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.
- Consider A Single Pump Circuit As Shown In The Figure The Fluid Control Valve Fcv Fails Due To Erosion Caused By Cavi 1 (235 KiB) Viewed 23 times