Media-Bed Filtration: Painstaking Analysis of Situation Leads to a Long-Term Solution
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:
- Leaking resin or media
- Excessive pressure drop
- Filter screen clogging
- Excessive backwash requirements
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.
The Nonspecific Inquiry
A typical inquiry to the filtration product manufacturer might ask for a set of header laterals to form a distribution system for an activated carbon column or demineralizer tank. The manufacturer may be supplied with:
- Request for the lateral system
- Overall size and configuration of desired system
- Flow rate
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."
Ask the Right Questions
In order to make sure that the product being ordered for the system is suitable for the process, the plant engineer should ask, or make sure that his purchasing agent asks, questions like:
- Will the product withstand system pressures?
- Is it designed to avoid clogging?
- If it does clog, will it withstand the strain or is it likely to collapse and lose the media?
- Is it sized to retain the particles in normal use?
- Will it pass the desired flow?
- Is it designed to minimize pressure loss?
- Has corrosion been considered? Are proper alloys available and specified?
- Will the product fit the system without necessity for field modification?
- Can it be successfully backwashed, or must it be removed for cleaning?
- If it must be removed, is it easy to take out and put in again?
- What is its expected life?
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.
Uniform Flow - The "Silent" Problem
The pulp and paper industry employs large, horizontal sand filters, gravity-fed tanks and river water. Efficient, uniform collection is important because channeling leads to liquid concentration and a premature regeneration need. This is a problem not recognized by many plant operators. The result is their system is demanding excessive downtime and wasted backwash water.
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.
EDA Predicts Results
Vessel internals determine flow characteristics and therefore its ultimate performance. This has been demonstrated in many retrofit applications, where operating time has been increased fifteen to thirty percent before backwash is required.
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.
Opening size: In water treatment processes like ion exchange, the function of the screen is to keep the resin beads in place, while facilitating fluid flow through the bed. Ion exchange resins are tiny beads of a very uniform size. If the specification calls for 12/1000ths openings, it is important to maintain that size. An opening 15/1000ths is too large, allowing the resin to leak out. On the other hand, 8/1000ths is too restrictive, impeding flow and leading to clogging. Either concession will involve a noticeable performance drop.
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.
Con Ed: The Proof is in the Use
Utility stations are some of the highest-volume users of media bed filtration. For example, the East River Station of Consolidated Edison Company of New York, NY employs three, two-tank trains of ion exchange units. Each train includes a cation tank and an anion tank, and each is capable of processing up to a million pounds (approximately 120,000 gallons) of water an hour in an emergency, although 700,000 to 800,000 pounds is the usual rate.
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.
Following that outage, John Forte, the Results Supervisor of the Performance and Chemical Groups at the East River Station, had enough. Forte is responsible for monitoring the performance of the station's vital water treatment plant. During the outage, Forte and his crew of one technician and four mechanics worked virtually around the clock to get the train on line again.
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.
In today's plants, filtration products are just one component among many - but very important to plant functioning. In some circumstances, the failure of just one element can bring down an entire operation.
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.
Know Your Wedgewire
The major benefits of wedgewire are strength, and ease of cleaning by backwash, both due to the triangular cross-section of the wire.
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.