Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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plant engineering, filtration & purification 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 to deliv- er and maintain the desired flow rate and pressure as the dirt builds up on the fil- ter medium. Proper pump and seal selection is critical and requires the following considerations: 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 temperature All construction materials must be compatible with the solution being pumped. In some cases, it is advisable to specify a construction material that will corrode slow- ly within tolerable limits if the material of ideal chemical resistance is too costly. In addition to the initial investment, careful consideration must be paid to the costs of pump operation, down time, parts, and labor. One should also consider, for each application, the relative advantages and disadvantages of the various styles. Fig. 1. Horizontal centrifugal pump. The rotation of the impeller imparts velocity to the liquid. Centrifugal force moves the liquid to the periphery of the casing and toward the discharge port. When the liquid in the impeller is forced away from the center of the liquid, a reduced pressure is produced and consequently more liquid flows forward. PUMP TYPES Horizontal centrifugal pumps (Fig. 1) are the most common pumps used in the plat- ing industry. Usually, the only part that wears is the seal. Flow rates are high, and pressure is moderate; thus, this pump is suitable for most filtration requirements. Care must be taken when pumping liq- uids with a specific gravity higher than 1.0 to ensure that the motor is not over- loaded. A valve on the discharge adjusts the flow and thus the required power when the centrifugal pump is working against virtually no restriction, such as when oper- ating with a clean filter. Care is usually taken by the manufacturer to supply a suf- ficient amount of horsepower to prevent this overloading, and also protection is provided in the motor starter. Some users de-rate the system by using a motor of lower horsepower to save on operating cost. To guard against overload, the dis- charge valve must be employed. Close-coupled, horizontal pump-motor units are available in all price ranges and sizes and offer the greatest advantage in 659

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