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Pumps for plating applications
Pumps for Plating Applications by Mark Betchaver Setheo Diu., Met-Pro Corp., Hauppauge, N.Y.
M
any types of pumps are used in a metalfinishing facility. These include filter pumps, sump pumps, transfer pumps, and metering pumps to name a few. Each pump is important but the filter pump is among the most critical. They run continuously during all hours of production and under varied pressure conditions. When they are down for service the tank is not being filtered and costly rejects can be the result. It is important, therefore, to understand the function of a filter system and the different styles of pumps that are used to recirculate the solution through the filter. We will review the filtration process first then explore the various types of pumps that can be used. TYPES OF CONTAMINANTS
There are two basic types of contaminants that we are concerned about. One is insoluble particulate matter, which is suspended in the plating bath and can be removed by filtration. These suspended solids are introduced into the tank from airborne dust, anode corrosion, drag in on work, and from make-up chemicals. Anode bags greatly reduce the amount of anode sludge introduced in the tank and can prolong the life ofyour filter media. Chemical manufacturers are providing increasingly pure make-up chemicals, Which also lowers the amount of insoluble solids from this source. The other types of impurities are soluble contaminants and these cannot be removed by ordinary filtration. These include both organic and inorganic impurities. Organic impurities can be removed by treatment with activated carbon, which will be discussed later. Inorganic soluble contaminants, such as dissolved base metals, are removed by dummy plating or by chemical methods as they cannot be removed by filtration or carbon treatment. Filtration can remove suspended, insoluble impurities very efficiently and te> a controlled micron rating. Filtration is accomplished by passing the solution through a filter media, usually contained in a filter chamber, which traps the solids while allowing the solution to pass through it. Pumps are used to recirculate the solution through the chamber and the media. October 2000
FACTORS INFLUENCING THE SIZE OF A FILTER SYSTEM
Dirt-holding capacity is a function of the amount of surface area your filter has. It determines the amount of solids the filter can retain before the flow rate is reduced to an inadequate level, which then requires replacement of the media. As the filter loads up with dirt, the back pressure increases and a corresponding drop in flow rate occurs. Undersizing the surface area results in rapid loading of the filter, lower flow rates, increased pressure, and possible accelerated wear to filter system components. The flow rate can actually drop to a point where your filter is not removing dirt as quickly as it is being introduced to the tank. For most plating applications, 2 to 3 ten-inch filter cartridges or their equivalent will provide adequate surface area for 100 to 150 gallons of solution and allow for adequate turnover rate. Consideration for conditions in your plant and for your particular plating process should be taken into account when sizing a filter. High-production shops, or shops where unusually high rates of solids are introduced to the tank, may require even more surface area per 100 gallons. The greater the amount of surface area a filter has, the more dirt it can collect. Because of this, some individuals deliberately oversize their filter as that will result in a higher average turnover rate for the filtration cycle and lower labor cost as the filter will not have to be serviced as often. In many cases it costs very little more to double the dirt-holding capacity of a filter chamber. Turnover rate is a method of comparing the flow rate of a filter system in gallons per hour to the capacity of the plating tank. It is determined by dividing the filtration rate ofthe filter in gallons per hour by the number of gallons of solution in the tank. For example, a filter system with a rate of 2,000 gallons per hour being used on a 1,000 gallon tank would provide a turnover rate of two times per hour. But why is that important to know? The answer is that the turnover rate determines the percentage of solids removed from. the tank when the filter is continually recirculating the bath. Let's assume there is no new introduction of dirt into the tank, which of course is unlikely. A tank being filtered at one turnover per hour will have approximately 60% of the solids removed. Increase the turn29
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Figure 1. Relationship between pump flow rate and pressure head.
over to twice an hour and about 85% of the solids are trapped. Three turnovers over 90% and four turnovers greater than 95%. Increasing the turnover rate greater than 4 to 5 times an hour does not dramatically improve the percent of solids removed and is not usually cost effective unless new solids are introduced into the tank at a high rate. Some applications, however, most notably electroless nickel, are filtered at turnover rates of up to 10 times/hr, sometimes even higher. Even with a high turnover rate, in some cases dirt can be introduced into the tank during working hours at a rate that exceeds the filters ability to remove it. Under those conditions it is advisable to run the filter during nonplating hours to allow the filter to catch up. Since both the flow rate and the dirt-holding capacity of the filter are determined by the surface area these three factors should be considered when sizing your filter system. Both your chemical supplier and filter supplier can suggest turnover rates for the various plating solutions, but a good general rule of thumb is to allow 2 to 3 tank turnovers per hour for acid baths or 1 to 2 turnovers per hour for alkaline baths under "normal" shop conditions. When selecting a filter system it often is not a good idea to choose one whose maximum flow rate is equal to your required gallons per hour to meet your turnover rate. An example may help make this clear. Say you have a 500-gallon acid copper tank that requires three turnovers per hour. The flow rate of the filter should then be 1,500 gph; however, if you select a filter whose maximum flow is 1,500 gph you may be quickly disappointed. As mentioned previously, as the filters clog, the back pressure increases and there is a reduction in flow rate. (See Fig. 1.) 30
The filter's flow rate gradually decreases from the desired 1,500 gph to 1,200, then to 1,000, and so on. Your desired turnover rate of three is soon down to 2, then 11/2, then maybe only one per hour. To restore the flow back up to the desired 3 turnovers you would have to change the filter media, possibly even before it has fully used up its dirt holding capacity. This results in both increased cost for filter media, increased labor cost to change it, and increased disposal cost. It is much more beneficial to purchase a filter of2,000 gph capacity so the average flow rate through your filtration cycle will be closer to the three turnovers you wish to achieve. In this example the 2,000 gph filter could lose 25% of its flow rate and still provide three turnovers per hour. At 1/2 its rated flow it still provides two turnovers, which translates results in up to 85% solids removal as mentioned above. Increasing the dirt-holding capacity of the filter chamber has a similar effect as filter media loads up slower, keeping the flow rate higher. Remember that both dirt-holding capacity and flow rate are determined largely by the surface area of the media. The greater the surface area, the greater the dirt-holding capacity and the higher the flow rate can be; therefore, when comparing the relative merits of different filter systems, it is important to consider all three factors. For example, a filter that is stated to have a high flow rate, yet offers a relatively small amount of surface area, can be expected to have a low dirtholding capacity; therefore, frequent changing of the media is necessary to maintain that flow rate. TYPES OF FILTER MEDIA
As the heart of any filter system is the filter media, let's first take a look at the various media usually used for plating solutions.
