Page 446 - Boiler_Operators_Handbook,_Second_Edition

Why They Fail 431

two forms, external and internal. There are arguments THERMAL SHOCK
for each installation and you should encounter some
boilers with both. Of all the modes of boiler failure thermal shock
The failures of boilers due to low water continues seems to be the one that can happen at any time. I’ve seen
despite the provisions of extra low water cutoffs and boilers that didn’t make it past their initial week’s opera-
regular testing of them. Perhaps one principle reason tion without failing as a result of thermal shock and boil-
is the failure to test them regularly so a problem is de- ers that failed after years of operation due to an incident
tected before a failure occurs. Whatever else you choose of thermal shock. I also saw one that was replaced and
to let go, never fail to test the low water cutoffs imme- repaired by the manufacturer under warranty three times
diately after arriving on the job. They can fail because before the manufacturer found an installation mistake
mud builds up in the piping connecting the cutoff to the that allowed them to refuse additional repairs.
boiler, or an accumulation of mud in the cutoff housing. It’s important to understand exactly how thermal
The mud is dirt that enters with the makeup and ac- shock destroys a boiler because there are several situa-
cumulates in the boiler water. It’s usually suspended in tions that are called thermal shock that aren’t consistent
the boiler water by the rapid circulation but will settle with the normal perception. Thermal shock can destroy
out in the water column and cutoff piping and chambers a boiler in a single incident or it can take several shocks
because the water moves slowly in them. to produce evident damage. There is a specific combina-
Float operated low water cutoff failures include the tion that must exist for thermal shock damage. First the
normal problems of mud collecting in the piping between metal of the boiler (or refractory) must be exposed to a
boiler and the float housing where the float chamber can’t change in temperature that’s enough to produce a range
drain so the level is higher than that in the boiler, (This of stress in the material.
happens if either the water leg or the steam leg is plugged, The best example of thermal shock is pouring wa-
the chamber fills with condensate and can’t drain) mud ter over ice cubes fresh out of the freezer. What happens
filling the bellows and hardening to resist transmission of to the ice cubes? They crack! Even if you use cold water
the float position, friction preventing operation of magnet stored in the refrigerator they crack. When you consider
actuated switches, also the stiffening with age of wiring the fact that steel is only about 7% stronger than ice (ever
connected to magnet actuated switches, fusing of contacts try to chop a fishing hole with a plain piece of steel?)
due to excessive electrical current, freezing of the switch you can understand that thermal shock can destroy a
actuating mechanism due to corrosion from boiler water boiler. The reason for the ice cracking can be explained
leaks or leakage into the switch housing. by noting how the cracks form. When the water hits the
Probe types, using conductance, can fail because ice there’s a rapid transfer of heat from the water to the
deposits coat the probe to simulate the presence of wa- surface of the ice. Keep in mind that ice contracts as it is
ter. The opportunities for a low water cutoff to fail are heated, and the operation is just the opposite for steel.
so many that regular testing (to detect problems) is the The inside of the cube remains cold because the heat
most important thing you can do. doesn’t transfer through the ice as fast as the outside
Remember that, despite the many schemes for is warmed by the water flowing over it. Because it’s
testing the low water cutoff, the only sure proof that warmed and tends to shrink the outer layer of the ice
the low water cutoff works is gradually dropping water cube is placed in tension, as if something was trying to
level with the burner operating until the cutoff shuts the pull it apart. The result is it is pulled apart, cracks form
burner down. Do it as often as possible while keeping and as the rest of the cube shrinks the crack continues.
a close eye on the water level. Other tests to check it, The second important element of thermal shock is
explained in the normal operation description, should thickness of the material. Shaved ice doesn’t crack when
be performed with the recommended frequency. Always cold water is poured over it. When the metal is thin
watch the level until cutoff occurs because the odds are enough the difference in temperature across it is not ad-
rather high that it will not work. equate to produce enough stress to produce cracking. The
Since incorporating timing of low water cutoff thicker parts of a boiler, tube sheets, shells, and drums are
testing into my burner management systems there have more susceptible to thermal shock than the tubes.
been no failures of the boilers with those systems. There The third element is frequency. One violent shock
were, however, three incidents of the testing revealing a may not be good for a boiler but hundreds of little ones
problem with a low water cutoff! repeatedly occurring will eventually result in failure be-
cause tiny microfissures (very little cracks) that form in