Condensation and corrosion can spell disaster for any component of a wastewater treatment plant, especially the digester complex, an area of the facility where a network of pipes and tanks has materials flowing through it at temperatures of 110 to 120 °F (43.3 to 48.9 °C). The digester complex is a critical part of the water treatment plant as it provides an environment for the oxygen-free decomposition of organic material by bacteria. Because of the extreme temperatures of the sludge and other materials contained with the pipes and tanks, the entire area’s temperatures must be regulated not only to prevent heat loss but also for worker safety and to ensure that condensation does not occur.

With the construction of a new digester complex for a Williston, North Dakota, USA wastewater treatment plant, it was imperative that the walls and pipes be properly insulated. Unfortunately, the insulation and pipe wrapping line item was accidentally missed on the bid. According to Kevin Renslow, coating consultant and Tnemec representative, the general contractor was willing to think outside the box to save the city of Williston from being hit with a huge change order. “The last thing Rice Lake Construction Group, the general contractor on the job, wanted was to cost the client more money because of an accidental miss on the bid. The original specifications called for the pipes to be wrapped and the walls to be insulated when they were studded out but not closed up,” said Renslow. Luckily, a cost-effective solution presented itself in the form of a Tnemec thermal insulating coating system.

Value-Added Solution

Renslow worked closely with Rice Lake Construction Group, Tnemec, and Mongan Painting and Sandblasting to come up with a solution that would not break the bank for the city of Williston. The solution did need to provide the same — if not better — insulation properties as those laid out in the original specifications. It was decided that Tnemec’s Aerolon thermal insulating coating system would be a cost effective and value-added solution to the digester complex’s issue. “The Aerolon coating system was a perfect choice for this job. It could be applied to both the walls and the pipes and it offers stratified, bonded layers of protection. Installing coatings on the metal pipes and concrete walls was a much more cost-effective solution than having to go back and insulate the interior of the walls and wrap all the pipes,” explained Renslow.

The Aerolon system is specifically designed to prevent heat loss, mitigate condensation, and reduce the risk of corrosion. “The system also has a low K-value, which means that the material has excellent insulating properties. (The lower the K-value of a material, the better the insulating properties.) In other words, although the material flowing through the pipes is extremely hot, it’s safe to touch the exterior of the pipes. There is very little heat transfer from the liquid in the pipe to the pipe itself,” said Renslow.

In addition, there are other advantages to the Aerolon coating system that would save time and money in the long term. According to Renslow, pipe wraps add about 4 inches (101.6 mm) to the diameter of the pipe. However, with the Aerolon coating system, only about a ½ inch (12.7 mm) is added to the diameter of the pipe. “This allows for more space in the digester complex for day-to-day maintenance, as well as dealing with any larger maintenance issues that may arise. Also, pipe wraps tend to ‘hide’ problems that may be occurring with the pipe, including any corrosion issues,” said Renslow.

Masking and Maintaining

Before the Mongan Painting and Sandblasting crew could begin the prep and application work on the 700 linear feet (213.4 m) of 24-inch (609.6 mm) stainless steel and cast iron piping and the 2,500 square feet (232.3 m²) of concrete walls, there was the matter of masking off the entire digester complex area. This proved to be no small endeavor!

“It took us six weeks to mask off the area, put up plastic sheeting to protect the other equipment in the space, and put tarpaper on the floors. Compare that with the fact that we only took 10 days to apply all the layers of the Aerolon coating system,” said Supervisor Larry Joines of Mongan Painting and Sandblasting. According to Joines, the masking process was by far the most challenging — and tedious — part of the job.

While the masking was certainly a challenge, it was not the only portion of the job that required the six-man Mongan crew members to remain on their toes. “We had to monitor the environmental conditions, including air temperature, dew point, surface temperature, and relative humidity, every three hours during the coating application,” said Joines. According to him, it was critical that the conditions remain within the correct range or the coatings would not achieve proper adhesion.

“The relative humidity couldn’t be above 85 percent, the surface temperature couldn’t dip below 50 °F [10.0 °C], the material temperature needed to be at least 70 °F [21.1 °C], and there needed to be at least a 5 percent difference between the dew point and the surface temperature,” stated Joines. If the dew point and the surface temperature were the same or too close, condensation would occur on the substrate, which would hinder the proper adhesion of the coating system. To track and record the environmental measurements, the crew utilized a DeFelsko Positector 6000.

One of the biggest obstacles the crew faced in maintaining the proper conditions was the fact that it was the middle of winter in North Dakota, and the boiler in the facility had not yet been turned on. Luckily, the crew had power and the necessary equipment to keep the temperature regulated. “We had Heat Wagon indirect, diesel-fired heaters that blew heat into the building but kept the exhaust outside,” said Joines.

Prep and Coat

Although the concrete walls were a new pour, they still needed some surface prep to ensure proper adhesion of the primer and subsequent coating layers. “We brush blasted the walls and then sand blasted them using Schmidt blast pots and Ingersoll Rand compressors. We needed to rough up the new concrete since it was too smooth for coating application,” said Joines. As for the pipes, the crew used grinders and heavyweight sandpaper to scarify the metal surface and ready them for the application of the primer.