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Automatic Self-Cleaning Scraper Strainers Filter Smallest Particles to Largest Debris—
Unlike backwash systems, scraper strainers reliably resist clogging and fouling when faced with micron-sized particles, oversized solids, and high solids concentration
Robert Presser
One key to providing reliable energy to the Kansas City area was using cooling water strainers that would not foul during the Missouri River’s high tide.
When a power plant was supplying energy as a backup for a power provider in the city, the plant used river water for cooling, utilizing large basket strainers. The plant is now part of an energy solution partner with multiple district energy networks across the United States, and sought more-efficient operation and maintenance from a downtown steam loop that produced chilled water.
District energy uses a centrally located facility, or facilities, to generate thermal energy—heat, hot water, or chilled water—for a number of nearby buildings. These resources are transported through underground pipes to meet the needs of communities, cities, and campuses, such as colleges, hospitals, airports, and office parks.
“To provide condensing water for steam production, as well as cooling water to the chillers and condensers, water is taken from the river and put into a once-through cooling system,” says Keith Williams, P.E., who was involved with the project and is a manufacturer’s representative for a company that represents North American manufacturers of heating, cooling, and hydronic equipment.
According to Williams, the river’s fast flow, along with the high tide, complicated the straining of river water for the plant.
Williams notes that the power plant’s
Automatic, self-cleaning scraper strainers can filter smallest particles to largest debris and resist clogging and fouling from micron-sized particles, oversized solids, and high solids concentrations.
Firms can custom-manufacture pressure vessels to fit within the existing piping arrangement, which minimizes scraper strainer installation costs. Plugging and fouling from large solids and small particulates can cause unscheduled downtime, excessive maintenance, and costly premature replacement, but automatic scraper strainers can remove waste from raw water before the debris enters heat exchangers and cooling systems.
previous basket strainers required excessive maintenance. “Workers had to clean the manual strainers every shift, three times a day, during high tide, and it was a very dirty, disgusting job that no one wanted to do.”
Backwash strainers are often used, but are not ideal for removing large solids from perforated screen elements. The backwash arm must be close to the screen to function properly, which prevents passing larger particles.
When solids are bigger than the gap between the screen and backwash arm, they remain in the vessel and must be removed manually. Biological film can also adhere to the screen, so frequent cleaning is usually required by maintenance crews.
In this case, a basket strainer was used, but was continuously clogged during the river’s high tide.
Williams was originally contacted as part of a plan to install new cooling towers. He instead advised that, for this application, a more-cost-effective solution was to install more-efficient strainers.
“I advised using an automatic scraper strainer that is capable of very fine straining, while still passing very large debris,” he says. “It can strain to the micron level, yet pass debris that’s surprisingly large.”
The automatic scraper strainer is a motorized unit designed to continually remove very large and very small suspended solids from cooling water. A spring-loaded blade and brush system provide cleaning, managed by a fully automatic control system.
Four scraper brushes rotate at eight revolutions per minute, cleaning 32 times per minute. The scraper brushes get into wedgewire slots and dislodge resistant particulates and solids. This enables scraper strainers to resist clogging and fouling when faced with large and high solids concentration. It ensures a complete cleaning and is very effective against biofouling.
Blowdown occurs only at the end of the intermittent scraping cycle when a valve is opened for a few seconds to remove solids from the collector area. Liquid loss is well below 1 percent of total flow.
According to Williams, the scraper basket also allows the strainer to bypass extremely large particles and debris. “There are very few manufacturers that can pass such large particles, while straining out the fine particles.”
The automatic scraper strainer had to be customized to install on a 30-inch inlet and an outlet with a 12-inch blowdown line for solids removal. Although industrial facilities with existing systems may be hesitant to replace under-performing backwash strainers due to the misperception that the installation modifications can be costly, some firms can custom-manufacture pressure vessels to fit within the existing piping arrangement, which minimizes installation costs. They can even deliver units with backwash arms.
“The line they wanted to install it in was at a 15-degree angle in a very tight space with a very short line, so we made a strainer where the inlet and outlet angled at 15 degrees,” says Williams. “This enabled them to just cut the pipe, install the flanges and the strainer, and be done. It was a custom-made vessel.”
The facility’s staff was impressed at how much the automatic scraper strainer simplified the maintenance of the strainer for cooling, despite the size range of river water debris it has to catch. “Now, they only open it for annual inspection and maintenance, and no one has to manually clean it anymore,” he concludes.
Robert Presser is vice president of Acme Engineering Prod. Inc. in Mooers, N.Y. For more information go to acmeprod.com. S
