How to Destroy a Boiler

7/28/2019 How to Destroy a Boiler

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How to Destroy a Boiler -

Part 1

Originally posted for the Web May 7

This article is part of the National BoardClassic Series. This installment by WilliamL. Reeves, P.E. was originally published inthe Winter 1999 National BoardBULLETIN.

Editor's note: The following article is reprinted from the Winter 1999 National BoardBULLETIN. SomeASME Boiler and Pressure Vessel Code requirements may changebecause of advances in material technology and/or actual experience. The reader iscautioned to refer to the latest edition and addenda of theASME Boiler and PressureVessel Code for current requirements.

How to Destroy a Boiler - Part 1

William L. Reeves, P.E.President, ESI Inc. of Tennessee

This article covers the four most common ways to "destroy a boiler," including fuel

explosions, low-water conditions, poor water treatment, and improper warm-up. Parts 2

and 3 in the series will be posted on the Web site in conjunction with the release of theSummer and FallBULLETINs, respectively.

The design and construction of power and recovery boilers represent one of the largestcapital expenditures in the industrial utilities arena. The operational reliability andavailability of these boilers is often critical to the profitability of the facility. Safeoperation of these units requires careful attention to many factors. Failure to follow a fewwell-established practices can, and likely will, result in a catastrophe. The most commonways to "destroy a boiler" include the following:

Fuel Explosions

Contaminated Feedwater

Low-Water Conditions

Improper Blowdown Techniques Poor Water Treatment

Improper Storage

Improper Warm-up

Pulling a Vacuum on the Boiler

Impact Damage to Tubes

Flame Impingement

Severe Overfiring


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Fuel Explosions

One of the most dangerous situations in the operation of a boiler is that of a fuelexplosion in the furnace. The photo above shows the complete devastation of a utilityboiler.

Conditions have to be just right for an explosion to occur and when a boiler is properlyoperated, it is not possible for such an event to take place. The most common causes of afuel explosion are:

Fuel-rich mixtures - The danger of a fuel-rich mixture is that high concentrations ofunburned fuel can build up. When this unburned fuel ignites, it can do so in a very rapidor explosive manner. Fuel-rich mixtures can occur any time that insufficient air issupplied for the amount of fuel being burned. Never add air to a dark smoky furnace. Tripthe 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 notefficient, is not dangerous.

Poor atomization of oil - Just as fuel-rich mixtures could allow accumulation of unburnedcombustibles, any inventory of a combustible fuel in the furnace can result in anexplosion. Boilers are blown up every year as a result of poor atomization of oil whichresults in incomplete combustion and can lead to unburned oil puddling on the floor ofthe furnace. To prevent this, the oil tips must be clean, the oil temperature must becorrect, the oil viscosity must be in spec, and the atomizing steam (or air) pressure andfuel oil pressure must be properly adjusted.

Improper purge - Many of the explosions occur after a combustion problem which hasresulted in a burner trip. Consider the following example: suppose that the oil tipbecomes plugged, which disturbs the spray pattern, causing an unstable flame that resultsin a flame failure. The operator attempts to relight the burner without investigating thecause 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 whenthe operator initiates another trial for ignition. The pilot then ignites the large inventoryof 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 gasesuntil the concentration is below the explosive limits. Purge, purge, purge!


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Low-Water Conditions

The potential for severe and even catastrophic damage toa boiler as a result of low-water conditions is easy toimagine considering that furnace temperatures exceed

1,800F, yet the strength of steel drops sharply attemperatures above 800F. The only thing that allows aboiler to withstand these furnace temperatures is thepresence of water in all tubes of the furnace at all timesthat a fire is present. Low-water conditions will literallymelt steel boiler tubes with the result closely resemblinga spent birthday candle, as shown above.

Typical industrial boilers are "natural circulation" boilers and do not utilize pumps tocirculate water through the tubes. These units rely on the differential density between hotand 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, sufficientheat 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) willillustrate:

1. Boiler feedwater being introduced into the steam drum.2. Cooler water sinking through tubes called "downcomers."3. Water absorbing heat from the tubes, then the heated water rising to the steam

drum.

Due to the critical need for water,

modern boilers are equipped withautomatic low-water tripswitches. Some older boilers maynot have these relativelyinexpensive devices. If yourboilers do not have low-watertrips, run, don't walk, to thephone and initiate theirinstallation. You have anaccident and expensive repairswaiting to happen. The neededrepairs can range from retubingto total destruction of the unit ifthe drums overheat.

In the event of low water, thelow-water trips will trip theburner (or fuel flow for solid fuelboilers) and shut down the forced draft fan. This shuts down the heat input.


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The trips should be installed at a water level that will prevent damage. Normal operatinglevel is generally near the centerline of the steam drum. Low-water trips are generallyinstalled approximately 6" lower, but the manufacturer's drawings usually indicatenormal and minimum water levels which vary from unit to unit.