Depth Filters String-wound cartridge media are referred to as depth filter media and are one of the most popular and least expensive choices, because they offer a relatively high amount of equivalent surface area in a small space. (See Fig. 2.) String-wound cartridges, usually made from polypropylene, cotton, or similar yarn, are wound in a honeycomb or diamond pattern over a supporting core and are referred to as depth cartridges. This diamond pattern gets increasingly smaller toward the center or core of the cartridge allowing larger particles to be caught towards the outside diameter and finer particles toward the core of the cartridge. As most of the solids are actually trapped inside the cartridge, they cannot be effectively back washed and are simply disposed of, and Metal Finishing
Figure 3. Surface-type media.
include the following types: pleated cartridges with deep folded pleats to extend surface area (and thus dirt-holding capacity), and cartridges that utilize a replaceable rigid sleeve or flexible bag over a supporting core.
Figure 2. String-wound cartridge media.
replaced when they have reached their dirt-holding capacity. String-wound cartridges are available in standard 6-, 10-, 20-, 30·, and 40-in. lengths and in ratings between 0.5 and 150 micron. One micron equals 39 millionths of an inch. A lO-in. wound cartridge with a 21/2-in. outside diameter has an equivalent surface area of 31/2 ft 2 and a dirt-holding capacity of between 1/2 to 1 lb of solids. The dirtholding capacity will vary somewhat with the micron rating. A coarser tube has greater dirt-holding capacity than a finer tube.
Surface Filter Media Surface-type media are the other major type of filter media and several styles are currently in use for plating applications. (See Fig. 3.) One type is a cartridge that uses pleated polypropylene media. It can be used in place of wound depth cartridges in most chambers designed for cartridges. They differ from the wound tube in that they are not a depth filter. All the particulate matter is trapped on the surface of the cartridge and, therefore, it is possible to backwash or clean off the media for reuse. With each successive cleaning, however, more fines are embedded in the cartridge, which leads to the eventual need to replace it. Surface cartridge media October 2000
Horizontal Disk Surface Media Horizontal disk surface media are another major media used for filtration of plating solutions. The media themselves are usually a paper or cloth membrane supported by a frame or disk. These supporting disks and the chosen media are generally stacked horizontally one on top of the other inside the housing (some chambers mount these media vertically). The surface area is a function of the diameter of the disk and the number of the disks. The larger the diameter of the disks and the greater the number of disks, the greater the surface area, and, thus, the dirt-holding capacity. These types of media can be precoated with a filter aid, which forms a cake on the outer surface. The particulate matter is then collected on the cake and eventually both the cake and trapped solids are backwashed from a cloth membrane. The cake is usually discarded with the paper membrane if that material is used. Bag Filters Bag filters are the last major type of surface media in wide usage. (See Fig. 4.) Bags are available in several materials; however, polypropylene is the most widely used for plating applications. Like other forms of surface media bags can be cleaned and reused several times prior to replacement. Bag filters are available in two differing styles. One style is the "free hanging bag" used frequently for electroless nickel. The bag is clamped to a disk or head and is positioned directly over the solution in a nonworking area of the tank. The solution is pumped through the bag and recirculated in that manner. Manufacturers also offer bag filter chambers, which enables the bag to be supported on all sides. This allows the filter to operate at higher pressure differentials 31
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Figure 4. Bag filters.
without tearing the bag. The use of a chamber allows the bag to be placed outside the tank, saving valuable in-tank space. It also allows the user to direct the discharge through a sparger if desired. On high-temperature applications, such as electroless nickel, less heat loss occurs if the bag is contained in a chamber. Compared to cartridges, bags have a low pressure drop and are capable of high flow rates in a relatively compact housing, which can result in increased turnover rates and use of smaller equipment. The importance of turnover rate has already been discussed. Filter media are available in varying densities, which determines the size of the solids that can be retained. The denser the media, the smaller the particle that can be trapped, and the greater the pressure drop. Generally, the denser the media, the lower the dirt-holding capacity of that media and the lower the flow rate. Most manufacturers state a filter system flow rate based on l5-micron media. If you will be using denser (also called finer) media you should expect a somewhat lower flow rate. It is always best to select the coarsest micron density that can be used for your applications, consistent with achieving good plating results. Coarser tubes have a higher dirt-holding capacity and, therefore, last longer. CARBON TREATMENT
Earlier we pointed out that carbon treatment can be used to remove soluble organic impurities from plating tanks. Carbon treatment is performed by two basic methods. Batch treatment is where the solution is transferred to a holding tank, then powdered carbon is added directly into the tank. Sometimes, hydrogen peroxide is added to assist carbon adsorption. The solution is then agitated for a few hours. The carbon is allowed to settle out, then the solution is pumped back to the plating tank, usually through October 2000
a filter to remove any carbon fines that did not settle out . If you choose the batch treatment method you need not consider carbon treatment when you select a filter. The batch method is performed when the bath reaches a point where organic contamination gets to such a high level that rejects are about to occur or are already occurring. It is then given a "shock treatment" where all organics are removed at one time. Continuous carbon treatment or frequent carbon treatment is performed on a regular basis and removes the organics as they are introduced into the tank, before their level increases to such a point to where you have to shut down the plating operation to perform a batch treatment. This type of carbon treatment is performed by three methods-either with filter media (cartridge or disk) containing activated carbon; with a chamber, which is designed to use bulk, granular carbon; or by precoating surface type media, such as disks. Treatment with carbon cartridges can be performed in any filter chamber designed to accommodate standard 6-, 10-, 20-, or 30-in. wound cartridges. Most carbon cartridges contain 6 to 8 ounces of granular carbon per 10-in. length and will also simultaneously filter your solution to about 10 micron. Similarly, carbon impregnated disks can be used in place of standard disk media to carbon treatment with a disk-type chamber. While turnover rate is important for good filtration , a high flow through carbon has a negative effect. For effective carbon treatment, the solution must have adequate contact time to allow the carbon enough time to adsorb the dissolved organics; therefore, when carbon treating with carbon cartridges or other carbon media it is desirable to reduce the flow rate to achieve optimum carbon treatment. Keep in mind that doing this may lower your turnover rate sufficiently enough to lower the effectiveness of your filtration. Because carbon cartridges are relatively expensive, $9 to $10 dollars for a 10-in. tube, they are generally used on tanks up to a few hundred gallons. For larger tanks, systems that take bulk granular or powdered carbon are usually employed as they are much more economical. Bulk carbon is sold by the pound. Good grades of acid washed carbon, suitable for plating applications, cost $3 to $4 dollars a pound, which is much less expensive than the $18 to $20 per 16 oz. of carbon cost in a carbon cartridge. Carbon chambers are available that hold up to about 100 Ib of carbon and they are available in two different styles. One is a single-chamber unit, available with a varying number of filter cartridges that 33
