ND Engineering triumphs in the Duplex Category
ND Engineering, a stalwart in the South African stainless steel sector with more than 55 years of experience, has earned the prestigious 2024 Sassda & Columbus Stainless Award in the duplex category.
This accolade was awarded in recognition of its outstanding work on the multi-purpose pulp tank (MPPT) for Sappi Ngodwana in Mpumalanga - a project that exemplifies the benefits of duplex stainless steel SAF 2205 in production efficiency and environmental impact.
ND Engineering GM Mark Ackerman comments: “In the late 1980s, we were one of the first in the country to really push the use of duplex stainless steel. We saw the potential immediately, especially in cyclic pressure vessels where the material’s strength and reduced thickness could make a significant difference. It was a game changer allowing us to improve performance and safety without relying on older, more cumbersome methods”.
Project overview
ND Engineering’s award-winning MPPT at Sappi Ngodwana is a colossal structure standing 32 metres tall with a diameter of 9 500mm, tapering into a 60° cone at the bottom. The tank, housed in a base skirt with double anchoring rings, boasts an impressive capacity of 1.7 million litres.
Constructed using duplex stainless steel SAF 2205, this project is not only a testament to ND Engineering’s expertise but also reflects the material’s superior qualities in industrial applications.
“When we were tasked with the MPPT project, we knew it was going to be challenging, especially in such a congested and active plant, but that is where our experience with duplex stainless steel shone,” Ackerman says. “We have been working with this material for many years, which demonstrated its incredible benefits in this project, allowing us to deliver on time, within budget and without compromising on safety or quality.”
Production benefits
The MPPT has significantly enhanced Sappi Ngodwana’s operational efficiency. Prior to this upgrade, the mill faced a substantial post-shutdown chemical make-up cost of R8.5 million and experienced a slow startup curve, reducing fibre line production rates until full capacity was achieved.
The new tank allows the mill to avoid these costly and timeconsuming issues, enabling smooth annual shutdowns. In addition, the MPPT has eliminated the need for the mill to lower its soda stock before maintenance shutdowns - a process that previously hampered production. Additional recovery of sodium from the effluent stream is another notable benefit, generating annual savings of R600 000 by reducing the need for salt cake makeup.
Environmental impact
The environmental advantages of the MPPT are equally impressive. Installation of the tank has significantly reduced effluent volume, sodium and sulphate during upset conditions, thereby improving the mill’s sodium effluent compliance by 0.1 tons per day. The tank has decreased sodium effluent by 0.5 tons per day, aligning with Sappi Ngodwana’s environmental sustainability goals.
One of the most crucial environmental benefits is that the tank enables statutory maintenance during annual outages without disposal of soda into the effluent stream. This feature not only supports environmental compliance but also enhances the overall sustainability of the mill’s operations.
Fabrication challenges
Constructing such a large and complex structure within a live, highly congested operating plant presented numerous challenges. ND Engineering’s field erection teams had to navigate a host of difficulties including limited space, environmental hazards like airborne contaminants and weather conditions and the logistical constraints of working in an active industrial environment.
The tank was surrounded by vessels, equipment, pumps and large structures, leaving no access to any side of the plinth. The nearest assembly area was more than 25 metres away across a road frequently used by chemical tankers, which could not be blocked for extended periods. The project also required lifting prefabricated tank sections over structures standing at least 15 metres high – all while minimising disruptions to the plant’s operations.
Innovative solutions
To overcome the challenges, ND Engineering adopted a piece-large approach: fabricating large sections of the tank off-site and assembling on-site using a tower crane with 14-ton lifting capacity. This crane was strategically positioned to avoid blocking roads and to minimise disruptions to the plant’s daily operations.
This innovative construction approach not only allowed ND Engineering to complete the project within an extremely short timeframe of 2,5 months but also enabled significant cost savings by eliminating the need for a massive main lift crane. Despite the complexity and risks associated with working in such a constrained environment, ND Engineering completed the project without a single lost-time injury, underscoring commitment to safety and excellence.
Conclusion
The MPPT project is a shining example of ND Engineering’s expertise in stainless steel fabrication and ability to deliver innovative solutions under challenging conditions. By harnessing the strength and versatility of duplex stainless steel, ND Engineering has not only improved Sappi Ngodwana’s operational efficiency and environmental footprint but also set a new standard of excellence in the industry.