Consider a single-pump circuit that transports a process fluid containing some solids from a storage tank to
Question:
Consider a single-pump circuit that transports a process fluid containing some solids from a storage tank to a pressurized tank. A heat exchanger heats the fluid, and a control valve regulates the rate of flow into the pressurized tank to 80 cubic meters per hour or 350 gallons per minute (gpm). The process is depicted in Figure 6.8. The plant engineer is experiencing problems with a fluid control valve (FCV) that fails due to erosion caused by cavitations. The valve fails 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 cavitations. Before the control valve is repaired again, the project engineer wants to look at other options and perform an LCCA on alternative solutions.
Engineering Solution Alternatives The first step is to determine how the system is currently operating and why the control valve fails. Then the engineer can see what can be done to correct the problem. 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: (485 gpm) instead of (350 gpm). This resulted in a larger pressure drop across the control valve than was originally intended. As a result of the large differential pressure at the operating rate of flow, and because the valve is showing cavitation damage at regular intervals, 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.
For each option, the major cost elements identified are as follows:
• Option A. The cost of a new control valve that is properly sized is $5,000. The cost of modifying the pump’s performance by reducing the diameter of the impeller is $2,250. The process operates at for 6,000 h/year. The energy cost is $0.08 per kWh and the motor efficiency is 90%.
• 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.
Given: Financial data as summarized in Table 6.1. Find: Which design option to choose. 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
Expert Answer:
Assumptions The current energy price is 008kWh The process is operated for 6000 hoursyear The company has a cost of 500 per year for routine maintenan... View the full answer
