Hot, Hot, Hot: Insulating the Inside of a Digester Complex

The digester complex is a critical part of the water treatment, as it provides an environment for the oxygen-free decomposition of organic material by bacteria.

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 to 49 °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 (Kansas City, Missouri, USA) 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,” says 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 (Deerwood, Minnesota, USA), Tnemec, and Mongan Painting and Sandblasting (Cherokee, Iowa, USA) 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 that having to go back and insulate the interior of the walls and wrap all the pipes,” explains 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,” says 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 in (102 mm) to the diameter of the pipe. However, with the Aerolon coating system, only about a ½ in (13 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,” says Renslow.

Masking, and Maintaining

Before the Mongan Painting and Sandblasting crew could begin the prep and application work on the 700 linear ft (213 m) of 24-in (610-mm) stainless steel and cast iron piping and the 2,500 ft² (232 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,” says 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,” says 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%, the surface temperature couldn’t dip below 50 °F [10 °C], the material temperature needed to be at least 70 °F [21 °C], and there needed to be at least a 5% difference between the dew point and the surface temperature,” states 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 (Ogdensburg, New York, USA) 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 [Valparaiso, Indiana, USA] indirect, diesel-fired heaters that blew heat into the building but kept the exhaust outside,” says Joines.

Prep and Coat

The piping in the digester complex was color-coded based on what was going to be flowing through them.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 [Fresno, Texas, USA] blast pots and Ingersoll Rand [Davidson, North Carolina, USA] compressors. We needed to rough up the new concrete since it was too smooth for coating application,” says 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.

With the walls and pipes ready to receive the Aerolon coating system, the crew began by priming the nuts and bolts of the pipes with Tnemec’s Series 115. This hydrophobic acrylic, rust-inhibitive coating was applied by brush and roller at an average thickness of 5 mils (127 µm).

The walls and pipes were then sprayed with approximately 6 to 7 mils (152 to 178 µm) of Series 1224 using a 1595 Graco (Minneapolis, Minnesota, USA) airless sprayer. This inorganic hybrid water-borne epoxy is low odor and low volatile organic compound (VOC) and is specifically formulated to provide a corrosion-resistant barrier under fluid-applied insulating coatings.

After the 1224 was applied, the crew installed the Aerolon thermal insulating coating, Series 971, at an average thickness of 100 mils (2,540 µm) using a Graco RTX 2000.

It was then time for the crew to apply the Series 1028 HDP acrylic polymer topcoat. This water-borne, low VOC, high dispersion pure acrylic polymer coating provides long-term protection from mildew and offers gloss and color stability. The crew used a Graco 1595 airless sprayer with a 415 tip to apply the coating at a thickness of 5 to 8 mils (127 to 203 µm).

Throughout all aspects of the job, the crew put safety first, wearing Tyvek (Wilmington, Delaware, USA) suits, and 3M (St. Paul, Minnesota, USA) half-faced respirators with carbon filters, gloves, safety glasses, and hard hats. The Mongan team has received Occupational Safety and Health Administration (OSHA) (Washington, DC, USA) training and went through Rice Lake Construction Group’s on-site training that was specific to the Williston wastewater treatment plant job.

Colorful Complex

Perhaps one of the more unique aspects of the job was the color coding that was done with the topcoat. “The pipes needed to be color-coded based on what was going to be flowing through them. We were able to customize the Tnemec 1028 so that there were six different colored pipes, corresponding to the wastewater plant’s specifications,” explains Joines.

According to Renslow, while some of the colors adhere to industry and American Water Works Association (AWWA) (Denver, Colorado, USA) standards, others are determined by state, regional, and local governing bodies. “For example, pipes containing air are always painted a certain shade of green, but other colors may change from region to region,” stats Renslow.

Turning It Around

The beginning of the job at the Williston wastewater plant’s digester complex was a bit rocky, necessitating some outside-of-the-box thinking, a coating system that provided excellent thermal insulation, and a dedicated application crew. Despite that, the job was completed successfully. “What started out as a missed specification, ended up saving the client a lot of money in the end,” says Joines. “We were happy with how everything turned out—seeing how great the finished product looked and performed was very rewarding. We left the jobsite feeling good about all we had accomplished.”

Reprinted with permission from the October 2018 issue of WaterCorr, a new online digital publication from NACE International pertaining to corrosion in the water and wastewater industry.

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