Beyond its cost, the value of maintenance: An analytical framework for capturing its net present value

Maintenance planning and activities have grown dramatically in importance across many industries and are increasingly recognized as drivers of competitiveness if managed appropriately. Correlated with this observation is the proliferation of maintenance optimization techniques in the technical literature. But while all these models deal with the cost of maintenance (as an objective function or a constraint), only a handful addresses the notion of value of maintenance, and seldom in an analytical or quantitative way.In this paper, we propose that maintenance has intrinsic value and argue that existing cost-centric models ignore an important dimension of maintenance, namely its value, and in so doing, they can lead to sub-optimal maintenance strategies. We develop a framework for capturing and quantifying the value of maintenance activities. Our framework is based on four key components. First, we consider systems that deteriorate stochastically and exhibit multi-state failures, and model their state evolution using Markov chains and directed graphs. Second, we consider that the system provides a flow of service per unit time. This flow in turn is “priced” and a discounted cash flow is calculated resulting in a present value (PV) for each branch of the graph—or “value trajectory” of the system. Third as the system ages or deteriorates, it migrates towards lower PV branches of the graph, or lower value trajectories. Fourth, we conceptualize maintenance as an operator (in a mathematical sense) that raises the system to a higher PV branch in the graph. We refer to the value of maintenance as the incremental PV between the pre- and post-maintenance branches of the graphs minus the cost of maintenance. The framework presented here offers rich possibilities for future work in benchmarking existing maintenance strategies based on their value implications, and in deriving new maintenance strategies that are “value-optimized.”     

The development of natural gas supply costs to Europe,the United States and Japan in a globalizing gas market—Model-based analysis until 2030

Quickly declining natural gas reserves in some parts of the world, increasing demand in today's major gas consuming regions, the emergence of new demand centres and the globalization of natural gas markets caused by the rising importance of liquefied natural gas (LNG) are changing global gas supply structures and will continue to do so over the next decades. Applying a global gas market model, we produce a forecast for global gas supply to 2030 and determine the supplier-specific long-run average costs of gas supplied to three major consuming regions. Results for the three regions are compared and analysed with a focus on costs, supply diversification and the different roles of LNG. We find that while European and Japanese external gas supply will be less diversified in international comparison, gas can be supplied at relatively low costs due to the regions’ favourable locations in geographic proximity to large gas producers. The US market's supply structure on the other hand will significantly change from its current situation. The growing dependency on LNG imports from around the world will lead to significantly higher supply costs but will also increase diversification as gas will originate from an increasing number of LNG exporting countries.     

Pharmacokinetics of triptorelin after intravenous bolus administration in healthy males and in males with renal or hepatic insufficiency

PURPOSE: The aim of the present study was to validate a simple MRI-procedure for semiquantitative assessment of regional cerebral blood flow. MATERIALS AND METHODS: Unilateral cerebral ischaemia (30 minutes) in the territory of the middle cerebral artery was induced in 14 anesthetised rates. The MRI-experiment consisted in an intravenous bolus injection of gadolinium-DTPA, recording of the cerebral contrast kinetics with a T2*-weighted pulse sequence, and measurement of the maximal concentration change at a chosen reference point of time. To measure perfusion quantitatively, a microsphere technique, an accepted reference technique was used. With both methods a perfusion index related to the contralateral side was calculated. RESULTS: In all cases decreased perfusion was detected by the MRI technique. The perfusion indices correlated with a coefficient of correlation of r = 0.89 (p < 0.001). CONCLUSION: The results demonstrate that contrast-enhanced MRI with bolus injection can be implemented with clinical potential as a semiquantitative instrument for the assessment of cerebral perfusion. Regional cerebral blood volume and collateral blood flow may interfere with the estimate of blood flow.     

Phenotypic variation among familial hypercholesterolemics heterozygous for either one of two Afrikaner founder LDL receptor mutations

Two common founder-related gene mutations that affect the low-density lipoprotein receptor (LDLR) are responsible for approximately 80% of familial hypercholesterolemia (FH) in South African Afrikaners. The FH Afrikaner-1 (FH1) mutation (Asp206-->Glu) in exon 4 results in defective receptors with approximately 20% of normal activity, whereas the FH Afrikaner-2 (FH2) mutation (Val408-->Met) in exon 9 completely abolishes LDLR activity (< 2% normal activity). We analyzed the contribution of these mutations and other factors on the variation of hypercholesterolemia and clinical features in Afrikaner FH heterozygotes. The type of FH mutation, plasma triglyceride levels, and age of patients each contributed significantly to the variation in hypercholesterolemia, whereas smoking status, high-density lipoprotein cholesterol levels, and gender had no influence. Although all FH heterozygotes had frank hypercholesterolemia, patients with the FH1 mutation had significantly lower cholesterol levels than those with the FH2 mutation. FH1 heterozygotes also tended to have milder clinical features. The differences between the two FH groups could not be explained by a difference in the common apolipoprotein E variants. This study demonstrates that mutational heterogeneity in the LDLR gene influences the phenotypic expression of heterozygous FH.     

