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Con Edison lines replaced by new multiwall piping system

Reprinted from the April 1997 issue of PIPE LINE & GAS INDUSTRY magazine.

Used with permission:
Natarajan K. Raman, Senior Engineer, General Studies & Development, Con Edison Co. of New York, Inc., New York, New York .
Kenneth Tierling, President, and Matthew J. Feehery, Vice President, Unisert Multiwall Systems, Inc., Conroe, Texas

Consolidated Edison Company of New York recently replaced two corroded fuel oil pipelines using a new multiwall piping system.

The project followed more than three years of engineering and evaluations by the utility company to find a suitable replacement fuel oil supply system to connect the fossil fuel power plants serving New York City.

The fuel oil supply lines run beneath the streets of New York City's Manhattan, Queens and Brooklyn boroughs and across the East River in two locations connecting the power plants and fuel storage facilities on either side of the river.

Unisert Multiwall Systems, Inc., a Texas-based multiwall pipe fabricator and installer, replaced the lines for the utility company. The replacement lines are located in Manhattan.

Problem and solution overview
The two fuel oil pipelines were approximately 30 years old and were due for an overhaul. The system had begun to experience some corrosion primarily caused by the pipe insulation that shielded the cathodic protection system from operating properly. During inspections and other normal operations, the utility company observed that while the coated pipe was in excellent condition, the pipe wrapped with insulation needed to be replaced. Other extraneous conditions contributing to the degradation of some pipe segments located along the banks of the East River included tidal action stray current. 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 holding in 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 find a way 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 system serves two primary functions simultaneously. The double-wall system features an internal fiberglass liner that is inert to galvanic corrosion and, therefore, does not cause the internal or external pipes to become sacrificial in the galvanic process. Furthermore, the insulating cement in the annular space drives the steel into a passive pH range acting as a corrosion inhibitor. The system can also be monitored for leak detection by applying a low-pressure detectable gas to the annular space. A pressure drop would indicate a leak in the external case while line pressure equalization between the annulus and the internal liner would indicate a leak in the carrier pipe.

Either of the two Unisert approaches, insitu/rejuvenation or replacement with new piping, would work in these applications. Condition of the existing external steel pipe case was the major factor in determining which approach to incorporate.

Project description
. The first project involved replacing a 6-in. diameter fuel oil supply line with a new 6-in. by 12-in. multiwall piping system. The supply line connects the East 74th Street Generating Station to the head house located at East 71st Street near the East River tunnel crossing which in turn connects the line to the Rainey Tank Farm in Queens. Product temperature averages 165° F and the operating pressure is 80 psi. Test pressure was rated and performed at 600 psi.

The second project called for replacing two lines running parallel: a 10-in. diameter supply line and a 4-in. diameter return line serving the Waterside Generating Station and Kips Bay, 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 company'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 C-factor of the fiberglass pipe, which compensates for the smaller diameter. The fuel oil supply line is an auxiliary line used to provide back-up fuel for the boilers during high demand times during the winter months when gas is in short supply. Product temperature averages 165° F and the operating pressure is 260 psi. Test pressure was rated and performed at 600 psi.

Design criteria
. Con Edison engineers reviewed numerous approaches to rehabilitating or replacing the existing pipelines, including steel-inside-steel double-wall piping, flexible liners, and insitu-cured liners. Ultimately, the company selected the multiwall technology because it offered the utility company the best options for system longevity and low maintenance, exceptional resistance to galvanic corrosion and related failures, and met double-wall containment requirements. Because of the location of the pipelines under city streets and among a congested system of pipes and conduits, excavation, installation and restoration would be difficult and expensive. Therefore, enhancing the system longevity by reducing the impact of galvanic corrosion and preventing costly leaks was a top priority.

The Unisert IT3 Multiwall Piping System consists of sliding one pipe system inside another and then filling the annular space with a suitable material 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 and 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. When preferred, a flanged connection is available.

Other factors that played an important role in the decision to utilize the multiwall technology included the fabricator's ability to meet specific design parameters, some of them unique. The system should handle both the temperature of the product and the outside environment, so the cementatious material used in the grout should have added insulating capabilities while maintaining the required compressive strength characteristics. 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 that the outer pipe in either new or insitu/retrofit applications is used. This insulation requirement was included because of the necessity of pumping #6 fuel oil in cold conditions. The lines are buried approximately 4 to 6 feet below ground. With the liner located so close to the East River, they also can experience the highly corrosive effects of a high water table and heavy tidal action. Other design parameters included the system's ability to handle high-pressure operations, start-ups and surges, some possibly caused by operator error. Allowing for the future capability of using alternative fuel sources was one reason the design pressures were rated at 600 psi.

