Long-term exposure to fluid scouring, pitting corrosion and uniform chemical erosion will gradually reduce the wall thickness of 316 stainless steel heating tubes. Once the residual wall thickness drops below the minimum safe design value, the heating tube may deform, crack or burst under thermal pressure, resulting in production shutdown, equipment damage and even safety accidents. Conventional visual inspection can only find obvious surface rust and leakage points, which cannot reflect the internal wall loss caused by hidden corrosion. Ultrasonic thickness testing is a mature non-destructive testing technology that uses high-frequency sound waves to penetrate the tube wall and calculate the remaining thickness by measuring the propagation time of sound waves. Regular thickness detection can track the thinning rate of key pipe sections, predict the remaining service life, and arrange maintenance or replacement plans in advance. Many sudden burst accidents of heating tubes are caused by long-term neglect of wall thickness monitoring, leading to cumulative corrosion thinning beyond the safety threshold. Therefore, establishing a standardized ultrasonic thickness inspection system is a key preventive maintenance measure to ensure the safe operation of heating equipment.
Ultrasonic thickness measurement relies on the reflection principle of sound waves at the interface of different media. The ultrasonic probe emits sound waves into the metal substrate; part of the sound wave is reflected back from the outer wall, and the other part is reflected from the inner wall of the heating tube. The instrument calculates the wall thickness through the time difference of the two reflected signals. However, surface fouling, rust layers, uneven roughness and improper coupling agent use will cause signal attenuation and measurement deviation. In addition, corrosion pits on the inner wall will lead to local thickness mutation, and single-point detection is easy to miss dangerous areas. Only by setting reasonable detection points, standardizing operation steps and establishing thickness grading standards can enterprises accurately evaluate the safety state of heating tubes.
Table 1 Wall Thinning Grading Standard and Disposal Strategy for 316 Stainless Steel Heating Tubes
表格
| Safety Grade | Residual Wall Thickness | Annual Thinning Rate | Corrosion Risk Level | Recommended Disposal Measure |
|---|---|---|---|---|
| Grade A (Safe) | ≥90% of original design thickness | ＜0.05 mm/year | Negligible | Normal operation, annual routine inspection |
| Grade B (Attention) | 75%–90% of original design thickness | 0.05–0.10 mm/year | Low | Shorten inspection cycle to every 6 months |
| Grade C (Warning) | 60%–75% of original design thickness | 0.10–0.15 mm/year | Medium | Strengthen water quality control and anti-corrosion maintenance |
| Grade D (Dangerous) | ＜60% of original design thickness | ＞0.15 mm/year | High | Replace the heating tube before next operation cycle |
Before inspection, the detection area of the heating tube must be polished to remove scale, rust, coating and oxide layers, ensuring close contact between the probe and the metal surface. Coupling agent should be evenly coated to eliminate air gaps and guarantee stable ultrasonic signal transmission. Key detection positions include welding seams, bent sections, fluid impact areas and the lower part of pipelines where fouling easily accumulates. Each key position needs multiple point measurements to record the minimum thickness value and mark the specific location. All measured data shall be archived to compare the thickness change trend year by year and calculate the average annual thinning rate.
For Grade A heating tubes, regular annual inspection can be followed normally. Grade B and Grade C equipment must be managed with warning labels, and relevant anti-corrosion measures such as water quality adjustment, regular chemical cleaning and local passive film reinforcement should be implemented to slow down the corrosion thinning rate. Any heating tube falling into Grade D must be replaced immediately to avoid pressure leakage accidents caused by insufficient bearing capacity. Combined with eddy current pitting detection and salt spray performance verification, ultrasonic thickness testing forms a complete safety early warning system, effectively avoiding equipment failure caused by long-term cumulative corrosion thinning and safeguarding the stable and safe operation of industrial anti-corrosion heating systems.
