12 Ways To Avoid Boiler Tube Corrosion
Corrosion often occurs unnecessarily in horizontal fire tube boilers, which operate at steam pressures less than 15 psig or water pressures below 30 psig.
Corrosion is very rarely caused by a defect in the tubing. In most cases, the need to replace has been caused by environmental conditions. Corrosion of the type found in heating boilers is rare in power boilers. Operators of power boilers understand the importance of proper water and fire side conditions and take measures to prevent such problems.
Firstly, most users of heating boilers are unaware of the possibility of corrosion. Often they are unaware of what causes it, and lack the knowledge and experience to combat it. In low pressure boilers, scale is not a major factor, however a buildup of scale at tube ends has occasionally caused grooving of the tube sheet. Over the past 60 years, we have had many occasions to examine boiler tubes to determine their cause.
Intermittent stand by
Generally, the water level maintained in the boiler under these circumstances corresponds closely to operation or reduced firing, and the temperature is maintained close to steaming temperature. Circulation, however, is very little.
By keeping the boiler water in an alkaline state, and by circulating the water rapidly, segregation of oxygen-containing water is prevented during operation. During the standby period, some loss of water occurs, for instance, a small leak through the blow down valve, which is replaced Water for feeding. If the Providing the feed water is sufficiently oxygen-free and alkaline (pH value), conditions leading to corrosion will not arise. If the conditions are less than ideal, delayed mixing of boiler water and feed water may result in oxygen-rich water forming at the boiler surfaces and initiating corrosion. The boiler water alkalinity may completely disappear if the feed water is not made up to replace losses over a considerable period of time, and general corrosion will result.
No A single rule can be given to ensure proper conditions in the boiler. On these boilers, regular boiler water tests must be performed as carefully as on the operating boilers. The reason for this is that standby boilers cannot be adjusted as easily as operating boilers, so they can be severely damaged by water problems much more easily. While testing multi-drum bent-tube boilers, it is very beneficial to take samples from the rear drum when the boiler is in intermittent operation.
Acid corrosion often affects the condensate return part of your boiler system. The water in your boiler can begin to corrode if it is highly acidic with a low pH. In most cases, dissolved gases cause the water in your boiler to become acidic and wear it down. Carbon dioxide, for instance, can cause your water to become too acidic, resulting in metal pitting in your boiler.
If you have a high-pressure boiler, soluble nickel or magnesium salts can form acids that corrode the inside of the unit.
With a deaerator tank, you can prevent gas buildup in your boiler feedwater. By decreasing the amount of carbon dioxide and dissolved oxygen in your boiler, deaerators protect your steam system from corrosive gases.
Continuing evolution of film-forming products
The development of film-forming products (both amine-based and those based on other compounds with alternative active groups) for steam system metal protection continues. In the past, filming amine chemistry was applied to steam generators, with octadecylamine (ODA, C18H39N) being a common compound. As the amine group attaches to the metal surface, the hydrophobic organic “tail” extends into the fluid to shield it.
Poor control and lack of knowledge often result in problems with ODA applications, including formation of gelatinous spheroids or “gunk balls,” which clogged steam generators. New amine and non-amine film-forming products (FFP) have been developed. Most of these work best in mildly basic conditions, so amine neutralisation is still necessary. The product feed is based on the residual concentration that best protects metal surfaces. Iron monitoring is vital for evaluating and fine-tuning programs. Some products actually dissolve iron oxide deposits during initial application, which can sometimes be mistaken for base metal corrosion.
Feedwater and condensate return system protection
The pH control of steam generating systems is critical to minimising general corrosion. This research involved high-purity water samples. The chart shows that carbon steel corrosion significantly decreases with pH elevation from 8.75 to 9.6. Please note that the results are based on ammonia (NH 3), the most common feed water pH-conditioning chemical for power plants, especially those that do not use copper alloys.
In many industrial condensate systems, there are multiple metallurgies, including copper alloy heat exchanger tubes. Copper is very corroded by ammonia and dissolved oxygen in combination. Furthermore, the optimal pH range for general copper corrosion control is 8.8-9.1, a little lower than the pH range for carbon steel. A pH range of 9–9.3 is often recommended in systems containing carbon steel and copper alloys. Therefore, neutralising amines may be used to replace ammonia in many industrial units. Neutralising amines are small-chain organic molecules with an ammonia group attached to or embedded within them
As amines have a higher basicity than ammonia, they can raise the pH if necessary. One of the most important properties of these chemicals is the distribution ratio, i.e., the amount of amine carried by steam versus the amount left in the water. As boiler temperature and pressure vary, some products tend to remain in the boiler water while others significantly partition with the steam. The right blend of products can provide comprehensive pH conditioning to boilers, steam systems, and condensate return networks.
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