can be used to simultaneously carbon treat and filter the tank. When one wishes to discontinue carbon treatment the bag of carbon can simply be removed from this type of chamber and it then can be used for filtration only. Another popular type of system has a separate carbon chamber and a separate filter chamber. In this type of arrangement a portion of the pump discharge is bled into the carbon chamber while the balance of the pump output goes through the filter chamber. When carbon treatment is not required the valve to the carbon chamber is simply closed. Because this is a twin-chamber system they generally take up more floor space and are more expensive than the single-chamber style. A third method of carbon treatment is with a portable carbon treatment system, which can be moved from one tank to another as carbon treatment becomes necessary. This style system usually only has a few small trap filters on the discharge side of the carbon to prevent carbon fines from entering the bath. This type of unit is usually used when your plating tanks are already equipped with existing filters. The first two types we discussed perform both filtration and carbon treatment and are, therefore, usually dedicated to one tank and are the only filtercarbon unit used on the tank. The last method of carbon treatment is by precoating surface filter media. Typically, horizontal disk filters are precoated with a filter aid and then a slurry of powdered carbon. Now that we've examined all the elements to consider when selecting a filter system lets put them all together to see how they relate to each other, when you, the plater, must decide what to purchase. PUMP TYPES
The last main consideration when specifying a filter system is the pump and motor, which will recirculate the solution through the media. Pump selection is very important to consider when choosing a filter system as all pumps are subject to some degree of maintenance. There are many types of pumps available. We will examine the features and benefits of the main types used on filter systems-centrifugal and positive displacement pumps. Centrifugal pumps come in several styles and are ideally suited to the ever-changing flow and pressure conditions created by the filter chamber. With clean filter media the pumps reach a flow rate approaching their maximum open-pumping rate and as the filter media clogs the resulting back pressure reduces the flow rate without causing any apprecia34
ble increase in wear to the centrifugal pump or motor. The "work" being done by a centrifugal pump and motor is based on the flow rate. The higher the flow the greater the amount of work and the more current the motor will draw. Manufacturers will usually oversize their motors on filter pumps so they will be nonoverloading at the maximum filtration rate, even for specific gravities of up to 1.2 or 1.4. The lower the flow the less current a motor will draw so, should a motor reach an overload condition at full flow (for instance after the filter media are changed on a high specific gravity application), the flow rate and the current draw can both be reduced to avoid the overload condition simply by partially closing the flow regulating valve between the pump discharge and filter inlet. As the filters clog, the valve can then be opened to restore the flow rate. Never regulate the flow with a valve on the suction side of the pump. Horizontal centrifugal pumps are available in two major styles-either direct drive or magnetically coupled. Direct-drive pumps use a mechanical seal to prevent leakage of the solution from the back of the pump. The mechanical seal is generally the only normal wearing part on this type of pump and it must be expected that it will require occasional replacement. It is always a good idea to have a spare mechanical seal on hand. Failure to replace a mechanical seal as soon as it is necessary can lead to both costly downtime and additional damage to the pump. Wear of a mechanical seal increases as the pump pressure increases; therefore, to extend the life of the seal it helps to change the filter media before the back pressure gets excessive. Many pumps using a mechanical seal, especially larger pumps, require a fresh-water seal flush of 1/4 to ¥2 gpm. This should be taken into account before specifying this type. Direct-drive pumps are offered in two different configurations. Closed-coupled pumps are built right to the face of the motor and the pump shaft is attached directly to the motor shaft. This leads to a very compact design, insures alignment between the pump and motor shaft, and helps lower the cost of the pump. Frame-mounted pumps are built onto a ball-bearing pedestal, which is connected to the motor shaft with a flexible coupling. Since the pump and motor are two separate items they must be mounted to a baseplate with a coupling and a coupling guard. Care must be taken to insure that the pump and motor shaft are aligned, otherwise vibration will result, which will damage the pump and motor bearings, mechanical seal, coupling, and posMetal Finishing
Figure 5. Horizontal magnetic-drive centrifugal pumps.
sibly other components. Frame-mounted pumps area usually more expensive than their close-coupled counterparts because of the increased number of components. The second type of horizontal pump widely used in our industry is the magnetically coupled sealless pump. (See Fig. 5.) There is no direct coupling between the pump and motor shaft and no mechanical seal is required. Because there is no seal, magnetic pumps are said to be leakproof. They are generally close coupled, that is, built to the face of the motor. The drive magnet is attached to the motor shaft, which turns the driven or slave magnet, which is either connected to or an integral part of the impeller. Between the two magnets is a cup or disk that presses against an O-ring between it and the pump casing, which seals the pump. Magnetic pumps are usually suitable for specific gravities up to 1.2 at open pumping. Since a filter reduces the flow rate that a pump will develop, they can even be used as a filter pump for heavier solutions, such as nickel, that can have specific gravities up to 1.4. Unless properly equipped for higher specific gravity solutions, magnetic-drive pumps can be difficult to prime, especially on these heavier solutions. The coupling may slip, leaving the motor running but the impeller standing still. If this occurs close the valve on the discharge side of the pump prior to starting the motor. This allows the two magnets to get up to speed. The valve can then gradually be opened to reach the desired flow. If the valve is opened to a point where the coupling slips, repeat the procedure; however, open the valve only to the point just before the coupling slipped. This is similar to the way a October 2000
Figure 6. In-tank vertical pump.
