Detection of Pipe Thinning Due to Hydrogen Sulfide Deterioration

In service waste water pipe lines are evaluated for hydrogen sulfide deterioration with sonic/ultrasonic resonant frequency data. One of the causes of concrete waste water pipe deterioration and failure is the thinning and weakening of concrete due to hydrogen sulfide that accumulates at the high points of the pipe.

Stress wave measurements in the sonic/ultrasonic frequency range are made on the exterior of the pipe at excavated locations. The sonic/ultrasonic data is a direct measurement of the transmission velocity of both the compressional and shear waves and the reflected phases of the compressional wave from the inside face of the pipe. The transmission velocity values determine the elastic deformational characteristics of the concrete, including Young's, shear, and bulk moduli, Poisson's Ratio, and calculated strength values. The reflected signals resonate at a frequency that is related to the thickness and compressional wave velocity of the concrete liner. Since the longitudinal velocity is measured directly and average thickness of the pipes are known, the average strength of the concrete pipe core as well as the presence of delamination, spalls or thinning pipe can be determined from the measured resonant frequency.

Sonic/ultrasonic direct and resonant frequency measurements are obtained using a projectile impact energy source, a hand-held four-sensor array (pictured below). A straight sensor array or a curved sensor array can be used to collect the sonic data. Typically data are obtained on the exterior of the pipe at a one-foot spacing at 12:00, 1:00, 3:00, 9:00, and 11:00 o’clock positions.