Fluid Solutions Case Study
Underwater Robot Speeds Sediment Removal at Power Plant
Out-of-the-box solution removes sediment from cooling tower basin, helping nuclear plant avoid derating.
Summary
A nuclear power plant experiencing heavy sediment buildup in its cooling tower basin utilized an underwater remote-control robot provided by United Rentals to efficiently and safely remove sediment without plant shutdown or the use of chemicals, ultimately reducing outage time and boosting revenue. This innovative solution proved to be a more effective and cost-efficient alternative to traditional diver-based sediment removal.
Challenge
The traditional method of using divers to remove sediment building in the cooling tower basin of a nuclear powerplant was ineffective due to poor visibility and the sheer volume of sediment. This was impacting the plant's efficiency and requiring costly and time-consuming removal methods.
At a large East Coast nuclear power generation plant, a local river feeds the cooling tower basin. Sediment in the river regularly causes buildup in the basin, which the plant has removed every 18 months during planned outages. Sediment reduces the water’s ability to transfer heat, clogs the cooling tower’s spray nozzles and increases the need for pump and pipe maintenance.
Heavy sediment buildup
When the amount of sediment in the river rose sharply one summer, sediment in the basin increased nearly tenfold.
Previously, the plant had sent in divers armed with vacuum hoses to suck out the sediment. The approach required hot work permits and was costly and time-consuming. In this case, the divers had zero visibility and were able to remove only a fraction of the sediment. As a result, the plant had to derate, which caused a significant loss of revenue.
The operators needed a better way to solve the sediment problem.
An underwater remote-control robot
The plant asked United Rentals, its long-time rental equipment partner, to develop a safer and more efficient sediment removal system.
The project presented two major challenges, in addition to the unusual amount of sediment. The first was the volume of water. Some 550,000 gallons—roughly the amount in an Olympic-sized swimming pool—entered the basin every minute. The second was the size of the basin. At 430 feet in diameter, it was too large for hoses to reach the center.
To find a better solution, United Rentals reached out to a partner that manufactures small underwater robots designed to remove sludge from water. The electric, remote-control robot could be safely deployed without a plant shutdown, and humans would not have to enter the basin.
The robot featured a 36-inch-wide horizontal auger on the front that cuts through layers of sediment up to 3 feet deep, loosening the sediment without stirring it up and sending it back into the system. The sediment was pumped through a hose on the robot to the first of two weir tanks.
Mechanical separation avoided flocculant use
To reduce environmental and chemical reactions in the cooling tank, and to avoid the need to obtain permits, the customer wanted the water and sediment separated without using flocculants.
United Rentals designed a mechanical separation system that retained water in the first weir tank for three hours until almost all the sediment sank to the bottom. The water was then pumped into a second weir tank for additional separation. Once the sediment level reached nearly zero, the water was returned to the cooling tower basin for reuse.
A successful pilot
After thorough examination of the plan, plant managers agreed to a pilot. The rate at which the system removed sediment during the pilot—about three times faster than divers removed it—impressed plant managers.
“The robot is like a lawn mower; it makes passes back and forth all day long, grabbing sediment,” said Otto Steffers, regional product development manager at United Rentals.
After reviewing the results, the customer decided to use the process for sediment removal every 8 months.
“Since plant operations won’t be disrupted by this ongoing maintenance, the customer is now able to reduce planned outage time in the future,” said Steffers.
By thinking creatively, United Rentals was able to offer a turnkey sediment removal and filtration solution that is cheaper, safer and more effective than using divers. In addition to reducing planned outage time, it is expected to help the plant boost its revenue by allowing it to run at full capacity during the summer months.