valve is used to avoid an overload situation of a motor that was earlier reviewed. Because of the magnetic pumps' "leakproof' characteristics, they are frequently used on precious metal plating baths to avoid losing expensive solutions from a mechanical seal leak. Most horizontal centrifugal pumps are not selfpriming; therefore, the operator must flood the pump body and inlet line prior to starting the pump. Failure to do so can result in the pump being run dry, which quickly leads to heat buildup and costly damage to the pump-especially mechanical seals and seal housings. Some manufacturers of magnetic pumps now offer self-priming versions, which will prime themselves once a priming reservoir is filtered. These pumps offer a form of run dry protection once the reservoir has been filled, and they can pull up to a 10- to 20-ft suction lift. They are available in sizes up to 85 gpm (5,100 gph). The last major category of centrifugal pumps is the vertical pumps, which have gained wide acceptance for plating and filtration applications for several reasons. (See Fig. 6.) They are of the cantilever design where oversized motor bearings support the pump shaft. Several manufacturers use a sleeved, extended motor shaft, which serves as pump shaft. This helps insure vibration-free performance. 35
Pumps ofthis style have no pump bearings and no mechanical seals, meaning they can be run dry indefinitely without damage. Because there are only a few component parts, usually a motor, impeller, body, and inlet cover, they are relatively inexpensive compared to some horizontal pumps of similar performance. When mounted inside the tank vertical pumps are self-priming as the impeller is usually below the liquid level. Most of these vertical pumps incorporate an opposing twin-impeller design. The function of the upper impeller is to pump any solution going up the column back down and out the discharge, preventing any leakage. This feature allows the twin-impeller pump to be mounted vertically, either inside or outside the tank. The ability to mount the pump outside the tank is important in that it saves valuable in-tank space and keeps the pump away from heaters, hoists, barrels, and the like. It also avoids placement of the motor directly over the fumes or vapors that are developed in the process tank. Vertical pumps also save valuable floor space as they are usually "C" clamped or bolted directly to the tank flange and can be used with a filter chamber that mounts similarly. Vertical-bearing free pumps are suitable for any plating solution, including electroless nickel and copper, and when mounted inside the tank can be used for precious metal solutions. Using the pump in tank insures that, in the unlikely case of a leak, the fluid would be contained by the tank, avoiding loss of these expensive solutions. The other major type of pump used for filter pumps is the self-priming or positive-displacement pump. They usually are designed with a flexible rubberlike impeller or liner. The most popular ones utilize a mechanical seal to prevent leakage. To achieve the self-priming feature, rubbing of the impeller must occur on the pump body, end cover, and backplate to develop the positive seal necessary to create the vacuum required for a suction lift, This rubbing produces wear and eventual replacement of the flexible impeller and the seal must be expected. This wear increases as the pressure the pump produces increases. Increasing the surface area of the filter helps to reduce the back pressure and, therefore, the wear that occurs. The convenience of the self-priming pump should be weighed against the increased costs of maintenance that can be expected. The development of sealless magnetic-drive self-priming pumps has reduced the use of positive displacement pumps. If a filter system is frequently going to be moved from 36
one tank to another, the convenience of a self-priming pump may be preferable to other styles. SELECTION OF A PUMP AND FILTER SYSTEM
First, of course, you must know your application. That doesn't just mean the solution, tank size, and temperature. Consider conditions unique to your plating operation that will influence your decisionfactors, such as rack or barrel plating, geometric shape of part or type of part (tubular pieces or parts that are hard to clean), amount of drag in, and the general environment of the shop. With that information you should be able to decide the turnover rate necessary for this bath. As mentioned, turnover rate and dirt-holding capacity are related to the surface area of the media. You may find a filter having 100 ft.2 of surface area will provide the desired turnover rate, but you may want to increase the surface area to 150 or 200 ft 2 to increase your dirt-holding capacity. By doing this you will reduce the amount of changes of the filter media you will have to make while you increase your average filtration rate as your media does not clog up as quickly. Next, decide whether or not carbon treatment is required and select the style of carbon treatment that will be the most effective and economical for your size tank. For example, it would not be wise to try to carbon treat a 1,500 gallon bright nickel tank with carbon cartridges unless you're expecting an expensive Christmas gift. from your carbon tube supplier. Using bulk carbon systems will be much more economical and you may get a Christmas bonus from your employer. Once you have decided what type and size filter chamber you require, the last thing is to select the pump style that is most appropriate for your needs. When you purchase a filter system (see Fig. 7), other than replacement of the filter media, not too much maintenance is necessary on the filter chamber itself. The pump is the component of the system that can either be a maintenance headache or a relatively pleasant experience. The wrong choice of pump style can be very costly over the years that you expect to own the filter. Some of the factors you should consider when selecting a pump for your filter are: 1. Material compatibility of pump and solution
2. 3. 4. 5. 6.
Temperature Flow rate and pressure required How heavy is the dirt load Is water for a seal flush available Does solution crystallize Metal Finishing
Figure 7. Typical filter system.
7. Is self-priming necessary 8. Specific gravity 9. Any constraints concerning space for mounting, electrical service, etc. 10. Any other in-plant considerations that will affect choice of pump As mentioned, most manufacturers will provide a pump whose capacity in gallons per minute matches the capacity of the filter chamber. If everything else is equal, select the pump that delivers the highest pressure for the flow conditions of your filter. That will insure the longest life of your filter media. In general, unless you will be moving your filter from tank to tank, stay away from the positive placement self-priming pump as you can expect them to have the highest maintenance cost. Magnetic-drive selfpriming pumps are usually a better choice. Vertical centrifugal pumps usually require the least maintenance as they have no norma! wearing parts and can even be run dry. Even solutions that crystallize when standing, such as nickel and acid COpper, will not cause appreciable damage on start-up as there are no parts rotating against each olher. The abrasiveness of crystallized salts can lead to premature seal and impeller wear in other styles. If an out-of-tank, floor-mounted pump is required, magnetic pumps are likely to be more leakproof, and
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October 2000
generally require less maintenance, than an equivalent pump with a mechanical seal; however, they are limited in the flow rate, although newer, larger models are being introduced, and they can be finicky when used with higher specific gravity solutions unless properly equipped. Direct-drive centrifugal pumps handle higher specific gravities well and are available in almost any conceivable flow rate and pressure range the operator may require. As centrifugal pumps vary widely in efficiencies, never state the size of the pump you require in terms of horsepower. Always state the flow and pressure desired. After deciding what features you require ask for recommendations, prices, and above all literature. Review the literature carefully to be sure that when you are comparing prices, you are comparing similar products. Check for ease of filter change, flaw rate and pressure of the pump, and maintenance requirements. Compare the surface area and carbon capacity. See what the cost would be to increase the size of the filter chamber. Very often you will find that you can increase your surface area without adding much to the cost of a filter system. Lastly, remember that this new filter and pump will hopefully be the last one you will buy for this tank for a very long time. If you undersize the capacity of the filter, or compromise on the pump, you'll have to live with it for several years. It's wise to get exactly what's necessary, rather than saving a small amount and spending many times that amount reworking rejects or replacing media or worn out pump parts. If the filter is to be dedicated to this one tank rather than moved around, select/specify a centrifugal pump style that will be best for your in-house considerations. A vertical or magnetic will give good service and require the least maintenance, and they are available in the size required. One of these will probably be your best bet. Now rather than ask your distributor or filter manufacturer to simply quote a filter for a 500gallon nickel tank, you can instead state your solution and specify that the system should have a flow rate of at least 1,500 gph, have 10 to 15 filter tubes, have a carbon capacity of about 10 lb, and be equipped with a vertical pump for in-tank mounting. In this way you will be sure of getting a system that not only has all the features you want, but one that has all the specifications necessary to do the only job it will ever perform, removing suspended solids and organic contamination from your tank without nceding a lot of attention from you or your maintenance personnel. MF