While these two projects called for the fabrication and installation of an entirely new pipe system in place of the existing lines, the Texas company had previously performed an insitu lining project for Con Edison in 1993 at the Astoria Power Generating Station. This experience coupled with the supplier's 29-year history of success without system failures, also contributed to the utility company's decision to utilize the multiwall technology in place of a more traditional replacement method.

For these replacement projects, the fabricator used Red Thread® II fiberglass pipe and Red Thread® II Performance Plus fiberglass pipe from Smith Fiberglass Products mated with .500 wall steel pipe (used as a corrosion allowance). A cementatious, insulating grout was used to fill the annular space. The components for fabricating the numerous 22½°, 45° and 90° steel and fiberglass 3-D radius bend elbows were matched to accommodate intelligent pigging of the lines. Hicks Industrial Fiberglass of Santa Ana, California assisted in the manufacturing of the custom-wound fiberglass elbows for the fabricator/installer. All pipe, elbows and fittings were then fabricated at the supplier's plant in Conroe, Texas. Bredero Price coated the finished pipe at the coater's Morrisville, Pennsylvania, plant using Priticâ coating while the elbows, fitting and welded connections were field coated with Scotch Koteâ 312. The fabrication and installation were performed under Unisert's Quality Control Level One specifications, which indicates that the system design will allow for the liner to handle the fuel oil system operating criteria on its own.

System design life
. As noted previously, life expectancy of the IT3 multiwall piping system was a primary consideration in the decision to use this product and approach. It is estimated 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. For example, the failure mode of the fiberglass pipe is dependent on a number of factors. Four specific factors considered include the shear between the glass and resin, the high temperature deterioration of the resin, erosion/corrosion and chemical deterioration.

  • Shear between the glass and the resin. This failure mode is a function of hoop loading and cyclic pressure causing shear between the glass and the resin. In the IT3 multiwall piping system the hoop loading is transferred entirely to the steel via the cement. This particular failure mode is negated until such time as the steel deteriorates to the point of yield, allowing hoop loading or strain on the fiberglass. With proper cathodic protection on the system, failure of the .50-in. wall steel pipe as designed in the system is very difficult to calculate or predict.

    Using a worst case scenario, the 3/8 -in. wall steel pipe used in the existing systems had lasted approx-imately 30 years to any pinholing and pitting. Increasing the pipe size to .50-in. wall steel should allow for a 40-year life prior to any pinholing of the steel. In order for there to be significant enough general corrosion on the steel pipe wall to affect hoop strain on the fiberglass, it is estimated to take double that time frame, thereby extending the life expectancy to 80 years prior to obtaining significant strain on the glass and a conservative 20-year hoop loading on the fiberglass will extend the system life to 100+ years.

    The only difference with an in-situ approach would be that the existing steel had already reached its half-life for providing a fully intact barrier prior to subjecting the liner to any actual strain. Under this scenario, all other parameters apply as with the new steel pipe and life expectancy is 50 years before strain becomes a factor, giving the in-situ lines system a 70+ year design life.

    Normal operating pressures of the system do not come close to the 20-year cyclic design life of the liner material as there is primarily static pressure loading on the system. Centrifugal pumps are used and cyclic loading is not a major concern. Given the worst-case scenario of having the pressures increased to the 20-year design life, it would still provide for a 100-year life expectancy on the system.

  • High temperature deterioration of the resin. The liner material is designed to function at 210° F maximum. As long as this temperature range is not exceeded, breakdown of the molecular chain will not occur and the tensile strength of the material will remain unchanged.
  • Erosion/corrosion. Fuel oil does not have any significant particles of sufficient hardness to impinge or erode the liner material. The viscosity and flow characteristics of the heavy fuel oils in the system reduce the velocity at the pipe surface and provide some natural inhibition to erosion.
  • Chemical deterioration. The epoxy materials utilized in the pipe system are UL approved for gasoline and fuel oil transportation because they are chemically resistant to these products.

Thermal expansion. Another important characteristic of the multiwall system 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. The empirical data on all tests performed have indicated no degradation or detrimental effects to the system.

Conclusion. By utilizing the new multiwall piping system and having the insulation properties built into the annular space, the cathodic protection system was greatly improved, therefore extending the system's design life to well over the 100-year mark.

 

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