Cooling Water System, Boiler Water & Boiler Feed Water PDF

Topic 1 Cooling Water System, Boiler Water and Boiler Feed Water Introduction Understanding water and how it affects industrial plant and equipment is important for many industrial processes. Water in these processes can play the roles of...

Topic 1 Cooling Water System, Boiler Water and Boiler Feed Water Introduction Understanding water and how it affects industrial plant and equipment is important for many industrial processes. Water in these processes can play the roles of cooling, solvent, and chemical reagents. Poor water quality can affect the performance of equipment. The study of friction, lubrication and wear has become the basis for selecting lubricants. In this regard Tribology plays an important role in the selection of high-quality marine fuels and lubricants that can contribute to outstanding performance in shipboard equipment. In the past, the lubrication requirements for a specific application could be satisfied by using general-purpose lubricants. Lubricant selection was typically based on experience and knowledge. Today, this approach is no longer viable due to the requirements of the current demanding environments to run faster, longer, and hotter. Today's lubricants must satisfy extreme requirements that are specific to each application. Lesson 1. Water Water is a tasteless, odorless substance that is essential to all known forms of life and is known as the universal solvent. It appears colorless to the naked eye in small quantities. It covers nearly 70% of Earth's surface. Water is the chemical substance with chemical formula H2O, one molecule of water has two hydrogen atoms covalently bonded to a single oxygen atom. The water on earth is 97 % ocean (saline) and the fresh water, amounts only 2.7%. Saline water contains vast amount of dissolved minerals. The mineral contents of sea water are chlorine, sodium, magnesium, and bromine which are commercially obtained from the sea. Unfortunately, most of the fresh water is not easily accessible since it is locked up in glaciers, frozen lakes or under the ground. The fraction of water available for human use is only 0.03% of the total global supply. Hence, it is the need of the hour to use the available water carefully and economically. 1.1. Properties of Water 1.1.1 Physical Properties of water Water is called the "universal solvent" because it dissolves more substances than any other liquid. This means that wherever water goes, either through the ground or through our bodies, it takes along valuable chemicals, minerals, and nutrients. Pure water is neutral and has a pH of 7, which is neither acidic (less than 7) nor basic (greater than 7). Physical properties of water are related to the appearance of water, namely, the color, temperature, turbidity, taste, and odor. The molecules of water have extensive hydrogen bonds resulting in unusual properties in the condensed form. This also leads to high melting and boiling points. As compared to other liquids, water has a higher specific heat, thermal conductivity, surface tension, dipole moment, etc. These properties form the reason for its significance in the biosphere. Water is an excellent solvent and therefore it helps in the transportation of ions and molecules required for metabolism. It has a high latent heat of vaporization which helps in the regulation of body temperature. Surface Tension Surface tension is the property of the surface of a liquid that allows it to resist an external force, due to the cohesive nature of its molecules. Water molecules want to cling to each other. At the surface, however, there are fewer water molecules to cling to since there is air above (thus, no water molecules). This results in a stronger bond between those molecules that actually do come in contact with one another, and a layer of strongly bonded water (see diagram). This surface layer (held together by surface tension) creates a considerable barrier between the atmosphere and the water. Insects could walk in the water because of surface tension Examples of surface tension Walking on water Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface. Floating a needle A carefully placed small needle can be made to float on the surface of water even though it is several times as dense as water. If the surface is agitated to break up the surface tension, then needle will quickly sink. Washing with cold water The major reason for using hot water for washing is that its surface tension is lower and it is a better wetting agent. But if the detergent lowers the surface tension, the heating may be unnecessary. Why bubbles are round The surface tension of water provides the necessary wall tension for the formation of bubbles with water. The tendency to minimize that wall tension pulls the bubbles into spherical shapes. Surface tension and droplets Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer. Adhesion and Cohesion Adhesion and Cohesion are water properties that affect how water interacts with itself and with other things. Essentially, cohesion and adhesion are the "stickiness" that water molecules have for each other and for other substances. Adhesion and cohesion are important water properties that affects how water works everywhere, from plant leaves to your own body.  