مقالات تکنولوژی و تجهیزات ابکاری

finishing equipment & plant engineering

SELECTION AND CARE OF PUMPS

BY JACK H. BERG

SERFILCO LTD., NORTHBROOK, ILL.; WWW.SERFILCO.COM

Since the pump is the heart of the filtration system, it must have the ability todeliver and maintain the desired flow rate and pressure as the dirt builds up onthe filter medium. Proper pump and seal selection is critical and requires the followingconsiderations:

1. Flow rate required (tank turnovers per hour in gph)

2. Location (in or out-of-tank)

3. Discharge head and distance

4. Filter medium and pressure drop

5. Solution corrosivity

6. Solution temperatureAll construction materials must be compatible with the solution being pumped.In some cases, it is advisable to specify a construction material that will corrodeslowly within tolerable limits if the material of ideal chemical resistance is toocostly. In addition to the initial investment, careful consideration must be paidto the costs of pump operation, down time, parts, and labor. One should alsoconsider, for each application, the relative advantages and disadvantages of thevarious styles.

 PUMP TYPES

Horizontal centrifugal pumps (Fig. 1)are the most common pumps used in theplating industry. Usually, the only partthat wears is the seal. Flow rates are high,and pressure is moderate; thus, this pumpis suitable for most filtration requirements.Care must be taken when pumpingliquids with a specific gravity higher than1.0 to ensure that the motor is not overloaded.A valve on the discharge adjuststhe flow and thus the required powerwhen the centrifugal pump is workingagainst virtually no restriction, such aswhen operating with a clean filter. Careis usually taken by the manufacturer tosupply a sufficient amount of horsepowerto prevent this overloading, and also protectionis provided in the motor starter.Some users de-rate the system by usinga motor of lower horsepower to save onoperating cost. To guard against overload,the discharge valve must be employed. Close-coupled, horizontalpump-motor units are availablein all price ranges and sizesand offer the greatest advantagein always assuring proper alignmentbetween the pump and themotor. They are compact and,therefore, require less floor space.Long-coupled pump-motor unitsuse standard motors and usuallyrequire an additional mountingplate to assure proper alignment.Improper alignment causes vibrationof the pump and motorassembly, which, in turn, causesfailure at the motor and pumpbearing; it also has an adverseeffect on the pump seal.Turbine pumps are similar to centrifugal pumps in basic design. These pumpsprovide high-discharge head at lower flow rates than do centrifugal pumps.However, they should only handle clean, low-viscosity liquids.Vertical sump pumps are usually of the centrifugal type and, depending upondesign, may have no bearings at all. This first type is referred to as a cantilever or bearinglessvertical pump. They are capable of running dry at high speed but are limitedto a length of 1 ft. If pumping is initiated only after the pump casing is immersed, asuction extension will allow up to 10 ft of deep drainage from a 1-ft long cantileveredpump. Cantilever-type pumps can also be mounted external to the tank.The short plastic cantilever pump is well suited to mixing, agitating, or transferringmany types of solutions. The performance is like that of their horizontal counterpart;however, there are no wearable parts. The short cantilever shaft requires nosupport and has neither seals nor bearings. A double impeller prevents the solutionfrom being pumped up the column, even at no flow and maximum head. Since thesepumps are seallessand have generous clearances, they are suitable for electrolessnickel and can even run dry. These pumps are said to be maintenance free.Longer pumps require one or more bearings, which may also act as seals. Verticalpumps with sleeve bearings should be specified with as short a column length as ispractical. They should be driven by 1,725 rpm motors where possible to reduce theload and subsequent wear on the bearings; however, loss of performance should beexpected at 1,725 versus 3,450. For the best results, bearings should receive freshwater rather than product flush. Long pump columns with multiple bearing setsdemand perfect motor-bearing-pump alignment.Magnetic-coupled pumps (Fig. 2) are unique because they require no directmechanical coupling of the motor to the pump impeller or shaft, and therefore noseals are needed, making them truly leakproof. The pump body is generally constructedof various plastics, and the impeller magnets are encapsulated in plasticto eliminate any metal contact with the solution. Those without internal carbonbearings are used for electroless solutions. Magnetic pumps are also available withencapsulated motors, so that the entire unit may be submerged in the liquid. Thisis an extremely desirable feature for use in precious metal plating, to avoid loss ofTo efficiently provide a self-priming feature, close tolerances or actual rubbingmust occur on both impeller and/or moving parts on the body of the pump. Mostnoteworthy is the fact that the greatest amount of wear occurs when the pump isdeveloping its greatest amount of pressure as the plating filter is approaching maximumreduction of flow due to dirt pickup. Therefore, oversizing the filter will reducethe frequency of this occurrence. The flexible impeller (Fig. 3) and the liner impeller(Fig. 4) are both self-priming. They develop pressures up to 20 psi but requirerelatively frequent impeller or liner replacement when used continuously. Also, theycannot be used on abrasive solutions or where dry-running capability is required.Air-operated diaphragm pumps (Fig. 5) do not have rotating seals, impellers, orother internal parts. They depend on a pulsing, intermittent reciprocating motionacting on an elastomeric membrane to form a liquid chamber between two checkvalves and thus produce low flow rates at high pressure. The air supply can be regulatedto produce certain performance requirements. Because of their self-primingfeature, capability to run dry, and ability to handle extremely viscous liquids ormaterials with a high solids content, they are widely used in waste treatment andin other industrial applications. However, since these pumps pulsate, the filter andpiping require pulsation dampening.Another common self-priming pump design is the progressive cavity design (Fig 6). This design uses a rotor, which has a helix turning inside a stator with a similarhelix at a set pitch. Liquid is passed from one chamber to another along the lengthof the rotor. These pumps are well suited for high-pressure, low-flow conditions oneither low- or high-viscosity liquids.Horizontal centrifugal pumps not normally thought of as self-priming can bemade self-priming by the addition of a priming chamber to the suction or dischargesides (or both) of the pump. Once the chamber is filled with liquid and the fill portsecurely sealed, suction lifts of up to 25 ft (depending on individual pump characteristics)may be achieved. Some pumps are capable of only a few feet of suction liftwhen a priming chamber is used. Basket strainers are available for priming chambersto prevent large solids from damaging pump internals.

