Trenchless solves Miami vice – pipe corrosion

Within a few hours of discovering a leak that suggested a serious problem at the South District Black Point Wastewater Treatment Plant, two-thirds of the 90-mgd facility had been shut down, and the Miami-Dade Water & Sewer Department (WASD) was initiating a trenchless cured-in-place pipe (CIPP) solution that would completely rehabilitate the failing segments and avoid the dig-and-replace methods that would have added four full months to the system’s downtime.

Fortunately, WASD had recently used the method to solve a complex failure problem involving pipe diameter transitions ranging from 72 inches to 92 inches with a variety of bends and crossovers at its North District Interama plant. That experience enabled the department to react quickly to the even more complex situation at Black Point, which involved multiple bends and crossovers and diameter transitions of 48 inches to 72 inches to 96 inches. Based on the results of the Interama project, WASD officials asked Lanzo Lining Systems, Pompano Beach, Fla., to handle the Black Point emergency with cured-in-place pipe developed by InLiner USA, Houston.

Inspections revealed a six-inch-by-24-inch hole in the crown of one manifold section and the original cause of the problem, which had started years earlier when the plant had operated at less than its design flow. A combination of circumstances had produced what WASD Sewage Collection Division Chief Rod Lovett called, “a perfect hydrogen sulfide generator.”

The repair project began with a thorough cleaning of the crossover and manifold. Then, the voids and pockets of the prestressed concrete were filled with calcium aluminate cement, and the ductile iron pipe was treated with epoxy.

Black Point still had two oxygenation trains in operation, and WASD was able to divert some flow to other treatment plants during rehabilitation of the 328 feet of failed pipe. After a thorough cleaning of the crossover and manifold, voids and pockets of the prestressed concrete were filled, and ductile iron pipe was treated with epoxy.

The liner for each phase was impregnated with vinyl ester resin, which WASD specifies for all CIPP projects, and transported to the job site in ice-filled, insulated carriers.

A twin-crane winching technique was used to ensure precise positioning of the 96-inch-diameter tube, eliminating any potential for twisting within the asymmetrical transitions of the manifold. A special felt laminated to the exterior of the proprietary tubes protected the liner from damage during installation. After placement, a calibration hose was everted through each tube and filled with water to secure the liner tightly against the interior wall of the host pipe. The circulating water was heated and carefully monitored with thermocouples placed along the tube.

Within 24 hours to 48 hours of placement, each segment was cured and cooled to produce a stand-alone cured-in-place pipe with a 50-year design life and the ability to withstand water, saturated soil and live loads.

At the job site, a crane was placed at each end of the section to be lined. While one crane pulled on the larger end of the tube, the other crane lifted the material and fed it into the pipeline. A special felt laminated to the outside of the tubes protects the liner from damage and reduces friction as it is pulled into the host pipe.

The twin-crane arrangement allows the tube to be positioned before inflation begins. This feature was especially important at Black Point because no twisting of the tube could be tolerated. The asymmetrical transitions of the manifold made proper alignment critical.

When the liner is in place, a calibration hose is everted through it and filled with water. In addition to pressing the impregnated tube tightly against the walls of the host pipe, the water is also used to cure the resin and tube into a structural pipe.

In order to optimize the long-term CIPP performance, the tube is cured with a multi-step thermal cure. After the tube is expanded, the water is circulated through a heat exchanger and the temperature is carefully monitored with thermocouples placed in and around the tube. After an extensive cure, the final step in the curing process is a slow, controlled cooling period.