Handling Abrasive Sump and Tank Applications With Vertical Pumps

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August 2011

Handling Abrasive Sump and Tank Applications With Vertical Pumps

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Many sump and tank applications with abrasive solids specify Vertical type centrifugal pumps. Depending on the application parameters, a variety of options are available.
Mark Pawlik - Editor	Printable Version
HANDLING ABRASIVE SUMP AND TANK APPLICATIONS WITH VERTICAL PUMPS The vertical cantilever is well suited for these applications as there are no close clearance submerged bushing's or sealing rings that would be subject to rapid erosive wear from the solids in the liquid being pumped. This being the case, the pump remains in its “as new” condition, from a performance standpoint, for a long time. These pumps can run dry and handle harsh environments with high temperatures quite easily. However, vertical cantilever pumps operating below first critical speed are typically restricted in their overall setting (distance from the underside of the support plate to the centerline of the impeller) to a practical limit of 6 or 7 feet, depending on their operating speed, impeller weight, casing type (circular volute, expanding volute, double volute or dual discharge) and other factors. When the operating levels, and in particular the start level, are lower than the practical limit of the impeller centerline, several options are available to manage such situations. 1. Hip or Shoulder Mounting of the bearing frame to lower the radial bearing below the mounting plate. Hip mounting is when the thrust bearing is above the cover plate and the radial bearing is below the mounting plate. Shoulder mounting is when thrust bearing is at or below the mounting plate (see figure 1). By using a long spacer coupling or drive shaft (cardan shaft) the bearing frame can be lowered to significant distances below grade (see figure 2). Care must be taken to make sure the operating levels cannot flood the bearing frame. If there is such a possibility of flooding the bearing frame can be constructed to be air tight and pressurized with air at a pressure slightly higher than those conditions expected during occasional flooding situations. 2. Priming Inductor: A priming inductor is a fluid driven jet pump adapted for use on the entrance to a pump suction pipe. The inductor primes the pump by injecting an externally pressurized stream of fluid through the entrance throat of the inductor body to fill the suction pipe and pump with fluid after which pumping can be initiated. Priming inductors provide positive fast and reliable priming. They have no moving parts and therefore are quite simple in design. An additional feature of the priming inductor is that the operation is possible even when the suction line is embedded in sediment. (See figure 3). Vertical Line Shaft Pumps with water filled columns from clean service water offer a means to handle abrasive laden sumps and tanks that have settings which are impractical for cantilever pumps. Despite the advantages of cantilever pumps in abrasive sump applications, some vertical pump applications are beyond the reach of hip or shoulder mounted designs and the practicality of suction lift via suction inlet (tail) pipes. When faced with this and the desire or need to have a vertical configuration that is not a submersible, the most ideal situation is to provide a clean source of external fluid to fill the support columns and provide a clean contaminant free environment for the journal bearings to operate in. In this configuration either packing or mechanical seals are use at the top of the support column and typically a close clearance bushing is used at the lower most support column just behind the impeller to control the leakage of liquid from the column into the sump or tank (see figure 4). For settings longer than 20 feet, it is necessary to employ multiple shafts that are connected with rigid couplings. In cases where leakage from the column into the process must be kept to a minimum, a mechanical seal can replace the throat bushing at the bottom support column. In this case temperature rise of the liquid captured in the support column must be considered. If necessary, this captured body of fluid can be directed to an external heat exchanger for adequate cooling. Virtually all abrasive sump applications can be successfully handled by a properly applied vertical pump design.
Copyright 2011 Lawrence Pumps Inc. 371 Market Street Lawrence, Massachusetts U.S.A. 01843, Tel: 800-434-5248, Fax: 978-975-4291