How to Avoid Overcooking a Liner When Steam Curing

Steam can be a fast and effective way to cure a liner. But be sure you’re doing it properly, or you could have unhappy customers in the future.

“I did a couple of shots with my new system and cured it with steam. I was really impressed with how quickly I could cure the liner and the amount of steam my new unit was generating. The only glitch was the finished liner appearance. It had a waffled look to it. Not everywhere, but enough of it that I wondered why.”

To answer the question, let’s first look at the mechanics of steam. We all know that steam is made when water is boiled and transformed from a liquid to a gas. We also know that water boils at 212 degrees F at sea level. At elevation the boiling temperature decreases. For example, at 7,500 feet the actual boiling point is about 198 degrees F. So that will give you a range depending on the elevation you work in.

We also know that steam under pressure actually increases the temperature of steam. For example, at 8 psi, steam temperature is actually at 234 degrees F. At 15 psi, the temperature at sea level is closer to 250 degrees F. With those parameters, you can produce some serious temperatures when curing a liner.

Knowing these facts, we can now look at thermoset resins and how they cure at differing pressures and temperatures. You can look at the specifications sheet of the thermoset resins from any manufacturer and learn a lot about curing temperatures. Most will tell you to maintain a temperature at or below 200 degrees F. Why? Most manufacturers know that keeping temperatures below that will ensure you don’t overdrive the forming of resin chains. When you overdrive the formation of the chemical chains that harden resin, you are forcing them to form too quickly. When they form too quickly, they bump into each other too quickly and cause stress cracking. We all know what stress cracking does to resin in liners. It allows leaks and more importantly allows root intrusion.

I’ve developed a specific protocol for curing liners with steam. It includes monitoring temperature during the curing process and maintaining a temperature around 180 degrees F. This temperature gives you a cushion while trying to maintain a range that won’t spike and cause stress cracking. It also gives you about the same amount of time to cure as 250 degrees F does without the risk of overcooking the liner.

If you’ve acquired a steam boiler, you should resolve to learn the correct method to ensure you have a good liner with no stress cracking and no future failures caused from roots growing through the liner. I’ve experienced this issue myself. Many years ago, we overcooked a liner on a job in Southern California. A few years afterward — yes, years — I was called to explain to the sewer district why the liner we installed had roots growing through the supposedly impregnable product we had sold them. Embarrassing? Yes. Costly? Yes. A lot of customer repair work to let them continue CIPP lining in their agency? Yes. If you want to avoid my mistakes, heed my words. Follow your resin supplier’s recommendations for curing temperatures and pressures. Generally that’s going to mean keeping temperatures during the cure at around 180 degrees F and well below 212 degrees F. You do this by introducing more air and reducing steam volume when the resin is exotherming.

That said, while steam is a great way to cure liners, particularly large-diameter liners, if you are looking for a fail-safe method, hot-water recirculating through the liner may be the way to go. It is much easier to set and forget versus the continued monitoring you should be doing with 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.



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