Adhesion Adhesion is the ability of Water to attract to other substances. Water molecules are adhesive in that they stick to other surfaces. Cohesion Water is attracted to water. The water molecule is highly cohesive, it is very sticky, meaning water molecules stick to each other. Water is the most cohesive among the non- metallic liquids. Hydrogen bonds between water molecules are constantly breaking and reforming with other water molecules. Water’s polarity also gives water high adhesion. The adhesive forces are stronger than cohesive forces. Capillary Action Plants and trees couldn't thrive without capillary action. It helps bring water up into the roots. With the help of adhesion and cohesion, water can work its way all the way up to the branches and leaves. Even if you've never heard of capillary action, it is still important in your life. Capillary action is important for moving water (and all of the things that are dissolved in it) around. It is defined as the movement of water within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension. Capillary action occurs because water is sticky, thanks to the forces of cohesion (water molecules like to stay close together) and adhesion (water molecules are attracted and stick to other substances). Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus, which turns upward. The surface tension acts to hold the surface intact. Capillary action occurs when adhesion to the walls is stronger than the cohesive forces between the liquid molecules. The height to which capillary action will take water in a uniform circular tube (below) is limited by surface tension. The attraction of water molecules to the sides of a narrow vessel (adhesion) is stronger than the cohesion drawing water molecules together. The result is capillary action, in which the force of adhesion pulls the fluid upwards.  Excellent Solvent Water is an excellent solvent. Water has the unique ability to dissolve many polar and ionic substances. This is important to all living things because, as water travels through the water cycle, it takes many valuable nutrients along with it! High Heat Capacity Water has high heat capacity. It takes a lot of energy to raise the temperature of a certain amount of water by a degree, so water helps with regulating temperature in the environment. For example, this property allows the temperature of water in a pond to stay relatively constant from day to night, regardless of the changing atmospheric temperature. Heat of Vaporization Water has high heat of vaporization. Humans (and other animals that sweat) use water’s high heat of vaporization to cool off. Water is converted from its liquid form to steam when the heat of vaporization is reached. Since sweat is made mostly of water, the evaporating water absorbs excess body heat, which is released into the atmosphere. This is known as evaporative cooling. Boiling Point The boiling point is defined as the temperature at which the vapor pressure of the liquid is equal to the pressure surrounding the liquid, and thus the liquid changes to vapor. It is known to us that the boiling point of water is 100°C.  Freezing Point The freezing point is the temperature at which the substance changes state from liquid to solid. So, for water, the point at which liquid state water turns to solid-state ice is the freezing point of water, which is 0°C or 32°F. Density Density is the ratio of the mass of the substance to its volume. An unusual characteristic of water is that, unlike most solids, ice is less dense than liquid water. For this reason, ice cubes float in a glass of water rather than sink to the bottom of the glass. The density of water is about 1 gm/cm3 and it varies with temperature in an unusual pattern. The density of water is different in different states – solid and liquid. In solid-state, the density is 0.9gm/cc. As temperatures drop, lake and ocean water freezes from the top down, and the ice layer floats on top, insulating water below it from further cooling. Salt water is denser than freshwater because of the presence of salt. 1.1.2 Chemical Properties of Water Natural waters always contain dissolved salts, micronutrients, some metals, and gases. In fact, so many substances dissolve in water that it is sometimes (mistakenly) referred to as the "Universal Solvent." While