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When thinking about the safe operation of boilers (and don't we all?), several systems can readily be named; flame control, fuel/air ratio; steam pressure control, levels in the vessel, etc. What about the water? It seems so passive, as long as there is enough for level control, what's the big deal? Well, it turns out, that as the steam produced by a boiler is used in the process, the condensate from that steam is returned to the boiler as feedwater. However, since 100% of the condensate is not returned, whatever solids had been in that water before it evaporated to form steam are left in the remaining water. Fresh feedwater is added to maintain levels, but even fresh water contains some dissolved solids. So over time, the water in the boiler system gets saturated with all sorts of dissolved minerals.
A common cause of failures in various types of equipment is fatigue failure of bolts that secure parts together. Fatigue in any type of dynamically loaded mechanical component accounts for more than 80% of all failures. This post reviews some important factors in fatigue failure of metallic bolts.
Clearly it is advantageous to know when corrosion is occurring in operating equipment to avoid future failure. There is extensive literature on this topic. This post is a brief summary of information selected from the four references cited below. Each source includes several other references.
Most engineers know about the danger of galvanic corrosion caused by physical contact between dissimilar metals exposed to a corrosive liquid. However, this possibility is often overlooked and other features of this type of corrosion may not be widely known.
Aluminum alloy materials are well known for their light weight and strength-to-weight ratios. They are also susceptible to several forms of corrosion. Relative to other metals, their natural electrochemical potential places them adjacent to other very susceptible (active) metals such as magnesium (Mg) and zinc (Zn) in a traditional galvanic series in seawater. However, Al alloys naturally form a very thin, protective (passive) oxide film on their surface when exposed to oxygen. This film prevents corrosion as long as it is maintained over the surface. Pitting corrosion is a common form of attack on aluminum. This is typically caused by exposure to halide ions, chloride ions being the most common type found in corrosive media, that breakdown the passive film at discreet spots. This permits pits to initiate and grow.
Most realize that damage due to corrosion of metals is costly - but what specifically is that cost and what is included in the total? A study completed in 2002 sponsored by the Federal Highway Administration (FHWA), the National Association of Corrosion Engineers (NACE International) and implemented by CC Technologies Laboratories (now part of DVN) addressed those questions.
Material selection entails picking an engineering material - either metal alloy or non-metal - that is inherently resistant to the particular corrosive environment and also meets other criteria. Variables that will affect corrosion are established along with materials that may provide suitable resistance for those conditions. Obviously other requirements such as cost and mechanical properties of the potential materials must be considered.
Pin-hole leaks from a fire-control sprinkler piping system just above the false ceiling of multiple rooms in an office building resulted in a suit against three parties involved with supplying the system. The plaintiff - a financial service organization that owned the building - had much computerized transaction data stored on magnetic tapes in several rooms damaged or destroyed by the incident. The monetary loss was major. The leaks occurred within two years after the system was installed. This period is clearly far short of the expected life of the system. Defendants in the case included the supplier of the piping used in the system, the engineering firm that designed it and the installation contractor.
A variety of low-voltage electronic devices may fail to operate properly because of exposure to certain corrosive, gaseous environments. Susceptible items include consumer products such as microwave ovens, audio equipment, TV's and personal computers as well as many types of industrial sensors and automatic control devices. Failure generally occurs because of specific application factors. These include the severity of the environment, the presence or absence of atmospheric sealing of components within the device and the metals used at critical internal locations. Frequently complete failure doesn't occur but normal operation is erratic.
An ominous cloud of smoke continues to billow as the sprinklers rain water throughout the facility. The production manager's mechanical and electrical equipment is rapidly deteriorating, and within hours, the previously pristine equipment normally used to manufacture precision circuit boards will have a thin coating of flash rust with further damage forthcoming.