Tuesday, 6 October 2015
ID is "Induced Draft" and FD is "Forced Draft." In an induced draft system, the fan is at the exit end of the path of flow, and the system is under negative pressure - that is, the pressure in the flow area is below atmospheric, because the air is being drawn through the fan.
William L. Reeves, P.E
President, ESI Inc. of Tennessee
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 BoardBULLETIN. (4 printed pages)
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.
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:
Fuel ExplosionsContaminated FeedwaterLow-Water Conditionsmproper Blowdown TechniquesPoor Water TreatmentImproper StorageImproper Warm-upPulling a Vacuum on the BoilerImpact Damage to TubesFlame ImpingementSevere Overfiring
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:
Fuel-rich mixtures - 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.
Improper purge - 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, 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) will illustrate:
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 drum.
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 to 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 failureControl valve failureLoss of water to the deaerator or make-up water systemDrum level controller failureDrum level controller inadvertently left in "manual" positionLoss of plant air pressure to the control valve actuatorSafety valve liftingLarge, 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 itself.
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
(psig)Total Dissolved Solids
(ppm Si02)Total Suspended Solids
(psig)Total Dissolved Solids
(ppm Si02)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 resultant carryover.
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.
Preventive measures - 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 "destroyers."
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 of failure.
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 drums.
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.