Civil Engineering Reference
In-Depth Information
8.1 Relation between Safety and Technical Risk
Technical safety is a very important ability of subject
machine, technology, operation to perform its func-
tions without any hazardous situations for persons or environment. Hazardous situation is such circum-
stance in which a person is exposed to a hazard. Hazard is a potential source of harm. Harm is physical
injury or damage to the health of people either directly or indirectly as a result of damage to property or
to the environment.
All methods used for analysis of global safety must take into consideration not only technical aspects of
machines safety and devices safety but also protection of health and environment.
Technical risk is defined as a combination of the probability of occurrence of harm and the severity of
that harm
/
weight factor. Risk is generally defined as follows:
/
R
¼
P
D
(8
:
1)
where P is a probability of risk situation occurrence, D is a consequence after risk situation occurrence
(damage in the whole man — machine — environment system).
Also it is possible to write:
R
¼
P
D
W
E
(8
:
2)
where W is a prohibition possibility of the observed phenomenon, E is an exposure time of the
phenomenon.
It is the task of statistical methods to define probability P. But it is not so easy to define
the consequence D. We can say that all damages are most frequently expressed in terms of economical
effects.
For the computation of risk level, we must know the real values of the probability of occurrence of
harm and values of severity of harm. If the final computed risk value is greater than the acceptable
level, the necessary measures for risk reduction or elimination must be performed. This shortly described
process is risk management and at the same time it is one subsystem of global management of work safety.
Safety, in this meaning, is a freedom from unacceptable risk. Risk management or in other words control
of risk is based on three points:
1. Identification of hazards
2. Risk analysis
3. Reduction or elimination of risk
8.2 Evaluation of Technical Risk
8.2.1 Example 1
The example considered here are big gearboxes with a power output of about 3.6 to 5.4 MW. We analyzed
two basic conditions for failure in risk evaluation of parts of gearboxes:
- Current operation with all suitable parameters
- Operation with improper maintenance
8.2.1.1 Description of the Mill Train Set
The observed gearboxes (six gearboxes with constant speed and three with variable speed) are working in
the hot mill train set and they transmit power from 3.6 to 5.4 MW. Gearboxes are one- or two-stage types
with helical gears and with a synchronous motor (Figure 8.1). According to the last analysis, it is just the
gearbox that is the part with the highest number of failures.
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