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Fig. 6. Progressive cavity pumps. As one cavity formed by the offset helix diminishes, the opposite cavity increases. The result is constant, uniform flow over the length and out the discharge port. of the rotor. These pumps are well suited for high-pressure, low-flow conditions on either low- or high-viscosity liquids. Horizontal centrifugal pumps not normally thought of as self-priming can be made self-priming by the addition of a priming chamber to the suction or discharge sides (or both) of the pump. Once the chamber is filled with liquid and the fill port securely sealed, suction lifts of up to 25 ft (depending on individual pump char- acteristics) may be achieved. Some pumps are capable of only a few feet of suction lift when a priming chamber is used. Basket strainers are available for priming cham- bers to prevent large solids from damaging pump internals. CENTRIFUGAL PUMP PRIMING Priming of centrifugal pumps can be made easier if the following precautions are taken. Avoid all sharp bends or crimps in the suction hose. Prevent small parts from entering or restricting flow to the suction hose. Prevent air from getting into the pump by checking for poorly connected hose or flanged fittings, which may have vibrated loose. The slightest amount of air coming from an insufficiently tight 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. (See tips on pump pack- ing 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 two loca- tions, and sooner or later priming will become more difficult. Air in the filter cham- ber is also an indication that the suction from the tank may be too close to an air out- let being used for solution agitation. A pump discharge fitted with a set of eductors could eliminate the problems associated with air agitation. Remember, the larger the pump, the more velocity is created and the more tendency to pull air into the suction opening. Priming is made easier with a slurry tank or priming chamber above the pump, making it possible to always have a flooded suction. Recirculating through the pump, filter, and slurry tank and then slowly opening the line to the plating tank grad- ually purges the system of air. The suction valve from the plating tank should initially be opened only a crack, so that the pump does not get a slug of air at one time. This air also collects in the filter chamber and must be released by venting. In a precoat- ed filter, any constant collection and venting of air soon results in ineffective filtra- tion. As air collects, the cake falls away and is redeposited elsewhere. Subsequent vent- ing returns solution to the unprecoated surface, where there 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 662