INTERNATIONAL PROTECTIVE

Internal protection, used by van Tamelen in a synthesis of colchicine, may< in... 

A critical review of internally protected capacitor units 25/812... 

Figure 25.3 Cascade failure of healthy elements in the absence of an internal protection...

In units with internal protection the fault is not severe, as the fuses with each element will isolate the element... 

For loads, such as on motors where the capacitor unit is being switched with the machine, or where close monitoring of the capacitor health is a prerequisite to avoid an eventual outage of the capacitor unit by gradual depletion of its capacitance, internally protected capacitor units may be preferred. 

This is applicable to both LT and HT capacitors. But it is more important in HT banks, which are relatively much larger and are built of a number of single units eonnected in series-parallel. These may encounter much higher fault currents in the event of a severe internal fault, even in one unit and are thus rendered more vulnerable to such ruptures. This phenomenon is more applicable to units that are externally protected w here (he intensity of fault may be more severe, than internally protected units. 

Protection with internal fuses is easier, as fuses are provided for each element which can contain the severity of the fault well within the safe zone in all probability. Some users even recommend capacitor units 250/300 kVAr and above with internal fuses only. Figure 26.1 shows a typical operating band of (he internal fuses for an internally protected unit. It demonstrates a sufficient margin between the operation of (he fuses and (he shell s safe zone. The fuse characteristics are almost the same for all manufacturers. 

Mtiking a ctipacitor element A critictil review of internally protected capacitor units Self-healing capacitors Making a capacitor unit from elements Making capacitoi banks from eapticitor units Rating... 

In general, in the case of coated materials, interaction with protection currents must be taken into account [106,107] (see Section 5.2.1). In internal protection, the effects of products of electrolysis must be checked [108] (see Chapters 20 and 21). 

Enamel coatings are applied for internal protection of storage tanks (see Section 20.4.1). Enamel is impervious to water, i.e., it separates the protected object and corrosive medium. Corrosion protection can fail only at defects in the enamel coating and through corrosion of the enamel (see Section 5.4). 

There are numerous publications [9,10,16,19-24] and test specifications [8,25] on the formation of cathodic blisters. They are particularly relevant to ships, marine structures and the internal protection of storage tanks. Blister attack increases with rising cathodic polarization. Figures 5-6 and 5-7 show the potential dependence of blister density and the NaOH concentration of blister fluid, where it is assumed that c(Na ) and c(NaOH) are equal due to the low value of c(Cr) [23]. 

Enamel coatings are used for the internal protection of storage tanks that in most cases have built-in components (e.g., fittings with exits, probes, temperature detectors) that usually exhibit cathodic effectivity. These constitute a considerable danger of pitting corrosion at small pores in the enamel. Corrosion protection is achieved by additional cathodic protection which neutralizes the effectiveness of the cathodic objects. 

In spite of a low driving voltage of about 0.2 V, about 90% of all galvanic anodes for the external protection of seagoing ships are zinc anodes (see Section 17.3.2). Zinc alloys are the only anode materials permitted without restrictions for the internal protection of exchange tanks on tankers [16] (see Section 17.4). 

Fig. 6-12 (a) Block-shaped anode with screw fixing and (b) block-shaped anode for internal protection. 

Fig. 6-14 (above) Anode shapes for the internal protection of tanks, (a) Semicircular cross-section, (b) rectangular cross-section, (c) trapezoid cross-section. 

The anodes for internal protection of containers and tanks are frequently fixed by screws because of the danger of explosion, welding or brazing is not allowed. 

In internal protection, attention must be paid to the fact that mixed gases that contain hydrogen and oxygen are evolved by the protection current. Noble metals and noble metal coatings can catalyze an explosion. The safety measures required in this case are presented in DIN 50927 [32] (see also Section 20.1.5). 

The advice given in Section 17.3.1 and Eqs. (17-2) to (17-4) applies in determining the current requirement and number and weight of anodes, taking into account the different current requirements of individual surface areas Sj. The number of anodes is derived from Eq. (17-4), taking into account the maximum current output /max of the galvanic anodes, which cannot be given with certainty because of problems with internal protection and interaction with dirty electrolyte. 

In contrast to external protection, the anodes in internal protection are usually more heavily covered with corrosion products and oil residues because the electrolyte is stagnant and contaminated. The impression can be given that the anodes are no longer functional. Usually the surface films are porous and spongy and can be removed easily. This is achieved by spraying during tank cleaning. In their unaltered state they have in practice little effect on the current output in ballast seawater. In water low in salt, the anodes can passivate and are then inactive. 

The internal cathodic protection of pipes is only economic for pipes with a nominal width greater than DIN 400 due to the limit on range. Internal protection can be achieved in individual cases by inserting local platinized titanium wire anodes (see Section 7.2.2). 

Internal cathodic protection of water tanks and boilers is most economical if it is taken care of at the design stage. It can, however, be installed at a later stage as a rehabilitation measure to halt the progress of corrosion. Tanks and boilers in ships were described in Section 17.4. Further applications of internal protection are dealt with in Chapter 21. 

The range of applications for magnesium anodes includes the internal protection of boilers, feedwater tanks, filter tanks, coolers, pipe heat exchangers and condensers. They are mainly used in conjunction with coatings and where impressed current equipment is too expensive or cannot be installed. 

The impressed current protection method is used mainly for the internal protection of large objects and particularly where high initial current densities have to be achieved (e.g., in activated charcoal filter tanks and in uncoated steel tanks). There are basically two types of equipment those with potential control, and those with current control. 

Protection current devices with potential control are described in Section 8.6 (see Figs. 8.5 and 8.6) information on potentiostatic internal protection is given in Section 21.4.2.1. In these installations the reference electrode is sited in the most unfavorable location in the protected object. If the protection criterion according to Eq. (2-39) is reached there, it can be assumed that the remainder of the surface of the object to be protected is cathodically protected. 

In comparison with the external cathodic protection of pipelines, tanks, etc., internal protection has limitations [4] which have already been indicated in Section 20.1 but are fully listed here ... 

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