Recursive Operability Analysis Research Articles

Integration of Recursive Operability Analysis, FMECA and FTA for the Quantitative Risk Assessment in biogas plants: Role of procedural errors and components failures

With more than 350 GWh per year and thousands of installations around the world, biogas is an appealing strategy in the field of energy production and industrial waste optimization. In this sense, it is of paramount importance to address the risk associated with such plants, as an increasing trend of accidents have been recorded in the last 20 years. In this work, a representative biogas production plant was considered, and a risk assessment was carried out through the combination of Recursive Operability Analysis and Failure Mode and Effects Criticality Analysis. The methodology is rigorous and allows for both the identification and the quantification of accidental scenarios due to procedural errors and equipment failures, which miss in the literature for the case of biogas. The analysis allows the automatic generation of the Fault Trees for the identified Top Events, which can be numerically solved. Results show that the most critical accidental scenario in the biogas plant here considered is the formation of an explosive air-biogas mixture, which can occur in both anaerobic digester and condensate trap. The calculated probabilities agree with the results available in literature on similar plants. Pumps and Distributed Control System were found to be the most critical components.

Recursive Operability Analysis as a Tool for Risk Assessment in Plants Managing Metal Dusts

Recursive Operability Analysis as a Tool for Risk Assessment in Plants Managing Metal Dusts

Application of an Enhanced Version of Recursive Operability Analysis for Combustible Dusts Risk Assessment.

Organic dust explosions were and are still today a critical issue in the food, pharmaceutical, and fine chemical industry. Materials such as flour, corn starch, sugar and APIs represent a cause of severe accidents. In this framework, we investigated a modified version of Recursive Operability Analysis−Incidental Sequence Diagrams (ROA–ISD), called ROA Plus−ISD, specifically tailored to describe industrial processes involving organic combustible dusts. Compared to more classical techniques such as Hazard and Operability (HazOp), ROA−ISD allows for a direct generation of fault trees, providing a useful tool to connect Qualitative with Quantitative Risk Analysis (QRA). ROA Plus−ISD is very similar to ROA−Cause Consequence Diagrams (CCD), which has already proven to be an effective tool to perform both risk assessment on existing plants and reconstructing already occurred accidents, given its logical structure and width of the application fields. In this work, we modified specific parts of the standard ROA−CCD method: (1) the Failure Mode and Operability Analysis (FMEA) database has been structured in order to retrieve the well-known explosion pentagon (for dusts) and all the instruments, devices, apparatuses and controllers typical of industries which process organic dusts; (2) a new comprehensive list of process variables has been compiled. In this way, it is possible to tailor the information required for the generation of the fault trees concerning top events involving mainly dust explosions and fires. This method has been implemented in order to reconstruct the dynamics of the February 2008 Imperial Sugar refinery plant accident (Port Wentworth, GA, USA). Results demonstrated the applicability of the enhanced method by highlighting the criticalities of the process already showed by a previously detailed reconstruction performed by the Chemical Safety Board.

Reliability Assessment in a Freeze-Drying Process

A risk analysis for a pilot scale-freeze-dryer is shown in this paper, aiming to build the basis for the risk-based decision making in plant and process design of a (pilot scale) freeze-dryer, to be then exploited in the design of a full scale safer plant. The risk analysis was performed using the recursive operability analysis to point out the process top events and to identify their primary causes. The top events here considered are the undesired freezing velocity in the freezing stage and the undesired heating profile and/or pressure value in the whole process, as they can be responsible for product overheating and, thus, jeopardize product quality. After the extraction of the fault trees, the minimal cut-sets were identified. The unavailability at mission time was then calculated, and the contribution of the primary causes to the top events occurrence was evaluated. The more critical plant components were identified, and a suitable 2k Design of Experiments was set up to evaluate how the uncertainty on...

