It has been reported that in principle sludge and blockages in pipes can be detected and characterized by using ultrasonic guided waves. The model idealised the sludge to be an asymmetrically uniform layer that is well bonded to the internal surface of the pipe wall. However, in practice, sludge layers normally have a very irregular shape, an asymmetrical circumferential profile and also uncertain bonding state. These practical issues complicate the testing and perhaps bring some different new features to the guided wave modes. We investigated the different effects of these issues on the characteristics of guided wave. A general assessment of the potential of using guided ultrasonic waves to detect and characterize sludge blockage in practice is given.