ETL 1110-2-550
30 May 97
c. Uncertainty. A condition where
and the basic assumptions for the economic
analysis. Currently, reliability is the key factor in
indeterminacy exists in some of the elements that
determining whether there is a Federal interest in a
characterize a situation. Uncertainty may exist from
proposed replacement. If an equipment replacement
either probability uncertainty or outcome
is reliability-driven, the investment is generally
uncertainty or any of the pathways between the
Federally funded. An increase in output which is
initiating event and the consequences.
primarily incidental to the reliability work may also
d. Variability. The existence of differences in
be included in such a project. However, non-
Federal funding is required to fund the project if
the numerical quantities within the same population.
there are no reliability problems and the proposed
Uncertainty and variability have some of the same
project purpose is only to improve output beyond
connotations. With variability, the range of
the original design. Contact CECW-B for current
possible values is usually known, perhaps along
policy on non-Federal funding of generation
with other information such as the distribution.
improvements.
However, uncertainty allows the values for a
quantity to retain an element of vagueness that is
b. Hydropower major maintenance work items
not characterized in quantities exhibiting variability.
also require reliability analysis and economic
This suggests that if placed on a continuum from
justification. Major maintenance includes projects,
complete randomness to complete determinacy,
such as a generator rewind, with total estimated
variability is somewhere closer to certainty than
costs that exceed million and do not qualify as
uncertainty is.
Major Rehabilitation. Specific guidance on
e. Reliability of power plants. There are risks
hydropower major maintenance evaluation
requirements is being drafted by CECW-B.
associated with the possible failure of operating
power plants. The risks include repair costs and
higher power generating costs. A generating unit
5. Reliability Concepts
that has been derated because of previous problems
is not capable of producing the same amount of
There are some basic reliability concepts which
power that it could originally produce. That is a
arise from statistics and are utilized in evaluating
certainty. The exact amount of power the unit can
reliability. The definitions of the terms used to
produce in the derated condition is uncertain. The
represent these concepts and the definitions of terms
probability that a generating unit will fail after it
more specific to hydropower equipment reliability
has been on line for 20 years has variability. The
analyses follow.
engineering reliability analysis required for a major
maintenance rehabilitation proposal needs to
a. Risk. The exposure to a chance of loss or
consider these reliability concepts.
injury; the likelihood of adverse consequences.
f. Equipment reliability. Hydropower
Expressions of risk are composed of the following
two parts:
equipment reliability is defined as follows:
The extent to which the generating equipment can
(1) The existence of unwanted consequences.
be counted on to perform as originally intended.
This encompasses the confidence in soundness or
(2) The occurrence of each consequence
integrity of the equipment based on forced outage
expressed in the form of a probability.
experience and maintenance costs, the output of the
equipment in terms of measured efficiency and
b. Certainty. A condition where determinacy
capacity, unit availability, and the dependability of
exists in the elements that characterize a situation.
the equipment in terms of remaining service life
The likelihood of an event occurring and its
(retirement of the equipment).
consequences are known absolutely.
2
Previous Page