Comparison of classical and "cause consequence diagrams" Recursive Operability Analysis: The T2 Laboratories accident

On December 19, 2007, a powerful explosion and fire killed four employees and destroyed T2 Laboratories, Inc., a chemical manufacturer in Jacksonville, Florida. In the accident 32 people were injured, including 4 employees and 28 members of the public who were working in the surroundings. Debris was found up to one mile away, and the explosion damaged buildings within one quarter mile of the facility. After the accident, the question which arose was: could an even simplified risk analysis prevent such a tragedy? It is widely accepted that performing a detailed Quantitative Risk Analysis (QRA) is a complex and time consuming task because of all the steps which must be carried out: 1) hazards identification; 2) frequency estimation; 3) accident consequence evaluation; 4) individual and societal risk calculation.Specifically, Hazard Identification (HI) and Frequency Estimation (FE) represent two fundamental activities since: 1) not identified hazards can remain hidden until the occurrence of the related accidents; 2) the probabilistically quantification of the hazardous plant states frequencies, e.g. through Fault Tree Analysis (FTA), helps to support decisions making on risk reduction. Particularly, since generating FTs is a time consuming task, the Recursive Operability Analysis (ROA) has been ideated. ROA, both in its classical and revised version (called Recursive Operability Analysis – Cause Consequence Diagrams, ROA-CCD), is based on a procedure which allows collecting plant perturbations data in a structured way.The aim of this work is to apply both the classical and the new ROA-CCD analysis on the T2 Laboratories chemical plant (with particular reference to the reactor node) in order to identify all the possible top events and states of plant unavailability. In this way, it has been evidenced that even a simplified but reliable risk analysis could have been unearthed clearly all plant criticalities. Moreover, the results concerning the risk quantification have been critically analyzed showing that ROA-CCD achieves the same results, in terms of Minimal Cut Sets, of its classical version with a lower effort. This goal is fulfilled by avoiding the unnecessary subdivision of the plant into nodes prior to perform the analysis; in this way, considering only the process variables related to the “key piece of equipment” (in this case, the reactor), records in the ROA format are drastically reduced.

Catalytic after-treatment of lean VOC–air streams: Process intensification vs. plant reliability

This paper compares two different processes for the treatment of a gaseous stream containing small amounts of volatile organic compounds (VOCs), namely a traditional plant, where a heat exchanger is used to recover heat from the hot gas obtained after VOCs combustion, and a plant with a reverse-flow reactor, where the catalyst is used for VOC removal and for a regenerative heat recovery. The analysis is focused on the reliability and the safety of both plants: the recursive operability analysis (ROA) is firstly used for hazards identification, pointing out that the sintering of the catalyst and the discharge of a gas with high VOCs concentration are the most critical top events. The fault trees are then extracted and quantified. Finally, an integrated dynamic decision analysis (IDDA) is carried out, with the goal of evaluating in more detail the plant behavior, for both cases, in the event of failure. Results show that the probability of occurrence of catalyst sintering is higher in the traditional plant, due to the fact that the control system used in the reverse-flow reactor is more complex and with more redundancies. With respect to the release of VOCs, this event is more probable in the reverse-flow reactor than in the traditional plant, but the associated risk is lower, although the difference is not very large.

Are Intensified Processes Safer and More Reliable than Traditional Processes? An emblematic Case Study

One of the best ways of preventing accidents is to avoid hazards by inherently safer design. As many processes, particularly those in the chemicals, nuclear and oil industries, involve the production, handling and use of hazardous substances, process intensification (PI) apparently offers to the plant managers many benefits such as safer, cleaner, smaller, and cheaper equipments. Process Intensification could lead to a higher process flexibility, increased inherent safety and energy efficiency, distributed manufacturing capability, and ability to use reactants at higher concentrations. These goals are achieved by multifunctional reactors, e.g. reactive distillation or membrane reactors, and miniaturization that can be done by employing micro reactors and/or improving heat and mass transfer. Within the benefits of process intensification with reference to process safety the following can be considered: The use of process intensification can reduce the number of process operations, which leads to fewer transfer operations and less pipework, preventing source of leakages; It can be easier to design a smaller vessel to contain the maximum pressure of any probable explosion; For exothermic reactions, the enhanced specific surface area of intensified plants makes heat transfer, and thus heat removal, easier, minimising the triggering potential of a runaway reaction. Process intensification has thus the potential to be a significant factor in the implementation of inherent safety practices Although safety can benefit from process intensification, it should be ensured that new hazards are not created. This can be obtained applying at a early design stage the reliability and safety analysis that are traditionally used in process risk assessment. In this paper in particular the above techniques are applied to a VOC (Volatile organic compound) treatment plant, comparing the traditional plant (a fixed bed reactor) to the intensified reverse flow reactor. From the results of the application of a recursive operability analysis and the successive fault trees fully quantified, some conclusions are drawn from the point of view of the reliability of intensified processes and from the adequateness of risk assessment methodologies

