Modeling erosion-corrosion

This paper focuses on how to model the deterioration of static pressurized process equipment to enable efficient inspection and maintenance planning. Such equipment tends to gradually deteriorate over time from erosion, corrosion, fatigue and other mechanisms, and at some point of time inspection, repair or replacement is expedient with respect to safety, production and costs. The deterioration of the equipment is influenced by many factors such as type of equipment, system design, operation and process service, material and environment. For hydrocarbon systems, the most critical deterioration mechanisms are corrosion due to CO2 and H2S, microbially influenced corrosion, sand erosion and external corrosion (DNV 2002). In general, CO2 is the most common factor causing corrosion in oil and gas system of low alloy steel (Singh et al. 2007). 

Erosion-corrosion is another example of a process with a stochastic trigger with a deterministic evolution. In this case, the trigger is the impact of a particle onto the surface, and the resultant exposure of unpassivated metal (for a passive system) or the removal of seale or a change in mass transport may cause a current pulse, the time-evolution of whieh can be modeled according to the metal-solution system being examined [59-64]. Unfortunately, most of the current induced by an impact is not captured by an external eleetrode, and this makes interpretation complex. Indeed, correlation with the results of... 

Erosion corrosion is mainly observed in hydraulic installations (pumps, turbines, or tubes). This form of corrosion appears at a point where the flow velocity of the bulk solution exceeds a critical limit, or where this limit is exceeded by local turbulence. Erosion corrosion results from an interaction between mechanical and chemical influences. Fig. 1-26. One model describing the mechanism of erosion corrosion assumes that local shear forces acting on the metal surface as a result of the high flow velocity forms pores or unprotected areas. Accelerated mass transfer then occurs in these areas and aggravates corrosion damage. 

Finally, the gravel pack and the erosion/corro-sion allowance are not considered in the analysis because there cannot be a direct control on them and their performance cannot be actively improved if needed. Thus, this model makes the assumption that these two barriers performance is constant and does not affect the dynamic assessment of loss of containment risk. However, the ageing of the two systems is an important aspect to be considered. In particular, if the erosion/corrosion allowance is in a critical state, decision-makers will have to take different decisions and properly respond to this serious issue. 

An example is given, considering the threats of erosion/corrosion in the petroleum production. This example shows how the proposed model deals with different operative conditions and how the risk of loss of containment is affected by them. In this way, the importance of both the state of the threats and the performance of the barriers is demonstrated. An effective action of risk prevention should be then focused on both of them by giving priority to the improvement of poor barriers connected to critical threats. 

VFO works well in gas turbines. In a nine-month test program, the combustion properties of VFO were studied in a combustion test module. A gas turbine was also operated on VFO. The tests were conducted to study the combustion characteristics of VFO, the erosive and corrosive effects of VFO, and the operation of a gas turbine on VFO. The combustion tests were conducted on a combustion test module built from a GE Frame 5 combustion can and liner. The gas turbine tests were conducted on a Ford model 707 industrial gas turbine. Both the combustion module and gas turbine were used in the erosion and corrosion evaluation. The combustion tests showed the VFO to match natural gas in flame patterns, temperature profile, and flame color. The operation of the gas turbine revealed that the gas turbine not only operated well on VFO, but its performance was improved. The turbine inlet temperature was lower at a given output with VFO than with either natural gas or diesel fuel. This phenomenon is due to the increase in exhaust mass flow provided by the addition of steam in the diesel for the vaporization process. Following the tests, a thorough inspection was made of materials in the combustion module and on the gas turbine, which came into contact with the vaporized fuel or with the combustion gas. The inspection revealed no harmful effects on any of the components due to the use of VFO. 

The principle of the human skin model test is that the test material is apphed topically for up to 4h to a three-dimensional human skin model, comprising at least a reconstructed epidermis with a functional stratum comeum (outermost layer of the skin). The human skin models can come from various sources, but they must meet certain criteria. Corrosive materials are identified by their abdity to produce a decrease in cell viabdity (as determined, e.g., by using a dye reduction assay) below defined threshold levels at specified exposure periods. The principle of the test is in accordance with the hypothesis that corrosive chemicals are able to penetrate the stratum comeum (by diffusion or erosion) and are sufficiently cytotoxic to cause cell death in the underlying cell layers. 

The antiulcerogenic effect of the bark of V. africana was also reported using different experimental models. Tan and collaborators (2, 49) showed the cytoprotection of aqneons extracts for the gastric mucosa against erosive action of corrosive agents (HCl/ EtOH). Thns, V. africana possess anti-ulcer properties and these properties were not related with an increase on mucus production, or reduced pepsin activity, however the cytoprotection was related to a mechanism involving the physico-chemical re-enforcement of the gastric mucous layer. 

Heavy fuel deposits were expected in boiling systems, and therefore the initial studies of deposition and activity transport for power reactors concentrated on the CANDU-BLW concept until the fields at Douglas Point became a concern. The deposit thickness was proportional to iron concentration in the coolant and to the square of the heat flux (69) deposition was reversible and quickly reached a steady value set by the local conditions. The corrosion products initially deposit by hydrodynamic and electrostatic effects then boiling accelerates deposition by drawing water and its contained iron into the deposit to replace the steam that leaves. Local alkalinity gradients within the deposit determine whether iron crystallizes to cement the deposit or dissolves to weaken it, and erosion processes then define the equilibrium thickness (70), This model works well in explaining deposition under boiling conditions. 

Fig. 4. Corrosion/erosion patterns of chamber materials under plasma etching (pictures are at 10,000x magnification). Model A indicates a uniform corrosion/erosion which can either be higher or low Model B shows the attack at grains of materials and Model C shows the attack at grain boimdaries of materials.

Part A gives general guidelines for the design of large commercial fluidized bed reactors with respect to the following aspects (1) solids properties and their effect on the quality of fluidization (2) bubble size control through small solid particle size or baffles (3) particle recovery by means of cyclones (4) heat transfer tubes (5) solids circulation systems (6) instrumentation, corrosion and erosion, mathematical models, pilot plants and scale-up techniques. 

Corrosion rate would by this model be proportional to concentration, velocity, target efficiency, and erosivity [95] (the mass of metal removed per unit mass of particles striking the metal surface). Erosivity would be expected to be a function of the flow characteristics, particle energy and abrasiveness, and other properties of the particle and the metal, although a general formula for it has not been developed. 

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