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Multiwall Piping Solves Corrosion Problems

Reprinted from the March 1998 issue of ELECTRIC LIGHT & POWER magazine.

Used with permission: Natarajan K. Raman, Con Edison Co. of New York, Inc.
Kenneth Tierling, and Matthew J. Feehery, Unisert Multiwall Systems, Inc.

Con Ed recently replaced two corroded fuel oil pipelines in Manhattan using a new multiwall piping system. The project followed more than three years of engineering and evaluations by the utility to find a suitable replacement fuel oil supply system to connect the fossil fuel power plants serving New York City. Unisert Multiwall Systems Inc., a Texas-based multiwall pipe fabricator and installer, replaced the lines for Com Edison.

Pipe Corrosion
The two fuel oil pipelines were approximately 30 years old and were due for an overhaul. The system had begun to experience corrosion primarily caused by pipe insulation that shielded the cathodic protection system. During inspections and other operation, the utility observed that although the coated pipe was in excellent condition, the pipe wrapped with insulation needed to be replace.

Other extraneous conditions, including a high water table and tidal action, contributed to the degradation of pipe segments located near the banks of the East River. Since much of the original installation took place in rocky granite beds and the trenches were backfilled with sand, the rock troughs were more susceptible to retaining moisture. The pipelines, which are located in a metropolitan area among a congested maze of underground electrical lines and other conduits, are also exposed to higher than normal amounts of stray current.

The utility's solution was to reverse the insulation process. By using a multiwall pipe, the coated steel pipe would be on the outside and the insulation would be located between the carrier pipe (fiberglass liner) and the outside steel.

This type of system served two primary functions simultaneously. The double wall system featured an internal fiberglass liner that was inert to galvanic corrosion and, therefore, would not cause the internal or external pipes to become sacrificial in the galvanic process.

Furthermore, the insulating cement in the annular space would drive the steel into a passive pH range acting as a corrosion inhibitor. The system could also be monitored for leak detection by applying a low-pressure detectable gas to the annular space. A pressure drop indicated a leak in the external case while line pressure equalization between the annulus and the internal liner indicated a leak in the carrier pipe.

Either of the two approaches, in situ/rejuvenation or replacement with new piping, would work in these applications. The existing external steel pipe case's condition was the major factor in reaching the decision to replace.

Project Design
The first project involved replacing a 6-in. diameter fuel oil supply line connecting a generating station to a head house with a new 6-in. by 12-in. multiwall piping system.

The second project called for replacing two lines running parallel: a 10-in. diameter supply line and a 4-in. diameter return line serving a generating station and a storage and pumping facility. Unisert installed a new 8-in. by 12-in. multiwall pipe system to replace the existing 10-in. supply line and a new 4-in. by 8-in. multiwall pipe system to replace the existing 4-in. return line.

The utility's engineering team decided that reducing the diameter of the supply line from 10 in. to 8 in. would not significantly affect the throughput because of the improved C-factor (smoothness of the pipe's internal surface) of the fiberglass pipe, which compensated for the smaller diameter.

Con Ed engineers reviewed numerous approaches to rehabilitating or replacing the existing pipelines, including steel-inside-steel double wall piping, flexible liners and in situ-cured liners. Ultimately, they selected the multiwall technology because it offered the best options for system longevity and low maintenance, resistance to galvanic corrosion and related failures, and it met double wall containment requirements.

The Unisert IT3 multiwall piping system consists of sliding one pipe system inside another and then filling the annular space with a suitable material (grout) to create a multiwall pipe. Perhaps the most unique aspect of this approach is the patented SK Collar used for connecting the pipe joints and elbows of new pipe systems.

The SK Collar is a dual material fitting that is epoxied an inserted into the annulus prior to the two connecting segments being joined and welded. This precision process allows for the simultaneous joining of both the internal liner and the outside steel pipe. The completed weld makes for true double wall containment, even at the connections and eliminates any portion of the liner from being exposed to external conditions and impact.

Other factors that played an important role in the decision to utilize the multiwall technology included the fabricator's ability to meet specific and unique design parameters. The system must handle both the temperatures of the product and the outside environment, so the cementatious material used in the grout should add insulating capabilities while maintaining the required compressive strength characteristics. Grout can include various cement products, flyash, plasticizers and insulation agents. This accounts for the system's larger than normal annular space between the fiberglass liner and the outside steel pipe. Typically, a liner one nominal pipe size smaller than the outer pipe is used. This insulation requirement was included because of the necessity of pumping No. 6 fuel oil in cold conditions through lines buried approximately 4 to 6 ft. below ground.

System Design Life
Life expectancy of the IT3 multiwall piping system was a primary consideration in the decision to use this product and approach. Estimates show that the system's life expectancy should surpass 100 years.

By combining the components of the multiwall piping system in this configuration, limitations of the individual components are practically eliminated. Its operating parameters support the choice of the specified liner material for handling temperature, preventing erosion/corrosion and resisting chemical deterioration. The failure mode of shear between the glass and the resin, which can result from hoop loading and cyclic pressure, is eliminated because the hoop loading is transferred entirely to the steel via the cement.

Another important characteristic is how the three primary materials used - fiberglass, cement and steel - respond to thermal expansion. The cement grout acts as an anchor and locks both systems together, allowing them to thermally expand and contract as a single unit. All three are virtually identical in thermal expansion factors. Empirical testing results have indicated no degradation or detrimental effects to the system.


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