Electrical separations

John Newman/ Ph.D./ Profe.s.sor of Chemical Engineering, University of California, Berkeley Principal Inve.stigatoi Inorganic Materials Re.search Division, Lawrence Berkeley Laboratory. (Separation Proce.s.se.s Based Primarily on Action in an Electric Field Theory of Electrical Separation.s)... 

A particular type of anodic danger arises in the interiors of pipes and storage tanks that are filled with an electrolyte and consist of similar or different metals, which, however, are electrically separated by insulating units. Potential differences are produced from external cathodic protection and are active in the interior [29,30]. These processes are dealt with in more detail in Sections 10.3.5,20.1.4, and 24.4.6. 

Recently in some types of refuelling stations, corrosion-resistant materials have been used between the filter-water separator and the outlet pipeline and valves, usually stainless steel, rarely aluminum. If these are buried, they must have good insulating coating and be electrically separated from other tank installations by insulating couplings. 

Protection currents of a few amperes are needed for the cathodic protection of assemblies of storage tanks or refuelling stations. In this case, electrical contact with grounded installations is the main problem. For cathodic protection, these contacts must be located and electrically separated. If this is not possible, then local cathodic protection should be installed (see Chapter 12). 

Pumping or compressor stations are necessary for the transport of material in pipelines. These stations are usually electrically separated from the cathodically protected long-distance pipeline. The concrete foundations are much smaller than in power stations and refineries. Since the station piping is endangered by cell formation with the steel-reinforced concrete foundations, local cathodic protection is recommended. 

Tank installations with underground storage tanks and station piping should, if possible, be provided with conventional cathodic protection [3]. This is sometimes not possible because electrical separation cannot be achieved between the protected installation and other parts of the plant (see Section 11.4). The necessity for cathodic protection can be tested as in Ref. 13. In tank farms, a distinction should be made between coated, buried storage tanks and aboveground, flat-bottomed tanks in which the base contacts the soil. 

The railway lines on bridges are often electrically connected with steel or reinforced steel structures which usually have a very low grounding resistance. In new installations, an electrical separation of the rails from the bridge structure is required according to the grounding resistance of the structure and the type of rail bed. Independent of this, pipelines and metal sheathing of cables are always electrically separated from the structure in order to exclude direct transmission of stray current from the rails in these conductors. 

Steel constructions and pipelines must either be electrically connected to the reinforcement of reinforced concrete structures or electrically separated. If they are connected, a current density of about 5 mA m should be applied to the external reinforcement and calculated on the total area of the concrete surface. 

Fig. 20-2 Insulated installation of heating pipes in enamelled containers with electrical separation of container and ground.

Fig. 20-3 Heating pipe (HE) in a boiler with a high-resistance coating with electrical separation from the boiler and from the ground by insulating units and a balancing resistor.

Heat exchangers with flowing cold electrolyte in fresh-water heat pump reservoirs must be treated similarly [9]. Here the cold electrolyte circuit need not have expensive insulated bushings installed as shown in Fig. 20-3, and the entire water pump equipment does not have to be grounded but must be covered to prevent contact. Electrical separation as in Fig. 20-3 is only undertaken with hot-water heat exchangers. 

Figure 20-9 shows the negative effect of uninsulated heating elements on corrosion protection. In a 250-liter tank, an electric tube heating element with a 0.05-m surface area was screwed into the upper third without electrical separation, and in the lower third a tinned copper tube heat exchanger with a 0.61 -m surface area was built in. The Cu heat exchanger was short-circuited for measurements, as required. For cathodic protection, a potential-controlled protection system with impressed current anodes was installed between the two heating elements. The measurements were carried out with two different samples of water with different conductivities. 

As the measurements show, the small heater without an electrical separation (from the boiler) is not detrimental to cathodic protection. However, with the uninsulated built-in Cu heat exchanger without an electrical separation, cathodic protection was not achieved. As expected, the polarization increased with increasing conductivity of the water. It should be pointed out that the Cu tube was tinned and that the tin could act as a weak cathodic component. Apart from the unknown long-term stability of such a coating, the apparent raising of the cathodic polarization resistance of tin is not sufficient to provide cathodic protection with such a large fixture. This applies also to other metal coatings (e.g., nickel). 

Cata, M., Ciccu, R., Delfa, G., 1973. Improvement in electric separation and flotation by modification of energy level in surface layers. 10th International Mineral Processing Congress, London, 1 1 - 22... 

Rao, R. B., Chattopadhyay, P. Banerjee, G. N. 1999. Removal of iron from fly ash for ceramic and refractory applications. Magnetic and Electrical Separation, 10, 21-27. 

The features, advantages, and disadvantages of the various types of thermocouples are tabulated in Table 3.178. For most process applications, the TC is manufactured with a protective outer sheath that uses an insulating material to electrically separate the TC from the sheath and provide mechanical and environmental protection. In some cases, the TC junction is placed in direct contact with the tip of the sheath to increase speed of response. Recommended practice is to always use an instrument with full isolation. The minimum span of even the best TC transmitters is limited to about 35°C (60°F). 

So, the third feature of intrinsic safety is a strict electrical separation between... 

Electrostatic-Separation Machines The first electrostatic machines to be used commercially employed the principle of contact electrification. These were free-fall devices incorporating large vertical plates between which an electrostatic field was maintained. Tribo-electric separation (contact charging) has experienced an increase in applications due to advances in mechanical self-cleaning and electrical design as well as the development of efficient precharging techniques. 

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