Bio-fouling occurs when organisms attach themselves to submerged structures in the aquatic environment, resulting in increased drag and corrosion. Within the Department of Defense, bio-fouling is a problem on components of U.S. Navy ships and submarines (e.g., heat exchangers, condensers, and seawater piping systems) and for infrastructure of the U.S. Army Corps of Engineers (e.g., dams, locks, and hydroelectric plants). Bio-fouling adversely affects system performance by decreasing heat transfer and blocking the flow of water. Chlorination is an effective antibiofouling method, but its negative environmental impacts necessitate an alternative method.
The objective of this project was to demonstrate the feasibility of using pressure pulses from an acoustic sparker to control microfouling (i.e., slime) in heat exchanger pipes.
The project used a pulsed electric discharge between two sparker electrodes to generate a pressure pulse for control of bio-fouling. One sparker was placed in front of the pipe (i.e., "wet-well") and another was placed "in-line" to prevent growth of microfouling inside the wall of the heat exchanger pipes. The sparkers were controlled remotely and powered through cables. The sparker was self-contained and did not touch the pipe. As such, it could be used with any pipe material. The pulsed electrical discharge vaporized a small volume of the surrounding water, producing a strong pressure pulse. The vaporized water formed a high-pressure gas cavity that expanded and collapsed, producing additional pressure pulses. After the first electrically driven pressure peak, two additional peaks from cavity collapses occurred in a short period. Each peak was on the order of 20 microseconds long, with a maximum pressure of about 100 atmospheres at a location of several inches from the sparker. The propagation of these high peak pressures, bounced along the inside surface of the pipe, prevented biological organisms from attaching or growing.
This project successfully demonstrated that sparker pressure pulses can control slime growth on the inside of heat exchanger pipes. Two sparker units were built and tested in the laboratory and then used in feasibility tests of bio-fouling control at the Naval Surface Warfare Center-Dahlgren Division (NSWCDD) Corrosion Test Facility. Each sparker was integrated with a 20 foot long and 5/8 inch diameter titanium pipe, along with three control pipes, and operated for approximately one month. Ocean water flowed through the pipes during the tests and slime growth was monitored in one pipe. At the end of the tests, the pipes were sectioned and the biomass accumulation was determined for all pipes. Results of the field test demonstrated the feasibility of controlling slime with sparker pressure pulses. The in-line sparker was more effective than the wet-well in eliminating bio-fouling. However, the control pipes showed a large variability in biomass that precluded quantitative assessment. Additional laboratory tests are recommended to quantify sparker slime control and determine sparker operational parameters that are most effective. This SEED project was completed in FY 2002.
Appropriate implementation of pulsed acoustic sparkers can eliminate use of harmful chemicals such as chlorine for microfouling control. Furthermore, a sparker biocontroller would provide an economically attractive alternative to current environmentally deleterious practices.