A procedure for sizing pump-pipe systems with regard to minimising life cycle costs manageable on excel spreadsheets

Abstract

Pump-pipeline systems are a common feature of every industry and account for about 20% of the world’s electrical energy demand. Pump and pipe selection should happen simultaneously in pump-pipe system design rather than first sizing the pipe and then finding the pump to go with the pipe. Further, proper selection of systems should go beyond just considering only the initial cost but the total cost of ownership- the life cycle cost (LCC). In this paper a spreadsheet tool is developed for pump-pipe system technical analysis with the output design parameters facilitating LCC analysis. The program can calculate the operating point of any size of pump, at any given speed and with any pipe size and by use of appropriate cost models determine the unit cost of pumping for each system. Dimensionless pump characteristic curves that are generic for all radial flow pumps are modelled by multi-polynomial equations. Dimensional similitude can then be used to determine the actual characteristic curves for any pump of given impeller diameter and rotational speed. The system resistance curve is calculated from well-known hydraulic formulae and represented by a quadratic equation. The operating point of the pump-pipe system is obtained from a simultaneous solution of the quadratic equations representing the pump and the pipe resistance curves. Best practice technical constraints, like maximum deviation from best-efficiency point (BEP), allowable net positive suction head and maximum allowable operating hours, can be set by the designer. Operating the pump-pipe system near best efficiency point is desirable since it reduces both energy and pump maintenance costs. A pump-pipe system is selected only if it falls within designer-specified best practice constraints. The life cycle cost of each system that passes the first test is then calculated using discounting techniques and the pump-pipe system with the least LCC is adopted.