The RAMS (Reliability, Availability, Maintainability and Safety) disciplines are a set of tools that make it possible to ensure that a product, process or system fulfils the mission for which it was designed, all under the conditions of reliability, maintainability, availability and well-defined safety.
Difference between Reliability and Availability
Reliability is defined as the probability that a device’s performance will remain unchanged over time, after determining the conditions of use. The fundamental parameter for determining the reliability of an object is its failure rate, i.e. the number of failures it undergoes in the set time of one hour. Reliability forecasting techniques make it possible, from the knowledge of the failure rates of individual elements, to determine the failure rate and therefore the reliability of an entire system, whatever its scope of application. If carried out during the design phase, these analyses make it possible to identify the components most prone to failure and intervene with replacements or the inclusion of redundancies.
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To be competitive in the marketplace, a product, process or system must not only be reliable, i.e. subject to failure as little as possible, but also available, i.e. operational.
Availability is defined as the probability that a device’s performance will be unchanged over time, after determining the conditions of use and assuming that any necessary external means are secured. Availability studies take into account the maintenance to be carried out on the system and the time needed to restore it; the aim is to ensure maximum availability of the system under study, identifying the most critical elements which, due to a higher failure rate or longer repair times, make a system unavailable, thus also affecting costs.
Techniques to study the reliability and availability
There are many techniques to study the reliability and availability of products, processes or systems, including the best known:
FMEA (Failure Mode and Effects Analysis)
through a hierarchical breakdown of the product it analyzes the failure modes and their effects;
Techniques derived from FMEA (FMECA, FMEDA)
that includes a criticality analysis of failures to assess the severity of the consequences of a failure related to its probability of occurrence (FMECA), and an analysis of the diagnosability of detected failures (FMEDA);
FTA (Fault Tree Analysis)
a technique that allows the calculation of the probability of occurrence of what has been defined as a Top event from the probabilities of the basic events, taking into account how they combine (logical relations And, Or, etc.);
RBD (Reliability Block Diagram)
taking into account the architecture of the product/process or system, it allows the calculation of its overall reliability and availability from the values for the individual elements.
Safety is defined as the departure from an unacceptable risk, where the risk is given by the product of the probability of the adverse event occurring due to the severity of the event. Risk analysis techniques aim to estimate all the risks present and assess them in order to make them acceptable. These techniques, therefore, allow, if carried out correctly in the early stages of design, for the elimination of critical issues before commissioning or being placed on the market, thus reducing costs.
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Risk Analysis Techniques
Among the techniques of risk analysis the best-known ones are:
HAZOP (HAZard and OPerability analysis)
based on group work, carried out in several sessions, aimed at identifying existing hazards in a given work process. These hazards are identified on the basis of the concept of deviation of key process parameters;
LOPA (Layer Of Protection Analysis)
a risk analysis technique developed around the need to verify the effectiveness of the safety measures taken understand how many safety barriers (layers) are needed and what risk reduction they should provide;
PHA (Process Hazard Analysis)
used in the initial design and construction phases of the system, it allows the identification of the hazards related to system components, incoming and outgoing substances, system layout, maintenance activities, safety systems, and causes due to the surrounding environment and natural events.