What are the common faults of platinum resistance thermometers with junction boxes mounted on movable flanges

The failure modes of platinum resistance thermometers with junction boxes mounted on movable flanges display unique dynamic structural properties. The main reasons for failure include mechanical fatigue produced by temperature displacement, wear and tear on the sealing system, and wear and tear on the electrical connections, not just the age of the sensing element. Based on statistics from the industrial area and authorized literature, the following are four common types of faults:




I. Lead wire fatigue fracture (around 35%)



The movable flange moves all the time because of thermal expansion and contraction. This puts stress on the lead wire at the bend within 50mm of the junction box outlet, which causes the metal to wear out quickly.



The DCS system shows "OL" (open circuit), the multimeter shows strange resistance readings for R12 and R13, and an open circuit in R23. The lead wire has visible metal wire drawing, discolouration, or micro-cracks that can only be seen using a magnifying glass.



High-risk situations include heating furnaces, steam pipelines, and vibrating machinery that start and stop often.



Important Information: Installations without a flexible arc section of at least 200mm raise the rate of lead wire breakage by 6 times.







II. Junction Box Sealing Failure and Insulation Deterioration (About 30%)



Causes: The moving flange moving around makes the sealing ring compress and relax over and over again, which speeds up the aging and cracking of fluororubber (FKM) or silicone rubber (VMQ).



Symptoms: The insulation resistance lowers quickly from more than 100MΩ to less than 10MΩ when measured with a 500V megohmmeter. Water building up on the inside wall of the junction box, copper terminals rusting, and wire insulation that is sticky. The temperature readings don't show the real working conditions because they drift and spike.



High-Risk Situations: locations with a lot of humidity, pipelines with a lot of condensation, and locations that have been chemically damaged.







III. Contamination and Jamming of Moving Contacts (About 20%)



Long-term exposure of the copper contacts inside the moving joint to oil mist, dust, or hydrogen sulfide conditions leads to the creation of a conductive carbon film or oxide layer.



Symptoms: The resistance stays the same, but the temperature changes in an odd way (not an open circuit). When you press the flange by hand, it makes a noise or has a lot of resistance. The thermal reaction time is longer than 45 seconds.



Places that are high-risk include oil refineries, food processing factories, and places near lubricating systems.







IV. Thermal Stress Cracks in the Welded Area (around 15%)



The difference in thermal expansion coefficients between stainless steel and carbon steel causes cyclic stress at the weld between the protective pipe and the flange. This causes microcracks in the weld heat-affected zone (HAZ).



Symptoms: There are no electrical problems, but the temperature measurements steadily drift; there are no obvious leaks, but ultrasonic testing shows cracks in the weld. This happens a lot when the machine is running continuously at high temperatures (>300℃).



High-risk situations include catalytic cracking equipment, hot oil pipelines, and high-pressure steam systems.



Common Fault Distribution and Operating Condition Correlation Table



Table Fault Type Main Causes High-Risk Operating Conditions Detection Methods



Lead Wire Fatigue Fracture Displacement Stress, No Redundant Arc Segment Vibrating Equipment, Frequent Start-Stop Resistance Measurement, Visual Inspection of Bending Points



Sealing Failure Aging of Sealing Rings, Misalignment Humid, Corrosive Media Insulation Resistance Test, Leakage Observation



Contact contamination oil mist, H₂S, dust intrusion oil refining, food, chemical signal stability analysis, and contact cleaning inspection



Welding Cracks Thermal Stress, Welding Defects High Temperature (>300℃), High Pressure Ultrasonic Testing, Penetrant Testing



The essential point is that 85% of problems can be avoided with preventative maintenance, which includes checking the sealing rings every 6 to 12 months, making sure the lead wire layout is flexible, and checking the junction box sealing.



Fatal Misconception: The most prevalent human-caused reasons for early failure are treating moveable flanges like fixed flanges, forcing leads to be straight, or over-tightening.