Muhammad Younas

A risk based heat exchanger analysis subject to fouling Part I: Performance evaluation

Heat exchangers operating in the power and process industries are fouled to a greater or lesser extent depending on surface temperature, surface condition, material of construction, fluid velocity, flow geometry and fluid composition. This fouling phenomenon is time-dependent and will result in a decrease in the thermal effectiveness of a heat exchanger. Once the thermal effectiveness decreases to a minimum acceptable level, cleaning of the equipment becomes necessary to restore the performance. In this paper, we present a simple probabilistic approach to characterize various fouling models that are commonly encountered in many industrial processes. These random fouling growth models are then used to investigate the impact on risk-based thermal effectiveness, overall heat-transfer coefficient and the hot- and cold-fluid outlet temperatures of a shell-and-tube heat exchanger. All the results are presented in a generalized form in order to demonstrate the generality of the risk-based procedure discussed in this paper.

Truncated extreme value model for pipeline reliability

Abstract A truncated extreme value distribution has been developed to characterize pipeline reliability by considering the distribution of initial pit depths on the surface of a pipeline exposed to corrosive environments. This model can be used as an alternative to the Weibull model when a pipe is subjected to a severe corrosive environment. The model yields an exponential type hazard function with a possibility of a minor damage to the pipe surface at the time of commissioning. The probability model is analyzed thoroughly and its characteristic features such as mean, variance, median, quantile, and coefficient of variation are presented in graphical form. The model parameters, α and θ, are related to the pipe geometry and the average pit depth of the parent distribution.

A risk based heat exchanger analysis subject to fouling Part II: Economics of heat exchangers cleaning

Fouling is one of the major uncertainties associated with the operation and maintenance of heat exchangers in the power and process industries. The decision regarding periodic maintenance (cleaning) to meet the target performance level is generally based on both thermal and economic behavior of the process. In this paper, we present a cost model, which includes the risk level and the scatter parameter of random fouling growth models. Four models, namely linear, power law, falling rate and asymptotic fouling growth are integrated in the model. The non-dimensional cost function Γ as a function of reduced time t/M is examined by considering the dimensionless cost parameters γ1, γ2 and γ3, representing additional fuel cost, antifoulant cost and miscellaneous costs, respectively. These dimensionless cost elements are examined for a heat exchanger that is used in a crude oil preheat train. The results are presented in terms of risk level p and scatter parameter α for the underlying fouling models. Furthermore, a simplified closed-form solution is also obtained to study the optimal cycle time, representing minimum cost of operation and maintenance of heat exchangers.

Statistical aspects of fouling processes

Abstract Fouling in heat exchangers is traditionally characterized by deterministic (linear or nonlinear) kinetic models of fouling deposition and removal processes. This deterministic approach to fouling does not reflect the real situation of heat exchangers subject to fouling. The observations in a real situation of fouling of heat exchangers, when compared with the results obtained from predictive models, show a large discrepancy. This discrepancy in the fouling literature is normally referred to as uncertainty of the process. In this paper an attempt is made to model this uncertainty by characterizing the fouling as a correlated random process. The deterministic kinetic models (available in the literature) are randomized by treating their parameters as random quantities. Three fouling patterns are characterized by Rf(t) = Bt for the linear process, Rf(t) = mtn for the power law process with a falling rate (0 n ≤ 1) and Rf(t) = Rf∗[1 − exp (—t/τ)] for an asymptotic process, where t > 0 and B, m, Rf∗ and τ are the random process parameters with associated distributions. Fouling causes the performance loss of heat exchangers which can be tolerated up to a certain limit (i.e. critical level of fouling, Rfc), and thus it is of interest to find P[R(t) ≤ Rfc] = P(T > t) where T is the time to reach Rfc. Such distributions are developed in this paper, which are validated against the available data in the literature. It is demonstrated that alpha, modified alpha and Weibull are the most appropriate models to characterize the time to reach a critical level of fouling, if the underlying random fouling growth laws are linear, power law and asymptotic respectively. Knowledge of these distributions and the methods to determine their parameters is useful for devising appropriate maintenance and cleaning schedules in a probabilistic framework.

