Environmentally-responsible tank cleaning usedat Delmarva
Delmarva Power & Light Co. officials wanted a more environmentally responsible alternative to conventional tank cleaning when they faced the need to remove and dispose of sludge from a No. 6 oil storage tank at its Vienna, Md., station. The 23-year old, cone-roof tank contained approximately 6,300 barrels of hydrocarbon material. Conventional tank cleaning is a labor-intensive task that recovers little or no oil. Sludge buildup in tanks can be several feet deep and usually consists of a mixture of hydrocarbons, water, sand, rust, ash and other solid contaminants. The sludge`s consistency varies from a free-flowing liquid to a semi-solid, unpumpable material. The sludge restricts usable tank capacity and wastes oil, trapping it in solution. It can lead to unscheduled outages and increased maintenance costs from downstream contamination problems, such as plugged strainers on fouled combustion turbines.
In conventional tank cleaning projects, workers enter the tanks with shovels or heavy equipment, and material from the tank is hauled to a landfill. Such projects often take several months, and can require cutting large holes in the tank`s walls to allow cleaning. Delmarva chose Landry Service Company Inc. (Lansco) to perform the project. The company`s tank cleaning process employs a patented technique to separate oil from solid materials, yielding minimal waste for disposal and recovering up to 98 percent of the fuel oil.
“This process not only cleans tanks effectively, but also enables us to minimize waste and recycle virtually all of the fuel oil in the tank,” said Tom Evans, Delmarva project engineer. Lansco`s proprietary tank entry devices (TED) were used to increase efficiency and improve the safety of the sludge removal process. A remote-controlled TED robot blends hydrocarbon diluent with sludge deposits, then pumps the liquefied material from the tank for further processing.
TED robots are operated by video remote control, minimizing the personnel time spent in the tank. Liquefied sludge pumped from the tank runs through screeners to remove debris and large solids. Remaining solids are then separated from the oil by a proprietary centrifugation technique, removing particles as small as five microns. Recovered oil is clarified and stored for use by the power plant. Lansco completed the project in just more than a month, with the tank certified clean and gas-free. A total of 6,360 barrels of sludge was processed, with 6,355 barrels of oil reclaimed. Dry solids of approximately 1 1/2 cubic yards were discarded.