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Many industries utilize media-bed filtration. This use may take the form of ion exchange for water softening/demineralization, sand filtration, activated carbon, and other fluid treatment processes. However, whether the application is in a water utility, a power utility, pulp and paper, steel, or any of the process industries, chemicals or pharmaceuticals, the problems tend to be very similar.
Time and again, the same common complaints arise in the media bed filtration process:
These complaints can be relieved by the use of properly designed and specified equipment and equipment internals (arguably the most important investment in media bed filtration). Unfortunately, most new equipment and retrofit inquiries do not include enough hard and specific data to ensure the proper product selection.
This data can certainly be used to produce a set of header laterals. In fact, a great variety of header laterals might be produced, all conforming to these rather nonspecific specifications. The question is: Which design would do the best job for the specifier?
In the ideal situation, the manufacturer's application engineers will return to their customers with a set of questions designed to elicit more specific information about the customers' overall requirements. For example, they may want to discuss ramifications of the filtration process to make certain the process is fully understood and the problem is pinned down.
These engineers may also suggest specific upgrades. Wedgewire is often recommended instead of wire-cloth mesh, for its characteristics of strength and cleaning ease. Wedgewire can also be installed over drilled pipe. This design offers improved support and better flow into the lateral, ensuring uniform distribution so that all media is used throughout the bed. Of course, this design costs more than an off the shelf "quick-fix."
Obviously, this is a lot to consider. However, considerations like these are all part of obtaining an optimum product for the desired application. In fact, it is precisely because such questions are addressed, that a retrofit may often be a significant improvement over the original design.
In any process where a fluid is being distributed through media, it is important to retain uniform cross-sectional velocity - forcing the fluid to flow uniformly and contact all media in the bed. Channeling is always the enemy. If the fluid is allowed to run into channels, the media will be exhausted in the neighborhood of the channels while it is still fresh and unused (essentially wasted) in other areas. Channeling leads to the premature need for regeneration.
However, uniform flow and a channeling tendency are difficult to predict on the basis of experience alone. Technology-oriented manufacturers use electronic design automation (EDA) to supply the answers. For example, finite element analysis (FEA) is used to computer-create the specific working environment of any existing or projected filtration unit.
The FEA software package allows application engineers, working with customer-supplied process data like flow rates and pressures, to perform a full analysis of the specific situation and/or problems, and suggest optimal solutions.
Materials of construction: The typical liquid "seen" by a filtration system is water. It can be wastewater or boiler feed water, river water, brackish water, sea water or ultrapure water. In the process industries, filtration may also involve a variety of chemicals. Fortunately, most of these fluids can be handled by elements made of the appropriate stainless steel, nickel alloy, or titanium. The customer should make sure the supplier can provide a material that functions well with the specific system fluid used.
PVC internals: Some older filtration systems were supplied with PVC (polyvinyl chloride plastic) laterals slotted to retain the media. PVC offers good corrosion resistance but poor structural stability. Flow velocity tends to cause the plastic to flex, altering slot size and configuration and resulting in pressure drop, media loss - or sometimes both. Nevertheless, mesh-wrapped PVC rods can sometimes be applied successfully.
Wire cloth vs. wedgewire: Wire-cloth screening is often used with good results in filtration applications, but does not provide the reliability of wedgewire. Since the wires that make up the cloth are round in cross-section, there is always a tendency for it to clog. Triangular cross-section wedgewire is easier to backwash. In addition, water pressures, system stresses and careless handling can cause abrasions or tears in the wire cloth. For these reasons, many engineers prefer the strength, clog resistance, ease of backwashing and dimensional stability of wedgewire. (See "Know Your Wedgewire.")
These considerations make it clear that the manufacturing of filter internals is as important as the design of the equipment itself.
The trains are used two at a time, with the third on standby or undergoing regeneration. Each of the Con Ed tanks is served by thirty-four inlet and outlet laterals. Until recently, these laterals were wrapped with wire mesh screen to retain the resin beads and pieces of broken resin.
In the extremely cold winter of '93-94, the station had a forced outage on a cation tank, caused by screen erosion. This allowed resin to leak into the system. The station was provided with spare laterals, so the shutdown lasted only about 24 hours. However, this was still a painfully long time to be shut down during a period of heavy operation.
With the concurrence of Mario Bulzacchelli of Con Ed's Main Chemical Group, Forte decided to test wrapped laterals. Made of wires with triangular-shaped profiles ("wedges"), wrapped around and welded to long, parallel support rods, wedge-wire screens are strong and clog-resistant. Abrasion, which damages mesh screens, has little effect on the wedge-shaped wires.
These laterals were tested in one ion exchange tank at the East River Station. All thirty-four mesh-wrapped laterals were replaced with Wedge-Flow laterals with a 0.010 slot opening and a 0.060 in. wrap.
The test laterals were installed in June 1994. So far, they have come successfully through two hard winters. In fact, operators now find they can increase throughput to 1.2 million pounds of water per hour in an emergency situation, with no damage to the laterals. The result is increased emergency capacity combined with peace of mind for the operators and supervisors. As operation permits, all laterals in the ion exchange vessels are being refitted with the Wedge-Flow laterals.
For long-term, reliable operation, it is important to obtain the optimum product. It is also wise to obtain that product from a supplier who can offer engineering and application assistance, including the latest computer aided design analysis, to ensure the best possible match with the customer's own conditions of service.
Wedgewire was originally developed for use in wells. It was designed to let water come through and keep sand out, and worked well in that application. However, the filtration needs of modern industry are much more critical.
Modern Wedge-Flow material supplied by LEEM Filtration Products, Inc. of Ramsey, NJ, is an improvement on conventional wedgewire or wellscreen. It is produced with attention to proper depth of weld penetration, uniform slot size, and selection of corrosion-resistant alloy to match the conditions of service. The filters are available in a range from 20 micron up to about 3/8 in.
Wedge-Flow material is composed of wire, helically wound over a series of longitudinal rods, and welded at every intersection. A panel-type Wedge-Flow material is also available.