ETL 1110-2-550
30 May 97
can be established from model tests and acceptance
(2) Hydropower equipment is typically
test data. It is important to fully investigate the
operated until it fails or is retired for some other
calibrations and calculations of the data in order to
reason. Failure meaning that it ceases to function
truly compare the original and current performance.
properly under the stresses applied. Replacement
and refurbishment are both considered as con-
(3) The information derived from this testing
stituting the effective retirement of a piece of
and analysis is provided as input to the
equipment. The first major reason for equipment
hydroelectric power benefits analysis, which is
retirement is physical condition, which includes
discussed in Appendix D. The benefits analysis
deterioration with time, wear from use, and failure
estimates the power system production costs using a
in service. The second reason for retirement is
full range of unit availability which can be applied
related to functional situations, which include
to the base case and each alternative.
inadequacy to perform required functions, potential
for improvement (uprating), and obsolescence.
c. Availability. Availability is the annual
These may occur due to a change in environment,
percentage of time that the generating equipment is
operating conditions, or load requirements. The
available for power production. Records of
first category, physical condition, is the primary
availability are maintained by each project on a
reason that the Corps developed the Major
unit-by-unit basis. The current level of availability
Rehabilitation Program. This program establishes a
must be compared with previous data to establish
standardized method of considering and evaluating
the extent of degradation. Historical trends can be
the deterioration and wear of equipment in an effort
extrapolated to project future changes in the unit
to optimize rehabilitation actions. Failures in
availability rate. Availability data are also used as
service are generally not evaluated under the Major
input to the hydroelectric power benefits analysis.
Maintenance and Rehabilitation Programs, but are
funded through reprogramming Operation and
d. Dependability (reliability).
Maintenance funds. Reliability is the key factor in
determining whether there is a Federal interest in a
(1) The final area of consideration concerning
proposed replacement. As previously stated, if
equipment reliability is dependability.
there are no reliability problems and the proposed
Dependability is ascertained by a risk analysis that
project purpose is to solely improve output beyond
determines the probability that the equipment will
the original design (improvement in functional
not perform satisfactorily in any given year. The
situations), non-Federal funding is required to fund
output from this risk analysis is used in the
the project. It also may happen that a replacement
probabilistic life-cycle cost analysis. One way to
is reliability-driven, Federally funded, and there is
graphically represent the probabilistic life-cycle cost
increased output which is primarily incidental to the
model is with event trees. A discussion of event
reliability work.
tree models is presented in Appendix E. Two
methods of probabilistic risk analysis are frequently
7. Risk Analysis Using Reliability Curves
used. The first method uses historical data and an
evaluation of the condition of the equipment to
determine a statistical distribution of age at
Historically, engineering judgment has been used to
retirement. This method is characterized by the use
predict remaining unit life and determine the
of reliability curves. The second method is similar
probability that the unit will perform unsatis-
to that used in structural evaluations. It extends the
factorily. The Corps has embarked on a program to
safety factor concept by using a probabilistic
attempt to structure these predictions and
approach to determine a reliability index. The
determinations. Methods of determining reliability
method that is most appropriate depends upon the
are well established for many types of physical
type of equipment being evaluated and the specific
properties. A useful way of expressing reliability
situation.
for the Corps' economic evaluations is the annual
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