Recursive Operability Analysis as a decision support tool for Risk-Based Maintenance

This work was developed with the support of MEMC, one of the most important producers of ultra-pure silicon wafers for electronic applications throughout the world. The availability of ultra-pure water is of prime importance in the silicon production process. In order to maximize the availability of UP water, MEMC has developed a preventive maintenance program and a detailed record of each maintenance intervention is recorded. This has allowed a complete failure rate data bank to be developed. In order to optimize the maintenance intervention, a Recursive Operability Analysis (ROA) has been used as a decision support tool. The results of the ROA, coupled with the failure rates, have made it possible to calculate the expected number of events (ENE) of various top events (TEs). The magnitude of each TE has been estimated on the basis of the monetary losses provoked by each event. The risk then has been calculated and the events ranked on this basis. Maintenance policies have been optimized with the aim of reducing the risk of the top ranked events.

Risk based decision‐making in plant design

AbstractAccidental events that have occurred in chemical and nuclear plants show that the application of technical regulations on its own, even when extremely detailed, does not assure that adequate safety levels are reached and, above all, maintained in time. The paper illustrates, with a simple example, how a risk analysis can be considered as an essential tool for a correct plant design, above all when dealing with major risk installations. In particular the use of the Recursive Operability Analysis is shown, in the phase of accident identification, since it allows to directly draw the logic trees, both Fault and Event Trees for the quantification phase, thus assuring the congruence of the whole analysis.

The systematic integration of human factors into safety analyses: An integrated engineering approach

The performance of the human reliability analysis (HRA) and integration of its outcomes into quantitative risk assessment schemes remains quite a difficult and complex task to perform. Even worse is the assessment of organisational reliability assessment. The reasons of this difficulty mainly lay on the absence of a generically accepted paradigm that enables engineers to include systematically human and organisational factors (H&OF) into the analysis. Broadly speaking, engineering approaches very often account for error of omission forgetting the errors of commission (EOC), and, on top of that, they do not make any macro distinction between pre- and post-initiating human failures. This paper offers a paradigm on how to integrate H&OF into safety analysis by means of the recursive operability analysis (ROA), which has been adapted to accommodate H&OF, and renamed integrated recursive operability analysis (IROA). By means of a practical example, the method will illustrate how to account for H&OF in a systematic and consistent manner using an engineering approach. The paper will even provide a paradigm for the construction of integrated fault trees consistent with the IROA framework.

Recursive Operability Analysis of a pilot plant gasifier

This paper describes the utilization of the Recursive Operability Analysis for the identification of accident scenarios of a pilot plant gasifier fed with solid wastes and alternative fuels. The procedure was used for the identification of situations of potential risk during the operation of the plant and, coupled with the Fault Tree algorithm, in the successive quantification of probability of occurrence of the different potential Top Events. The results indicated that Recursive Operability Analysis can be usefully applied in the design as well as in the commissioning and in the start-up stages of the plant construction. The quantification of the actual effects of the implementation of new management criteria and design solutions can also be easily obtained.

