PROCESS SAFETY
OUR SERVICES
- Compatibility Study
- Preliminary Hazard Analysis (PHA)
- Hazard and Operability Study (HAZOP)
- Failure Mode and Effect Analysis (FMEA)
- Hazard Analysis and the Critical Control Points (HACCP)
- Fault Tree Analysis (FTA)
- Quantitative Risk Assessment (QRA)
- Risk Management Plan (RMP)
- Inherent Safety
- Consequence Analysis for Events with Flammable and Toxic Chemicals
- Equipment and Process Plants Reliability Studies
- Control of Static Electricity in Chemical Plants
- Training and Information Courses in the Risk Assessment and Management Techniques
- Safety Integrity Level (SIL) Assessment
- Layer of Protection Analysis (LOPA)
Safety Integrity Level (SIL) Assessment
Safety integrity is defined as the probability of a safety-related system satisfactorily performing the required safety functions under all the stated conditions within a stated period of time (3.5.4 of IEC 61508-4). Safety integrity relates to the performance of the safety-related systems in carrying out the safety functions (the safety functions to be performed will be specified in the safety functions requirements specification).
Safety integrity is considered to be composed of the following two elements.
– Hardware safety integrity; that part of safety integrity relating to random hardware failures in a dangerous mode of failure (see 3.5.7 of IEC 61508-4). The achievement of the specified level of safety-related hardware safety integrity can be estimated to a reasonable level of accuracy, and the requirements can therefore be apportioned between subsystems using the normal rules for the combination of probabilities. It may be necessary to use redundant architectures to achieve adequate hardware safety integrity.
– Systematic safety integrity; that part of safety integrity relating to systematic failures in a dangerous mode of failure (see 3.5.6 of IEC 61508-4). Although the mean failure rate due to systematic failures may be capable of estimation, the failure data obtained from design faults and common cause failures means that the distribution of failures can be hard to predict. This has the effect of increasing the uncertainty in the failure probability calculations for a specific situation (for example the probability of failure of a safety-related protection system). Therefore a judgement has to be made on the selection of the best techniques to minimize this uncertainty. Note that it is not the case that measures to reduce the probability of random hardware failure will have a corresponding effect on the probability of systematic failure. Techniques such as redundant channels of identical hardware, which are very effective at controlling random hardware failures, are of little use in reducing systematic failures such as software errors.
According to the standards IEC 61508 and DIN V 19250 the allowable probability of failure on demand (PFD) for a tripping system for each SIL category must be:
The Safety Integrity Level (SIL) required will be a function of: frequency of the unwanted occurrence or demand for the tripping system (W), the potential consequence severity to human beings in case of failure on demand (S), the frequency of people, and exposure time, in the hazardous zone (A), the possibility of avoiding exposure to the hazardous event (G), the consequence severity concerning the production loss and plant damage in case of failure on demand (L) and the consequence severity concerning the environment in case of failure on demand (E).