A water pipeline consists of three pump stations, each having one constant-speed motor-driven pump of 2000 HP. The analysis of current operations shows that the pipeline has been operating at a fairly constant flow rate of 3000 gal/min, and the pump throttle pressures are as follows:
Pump station 2:75 psi
Pump station 3:85 psi
Pump station 1 has zero throttle pressure. Trimming the pump impellers to the correct size so as to eliminate wasted HP from pump throttle is estimated to cost $50,000 per pump station. Retrofitting each pump station with VFD motor-driven pumps is estimated to cost $500,000 per pump station. The operating cost for the VFD units is $50,000 per site.
(a) Determine the economics of installing trimmed impellers and the payment period based on 8% interest rate. Assume 80% pump efficiency for HP calculations and an electricity cost of 10 cents/kWh.
(b) If it was decided not to trim the pump impellers, compare the VFD motor option with the constant-speed pump option. Consider the control valve annual operating cost to be $5000 per site.