37
M
any types of pumps are used in a metalfinishing facility. These include filter pumps, sump pumps, transfer pumps, and metering pumps to name a few. Each pump is important but the filter pump is among the most critical. They run continuously during all hours of production and under varied pressure conditions. When they are down for service the tank is not being filtered and costly rejects can be the result. It is important, therefore, to understand the function of a filter system and the different styles of pumps that are used to recirculate the solution through the filter. We will review the filtration process first then explore the various types of pumps that can be used. TYPES OF CONTAMINANTS
There are two basic types of contaminants that we are concerned about. One is insoluble particulate matter, which is suspended in the plating bath and can be removed by filtration. These suspended solids are introduced into the tank from airborne dust, anode corrosion, drag in on work, and from make-up chemicals. Anode bags greatly reduce the amount of anode sludge introduced in the tank and can prolong the life ofyour filter media. Chemical manufacturers are providing increasingly pure make-up chemicals, Which also lowers the amount of insoluble solids from this source. The other types of impurities are soluble contaminants and these cannot be removed by ordinary filtration. These include both organic and inorganic impurities. Organic impurities can be removed by treatment with activated carbon, which will be discussed later. Inorganic soluble contaminants, such as dissolved base metals, are removed by dummy plating or by chemical methods as they cannot be removed by filtration or carbon treatment. Filtration can remove suspended, insoluble impurities very efficiently and te> a controlled micron rating. Filtration is accomplished by passing the solution through a filter media, usually contained in a filter chamber, which traps the solids while allowing the solution to pass through it. Pumps are used to recirculate the solution through the chamber and the media. October 2000
FACTORS INFLUENCING THE SIZE OF A FILTER SYSTEM
Dirt-holding capacity is a function of the amount of surface area your filter has. It determines the amount of solids the filter can retain before the flow rate is reduced to an inadequate level, which then requires replacement of the media. As the filter loads up with dirt, the back pressure increases and a corresponding drop in flow rate occurs. Undersizing the surface area results in rapid loading of the filter, lower flow rates, increased pressure, and possible accelerated wear to filter system components. The flow rate can actually drop to a point where your filter is not removing dirt as quickly as it is being introduced to the tank. For most plating applications, 2 to 3 ten-inch filter cartridges or their equivalent will provide adequate surface area for 100 to 150 gallons of solution and allow for adequate turnover rate. Consideration for conditions in your plant and for your particular plating process should be taken into account when sizing a filter. High-production shops, or shops where unusually high rates of solids are introduced to the tank, may require even more surface area per 100 gallons. The greater the amount of surface area a filter has, the more dirt it can collect. Because of this, some individuals deliberately oversize their filter as that will result in a higher average turnover rate for the filtration cycle and lower labor cost as the filter will not have to be serviced as often. In many cases it costs very little more to double the dirt-holding capacity of a filter chamber. Turnover rate is a method of comparing the flow rate of a filter system in gallons per hour to the capacity of the plating tank. It is determined by dividing the filtration rate ofthe filter in gallons per hour by the number of gallons of solution in the tank. For example, a filter system with a rate of 2,000 gallons per hour being used on a 1,000 gallon tank would provide a turnover rate of two times per hour. But why is that important to know? The answer is that the turnover rate determines the percentage of solids removed from. the tank when the filter is continually recirculating the bath. Let's assume there is no new introduction of dirt into the tank, which of course is unlikely. A tank being filtered at one turnover per hour will have approximately 60% of the solids removed. Increase the turn29
...
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Figure 1. Relationship between pump flow rate and pressure head.
over to twice an hour and about 85% of the solids are trapped. Three turnovers over 90% and four turnovers greater than 95%. Increasing the turnover rate greater than 4 to 5 times an hour does not dramatically improve the percent of solids removed and is not usually cost effective unless new solids are introduced into the tank at a high rate. Some applications, however, most notably electroless nickel, are filtered at turnover rates of up to 10 times/hr, sometimes even higher. Even with a high turnover rate, in some cases dirt can be introduced into the tank during working hours at a rate that exceeds the filters ability to remove it. Under those conditions it is advisable to run the filter during nonplating hours to allow the filter to catch up. Since both the flow rate and the dirt-holding capacity of the filter are determined by the surface area these three factors should be considered when sizing your filter system. Both your chemical supplier and filter supplier can suggest turnover rates for the various plating solutions, but a good general rule of thumb is to allow 2 to 3 tank turnovers per hour for acid baths or 1 to 2 turnovers per hour for alkaline baths under "normal" shop conditions. When selecting a filter system it often is not a good idea to choose one whose maximum flow rate is equal to your required gallons per hour to meet your turnover rate. An example may help make this clear. Say you have a 500-gallon acid copper tank that requires three turnovers per hour. The flow rate of the filter should then be 1,500 gph; however, if you select a filter whose maximum flow is 1,500 gph you may be quickly disappointed. As mentioned previously, as the filters clog, the back pressure increases and there is a reduction in flow rate. (See Fig. 1.) 30
The filter's flow rate gradually decreases from the desired 1,500 gph to 1,200, then to 1,000, and so on. Your desired turnover rate of three is soon down to 2, then 11/2, then maybe only one per hour. To restore the flow back up to the desired 3 turnovers you would have to change the filter media, possibly even before it has fully used up its dirt holding capacity. This results in both increased cost for filter media, increased labor cost to change it, and increased disposal cost. It is much more beneficial to purchase a filter of2,000 gph capacity so the average flow rate through your filtration cycle will be closer to the three turnovers you wish to achieve. In this example the 2,000 gph filter could lose 25% of its flow rate and still provide three turnovers per hour. At 1/2 its rated flow it still provides two turnovers, which translates results in up to 85% solids removal as mentioned above. Increasing the dirt-holding capacity of the filter chamber has a similar effect as filter media loads up slower, keeping the flow rate higher. Remember that both dirt-holding capacity and flow rate are determined largely by the surface area of the media. The greater the surface area, the greater the dirt-holding capacity and the higher the flow rate can be; therefore, when comparing the relative merits of different filter systems, it is important to consider all three factors. For example, a filter that is stated to have a high flow rate, yet offers a relatively small amount of surface area, can be expected to have a low dirtholding capacity; therefore, frequent changing of the media is necessary to maintain that flow rate. TYPES OF FILTER MEDIA
As the heart of any filter system is the filter media, let's first take a look at the various media usually used for plating solutions.
