Cured in Place Pipe (CIPP) can be used to rehabilitate gravity sanitary sewers, storm drains and pressure pipelines for water, gas and process effluents. Circular pipe from 75 mm to 2,700 mm and a variety of noncircular pipe such as egg shapes, ovals, and box culverts can be lined. Lining with CIPP removes the pipe from service for the duration of the installation and reinstatement process so overpumping or provision of an alternate source of supply may be necessary. Prior to lining, the pipe must be cleaned by jetting to remove corrosion debris and detritus. Protruding lateral connections also must be removed. Local repairs may be required where the existing pipe is substantially deformed or damaged. After lining, the service connections or laterals are reinstated and the pipe is returned to service, usually within the same day, depending on lining material. If reinforcing fibres are used instead of resin carrier material like polyester felt, the lateral connections can be reopened after the curing process is carried out . Lined water mains must be disinfected before returning to service. CIPP liners of non-woven polyester felt or fibre reinforced fabric are manufactured to fit the host pipe. The liners are impregnated with a polymer resin, which when cured will form a close fitting liner pipe within the host pipe. For gravity applications, the liner may be designed with sufficient thickness when cured to sustain the loads imposed by external groundwater and internal service pressure, and by soil and traffic acting on the pipe. The final wall thickness needed can vary, depending on the fibre type and the resin used. Reinforcing fibres will increase the flexural modulus, which reduces the required wall thickness. The liner is thoroughly saturated with polyester, vinyl ester, epoxy or silicate resin using vacuum, gravity or other applied pressure. The resin includes a chemical catalyst, photo initiator or hardener to facilitate curing. The outermost layer of the liner tube is coated with a polymer film to protect the liner during handling and installation. The impregnated liner may be chilled for transportation to maintain stability until installed. For UV cured liners, the uncured liner must always be kept out of daylight and can be stored and distributed at room temperature, the liner is normally protected with a UV protection made out of polymer film. For UV cured liners, (where the chemical catalyst and photo initiator is mixed with a hardener), storage and handling must be maintained under controlled temperature and light radiation conditions.
As shown above, CIPP liners may be installed by winching into place (WIP) (also called pulled in place) through a manhole or specially prepared access point and inflated to come into close contact with the circumference of the existing pipe. Usually, a Gliding Foil is first pulled in to reduce friction and protect the liner.
CIPP liners may also be installed using the inversion method as shown above. The inversion method employs a scaffold tower or pressure vessel to apply air or water pressure to turn the liner inside out and push it along the host pipe. The liner is restrained in any intermediate manhole. CIPP liners may be inflated or inverted with air or water pressure. Lengths installed may vary from short sections over a joint or defect, to full length linings typically 30 m to 150 m. Lengths of up to 900 m have been installed in a single shot. The resin impregnated liner may cure at ambient temperature, by heat or by light. Heat curing is by circulation of the inversion water through a boiler or by blending steam with the inflation air. Light curing uses lights typically in the range of 360 nm to 450 nm or by pulling a UV light train through the pipe. After the pipe has been cured, the liner is cooled and the ends removed flush with the pipe ends, and sealed where necessary. The curing water or steam condensate and the trimmings cut from the liner are removed for safe disposal. A sample of the CIPP may be taken from pipe ends or from a mould cured under the conditions of installation for testing and verification that the liner meets performance specifications.
Insertion Process
The first CIPP liner installed in London in 1971 was pulled into position within the host pipe and then inflated and cured. However, within a few years, an inversion installation process had been developed in which the impregnated liner was contained within a plastic handling layer and was unfurled (inverted) within the host pipe using water or air pressure. This method causes the liner to unwrap itself within the host pipe without the need to drag the prepared liner through the host pipe. The first commercial installation of UV cured CIPP was installed in Sweden in 1985, by the inversion process using air. The handling layer remains on the inside of the finished liner and the impregnated felt/fabric winds up in direct contact with the host pipe. In both cases, Winched-In-Place or Inverted process, there is normally a handling/sealing layer on both the outer and inner surfaces of the liner. The inside polymeric layer is often a temporary internal membrane used to separate process fluid (typically water or air) from the resin system during liner insertion, and is withdrawn after the resin cure. This allows the liner to be pulled in place and then for the inflation and curing process to be accomplished from inside the liner. Both methods are effective for thermal curing and UV curing, the choice can depend on job circumstances or contractor preference.
