Boiler has emerged as an important tool of industry with a high degree of versatility, a source of hot water and or steam. It has found applications in textile, pulp and paper, sugar, organic and inorganic chemicals, dyes etc. The requirement of process temperature may vary from 40c to 200c or even more in some cases. The terms of ratings and steam pressure, the corresponding requirements are up to 15 tons/hour and 55 bar respectively. This range and diversity of demand placed on its performance has given a fill up to the evolution of its design over the years. While a modern industrial boiler in the shape of a shell type multi-tubular package boiler is an example of optimized design and efficiency, no single design can obviously satisfy the need of such a wide range of requirements.
The textile dyeing and finishing sector uses large volumes of water and substantial quantities of complex chemicals. Companies operating in this sector face lot of challenges, many associated with the acquisition and disposal of these essential raw materials. In particular, the charges incurred for mains water supply and effluent disposal are increasing, and companies need to address these issues to save money and remain competitive in business. Steam is vaporized water, being partly gas, partly liquid. Steam itself is usually interspersed with minute, droplets of water in its liquid state, which gives it a white, cloudy appearance. In industrial and process situations, steam is often generated using water boilers that are heated to create steam under controlled conditions. The energy generated is then transferred and used in many different ways. In nature, steam is produced by the heating of underground water by volcanic processes and is emitted from hot springs, geysers, fumaroles, and certain types of volcanos. Steam is used as a heat transfer medium in several industries like textile, pulp and paper chemical manufacturing to name a few. In the industry, steam is produced by water boilers which come in all shapes, sizes, types and pressures and the water within them likewise. Steam under pressure is at elevated temperatures, greater than 100C and therefore can be used to transfer energy to different parts of a system. This energy can then be used to cook, create energy for chemical reactions, heat water and a myriad of other uses. Steam being "pure" water dissolves other materials into it very easily and it is these other materials that cause problems requiring treatments. The primary function of a boiler is to transfer heat from hot gases generated by the combustion of fuel into water until it becomes hot or turns to steam. The steam or hot water can then be used in building or facility processes.
Except for a small number of specialty models, boilers generally fit into one of the two common categories: fire-tube boilers and water-tube boilers. Fire-tube boilers pass hot combustion gases through tubes submerged in water. Water-tube boilers, on the other hand, circulate water inside the tubes in a closed vessel filled with hot combustion gases. In either category the boiler feed water and fuel often contain impurities, which impairs boiler operation and efficiency. Chemical additives can be used to correct the problems caused by these impurities. To improve feed water quality, fuel oil condition, and steam purity, these chemicals can be injected directly into the feed water, steam or fuel oil.
There are several problems within a boiler system some of
which require chemical treatment or other mechanical means to overcome them.
The major problems are:
- Boiler water carryover
- Sludge Deposition
Preventing scale problems
These can cause problems in all parts of the system starting with the feed tank leading through the boiler and into the condensate system. The problems arise from the quality of the water used within the steam raising system and the manner in which the system is operated and hence it is not only chemical issues that need to be addressed. In extreme cases it has been known for steam boilers to explode causing much damage and even death. So it is for this reason that strict standards have arisen on how to treat and maintain boilers and associated systems. Then there are efficiency problems with excess fuel being used to raise the steam or leaks causing loss of water, chemicals and energy from the system. All of these issues are avoidable with simple chemicals and good maintenance regimes.
Advantages of chemical treatments
- Increase boiler efficiency;
- Reduce fuel, operating and maintenance costs;
- Minimize maintenance and downtime; and
- Protect equipment from corrosion and extend equipment lifetime.
The feed water is composed of makeup water (usually city water from outside boiler room/ process) and condensate (condensed steam returning to the boiler). The feed water normally contains impurities, which can cause deposits and other related problems inside the boiler. Common impurities in water include alkalinity, silica, iron, dissolved oxygen and calcium and magnesium (hardness). Blow down, a periodic or continuous water removal process, is used to limit the concentration of impurities in boiler water and to control the buildup of dissolved solid levels in the boiler. Blow down is essential in addition to chemical treatments.
