| dc.description.abstract | The essential elements to be reviewed when considering a centrifugal pump application are highlighted. The purpose is to ensure the procurement and/or application of a centrifugal pump which optimizes reliability, energy usage, maintenance costs, and achieves this in a safe manner. The need to understand the system hydraulics and complete process requirements is emphasized, as is the impact each system parameter has on pump design. The test is broken into three basic areas: pump boundaries conditions, flow requirements, and pump specification – key elements. Pump boundary conditions addressed NPSH considerations as they relate to flow requirements, potential for cavitation damage, suction specific speed, and mechanical seal temperature margin. This section also addresses system resistance variations, and addresses the effects of variations in dynamic and static system heads and the potential prime causes of variations in these heads. Flow requirements discusses the determination of normal, maximum (or rated), and minimum flow requirements. A method of approximation of a pump’s minimum acceptable flowrate is presented (based on the pump’s hydraulic specifications), which utilizes suction specific speed and moderating factors such as impeller head, NPSH margin, specific gravity, etc. Attention is paid to the effect of excessive specified flow on NPSHA, flow rangeability, and suction specific speed. Mechanical and hydraulic interrelationships are discussed to emphasize how flowrate variations affect radial and axial thrust, impeller suction and discharge recirculation, and potential for cavitation; also covered are the effect of variation in running clearances and changes in specific gravity. Critical aspects of both parallel and series pump application are addressed. The dangers of incorrect parallel pump application are highlighted and guidelines are offered for correct application of parallel pumps. Series pump operation is discussed primarily in terms of percentage split in pumps’ combined total head, pump protection and system protection. Pump Specification – Key Elements addresses communicating to prospective pump vendors the key parameters that may determine the metallurgy, mechanical seal design and mechanical seal peripheral’s design, and pump hydraulic design best suited to the user’s requirements. The liquid specification is highlighted, the important site and operating conditions are addressed, and the importance of clearly defining the key performance requirements in full is discussed. The wrong pump, operated incorrectly coupled with poor maintenance practices, results in unsatisfactory hydraulic performance, high energy costs, high maintenance costs, poor reliability, and increases the potential for an unsafe failure. Proper specification and selection principles, coupled with the implementation of correct operating and maintenance practices, will result in optimized centrifugal pump application. The net result will be a pumping application that meets all process demands, with a minimum of energy usage, and low frequency of repair (high reliability), low overall maintenance costs, and low process debits. Key hydraulic and Performance Criteria are addressed along with their effects on performance, energy, reliability, energy, reliability, maintenance costs, and safety. The interrelationships that exist between hydraulic characteristics and the mechanical reliability is highlighted to ensure that neither are treated separately. | en |
