Page 447 - Boiler_Operators_Handbook,_Second_Edition

432 Boiler Operator’s Handbook

thinner metals or where the temperature differences are that is more than fifty years old. I know where there are
not dramatic will, if constantly bombarded with thermal a few but they’re few and far between and, when they’ve
shock conditions, eventually grow into large cracks that had good care and water treatment, it’s predominantly
finally result in boiler failure. because of corrosion and wear. I could argue that a boil-
Many people don’t realize that thermal shock er doesn’t have any moving parts so it can’t wear out. I
doesn’t have to happen on the water side of a boiler. I just finished a project where we replaced all the tubes
normally differentiate it by calling it firing shock but it’s and casings in thirty year old boilers and I have every
really thermal shock. Any boiler that trips while running reason to believe that they’ll last another thirty years but
at high fire and immediately goes into a purge is sub- they’re an exception because they’re well cared for. The
jected to thermal shock because the metal of the boiler’s normal end of a well maintained boiler’s life is almost
heating surface is immediately subjected to contact with always due to a decision to replace them, not wear.
cold purge air right after it was exposed to the hottest There are areas in a boiler that can’t be reached
flue gas of normal operation. Add to that the trip oc- to monitor and prevent corrosion, sometimes they’re
curring near the maximum operating temperature (and due to installation and sometimes to manufacturing
related pressure) and there’s potential for failure. but they’re there. In many cases, as in the project I just
I’ve never seen a significant indication in water tube finished, the only way to address those spots is a major
boilers but that doesn’t mean they can’t experience it. The rebuild of the boiler to reach them and clean, protect,
most common failure in this mode occurs with the ends and recover them to extend the boiler’s life. That’s a
of the fire tubes at the inlet of the second pass of a fire tube sound decision in many cases but many boiler owners
boiler. The reason they fail is because they’re sticking out just won’t do it.
into the hot flue gas where their temperature is elevated I’ve seen failures due to rubbing in a boiler where
by the high fire glue gases and then they suddenly en- each time it heats up and cools off metal to metal rub-
counter the cold purge air. That failure is usually one that bing resulted in cutting through a tube. In one case I
results in gradual growth of microfissures in the ends of discovered three boilers were lost in a matter of three
the tubes and will even happen in tubes that are welded years due to lack of adequate combustion air openings,
to the tube sheet. The primary reason for this type of fail- completely destroyed by alternating corrosion and re-
ure is improper adjustment of the firing rate controls such duction. Those, however, are the unusual cases. Most of
that the boiler cycles off while the modulating controls the time the problems with wear are all at the burner.
are still at high fire or just left high fire. When the control valve on a boiler has run from
Hydronic heating systems can operate to produce high fire to low fire six or seven times a day, 365 days a
significant thermal shock by returning water from idle year for 10 years it’s run over 21,000 cycles. Did you know
sections of the system (where the water got very cold) to that the ASME Code has factors for operating cycles with
the hot boiler. A slug of cold water is directed against the no additional allowance for boilers that are expected to
boiler heating surfaces. In some cases this can be caused cycle less than 7,000 times in their lifetime? Now won-
by automatic controls operation, especially day/night der how many engineers allow for more than that many
controls. Sources of the problem are usually close to the cycles. Exactly how long do you expect that system to run
boiler because any slug of cold water in a remote system without failing? A major revamping of a boiler’s burner
will be heated by the metal in the piping as it returns to and controls on a five year cycle should prevent failures
the boiler. Service water heating with a hydronic boiler due to wear but they never seem to happen.
has a high potential for thermal shock if the heating wa-
ter to the service water heater is cycled on and off. It’s
better to use a constant flow to the service water heat OPERATOR ERROR AND POOR MAINTENANCE
exchanger with other provisions to prevent overheating
the service water. See the section on service water heat- Regrettably the National Board statistics, which
ing for more on thermal shock in that application. are quoted here, don’t provide enough breakdown to
clearly indicate why trends exist or to detect reasons for
trends. I’ve seen a considerable increase in the elimina-
CORROSION AND WEAR tion of central plants with licensed boiler operators.
Their replacement multiple low pressure heating plants
Nothing lasts forever and that’s very true for boil- are maintained by individuals without a license so the
ers. You will be hard pressed to find a boiler in operation increasing contribution of operator error to boiler fail-