Failures caused by short-term overheating are confined to steam- and
water-cooled tubes including downcomers, waterwalls, roofs, screens, su-
perheaters, and reheaters. Because of their high operating temperatures,
superheaters and reheaters are common failure sites. Failures due to
short-term overheating almost never occur in economizers, where temper-
atures are limited.
When low water level is the cause, failures will often occur near the top of
waterwalls near steam drums. A single ruptured tube in the midst of other
apparently unaffected tubes suggests pluggage or other flow-related prob-
Poor attemperation usually will not cause short-term overheating, al-
though long-term overheating may occur. Failures of superheaters and
reheaters can also occur during start-up, when steam flow is limited.
Short-term overheating occurs when the tube temperature rises above
design limits for a brief period. In all instances, metal temperatures are at
least 85O0F (4540C) and often exceed 13500F (7300C). Depending on tem-
perature, failure may occur in a very short time. Failure is usually caused by
a boiler operation upset. Conditions leading to short-term overheating are
partial or total tube pluggage and insufficient coolant flow due to upset
conditions and/or excessive fire-side heat input.
An occurrence of short-term overheating is caused by an unusual set of
circumstances, such as an upset, occurring during a brief period. There-
fore, pinpointing unusual events immediately preceding failure may be
extremely important in identifying the cause of failure.
Since short-term overheating frequently has little to do with water
chemistry, efforts should be concentrated on operating procedures and
system design. Did failure occur on start-up or shutdown? Was there a
recent acid cleaning? Were headers or U-bends filled with debris upstream
of the failure? Did another failure immediately precede this one? Was the
firing pattern changed? Was th