Types of Protective Layers

The primary purpose of LOPA is to determine whether there are sufficient layers of protection against a specific accident scenario. As illustrated in Figure 11-16, many types of protective layers are possible. Figure 11-16 does not include all possible layers of protection. A scenario may require one or many layers of protection, depending on the process complexity and potential severity of an accident. Note that for a given scenario only one layer must work successfully for the consequence to be prevented. Because no layer is perfectly effective, however, sufficient layers must be added to the process to reduce the risk to an acceptable level. 

Most borides are chemically inert in bulk form, which has led to industrial applications as engineering materials, principally at high temperature. The transition metal borides display a considerable resistance to oxidation in air. A few examples of applications are given here. Titanium and zirconium diborides, alone or in admixture with chromium diboride, can endure temperatures of 1500 to 1700 K without extensive attack. In this case, a surface layer of the parent oxides is formed at a relatively low temperature, which prevents further oxidation up to temperatures where the volatility of boron oxide becomes appreciable. In other cases the oxidation is retarded by the formation of some other type of protective layer, for instance, a chromium borate. This behavior is favorable and in contrast to that of the refractory carbides and nitrides, which form gaseous products (carbon oxides and nitrogen) in air at high temperatures. Boron carbide is less resistant to oxidation than the metallic borides. 

Various types of protective layers have been developed to improve poisoning resistance and thermal aging behavior against lifetime (Section 5.6.4, Fig. 5.6.7). 

LOPA is a simplified risk assessment to determine if there are sufficient IPLs against an accident scenario. As illustrated in Fig. 1, many types of protection layers can be considered against an unwanted accident. The thickness of the arrows represented in Fig. 1 indicates the frequency of the specified consequence for the initiating event. The results of LOPA can be used for the decision-making for numerical criteria and the number of IPL credits althou LOPA does not suggest which IPLs to add or which design to choose [William G. Bridges. 2001]. 

Electrochemical methods of protection rest on different precepts (1) electroplating of the corroding metal with a thin protective layer of a more corrosion-resistant metal, (2) electrochemical oxidation of the surface or application of other types of surface layer, (3) control of polarization characteristics of the corroding metal (the position and shape of its polarization curves), and (4) control of potential of the corroding metal. 

The schematic circuit diagram of an intrinsically safe fieldbus according to IEC 61158-2 (physical layer) is given in Fig. 6.212. The field devices are certified as intrinsically safe apparatus. Four-wire transmitters with an external power supply are explosion protected by an additional type of protection,... 

Pressurization techniques are similar to those used in zones 1 and 2, with the exception that purging shall not be applied. The high flow rate of protective gas during the purging period accompanied by a high velocity of gas may stir up dust layers inside the p-room on components and thus adversely affect the safety of this type of protection. Static pressurization, leakage compensation... 

Frequency 1 x 10 4 to 1 x 10"3(1 in 10,000 years to 1 in 1,000 years) Hazard Scenario This particular scenario is likely to occur somewhere in the industry during the life of this general type of process. Layers of Protection Two independent highly reliable safeguards are in piace, failure of one safeguard would not initiate an unwanted event. 

The samples from test II show one of the two trends observed for test I. The compositional distribution spreads out towards the Mg+Ca+Ti+Fe-comer due to the formation of coating structures, as discussed above (see Fig. 7), The chemical compositions of different coating layers that have been determined by the use of point analyses are illustrated in Fig. 8. The comparison of Figs. 7 and 8 shows that the content of calcium- and phosphorus-rich material increases as test proceeds. The presence of this type of phases is also indicated by the XRD results. It has previously been deduced that this type of coating layers protects the bed from agglomeration. 

Encapsulation of an electroactive area with a polymer which should serve as a barrier layer is very often the most challenging step in the whole sensor fabrication process. The most needed barrier is a layer that protects the electroactive area and does not allow passage of a charge through it. The second type of barrier layer is semipermeable, allowing a transport of charge... 

The issue with this type of superimposed layer strategy is that it creates a bulky and heavy protective clothing system, with a negative impact of comfort, ergonomics, and function. The same problem stands with other types of PPE, for example, helmets, boots, etc. On the other hand, if the PPE material or layer can combine several functions and protect against the whole list of hazards associated with a type of activity, for example, through the use of smart materials, gains in comfort and efficiency can be made (Peltonen et al., 2012). 

The aforementioned can be summed up as follows far larger settlements are permissible over the long term for the HDPE geomembranes than for nearly all other liners components, in particular for compacted clay liners or asphaltic concrete liners. Geomembranes must, however, be reliably protected from penetrative deformations with small radii of curvature, such as those due to pieces of gravel. Types, appropriate design and tests of protective layers will be dealt with in Chap. 8. 

Stringent use conditions are essential. This type of protection will need information and training for all the users. Protective creams should be applied on a clean and well-dried skin. Their application must be careful, in thin layer, with emphasis on spaces between fingers and around nails. They can also be used to protect the forearms, neck, or face from potential contact with dust or chemical fumes. It is recommended to let the cream dry, after applying it, for a few seconds to make the protective film stable. The application should be repeated at least every 2-3 h and, more often, in cases of mechanical wear removing the deposited layer. 

A third type of protective outer coating, stainless steel, for fused silica offers an alternative to aluminum-clad fused silica for elevated column temperatures. This technology is the inverse of that for polyimide-clad fused-silica capillary where a layer of fused silica is deposited on the inner surface of a stainless capillary. In Figure 3.19 scanning electron micrographs are displayed to compare the rough surface of stainless steel with the smooth surface of untreated fused silica and the surface of stainless steel after a micron meter layer of deactivated fused silica... 

No specific reliability/availability targets are set against each of these categories or classes. There is however a maximum limit set for software-based systems of 10 " PFD. More generally the reliability/availability targets are set in the Plant Safety Design Base and can be set either quantitatively or qualitatively. There is a preference for quantitative plus basic requirements on layers and types of protection. 

Enzyme sensors in thick-film technology, according to the design in Fig. 7.36, are commercially available. Most common are glucose sensors of this type. The protecting layer allows one to use the sensors for glucose determination in blood samples without pretreatment. 

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