Pipes carrying fluids under pressure are critical components in infrastructure and industry. Piezoelectric transducers bonded to the pipe produce guided waves that propagate long distances and illuminate the whole pipe, providing a promising tool for pipe structure health monitoring. However it is difficult to recognize the change produced by a scatterer because of the many wave modes and wave paths. Moreover, under operating conditions, the environmental and operational variations produce significant changes in pitch-catch signals, which would produce false-positive results with conventional change detection methods. We instrumented pressurized pipe segments in a working hot-water supply system that experiences ongoing variations in pressure, temperature, and flow rate in an environment that is noisy mechanically and electrically. We conducted pitch-catch tests between transducers located roughly 16 diameters apart on a 10-in. pipe. We show significant environmental and operational variations, even after temperature compensation. At several different time intervals we applied and removed a grease-coupled mass scatterer on the pipe as a physical simulation of damage. We then use singular value decomposition (SVD) to build a change detector that is sensitive to the mass scatterer but insensitive to the changes produced by operational and environmental variations, and we show examples of its successful performance on field experiments data. We show that specific components are associated with the changes produced by the mass scatterer, while others are associated with the environmental variations.