Information Technology Reference
In-Depth Information
Effects of a Periodic Maintenance on the Safety
Integrity Level of a Control System
Karol Rástočny and Juraj Ilavský
Department of Control and Information Systems, Faculty of Electrical
Engineering, University of Zilina, Univerzitná, 010 26 Žilina, Slovak Republic
{karol.rastocny, juraj.ilavsky}@fel.uniza.sk
Abstract. An approach to analysis of impacts of preventive mainte-
nance on safety integrity level of a control system is presented in the
paper. Suggested approach incorporates combination of Continuous
Time and Discrete Time Markov Chain which allows full employ-
ment of advantages of Continuous Time Markov Chains as a stochas-
tic modelling method. Deterministic behaviour of either periodic or
non-periodic preventive maintenance is consecutively implemented
through Discrete Time Markov Chain.
Keywords: Safety, Periodic Maintenance, Safety Integrity, Control System,
Markov Model.
1
Introduction
Many different quantitative methods can be employed if the safety integrity
level (SIL) of a control system is to be analysed during a safety evaluation
(even methods that were originally meant for reliability analyses - e.g. FMEA,
RBD, FTA). Those widely used methods are supported by a large number
of software tools that make possible not only a neat creation and editing
of created models, but also provide a possibility of simulation and ways of
presentation of achieved results [1].
The aim of a qualitative evaluation of the SIL of a system is to determine
probability of a hazardous state of a system (probability that a specific safety
function fails) that depends on many safety-influencing factors [2], [3], i.e.:
p
H
(
t
)=
f
(
λ, c, μ, t
D
,...,t
)
(1)
Given the fact that the SIL of a system rests on many miscellaneous
factors (random hardware failure rate
λ
, diagnostic coverage coe
cient
c
,
time needed to detect a failure
t
D
, recovery rate
μ
, . . . ), quantitative method
for the SIL evaluation that covers most of the factors is usually preferred. Such
This paper was supported by the scientific grant agency VEGA, grant No.
VEGA-1/0040/08“Mathematic-graphicalmodellingofsafetyattributesofsafety-
critical control systems”.