most of these substances are important for healthy aquatic ecosystems, as concentrations increase, they can have negative effects and we think of them as pollutants. Chemical properties of water involve assessing parameters such as pH , hardness and dissolved oxygen: pH value The term pH stands for power of Hydrogen Ion Concentration. It is really a measure of the relative amount of free hydrogen and hydroxyl ions in the water. The pH value of water is a measure of its acidity or alkalinity. Pure water is very slightly ionized into positively charged hydrogen ions (H+) and negatively charged hydroxide ions (OH–). Water is neutral when the numbers of hydrogen ions and hydroxide ions are equal. When the concentration of hydrogen ions exceeds that of hydroxide ions, the water is acidic and has a pH value less than 7. Conversely, when the concentration of hydroxide ions exceeds that of hydrogen ions, the water is alkaline and has a pH value greater than 7. Turbidity Turbidity is caused principally by inorganic matter in suspension including mineral sediments and oxides of iron or manganese but organic matter including algae can also cause significant turbidity. Most surface waters show particularly high turbidity following periods of heavy rainfall, whilst groundwater generally shows low to very low turbidity. However, variations following heavy rainfall, for example, may indicate rapid recharge bringing in contaminants from the surface. Turbidity measurement gives a quantitative indication of the clarity of water and analysis is carried out using a nephelometer. Nephelometers measure the intensity of light scattered in one particular direction, usually perpendicular to the incident light and are relatively unaffected by dissolved colour. The standard unit of turbidity is the nephelometric turbidity unit or NTU. Turbidity is removed because high turbidity can impair the efficiency of disinfection and for aesthetic reasons. The UK water quality regulations specify a standard of 4NTU at consumers’ taps with an indicator parameter value of 1NTU in water leaving a treatment works. A variety of filtration techniques can be successfully applied to small supplies, and cartridge filters are the most widely employed. Dissolved Oxygen The oxygen that makes aquatic life possible does not form bubbles, nor is it the oxygen that is part of the H2O water molecule. It is a separate O2 molecule that is dissolved in the water and invisible to our eyes. Natural Factors Influencing Dissolved Oxygen o Aquatic life- Animals living in water use up dissolved oxygen. Bacteria take up oxygen as they decompose materials. Dissolved oxygen levels drop in a water body that contains a lot of dead, decomposing material. o Elevation- Since streams get much of their oxygen from the atmosphere, streams at higher elevations will generally have less oxygen. o Salinity (saltiness)- Salty water holds less oxygen than fresh water. o Temperature- Cold water holds more dissolved oxygen than warm water.  o Turbulence- More turbulence creates more opportunities for oxygen to enter streams. o Aquatic Vegetation- Aquatic vegetation and algae directly release oxygen into the water during photosynthesis (during the day). At night, plants actually use oxygen for their metabolism. o Riparian Vegetation (plants along the stream)- Riparian vegetation shade the stream, decreasing water temperatures, and as temperature decreases dissolved oxygen increases. Hardness of Water Alkalinity and water hardness are fairly similar--essentially they both come from sources in nature. Water moves through rocks (and picks up minerals as it does so) on its way to rivers and lakes. When limestone and dolomite dissolve in water, one half of the molecule is calcium or magnesium (the "hardness") and the other half is the carbonate (the "alkalinity"). This means that the level of water hardness and alkalinity in a place will be very similar. However, they are very separate measurements, and have very different importance. Water is classified into two categories depending upon its behavior towards soap solution: Soft Water Water produces lather with soap solution easily is called soft water. It is a type of water that contains few or no calcium or magnesium ions. Although this soft water may contain sodium salt, but this type of salt has no capability to form scale. Soft water can also be produced by an artificial process that removes the calcium and magnesium. Examples: Distilled water, rain water Hard water Water which does not produce lather with soap solution easily is called hard water. Hard water is water that has a high mineral content, such as metal ions, mainly calcium (Ca+2) and magnesium (Mg+2). Examples: River water, sea water Hardness can be subdivided into: Temporary or Alkaline Hardness Alkaline hardness