Piping systems are a major part of the infrastructure in many industries. Because defects can cause pipe failure, maintenance of the pipe requires periodic inspection in order to locate potential defects. Nondestructive testing techniques are necessary to carry out such maintenance inspections. Techniques used for this task include ultrasonic and radiographic testing, but such techniques are labor intensive. A method for manufacturing helically formed and welded pipe was patented in 1940. Initial diameters ranged from 100 mm to 875 mm. After World War II, German machines were imported to the U.S., and these enabled construction of pipes with diameters as great as 3600 mm. Recently, spiral welded pipes have become more widely used in industry. The existing research done on use of microwave technology to detect defects on the inner wall of pipes has been done with smooth pipe acting as the waveguide. Calibration of the effects that a spiral weld may have on the reflected waveform within a spiral welded waveguide, has not been performed, until now. To determine the effect of real-world geometry on the proven techniques, we use Computer Simulation Technology (CST) to research microwave defect detection. We model and compare two types of pipe: smooth wall and spiral welded. The specifications of both pipes are: length 762 mm; inner diameter 152.4 mm; wall thickness 12.7 mm. To both types we will add fully circumferential wall-thinning defects, depth 6.35 mm. The thinning defects will have three profiles: rectangular, toroidal, one rounded and one right-angle corner. We see two significant effects: a presence of noise in the reflections in the spiral waveguide, and a change in dB levels between the smooth and spiral pipes. The waveform contour remains the same. The amplitude of peaks is only slightly affected.