means exhaustive. It is however a useful troubleshooting guide for pneumatic transmitters.

Read Also: Troubleshooting Guide for DP Transmitters

How a Pneumatic Pressure Transmitter Works

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The Foxboro 11GM pneumatic pressure transmitter is a force-balance instrument used to measure pressure and transmits it as a proportional 3 – 15psig signal.
Pneumatic Pressure Transmitter. (Photo Credit: Foxboro)

As shown in the diagram of the transmitter, the pressure being measured is applied to a bellows capsule. The force on the capsule is transmitted through a flexure to the lower end of the force bar. The metal diaphragm seal serves as both a fulcrum for the force bar and as a seal for the pressure chamber. The force is transmitted through the flexure connector to the range bar which pivots on the range wheel.

Any movement of the range bar causes a minute change in the clearance between the flapper and the nozzle. This produces a change in the output pressure from the pneumatic relay to the feedback bellows until the force on the feedback bellows balances the force on the bellows capsule.

The output pressure established by this force-balance mechanism is the transmitted signal and is proportional to the pressure applied to the bellows capsule. This signal can be transmitted to a pneumatic receiver or controller for recording/indication or control purposes.