Keeping a steam boiler running at peak performance doesn’t happen by accident. There are multiple factors that need to be assessed and addressed in order to operate a steam system properly; whether it is in a manufacturing plant, co-gen, at a hospital or in an apartment building. A poorly maintained steam boiler system will cost more to operate in the long run and could ultimately fail altogether.
Why Do Steam Boilers Fail?
In a steam boiler system, the two primary causes of failure are:
1. Departure from Nucleate Boiling (DNB) is caused by deposition (scaling) causing localized overheating of heat transfer surfaces. When scale aggressively forms on a boiler heat transfer surface, overheating occurs which lowers boiler fuel-to-steam efficiency, causes more fuel to be consumed because the boiler has to fire harder in order to make the same amount of steam per unit of fuel. This causes localized overheating eventually leading to premature boiler failure through the mechanism of tube rupture (i.e.: thin-lipped and thick-lipped ruptures.) This is controlled via preboiler treatment to remove or neutralize the offending scale forming minerals, coupled with effective chemical boiler water treatment to either sequester, complex, modify, or precipitate the scale forming contaminants, which are then removed via manual or automatic boiler blowdown. Devices such as water softeners, dealkalyzers, condensate polishers, and demineralizers are often installed in the makeup water and/or condensate return systems ahead of the boiler to remove offending contaminants. Boiler anti-scalant chemicals are introduced in proportion to level of contaminants expected to be encountered (either with or without pre-boiler treatment devices) in order to prevent energy-robbing and failure-causing internal scale formation.
2. Dissolved Oxygen (O2) Pitting Corrosion is the leading cause of premature failure of a steam boiler. In water, O2 is inversely soluble to the heat applied to it. This means that as a boiler is heated, any and all O2 is released within the confines of the boiler itself. As this O2 is released, it causes extreme and deleterious pitting corrosion, often actually working its way completely through boiler or economizer tubes in an effort to escape the heat which is being applied to it. This leads to aggressive and often rapid boiler failure. In an effort remove as much oxygen as possible from the feedwater before it is introduced to a steam boiler, many plants have implemented feedwater heating and oxygen removal devices such as feedwater heaters or deaerators. In a feedwater heater, boiler feedwater (which is comprised of steam condensate returns plus any makeup water, pretreated or not) is pre-heated, typically by an internal steam sparge line, to roughly 160-190 degrees Fahrenheit. By preheating the feedwater, much of the oxygen removed via heat is vented to the surrounding atmosphere. For a low pressure boiler, <15 psig pressure, this if often enough O2 removal for boiler operations. However, in a high pressure steam plant, 15 psig and above, a deaerator is often necessary in order to remove air and noncondensible gases in the feedwater to a level of .007 ppm (7 ppb) or less before it can be introduced to the boiler. However, even with a proper functioning deaerator, in order to prevent even the smallest amounts of entrained O2 from damaging a boiler system, most plants utilize a form of chemical oxygen scavenging.
As one of the top boiler water treatment companies, we are constantly asked to deal with the issues surrounding boiler scale and oxygen pitting. In many cases, the solution is very straightforward and can often be addressed by exercising standard water treatment best practices; but because not all water is the same, sometimes providing the correct boiler water treatment solution to a particular facility can become more of a challenge.
What Can I Do?
The first step in combating these issues at your facility is knowledge. Learning more about what potential issues your steam boilers could face is always a good idea. A good place to start is by checking out the wealth of informational resources regarding steam boilers that can be found online. As always, if you have any specific questions or concerns regarding your facilities water treatment program, please feel free to contact one of our water treatment consultants. Also, if you would like more information about common operational issues with steam boilers, please download our eBook “10 HUGE Mistakes Facilities Make in Boiler Operation and How to Avoid Them!” which you can download for free at the link below.
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