Since its introduction in the early 1970s, cured-in-place pipe (CIPP) lining has continuously evolved to become one of the most widely used forms of trenchless pipe rehabilitation and repair. This revolutionary technique allows deteriorated pipes to be quickly and efficiently restored with minimal disturbance to the surrounding environment. Its growth in popularity among the trenchless technology industry is due mainly to the numerous advantages it offers over conventional pipe rehabilitation methods. One of the major benefits of CIPP is its ability to perform structural renovations.
CIPP is considered to be a structural liner, in other words, once it is installed and fully cured, it creates a new pipe within the host pipe. The various properties of the host pipe, such as ring stiffness, flexural strength, tensile strength, etc., are replaced by the properties of the pipe liner.
Although CIPP liners are a highly effective method of pipe repair, their performance is heavily dependent on the thickness of the epoxy and the type of lining material used. Engineers must, therefore, consider several factors when determining the most appropriate CIPP solution for a given situation. In this article, we will discuss how CIPP liners are installed and look at the factors that affect the pipe lining decision-making process.
What Is a Cured-in-Place Pipe (CIPP) Liner?
CIPP is a seamless and jointless trenchless technique capable of rehabilitating a wide range of pipe diameters. It is commonly used to repair pipelines in the water/wastewater and petrochemical industries.
The liner itself is a felt tube constructed from polyester, fiberglass or any other suitable material. The interior of the liner is first vacuum impregnated with a specific resin. The coiled and resin-impregnated liner is then inflated into the host pipe using an inversion drum which turns the liner inside out such that the resin-saturated surface contacts and is pressed against the inner walls of the host pipe.
Once the insertion of the liner is complete, a heating medium (typically water or steam) is used to cure the resin and form a permanent bond between the liner and the host pipe. (For more on CIPP, see Why CIPP Is Growing Rapidly for Drinking Water Mains.)
Factors to Consider During CIPP Installation
Location of host pipe
One of the first items to take into consideration during CIPP lining installation is the location of the pipe to be repaired or rehabilitated, i.e., is the pipe above or below ground? Pipes that are located below ground are subject to external pressures from surrounding soil, hydrostatic forces, and above-ground infrastructure (surcharge). These pressures act on the outer walls of the pipe and can potentially affect the structure’s dimensional stability.
As a result, the pipe structure can become bent out of shape (similar to squeezing the outside of a balloon). The deeper the pipe is installed, the higher the pressures acting on it, thus the thicker the liner epoxy needs to be to resist deformation. Pipes that are located above ground, such as those in the walls of buildings, are not acted on by external pressures and are not susceptible to deformation. Above-ground pipes can, therefore, be repaired or rehabilitated using thinner epoxy thicknesses.
Diameter of the host pipe
The host pipe’s diameter is also a significant factor in determining the appropriate epoxy thickness for a given CIPP installation. The pipe’s diameter, like its location, can determine how susceptible the structure is to deformation. Ring stiffness, a property that indicates a pipe’s ability to resist external soil, hydrostatic and surcharge pressure, is inversely proportional to its diameter.
In other words, the larger the diameter, the smaller the ring stiffness, therefore, the more vulnerable the host pipe is to ring deflection. As such, pipes with bigger diameters will require thicker epoxy thicknesses to maintain dimensional stability.
Length of the host pipe
The length of the host pipe is an influential factor in two structural behavior characteristics: bending and deflection. Bending, which is quantified by an internal property known as a bending moment, is proportional to the length of the host pipe, i.e., the longer the length of the pipe, the higher the induced bending moment. Bending moments which exceed the capacity of the pipe of the liner can result in bending failure, which is characterized by cracking and fracture.
Pipe length can also influence the amount of longitudinal deflection a pipe experiences. This is especially significant in cases where the pipe may lose support due to soil settlement. Excessive deflections can cause a buildup of internal stresses in the liner, which can result in structural failure. Careful analyses must, therefore, be conducted to ensure that the appropriate epoxy thicknesses and liner materials are used for the given length of pipe.
Existing pipe condition
The host pipe’s current condition also needs to be taken into consideration during CIPP operations. Visual inspection and other analyses may be required to determine the capacity of the existing pipe. The condition of the pipe will give engineers and technicians an idea of how much strength loss has occurred in the pipe section.
The worse the existing condition, the less the load carrying capacity is expected to be. Pipes that are severely deteriorated will require greater epoxy thicknesses and more robust liner materials than pipes that have experienced little to no degradation. (To learn more about reasons CIPP is used, see Common Causes for CIPP.)
By thoroughly assessing the previously mentioned parameters, and performing the necessary analyses and calculations, the appropriate liner material and epoxy thickness for a particular liner installation can be determined. This helps ensure that the chosen CIPP lining solution can perform safely and effectively in the given environment and prevent avoidable structural failures.