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Using the criteria proposed here, operators can analyze hydraulic factors to boost pump reliability and reduce unscheduled maintenance and lifecycle costs
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Pump selection in the chemical process industries has historically been made based simply on hydraulic fit for the intended duty. Pump efficiency has long been a key criterion used during pump selection, and most pump-selection software presents candidate models in order of decreasing efficiency. Other criteria used include net positive suction head (NPSH), NPSH margin,1 operating speed and pump cost.
Recently, however, significant attention has been given to the lifecycle cost of pump ownership. Major components of this overall cost of ownership include the initial equipment cost, and the costs associated with installation, operation and ongoing maintenance.
In this article, we propose a new method for assessing pump reliability, called the NPSH-Margin Reliability Factor (NPSHRM-RF; discussed in detail below). This method mainly addresses the impact of cavitation on pump reliability, by quantifying the interaction between suction energy [3] and NPSH margin [4\. Readers should note that the various methodologies discussed in this article apply only to centrifugal pumps, not to rotary or reciprocating pumps.
A new approach
The cost of power is often the largest cost associated with pump ownership. While this would seem to validate the (historic) use of efficiency as the primary pump-selection factor, the relatively narrow spread of efficiencies minimizes the effectiveness of this approach (For example, because the efficiency of many pumps and motors have evolved to the point where they easily operate near their theoretical maximums, most pumps offered for a given application are not very far apart, in terms of efficiency.) Even though the cost of power is high, opportunities to trim the pump operating cost by selecting the most-efficient pump tend to be limited. Thus, further reductions in the power cost component will have to come from improvements in the process, or by selecting a pump to truly operate near its best efficiency point (BEP).
In practice, unscheduled maintenance is usually the most significant cost associated with pump ownership. The literature primarily addresses mechanical means of improving reliability; rarely is an objective guideline provided on how to boost pump reliability overall and minimize unscheduled downtime, or how to select the best...