Depth Filters String-wound cartridge media are referred to as depth filter media and are one of the most popular and least expensive choices, because they offer a relatively high amount of equivalent surface area in a small space. (See Fig. 2.) String-wound cartridges, usually made from polypropylene, cotton, or similar yarn, are wound in a honeycomb or diamond pattern over a supporting core and are referred to as depth cartridges. This diamond pattern gets increasingly smaller toward the center or core of the cartridge allowing larger particles to be caught towards the outside diameter and finer particles toward the core of the cartridge. As most of the solids are actually trapped inside the cartridge, they cannot be effectively back washed and are simply disposed of, and Metal Finishing
Figure 3. Surface-type media.
include the following types: pleated cartridges with deep folded pleats to extend surface area (and thus dirt-holding capacity), and cartridges that utilize a replaceable rigid sleeve or flexible bag over a supporting core.
Figure 2. String-wound cartridge media.
replaced when they have reached their dirt-holding capacity. String-wound cartridges are available in standard 6-, 10-, 20-, 30·, and 40-in. lengths and in ratings between 0.5 and 150 micron. One micron equals 39 millionths of an inch. A lO-in. wound cartridge with a 21/2-in. outside diameter has an equivalent surface area of 31/2 ft 2 and a dirt-holding capacity of between 1/2 to 1 lb of solids. The dirtholding capacity will vary somewhat with the micron rating. A coarser tube has greater dirt-holding capacity than a finer tube.
Surface Filter Media Surface-type media are the other major type of filter media and several styles are currently in use for plating applications. (See Fig. 3.) One type is a cartridge that uses pleated polypropylene media. It can be used in place of wound depth cartridges in most chambers designed for cartridges. They differ from the wound tube in that they are not a depth filter. All the particulate matter is trapped on the surface of the cartridge and, therefore, it is possible to backwash or clean off the media for reuse. With each successive cleaning, however, more fines are embedded in the cartridge, which leads to the eventual need to replace it. Surface cartridge media October 2000
Horizontal Disk Surface Media Horizontal disk surface media are another major media used for filtration of plating solutions. The media themselves are usually a paper or cloth membrane supported by a frame or disk. These supporting disks and the chosen media are generally stacked horizontally one on top of the other inside the housing (some chambers mount these media vertically). The surface area is a function of the diameter of the disk and the number of the disks. The larger the diameter of the disks and the greater the number of disks, the greater the surface area, and, thus, the dirt-holding capacity. These types of media can be precoated with a filter aid, which forms a cake on the outer surface. The particulate matter is then collected on the cake and eventually both the cake and trapped solids are backwashed from a cloth membrane. The cake is usually discarded with the paper membrane if that material is used. Bag Filters Bag filters are the last major type of surface media in wide usage. (See Fig. 4.) Bags are available in several materials; however, polypropylene is the most widely used for plating applications. Like other forms of surface media bags can be cleaned and reused several times prior to replacement. Bag filters are available in two differing styles. One style is the "free hanging bag" used frequently for electroless nickel. The bag is clamped to a disk or head and is positioned directly over the solution in a nonworking area of the tank. The solution is pumped through the bag and recirculated in that manner. Manufacturers also offer bag filter chambers, which enables the bag to be supported on all sides. This allows the filter to operate at higher pressure differentials 31
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Metal Finishing
Figure 4. Bag filters.
without tearing the bag. The use of a chamber allows the bag to be placed outside the tank, saving valuable in-tank space. It also allows the user to direct the discharge through a sparger if desired. On high-temperature applications, such as electroless nickel, less heat loss occurs if the bag is contained in a chamber. Compared to cartridges, bags have a low pressure drop and are capable of high flow rates in a relatively compact housing, which can result in increased turnover rates and use of smaller equipment. The importance of turnover rate has already been discussed. Filter media are available in varying densities, which determines the size of the solids that can be retained. The denser the media, the smaller the particle that can be trapped, and the greater the pressure drop. Generally, the denser the media, the lower the dirt-holding capacity of that media and the lower the flow rate. Most manufacturers state a filter system flow rate based on l5-micron media. If you will be using denser (also called finer) media you should expect a somewhat lower flow rate. It is always best to select the coarsest micron density that can be used for your applications, consistent with achieving good plating results. Coarser tubes have a higher dirt-holding capacity and, therefore, last longer. CARBON TREATMENT
Earlier we pointed out that carbon treatment can be used to remove soluble organic impurities from plating tanks. Carbon treatment is performed by two basic methods. Batch treatment is where the solution is transferred to a holding tank, then powdered carbon is added directly into the tank. Sometimes, hydrogen peroxide is added to assist carbon adsorption. The solution is then agitated for a few hours. The carbon is allowed to settle out, then the solution is pumped back to the plating tank, usually through October 2000
a filter to remove any carbon fines that did not settle out . If you choose the batch treatment method you need not consider carbon treatment when you select a filter. The batch method is performed when the bath reaches a point where organic contamination gets to such a high level that rejects are about to occur or are already occurring. It is then given a "shock treatment" where all organics are removed at one time. Continuous carbon treatment or frequent carbon treatment is performed on a regular basis and removes the organics as they are introduced into the tank, before their level increases to such a point to where you have to shut down the plating operation to perform a batch treatment. This type of carbon treatment is performed by three methods-either with filter media (cartridge or disk) containing activated carbon; with a chamber, which is designed to use bulk, granular carbon; or by precoating surface type media, such as disks. Treatment with carbon cartridges can be performed in any filter chamber designed to accommodate standard 6-, 10-, 20-, or 30-in. wound cartridges. Most carbon cartridges contain 6 to 8 ounces of granular carbon per 10-in. length and will also simultaneously filter your solution to about 10 micron. Similarly, carbon impregnated disks can be used in place of standard disk media to carbon treatment with a disk-type chamber. While turnover rate is important for good filtration , a high flow through carbon has a negative effect. For effective carbon treatment, the solution must have adequate contact time to allow the carbon enough time to adsorb the dissolved organics; therefore, when carbon treating with carbon cartridges or other carbon media it is desirable to reduce the flow rate to achieve optimum carbon treatment. Keep in mind that doing this may lower your turnover rate sufficiently enough to lower the effectiveness of your filtration. Because carbon cartridges are relatively expensive, $9 to $10 dollars for a 10-in. tube, they are generally used on tanks up to a few hundred gallons. For larger tanks, systems that take bulk granular or powdered carbon are usually employed as they are much more economical. Bulk carbon is sold by the pound. Good grades of acid washed carbon, suitable for plating applications, cost $3 to $4 dollars a pound, which is much less expensive than the $18 to $20 per 16 oz. of carbon cost in a carbon cartridge. Carbon chambers are available that hold up to about 100 Ib of carbon and they are available in two different styles. One is a single-chamber unit, available with a varying number of filter cartridges that 33
