ETL 1110-2-560
30 Jun 01
Appendix D: Mechanical Equipment Example
D-1. Description
For this analysis, the individual mechanical gate systems are considered subsystems to the overall lock
and dam system. The example lock miter gate and valve machinery subsystems are laid out as shown in
Figures D-1 and D-2. The dam gate machinery is laid out as shown in Figure D-3.
D-2. Reliability Block Diagram Formulation
Formulation of the system reliability block diagram (RBD) is in accordance with MIL-STD-756B. The
initial step in determining the reliability of the mechanical systems of the lock and dam is to identify the
function or mission of the machinery. The machinery function is to operate the gates. The major
components required for mission success are defined and organized into an RBD. The block diagrams for
the miter gate and tainter valve and dam gate components included in this evaluation are shown in
Figures D-4, D-5, and D-6. The RBD is simplified or expanded, if necessary, to sufficient detail to allow
determination of component failure rate from published data. The process continues until only blocks
with published component failure rate data remain in the block reliability model. In this example, the
structural supports are not included in the model. They are unique to each system, and no published data
are available. For the lock and dam gate and valve machinery shown in the figures, the failure of any one
component constitutes nonperformance of the mission. There are no parallel or redundant items. The
mission and basic block diagrams will be series models.
D-3. Reliability Calculation
The basic and mission reliability model blocks should be keyed with consistent nomenclature of elements.
Each model should be capable of being readily updated with new information resulting from relevant
tests, as well as any changes in item configuration or operational constraints. Hardware or functional
elements of the system not included in the model shall be identified. Rationale for the exclusion of each
element from the model shall be provided.
a. Duty cycle. The mission or function of the system should address the duty cycle or period of
operation. The miter gate equipment is considered to have a negligible failure rate during periods of non-
operation (ignoring barge impact). The failure rate can be modified by a duty cycle factor. The duty
cycle factor is the ratio of actual operating time to total mission time t. For example, the equation R(t) =
e-λtd is the exponential failure rate distribution with a duty factor d. The duty factor for lock mechanical
equipment is directly related to the number of lockages or hard operations that occur at a facility. The
number of lockages may vary over time, and hence the duty factor may vary. In this example, the
lockages or cycles increase with time. The duty factor is calculated for each year as follows: For year 5,
the lock performs 11,799 open/close cycles. Assuming the operating time of an open or close operation is
120 seconds (or 240 seconds per open/close cycle) and using a total mission time of 8760 hours per year
then,
Operating time = (240*11,799)/3600
= 786.6 operational hours/year
= 786.6/8760 hours/year
d = 0.0898
b. Environmental conditions. Environmental conditions shall be defined for the ambient service of
the equipment. An approximate approach (Green and Bourne 1972) multiplies failure data by various K
D-1
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