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7/1/1998 | 17 MINUTE READ

Choosing Pumps and Filters

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Basic considerations when selecting pumps and filters...


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Platers and finishers have special requirements compared to the average user of pump and filtration equipment. The solutions are toxic, precious, expensive and corrosive. A certain amount of thought goes into the specification of each component.

Each person in the company has concerns and requirements when selecting pumps and filters. However, each may have a different opinion about the application and equipment. It is important to consider all of the concerns.

Facility manager's concerns.

  • What are the utility requirements, such as the amp draw of the mo-tors, voltage requirements, power consumption?
  • Shop air requirements at scfm or psi?
  • Dimensions of the equipment or a footprint?

Maintenance manager's concerns.

  • What is required for proper installation?
  • In addition to items purchased, what components, such as pipe, fittings, valves, electrical components, switches, etc. are needed?
  • What type of pump is best for the particular application?
  • Will the pumps be mounted inside or outside the tank?

Questions about in-tank vertical pumps that need to be addressed.

  • How will chemicals and fumes affect the motors?
  • Will the motor's coating withstand the harsh environment without peeling and contaminating the solution?
  • Will the motor bearings hold up? Are the bearings sealed or unsealed and what service is required for them?
  • What is the service factor of the motors?
  • Are they insulated for chemical duty?
  • Will the pumps introduce air into the solution?
  • Are drip shields available for the motors?
  • Are all wet-end components compatible with each constituent of the chemical being pumped?
  • Can the pump run dry without damage?
  • In electroless plating where the metal tends to plate out and deposit on plastic, can the wet-end components of the pump withstand the effects of stripping solution?
  • What temperature range will the pump withstand without damage?
  • What are the psi to temperature ratios the pump will operate under?
  • Will the pump run against a closed discharge without damage? How long? Will the internal components warp under heat?

Questions to ask when considering a vertical pump mounted outside the tank.

  • Will the pumped liquid be kept in the pump column or will it pump up the column when under a closed discharge?
  • Does the pump have a port seal at the mounting plate to protect the motor bearing from corrosive fumes?
  • What spare parts should be kept in inventory?
  • Are there any parts susceptible to wear, and what preventive maintenance is needed to keep the pump running at its optimum?
  • When repairs are necessary, can the pump be removed and repaired at the shop, or does it need to be shipped back to the factory?
  • Are there factory authorized dealers in the area?
  • What are the return authorization procedures?
  • How long is the pump warranty, and what is covered?
  • Are there any special tools needed for disassembly and repair?
  • Does the pump have a parts list and operating instructions?

In addition to most of the preceding vertical pump concerns, there are more concerns relating to out-of-tank pumps.

The horizontal pump may be a magnetic-driven sealless centrifugal pump, a direct-drive single- or double-seal centrifugal pump, a self-priming centrifugal pump, an air-diaphragm pump or any of a number of other types.

Each of these pumps presents different concerns for the maintenance manager in addition to the concerns presented by the in-tank pumps. The list sometimes seems endless, but each concern is legitimate and must be answered.

  • What are the Net Positive Suction Heat (NPSH) factors of each pump and application involved?
  • Is a "flooded suction" to the pump available? If so, is the liquid being pumped or introduced to the suction casing by gravity? With flooded suction available, priming is unnecessary. If not available, either manual priming would be necessary or a priming chamber should be considered.
  • Can the pump handle abrasives or be run dry? For how long?

The magnetic-driven sealless pump may be considered if the solution contains precious metals or has a low specific gravity (1.4 or lower) or the impeller was trimmed to handle the specific gravity. Magnetic-driven pumps totally isolate the process solution. Since they do not have mechanical seals, packing rings or shaft lip seals, they are referred to as sealless or leakproof. This added measure of safety could reduce environmental concerns and help the user stay within EPA regulations. However, magnetic-driven pumps should never be used with electroless plating solutions or cleaners containing ferrous metal fines. The particles will adhere to the magnet and act like a grinding wheel, eventually destroying the pump.

The mechanical-sealed pump with a single mechanical seal might be considered if the solution is not extremely toxic or contaminated with abrasives, and where slight leakage due to wear or failure of the seal would not present a problem. If temperatures greater than 140F are anticipated, or the solution contains abrasives, a double water flushed seal should be considered to cool and flush the abrasives off the process seal to prevent premature failure.

