Boiler Water Treatment, Scale & Corrosion
In general there are two types of boilers, low pressure and high pressure:
A) Low pressure boilers will be found heating water or steam to be circulated around a building for heating purposes via radiators. These systems are "closed circuit" types and require little or no "make" water to top them up. Mostly low pressure boilers operate below 10 psi.
B) High pressure boilers will be found in industry, generating steam for a variety of uses, locomotion via a steam engine, or for end use of steam in laundries, rubber product manufacturing, wood pulp products, food manufacture etc. These boilers operate at over 10 psi and all require constant "make up" water as some of the steam is used.
Boiler feedwater ("BFW") may either be the same as the makeup water, or may consist of returned steam condensate, or (as in most cases) will be a mixture of both.
The relative amounts of makeup and condensate may vary. A typical figure is 5% makeup with 95% condensate, but this depends on how the steam plant is operated and how much of the steam is condensed and recovered for recirculation.
In order to be non-scaling, the BFW must be a softened or demineralised water . In order to be non-corrosive to the carbon-steel or low-alloy steel components from which boilers are usually constructed, the water must be thoroughly deaerated. This is partially achieved by thermomechanical means (use of deaeration heating tanks), while the last traces of dissolved oxygen are removed by chemical agents ("oxygen scavengers").
Oxygen scavengers include both volatile products (e.g., hydrazine, or other organic products like carbohydrazine, hydroquinone, diethylhydroxyethanol, methylethylketoxime, etc.) and non-volatile salts (normally: sodium sulphite, Na2SO3, or a derivative thereof). The latter salts often contain catalysing compounds to increase of rate of reaction with dissolved oxygen (e.g., cobaltous chloride).
While the oxygen scavenging salts tend to react rapidly with oxygen, even at lower temperatures, their reaction products result in increased levels of total dissolved solids in the boiler feedwater.
To minimize this disadvantage, the salt should be fed to the storage tank of the deaerating heater and care should be taken that no precipitates are introduced into the boiler feed.
As rule of thumb, one typically feeds 10 parts of sodium sulphite per part of oxygen (to compensate for reaction with atmospheric oxygen and for impurities in the sodium sulphate).
Volatile oxygen scavengers are normally employed for higher pressure systems (e.g., above 1000 psig). These materials may react directly with oxygen and or directly with carbon steel boiler surfaces to form gamma iron oxide or magnetite. The formation of such "oxygen-impervious" oxide films precludes excessive corrosion.
Except for the common hydrazine (N2H4), most other volatile organic oxygen scavengers are sold under some proprietary name. The use of hydrazine in boiler feedwater is very common and very well documented. Because hydrazine typically does not react with oxygen at a rapid rate at lower temperatures, it may not be the preferred oxygen scavenger for low temperature boiler feedwater systems. In addition, hydrazine is seriously toxic and great care should be taken in its handling. Food production sites should consider seriously the potential consequences of using highly poisonous chemicals.
As water is heated and converted into steam, contaminants brought into a boiler with makeup water are left behind. The boiler functions as a distillation unit, taking pure water out as steam, and leaving behind concentrated minerals and other contaminants in the boiler. Scale forms as a result of the precipitation of normally soluble solids that become insoluble as temperature increases. Some examples of boiler scale are calcium carbonate, calcium sulphate, and calcium silicate.
Corrosion is a general term that indicates the conversion of a metal into a soluble compound. In the case of boiler metal, corrosion is the conversion of steel into rust. In a boiler, two types of corrosion are prevalent:
1.) Oxygen pitting corrosion, seen on the tubes and in the preboiler section.
2.) Low pH corrosion, seen in the condensate return system.
Corrosion of either type can lead to failure of critical parts of the boiler system, deposition of corrosion products in critical heat exchange areas, and overall efficiency loss.
Carryover is caused by either priming or foaming. Priming is the sudden violent eruption of boiler water, which is carried along with steam out of the boiler, usually caused by mechanical conditions. Priming can cause deposits in and around the main steam header valve in a short period of time. Foaming causes carryover by forming a stable froth on the boiler water, which is then carried out with the steam. Over a period of time, deposits due to foaming can completely plug a steam or condensate line.
Typical Boiler Failures and Causes
The time when a boiler system is most vulnerable to oxygen pitting is during idle periods. In order to prevent oxygen pitting during these times it is important to utilize proper storage techniques. Please see our technical tip on this subject at Dry Storage of Boilers and Wet Storage of Boilers.
When a boiler is in operation, oxygen pitting is most likely to occur in feedwater heaters or economizer tubes, since this is this is where the water is first heated above the deaerator temperature. Maintaining a properly operating deaerator with sufficient oxygen scavenger is the best method of prevention.
If oxygen pitting is noticed, it is important to note if it is “old” or active pitting. Active oxygen pits can be distinguished by the red-brown tubercle which, when removed, exposes black iron oxide within the pit.
This type of failure is usually indicated by a "thin-lipped" burst of the boiler tube. These failures occur when water circulation in the tube is interrupted, and the flue gas temperatures cause a rapid overheating of the metal to a point where the metal becomes highly plastic and a violent burst occurs. Typical causes of short-term overheating are circulation problems caused by poor operation (sudden increase in steam demand or low water level) or design, and tube blockage. Tube blockage normally occurs from deposition in the tube or supply header.
This type of failure is usually indicated by a "thick-lipped" burst of the boiler tube. Long-term overheating can result from excessive deposition, flame impingement, mild flow restrictions, or poor water or flue gas circulation patterns. Probably the most common of these is excessive deposition, which prevents proper heat transfer and excessive metal temperatures. This prolonged overheating of the tube causes metal degradation to the point that in can no longer handle the operating pressure and a "thick-lipped" failure occurs.
Caustic gouging occurs when NaOH concentrates under porous boiler water deposits. An example of such deposition would be iron, which tends to be porous. Essentially, what occurs is that boiler water is present in the deposit. As steam escapes, the NaOH concentration increases dramatically, dissolving the protective magnetite and boiler tube metal.
In addition to the gouging of the boiler tubes you may also notice a white substance (sodium carbonate) outlining the edges of the original deposit.
There are other failure mechanisms such as caustic and hydrogen embrittlement, stress corrosion cracking and steam blanketing. In this tip we have dealt with those we feel are the most prevalent. If you believe that you have a failure that does not fit into one of the categories we discussed please feel free to contact our staff. Finally, for a complete analysis and understanding of a failure, a sample should be sent to an independent metallurgical lab.
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