sometimes referred to as temporary hardness, is due to the bicarbonates of calcium and magnesium dissolves in it. They rapidly decompose upon heating to form carbon dioxide and the corresponding carbonates which are precipitated then deposits as a soft scale or sludge. It can be removed simply by boiling. Ca(HCO3)2 → CaCO3 + CO2 + H2O Mg(HCO3)2 → MgCO3 + CO2 + H2O Permanent or Non Alkaline Hardness Non alkaline hardness sometimes referred to as permanent hardness, it is due mainly to sulphates and chlorides of calcium and magnesium, which are acid in nature. They can deposit under certain boiler conditions to form scale of varying degrees of hardness Permanent hardness of water cannot be removed by boiling. It is usually caused by the presence of calcium and magnesium sulfates and/or chlorides in the water, which become more soluble as the temperature rises. Units of Hardness: Parts per million (ppm) : It is defined as the parts of CaCO3 equivalent hardness present per 106 parts of water. Milligrams per Liter (mg/L): It is the number of milligrams of hardness causing substances in terms of CaCO3 equivalents per liter of water. Grains per Gallon (gpg) 1.2 Types of Water There are different types of water based on their chemical, physical and biological characteristics. Fresh Water Fresh water is found naturally on the Earth's surface as ice, as water in wetlands, ponds, lakes, rivers and streams, and as groundwater in aquifers. It usually has a low concentration of dissolved salts and solids Types of fresh water Potable water Potable water or drinking water is water fit for human consumption, which has undergone appropriate treatment and can be consumed without any risks. It is clean, transparent, has no unpleasant odor or taste and is free of any contaminants. Tap water Tap water is also classified as fresh water. It is the water that comes out of the faucets in your house. Communities usually get tap water from run-off from reservoirs, rivers, creeks, and streams. In some towns and cities, these bodies of water may contain pollutants from fertilizer, herbicides, and insecticides. Additionally, some communities use chemicals to kill bacteria in the water, which may cause cancer. Check with your local water bureau to determine the content of your tap water. Hard water Hard water contains calcium and magnesium. Though the incident of heart disease is lower in regions with hard water, experts do not believe that the type of calcium found in hard water is not beneficial because it is deposited on the outside rather than the inside of bones, arteries, and the heart. Soft water Soft water can be naturally occurring, but it is usually soft due to an artificial process that removes the calcium and magnesium. Though the water does not leave the residue of hard water, it is more likely to dissolve pipes. For houses with lead pipe, lead enters the tap water; with plastic or galvanized pipe, toxins enter the tap water; and with copper pipes, copper, iron, zinc, and arsenic enter the tap water. Water salinity based on dissolved salts in parts per thousand (‰) Salt water It is also called seawater, found in the Earth's oceans and seas. It has a concentration of dissolved salts of about 3.5%. Distilled water Distilled water is a type of water wherein all of its impurities as well as electrolytes were removed through distillation (that is, boiling the water and re-condensing the steam into a clean container, thus leaving contaminants behind). It is widely used in chemical and biological laboratories.  Mineral water Mineral water is bottled directly from a natural spring, without being pumped or forced. It is important to find out what minerals occur naturally in the water that you drink. If you are deficient in certain minerals, this type of water can be beneficial. But, having too much of a good thing can be harmful. “Natural” spring water is not supplemented with additional minerals and may not be filtered or treated after being bottled. Demineralized water Demineralized water is without heavy metals as well as nitrates, calcium, and magnesium due to a process in which the electrons of the water are neutralized. Filtered water Filtration in water may occur naturally or by an artificial process. The objective is to remove impurities. Though no method is foolproof because germs are smaller than the pores of even the best filtration system, there are dependable ways of making water more drinkable. Rain Water The heat of the sun naturally distills rainwater. But it becomes contaminated as it falls through air filled with bacteria, dust, smoke, chemicals, and minerals. By the time it reaches the earth as rainwater, it is so saturated with decaying matter, dirt, and chemicals that its color becomes a yellowish-white. Snow is even dirtier. Neither should be consumed.