The third article of a three-part series describing some potentially catastrophic events that power and recovery boilers are prone to if not properly maintained.
Editor's note: The following article covers the concluding topics of the most common ways to "destroy a boiler" which include improper blowdown techniques, improper storage, flame impingement, pulling a vacuum, and preventive measures. Part one and part two of the series were printed in the Winter and Summer 1999 BULLETINs.
Improper Blowdown Techniques
The concentration of undesirable solids in boiler water is reduced through proper feedwater treatment and the proper operation of a continuous purge ("blowdown") system, and by performing intermittent bottom blowdowns on a regular basis.
The sodium zeolite water softening process is the predominant method of water treatment for boilers operating at low pressures with saturated steam. In this ion exchange process, harmful scale-producing calcium and magnesium ions are exchanged for sodium ions. The resultant water has a total dissolved solids concentration equal to the previous combined total of sodium, magnesium, and calcium concentrations.
The main purpose of blowdowns is to maintain the solids concentration of the boiler water within certain acceptable limits. A blowdown system is shown in Figure 1. The blowdown rate can be determined by several factors which include total dissolved solids, suspended solids, silica, and alkalinity. Table 1 shows these maximum recommended concentration limits in the water of an operating boiler according to American Boiler Manufacturers Association (ABMA).
|Drum Operating Pressure (psig)
||Total Dissolved Solids (ppm)
||Total Alkalinity (ppm)
||Silica (ppm Si02)
||Total Suspended Solids (ppm)
As the operating pressure increases, the limits become substantially more stringent, which can potentially require an extremely high blowdown rate if sodium zeolite softening is the feedwater treatment method. To substantially lower the blowdown rate and control the concentration of silica, a total demineralized water treatment system should be used. A demineralized water treatment system removes the anions and cations instead of substituting them for other ions. This results in very low blowdown rate requirements.
The continuous blowdown rate is set to control the boiler water within these ABMA-recommended acceptable limits. A well-designed continuous blowdown system will constantly monitor boiler water conductivity (solids concentrations) and adjust the blowdown rate to maintain the control range. If the boiler water exceeds the recommended limits, potential problems can occur which include scale and sludge formation, corrosion, and moisture carryover due to foaming and poor steam drum separation equipment performance. When this occurs, solids and silica are carried over in the steam. This results in silica and scale formation on the superheater and other process equipment, including steam turbine blading. This foaming phenomena associated with high conductivity can also cause drum level instability leading to nuisance water level alarms and potential boiler trips.
Sometimes it is necessary to perform intermittent bottom blowdowns to dramatically reduce solid concentrations in the boiler water. Also, intermittent bottom blowdowns of water wall headers and the mud drum are critical to remove potential sludge buildup to keep all water circuitry clear. Generally, the only bottom blowdown that can be performed while the unit is being fired is from the mud drum. The blowdown of lower water wall headers, particularly the furnace wall headers, should not be performed while the unit is being fired. This action could potentially result in water wall tube overheat damage due to the interruption of the boiler’s natural circulation. The lower water wall headers should be routinely blown down every time the unit is brought out of service after fuel firing has been halted and the unit is still under pressure. Care should be taken to perform a blowdown of a limited duration to maintain visibility of the boiler water level in the sight glass. Additional bottom blows can be performed once feedwater is added to raise the level back up in the sight glass.
The single biggest problem caused by poor blowdown practices is the failure to periodically blowdown the boiler water columns. This results in sludge and debris buildup in the water columns, which renders the low water trips inoperative. All well-designed boiler installations should include a push-button momentary low water trip override system located at the water column blowdown valves. This system allows the low water trip devices to be blown down, thus cleaning the system and verifying that the low water trip alarm is activated without causing an actual boiler trip.
Steam plant operations frequently require the long-term storage of boilers either used as standby units or units operated only during maintenance periods. Attention to proper storage techniques can be critical to maintaining boiler longevity as a standby unit. The improper storage of a boiler can lead to corrosion on either the fire or water sides.
Fireside corrosion damage often occurs on a boiler that is in cold standby and that has previously fired sulfur-laden fuels. There are inevitably areas of the boiler where ash is not removed from the tube surface during normal sootblower operation. One of the most vulnerable areas is the interface where the tubes enter the drum at tube-baffle interfaces and refractory-to-tube interfaces. When the boiler is hot, corrosion is generally not a problem since moisture is not present; however, upon shutdown, this ash and refractory can absorb moisture and concentrated corrosive attack will occur over time in these areas. Localized pitting can be quite deep, rendering an otherwise sound tube in need of at least partial replacement.
When possible, store a standby boiler in a hot condition to prevent fireside corrosion of the tubes. Hot storage techniques such as utilizing mud drum heaters or routing the blowdown from an operating boiler through the inactive unit is generally sufficient to keep the temperature of the boiler tubes above the acid dew point. These same techniques for keeping a boiler hot are critical if a unit is required to quickly move from standby status to operation in the event of another unit failure. Maintaining the boiler in hot standby will prevent problems associated with improper warm-up.
