The Pulp & Paper Mill Engineer’s Guide to Advanced Scraper Strainer Technology
- Published: August 29, 2025
By Philippe Ellison, Project Manager, Acme Engineering Products
In the pulp and paper industry, mill engineers are often involved in specifying strainers, particularly in applications that require bid specs or system-level design decisions.
These industrial strainers separate unwanted suspended solids from liquids and slurries and are commonly used in whitewater applications to remove clumps of fibers and in black liquor straining before being sent to the burner. Strainers may also be used in the treatment of wastewater or to remove debris from process and cooling tower water.
In this endeavor, automatic scraper strainers offer numerous advantages over traditional choices such as backwash filters and basket strainers.
Strainer Selection and Sizing
Selecting the appropriate strainer begins with understanding the application’s process requirements, including the type and size of solids, solid loading, and what needs to be filtered.
Strainer sizing involves balancing flow rate, particle size, and solid concentration. Higher solid loading requires a larger filtration area and vessel size. As flow rate and particle concentration increase, so must the strainer’s capacity. Proper data on particle size distribution and operational conditions is critical for accurate equipment selection and sizing as well.
It is noted that pulp and paper mill engineers may specify the wrong type of strainer if they lack detailed information about the operating conditions. For example, backwash strainers are sometimes specified in applications where the solids are large, sticky, or difficult to remove – conditions that backwash systems struggle to manage effectively. In these cases, scraper strainers are the better option, as their mechanical cleaning action is specifically designed to handle challenging debris.
Oversizing backwash strainers can also reduce cleaning efficiency. In addition, no fluid processing or filtration system remains static. Treatment conditions continually change due to variable factors such as pressure, particle size, solids loading, and even the presence of sticky biologicals. A properly selected strainer must be able to handle the full range of operating conditions to ensure consistent and reliable performance.
Manual Basket Strainers
Basket strainers are manual filters used to remove large solids or debris from a fluid stream, typically in water or process piping systems. They consist of a pressure vessel housing that contains a perforated or mesh-lined basket. The basket acts as a screen to capture and retain particulates while allowing the fluid to pass through.
In continuous flow processes that cannot be shut down for cleaning purposes, duplex basket strainers with two distinct chambers that function independently are often employed. When one chamber needs cleaning, the flow is diverted to the other chamber, enabling the removal and cleaning of the first basket.
Cleaning is a messy, laborious process that involves equalizing pressure between the baskets, diverting flow to the off-line chamber, opening the cover, manually removing the clogged basket, and cleaning it before refitting the basket, ensuring the seal, and tightening the fasteners. If an operator fails to adequately clean the basket strainers, both can become clogged at the same time. For many processors, this can occur simply due to having insufficient personnel to keep basket strainers clean.
Backwash Systems
Pulp and paper mill engineers are generally most familiar with backwash systems. This often leads to their default specification, even in cases where scraper technology would offer a more effective solution.
Backwash filters remove suspended solids, sediment, and other particulate matter from water. They are designed to operate continuously with minimal manual intervention by automatically cleaning themselves through a backwashing process.
During backwashing, the drain valve opens, causing a reverse in flow across the section of the screen which is isolated by the backwash cleaning mechanisms openings. This dislodges the accumulated solids, which are then pulled into the backwash cleaning mechanism and flushed out through a drain. Once the filtration screen is clean, the system returns to normal filtration mode.
Backwash filters rely on a substantial amount of flow and constant pressure, which can compromise reliability if not always available. Industry data shows that backwash units do not operate well in backwash mode below 30 PSI. To compensate, some utilize complex, pressure-inducing tactics, but these do not always resolve the issue.
Additionally, conventional backwash units are not designed to effectively remove larger or irregularly shaped solids. Industry recommendations suggest that backwash systems are only recommended when filtration requirements are below 50 microns and solid loading is low.
Automatic Scraper Strainers
Automatic scraper strainers are a viable alternative to backwash systems in many scenarios.
Unlike backwash strainers, scraper strainers do not rely on a pressurized backwash to remove solids from the screen. Instead, blades and brushes provide more reliable cleaning under varying conditions.
One motorized unit on the market is offered with three screen types, selected based on the specific application. Reverse-formed wedge wire screens are the standard choice, valued for their durability and compatibility with brush cleaning systems. For applications requiring finer filtration, multilayer sintered metal mesh screens are recommended. In fibrous processes, perforated screens with round holes provide optimal performance.
Four blades/brushes rotate at 8 RPM, resulting in a cleaning rate of 32 strokes per minute. The scraper brushes get into wedge-wire slots and dislodge resistant particulates and solids.
Scraper strainers allow the solids to accumulate at the bottom of the vessel, where the blowdown valve will open periodically to clear them out. 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% of total flow.
If additional pressure is required to clean the screen, an inexpensive trash pump can be added to the blowdown line to assist in removing the solids from the strainer sump.
For applications with high solids loading that are prone to clogging, a macerator can be installed upstream of the automated scraper strainer to break down large solids into smaller fragments.
Pulp and paper mill engineers face increasing pressure to deliver systems that are not only high-performing and cost-effective but also reliable and low maintenance. The adoption of advanced scraper strainer technology represents a significant opportunity to meet these demands.
ABOUT THE AUTHOR
Philippe Ellison serves as Project Manager for Acme Engineering Products, a North American manufacturer of industrial self-cleaning strainers. Learn more at acmeprod.com or email This email address is being protected from spambots. You need JavaScript enabled to view it..
