Pump Replacement Under a Fixed Budget and Shutdown Window
A small water utility operates a booster pump that delivers 42 L/s against a 35 m head. The existing unit draws 22 kW at the shaft; the motor is rated 30 kW with an efficiency of 91%. The utility has a capital budget of 18,000 currency units and a single 6-hour shutdown window. Two replacement options are on the table. Option A: a high-efficiency pump costing 16,500, expected shaft power 17.5 kW, delivery lead time 9 weeks. Option B: a refurbished pump costing 7,200, expected shaft power 20.0 kW, available in 2 weeks. Electricity costs 0.14 per kWh and the pump runs 5,600 hours per year. The operations manager wants Option B because it leaves budget for spares; the finance officer wants Option A because of the efficiency gain. The maintenance team notes the existing pump has failed twice in the last 14 months and that the shutdown window cannot be extended.
1. Quantify the annual energy saving of Option A over Option B and assess whether the efficiency argument alone justifies the extra capital cost.
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Compute the electrical input for each option by dividing shaft power by motor efficiency (0.91).
- Option A: 17.5 / 0.91 = 19.23 kW input.
- Option B: 20.0 / 0.91 = 21.98 kW input.
- Difference: 2.75 kW.
Annual energy difference = 2.75 kW x 5,600 h = 15,400 kWh. At 0.14 per kWh this is 2,156 per year.
Extra capital cost of A over B = 16,500 - 7,200 = 9,300.
Simple payback = 9,300 / 2,156 = 4.3 years.
So the efficiency case is real but not decisive on its own: a 4.3-year payback may or may not clear the utility's hurdle rate, and it ignores the failure risk of the existing unit. The efficiency argument supports A only if the utility expects to run the pump well beyond 4.3 years and values the avoided downtime. It does not by itself settle the decision, because B is 9,300 cheaper and available 7 weeks sooner.
2. Recommend an option, stating the assumptions and the residual risks the utility must accept.
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The recommendation should weigh cost, availability, and failure risk rather than efficiency alone.
Option A fits the 18,000 budget (16,500 leaves 1,500) but its 9-week lead time means the failing pump must survive roughly two more months. With two failures in 14 months, that is a material risk: an unplanned outage during peak demand could cost far more than the 9,300 capital difference.
Option B is available in 2 weeks, fits within the 6-hour shutdown, and leaves 10,800 for spares and contingency. Its energy penalty is 2,156 per year, which is modest against the avoided outage risk.
A defensible recommendation is B, conditional on: (1) confirming the refurbished unit's expected service life and warranty; (2) holding the 2,156/year energy penalty as an accepted cost; (3) scheduling A's efficiency upgrade into the next capital cycle once the failing unit is replaced. If the utility can tolerate the 9-week exposure, A is preferable on lifecycle cost. The key judgement is that availability and failure risk dominate a 4.3-year payback in a system with a single pump and no standby.