Pumps with priming chambers are effective when piping from the process tank is plumbed over the side of the tank, and gravity does not introduce liquid to the suction of the pump. If the pump were to be mounted higher than the liquid level in the tank, caution would need to be exercised and the NPSH would need to be calculated and applied. Pumps with priming chambers require priming only at initial startup.

Air-diaphragm pumps are usually the choice for waste treatment to pump heavy or abrasive sludges. These pumps may be used whenever NPSH calculations reveal a shortcoming in centrifugal pumps. With air, the maintenance manager's additional concerns might be as follows.

  • Is sufficient shop air available to operate the pumps, or will an additional compressor be required?
  • Would the pumps need oil lubrication for the ball check valves? Are the oil lubrication components standard with the pump, or do they need to be purchased separately?
  • What noise levels are expected by the pulsation of the pump? Is a muffler included?
  • Are pulsation dampeners needed on either the pump inlet or discharge plumbing to reduce vibration? Are they included with the pump?

Positive displacement pumps, such as peristaltic, gear or metering pumps, involved in most of the prior mentioned considerations and the addition of bypass pressure-relief valves may be required on the pump discharge.

Process engineer or chemist's concerns.

  • Will the chemistry or operation of the process be changed by the operation of the pump, filtration or agitation system? If so, in what ways? Will filtering particles from the solution result in a smoother deposit?
  • Will the pumps introduce air into the solution that may adhere to the flat surfaces of parts?
  • Will increased solution flow give better throwing power in low-current-density areas?
  • Can plating time be reduced?
  • Will stratification in any area of the process tank be reduced and particles swept off the tank bottom by the agitation?
  • Will more or less brightener or chemicals be needed because of the increased agitation?
  • If anode bags are used or curtains installed in front of the anodes, will there be a detrimental effect on the finish due to particles being forced by the pump agitation through the bag and into the solution?
  • Will the filtration rate be sufficient to remove the particles before they cause roughness on the plated parts?

A turnover rate of 10 to 20 times per hour may be needed to achieve the particle removal speed necessary to prevent this condition from occurring.

The plater's concerns.

  • Are the pumps, filters, plumbing, valves, agitation eductors or any of the other components such as hoses, etc., in the way?
  • The anodes must not be shaded with added equipment in front of them that decreases the exchange of positive/negative ions interacting in the bath.
  • If internal or external auxiliary anodes are needed on occasion, there must be room for immediate insertion and removal without obstruction.
  • Would the pump suction or discharge interfere with plating?

The shop owner, CEO or general manager's concerns are the bottom line and the greatest return on investment. Concerns may be the following.

  • Can we get along without it? If not, how did we get along without it until now?
  • What is the justification for adding or replacing the equipment?
  • How much will the installation or the additional and/or replacement equipment cost?
  • How long is the pay back?

Some of the previously cited decision-makers with their various concerns may also have a staff than can influence the decisions. Every member of the staff may have questions about the application. Therefore, whether the selection of the product begins with shop maintenance workers or the CEO, the majority of these concerns need be addressed. Satisfying the needs of the various personnel before selecting the system helps avoid problems.

Filtration media. After the pumps have been specified and turnover agreed upon, filtration media should be considered, if necessary. There are several effective filtration media available that will remove particulates from 100 microns down to submicron levels. The degree of automation required for the use of the equipment should be considered as well.

Media choice is left to the user. However, based on successful installations and past experience, the supplier will usually offer alternative methodologies such as depth-wound filter cartridges, flat paper or cellulose discs, horizontal or vertical plates, bags, cleanable sleeves, disposable fabric or backwashable permanent media. Each media has its pros and cons.

String-wound depth cartridge is the choice of filtration for the average plater because of its simplicity, high-solids-holding capacity and wide range of porosities available to remove progressively smaller particles. A 10-inch by two-and-a-half-inch diameter cartridge is equal to three-and-a-half sq ft of surface media.

For instance, when filtering iron from nickel, acid zinc or cadmium solutions, a filter with dimensions previously described and 15 micron retention will easily hold eight ounces of iron before loading. Sized at one cartridge per 50 gal of plating solution with a suitable pump to achieve an initial solution turnover rate of twice per hour, eight to 12 weeks between cartridge changes is not uncommon. For just a small extra capital investment, the filter chamber size could be doubled and two ten-inch filters per 50 gal of solution could provide 16 to 24 weeks between filter changes.

