Buried systems

Fig. 3.7 Stagnant confined systems are former (fossil) through-flow systems (1) that got buried beneath the level of the terminal base of drainage and (2) were covered by new rock systems that eventually got buried. System (3) is presently active as a through-flow system.

In the early years of industrial development in the United States, many plants buried their outside pipelines. The initial cost for this type of installation is low because no supports are required and the earth provides insulation. However, location and repair of leaks are difficult, and other pipes buried in the same trench may make repairs impossible. Above-ground piping systems in industrial plants have proven to be more economical than buried systems, and, except for major water and gas lines, most in-plant piping systems in new plants are now located above ground or in crawl-space tunnels. 

Putting utilities underground does have drawbacks. There is an out-of-sight, out-of-mind problem— buried systems may not be inspected and checked as those above ground. They may also be more subject to corrosion than if they were above ground. A sensible compromise is to place utility lines below grade in open trenches. 

The resistivity values obtained represent the average resistivity of the soil to a depth equal to the pin spacing. Resistance measurements are typically performed to a depth equal to that of the buried system (pipeline) being evaluated. Typical probe spacing is in increments of 0.5 to 1 m. 

A third auxiliary effect is the deposition of compounds from the soil on the buried system. This effect is extremely pronounced in seawater, where a cathode scale resembling hard glossy enamel can build up over time and grow as thick as 2 cm. When deposited in seawater, this material is a complex mixture of calcium and magnesium oxides, hydroxides, and carbonates. The composition of this hard deposit depends upon CP current density, among other factors (see Chap. 8 for details on calcareous deposits). 

In fresh or brackish water cathode scale composition is even more variable than the scale formed in seawater since its formation is based on highly variable ion concentrations. This is also true in soils. Some soils will not form a visible scale, although there is enough calcium present in most soils to favor scale formation. Often, when a system has been polarized by CP, no scale will be visible until the system surface dries out. It then appears as a whitish coloration. On a buried system which has not been under CP, cathode scale is frequently found in an irregular mottled pattern this makes the actually active cathodic areas visible to the eye. 

When a buried system that has been in corrosive soil without adequate protection for some time is examined, it is usually foimd that by far the greater part of the area is unaffected. Where corrosion has taken place, it is in the form of pits, which are relatively small areas where the attack has been deep (Fig. 10.3). 

In general, there will always be one spot on any buried system where all of the conditions combine to give the highest rate of penetration. This hot spot is exactly where the first pit through perforation will occur. Since the underground system is invisible to the observer, the resulting leak will be the first place where corrosion will make its presence known. 

A true one-call notification system in which excavators can alert aU operators of buried systems... 

Corrosion due to stray current—the metal is attacked at the point where the current leaves. Typically, this kind of damage can be observed in buried stmctures in the vicinity of cathodic protection systems or the DC stray current can stem from railway traction sources. 

And last not least, we will have to see further improvements in the graphical user interfaces of software systems and the retrieval systems of databases in order to make software and databases more acceptable to the chemical community at large. Software and databases should speak the language a chemist is used to, with hand-drawn chemical structures and reaction equations, or even imderstand the spoken word - and only provide the desired information selectively, not buried in a phe of unnecessary output. 

The vastly increased acidity of superacidic systems resulted in the significant new field of superacid chemistry. I began to ask myself whether a similar but more general approach could be used to produce electrophiles of greatly enhanced electron deficiency and thus reactivity. Over the years, there were a number of unexpected results in my own research work, as well as some previously unexplained observations buried in the literature, that seemed worth pursuing. 

The next step is to apply a number of loss control credit factors such as process control (emergency power, cooling, explosion control, emergency shutdown, computer control, inert gas, operating procedures, reactive chemical reviews), material isolation (remote control valves, blowdown, drainage, interlocks) and fire protection (leak detection, buried tanks, fire water supply, sprinkler systems, water curtains, foam, cable protection). The credit factors are combined and appHed to the fire and explosion index value to result in a net index. 

Consumption of natural gas, as of the mid-1990s, was about 2000 x 10 /yr. Using seismic detection equipment, exploration firms search for gas reserves buried deep underground and beneath the sea floor. Advanced computer systems process the seismic data to pinpoint the most likely locations for reserves. These advanced systems have both cut the time required for data analysis, by 80%, and gready improved the success rate for new drill rigs. 

Low viscosity urethane polymers have been prepared from castor od and polymeric isocyanates (82). These low mix viscosity systems are extremely usehd for potting electrical components where fast penetration without air voids, and fast dispensing cycles are desirable. Very low viscosity urethane systems containing castor polyols have been prepared for use in reclaiming water-logged buried telephone cable and for encapsulating telephone cable sphces (83—86). 

Examples of the sacrificial-anode method include the use of zinc, magnesium, or aluminum as anodes in electrical contact with the metal to be protected. These may be anodes buried in the ground for protection of underground pipe lines or attachments to the surfaces of equipment such as condenser water boxes or on ship hulls. The current required is generated in this method by corrosion of the sacrificial-anode material. In the case of the impressed emf, the direct current is provided by external sources and is passed through the system by use of essentially nonsacrificial anodes such as carbon, noncor-rodible alloys, or platinum buried in the ground or suspended in the electrolyte in the case of aqueous systems. 

In the cathodic protection of storage tanks, potentials should be measured in at least three places, i.e., at each end and at the top of the cover [16]. Widely different polarized areas arise due to the small distance which is normally the case between the impressed current anodes and the tank. Since such tanks are often buried under asphalt, it is recommended that permanent reference electrodes or fixed measuring points (plastic tubes under valve boxes) be installed. These should be located in areas not easily accessible to the cathodic protection current, for example between two tanks or between the tank wall and foundations. Since storage tanks usually have several anodes located near the tank, equalizing currents can flow between the differently loaded anodes on switching off the protection system and thus falsify the potential measurement. In such cases the anodes should be separated. 

Corrosion likelihood describes the expected corrosion rates or the expected extent of corrosion effects over a planned useful life [14]. Accurate predictions of corrosion rates are not possible, due to the incomplete knowledge of the parameters of the system and, most of all, to the stochastic nature of local corrosion. Figure 4-3 gives schematic information on the different states of corrosion of extended objects (e.g., buried pipelines) according to the concepts in Ref. 15. The arrows represent the current densities of the anode and cathode partial reactions at a particular instant. It must be assumed that two narrowly separated arrows interchange with each other periodically in such a way that they exist at both fracture locations for the same amount of time. The result is a continuous corrosion attack along the surface. 

In postulating a statistieal model for a statie stress variable, it is important to distinguish between brittle and duetile materials (Bury, 1975). For simple stress systems, i.e. uniaxial or pure torsion, where only one type of stress aets on the eomponent, the following equations determine the failure eriterion for duetile and brittle types to prediet the reliability (Haugen, 1980) ... 

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