HYDRAULIC SELECTION TO MINIMIZE THE UNSCHEDULED MAINTENANCE PORTION OF LIFE CYCLE COST Authors:

Recently significant attention has been given to the life cycle cost of owning a pump. Major components of the cost of ownership are initial cost, installation cost, operating cost, and maintenance cost. For nearly all applications it has been found that the initial cost is a very small percentage of the life cycle cost. In most commercial and municipal applications, the cost of power is the largest component of the cost of ownership. This validates use of efficiency as the primary selection factor. Even though the cost is high, opportunities to significantly reduce the operating cost are limited because the efficiency of pumps and motors are near their theoretical maximums. Further reductions in this cost component will come from improvements in the process. In process plants it has been found that under many circumstances the cost of unscheduled maintenance is the most significant cost of ownership. Although numerous papers have been presented on the subject of pump reliability, that literature addresses mechanical means of improving reliability only, and does provide an objective guide for the user. There has been little published on the subject of the best hydraulic fit to optimize reliability. A test program was initiated to quantify the effects of three hydraulic reliability factors: operating speed, impeller diameter and operating point. Experimental measurements for each factor are compared to a published method that proposed a quantitative evaluation to guide the Application Engineer in making the best choice. Experimental measurements confirm the reliability chart for operating speed. The test data also confirms an optimum impeller diameter (between 60 –80% of the trim range) exists where reliability can be maximized. The operating capacity and impeller diameter charts show generally good agreement below best efficiency capacity and for impeller diameters below the optimum trim range, respectively. Recommendations are given for modifying the operating capacity and impeller diameter reliability charts for flows greater than best efficiency capacity and for impeller trims above optimum diameter, respectively. INTRODUCTION Reliability of centrifugal pumps has received considerable attention in recent years. The results of this attention have been an increase in the Mean Time 2 Between Failure (MTBF) for process plants. A survey conducted seven years ago by one of the authors revealed that in the North American chemical industry the MTBF for chemical process pumps was 15 months. Today the norm in the chemical industry is closer to 24 months and the refining industry approaches 5 years. Hrivnak [1] reported an increase in MTBF by a factor of 5.5 in a major chemical plant as a result of an extensive pump reinstallation project. His impressive results were achieved through improved installation practices, and increased attention to operating procedures. Efforts such as these will continue to yield improvements in MTBF, but will be limited in potential unless a holistic approach is used. Such an approach involves addressing all the factors of a Reliability Engineered System. Figure 1 illustrates the components of such a system. Figure 1. Factors in a Reliability Engineered System. The authors believe there are five significant factors that must be properly Engineered in order to achieve a reliable pump installation. Figure 1 illustrates these factors and gives examples of each. It is necessary to address each factor in a successful MTBF plan. The literature abounds with information on best practices for specifying the type, installation, and application of centrifugal pumps. Additionally, many courses are available that provide training on operation and maintenance. Specifying a pump with the optimum mechanical configuration is enabled through manufacturer’s literature, as well as many papers. Similarly, general guidelines exist for good hydraulic application. Rules such as “select a pump to operate at best efficiency point”, “slower speeds are better”, and “provide adequate NPSH margin” are common axioms. In most applications though, it is not possible to select a pump near best efficiency point, or impractical to select a slower speed pump. The Application Engineer is left to subjective judgement in making the hydraulic application. Bloch [2] has published a method that quantifies the effects of three hydraulic application factors: speed, impeller diameter tip clearance, and operating point. This method defines three Reliability Factors and proposes a quantitative evaluation of each to guide the Application Engineer in making the best choice. Although the method provides quantitative evaluations, it is itself subjective. It is the purpose of this paper to test the validity of the method using experimental measurements.