Page 323 - Improving Machinery Reliability

292 Improving Machinery Reliability

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The same spreadsheet will be used with more details when statistical uncertainty
is added in a section that follows.

Step 5: Gather cost estimates and cost models. This is the complicated section
where all the details are assembled. Of course, the more thorough the collection
process, the better the LCC model. For this text, the details have been shortened with
just enough information described to show the trends.

Alternative #I Do-nothing case-the datum. Use the following details from
:
plant experience.
Assume all the equipment follows the exponential distribution for reliability with
constant failure rates. Note the reciprocal of failure rate is the mean time to failure.
Since failure rates are constant, use one year time buckets to collect the cost of fail-
ures per year as the literal failure date is unknown. Use the following assumptions
based on an accounting principle that costs will follow activity-in this case it will
follow failure activity.
Capital cost are zero as the solo ANSI pump is currently a sunk cost and will not
change.
Lost gross margin occurs at US$4,000/hour when the process is down for repairs.
Annual power cost for running the pump is US$l65/year per horsepower. The
plant incurs 1.6 power outages each year for an average downtime of 0.5 hours, and
this cost is charged into plant overhead rather than to individual pieces of equipment.

Annual power costs are (US$165/hp-yr) * (100 hp) = US$16,500.
Pump seals have a mean time to failure of three years. When seal failure occurs,
eight hours of downtime is also lost production time. Maintenance crew costs for
labor, incidental materials, and expense are US$lOO/hr. Seal replacement costs are
US$1,500/seal plus US$300/incident for bearing replacements that occur as good
maintenance practice while the pump is disassembled. Seal and bearing transporta-
tion costs are usually expedited and cost US$l50 per incident.
Annual seal costs are (1 yr/3 yeardfailure) * (US$( 1500 + 300 + 150) +
(US$IOO/hr) * 8 hours + (US$4,000/hour * 8 hours)) = US$ll,583
Pump shafts have a mean time to failure of 18 years. When shaft failure occurs,
ten hours of downtime is also lost production time. Maintenance crew costs for
labor, incidental materials, and expense are US$lOO/hour. Shaft replacement costs
are US$2,500/shaft plus US$1 ,XOO/incident for seal and bearing replacements that
occur as good maintenance practice while the pump is disassembled. Shaft, seal, and
bearing transportation costs are usually expedited and cost US$450 per incident.
Annual shaft costs are (1 yeadl 8 yeadfailure) * (US$(2,500 + 1,800 + 450) +
(US$lOO/hour) * 10 hours + (US$4,000/hour * 10 hours)] = US$2,542