Sewer Pipe Inspection: Mandrels vs. Laser Profiles

How is inspecting a sewer pipe like creating a document? Well, back when typewriters were the norm, you could type an entire page and then look for mistakes.  If you found one, you would have to re-type the entire page (for a professional-looking document).  However, we now have word processing:  you can create the entire document and run spell check, enabling you to quickly change only what’s necessary before printing.  Similarly, we can compare the use of mandrels in sewer pipe inspection to the use of lasers.  Mandrels find one problem at a time and force you to fix the problem and start over.  Lasers give you a comprehensive view and allow you to find all of the problems in one shot.

Sewer Mandrels: The Old Way

Currently, most contractors who must verify new pipe installation for ovality and deflection use mandrels.  A mandrel (pictured here) is a specifically sized and shaped device constructed of metal or wood that is designed to physically stop at any ovality or deflection that exceeds design tolerance. For example, let’s say you have a round, 40″ Centrifugally Cast-Glass Fiber Pipe (CC-GFP).  It has been installed and requires deflections after backfill not to exceed 5%.  The most common method to verify the pipe’s installation is to try to pull a 38″ diameter mandrel through the pipe. If the mandrel makes it through the entire pipe segment without getting stopped, the segment has passed the inspection test. Every time the mandrel cannot be pulled through the pipe, something is out of specification and must be repaired or replaced. You must repeat the process until the mandrel passes through the entire segment.

This procedure, while conceptually simple, leaves much to be desired operationally and technically. A properly designed mandrel is guaranteed to stop at every defect that exceeds design tolerance. However, it can only locate one defect at time (since it stops at the defect). This is a major operational problem if the pipe under inspection has multiple defects. In the worst case, this pipe will have to be inspected once for each located defect in order to guarantee installation. Furthermore, while mandrels locate well, they do not provide any measure of the magnitude of the defect – a 5% deflection stops the mandrel in just the same way as a 50% deflection.  Additionally, mandrels often stop for other reasons unrelated to installation defects, such as construction debris, pipe joints, and installed bends. Distinguishing between false alarms and true installation defects often requires man entry for resolution.

A Better Way: Lasers

Profiles derived from laser devices have all the strengths of mandrel inspection and none of the limitations. The adoption of laser technologies in the waste water market has been rapid for use in existing pipes, and provides many benefits to owner/operators. However, adoption has been slower in new construction.

A laser can travel through an entire pipe with numerous deflection defects in a single pass. This is possible because laser equipment is small in comparison with mandrels, and can pass even large deflections with ease. Furthermore, laser data can be used to measure the size and extent of defect that they pass to high levels of precision (depending on the type of laser and deployment methodology). Measurements are critical to diagnosing the cause of installation defects. I have seen case studies where deflection size was correlated directly to type of backfill used in the construction process at various segments; the understanding of this correlation then enabled changes to backfill material, preventing further installation defects in 30,000 feet of pipe in one job alone.

Use of lasers for measurement also precludes the need for man entry in pipes. This is an important safety feature, as pipes under deflection due to design defects or construction damage could collapse or buckle.  Lasers also measure the magnitude of deformations and deflections, eliminating any ambiguity between true deflections and other obstructions or mobility impediments.

The use of lasers in verification of new construction is a natural extension of their ongoing adoption in inspection of wastewater pipelines. In many ways, the information obtained provides benefits in new construction that are superior to their use in operating pipes. For example, once you inspect the new construction with a laser, you have a permanent record of the as-built condition that serves as a baseline for future inspections. With this baseline, future inspections of this pipe will monitor changes in shape and progress of defects that will enable improved condition assessment and precision asset management.  There are several different types of lasers in use today, and most provide an advantage over mandrels.  They have various levels of accuracy and additional capabilities beyond profiling.

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