CENTRIFUGAL PUMP PRIMING

Priming of centrifugal pumps can be made easier if the following precautions aretaken. Avoid all sharp bends or crimps in the suction hose. Prevent small partsfrom entering or restricting flow to the suction hose. Prevent air from gettinginto the pump by checking for poorly connected hose or flanged fittings, whichmay have vibrated loose. The slightest amount of air coming from an insufficientlytight threaded fitting or a loose flanged fitting prevents successful priming.Fittings with an “O” ring provide for a positive seal. As the pump packing wears,it will also suck air and, depending on usage, must be adjusted as required. (Seetips on pump packing and the use of water lubrication to prevent sucking air.)If frequent venting of the filter chamber is necessary when the filter is running,it is likely that an air leak has developed some place at the previously described twolocations, and sooner or later priming will become more difficult. Air in the filterchamber is also an indication that the suction from the tank may be too close to anair outlet being used for solution agitation. A pump discharge fitted with a set ofeductors could eliminate the problems associated with air agitation. Remember, thelarger the pump, the more velocity is created and the more tendency to pull air intothe suction opening. Priming is made easier with a slurry tank or priming chamberabove the pump, making it possible to always have a flooded suction. Recirculatingthrough the pump, filter, and slurry tank and then slowly opening the line to theplating tank gradually purges the system of air. The suction valve from the platingtank should initially be opened only a crack, so that the pump does not get a slugof air at one time. This air also collects in the filter chamber and must be releasedby venting. In a precoated filter, any constant collection and venting of air soonresults in ineffective filtration. As air collects, the cake falls away and is redepositedelsewhere. Subsequent venting returns solution to the unprecoated surface, wherethere is no filtering action, and the contaminated solution passes through.To prime a centrifugal pump, if a hose is used on the suction side of the pump(without a slurry tank), liquid may be introduced through the hose and pump intothe filter chamber. The filter need not be filled completely, but most contain a sufficientvolume of liquid so that, as the hose is lowered to approximately the sameheight as liquid in the chamber, the hose will gradually fill with solution. Shakethe hose to make certain any air trapped in the top of the pump or in other highpoints is completely expelled. When the liquid level completely fills the hose, keepthe tip of the hose at the same position, but close the valve between the pump andthe filter chamber. Now insert the hose in the tank (since the valve is closed, virtuallyno liquid will run out of the hose if a gloved hand is cupped over the end). Startthe motor and wait until the motor has reached its proper speed; then slowly openthe valve to the filter. This is a further precaution, which will enable the pump tocreate enough suction to handle the small amount of air that may still be in the line.When transfer pumping out a tank, it is advisable to connect a 90O hose barbor a strainer to the suction end of the hose so that it may be lowered as solutionlevel drops. This prevents cavitating the pump, which could occur if the end ofthe hose rested flat on the bottom or against the side of the tank. If the hose hasa tendency to curl, insert a length of straight, corrosion-resistant pipe into theend to accomplish the preceding purpose. Since the most difficult time to primea pump is after most of the solution has been removed from the tank, operatorsoften dump this remaining heel, which is a needless waste of solution. Platingtanks with sumps at one end minimize this loss when solution transfer is necessary.Small self-priming pumps, such as drum pumps, may be used to salvage theheel left in the plating or treatment tank..

