Eddy current monitoring of wall thickness and defects in pipes - using probe made of cobalt, iron, vanadium alloy and carrying magnetising and search coils

摘要

The pipes are straight and made of magnetisable material; they are used esp. in heat exchangers or steam generators, and are inspected by feeding a probe through each pipe. The probe (1) consists of three parts (2,3,4), each with the same length (L), where the two ends (2,4) form pole pieces used to magnetise the pipe via a pre-magnetising coil (5) located on the middle zone. In the middle of coil (5) is a search coil (9). Length (L) equals the outer dia. (D) of poles (2,4), whereas the dia. (d) of zone (3) is ca. 10% smaller than (D); and (D) is 0.3-0.9 mm smaller than the bore (D1) of the pipe (6) being inspected. Probe (1) is pref. made of material with a saturation magnetisation of ca. 25 kG, esp. an alloy contg. 49% Co, 48.6% Fe, 1.9% V. The probe can be fitted with fins providing self centering in the pipe (6), so the probe (1) can travel at 30-40 m/minute during accurate and reliable inspection.