1, boiler feed water to adjust the pH value of the reason
Nowadays, most boilers in China use reverse osmosis demineralized water or sodium ion resin exchange softened water, reverse osmosis demineralized water or sodium ion resin exchange softened water pH value is mostly low and acidic, reverse osmosis demineralized water pH value is generally 5-6, sodium ion resin exchange softened water pH value is generally 5.5-7.5, in order to solve the corrosion of acidic water supply to boilers and pipes, According to the provisions of the national standard BG/T1576-2008, the pH value of the industrial boiler water is between 7-9 and the pH value of the demineralized water is between 8-9.5, so the boiler water supply needs to adjust the pH value.
2, the basic principle of adding sodium carbonate to the boiler feed water to adjust the pH value
Sodium carbonate is commonly known as soda, soda ash, soda ash, washing alkali, classified as salt, not alkali, chemical formula Na2CO3, under normal circumstances for white powder or fine salt. The basic principle of adding sodium carbonate to the boiler feed water to adjust the pH value is to use sodium carbonate to dissolve in water and be alkaline, which can neutralize carbon dioxide in the acidic feed water and solve the corrosion of acid softened water or salt water on the boiler and pipeline. Sodium carbonate is a weak electrolyte, dissolved in water to form a buffer solution of sodium carbonate and sodium bicarbonate, there is an electrolytic balance in the solution, with the consumption of the electrolytic hydroxide, the balance will continue to move to the right, so the pH present in the reaction does not change much.
Sodium carbonate primary hydrolysis process:
Na2CO3 Sodium carbonate +H2O water = NaHCO3 sodium bicarbonate +NaOH sodium hydroxide
Sodium carbonate secondary hydrolysis process:
NaHCO3 sodium bicarbonate +H2O water =H2CO3 carbonic acid +NaOH sodium hydroxide
Sodium carbonate primary hydrolyzed ion equation:
(CO3) 2-carbonic acid +H2O water =HCO3- bicarbonate +OH- hydroxide
Sodium carbonate secondary hydrolyzed ion equation:
HCO3- bicarbonate +H2O water =H2CO3 carbonic acid +OH- hydroxide
3, the basic principle of adding sodium hydroxide to the boiler water to adjust the pH value
Sodium hydroxide is also called caustic soda, caustic soda, caustic soda, caustic soda, usually white flake, chemical formula NaOH, sodium hydroxide has a strong alkaline, extremely corrosive.
Ionization equation for sodium hydroxide: NaOH=Na++OH-
Adding sodium hydroxide to the water of the boiler can stabilize the protective film on the surface of the metal, improve the pH value of the boiler feed water and the furnace water, so as to solve the corrosion of the acid softened water or demineralized water on the boiler and the pipeline, and protect the metal equipment from corrosion.
4. The advantages and disadvantages of using sodium carbonate or sodium hydroxide to adjust pH value for boiler feed water are compared
4.1 The speed of raising pH value with sodium carbonate and sodium hydroxide for boiler feed water and the time of maintaining the use effect are different
The speed of adding sodium carbonate to the boiler water supply to increase pH value is slower than that of sodium hydroxide. Because sodium carbonate generates buffer solution, it has small fluctuation and is relatively stable and easy to adjust. However, the range of pH adjustment is limited. When adjusting the same pH value, the amount of sodium carbonate will be larger than that of sodium hydroxide. The use effect is maintained for a long time, and the pH of the water is not easy to drop.
Sodium hydroxide is a strong base and a strong electrolyte, sodium hydroxide to adjust the pH value of the volatility is large, sodium hydroxide after the addition of water pH is easy to increase, adjust the pH value faster and more direct, but also easy to overthrow can not put too much, compared with sodium carbonate to add much less can reach the pH index requirements, that is to say, although sodium hydroxide pH value increased, However, the amount of sodium hydroxide added is not large, that is, the ability of the water to neutralize the acid of the hydroxide group does not increase much, the pH will soon drop.
4.2 The harm caused by excessive addition of sodium carbonate and sodium hydroxide to increase pH value for boiler feed water is different
Adding too much sodium carbonate to the boiler water to adjust the pH value will increase the salt content of the pot water and the conductivity; There are more bicarbonate ions in the pot water, and bicarbonate ions are easily decomposed into carbon dioxide when heated. CO2 enters the heat exchanger and condensate water with the steam. Sodium carbonate not only cannot adjust the pH value of the steam and steam condensate return water, but also reduces the pH value of the steam and condensate, corroding the heat exchanger and condensate pipeline. The reason why iron ions in steam condensate return water exceed the standard color yellow or red color.
Adding too much sodium hydroxide to the furnace water to adjust the pH value will cause the pot water alkali to be too high, and the water and soda will appear. The amount of sodium hydroxide is not easy to control, and excessive free NaOH will cause large relative alkalinity, and alkali embrittlement will also cause corrosion to the equipment. The author has seen a glass fiber reinforced plastic debrine tank full of patches at a user’s site, which was corroded and perforated because of the use of sodium hydroxide to regulate the pH value of debrine. Sodium hydroxide can not adjust the pH value of steam and steam condensation return water, and can not control the corrosion of steam and steam condensation return water system equipment and pipe network.
4.3 The safety of sodium carbonate and sodium hydroxide used in boiler feed water to raise pH value is different
Sodium carbonate is relatively mild, belongs to food grade material, small stimulation, slight corrosion, normal can be touched by hand, long-term need to wear gloves.
Sodium hydroxide is a dangerous material, corrosive, and its solution or dust splashed on the skin, especially on the mucous membrane, can produce soft scabs, and can penetrate deep tissues. A burn leaves a scar. Splashing into the eye, not only damage the cornea, but also damage the deep tissue of the eye. Therefore, the operator should apply neutral and hydrophobic ointment on the skin, and must wear work clothes, masks, protective glasses, rubber gloves, rubber aprons, long rubber boots and other labor protection supplies to do a good job of personal protection.
There are use and test cases that show: Sodium hydroxide and sodium carbonate are used alternately, or mixed, its economy and effect are better than using a certain pH regulator alone. When the pH value of the boiler feed water is found to be too low, some sodium hydroxide can be appropriately added to quickly increase the pH value. After sodium hydroxide is completely dissolved, some sodium carbonate can be added to raise the carbonate in the water. This can ease the decline of the pH value of the feed water; Because the amount of sodium carbonate can be put in more, the ability to maintain the carbonates in the water is more, so usually sodium carbonate can be used to maintain the pH value of the water supply and pot water, only when the pH value of the water is too low, the author recommends the use of sodium hydroxide to rapidly increase the pH value, so that the two are alternately mixed, both economic and good effect.
Post time: Mar-01-2024