Composition
The most common resin used today in CIPP is polyester resin, normally an ISO-NPG resin. It has excellent properties for many applications and is economical, but other resins are used for specific applications. In particular, vinyl ester resins may be used for better resistance to chemical attack and epoxy resins for higher strengths and use in potable water applications. Fillers may be added to the resin to increase the modulus of the cured resin and to reduce costs. To maintain the resin position during storage and transport, the resin viscosity is normally increased by the use of a thickening agent. The common requirement is that the resin composition be designed so that the liner can be installed in the uncured and flexible state and then that the curing and hardening will be initiated while the liner is expanded against the interior of the host pipe. The flexibility in the exact composition of the resin means that installations can be tailored to specific requirements but also implies that adequate quality control measures be used to document the installation.
Carrier Material / Reinforcement
With CIPP using felt and resin, the uncured resin is impregnated into an unwoven felt fabric which essentially serves to hold the resin in place while the liner is installed in the pipe, (the felt is defined as a “carrier material”). The felt fabric does not contribute to the strength or modulus of the liner once the liner is cured. With CIPP using reinforced lining material and resin, often the woven fabrics are made out of Fibre Glass with greatly enhanced structural properties. These can be designed to allow thinner liner wall thicknesses of liners with superior strength and Flexural Modulus properties.
When the CIPP is designed by woven or non-crimp fabrics made out of fibreglass, the continuous fibres are normally ECR (Electro Chemical Resistance) to prevent the structural design from corrosion attacks, especially cut and unsealed ends. The ECR fibers in combination with a high grade polyester- (ISO-NPG), Vinyl ester or Epoxy resins will ensure long term strength even under strong influences of for example Sulfuric acids.
When the CIPP is based on reinforcing fiber like fibreglass, the cured CIPP samples are preferably tested under influence of Strain Corrosion, which not applicable for synthetic fibres/felt fabrics.
Curing Process
The uncured resin is designed so that, once the liner has been installed, the curing process can be completed while the liner is held against the interior of the host pipe and within a reasonable length of time. One approach is simply to allow the resin to cure under ambient conditions (i.e. at the temperature that will be present when the liner is installed in the host pipe). This can work but is much slower than the other approaches used and may be vulnerable to unexpected temperature variations during installation. In the past the most common curing process used for CIPP has been a hot water cure, today light cured liners are the most common curing technology used with preference to light in the range of 360 nm to 420 nm. This type of liner has a long shelf life, is inverted or inflated by air pressure and has the light train inserted within the inflated liner to accomplish the cure. When using the inversion method combined with heat curing, the hot water is used to both inflate the liner against the host pipe and to initiate the resin cure. To speed liner cure, steam can be used instead of hot water for the cure with the steam pressure inflating the liner rather than the hot water pressure. The curing process for resins is exothermic and hence this heat released contributes to the curing process once the curing is initiated. The curing speed with light curing is comparably faster than heat curing done by water or in some cases steam. For specialized applications, the heat from electric wires embedded in the liner can be used to initiate the cure. Combinations of curing can also be used – for instance for very thick liners. Control of the liner installation and curing process is important for consistent and high quality liners. When UV cured liners are used, the all important parameters can be read and recorded to ensure a fully cured liner after the installation process is finished. For heat curing, embedded temperature sensors can monitor the thermal evolution of the liner during curing to make sure that adequate curing is achieved. The ultra-violet cure system allows an inspection of the full length of the interior of the liner before the curing process is started.
Sealing against groundwater infiltration
If the object of the rehabilitation is to reduce water infiltration, then this must be planned before the liner installation. Typically, there is an annulus between the liner and the host pipe, due to the liner material shrinking slightly on curing and pipe size variations. Groundwater infiltrates through the host pipe into the annulus and will drain back into the host pipe at any openings in the liner.
The sealing of the liner ends after installation is a finishing operation that is not sufficient to stop infiltration water leaking out of the annulus. Better sealing is achieved using CIPP hydrophilic end seals which must be installed in the host pipe prior to inserting the liner. The hydrophilic seals absorb water and expand, sealing the infiltration water within the annulus. Mechanical end seals are also available to seal the gap between the CIPP liner and the host pipe.
These end seals are adequate if there are no penetrations made through the liner for lateral connections. If there are such lateral connections to be re-established, then a comprehensive sealing system must be installed to ensure watertightness. This would include hydrophilic sealing of lateral connection junction liners (top hats) as well as ensuring the existing lateral connection pipe is watertight or lined with its own end seals.