Pipelining is one of the oldest and most popular trenchless rehabilitation methods which involves inserting material into the host pipe, thereby creating a new pipe within the damaged pipe. Traditionally, lining methods would consist of PVC pipes (also known as liner tubes) or cured-in-place-pipe liners (CIPP).
However, Kevlar reinforced composite lining systems are providing engineers and contractors with an alternative rehabilitation option.
What is Kevlar?
Kevlar is a polymeric material with extraordinarily high tensile strength properties (over eight times greater than that of steel). This synthetic fiber is also known for its superior heat and chemical resistance, with the ability to withstand temperatures up to 800°F (430°C). These properties make Kevlar more than ideal for pipelining systems, particularly in high-temperature or high-pressure environments.
The strength properties of Kevlar liners allow significantly lower liner thicknesses to be used (6mm -8mm), thus reducing the cross-sectional losses typically associated with HDPE or PVC liners. Kevlar’s flexibility also allows the liner to achieve bends up to 90 degrees, allowing lengthy installations (hundreds of feet) to be performed in a single step.
Composition of Kevlar Reinforced Composite Lining Systems
Kevlar reinforced composite lining systems are composed of two critical elements: the liner and the fittings.
The composite liner consists of three distinct layers: an inner layer, a middle layer, and an outer layer. The inner layer is composed of low-density polyethylene (PE), which makes it ideal for potable water applications in accordance with several international standards, including the AS/NZS 4020:2005 and NSF/ANSI 61.
The middle layer can be constructed from a hybrid of polyester and Kevlar for low-pressure applications or pure Kevlar for high-pressure operations. For low pressures, the polyester/Kevlar hybrid is available in 6-inch to 12-inch diameters with maximum operating pressures ranging from 175 psi to 360 psi. For high-pressure applications, the pure Kevlar middle layer is available between 6 inches and 20 inches and has maximum pressure capacities between 230 psi and 810 psi.
Typically, the thickness of the two liner types is 6mm; however, for extreme pressures, a second Kevlar layer can be added for additional reinforcement. For dual-layer Kevlar construction, the thickness of the middle layer increases to 8mm.
Finally, the outer layer is constructed of polyethylene (PE) to protect the internal layers during the installation process.
The liners are installed in a bent U-Shape and are expanded upon pressurization. The liners are able to maintain their round shape without the application of further external pressure. Additionally, there is no connection between the liner and the host pipe; therefore, the Kevlar liner is capable of operating independently.
Figure 1: Layers of the Kevlar composite lining system (source)
The second part of the system is the specially designed termination fittings. The fittings are classified according to the size of the liner used and the anticipated operating pressures. Low-pressure fittings are available for liner sizes up to 12 inches.
The installation of these fittings is performed mechanically using a compression style fitting. Medium- to high-pressure fittings are available in liner sizes up to 20 inches. In this case, the installation is performed using a two-component epoxy resin.
Application & Benefits of Kevlar Reinforced Composite Liners
As with other lining solutions, Kevlar liners are ideal for rehabilitating pipes which traverse challenging terrain.
Relatively small pits can be excavated on either side of the terrain, allowing rehabilitation operations to occur without disturbing infrastructure on the surface. This feature makes Kevlar liners especially ideal for rehabilitating pipes in urban environments.
In addition to rehabilitating existing pipes to their original intended structural function, Kevlar reinforced liners are capable of increasing the pressure rating of the existing piping network since the liners have their own pressure capacity.
In other words, the host pipe ceases to contribute to the structural integrity of the system and merely acts as a conduit for the liner. As such, the lining material can be installed in host pipes with significantly larger diameters provided that a reduction in hydraulic capacity is feasible. For example, a 20-inch liner can be easily used to rehabilitate a 24-inch host pipe.
These types of liners are also a safe and cost-effective solution for rehabilitating asbestos cement water mains since the cost of removing and disposing of this material using conventional excavation methods can be time-consuming and costly. (Read Why CIPP Is Growing Rapidly for Drinking Water Mains.)
As with other sliplining projects, an entry pit and an exit pit is first excavated to gain access to the host pipe. A mobile CCTV camera is deployed inside the pipeline to inspect it for several features, including obstacles, debris, sudden changes in cross-section, directional bends, etc.. (Read Using CCTV to Inspect Pipes.)
The results from the camera inspection are used to determine the type of cleaning method to be used to establish a free inner diameter. High-pressure water jetting, steel scrapers, and cleaning pigs are some of the most common cleaning methods used to clear the pipe’s interior before liner installation.
In addition to inspecting the pipe, the camera is also used to establish a rope connection between the entry and exit pits. The rope is then used to pull the coiled and pre-folded liner through the host pipe using a pulling head. The liner is drawn from the exit pit to the entry pit without adhering to the host pipe; therefore, an annulus remains between the pipe and the liner.
The U-shaped liner then achieves its round shape by pumping it with compressed air. After inflation, the terminal fittings are applied at the end of the host pipe. A leak test is then performed on the renovated section of the pipeline.
What We've Learned
The desirable properties of Kevlar make it ideal for lining systems. Kevlar’s corrosion, heat, and chemical-resistance ensure the efficiency and durability of the rehabilitation process. In addition, its high strength-to-weight ratio allows it to replace the properties of the host pipe with minimal hydraulic capacity reduction.