A study of aerosol sampling at high values of the velocity ratio

A study of aerosol sampling at high values of the velocity rati

A numerical study of the aspiration efficiency of a thin-walled sampler facing the wind for high velocity ratios

In this work the fluid flow and particle trajectories for a thin-walled sampling probe facing the wind are considered. The flow around the sampler and into the sampler has been determined numerically and the paths of the particles in the flow are then traced and the efficiency of the sampler investigated. A variety of operating conditions have been considered, in particular large values of the velocity ratio, R, which is equal to the ratio of the undisturbed free stream air velocity to the average sampling velocity. The situation of large R values is becoming increasingly important as samplers are developed with low flow rates. Previous experimental results have shown that the empirical model developed for sampling in moving air does not accurately predict the efficiency of samplers operating at these high values of R. The numerical results show that the aspiration efficiency for high R is not significantly affected by gravitational effects for the majority of cases of interest but it is dependent upon the magnitude of the reversal of the flow within the sampling tube

An algorithm for automated cause-consequence diagram construction

The cause-consequence diagram (CCD) method is a system safety technique which determines the logical combinations of causes of events, by the use of fault trees, and identifies all possible consequences of the events. Traditionally cause-consequence analysis is based on the manual construction of the CCD which is very time consuming, expensive and also a source of human errors. A way of overcoming these drawbacks is to have an automated method of constructing the diagram. Hence in this paper the development of an automated CCD construction algorithm is presented. The algorithm created is based on methods previously developed for reliability techniques. Using a model for each component in the system a set of rules are developed which automatically construct the CCD in an efficient manner. The procedure has been validated by testing it on a variety of industrial systems

Analysis of fault trees with secondary failures

The Fault Tree methodology is appropriate when the component level failures (basic events) occur independently. One situation where the conditions of independence are not met occurs when secondary failure events appear in the fault tree structure. Guidelines for fault tree construction, which have been utilised for many years, encourage the inclusion of secondary failures along with primary failures and command faults in the representation of the failure logic. The resulting fault tree is an accurate representation of the logic but may produce inaccurate quantitative results for the probability and frequency of system failure if methodologies are used which reply on independence. This paper illustrates how inaccurate these quantitative results can be. Alternative approaches are developed by which fault trees of this type of structure can be analysed

Use of Petri nets to model the maintenance of wind turbines

With large expansion plans for the offshore wind turbine industry there has never been a greater need for effective operations and maintenance. The two main problems with the current operations and maintenance of an offshore wind turbine are the cost and availability. In this work a simulation model has been produced of the maintenance process for a wind turbine with the aim of developing a procedure that can be used to optimise the process. This initial model considers three types of maintenance; periodic, conditional and corrective and also considers the weather in order to determine the accessibility of the turbine. Petri nets have been designed to simulate each type of maintenance and weather conditions. It has been found that Petri nets are a very good method to model the maintenance process due to their dynamic modelling and adaptability and their ability to test optimisation techniques. Due to their versatility Petri net models are developed for both system hardware and the maintenance processes and these are combined in an efficient and concise manner

A binary decision diagram method for phased mission analysis of non-repairable systems

Phased mission analysis is carried out to predict the reliability of systems which undergo a series of phases, each with differing requirements for success, with the mission objective being achieved only on the successful completion of all phases. Many systems from a range of industries experience such missions. The methods used for phased mission analysis are dependent upon the repairability of the system during the phases. If the system is non-repairable, fault-tree-based methods offer an efficient solution. For repairable systems, Markov approaches can be used. This paper is concerned with the analysis of non-repairable systems. When the phased mission failure causes are represented using fault trees, it is shown that the binary decision diagram (BDD) method of analysis offers advantages in the solution process. A new way in which BDD models can be efficiently developed for phased mission analysis is proposed. The paper presents a methodology by which the phased mission models can be developed and analysed to produce the phase failure modes and the phase failure likelihoods

Analysis methods for fault trees that contain secondary failures

The fault tree methodology is appropriate when the component level failures (basic events) occur independently. One situation where the conditions of independence are not met occurs when secondary failure events appear in the fault tree structure. Guidelines for fault tree construction that have been utilized for many years encourage the inclusion of secondary failures along with primary failures and command faults in the representation of the failure logic. The resulting fault tree is an accurate representation of the logic but may produce inaccurate quantitative results for the probability and frequency of system failure if methodologies are used that rely on independence. This paper illustrates how inaccurate these quantitative results can be. Alternative approaches are developed by which fault trees of this type of structure can be analysed

Application of a reliability model generator to a pressure tank system

A number of mathematical modelling techniques exist which are used to measure the performance of a given system, by assessing each individual component within the system. This can be used to determine the failure frequency or probability of failure of the system. Software is available to undertake the task of analysing these mathematical models after an individual or group of individuals manually create the models. The process of generating these models is time consuming and reduces the impact of the model on the system design. One way to improve this would be to automatically generate the model. In this work the procedure to automatically construct a model, based on Petri nets, for systems undergoing a phased-mission is applied to a pressure tank system, undertaking a four phase mission

Automated generation of a reliability model for a system undertaking phased missions

There are various mathematical models available to assess the reliability of a given system, these models relate the performance of the system to the performance of the components of which it is comprised and can be used to determine the failure probability or failure frequency of the system in question. Currently there is software available to perform the mathematical analysis of the model but its construction, which is used as input to the software, is undertaken manually. This is quite a lengthy process and can limit the usefulness of the model. One way of improving this situation would be to automate the construction process. In this work a procedure is developed to automatically generate a reliability model, based upon Petri Nets, for a system undertaking a phased mission

Application of a reliability model generator to a pressure tank system

A number of mathematical modelling techniques exist which are used to measure the performance of a given system, by assessing each individual component within the system. This can be used to determine the failure frequency or probability of failure of the system. Software is available to undertake the task of analysing these mathematical models after an individual or group of individuals manually create the models. The process of generating these models is time consuming and reduces the impact of the model on the system design. One way to improve this would be to automatically generate the model. In this work the procedure to automatically construct a model, based on Petri nets, for systems undergoing a phased-mission is applied to a pressure tank system, undertaking a four phase mission