Frozen instrumentation lines frequently lead to inaccurate sensor readings and emergency equipment shutdowns in sub-zero temperatures. This guide provides the industry-standard procedures for identifying, thawing, and protecting your pneumatic and sensing lines.
Step 1: Confirm the suspected frozen line
Compare the local gauge reading against the control room Distributed Control System (DCS) feedback for discrepancies. If the instrument remains static despite process changes, suspect a blockage caused by hydrate or ice formation in the tubing. Note the ambient temperature and the specific instrument affected for diagnostic logging.
Step 2: Perform a physical instrument check
Visually inspect the sensing line tubing for visible signs of bulging or frost accumulation. Verify if the instrument housing feels abnormally cold to the touch. An ice plug often creates a distinct localized temperature drop along the tubing segment.
Step 3: Isolate the instrument loop
Close the root valves to isolate the instrumentation line from the primary process fluid to prevent further surge. Ensure all upstream and downstream isolation points are properly tagged out. Notify central control of the intent to bypass the line to prevent false alarms.
Step 4: Bleed off trapped pressure
Slowly open the vent or drain valves to relieve any residual trapped pressure within the isolated tubing section. Listen for the sound of escaping air or fluid; the absence of flow confirms a complete blockage. Wear appropriate personal protective equipment to shield against potential chemical exposure during this step.
Step 5: Apply heat to thaw lines
Use a specialized heat gun or wrap the tubing in controlled heat trace elements to slowly raise the temperature. Do not use open flames or torches, as this risks compromising the integrity of the tubing seals and creating a fire hazard. Monitor the lines continuously until liquid begins to drain from the low-point weep holes.
Step 6: Avoid using high-pressure air
Never attempt to clear frozen instrumentation lines by forcing high-pressure air into the obstruction. This can result in tubing failure or damage to sensitive internal diaphragms. Rely on controlled thermal energy to melt the ice naturally.
Step 7: Verify clear flow after thawing
Once the temperature has reached safe levels, open the upstream valves incrementally to ensure fluid moves through the line as expected. Confirm that pressure gauges or transmitters reflect live process data accurately. Observe the tubing for any leaks that may have developed due to ice-induced stress.
Step 8: Secure the instrumentation system
Ensure all vent and drain valves are fully closed and tightened after the thaw is complete. Verify that the transmitter signal is stable and matches the process reality. Document the incident and ensure all bypasses are removed from the control system.
Confirm the stability of the instrument readings over the next shift to ensure no residual moisture remains. Implement winterization covers or verify the functionality of heat trace systems to prevent recurrence.
Need help finding vendors for your project?
Explore Trunkline’s Top 10 vendor rankings to find companies with the experience and capabilities to get the job done.