Pipe Ovality: Better Ovality for Lower Costs

Pick a number from 1 to 10. What are the chances you would be able to choose the correct number? Only 10%, right? What if an inaccurate answer might cost you thousands if not hundreds of thousands dollars?

Chances are you would want to enhance your odds from a 10% guess to a 99% certainty for the correct number. This is the value-added component that measurable information in pipeline condition assessment can provide. It takes the guesswork out of the bid work. Providing critical pipe ovality dimensions translates into right-sized trenchless rehabilitation technologies. Translation: you save money.

Most municipalities waste a great deal of money when they rehabilitate pipes. They typically use a conservative guess when designing the lining for an existing sewer using a cured-in-place thermosetting liner. The majority of cities use the ASTM F1216 Standard to calculate pipe ovality:

Ovality = (Maximum diameter- Mean diameter)x100%

… all of which is divided by the Mean diameter

However, typically they have to make a best guess at some of the variables in the design equation. The final designed liner thickness can vary, and the increase of liner thickness by mm’s or inches more than required can result in hundreds of thousands of dollars of unnecessary cost. Just how much excess cost, of course, depends on the diameter of the pipe, the linear feet, and the liner thickness.

Today’s newer trenchless rehabilitation technologies have allowed authorities and engineers to approach underground construction projects from a different angle.

Laser and SONAR cross sections obtained from precision scans provide accurate, quantitative information on pipe ID, including ovality and eccentricity. Ovality and eccentricity are measured from the horizontal and vertical pipe diameters. While new construction verification is typically straightforward, obtaining vertical diameter in a partially charged pipe is more difficult. With laser technology, you can disaggregate the vertical diameter into two radial segments. The first vertical radial segment is called the crown radius — it can be obtained directly from the 3D spinning laser data. The second vertical segment is called the invert radius. The invert radius is currently taken from the sonar data or derived from as-built data. With the crown and invert radius known, precise numbers for the ovality calculation is now possible. Precise dimensions and numbers translate into liners that are designed as thick as they need be, but not overly thick and needlessly expensive.

A more accurate value for sewer pipe’s ovality, obtained by means of laser technology throughout the sewer pipe length, will give a more accurate computation of the liner’s wall thickness. A more accurate liner wall thickness correlates into a safer and more cost effective trenchless technology rehabilitation project.

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