Perfect Balance

uclear power remains an alternative to fossil fuels, and while the global economy adjusts to fallout from COVID-19, several countries are looking for ways to boost a faltering economy. Davi recently engineered and installed its largest rolling machine for a customer making nuclear reactor components. It had to navigate supply chain challenges as well as logistics to build a machine capable of bending 7,300 tons.

Few forges in the world are able to handle a roller with a top roll measuring 20 ft. long, with an 80-in. diameter, and weighing 140 tons. Finding forge shops capable of handling the massive size—while adhering to quality and production requirements—proved challenging, says Dario Mulazzani, project manager.

The machine has a nominal capacity of rolling 14-in.-thick carbon steel. Davi’s customer will fabricate reactor pressure vessels (primary and secondary heat exchangers) with wall thicknesses up to 11 in., using a vanadium-steel alloy with a yield up to 800 MPa.

“We had to demonstrate the validity of a huge investment like this,” explains President Alex Soffritti, Davi Inc. “In the nuclear sector, vessels with very high thicknesses are often forged, but the value [of Davi’s method], on tight diameters in particular, proved the customer’s investment in the rolling machine worthwhile.”

Hot rolling introduces heat to material as does rerolling, where even calibration can be hot. In order to achieve repeatability and reliability, Davi’s technical proposal tried to limit challenges associated with rerolling.

During the rolling and calibration phase, the capacity of the machine is designed to maintain control over pressure to prevent material thinning and lamination. Rerolling is necessary when the machine doesn’t apply enough force. Engineers designed a plate roll with enough capacity that heat generated during rerolling is significantly reduced.

In the few cases where rerolling and calibration is necessary, results are accurate because the machine maintains continuous control of working pressures, which prevent the plate from thinning or laminating. Otherwise, the plate won’t match specs and will be rejected, a costly blunder.

“The machine is particularly suitable for calibration in high thickness and small-diameter products,” says Stefano Santoni, director of Davi’s heavy-duty business unit. “The patented solution, Servo-Tronic, for the parallelism system is how the machine achieves high accuracy in rolled products.”

Versatile usage

Three engineers devoted to this projected were supported by 25 technical department employees. The team had to figure out how to roll heavy-gauge materials with high yield.

The variable axis on the rolling machine allows users to find the ideal geometry using the optimum force distribution for each specific job. “Similar to [our] regular rolling customers, the process of the three-roll variable axis is always the same,” explains Santoni. “The roller functions are very versatile,” making them ideal for many heavy industrial applications.

The machine’s frame design is perfectly symmetrical and can balance load distribution to achieve tight tolerances on rolled products. “That includes motorization with reliable multi-gear technology,” Santoni adds. “The market is constantly looking at higher thicknesses so our largest plate roll ever has sparked great interest.

“We are already studying solutions for machines with performances that are even higher,” he continues. “Some clients are visiting our plant in order to take a look at all the processes.”

Open communication

The project spans 11 months: three months dedicated to the design and development of the machine and eight months for fabrication, delivery and installation. COVID-19 has caused challenges but strong existing long-term relationships have allowed the machine manufacturer to overcome any delays with its key suppliers.

Davi communicates extensively throughout the process, including submitting monthly reports with updates on project status. “Since the machine is destined for a sector that is an essential business, we’ve had the chance to provide work for our suppliers during the lockdown,” Mulazzani says.

The sheer size of the rolling machine resulted in different supply chain needs. “Many suppliers don’t have the necessary machine tools and equipment to handle large and heavy components,” Mulazzani says. “In our warehouse, we have cranes with a combined capacity of 150 tons and spanning 52.5 ft. tall by 46 ft. wide.”

Davi fabricated and tested the machine in its factory. “During the design, the three engineers worked together and once in construction, teams of mechanical, electric and testing experts alternated work on the machine,” Mulazzani says.

Installation of the rolling machine is ongoing. Twenty-four trucks transported the components with some of the trucks equipped with rigs to move special added features of the roller. Special transportation equipment was used for the bulkiest and heaviest components in particular, Mulazzani says. “The design of the machine took into account the limits related to transportation, but without compromising machine functionalities.”

Installation is expected to be completed in 2020. “We foresee a very challenging installation phase,” Mulazzani says.

Davi has team members stationed across five continents. “Our Service and After Sales Division (SAS) is extremely organized and able to give support all over the world,” Mulazzani says. “We can give support to multinational companies since our employees are multilingual. In the nuclear and defense sectors—both navy and aerospace areas, there are also citizenship requirements for workers installing equipment on premises so our global presence is a big advantage for us.”

According to Soffritti, “Demand for high thickness rolling capabilities is on the rise. The market leads us to invest in machines with capacities that are higher and higher,” he adds. “In this way, we can supply equipment quickly and continuously invest in technologies.”