Risk-Based Design of a Regenerative Thermal Oxidizer

Following an explosion in a piece of equipment [a thermal oxidizer (Loss Prev. Bull. 2003, 170, 8−10)] employed for the purification of gas streams contaminated by volatile flammable compounds, a similar plant that is under construction was redesigned with a risk-based approach, to increase both the reliability and operational safety of the regenerative thermal oxidizer (RTO) and venting collection system (VCS). This paper, in particular, describes the methodologies that were used and the results that were obtained in terms of the reduction of the frequency of occurrence of the top event (TE) “Explosion in the venting collection system”. A recursive operability analysis (ROA) was performed to identify “primary events” that could lead to a possible explosion in the RTO or VCS, with their evolution toward the TE. Fault trees were then directly derived from the ROA tables, through an intermediate logic diagram, the incidental sequence diagram, and then quantified in order to assess the frequency of occurrenc...

How to avoid the generation of logic loops in the construction of fault trees

Generation of an infinite series of identical sub-trees may occur during the construction of a Fault Tree (FT) when one item of equipment in a plant is considered several times in the same sub-tree in the course of the tree extraction from a HazOp (Hazard Operability analysis) analysis. Generation of loops in the construction of an FT can be avoided by means of an ad hoc logical analysis in which certain simple rules of syntax are taken into account. A radical solution, however, can be obtained if identification of unwanted events in a process plant is not undertaken with conventional procedures, such as HazOp (Operability Analysis with guide words, failure mode and effect analysis (FMEA) etc.), but with a more modern and structured version, such as Recursive Operability Analysis (ROA), which is both systematic and complete, and allows direct extraction of logic trees, (FT, event trees, etc.) for subsequent quantification. This feature means that, by contrast with conventional operability analysis, the congruence of the ROA itself can be checked. The ROA method is illustrated in this paper with the aid of some simple examples.

On the numerical solution of fault trees

Abstract In this paper an account will be given of the numerical solution of the logic trees directly extracted from the Recursive Operability Analysis. Particular attention will be devoted to the use of the NOT and INH logic gates for correct logical representation of Fault Trees prior to their quantitative resolution. The NOT gate is needed for correct logical representation of events when both non-intervention and correct intervention of a protective system may lead to a Top Event. The INH gate must be used to correctly represent the time link between two events that are both necessary, but must occur in sequence. Some numerical examples will be employed to show both the correct identification of the events entering the INH gates and how use of the AND gate instead of the INH gate leads to overestimation of the probability of occurrence of a Top Event.

Recursive operability analysis of a complex plant with multiple protection devices

Abstract Recursive operability analysis (ROA) is a powerful hazard evaluation method that is particularly suitable for the safety analysis of plants with multiple protection levels activated by the same process variable. The propagation of deviations of the variable along the process lines can be readily determined with this method, and hence the correct sequence in which the protection means must intervene can be appropriately followed. This paper illustrates the versatility of a ROA through its application to a 900 kg/h liquid chlorinated waste furnace. The following process variable deviations were assessed: primary air flow rate, flow rate and lower calorific power of the treated wastes and furnace temperature. It has been shown that the analysis requires a precise determination of how the variables evolve, especially when they are correlated, as is the case with the temperature and oxygen concentration. The ways in which deviations can develop in quantitative terms and the correct sequence in which the protection means must intervene are deduced from a correct study of the process thermodynamics.

Dot chart analysis as a means to develop a fault tree: application to a catalytic hydrogenation unit

This paper describes the application of dot chart analysis to a semicontinuous catalytic hydrogenation unit. Dot chart tables have been used as a basis for developing the recursive operability analysis and the fault trees (FTs), whose aim is to determine the safety of both the unit and its operators. The unit is formed of two reactors in parallel: the transfer of operations from one reactor to the other when its catalyst is exhausted is performed by means of the isolation systems installed for this purpose on the inlet and outlet lines. FTs assessed the expected number of leak at 3×10−3 occurrences per mission time. The study clearly showed that the operations could be regarded as safe, since, with minor modification to control system and operative procedure, these leaks would be of pressurised nitrogen and hence without consequences for the unit and its operators.