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6.2.2.Mechanical systems
Cutters and scrapers are used to remove difficult foulants. Cutters come in a variety of
configurations and most are equipped with flexible shafts or universal joint shafts. Equipment
being cleanedmust be taken out of service in order to operate the cutter. The cutters can damage
and gouge the pipes if used aggressively or incorrectly. Cutters are also significantly slower than
other o cleaners (Rice et al. 1993).
Scrapersmatch the pipe diameter, and operate by using spring loaded scraping edges to clean the
pipes. Equipment being cleanedmust be taken out of service in order to operate the scraper. The
scrapers can damage and gouge the pipes if used aggressively. If the foulant buildup is thick and
hard, high-pressurewater lancing equipment also needs to be used (Rice et al. 1993).
6.3. Debris Filters
Inline strainers/debris filters installed between thewater source and the condenser inlet waterbox
or other heat exchangers have been used to reduce debris. Debris filters removematerial that has
passed through preliminary and secondary screening technologies or has originated within the
piping (Stone and Webster Engineering Corp. 1992). Filter types available in the US include
turbulence, waterbox, backwash, and inline debris separator. Most of the filters used in the US
are the backwash design and intended for automatic operation.
In the turbulence filters, water and debris enter the annulus between the cylindrical screen filter
basket and screen housing. During flushing mode, tangential flow created by inlet valve action
removes the debris from the filter element. The debris is flushed into a discharge pipe. This type
has some difficulty flushing off heavy debris influxes and stringy debris (Stone and Webster
EngineeringCorp. 1992).
The backwash filter uses four separate basket sections that sequentially close off cooling water
flow and backflush during flushingmode. Debris is removed from the baskets by flushing clean
(filtered) water through and carrying it to the discharge pipe (Stone and Webster Engineering
Corp. 1992). The strainer screen rotates a section of the screen to a position where a stationary
suction scoop pulls debris off the strainer mesh. Strainers may not improve operating water
levels for the circulating water pumps and may not perform adequately during periodic algal
loading events. These filters work well in areas where biofouling is occurring in the piping
downstream of the circulating water pumps (Perkins et al. 2009). More research is needed to
reduce head loss associatedwith these filters.
Waterbox and inline debris separators use conical strainers with external rotating jet spray at the
cleanwater side. The jet spray transfers debris from the strainer surface to the cone tip, cleaning
the filter surface. Periodically, debris is flushed through the debris dischargemanifold.
Filters have some disadvantages. System head (pressure) loss increases approximately 1.5 ft.
with clean filters and 2 ft. when the filters are 30% fouled (water velocity 9 ft. s
-1
). When the
filters are retrofitted to an existing system the system pressuremust be increased thus increasing
station pumping costs. If the intake pipe cannot be adjusted to compensate for the pressure drop,
the unit is not able to run efficiently and the cooling water flow drops. These filters work best