The housing careers of black middle-class residents in a South African metropolitan area

Under apartheid, black African households could not own land or homes in most major urban centres in South Africa. This limited residential mobility and locked many households into state rental accommodation in townships. Homeownership for all South Africans was restored in the mid-1980 s and the Group Areas Act was repealed in 1991. Democracy opened up economic opportunities previously unavailable to black people. This paper investigates the effect on black middle-class South African households’ residential mobility and housing careers. A retrospective cross-sectional survey of 244 such homeowners in the Mangaung Metropolitan Municipality determined their last five housing states. Changes in housing state indicated a steady improvement in housing quality, but tenure changes were not necessarily unidirectional—some had reverted to rental. More than 85% of the study participants had used mortgages to finance their housing career. Very few had financed their housing using own savings, an inheritance, or sale of a previous house, and not many had used the government subsidy. We found that housing careers are bridging the historical spatial racial divide in this municipality.     

The activated sludge process—3 single sludge denitrification

The general aerobic bi-substrate active-site death-regeneration activated sludge model including nitrification of Dold et al. (Prog. Wat. Technol.12, 47–77, 1980) is extended to include the kinetic behaviour of the denitrification process in single sludge systems. The extension requires a change in the value of only one of the kinetic constants (K′_mp) in the expression for the particulate substrate utilization rate when the environment becomes anoxic. The extended model simulates very closely the response of the multi-reactor nitrification-denitrification process configurations under both constant and cyclic flow and load conditions. Under constant flow and load conditions, the denitrification response predicted can be reduced to that approximated by a zero order reaction dN/dt = ?KX_a with two rates in the primary and one in the secondary anoxic reactor respectively.     

Calcium carbonate crystallization kinetics

The kinetics of crystallization of calcium carbonate were studied by following changes in pH only on batch solutions of CaCl₂, NaHCO₃ and NaOH in the presence of calcite seed crystals. The Ca⁺² and CO⁻²₃ concentrations were derived by taking account of the equilibrium relationships between the species of the carbonic—and calcium systems with changes in pH and ionic strength. Reactant concentrations and seed crystal masses studied were in the same range as that found in water softening practice. At these high concentrations the crystallization kinetics continued to conform to the crystallization model of Reddy and Nancollas. Removal of Ca⁺² from solution follows a rate equation first order with respect to Ca⁺² and CO⁻²₃ concentrations, the activation energy for crystal growth is 10·3 ± 0·9 kcal mol⁻¹ and the rate is independent of the stirring speed, all suggesting a surface controlled process. The rate constant is a function of the pH established immediately after mixing of the reactants. The rate of Ca⁺² removal from solution is directly proportional to the mass of crystals present in the reaction solution. Numerical integration of the rate equation gives a solution for the design of calcium carbonate crystallization reactors.     

Adsorption of 4-nitrophenol onto Amberlite IRA-900 modified with metallophthalocyanines

The adsorption of 4-nitrophenol using commercially available Amberlite IRA-900 modified with metal phthalocyanines (MPc) was investigated. The metallophthalocyanines immobilised onto the surface of Amberlite IRA-900 include Fe (FePcS4), Co (CoPcS4) and Ni (NiPcS4) tetrasulphophthalocyanines, and differently sulphonated phthalocyanine mixtures of Fe (FePcSmix), Co (CoPcSmix) and Ni (NiPcSmix). Adsorption rates were fastest for the modified adsorbents at a loading of 1x10(-3)g MPc/g Amberlite, at pH 9. The highest amount of 4-NP removal was obtained on FePcSmix modified Amberlite IRA-900 with Qt=42.9mmolg(-1) and adsorption efficiency of 86%. The recovery efficiency of 4-NP within 150min was 76%. Using the Langmuir-Hinshelwood kinetic model, the complexes showed an order of 4-nitrophenol adsorption to be as follows: CoPcSmix>NiPcS4>NiPcSmix>FePcS4>FePcSmix>CoPcS4. The MPc modified Amberlite IRA-900 was used repeatedly, following removal of 4-NP by nitric acid, without any significant loss of activity.