A boiler stored in hot standby with the unit full of water must be properly managed to prevent oxygen corrosion of the unit. The unit must be slowly brought down in rating while raising the water level as high in the gauge glass as possible while still delivering export steam to the line. As the steam pressure stabilizes at the hot standby pressure, make sure deaerated feedwater is introduced into the unit to maintain the water at the proper level so that immediate firing can commence when necessary.
If a unit is in cold wet standby, follow the procedures above in bringing the unit out of service. The most effective way to control corrosion is to build up a sodium sulfite concentration of 100 ppm in the boiler water. A nitrogen source should be attached to the drum vent once the pressure is almost depleted. Pressurizing the boiler to 5 psig with a nitrogen blanket system will ensure that oxygen will not be introduced into the boiler. Cold storage is recommended using the wet procedures above; however, if dry storage is necessary, make sure that ample quantities of desiccant are placed in the drums and that the waterside is closed up tight.
Flame impingement is a subtle problem that is primarily characteristic of high-capacity package boilers. Since shipping clearances dictate the design geometry of package boilers, the boilers naturally get long and narrow as size increases. This results in a significant challenge for burner manufacturers to shape the flame so that it is long and narrow, while simultaneously trying to stage combustion to mitigate NOx formation.
When the flame washes the furnace side walls, the result is potential corrosion on the tubes at the flame interface, particularly if firing heavy oils with contaminants. The corrosion is accelerated due to high metal temperatures associated with flame impingement and chemical deposits placed on the tubes resulting from quenching the flame when it touches the tube wall. Water treatment problems can accentuate the problems associated with flame impingement because internal deposits at this localized high temperature zone are formed on the inside tube wall driving the tube operating temperature even higher.
Pulling A Vacuum
When boilers are designed to operate at very high pressures, they are not designed to operate under even the slightest vacuum. A potential vacuum is created when a boiler is shut down. As the unit cools, the steam condenses and water level drops, which allows the pressure to drop.
If the steam drum vent is not open when the unit is cooling, a vacuum condition can result. A vacuum on a boiler can cause problems with leaks on rolled tube seats of generating tubes, which are designed for a mechanical fit to withstand positive pressures.
In conclusion, some common practices that should be followed in order to avoid "destroying a boiler" include:
- Frequent observation of the burner flame to identify combustion problems early.
- Investigate the cause of any trip before numerous attempts to relight.
- Before lighting a boiler, always purge the furnace 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. If in doubt, purge, purge, purge!
- Verify that the water treatment system is operating properly, producing boiler feedwater of sufficiently high quality for the temperatures and pressures involved. Although zero hardness is always an absolute criteria, other water quality standards based on operating pressures and temperatures as recommended by ABMA should be followed. Never use untreated water in a boiler.
- Blowdown all the dead legs of the low water trips, water column, etc., on a regular basis to prevent sludge buildup, which leads to device malfunction. Never under any circumstance disable a low water trip.
- Verify that the water leaving the deaerator is free of oxygen, that the deaerator is operated at the proper pressure, and that the storage tank water is at saturation temperature. A continuous vent from the deaerator is necessary to allow the discharge of non-condensable gases.
- Continuously monitor the quality of condensate coming back from the process to enable the diversion of the condensate in the event of a catastrophic process equipment failure.
- Adjust continuous blowdown to maintain conductivity of the boiler water within required operating limits and operate the mud drum blowdown on a regular basis (consult your water treatment specialist). Never blowdown a furnace wall header while the boiler is operating.
- The boiler waterside should be inspected on a regular basis. If there are any signs of scaling or buildup of solids on the tubes, water treatment adjustments should be made and the boiler should be mechanically or chemically cleaned.
- The deaerator internals should be inspected for corrosion on a regular basis. This is an important safety issue because a deaerator can rupture from corrosion damage. All the water in the deaerator will immediately flash to steam in the event of a rupture, filling the boiler room with deadly steam.
- The boiler's warm-up curve should be strictly followed. The standard warm-up curve for a typical boiler is not to 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, during start-up, the burner must be intermittently fired to ensure that this rate is not exceeded.
- Make sure all personnel working on boilers understand that the thin tubes are quite fragile. Encourage workers to report any accidental damage so that it can be inspected and/or repaired as necessary.
- If production demands necessitate overfiring of the boiler, make periodic assessments of potential effects of overfiring and communicate these to management.
- When a boiler is shut down for an extended period of time, a nitrogen blanket system should be used to prevent the introduction of air and oxygen into the boiler during cooling and storage, and sodium sulfite should be injected to react with any free oxygen in the boiler water. When a boiler is stored dry, desiccant should be placed in the boiler drums along with the nitrogen blanket to absorb any free moisture.
- Always ensure that the steam drum vent valve is opened whenever the boiler pressure is less than 5 psig.
In summary, a boiler is much like the human body. If properly cared for, it will give many years of reliable service. It will often withstand abuse and keep on functioning. However, certain seemingly minor mistreatment can have catastrophic effects. You can impose a nasty cut on most parts of your body with little more than minor discomfort. However, if the cut severs the carotid artery in your neck, it is fatal. A boiler as well has its particular vulnerabilities.
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.