The potential for damage is more critical with solid fuel-fired boilers. A gas/oil boiler hasno inventory or bed of fuel. When you trip the burner, for all practical purposes, the heatinput 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 systemscannot always prevent a low-water condition. The "water level" in a steam drum isactually a fairly unstable compressible mixture of water and steam bubbles that willshrink 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 aredestroyed each year. Common reasons:

Disabled trip circuits - very common - a $39 jumper cable will readily foil the best-madeplans (with repairs often exceeding $100,000, this represents an attention-grabbing returnon investment for a $39 expenditure!). A typical scenario involves disabling the trips toeliminate nuisance trips due to improperly tuned controls, etc. This is a "band-aid" tocover 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. Sludgewill eventually plug the piping or the switch itself.

Have you checked your trips today? Nuisance trips should not be a concern with aproperly tuned boiler with proper drum internals, so this is not a valid reason to disablelow-water trips. Dysfunctional low-water trips should be a "no go" item and should becorrected before the boiler is fired.


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Poor Water Treatment

Boiler feedwater is treated to protect it from two basic problems: the buildup of soliddeposits on the interior or water side of the tubes, and corrosion.

Prevention of scaling or buildup - The need for proper feedwater treatment is obvious ifyou will consider the comparison of a boiler and a pot of boiling water on the stove. Theboiler is actually an oversized distillery in that the water that enters the boiler isvaporized to steam, leaving the solids behind. Depending on the amount of solids in thewater, or hardness, the residue is sometimes visible when a pot containing water is boileduntil all water is vaporized.

This same thing occurs inside the boiler and, if left unchecked, can destroy it. Boilers relyon the water to protect the steel boiler tubes from the temperatures in the furnace whichgreatly exceed the melting point of the tube material. A buildup of deposits inside thetubes 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 ofthe tube and eventual blowout.

In order to prevent the buildup of deposits on the tubes, the level of solids in the boilerfeedwater must be reduced to acceptable limits. The higher the operating pressure andtemperature of the boiler, the more stringent the requirements for proper feedwatertreatment. Refer to the table below for the maximum recommended concentration limitsin the water of an operating boiler according to ABMA.

Drum Operating Pressure

(psig)

Total Dissolved Solids

(ppm)

Total Alkalinity

(ppm)

Silica

(ppm Si02)

Total Suspended Solids

(ppm)

0-300 3,500 700 150 15

301-450 3,000 600 90 10

451-600 2,500 500 40 8

601-750 1,000 200 30 3

751-900 750 150 20 2

901-1,000 625 125 8 1

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 simplewater softeners for feedwater treatment. At higher pressures and when turbines and

superheaters are involved, more complex feedwater treatment systems such as reverseosmosis, demineralizer systems, etc., are required to treat the feedwater. A state-of-the-artdemineralization system is shown in the photo on the opposite page.

Solids are also removed from the boiler through proper operation of the continuousblowdown system and by the use of intermittent or bottom blowdown on a regular basis.Blowdown flows reduce the solids by dilution.


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Proper treatment of the boiler feedwater is absolutely critical to enable a normal lifeexpectancy of the unit. This is one of the most serious boiler "destroyers."

Improper Warm-up

This is a common problem because management and production often exert extremepressure on utilities to complete forced or scheduled outages so that production canresume. 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 mustendure. Going through the cycle of start-up, operation, and shutdown for any boilercreates higher equipment stresses and, consequently, much more maintenance-type issuesthan continuous operation at maximum rated capacity. Any piece of equipment such as aboiler, airplane fuselage, or combustion engine that undergoes an extreme transformationfrom ambient out of service conditions to operating conditions is subject to fatigue andfailure. Good design and the process of making a slow transition between these

conditions is essential for prolonging boiler life and reducing the possibility of failure.

A typical boiler is constructed of different types of materials which operate in totallydifferent 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 andcooling from water, steam, and air on the other side,

Insulation materials which are specially designed to operate at a much higher temperatureon one side than on the other side, and

Thick cast-iron castings such as access doors that are refractory-lined which see the fulltemperature 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. Thissituation 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 ofthe 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 drumand the bottom half of the drum will expand much more quickly than the top half whichis not in contact with water. Consequently, the bottom of the drum will become longerthan the top, causing the drum to warp. This phenomenon called "drum humping" canlead to stress fractures of the generating tubes between the steam and mud drums.

Refractory damage is the most prevalent damage associated with a quick warm-up of aboiler from a cold start. Refractory by design transfers heat very slowly and therefore


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heats up much more slowly than metal. Also, as the air inside the furnace and refractorycool, moisture is absorbed from the air in the refractory. A gradual warm-up is requiredto prevent refractory from cracking; this allows adequate time for the moisture to bedriven from the refractory. Trapped moisture quickly becomes steam and causes therefractory to spall as the steam escapes.

The standard warm-up curve for a typical boiler does not increase the boiler watertemperature over 100F per hour. It is not unusual for a continuous minimum fire toexceed this maximum warm-up rate. Consequently, the burner must be intermittentlyfired to ensure that this rate is not exceeded.

Correct planning and education will allow a boiler to be started properly, which willprolong the boiler life and eliminate costly maintenance repairs.

2004, The National Board of Boiler and Pressure Vessel Inspectors. All rights reserved.

How to Destroy a Boiler

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