Typical Boiler Failures and Causes

Oxygen Pitting

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.

Short-Term Overheating

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.

Long-Term Overheating

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

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.