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