Boiler waterside fouling: Scale is one of the most common deposit related problems. Scale is a buildup of solid material from the reactions between the impurities in water and tube metal, on the water-side tube surface. Scale acts as an insulator that reduces heat transfer, causing a decrease in boiler efficiency and excessive fuel consumption. More serious effects are overheating of tubes and potential tube failure (equipment damage). Fuel wasted due to scale may be approximately 2-5 percent depending on the scale thickness.
Oxygen attack is the most common causes of corrosion inside boilers. Dissolved oxygen in feed water can become very aggressive when heated and reacts with the boiler's internal surface to form corrosive components on the metal surface. Oxygen attack can cause further damage to steam drums, mud dams, boiler headers and condensate piping.
Acid attack is another common cause of corrosion. Acid attack happens when the pH of feed water drops below 8.5. The carbonate alkalinity in the water is converted to carbon dioxide gas (C02) by the heat and pressure of the boilers. CO2 is carried over in the steam. When the steam condenses, CO2 dissolves in water to form carbonic acid (H2CO3) and reduces the pH of the condensate returning to the boilers. Acid attack may also impact condensate return piping throughout the facility.
Lime softening and soda ash: Quick or slaked lime (usually calcium hydroxide) is added to hard water to precipitate the calcium, magnesium and, to some extent, the silica in the water. Soda ash is added to precipitate non-bicarbonate hardness. The process typically takes place in a clarifier followed by a hydrogen cycle cation exchange and a hydroxide cycle anion exchange demineralization.
Phosphate: Mono-, di-or trisodium phosphate and sodium polyphosphate can be added to treat boiler feed water. Phosphate buffers the water to minimize pH fluctuation. It also precipitates calcium or magnesium into a soft deposit rather than a hard scale. Additionally, it helps to promote the protective layer on boiler metal surfaces. However, phosphate forms sludge as it reacts with hardness; blow down or other procedures should be established to remove the sludge during a routine boiler shutdown.
Chelates: Nitrilotriacetic acid (NTA) and ethylenediamine tetraacetic acid (EDTA) are the most commonly used chelates. Chelates combine with hardness in water to form soluble compounds. The compounds can then be eliminated by blow down. The preferred feed location for chelates is downstream of the feed water pump. A stainless steel injection quill is required. However, chelates treatment is not recommended for feed water with high hardness concentration.
Polymers: Most polymers used in feed water treatment are synthetic. They act like chelates but are not as effective. Some polymers are effective in controlling hardness deposits, while others are helpful in controlling iron deposits. Polymers are often combined with chelates for the most effective treatment.
Oxygen scavengers: A deaerator removes most of the oxygen in feed water; however, trace amounts are still present and can cause corrosion-related problems. Oxygen scavengers are added to the feed water, preferably in the storage tank of the feed water, to remove the trace a mounts of oxygen escaped from the deaerator. The most commonly used oxygen' scavenger is sodium sulfite. Sodium sulfite is cheap, effective and can be easily measured in water.
Neutralizing amines: Neutralizing amines arc high pH chemicals that can be fed directly to the feed water or the steam header to neutralize the carbonic acid formed in the condensate (acid attack). The three most commonly used neutralizing amines are morpholine, diethyleminoethanal (DEAE) and cyclohexylamine. Neutralizing amines cannot protect against oxygen attack; however, it helps keep oxygen less reactive by maintaining an alkaline pH.
Filming amines: Filming amines are various chemicals that form a protective layer on the condensate piping to protect it from both oxygen and acid attack. The filming amines should be continuously fed into the steam header with an injection quill based on steam flow. The two most common filming amines are octadecylamine (ODA) and ethoxylated soya amine (ESA). Combining neutralizing and filming amine is a successful alternative to protect against both acid and oxygen attack.