Passive failures

The development of computer capabiUties in hardware and software, related instmmentation and control, and telecommunication technology represent an opportunity for improvement in safety (see COMPUTER TECHNOLOGY). Plant operators can be provided with a variety of user-friendly diagnostic aids to assist in plant operations and incipient failure detection. Communications can be more rapid and dependable. The safety control systems can be made even more rehable and maintenance-free. Moreover, passive safety features to provide emergency cooling for both the reactor system and the containment building are being developed. 

Minimal cut sets are then ranked. Two factors are considered in the ranking procedure. The first factor considers stmcture, ie, a one-event minimal cut set is more important than a two-event minimal cut set. The implication is that one event is more likely to occur than two events, two events are more likely than three events, and so on. The second factor considers ranking within equal-size minimal cut sets. The general ranking rules consider the probabihty of human error, active equipment failure, and passive equipment failure (73). 

This ranking implies that human errors are more likely to occur than active equipment failures (functioning equipment, such as a mnning pump) and that active equipment failures are more likely to occur than passive equipment failures (static, nonfunctioning equipment, such as a storage tank). 

This ordering by singles, doubles and triples takes added meaning when the failure rates of the active and passive components (Table 1.4.3-1) are included. A doublet has a failure frequency that is the product of the two failure rates a triplet is the product of three... 

By identifying the potential sources of failures, it is possible to develop controls to address those hazards. These controls might be passive physical items (e.g., dikes, walls, vents), active physical systems (e.g., fire suppression, pressure limiters, temperature controls), or administrative procedures. 

Failures can occur in two general types of equipment—active and passive— explained as follows ... 

There are a number of failure modes for the three failure severities and for active and passive equipment. Figures 2.1 and 2.2 illustrate these failure modes and severities by type of equipment. 

Figure 2.2 Passive equipment failure modes. Reprinted from ANSI/IEEE Std. 500-19S4, 1984 by the IEEE, with permission of the IEEE Standards Department.

Chloride salts (sodium chloride, potassium chloride) tend to interfere with the formation of a protective layer over metals. Chloride salts destroy the passivity of some stainless steels and cause them to fail by rapid cracking under tensile stress at temperatures higher than about 176°F (80°C). This type of failure is called chloride stress cracking (CSC) [186,194]. 

Steels and stainless steels show preferential nucleation of pits at inclusions, most notably sulphide inclusions ". Other sulphur-rich regions in ferrous and nickel-based alloys may also lead to premature failure. It has been shown that accumulation of sulphur on the surface of these materials retards passivity and enhances dissolution of the metal. These effects occur in any solution in which the metal shows an active region and they are also preferential pitting sites in the presence of chloride. A recent notion for... 

The arbitrary division of behaviour has been made because of the extreme behaviour of some chemicals that initiate small areas of attack on a well-passivated metal surface. The form of attack may manifest itself as stress-corrosion cracking, crevice attack or pitting. At certain temperatures and pressures, minute quantities of certain chemicals can result in this form of attack. Chloride ions, in particular, are responsible for many of the failures observed, and it can be present as an impurity in a large number of raw materials. This has led to the development of metals and alloys that can withstand pitting and crevice corrosion, but on the whole these are comparatively expensive. It has become important, therefore, to be able to predict the conditions where more conventional materials may be used. The effect of an increase in concentration on pitting corrosion follows a similar relationship to the Freundlich equation where... 

Passivation at the metal/active mass interface, or of the active mass itself can also lead to failure. Detrimental changes in the morphology of the active mass and microstructural changes in the grid material can also occur. 

The capacity of an anode is dependent on the anode current density. To some extent it will be governed by the exposure environment but, in part, is within the control of the design. Certainly wholly unsuitable current densities can usually be avoided. At lower operating current densities some anodes exhibit reduced capacity this is shown in Fig. 10.17. Long periods of low operating current density can lead to passivation. This may result in failure to activate when the current demand increases (as can occur with anodes on coated structures when the coating deteriorates). 

The Operational Characterisics of Platinised-Titanium Anodes Platinised-titanium anodes have the disadvantage that the protective passive him formed when titanium is made anodic in certain solutions can breakdown. This could result in rapid pitting of the titanium substrate, leading ultimately to anode failure. The potential at which breakdown of titanium occurs is dependent upon the solution composition, as is evident from Table 10.16. 

Stainless steel has been tried as an inert anode, mainly under laboratory conditions and with only partial success. Even at low current densities in fresh water the majority of alloys pit rapidly, although others show the ability to remain passive at a low current density . However, at practical current densities, the presence of chloride ions, deposits on the anode or crevice corrosion at the anode support lead to rapid failure , but it may be possible that stainless steel could give useful service under certain conditions and with particular alloys . ... 

Tests have indicated failure to form PbO rapid deterioration, although at 100 Am it slows down after several weeks. Increasing silver content results in some improvement ". Anode passivated in 0-163 ohm m water continues to operate whilst PbO, is undamaged "". ... 

Although the first industrial application of anodic protection was as recent as 1954, it is now widely used, particularly in the USA and USSR. This has been made possible by the recent development of equipment capable of the control of precise potentials at high current outputs. It has been applied to protect mild-steel vessels containing sulphuric acid as large as 49 m in diameter and 15 m high, and commercial equipment is available for use with tanks of capacities from 38 000 to 7 600000 litre . A properly designed anodic-protection system has been shown to be both effective and economically viable, but care must be taken to avoid power failure or the formation of local active-passive cells which lead to the breakdown of passivity and intense corrosion. 

Table 19.S). In particular, they have established that the presence of dissolved O2 enhances passivity thus causing E, to become more positive, and consider that this explains the failure of France and Greene to obtain accord between controlled potential tests in hydrogen-saturated chloride solutions and immersion tests in oxygenated chloride solutions at the same potentials. 

Although the need to properly soften the FW control BW variables such as alkalinity, TDS, and sludge and limit carbonic acid corrosion remains vital, these other problems generally are secondary to effective control over DO. If severe boiler plant damage and system failure are to be avoided, it is necessary to continuously maintain waterside reducing conditions and properly passivated boiler surfaces. 

This model was developed after pioneering experiments carried out in the USA by Overmier and Seligman (1967) who reported profound behavioural changes in dogs after their exposure to inescapable, uncontrollable stress (footshock). Subsequent work has concentrated on rats and mice, which show a similar behavioural response. This is expressed as appetite and sleep disturbance, general passivity and, on re-exposure of subjects to the stress, a failure to attempt to escape ( escape deficits ), even when this is feasible. 

---