HOW TO NOT DESTROY A BOILER
According to the National Board of Boiler and Pressure Vessel Inspectors, there are many ways to destroy a boiler. The article below was originally published in the Winter 1999 National Board Bulletin. It covers four of the most common ways to “destroy a boiler,” including fuel explosions, low-water conditions, poor water treatment, and improper warm-up. It is part of the National Board Technical Series.
The design and construction of power and recovery boilers represent one of the largest capital expenditures in the industrial utilities area. The operational reliability and availability of these boilers is often critical to the profitability of the family. 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 Explosions
- Contaminated Feed Water
- 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
One of the most dangerous situations in the operation of a boiler is that of a fuel explosion in the furnace.
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 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 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
|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 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.”
To continue reading How to Destroy a Boiler — Part 1 CLICK HERE