Effective Industrial Water Screening Starts With Choices
Industries around the world use an astonishing variety of screens for countless purposes. For example, within two broad screen categories – wet screens and dry screens – are subcategories such as vibrating screens, moving screens, stationary screens, horizontal screens, inclined screens, and many more.
Each of these subcategories has its own subcategories. For example, moving screens include rotating screens, cup screens, drum screens, and traveling screens.
A power plant might use both stationary screens and traveling screens to protect pumping equipment from debris in the raw water it uses for cooling. A mining plant, on the other hand, might use interstage retention screens to retain media in carbon-in-leach circuits, while the agriculture plant might use vibrating screens to separate produce by size.
Likewise, a steel plant might use vibrating screens to ensure a plentiful supply of sized coke for its furnaces in addition to raw-water intake screens to protect the pumps it uses in its cooling processes.
The one thing that remains constant across all industries and all types of screens is this: To be effective, each screen must perfectly align with its purpose and the conditions under which it must function. This form-and-function alignment is where choice begins.
Common Industrial Screens
The following sampling provides a broad overview of screens that are widely used for water treatment in the power, precious metals recovery, petrochemical, and steel manufacturing industries.
A type of stationary raw-water intake screen, the bar screen is a large, stationary device that uses regularly spaced metal bars to block waterborne debris while allowing water to pass freely. Industries primarily use them to protect downstream equipment – including other types of screens – from large objects that would otherwise damage it. The objects in question can encompass everything from logs to leaves to aquatic vegetation, depending on the water source and equipment location.
When water pressure pins debris against bar screens, plants must remove and dispose of the debris by either manual or mechanical means. As a result, some bar screen vendors also offer mechanical trash rakes that remove debris.
For example, WTR® Engineering’s Talon Rake® is a trash rake that operates on a monorail. Plant staff can operate it manually as needed or can set it to automatically operate at intervals. The trash rake captures pinned debris, then transports it along the monorail and deposits it into a waste receptacle for disposal.
Bar screens and trash rakes are useful for power plants, manufacturing plants such as steel mills, petrochemical operations, land drainage pumping stations, and virtually any other industry that uses large volumes of raw water.
Cup Screens and Drum Screens
Like bar screens, cup and drum screens are excellent choices for industries that need to protect downstream equipment from water-born debris. However, unlike bar screens, these screens are capable of preventing smaller objects – 2 to 10 millimeters (mm) – from clogging pumps, condensers, and other equipment.
Cup screens (single entry dual exit) are usually composed of mesh screening material that is sealed to one end of a circular frame. Water flows into this mesh “cup” through the center of the open end. The downstream end is closed, which prevents anything that is larger than the screen’s openings from flowing through.
Drum screens (dual entry single exit) are also composed of mesh screening material on a circular frame. However, these screens are open on both sides to accept a dual-flow pattern. As with cup screens, the influent and effluent are completely separate so nothing larger than the mesh aperture can pass downstream.
Both cup screens and drum screens prevent carryover debris from entering waters that house sensitive equipment, making them ideal choices for power plants and manufacturing operations.
Linear screens are popular in the gold recovery industry for their ability to remove wood fiber and tramp iron from ore that is crushed and ready for the industry’s carbon-in-pulp (CIP) process. These screens, including WesTech’s Linear Screen, use a linear belt of polyester screen cloth that allows slurry to pass through.
Also called carbon retention screens and media retention screens, interstage screens are used primarily in carbon-in-leach (CIL), resin-in-leach (RIL), carbon-in-pulp (CIP), and resin-in-pulp (RIP) gold recovery circuits. These cylindrical screens include a rotating cage that uses water pressure to clean the screen and an impeller that draws slurry up through the center of the cylinder. In this case, the screen’s function is to prevent gold-laden carbon or resin from leaving the tank from which it is to be collected.
The screen baskets that prevent the gold-collection media from flowing to the next tank in the circuit are either metal mesh or, like WesTech’s Interstage Screens, closely spaced metal strands that are reinforced with backer rods.
Also known as static screens, stationary screens provide yet another means of keeping debris (windblown trash, cooling tower elements, common aquatic vegetation, and so forth) from jeopardizing downstream equipment in industrial applications, typically in closed loop systems.
As the word stationary suggests, these screens are non-mechanical and sit idle in the flow. Generally housed in a steel framework, many stationary screens are composed of metal mesh screening material.
Most stationary screens require manual debris removal. WTR’s Stationary Screens can be installed within a framework that accommodates a primary and secondary screen, allowing organizations to lift one screen out of the flow for cleaning while the other continues to protect equipment.
Like cup and drum screens, traveling screens move continuously to both capture debris and deposit it in a container for disposal. Traveling screens are widely used in industries that require continuous, reliable protection for downstream equipment – including the power, steel manufacturing, and petroleum industries.