can be used to simultaneously carbon treat and filter the tank. When one wishes to discontinue carbon treatment the bag of carbon can simply be removed from this type of chamber and it then can be used for filtration only. Another popular type of system has a separate carbon chamber and a separate filter chamber. In this type of arrangement a portion of the pump discharge is bled into the carbon chamber while the balance of the pump output goes through the filter chamber. When carbon treatment is not required the valve to the carbon chamber is simply closed. Because this is a twin-chamber system they generally take up more floor space and are more expensive than the single-chamber style. A third method of carbon treatment is with a portable carbon treatment system, which can be moved from one tank to another as carbon treatment becomes necessary. This style system usually only has a few small trap filters on the discharge side of the carbon to prevent carbon fines from entering the bath. This type of unit is usually used when your plating tanks are already equipped with existing filters. The first two types we discussed perform both filtration and carbon treatment and are, therefore, usually dedicated to one tank and are the only filtercarbon unit used on the tank. The last method of carbon treatment is by precoating surface filter media. Typically, horizontal disk filters are precoated with a filter aid and then a slurry of powdered carbon. Now that we've examined all the elements to consider when selecting a filter system lets put them all together to see how they relate to each other, when you, the plater, must decide what to purchase. PUMP TYPES
The last main consideration when specifying a filter system is the pump and motor, which will recirculate the solution through the media. Pump selection is very important to consider when choosing a filter system as all pumps are subject to some degree of maintenance. There are many types of pumps available. We will examine the features and benefits of the main types used on filter systems-centrifugal and positive displacement pumps. Centrifugal pumps come in several styles and are ideally suited to the ever-changing flow and pressure conditions created by the filter chamber. With clean filter media the pumps reach a flow rate approaching their maximum open-pumping rate and as the filter media clogs the resulting back pressure reduces the flow rate without causing any apprecia34
ble increase in wear to the centrifugal pump or motor. The "work" being done by a centrifugal pump and motor is based on the flow rate. The higher the flow the greater the amount of work and the more current the motor will draw. Manufacturers will usually oversize their motors on filter pumps so they will be nonoverloading at the maximum filtration rate, even for specific gravities of up to 1.2 or 1.4. The lower the flow the less current a motor will draw so, should a motor reach an overload condition at full flow (for instance after the filter media are changed on a high specific gravity application), the flow rate and the current draw can both be reduced to avoid the overload condition simply by partially closing the flow regulating valve between the pump discharge and filter inlet. As the filters clog, the valve can then be opened to restore the flow rate. Never regulate the flow with a valve on the suction side of the pump. Horizontal centrifugal pumps are available in two major styles-either direct drive or magnetically coupled. Direct-drive pumps use a mechanical seal to prevent leakage of the solution from the back of the pump. The mechanical seal is generally the only normal wearing part on this type of pump and it must be expected that it will require occasional replacement. It is always a good idea to have a spare mechanical seal on hand. Failure to replace a mechanical seal as soon as it is necessary can lead to both costly downtime and additional damage to the pump. Wear of a mechanical seal increases as the pump pressure increases; therefore, to extend the life of the seal it helps to change the filter media before the back pressure gets excessive. Many pumps using a mechanical seal, especially larger pumps, require a fresh-water seal flush of 1/4 to ¥2 gpm. This should be taken into account before specifying this type. Direct-drive pumps are offered in two different configurations. Closed-coupled pumps are built right to the face of the motor and the pump shaft is attached directly to the motor shaft. This leads to a very compact design, insures alignment between the pump and motor shaft, and helps lower the cost of the pump. Frame-mounted pumps are built onto a ball-bearing pedestal, which is connected to the motor shaft with a flexible coupling. Since the pump and motor are two separate items they must be mounted to a baseplate with a coupling and a coupling guard. Care must be taken to insure that the pump and motor shaft are aligned, otherwise vibration will result, which will damage the pump and motor bearings, mechanical seal, coupling, and posMetal Finishing
Figure 5. Horizontal magnetic-drive centrifugal pumps.
sibly other components. Frame-mounted pumps area usually more expensive than their close-coupled counterparts because of the increased number of components. The second type of horizontal pump widely used in our industry is the magnetically coupled sealless pump. (See Fig. 5.) There is no direct coupling between the pump and motor shaft and no mechanical seal is required. Because there is no seal, magnetic pumps are said to be leakproof. They are generally close coupled, that is, built to the face of the motor. The drive magnet is attached to the motor shaft, which turns the driven or slave magnet, which is either connected to or an integral part of the impeller. Between the two magnets is a cup or disk that presses against an O-ring between it and the pump casing, which seals the pump. Magnetic pumps are usually suitable for specific gravities up to 1.2 at open pumping. Since a filter reduces the flow rate that a pump will develop, they can even be used as a filter pump for heavier solutions, such as nickel, that can have specific gravities up to 1.4. Unless properly equipped for higher specific gravity solutions, magnetic-drive pumps can be difficult to prime, especially on these heavier solutions. The coupling may slip, leaving the motor running but the impeller standing still. If this occurs close the valve on the discharge side of the pump prior to starting the motor. This allows the two magnets to get up to speed. The valve can then gradually be opened to reach the desired flow. If the valve is opened to a point where the coupling slips, repeat the procedure; however, open the valve only to the point just before the coupling slipped. This is similar to the way a October 2000
Figure 6. In-tank vertical pump.
