William L. Reeves, P.E
President, ESI Inc. of Tennessee
Summary: The following article is a part of the
National Board Technical Series. This article was originally published in the
Winter 1999 National Board BULLETIN. (4 printed
This article covers the four most common ways to "destroy a boiler," including
fuel explosions, low-water conditions, poor water treatment, and improper
The design and construction of power and recovery boilers represent one of the
largest capital expenditures in the industrial utilities arena. The operational
reliability and availability of these boilers is often critical to the
profitability of the facility. Safe operation of these units requires careful
attention to many factors. Failure to follow a few well-established practices
can, and likely will, result in a catastrophe. The most common ways to "destroy
a boiler" include the following:
mproper Blowdown Techniques
Poor Water Treatment
Pulling a Vacuum on the Boiler
Impact Damage to Tubes
One of the most dangerous situations in the operation of a boiler is that of a
fuel explosion in the furnace. The photo above shows the complete devastation
of a utility boiler.
Conditions have to be just right for an explosion to occur and when a boiler is
properly operated, it is not possible for such an event to take place. The most
common causes of a fuel explosion are:
- The danger of a fuel-rich mixture is that high
concentrations of unburned fuel can build up. When this unburned fuel ignites,
it can do so in a very rapid or explosive manner. Fuel-rich mixtures can occur
any time that insufficient air is supplied for the amount of fuel being burned.
Never add air to a dark smoky furnace. Trip the unit, purge thoroughly, then
correct the problem. By adding air with a fire in the unit, you may develop an
explosive mixture. While it is dangerous to have too rich a mixture, the
reverse is not true. A lean mixture which results in more air than necessary,
while not efficient, is not dangerous.
Poor atomization of oil - Just as fuel-rich mixtures could allow
accumulation of unburned combustibles, any inventory of a combustible fuel in
the furnace can result in an explosion. Boilers are blown up every year as a
result of poor atomization of oil which results in incomplete combustion and
can lead to unburned oil puddling on the floor of the furnace. To prevent this,
the oil tips must be clean, the oil temperature must be correct, the oil
viscosity must be in spec, and the atomizing steam (or air) pressure and fuel
oil pressure must be properly adjusted.
- Many of the explosions occur after a combustion problem
which has resulted in a burner trip. Consider the following example: suppose
that the oil tip becomes plugged, which disturbs the spray pattern, causing an
unstable flame that results in a flame failure. The operator attempts to
relight the burner without investigating the cause and during successive
attempts to relight the burner, oil is sprayed into the furnace.
The oil on the hot furnace floor begins to volatize and release its combustible
gases when the operator initiates another trial for ignition. The pilot then
ignites the large inventory of unburned combustible gases in the furnace, which
produces the explosion.
This entire scenario can be prevented by:
Investigating the cause of the trip before attempts to relight.
Allowing the furnace to purge thoroughly. This is particularly important if oil
has spilled into the furnace. The purge will evacuate the inventory of unburned
gases until the concentration is below the explosive limits. Purge, purge,
The potential for severe and even catastrophic damage to a boiler as a result
of low-water conditions is easy to imagine considering that furnace
temperatures exceed 1,800°F, yet the strength of steel drops sharply at
temperatures above 800°F. The only thing that allows a boiler to withstand
these furnace temperatures is the presence of water in all tubes of the furnace
at all times that a fire is present. Low-water conditions will literally melt
steel boiler tubes with the result closely resembling a spent birthday candle,
as shown above.
Typical industrial boilers are "natural circulation" boilers and do not utilize
pumps to circulate water through the tubes. These units rely on the
differential density between hot and cold water to provide the circulation. As
the water removes heat from the tubes, the water temperature increases and it
rises to the boiler steam drum. Eventually, sufficient heat is transferred and
steam is generated. Colder feedwater replaces the water that rises, which
creates the natural circulation. A typical boiler circulation (as shown below)
Boiler feedwater being introduced into the steam drum.
Cooler water sinking through tubes called "downcomers."
Water absorbing heat from the tubes, then the heated water rising to the steam
Due to the critical need for water, modern boilers are equipped with automatic
low-water trip switches. Some older boilers may not have these relatively
inexpensive devices. If your boilers do not have low-water trips, run, don't
walk, to the phone and initiate their installation. You have an accident and
expensive repairs waiting to happen. The needed repairs can range from retubing
to total destruction of the unit if the drums overheat.
In the event of low water, the low-water trips will trip the burner (or fuel
flow for solid fuel boilers) and shut down the forced draft fan. This shuts
down the heat input.
