Page 289 - Improving Machinery Reliability

260 Improving Machinery Reliability

Purchasing a new, nonredundant electronic governor might be the second option,
and installing a new, fault-resistant (fully redundant) electronic governing system
might represent the third possible course of action.
The yearly repair costs would be calculated by multiplying the average frequency
of failure by the cost of each failure event. The mean time between governor failure
(MTBF) and mean time to repair or replace a governor (MTTR) are typically needed
to perform a reliability analysis:

Cy = (CG) (8760)/(MTBF + MTTR) (5-1)

where Cy = annual cost of failures for a governor or associated (governed)
system
CG = cost per failure event
MTBF = mean time between failure, hours
MTTR = mean time to repair or replace, hours

The following information is known about the three options:
The 28 hydromechanical governors at this plant have failed a total of 33 times in
the last seven years, requiring an average repair or replacement time of 18 hours.
Thus, MTTR is 18 hours, and
(28) (7 yrs) (8,760 hrs / yr) - (33 failures) (18 hrs)
MTBF =
33 failures
= 52,011 hours or 5.94 years
In this example, the cost to repair or replace a hydromechanical governor is
$12,370. Production losses are primarily influenced by the need to flare huge
amounts of hydrocarbon feed for approximately 1% hours per outage event. This
costs the plant $72,820, plus $5,120 in lost profits and $4,960 in restarting, overtime,
and associated costs. Adding $20,000 for environmental fines, which will likely be
assessed against the plant, the total now stands at $115,270/5.9 years, or
$19,537/year.
For the nonredundant electronic governor alternative, the plant has to depend on
outside sources for projected failure data. It was found that others experienced one fail-
ure every 80,270 hours, or 9.16 years. The projected MTTR for nonredundant electron-
ic governors is only 4 hours and will have little influence on the MTBF expression.
Replacing the hydromechanical governor with an electronic alternative will cost
$49,700 in acquisition and conversion costs. In case of failure, troubleshooting and
component replacement costs are estimated at $3,700. Since the projected MTTR of
4 hours exceeds the necessary furnace tube cool-down period of 1% hours, it will
again be necessary to flare for 1% hours. Accordingly, pursuit of this option will
again incur the $72,820; $5,120; $4,960 and $20,000 components and the cumula-
tive total will be $106,600/9.16 years, or $1 1,638/year level annual costs.
Finally, we look at option 3, the redundant electronic governor system. Its acquisi-
tion and conversion costs are $71,870. In case of failure of one of the two modules,