November 2019 Vol. 74 No. 11
Rehab Technology
Operation and Maintenance of Pressure Pipe Linings
NASSCO’s Pressure Pipe Committee
EDITOR’S NOTE: This is the second installment of a three-part series on pressure pipe presented by the National Association of Sewer Service Companies (NASSCO) Pressure Pipe Committee. There will be an in-depth program track on these topics and more at UCT, Jan. 28–30, 2020, in Fort Worth, Texas.
Pressure pipe renewal technologies are designed for a variety of applications, from AWWA Class I corrosion barriers to fully structural, Class IV solutions. The main objective is to extend the service life of the pressure pipe system, while minimizing the carbon footprint and disruption to water supply.
A common question presented by utilities concerns maintenance of these renewed buried assets. Specifically, how future connections, repairs and cleaning are addressed without compromising hydrostatic integrity of the pressure pipe or long-term performance of the lining system.
Cleaning of lined pipe
Due to their chemical composition and essentially smooth interior, lining technologies are generally not susceptible to corrosion or material deposits. However, should material buildup or bacteriological growth form within the pipe, it may be removed through flushing or non-abrasive pigging. Hydraulic cleaning or water jetting at high velocity and pressure, abrasive pigging, drag scraping and other mechanical cleaning methods are not recommended as they can cause irreparable damage to most lining systems.
Flushing, a routine maintenance practice for unlined pipe, is the simplest and most commonly used pipe cleaning method. Conventional hydrant flushing is effective at reducing water age, raising disinfection residuals and removing sediment. Unidirectional flushing involves the isolation of pipe sections by closing valves and opening hydrants to increase scour velocities, which in turn helps to more efficiently remove sediment and biofilm.
Swabbing or non-abrasive pigging can be used to remove heavier biofilm or sediment in lined pipe. Non-abrasive pigging makes use of open cell, polyurethane, compressible (over-sized) foam pigs, which are pulled through the lined pipe when the system is out of service or propelled forward with water pressure. The pigs can be inserted through points within the system (pig launchers) and received at designated points downstream (pig catchers). Some hydrants can be used as entrance and exit points for pigging operations. Non-abrasive pigging can be used on most lined pipes. Analyzing value to risk should be made as there are times the pigs get stuck within the pipe, requiring excavation for removal and repair.
End treatments
Mechanical internal pipe joint seals have a successful history for repairing defective joints in pressure pipelines. These are typically comprised of an elastomeric material and stainless-steel sleeve or compression bands to provide a corrosion-resistant, watertight seal with the necessary flexibility and elongation to accommodate high-pressure applications and normal pipe movement. These products are also used to seal terminations of a lining system to prevent fluid movement between the host pipe and a close-fit lining.
End seals or treatments are a common requirement for pressure pipe and potable water applications, particularly with cured-in-place pipe (CIPP). The mechanical style end seals are expanded in place, compressing the elastomeric seal against the liner and host pipe to achieve water tightness. Each seal is sized to accommodate the inside diameters of the liner and host pipe, and the installed liner is trimmed back to a prescribed distance inside the host pipe to permit proper installation of the seal. This creates a uniform, concentric transition between the liner and host pipe.
The elastomeric seal is typically made of EPDM or nitrile rubber, depending on the application service. The end seal installation is accomplished using either a self-contained hydraulic hand pump or a pneumatic packer to expand the stainless-steel sleeve or retaining bands to achieve the desired pressure and compression. Once expanded, the retaining bands or sleeve are mechanically locked into position. Mechanical style end seals also have the capability of being pressure tested to ensure the compression of the elastomeric membrane is sufficient to create a watertight seal.
In addition to termination points of all pressure pipe liner installations, end seals should always be considered when new connections (valves, tees, hydrants) are made to previously lined pipe segments. In the unlikely event that a section of lined pipe is damaged due to construction or maintenance activity, the damaged pipe section should be removed, end seals installed and replaced in-kind with a like-diameter spool piece and appropriate mechanical fittings.
Tapping of lined pipe
With the growing popularity and feasibility of pressure pipe lining technologies, especially in potable water, industry leaders are working together to ensure these solutions are able to support future infrastructure growth. Functions such as line stopping, valve insertion and tapping into or through a lined pipe require careful consideration of equipment and attention to the process.
Maintaining water tightness at existing services can be challenging when failure of the host pipe or pressure pipe system occurs through galvanic corrosion or other means. Demonstration tests may be coordinated by the lining system manufacturer at the direction of the utility to simulate site conditions, confirm predicted performance and establish alternative repair measures when hydrostatic integrity is lost. This may include connecting a new service to the existing main by dry or wet tapping into the lined pipe or replacing the service in-kind.
Lining system manufacturers should be consulted to provide guidance to utility maintenance and operation departments on maintaining water tightness and reconnection when lined pipe is penetrated, removed or replaced. Tapping of a lined pipe should follow published guidelines from the lining system manufacturer. Methods and equipment for tapping lined pipe should be proven through relevant demonstration testing to the utility that simulate installation conditions.
This article starts the necessary conversation between utilities and those presenting lining for the renewal of pressure pipes. Although the need should be obvious, many times the requirements for ongoing maintenance and operation of lined systems is not addressed when the rehabilitation is chosen or made.
The long-term success for linings used to renew pressure pipes relies heavily on implementing operation and maintenance activities required to preserve the new pipe material (lining) and optimize system performance. Responsible lining system providers and manufacturers should provide published guidelines detailing the care and maintenance of the installed systems. •
FOR MORE INFORMATION:
NASSCO, (410) 442-7473, nassco.org
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