“I have both a steam curing unit and a hot water unit to cure CIPP liners. My liners using hot water always turn out well, but sometimes the ones I steam cure melt the calibration tube to the coating and when you look at the finished liner it doesn’t look smooth. I like the steam unit because I don’t have to deal with the water, but it doesn’t turn out the same. Why?”

Let’s talk about the functions of heaters. The job of a heater is to initiate the resin to form resin chains at a faster pace than they would at an ambient temperature. As the resin chains form, they give off heat. This process is called exotherm. This is the point where the heater has done its job and may no longer be needed to maintain temperature and cause all of the resin chains to form. And this is also where steam and hot water perform differently.

The hot water units have a thermostat that cuts off the burner if the water running through the chamber is at or above the temperature setting. When it reaches set temperature, the burner quits but the pump continues to circulate water. We know the ground temperature is below cure temperature and that it is cooling the water circulating through. When the resin begins exotherming, heat is being generated by the chemical binding process. This makes our heater now a cooling device as the burner is off while continuing to circulate water. This automatic configuration ensures that the resin temperature isn’t over driven.

Steam, on the other hand, is separate from the air pushing it. You have a steam unit generating steam and an air compressor tied in to distribute the steam through the liner. This system is not automatic as both units operate independently — as long as the air is pushing steam and the steam unit is generating steam, any added temperature will drive up the temperature. The steam temperature is a constant 212 degrees F (at sea level) and the compressor has the job of distributing steam and holding pressure on the calibration tube.

Here is where things get complicated because the exotherm process itself is generating heat. Coupled with the heat generated by the steam, under pressure, the temperature begins to spike. While the ground is still dissipating heat, it isn’t dissipating the heat as fast as the exotherm and steam unit producing it, so the resin exceeds the not-to-exceed temperature pretty quickly. This allows the calibration tube to deform and the finished tube/resin laminate to stress crack, creating a future area for roots to grow through. Since there aren’t any automatic controls, the installer must physically stop the generation of steam and at the same time increase the volume of air to take over the cooling process in order to keep the cure temperature in the ideal 180 to 200 degrees F range. Installation of thermocouples and continued monitoring by the operator is not only a good practice, it’s mandatory.

I hope this helps with the understanding of the physics behind the CIPP curing process and provides a pathway to future jobs where you can make all your liners turn out the same whether using hot water or steam.

About the Author

John Heisler is the owner of Pipe Lining Supply and Quik-Lining Systems Inc. He has 20 years of experience in the CIPP lining industry and over 40 years in the underground construction industry.