Consider a single-pump circuit as shown in the figure. The fluid control valve (FCV) fails due to erosion caused by cavitation every 10 to 12 months at a cost of $4,000 per repair. A change in the control valve is being considered: Replace the existing valve with one that can resist cavitation. Before the control valve is repaired again, the project engineer wants to look at other options and perform an LCCA on alternative solutions
Example 6.9 Cont. The control valve currently operates between 15 and 20% open and with considerable cavitation noise from the valve. It appears that the valve was not sized properly for the application. After reviewing the original design calculations, it was discovered that the pump was oversized 110 m3/hr instead of 80 m3/hr, Resulting in a larger pressure drop across the control valve. The engineer determines that the control valve is not suitable for this process. The following four options are suggested: ➢ Option A: A new control valve can be installed to accommodate the high pressure differential. ➢ Option B: The pump impeller can be trimmed so that the pump does not develop as much head, resulting in a lower pressure drop across the current valve. ➢ Option C: A variable-frequency drive (VFD) can be installed and the flow control valve removed. The VFD can vary the pump speed and thus achieve the desired process flow. ➢ Option D: The system can be left as it is, with a yearly repair of the flow control valve to be expected
Example 6.9 Cont. Cost Summary ➢ Option A: The cost of a new control valve that is properly sized is $5,000. ➢ Option B: By trimming the impeller to 375 mm, the pump’s total head is reduced to 42.0 m (138 ft) at This drop in pressure reduces the differential pressure across the control valve to less than 10 m (33 ft), which better matches the valve’s original design intent. The resulting annual energy cost with the smaller impeller is $6,720 per year. It costs $2,250 to trim the impeller. This cost includes the machining cost as well as the cost to disassemble and reassemble the pump. ➢ Option C: A 30-kW VFD costs $20,000 and an additional $1,500 to install. The VFD will cost $500 to maintain each year. It is assumed that it will not need any repairs over the project’s eight-year life. ➢ Option D: The option to leave the system unchanged will result in a yearly cost of $4,000 for repairs to the cavitating flow control value
Example 6.9 Cont. Assumptions ▪ The current energy price is $0.08/kWh. ▪ The process is operated for 6,000 hours/year. ▪ The company has a cost of $500 per year for routine maintenance of pumps of this size, with a repair cost of $2,500 every second year. ▪ There is no decommissioning cost or environmental disposal cost associated with this project. ▪ The project has an eight-year life. ▪ The interest rate for new capital projects is 8%, and an inflation rate of 4% is expected.