Wasting of snow or ice by melting or evaporation



Related Terms

CORROSION

The wasting away of metals as a result of chemical action. In a boiler, usually caused by the presence of O2, CO2, or an acid.
Most of the metal equipment in marine power plants is made up of steel or copper alloys (brass, copper-nickel, bronze and others). All of these metals will dissolve slowly in water unless the water is properly treated. This is called corrosion.

Some of the most important kinds of corrosion damage which can occur in marine power plant equipment are:

  1. Thinning of the tube metal. This is the result of corrosion that is continuous and over a fairly large area of metal. This kind of a damage is also called general corrosion. Thinning can progress to the point at which the metal can no longer contain the internal pressure which may cause the metal to swell and eventually burst.

  2. Pitting - when only a small area of metal is corroded the result is a deep hole called a pit. If pitting corrosion is not controlled, some pits may go all the way through the metal. This causes leaks. When these are many pits close together, they may become connected. The effect on the metal is the same as that of general corrosion.

  3. Corrosion cracking is another form of corrosion which can effect certain materials. In general, alloys, which are mixtures of metals are most susceptible to cracking. Stainless steel and brass such as Admiralty are particularly susceptible to cracking under certain conditions. cracking is a form of corrosion which occurs along a very narrow band through the metal.

  4. Some metal alloys are susceptible to exfoliation or de-alloying. Both of these types of corrosion are associated with the selective reaction of only one of the metals in a metal alloy. Exfoliation generally occurs in feedwater heaters. Nickel is selectively oxidised from the copper-nickel alloy tubing leaving layers of copper metal and nickel oxide. Brasses are mixtures of copper and zinc. When de-alloying occurs, zinc is removed from the metal leaving a spongy mass of copper behind. This is commonly referred to as dezincification.

  5. Embrittlement is an effect of corrosion that changes the physical properties of a metal. Some corrosion reactions cause metals to lose their normal strength and ductility and become brittle and weak. Embrittlement cannot be seen by inspecting a boiler tube that has not failed. However, an embrittled tube that has failed will have a crystallized appearance at the edge of the point of failure and usually there will be no evidence of bulging.

The study of corrosion considers reactions between a material and its environment. From the standpoint of the power plant water chemist, the study of corrosion also includes suppression of corrosion by altering or controlling the environment to which steam power plant materials are exposed. In order to understand the suppression of corrosion, one must first understand its causes.

The earliest studies of corrosion of iron (steel) showed that practically the only factor which limits the life of iron is oxidation. All of the chemical processes by which iron is corroded, eaten away, or rusted are covered by this term.

Oxidation implies the chemical reaction of a substance with oxygen. This is true, but this is a very specific application of the term. It also has a much broader and more important meaning.
The term reduction is often thought of as a reaction that involves the removal of oxygen from a substance. Again, this is true but it is also only a specific application of the term. The reduction also has a much broader and important meaning. The terms are often abbreviated as "REDOX" because they are closely related.

Simply stated oxidation involves the loss of electrons by a substance and reduction of the gain of electrons by a substance. By this definition, the element oxygen does not have to be involved in an oxidation-reduction reaction at all. The reason for this close relationship of the terms should also be apparent: If one substance gives up electrons another substance must gain them.

In experiencing oxidation, uncharged iron atoms pass into solution and become iron ions. This change involves the atoms giving up electrons. The oxidation of iron is therefore electric in nature because of the flow of electrons.

Oxidation and corrosion are therefore electrochemical processes. On this basis, corrosion can be looked at in a similar fashion to other electrical processes.

The basic nature of corrosion is almost always the same. A Flow of electricity occurs between certain areas of a metal surface through a solution capable of conducting an electric current. This electrochemical action causes the eating away of metal at areas where the electric current leaves the metal and the metal atoms enter the solution as ions.

EVAPORATION RATE

The number of pounds of water evaporated in a unit of time.

VAPOR

The gaseous product of evaporation.

EVAPORATION

The process of converting a liquid into a vapor with accumulation of latent heat.

BOIL-OFF

Boil-off is the vapor produced above the surface of a boiling cargo due to evaporation. It is caused by heat ingress or a drop in pressure.

VAPOR PRESSURE-REID (RVP)

Measure of the pressure of vapor accumulated above a sample of gasoline or other volatile fuel in a standard bomb at 100°F (37.8°C). Used to predict the vapor locking tendencies of the fuel in a vehicle's fuel system. Controlled by law in some areas to limit air pollution from hydrocarbon evaporation while dispensing.

HYDROLOGY

The scientific study of the waters of the earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of ground water.

WEATHERING

Processes of ablation and accumulation which gradually eliminate irregularities in an ice surface.

WET-BULB TEMPERATURE

The lowest temperature to which air can be cooled at any given time by evaporating water into it at constant pressure, when the heat required for evaporation is supplied by the cooling of the air. This temperature is indicated by a well-ventilated wet-bulb thermometer. See also FREE-AIR TEMPERATURE.

BOILER HORSEPOWER

The evaporation of 34 1⁄2 lbs. of water per hour from a temperature of 212oF into dry saturated steam at the same temperature. Equivalent to 33,475 Btu.

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