The trips should be installed at a water level that will prevent damage. Normal
operating level is generally near the centerline of the steam drum. Low-water
trips are generally installed approximately 6" lower, but the manufacturer's
drawings usually indicate normal and minimum water levels which vary from unit
The potential for damage is more critical with solid fuel-fired boilers. A
gas/oil boiler has no inventory or bed of fuel. When you trip the burner, for
all practical purposes, the heat input immediately stops. With solid fuel
units, however, a fairly large mass of bark, coal, etc., is still on the grate
and even though starved of air by the loss of the forced draft fan, these units
have more "thermal inertia" and will continue to produce some heat.
The control of the boiler drum level is tricky and even the best tuned control
systems cannot always prevent a low-water condition. The "water level" in a
steam drum is actually a fairly unstable compressible mixture of water and
steam bubbles that will shrink and swell with pressure changes and will
actually shrink momentarily when more "cold" feedwater is added.
Some common causes of low-water conditions include:
Feedwater pump failure
Control valve failure
Loss of water to the deaerator or make-up water system
Drum level controller failure
Drum level controller inadvertently left in "manual" position
Loss of plant air pressure to the control valve actuator
Safety valve lifting
Large, sudden change in steam load
Unfortunately, an alarming number of boilers equipped with low-water trips are
destroyed each year. Common reasons:
Disabled trip circuits
- very common - a $39 jumper cable will readily foil the best-made plans (with
repairs often exceeding $100,000, this represents an attention-grabbing return
on investment for a $39 expenditure!). A typical scenario involves disabling
the trips to eliminate nuisance trips due to improperly tuned controls, etc.
This is a "band-aid" to cover the real problem and should never be allowed.
Inoperative trip switches - the trip switches should be blown down
regularly to remove sludge. These switches are installed in "dead legs" where
no circulation occurs. Sludge will eventually plug the piping or the switch
Have you checked your trips today? Nuisance trips should not be a concern with
a properly tuned boiler with proper drum internals, so this is not a valid
reason to disable low-water trips. Dysfunctional low-water trips should be a
"no go" item and should be corrected before the boiler is fired.
Poor Water Treatment
Boiler feedwater is treated to protect it from two basic problems: the buildup
of solid deposits on the interior or water side of the tubes, and corrosion.
Prevention of scaling or buildup - The need for proper feedwater
treatment is obvious if you will consider the comparison of a boiler and a pot
of boiling water on the stove. The boiler is actually an oversized distillery
in that the water that enters the boiler is vaporized to steam, leaving the
solids behind. Depending on the amount of solids in the water, or hardness, the
residue is sometimes visible when a pot containing water is boiled until all
water is vaporized.
This same thing occurs inside the boiler and, if left unchecked, can destroy
it. Boilers rely on the water to protect the steel boiler tubes from the
temperatures in the furnace which greatly exceed the melting point of the tube
material. A buildup of deposits inside the tubes will produce an insulating
layer which inhibits the ability of the water to remove the heat from the tube.
If this continues long enough, the result is localized overheating of the tube
and eventual blowout.
In order to prevent the buildup of deposits on the tubes, the level of solids
in the boiler feedwater must be reduced to acceptable limits. The higher the
operating pressure and temperature of the boiler, the more stringent the
requirements for proper feedwater treatment. Refer to the table below for the
maximum recommended concentration limits in the water of an operating boiler
according to ABMA.
|Drum Operating Pressure
|Total Dissolved Solids
|Total Suspended Solids
Unless a power generation turbine is involved, or the water quality is
particularly bad, most industrial boilers operate at sufficiently low pressures
to enable the use of simple water softeners for feedwater treatment. At higher
pressures and when turbines and superheaters are involved, more complex
feedwater treatment systems such as reverse osmosis, demineralizer systems,
etc., are required to treat the feedwater. A state-of-the-art demineralization
system is shown in the photo on the opposite page.
Solids are also removed from the boiler through proper operation of the
continuous blowdown system and by the use of intermittent or bottom blowdown on
a regular basis. Blowdown flows reduce the solids by dilution.
High conductivity or contamination of the boiler feedwater can create other
problems such as drum level instability and foaming. This can result in high or
low-water alarms and an increase in the carryover of moisture droplets into the
steam header since the moisture separator of the drum cannot handle the
Prevention of corrosion - The most effective method of controlling
corrosion is proper deaeration of the water. The removal of oxygen from the
water drastically reduces the potential for corrosion. This is most often
accomplished through the use of deaerators. These units typically utilize steam
to both preheat the feedwater and remove the oxygen, carbon dioxide, and other
gases from the make-up water. Oxygen scavenging chemicals are also commonly
injected into the deaerator to provide an additional measure of protection.
