It has been previously reported that in principle sludge and blockages can be detected and even characterized by using guided ultrasonic torsional waves, based on an idealized model in which the sludge layer was simplified in terms of geometry and material properties. The work revealed that the presence of a layer inside a pipe scatters the guided wave propagating in the pipe and both the reflection and transmission of the guided wave can be used to effectively detect and characterize the layer. This paper proceeds the work by taking into account more realistic sludge characteristics, including irregular circumferential profiles of the sludge layer and imperfect bonding state between the sludge and the pipe. The influence of these issues is investigated to identify the critical factors that influence the detection and characterization capability of the two measurements.