valve is used to avoid an overload situation of a motor that was earlier reviewed. Because of the magnetic pumps' "leakproof' characteristics, they are frequently used on precious metal plating baths to avoid losing expensive solutions from a mechanical seal leak. Most horizontal centrifugal pumps are not selfpriming; therefore, the operator must flood the pump body and inlet line prior to starting the pump. Failure to do so can result in the pump being run dry, which quickly leads to heat buildup and costly damage to the pump-especially mechanical seals and seal housings. Some manufacturers of magnetic pumps now offer self-priming versions, which will prime themselves once a priming reservoir is filtered. These pumps offer a form of run dry protection once the reservoir has been filled, and they can pull up to a 10- to 20-ft suction lift. They are available in sizes up to 85 gpm (5,100 gph). The last major category of centrifugal pumps is the vertical pumps, which have gained wide acceptance for plating and filtration applications for several reasons. (See Fig. 6.) They are of the cantilever design where oversized motor bearings support the pump shaft. Several manufacturers use a sleeved, extended motor shaft, which serves as pump shaft. This helps insure vibration-free performance. 35
Pumps ofthis style have no pump bearings and no mechanical seals, meaning they can be run dry indefinitely without damage. Because there are only a few component parts, usually a motor, impeller, body, and inlet cover, they are relatively inexpensive compared to some horizontal pumps of similar performance. When mounted inside the tank vertical pumps are self-priming as the impeller is usually below the liquid level. Most of these vertical pumps incorporate an opposing twin-impeller design. The function of the upper impeller is to pump any solution going up the column back down and out the discharge, preventing any leakage. This feature allows the twin-impeller pump to be mounted vertically, either inside or outside the tank. The ability to mount the pump outside the tank is important in that it saves valuable in-tank space and keeps the pump away from heaters, hoists, barrels, and the like. It also avoids placement of the motor directly over the fumes or vapors that are developed in the process tank. Vertical pumps also save valuable floor space as they are usually "C" clamped or bolted directly to the tank flange and can be used with a filter chamber that mounts similarly. Vertical-bearing free pumps are suitable for any plating solution, including electroless nickel and copper, and when mounted inside the tank can be used for precious metal solutions. Using the pump in tank insures that, in the unlikely case of a leak, the fluid would be contained by the tank, avoiding loss of these expensive solutions. The other major type of pump used for filter pumps is the self-priming or positive-displacement pump. They usually are designed with a flexible rubberlike impeller or liner. The most popular ones utilize a mechanical seal to prevent leakage. To achieve the self-priming feature, rubbing of the impeller must occur on the pump body, end cover, and backplate to develop the positive seal necessary to create the vacuum required for a suction lift, This rubbing produces wear and eventual replacement of the flexible impeller and the seal must be expected. This wear increases as the pressure the pump produces increases. Increasing the surface area of the filter helps to reduce the back pressure and, therefore, the wear that occurs. The convenience of the self-priming pump should be weighed against the increased costs of maintenance that can be expected. The development of sealless magnetic-drive self-priming pumps has reduced the use of positive displacement pumps. If a filter system is frequently going to be moved from 36
one tank to another, the convenience of a self-priming pump may be preferable to other styles. SELECTION OF A PUMP AND FILTER SYSTEM
First, of course, you must know your application. That doesn't just mean the solution, tank size, and temperature. Consider conditions unique to your plating operation that will influence your decisionfactors, such as rack or barrel plating, geometric shape of part or type of part (tubular pieces or parts that are hard to clean), amount of drag in, and the general environment of the shop. With that information you should be able to decide the turnover rate necessary for this bath. As mentioned, turnover rate and dirt-holding capacity are related to the surface area of the media. You may find a filter having 100 ft.2 of surface area will provide the desired turnover rate, but you may want to increase the surface area to 150 or 200 ft 2 to increase your dirt-holding capacity. By doing this you will reduce the amount of changes of the filter media you will have to make while you increase your average filtration rate as your media does not clog up as quickly. Next, decide whether or not carbon treatment is required and select the style of carbon treatment that will be the most effective and economical for your size tank. For example, it would not be wise to try to carbon treat a 1,500 gallon bright nickel tank with carbon cartridges unless you're expecting an expensive Christmas gift. from your carbon tube supplier. Using bulk carbon systems will be much more economical and you may get a Christmas bonus from your employer. Once you have decided what type and size filter chamber you require, the last thing is to select the pump style that is most appropriate for your needs. When you purchase a filter system (see Fig. 7), other than replacement of the filter media, not too much maintenance is necessary on the filter chamber itself. The pump is the component of the system that can either be a maintenance headache or a relatively pleasant experience. The wrong choice of pump style can be very costly over the years that you expect to own the filter. Some of the factors you should consider when selecting a pump for your filter are: 1. Material compatibility of pump and solution
2. 3. 4. 5. 6.
Temperature Flow rate and pressure required How heavy is the dirt load Is water for a seal flush available Does solution crystallize Metal Finishing
Figure 7. Typical filter system.
7. Is self-priming necessary 8. Specific gravity 9. Any constraints concerning space for mounting, electrical service, etc. 10. Any other in-plant considerations that will affect choice of pump As mentioned, most manufacturers will provide a pump whose capacity in gallons per minute matches the capacity of the filter chamber. If everything else is equal, select the pump that delivers the highest pressure for the flow conditions of your filter. That will insure the longest life of your filter media. In general, unless you will be moving your filter from tank to tank, stay away from the positive placement self-priming pump as you can expect them to have the highest maintenance cost. Magnetic-drive selfpriming pumps are usually a better choice. Vertical centrifugal pumps usually require the least maintenance as they have no norma! wearing parts and can even be run dry. Even solutions that crystallize when standing, such as nickel and acid COpper, will not cause appreciable damage on start-up as there are no parts rotating against each olher. The abrasiveness of crystallized salts can lead to premature seal and impeller wear in other styles. If an out-of-tank, floor-mounted pump is required, magnetic pumps are likely to be more leakproof, and
-
October 2000
generally require less maintenance, than an equivalent pump with a mechanical seal; however, they are limited in the flow rate, although newer, larger models are being introduced, and they can be finicky when used with higher specific gravity solutions unless properly equipped. Direct-drive centrifugal pumps handle higher specific gravities well and are available in almost any conceivable flow rate and pressure range the operator may require. As centrifugal pumps vary widely in efficiencies, never state the size of the pump you require in terms of horsepower. Always state the flow and pressure desired. After deciding what features you require ask for recommendations, prices, and above all literature. Review the literature carefully to be sure that when you are comparing prices, you are comparing similar products. Check for ease of filter change, flaw rate and pressure of the pump, and maintenance requirements. Compare the surface area and carbon capacity. See what the cost would be to increase the size of the filter chamber. Very often you will find that you can increase your surface area without adding much to the cost of a filter system. Lastly, remember that this new filter and pump will hopefully be the last one you will buy for this tank for a very long time. If you undersize the capacity of the filter, or compromise on the pump, you'll have to live with it for several years. It's wise to get exactly what's necessary, rather than saving a small amount and spending many times that amount reworking rejects or replacing media or worn out pump parts. If the filter is to be dedicated to this one tank rather than moved around, select/specify a centrifugal pump style that will be best for your in-house considerations. A vertical or magnetic will give good service and require the least maintenance, and they are available in the size required. One of these will probably be your best bet. Now rather than ask your distributor or filter manufacturer to simply quote a filter for a 500gallon nickel tank, you can instead state your solution and specify that the system should have a flow rate of at least 1,500 gph, have 10 to 15 filter tubes, have a carbon capacity of about 10 lb, and be equipped with a vertical pump for in-tank mounting. In this way you will be sure of getting a system that not only has all the features you want, but one that has all the specifications necessary to do the only job it will ever perform, removing suspended solids and organic contamination from your tank without nceding a lot of attention from you or your maintenance personnel. MF
37