How Does the Tube Wall Thickness Measurement Frequency (Annually vs. Quarterly) for a Titanium Sheath Heater in 50% Phosphoric Acid at 100°C Affect the Probability of Detecting a Pit Before Perforation?

Grade 2 titanium corrodes uniformly in 50% phosphoric acid at 100°C at a rate of 0.08-0.15 mm/year, although it also experiences occasional pitting at a rate of 0.3-0.8 mm/year. Frequency of examination is a function of detection of a pit before it perforates the tube wall. Between examinations, pits that originate and expand to perforation are missed by the annual inspection. Quarterly check raises detection chance from 40-60% to 80-95%.

Inspection Interval and Quantitative Detection Probability

For a 1.5 mm wall, with pit propagation rate of 0.5 mm/yr:

Inspection interval 12 months: Pit depth 0,5 mm between examinations. 40-60% detection likelihood. Risk of perforation high.

Pit depth 0.25 mm. 6 months inspection period. 70-85% detection probability.

Pit penetrates 0.125 mm at 3 months inspection interval 2 Probability of detection 85 - 95%.

1 month inspection interval: Detection probability > 98 %.

Guide to Inspection Frequency

Uniform corrosion rate (mm/yr) Pit propagation rate (mm/yr)Annual 60–80 % Recommended Inspection Frequency Detection Probability <0.10 <0.3
0.10-0.15 0.3-0.5 85-95% Quarterly
0.15–0.20 0.5–0.8 Monthly >95% >0.20 >0.8Continuous surveillance N/A Engineering recommendation

50% phosphoric acid at 100°C. Quarterly ultrasonic thickness mapping at specified grid locations. The engineer raises the inspection frequency from annual to quarterly and improves the pit detection probability from 60% to 90%, thereby preventing unexpected perforation.