WTR’s Traveling Water Screens can accommodate several flow patterns, including thru flow (TF), dual flow conversion (DFC), dual flow (DF), and center flow (CF). WTR can configure all flow patterns of traveling screens to comply with the Environmental Protection Agency’s (EPA’s) 316(b) requirements for fish recovery.
Each screen type can accommodate a variety of screening materials, which are generally woven into a mesh but can also be implemented in the form of metal bars, a perforated plate, or a series of perforated pipes.
While bar, cup, drum, linear, interstage retention, stationary, and traveling screens are often constructed of either carbon or stainless-steel mesh, depending on environmental conditions, industries can also install these types of screens with meshes of copper-nickel alloy, Monel (an alloy of nickel), polyester, and nylon.
Copper-Nickel and Monel
Alloys of copper and nickel are highly resistant to the corrosive effects of seawater and acids. Therefore, industries that rely on raw water from the sea – including the desalination, power generation, and offshore oil industries – are more likely to use traveling, cup, drum, and stationary screens that are composed of these materials for corrosion resistance. Copper-nickel is also used for bio-fouling resistance.
Cost-effective and strong, meshes of uncoated carbon steel are ideal for industrial applications that do not involve corrosive materials such as acid and seawater. Industries that use carbon steel mesh screens include the mining industry (generally for sizing ore).
Uncoated stainless steel is strong, durable, and less likely to rust than uncoated carbon steel. While it is costlier than carbon-steel mesh, it is also less vulnerable to corrosive environments. This makes stainless steel a popular choice for industries that use corrosive raw water (seawater and water from brackish sources, for example) such as the power-generation, manufacturing, and petrochemical industries. Stainless steel is commonly used in cup, drum, stationary, and traveling screens.
Coated Carbon and Stainless Steel
Carbon and stainless steel that is coated with corrosive-resistant materials such as zinc (called galvanized steel), epoxy, and polyvinyl chloride (PVC) are more costly than uncoated carbon and stainless steel but are still less costly than Monel and copper-nickel screens. This makes coated steel screening an attractive alternative for industries that use raw water from the sea or that operate in other types of caustic environments. This media type is common in cup, drum, stationary, and traveling screens.
Durable and heat resistant, screens made of polyester mesh are also resistant to acid and alkali substances. Although polyester media are sometimes used in raw-water intake screens, their high tensile strength and accurately produced mesh patterns make them a good fit for the linear screens used in precious-metal recovery, such as WesTech’s Linear Screen, where the media’s ability to be woven to tight tolerances is also a plus.
Much like polyester media, nylon media are flexible and abrasion and corrosion resistant. While some raw-water intake screens use nylon media, its affinity for tightly woven patterns makes it particularly attractive to industries that need screens for oil, paint, and powder.
While selecting the correct form and media for a screen is important, if the size of the screen’s openings isn’t right, the screen cannot function as intended.
For example, say a gold recovery operation already uses a series of interstage screens that are sized for 4-millimeter (mm) carbon pellets. If it learns that it can recover more gold with a resin-in-leach circuit and switches to resin media that are slightly smaller and smoother, the existing screens will be subject to pinning and blinding. Both of these conditions impede flow and therefore reduce the rate of gold production. Worn resin media might also simply flow through the openings, which would reduce total gold recovery as well.
As another example, suppose that the raw water a coastal power plant uses for cooling contains sea grasses that can slip through the intake screen. In this case, the grasses will invariably clog the plant’s pumps, condensers, and heat exchangers. As a result, the plant will be forced to shut down its operation for cleaning, repairs, or both.
The Value of Expert Advice
Because each part of the screening equation requires careful consideration, a little help from a screening expert can add significantly to the screen’s success. Many reputable screen vendors learn everything they can about a screen’s intended use before they even offer a bid. They also stand ready to make recommendations based on the answers to their questions.
For example, WTR asks potential customers questions about everything from their water sources to the screening media they have in mind.
If a customer calls and starts chatting about a screen composed of a particular material and then casually mentions that the water contains chlorides, we let him know that he actually needs a different material, explains Trent Gathright, Director of Business Development at WTR Engineering, a wholly owned subsidiary of WesTech Engineering.
Such expert advice can save organizations money in the short and long term. For example, a WesTech engineer was able to save a mining organization seeking new resin-in-leach interstage screens hundreds of thousands of dollars simply by showing the organization that it could retrofit its existing interstage screens with a new pumping mechanism and new screen media with smaller openings.
Sometimes the optimal screening solution comes with a higher price tag in the short term while in the long term, it saves not only in longevity but also in reduced maintenance costs.
Look to the long term, not just the lowest capital dollar, and keep maintenance in mind, advises Gathright.
A Review of Water Intake Screening Options for Coastal Water Users With Recommendations for Ocean Thermal Energy Conversion (OTEC) Plants, by David L. Thomas, published by Argonne National Laboratory for U.S. Department of Energy Division of Central Solar Technology https://www.osti.gov/servlets/purl/6204853 (PDF)