Additionally, the boiler steam drum, or feedwater, has generally supplied
chemicals at a controlled rate for even further protection. A qualified water
treatment specialist is invaluable in determining the best method for your
plant and your site-specific water requirements.
- In order to prevent problems with poor water
treatment, the following are recommended:
Verify that your boiler feedwater is of sufficient high quality for the
temperatures and pressures involved. Water quality standards based on operating
pressures and temperatures as recommended by ABMA should be followed.
Verify that the water leaving the deaerator is free of oxygen, that the
deaerator is operated at the proper pressure, and that the water is at
saturation temperature for the pressure.
Verify proper operation of the water treatment systems on a regular basis. Loss
of resin from a softener or demineralizer can create problems if the resin
escapes into the feedwater. Such resins can melt on the tube surfaces,
resulting in overheated tubes, etc.
Never use untreated water in a boiler.
Adjust continuous blowdown to maintain the conductivity of the boiler water
within acceptable limits and blow down the mud drum on a regular basis.
It is also important to blow the sludge out of all the dead legs of the
low-water trips, water column, etc., on a regular basis to prevent sludge
buildup in these areas. The buildup of sludge can disable the low-water trips.
The boiler water side should be inspected on a regular basis. Should any signs
of scaling or build up of solids on the tubes be noted, adjustments to the
water treatment should be made.
The water side of the deaerator should be inspected on a regular basis for
corrosion. This is an important safety issue because a deaerator can rupture
from corrosion damage. All the water in the deaerator would immediately flash
to steam in the event of a rupture.
Proper treatment of the boiler feedwater is absolutely critical to enable a
normal life expectancy of the unit. This is one of the most serious boiler
This is a common problem because management and production often exert extreme
pressure on utilities to complete forced or scheduled outages so that
production can resume. As soon as the boiler is "capable" of producing steam,
they want it.
The improper warm-up of a steam boiler is one of the most severe hardships a
boiler must endure. Going through the cycle of start-up, operation, and
shutdown for any boiler creates higher equipment stresses and, consequently,
much more maintenance-type issues than continuous operation at maximum rated
capacity. Any piece of equipment such as a boiler, airplane fuselage, or
combustion engine that undergoes an extreme transformation from ambient out of
service conditions to operating conditions is subject to fatigue and failure.
Good design and the process of making a slow transition between these
conditions is essential for prolonging boiler life and reducing the possibility
A typical boiler is constructed of different types of materials which operate
in totally different environments, including:
Drums and headers fabricated of thick metal which contain water and steam,
Tubes fabricated of much thinner metal which contain water and steam,
Refractory materials that are exposed to high furnace temperatures on one side
and cooling from water, steam, and air on the other side,
Insulation materials which are specially designed to operate at a much higher
temperature on one side than on the other side, and
Thick cast-iron castings such as access doors that are refractory-lined which
see the full temperature of the furnace on one side and ambient air cooling on
the other side.
By design, all of these materials heat up and cool down at a much different
rate. This situation is made much worse when a component is exposed to
different temperatures. For example, a steam drum that is operating at normal
water level has the bottom half of the drum cooled by water and the top half by
air initially and steam eventually. If one starts to fire the boiler from a
cold start, the water will heat up very quickly in the drum and the bottom half
of the drum will expand much more quickly than the top half which is not in
contact with water. Consequently, the bottom of the drum will become longer
than the top, causing the drum to warp. This phenomenon called "drum humping"
can lead to stress fractures of the generating tubes between the steam and mud
Refractory damage is the most prevalent damage associated with a quick warm-up
of a boiler from a cold start. Refractory by design transfers heat very slowly
and therefore heats up much more slowly than metal. Also, as the air inside the
furnace and refractory cool, moisture is absorbed from the air in the
refractory. A gradual warm-up is required to prevent refractory from cracking;
this allows adequate time for the moisture to be driven from the refractory.
Trapped moisture quickly becomes steam and causes the refractory to spall as
the steam escapes.
The standard warm-up curve for a typical boiler does not increase the boiler
water temperature over 100°F per hour. It is not unusual for a continuous
minimum fire to exceed this maximum warm-up rate. Consequently, the burner must
be intermittently fired to ensure that this rate is not exceeded.
Correct planning and education will allow a boiler to be started properly,
which will prolong the boiler life and eliminate costly maintenance repairs.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.