 PUMP SEALS

The available types of pump seals vary from no seal at all to lip type, packedstuffing box, and mechanical. Since conventional pumps have an interconnectingshaft between the pump impeller and the motor, a suitable seal is necessary toprevent leakage during the rotation of this shaft. A magnetically driven impelleror vertical cantilever are perhaps the only truly seal-less pumps. Other pumps,which use a liner, or section of hose, are seal-less; but, since these componentsmay fail through usage, fatigue, and abrasive wear, the system, like any other,is subject to eventual leakage. It is always desirable to replace seal componentsbefore leakage occurs. Unfortunately, one never knows just how much longera seal will last before replacement is necessary. They may operate from a fewminutes to a more realistic several years.A lip-type seal consists of a molded, rubberlike material, which has a squeegeeaction in snugging itself around the shaft. A mechanical seal consists of two mirrorlikelapped surfaces, one rotating with the shaft, the other stationary in thepump, which are held together by a light spring pressure, preventing leakage. Thepreferred arrangement is an outboard mounted seal, so that exotic or nonmetallicseals are eliminated. A packing stuffing box consists of a suitable cavity, withthe rotating shaft in the center, around which a compressible-type material maybe inserted in alternating rings and held in place and adjusted by tightening thepacking gland. Both the mechanical seal and the stuffing box seal are availablewith provision for water lubrication or recirculation of the solution being pumped.Usually, water from an external pressure water line is desirable, because it assurescooling and lubrication of the seal components. It reduces wear by keeping filteraid and dirt out of the seal area. The water also prevents the solution from crystallizingon the seal faces during shutdown periods. Even while the pump is running,crystals may form as plate-out might occur with electroless solutions.On double-seal pumps, care must be taken through the use of a check valve, orsiphon breaker, so that no solution is pumped into the water system during anunexpected failure. Also, a regulator should be installed in the water line to controlthe pressure, because it will vary from low when the plating room is in operation tohigh during the weekend when no other water is being used. If the water pressureand flow to the seal are not regulated, it is possible to actually draw water throughthe packing into the plating tank, especially when the filter is clean, because anegative pressure exists at this point. This could cause chemical imbalance andeven overflow of the plating tank. Solutions requiring deionized water for the sealuse a double-seal arrangement, with an additional small pump recirculating thedeionized water in the seal area.When selecting the type of seal to use, consider the fact that a stuffing box sealor lip-type seal wears slowly, giving warning that replacement will be necessaryby gradually increasing constant leakage. A mechanical seal is more trouble-freeon a day-to-day basis and yet may fail without warning; thus, there is a need forpreventive maintenance. (See piping instructions to minimize solution loss.)Certain types of packing are more suitable for acid, and others are more suitablefor alkaline solutions. The construction materials in a mechanical seal, such as thetype of carbon and ceramic, along with what type of elastomer, also vary. Therefore,it is important to give the type of service to the manufacturer to assure suitablematerials of construction. Some seal wear has to be expected, and periodic replacementof components is necessary. Whenever replacing the seal or packing, the pump shaft should be inspected. If worn or scored, it must be replaced.