Dry Leg Concept
A gas-filled low-side impulse line used where liquid condensate would introduce errors.
Learning objectives
- Explain Dry Leg Concept in technician-friendly language.
- Identify where when dry legs are used and how to prevent liquid accumulation appears in live plant work.
- Connect the topic to pressure measurement, calibration, or loop checking.
- Avoid the main field mistake: allowing liquid to build up in a dry leg, causing a false high DP reading.
Dry Leg Concept is part of the Pressure Measurement master module. A gas-filled low-side impulse line used where liquid condensate would introduce errors.
The main focus is when dry legs are used and how to prevent liquid accumulation. In the field, the technician must read the tag, confirm the pressure reference and unit, check the process connection, and compare the instrument reading with the expected plant condition. The topic also links to calibration records, DCS scaling, alarm settings, and safe isolation before any instrument work.
A practical way to study this topic is to ask four questions: what pressure is being measured, what reference is used, how the signal reaches the DCS, and what failure would make the reading unsafe or misleading. This keeps the lesson tied to real maintenance and commissioning decisions rather than only theory.
Field examples
- Gas service DP flow transmitters use dry legs sloped back to the process to remain gas-filled.
- Compare the local gauge, transmitter HART value, and DCS indication before deciding which value is credible.
- Record the unit, range, LRV, URV, and tag number before calibration or loop checking.
Common mistakes
- allowing liquid to build up in a dry leg, causing a false high DP reading
- Trusting one reading without checking the pressure reference, unit, range, and process condition.
- Opening tubing, manifolds, or vents without proving isolation and zero stored pressure.
Safety
Unplanned liquid accumulation in gas lines is a condensate hazard; drain procedures must be followed.
Key takeaway
Dry Leg Concept helps a technician understand pressure readings, field installation, and safe troubleshooting. Always verify the unit, reference, range, and isolation state before acting on the measurement.
Imported study note from pressure_lesson_096. The app generates this text from a lesson seed. It is not a long textbook page.