Corrosion electrical systems

Dust-tight electrical systems should be used in areas where activated carbon is present, particularly powdered products (66). When partially wet activated carbon comes into contact with unprotected metal, galvanic currents can be set up these result in metal corrosion (67). 

Although transformers suitable for other industrial installations are generally suitable for producing applications, certain options may be desirable— primarily due to environmental considerations. At locations subject to harsh environmental conditions, and particularly at locations subject to washdown with high-pressure hoses, non-ventilated enclosures are desirable, if not necessary. Likewise, at locations subjected to salt water and salt-laden air, it often is desirable to specify copper windings and lead wires. Most manufacturers provide standard units with aluminum windings and lead wires. Even if aluminum coils are used, it is almost always desirable to require stranded copper lead wires. This will lessen corrosion and loose terminal problems when transformers arc interconnected to the facility electrical system with copper conductors. If the transformers are to be installed outdoors in corrosive environments, cases should be of corrosion-resistant material (e.g., stainless steel) or be provided with an exterior coating suitable for the location. 

A factor which previously limited installation of automatic corrosion monitoring systems was the cost of cabling between sensors and control room instrumentation-this was particularly relevant to the electrical resistance (ER) systems. Developments to overcome this have included transmitter units at the probe location providing the standard 4-20 mA output (allowing use of standard cable) for onward transmission to data systems or the use of radio linkage which has been successfully used for other process-plant instrumentation. 

Several attempts have been made to use other materials as antifreezes. Salts, such as calcium chloride or sodium acetate, depress freezing points, but are corrosive. They can cause severe corrosive damage to the engine. Leakage of these salt solutions can cause short circuiting of the electrical system. 

The grounding of household electrical systems to the plumbing can increase corrosion rates and the subsequent leaching of lead from the lead solder used for copper pipes. Areas where the pH of the water is less than 8.0 may have higher lead drinking water levels as well. 

The inherent disadvantages of the underground system are high first cost, inconvenience, and excessive cost of maintenance. Overhead construction is often used since it can be installed at about one-third the cost of the underground system, and repairs can be quickly made. However, such lines are exposed to impairment by weather, lightning, trucks, cranes, and corrosion, so that overhead construction should be avoided in congested areas where a reliable and safe electrical system is necessary. 

Agricultural buildings that house livestock require special care in selecting wiring materials, wiring methods because of the corrosive dust, gases, and moisture. Corrosion of metallic conduit, boxes, and fixtures lead to electrical system failure. Accelerated corrosion because of condensation occurs on electrical panels that are not properly designed. 

All electrical installations and installed equipment must be safe to use and free from the dangers of electric shock, but some installations or locations require special consideration because of the inherent dangers of the installed conditions. The danger may arise because of the corrosive or explosive nature of the atmosphere, because the installation must be used in damp or low-temperature conditions or because there is a need to provide additional mechanical protection for the electrical system. Part 7 of the lET Regulations deals with these special installations or locations. In this section we will consider some of the installations which require special consideration. 

The disciplines of mechtmical, electrical, and electronic engineering are needed by the chemical engineer to be able to consider process design, materials of construction, corrosion, and electrical systems for the motors and heaters. Electronic engineering is basic knowledge needed for control systems and computer uses. 

Seven primary corrosion prevention methods are available to reduce the corrosion of metals material selection, electrical isolation, electrical bonding, environmental modification, inhibitors, coatings, and cathodic protection. Corrosion prevention methods are often used in conjunction with one another. Coatings are used to reduce current requirements in cathodic protection. Zinc is used as a cathodic protection system in coatings. The effects of two or more corrosion prevention systems can be synergistic or antisyner-gistic. Two effects are synergistic if the combination of the two is more effective than the sum effect of the two. 

Cathodic protection is an electrochemical polarization process that is widely and effectively used to limit corrosion. Simply stated, it is an electrical system whose energy operates in opposition to the natural electrochemical decomposition process of corrosion. All cathodic protection systems require the artificial development of an alternative corrosion cell with (-) electrons flowing finm the artificially installed anode to the structure in the metallic path. It also requires the flow of (+) ions (atoms or molecules carrying electrical charge) from the anode to the structure by the electrolyte path and/or (-) ions in the opposite direction. For a constant current, the level of protection depends on the polarization slope of the cathodic reaction on the structure. Current can be supplied by a galvanic or impressed current system. In a galvanic system, the electrons flow because of the difference in half-cell potential between the metal of the structure and the cathodic protection anode metal, given that the anode metal is more reactive than the metal of concern. In an impressed current system, an... 

The absence of solvents in such solid-polymer-electrolyte photovoltaic cells presents the possibility of fabricating corrosion-free systems. The thin-film solid-state cells also allow fabrication of multispectral cells composed of more than one semiconductor in optical and electrical series. A solid-state photovoltaic cell, n-Si/Pt/PP/PEO(K.I/ l2)/Pt/ITO, was studied. The surface modifications of n-Si with PP can dramatically reduce the large activation energy barrier against efficient charge transfer between semiconductor and polymer-solid electrolyte. The efficiency of this cell is limited by a high surface recombination velocity associated with surface states of the n-Si. The cell had V = 225 mV and 11 niA cm at 100 mW cm illumination with junction ideality factor of 1.5. This implies the existence of deleterious surface states acting as recombination centres. 

Electricity is an ignition source ttiat can cause fires and explosions if equipment is installed or wired inadequately or if a static discharge occurs. Table 2-2 showed that electrical systems are the leading cause of fires. Improperly installed electrical systems may also cause accidental electrocution. Stray ciurents may result in corrosion of piping and equipment. 

Thermally conductive polymer composites offer new possibilities for replacing metal parts in electric systems. The advantages of polymer composites as compared to metals include improved corrosion resistance, lighter weight, and the ability to adapt the conductivity properties to suit the application needs [5]. Due to the fact that most polymers show a rather low thermal conductivity, the addition of conductive fillers is commonly used to enhance the thermal transport of polymer-based composites [2]. 

Corrosion Monitoring System Enables Utility to Avoid Relining Stack and Ducts, EPRI RP1871-17, Palo Alto, Calif, Electric Power Research Institute, 1990. 

Other Specialty Chemicals. In fuel-ceU technology, nickel oxide cathodes have been demonstrated for the conversion of synthesis gas and the generation of electricity (199) (see Fuel cells). Nickel salts have been proposed as additions to water-flood tertiary cmde-oil recovery systems (see Petroleum, ENHANCED oil recovery). The salt forms nickel sulfide, which is an oxidation catalyst for H2S, and provides corrosion protection for downweU equipment. Sulfur-containing nickel complexes have been used to limit the oxidative deterioration of solvent-refined mineral oils (200). 

In other designs, a diffused siUcon sensor is mounted in a meter body that is designed to permit caUbration, convenient installation in pressure systems and electrical circuits, protection against overload, protection from weather, isolation from corrosive or conductive process fluids, and in some cases to meet standards requirements, eg, of Factory Mutual. A typical process pressure meter body is shown in Figure 10. Pressure measurement from 0—746 Pa (0—3 in. H2O) to 0—69 MPa (0—10,000 psi) is available for process temperatures in the range —40 to 125°C. Differential pressure- and absolute pressure-measuring meter bodies are also available. As transmitters, the output of these devices is typically 4—20 m A dc with 25-V-dc supply voltage. 

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