Unistress Corp. designs and manufactures prestressed concrete building materials in the Northeast. With one of the largest facilities in North America, the company specializes in large-scale structures such as concrete cooling towers, industrial plants and retail buildings, and major roadway overpasses.
Among other ventures, Unistress was tapped for a major roadway reconstruction project: replacing the Tappan Zee Bridge, which has connected New York state’s Westchester and Rockland counties via a 3.1-mile-long span across the Hudson River since 1955. As a part of the bridge renovation, the constructors opted for precast concrete deck panels from Unistress.
At its Pittsfield, Mass.-based plant, Unistress designs and precasts the custom-reinforced concrete products for the bridge project. Once cured, the panels are shipped to the construction site, where they are erected. Precasting construction components off-site eliminates congestion on-site while reducing labor costs and waste.
The Tappan Zee project reflects a larger trend in roadway reconstruction projects. Construction companies have been moving to precast and prestressed component manufacturing to take advantage of its benefits: improved speed, quality control and reduced cost of construction. By engineering the casting and curing environment in a manufacturing location rather than on-site, variables such as temperature, moisture, humidity and curing time can be better managed and controlled.
Concrete curing is a physical and chemical process that is directly influenced by environmental factors. In order to cast products year-round, Unistress seasonally heats the precasting beds during the colder months to accelerate and control the curing of the concrete. A controlled and consistent curing process is key to ensuring high strength and high performance concrete as specified by the precaster.
When precasting concrete in a manufacturing environment, indirect heating with steam or heat transfer fluid can greatly enhance temperature control of the environment. Optimized temperature control helps engineer an accelerated and near-perfect cure, especially during colder weather. Using measurement and analysis of the process and resulting product, the optimized process can be repeated for consistent quality and efficiency.
Improving an Existing Hot-Oil System
Unistress’ engineered process for controlling the curing conditions of their slab manufacturing was first designed in the 1990s. An 8 million BTU gas-fired hot-oil system heats multiple tubes that run beneath the concrete curing beds. The coverage area is roughly equivalent to about two football fields.
When initially installed in the ’90s, the heating system used a brand of heat transfer fluid that had, over time, become severely degraded. In addition, corrosion had formed in the expansion tank due to the elevated acid level of the aging thermal transfer fluid.
Early in 2016, two Unistress team members — Daniel Kaufman, sourcing manager, and Trevor Leja, maintenance manager — contacted several thermal fluid manufacturers to investigate the requirements to service and refill the system. Their preliminary visual inspection of a sample of the casting beds’ heat transfer fluid — nearly eight years old — showed signs of age, wear and tear.
Each fluid company performed evaluations of the fluid and system condition. After Unistress managers deliberated over the solutions presented, Paratherm was chosen to help with the aging hot-oil system. Steve Beward, a sales engineer at Paratherm, and Jim Oetinger, the company’s director of technology, visited Unistress’ facility to inspect the entire operation first-hand and discuss potential solutions.
Fluid samples taken from the system were analyzed at Paratherm’s fluid-analysis laboratory. While there are several tests that can be performed depending on what problems are suspected, for the Unistress fluid, three core tests — viscosity, distillation range and total acid number (TAN) — provided the answers needed. A high TAN in the Unistress sample indicated that severe fluid oxidation had taken place.
The degraded fluid would require replacement. But more importantly, system repairs and a few design alterations were needed. The adjustments would help to ensure that the fluid and equipment work together to thwart the continuing deterioration of both as well as reduce or eliminate the prospect of unplanned downtime in the future.
“Companies often install a hot-oil system after years of working with more temperamental heating solutions — steam systems are a great example,” says Oetinger, who has 25 years’ experience analyzing and troubleshooting hot-oil applications in process manufacturing. “With hot-oil temperature control, there are no steam traps, essentially no pressurization, no water treatment or blowdown, or a dozen other routine maintenance practices that steam systems need, and need regularly. Also, hot-oil systems have minimal chance of problems related to freezeups if there’s a cold snap. With hot oil, you set it up, turn it on, and go about your other business. Routine maintenance is minimal compared to steam. All users need to do is log some key data once a day, test the pressure and temperature sensors once a month, and send a sample out for testing once a year.”
The Paratherm solution selected by Unistress had five main steps:
- Replace a calculated percentage of the system’s fluid volume with a liquid system-cleaning product. The in-place additive dissolves and suspends sludge and carbon deposits while the system runs.
- Run the system for a period of time, then drain out as much of the existing fluid — and suspended sludge deposits — as possible.
- Make system repairs and alterations.
- Refill, start up and run the system, observing performance.
- Retest the fluid to establish new baseline values for future predictive maintenance.
Performing each step in sequence helps ensure fluid longevity.
Steps to Clean, Drain, Charge and Repair a Hot-Oil Heating System
To clean the system, approximately 4 percent of the 8,000-gal total fluid volume was replaced with Paratherm LC system cleaner liquid. This mixture was circulated through the entire hot-oil system at room temperature for several days. System cleaner fluids help break up fouling and gradually begin to suspended settled sludge deposits. Next, the heater was turned on to allow for several weeks of hot-fluid circulation. This helps to further soften, break up and dissolve the sludge and carbon into the fluid/cleaner liquid mixture.
Once the fluid cleaner had done its job, it was time to drain the deteriorated fluid and sludge. With a large system with many loops and crannies — like that found at Unistress — complete drainage can be a challenge. The goal was to achieve 85 percent drainage, or 6,800 gal. Without resorting to measures such as cutting into piping runs or installing additional low-point valves, the team succeeded in draining approximately 71 percent, or 5,700 gal.
Once the degraded fluid and system cleaner were drained, it was time to address heater issues. The team, which now included technicians from the heater manufacturer, determined that the heater had been running in a sub-optimized manner. This had contributed to the fluid deterioration and the buildup of sludge and carbon. Adjustments were made to the system layout and piping to save energy, prolong the life of the heater and the oil, and improve the overall engineering of the application.
During the heater system evaluation, it was found that the gradual acidification of the original charge of heat transfer fluid had combined with high temperatures and exposure to air to rust out the system’s expansion tank. As a result, an extensive rebuilding of the 2,500-gal expansion tank was undertaken — a project that involved replacing and rewelding the tank’s upper half. Overall, during the cleaning, charging and design review phases, Paratherm worked with Unistress to make adjustments to procedures and decisions as more information was uncovered.
By August, the recharged and reinvigorated system was online. Because the heating system does its heaviest duty in the coldest months, as of this writing, it has been used to circulate the new charge of heat transfer fluid without the demands of full cold-season production. Operators will sample at key intervals to assess the condition of the mixture of old and new fluid during its first full cold-weather season of use. The test results will help determine whether they will be used to set a new benchmark of acceptable tested values for viscosity, acidity and distillation profile, or to determine that a further addition/replacement of fresh and new fluid would be advised. Paratherm will perform the testing and, in consultation with Unistress’ operators, make those decisions.
As of this writing, the heating season approaches. All of the preparations are expected to have brought the Unistress operation to an efficient and expense-balanced solution to continue producing precast concrete components. The team at Paratherm continues to provide technical suggestions to improve the system operation and extend the life of the oil.
Unistress plans to implement an annual program of fluid testing and equipment monitoring to maintain and adjust the fluid and system. These changes should help avoid any future challenges with the hot-oil system.