If mainly iron is filtered, the cartridges can be backwashed or soaked in a dilute acid solution to redissolve the iron. Also, an alkaline cleaner solution could be used to remove some oil and soil buildup in the filter. They could then be carefully rinsed and neutralized for several reuses prior to landfill disposal. If a spare change of filters is kept in stock, one set can be on-line while the other is soaked for reuse. Typically, two to four cartridges of 50 to 100 micron retention per 100 gal work best on cleaners.

One of the most common objections to using string-wound depth filters is that they are too bulky for landfill compared to disc bags. However, whenever any bulky material is sent to landfill for disposal, it is usually processed through a shredder or compactor first. Because of this, depth filters are no more of a problem to dispose of than any other media. Incineration may be considered in the future because polypropylene supports combustion and used cartridges can be used to reduce fuel costs.

Filter cartridges are also available in other configurations such as pleated material, melt-bonded poly-propylene free of organic sizing agents and a number of others, including membranes for electronic grade use. These membranes will filter down to 0.1 micron nominal or absolute, if necessary.

Carbon impregnated fibers as well as granulated carbon filter cartridges are also available in the same size configuration and may be mixed selectively with the other filter cartridges in the filter chamber to achieve carbon adsorption of unwanted organics. Where heavy dirt loads are encountered, it is recommended that the solution be filtered first and then carbon treated in a separate chamber downstream on a bypass.

Disc filter sets, composed of paper, polypropylene or cellulose fiber, are another type of filter media used in industry. The discs may be used alone with paper, or precoated with filter aid to achieve a faster and finer micron retention of the particulate filtered. The downside to discs is their low-solids-holding capacity. They need to be cleaned or serviced often, compared to depth-wound string filters. In heavy particulate applications such as acid zinc or cadmium plating with high iron content, daily cleaning is common, especially on barrel lines.

Wheel platers are the connoisseurs of disc filtration systems. They like them because of their rapid filtration of fine particulates. They buff and polish between steps in many shops, so introduction of buffing compound into the plating bath is common. However, since most buffing compound contains animal fat and greases, carbon needs to be used on the precoat of the discs to remove this organic contamination. When using powdered carbon on the discs, the brighteners in the acid copper and bright nickel baths need to be replenished continuously. If bulk granulated carbon is used on a by-pass in a separate chamber downstream from the filter discs, brightener depletion is much slower.

Bag filters are used in electroless nickel and copper plating because autocatalytic baths have a tendency to plate out on any surface they contact. Bags are the easiest filters to use in these applications, since they are a surface-type media. When they are plated with metal, the operator can see the plate-out and service the bags. When discs or depth filters are used in an enclosed chamber, the operator cannot see the plate-out. Plate- out of the entire bath could occur if the problem is not detected and corrected immediately.

Also, bags are a favorite where high loading of coarse solids occurs, such as in acid dip tanks and running water rinses. If the solution is not slimy and does not coat the bag surface, then the bag will load as vacuum cleaner bag does. However, alkaline cleaner baths are usually slimy, so the particles they contain blind off the bag surface quickly, requiring frequent service. On cleaner or acid dip tanks where oil contamination is a problem, an additional coalescing filter cartridge installed downstream of the bag filter may remove the oil. Auto-gravity filters are also ideal for this application.

Coalescing cartridges will separate any dissimilar liquids with a difference in specific gravity of 0.09 or greater. The prefilter removes the unwanted particulate so the coalescing filter will last indefinitely as long as no particulate is introduced. The oil coalesces from tiny droplets into large ones that float to the top of the water in the chamber. This oil can be manually or automatically bled off. A recycler can then recycle the concentrated oil.

Permanent-media filtration systems are a choice of platers for unattended operation and where disposable filter media or the labor to change it is objectionable. When used in a suitable system, the plating solution is kept clean and the filter media is restored to a clean state each time the system automatically backwashes itself. The up side of this system is that it operates in the top 20 pct of the filtration flow range at all times as compared to non-backwashed media where flow diminishes as the media loads. The backwashable sand filtration system is also an excellent choice for polishing clarified wastewater from the treatment system. A key advantage of the backwashable pressure filtration system is that it cleans itself automatically.