Reliability Based Spare Parts Forecasting and Procurement Strategies

Spare parts procurement strategies based on the maintenance engineering techniques of reliability engineering are merged with materials management discipline to provide a practical method to manage and control spare parts for industry. The well-known techniques of reliability engineering are used to determine failure rates for equipment and related parts. Then this information from the maintenance discipline is linked to the data of the materials management discipline. The results of this work will provide a scientific method of spare parts forecasting based on reliability of the parts, and more rational inventory management and procurement strategies with a minimum risk of stock out. As a consequence overstocking can be eliminated, and spare parts management can be streamlined on a rationale basis.

Corrosion-fatigue life prediction for notched components based on the local strain and linear elastic fracture mechanics concepts

An initiation-propagation model based on the local strain and linear elastic fracture mechanics concepts has been investigated for application to predict the fatigue life of notched components exposed to a corrosive environment. Estimates of the corrosion-fatigue crack initiation lives were obtained using strain-life relationships. The Paris power law was used to obtain the estimates of corrosion-fatigue crack propagation lives. Estimated corrosion-fatigue lives were compared with the experimentally obtained corrosion-fatigue life data using centre-notched specimens of three types of modified Al-2.5Mg alloy exposed to an Arabian Gulf seawater environment. Good fatigue life estimates were obtained both in air and in Arabian Gulf seawater environments for all three types of alloy. It is shown that good corrosion-fatigue life predictions can be made by determining the relevant fatigue parameters via a few simple constant-amplitude fatigue tests on smooth specimens and a few crack growth rate tests in the environment at the frequency of interest.

A reliability model for fatigue life characterization

Abstract A probabilistic characterization of the crack propagation process is used to develop a new two-parameter reliability model for predicting the life of products, where the life is defined as the time to reach the crack size up to a critical limit. Some characteristic features of this model are discussed. Methods of estimating the parameters from damage functions representing the crack growth or directly from time to failure data are presented. Using these methods, fatigue life data from literature have been used to develop the reliability of components or structures subjected to fatigue failure due to cyclic loading. Some implications of the model in fatigue life studies are also discussed.

Reliability and performance evaluation of extrusion dies

Extrusion dies are made of hot working tool steels and they can fail due to a variety of failure modes, such as wear, cracking and mechanical and thermal fatigue. Important features of the die damage processes are identified and approaches to minimise the tool damage are highlighted in this paper. In-service performance evaluation is done using industrial data of time to failure of dies of different complexity. It shows a significant variability around its average value. Keeping in view the average value and dispersion in the die life, the nature of their failure rate is explored and appropriate reliability characterisation is provided. Based upon the reliable life modelling, an interpretation of tool quality and its performance in view of Taguchis loss function is also provided to compare dies produced by two competing manufacturers, die materials or heat treatment processing cycle. Finally, some recommendations to enhance die life are provided.

Use of asphaltene filler to improve low-density polyethylene properties

ABSTRACT Asphaltene filled LDPE composite were prepared and characterized. The composites were studied to determine the reinforcement imparted by the asphaltene on the durability of LDPE polymer. The study showed that the composites have higher melting and crystalline behavior, delays the degradation induced by heat and acts as a thermal barrier limiting the emission of the gaseous degradation products, resulting in an increase in the thermal stability of the composites, higher tensile strength, have higher values of the storage and loss modulus. 5 wt% of asphaltenes added in the LDPE afforded the best dispersion in the polymeric matrix, larger crystallite size, enhanced thermal stability, highest relative degree of crystallinity and improved mechanical tensile or thermo-mechanical properties.

A fatigue-life prediction methodology for notched aluminum-magnesium alloy in gulf seawater environment

Local-strain and linear-elastic fracture-mechanics (LEFM) methodologies have been investigated for prediction of the corrosion-fatigue life of notched components of specially developed Al-2.5Mg alloys exposed to Arabian Gulf seawater environment. Corrosion-fatigue crack initiation life estimates were obtained using strain-life relationships; corrosion-fatigue crack propagation life estimates were obtained using LEFM relationships. The total corrosion-fatigue life was considered to be the sum of the crack initiation and crack propagation lives. Estimated corrosion-fatigue lives were compared with experimentally obtained corrosion-fatigue life data using center-notched specimens of three types of Al-2.5Mg alloys (containing different amounts of chromium) exposed to Arabian Gulf seawater environment. Two notch geometries, a circular notch (Kt= 2.42) and an elliptical notch (Kt= 4.2), were investigated. Good corrosion-fatigue life predictions can be obtained using local-strain and LEFM methodologies by determining the relevant material constants via a few simple fatigue tests on smooth specimens and a few crack-growth-rate tests in the environment at the frequency of interest.