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Western Dam Engineering Technical Note May 2016 14 from a boiler through the inside of the water-filled liner, heating the water and resin-impregnated liner, causing the resin to cure. Heated water can be circulated throughout the installed CIPP at the required temperature to initiate cure of the resin. The water temperature should be constantly monitored to maintain it above the required cure temperature for the minimum time required. The initial cure will be complete when the exposed CIPP is hard and temperature sensors show the resin has reached the required temperature. After the initial cure, the water temperature should be raised to the required post- cure temperature for the time recommended by the manufacturer. Steam The use of steam to cure CIPP liners is most often used for installing liners within pipes from about 6 to 36 inches in diameter. The steam cure method is normally faster than hot water cure, as heat transfer from steam occurs more rapidly. Steam is used in conjunction with pressurized air that inflates the liner against the host pipe and distributes the steam from one end of the liner to other. After the liner has been placed in the host pipe, steam cans, or other similar equipment, are attached at each end of the pipe to distribute steam uniformly throughout the CIPP. Steam with compressed air is then passed through the liner, pressing the liner tight against the host pipe and heating the resin until cure is complete. The steam generating system should be constantly monitored to maintain the steam temperature above the required cure temperature. The initial cure will be complete when the exposed CIPP is hard and temperature sensors show the resin has reached the required temperature. After the initial cure, the steam temperature should be raised to the required post- cure temperature for the period of time recommended by the manufacturer, then cooled in a controlled manner as described below. Steam is not recommended for pipes that have bellies or vertical bends (such as siphons), as the steam may condense and pool in low areas preventing that portion of liner from reaching the required curing temperature. Condensation also collects at the downstream end of the pipe; therefore, a fitting with a drain hole is usually installed at the downstream end. Steam curing is two to three times quicker than water curing and is better with restricted-access installations. Steam is also better for steeply-sloping pipes to avoid excessive hydrostatic pressure that can result with water-cured procedures. Cool-Down With both the heated water and steam curing methods, the temperature should be cooled below 100°F in a controlled manner before relieving the internal pressure on the CIPP to avoid unwanted shrinkage. The manufacturer will provide the recommended cooling process and temperature drop intervals based on the fabric, resin, and pipe size. The most common cool down process circulates cool water throughout the CIPP. The air or fluid pressure inside the CIPP should be monitored and relieved in a controlled manner so an internal vacuum does not occur and damage the CIPP. Environmental Some thermoset resins (e.g. vinyl ester) can contain up to 33 parts per million (ppm) of styrene. Styrene- contaminated resins and water must be disposed of properly and cannot be discharged into stormwater systems because of concerns with its odor and volatile organic compound (VOC) emissions. There are some newer formulations of thermoset vinyl ester resins that reduce VOC emissions. Epoxy resins are naturally VOC- free, but are generally more costly and must be mixed and applied on-site rather than in a controlled factory setting. UV There are resin systems that can be cured with UV light. These resins do not react to temperature but rather contain a photo-initiator that reacts to certain UV light wavelengths. UV-curable liners use a nonwoven glass fiber tube, rather than other nonwovens, because the translucent glass fibers permit light transmission through the liner’s thickness. A UV light is pulled through the CIPP at the required wavelengths initiating cure of the resin. UV cure is recommended in colder temperatures where heat cure would be more expensive and less efficient. UV may also be a beneficial alternative for remote locations with long transport times, as these products have a much longer shelf-life without refrigeration and