Disposable fabric filtration systems are often referred to as automatic indexing gravity filters. They use an array of tanks, conveyors, pulleys, motors, pumps and float level controls to index disposable filter media over a stationary conveyor. This type of system effectively removes particulates from phosphating solutions, carbonized deposits from quench oil, copper fines from printed circuit board deburring operations and high-solids solutions. The micron retention of the available fabric ranges from one to 125 microns.

Recessed plate filter presses are used in about every precipitation wastewater treatment plant to dewater the underflow of clarifiers. A typical tube-type or slant-plate clarifier with an inverted pyramid bottom or cone achieves approximately 0.5 pct solids on the bottom. If a flat bottom is used and a sludge rake or scraper sweeps the bottom of the clarifier, three pct solids may be achieved if proper flocculation and settling occur. However, the slurry still needs to be thickened to eight or nine pct solids through a sludge thickener before pumping through a filter press, otherwise a much larger press would be needed and would be cost prohibitive.

Where liquids having extremely high solids, usually five to 10 pct, need to be dewatered, an air-diaphragm pump operated from 60 to 90 psi usually is the choice to transfer the thickened slurry through the filter press. The polypropylene filter plates have recessed cavities of one-half to three-quarter inch each with a 20 micron polypropylene filter cloth. As the plates are sandwiched together under pressure, every pair of plates provides a one to one-and-a-half-inch cavity to retain the solids as the slurry is pumped through the plates. This type of filtration system is capable of achieving 30 to 35 pct solids with standard filter cloths. Using membrane-type filter cloths with high psi pumps, solids of up to 60 pct have been achieved. However, the cost of this type of system is much greater than the standard filter press.

Capacities of one-half to 50 or more cubic feet are readily available and applications may be sized accordingly. The general rule of thumb in sizing a filter press for the average plating or printed circuit shop is two-cu-ft capacity minimum for every 25 gpm flow through the clarifier. This sizing arrangement will usually provide a full eight-hour shift before dumping is necessary. The time required to dump and clean a filter press is minimal, usually 15 to 20 min by one worker.

Carbon purification is the choice of platers to adsorb organic contamination from plating solutions, rinses, etches and cleaners.

If total purification is required in a batch treatment, the solution is transferred to a treatment tank and temperature raised above 140F. Hydrogen peroxide is sometimes added to the solution. Three to 12-and-one-half lbs of powdered carbon per 100 gal of solution are added, stirred and allowed to sit four to eight hours. The carbon will settle to the bottom, along with the adsorbed contaminants. The clean liquid can be decanted or pumped back to the process tanks through a filtration media. The carbon sludge is then batch treated and disposed of with the rest of the hazardous waste.

The frequency of the batch carbon treatment procedure may be greatly reduced by circulating the plating solutions continuously through a chamber containing granulated carbon. In this way, a constant balance of brighteners to achieve uniform ductility will be achieved.

An effective way to accomplish purification is to install a separate canister containing granular carbon on a bypass with a valve controlling the flow. After the solution has passed through the filter chamber to remove particulates, a portion (about five to 20 pct) may be directed through the granulated carbon on a continuous basis to remove the unwanted organics and then through a coalescer to remove oil. Granular carbon will remove organic breakdown products of the brighteners as well as oil, grease, etc., without stripping the brightener system.

The canisters holding the carbon have either fine mesh screens or fine retention depth filters of one to three microns to trap any carbon attempting to exit the vessel. This type of granulated carbon canister is also widely used as a portable system on a cart with its own pump, hoses, valves, etc., to recirculate solutions needing carbon treatment.

Caveat Emptor. An experienced sales application consultant will attempt to gather the pertinent data for the application to satisfy all of the concerns of the team involved in the purchase. The ultimate goal is to have a successful installation.

MSDS supply the chemical composition of the materials and must be obtained wherever possible. These documents contain information as to flash points, toxicity and the constituents. It is essential that the chemical compatibility, pH, pressure, temperature, limitation and flow performance curves be researched for the pumps, filters and components of the applications. You also must determine what type and amount of solids are to be removed and how fast. And, are abrasives present or oil or dissimilar liquids that need to be separated? All of this data must be used to select the material and components for the application.

In the end, the rule of Caveat Emptor (let the buyer beware), usually applies. If you let the application consultant make the recommendation, and purchase the product recommended, a reputable manufacturer will stand behind the product. Real products applied to real applications repeatedly offer the customer the greatest assurance and the most peace of mind that the proper product and application have been recommended.