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By Thomas A. Westerkamp
January 2008 -
Boiler operation and maintenance are closely tied together. Good operation includes performing necessary daily and periodic maintenance. Low maintenance cost depends on good daily operating control, given that the system and fuel are compatible.
Operating a steam generator — whether it is a low-, medium-, or high-pressure design — is a complex undertaking. Important physical and chemical balances are necessary for safe and efficient control.
The primary duty of the boiler operator is to achieve optimum operating efficiency of the equipment consistent with high reliability and low cost. The steam generator’s efficiency depends on proper control of time, temperature, turbulence and oxygen.
Time and temperature. Before a boiler begins to achieve efficient operation, the technician must raise the furnace to operating temperature. The fuel-burning rate must be maintained to produce the desired number of pounds of steam per hour to run the stream of turbines — if generating electricity — and supply steam for heat and process needs.
Turbulence. The turbulence in fossil-fuel boiler systems results from the combination of forced-draft fans located in the fuel-supply section and the induced-draft fans located in the stack breeching. The drafts introduced by these large-volume air handlers produce the turbulence necessary for efficient operation. They also create a demand for emission controls, which are very important to air-quality improvements that are being emphasized today and will only be more important in the future.
Modern equipment contains very effective instrumentation to control these four key operating characteristics. Simultaneous with operating control, those same instruments provide the indicators that predict maintenance needs and timing.
During daily operations, operators prepare log books with information collected from instrumentation at frequent enough intervals to detect trends early. This early detection is critical to operating efficiently and at low cost.
To understand this control sequence, it is necessary to look at some common types of equipment and their makeups.
The furnace and steam-generating boiler are made up of a setting, or support structure, a fuel-handling and -supply system, a fuel-burning control system, space above the fuel for heat transfer by radiation and convection, boiler tubes for conducting heat to the water, boilers for steam generation and storage, air- and ash-handling equipment, and many support systems, such as condensers, pumps, deaerators, water softeners, and soot blowers.
For smaller steam requirements, highly efficient packaged boilers and steam generators are available. Where loads fluctuate greatly or where frequent startups and shutdowns are necessary, it might be preferable to install several smaller, packaged units rather than one large furnace and boiler. These units are usually gas or oil fired.
The primary cause for boiler failure during operation is low water. According to authorities on boiler explosions, an estimated 75 percent of boiler failures are due to this cause. The main cause for this high level of accidents is the assumption that boilers require little or no attention because of the redundant, automatic controls they feature.
But without regular operation and maintenance controls, a series of automatic-control failures can occur, preceding an explosion. First, the automatic feed device fails, causing the low-water condition.
Then, the low-water fuel cutout fails to sense the low-water condition and stop the fuel supply. Third, the safety pop valve fails to actuate to relieve the pressure buildup.
Although all of these devices are automatic, they have a finite life span under the conditions in which they operate. Mechanical wear, fatigue, corrosion and erosion take their toll.
These four steps will ensure more reliable and energy-efficient boiler operations and prevent failure.
First, match the best equipment available with the type of service and fuel required. Second, verify proper operation, including all necessary controls and safety equipment, by having the installation checked annually by the insurance company’s service representative.
Third, specify as a part of the installation contract that the system is inspected by an authorized insurance company or state or local inspector before acceptance. This step ensures the installation meets all ordinances and that installers followed good practices.
Finally, provide operators with a log book for recording daily events and a preventive maintenance program for regular, daily, weekly, monthly, semiannual, and annual maintenance procedures. These procedures should include repair, replacement, inspection, cleaning, and lubricating. Technicians should schedule these tests annually and perform them periodically.
Many institutional and commercial organizations also use unit heaters to provide heat to parts of their facilities. These units heat space by a combination of conduction, convection, and radiation, and steam and hot water are the most frequently used sources.
The most common reasons unit heaters fail include improper installation, installation in a corrosive environment, and lack of maintenance. Annual inspection and cleaning several months before the heating season is highly recommended.
Steam and hot-water heaters often fail due to internal corrosion and water hammer. Technicians can minimize corrosion by treating the makeup water with a filming amine, which protects the tube walls, due to the formation of carbonic acid.
Technicians can control water hammer by using the right type and size of steam trap for removing condensate from the heaters. They should check traps for proper operation and clean steam-line filters annually. Long drip legs correctly installed in the condensate lines help to keep a static head of condensate to overcome pressure loss across condensate piping, strainers and traps.
Technicians can remove the drip-leg caps annually for cleanout and inspection to determine the amount of scale buildup occurring. They can install vacuum breakers between the heating units and the trap if a control valve regulates the steam supply. This tactic prevents pressure in the tubes from dropping below atmospheric pressure.
Gas- and oil-fired heaters are subject to internal corrosion due to atmospheric conditions. The only solution is to move them to a location less vulnerable to corrosive substances. Overfiring caused by drafts can occur, causing the burners to fail prematurely. If technicians cannot eliminate the draft, adding outside air might reduce the overfiring problem.
Finally, technicians should clean the contactors and inspect them for oxidation and pitting, replacing them if they look badly burned. The coil itself can crack and break from fatigue, due to frequent cooling and heating. Technicians can make a temporary fix by reconnecting the broken ends with a conducting fastener and washers, but they should replace the faulty coil with the proper part as soon as possible.
Providing appropriate training and conducting operating-floor visits can help managers ensure technicians follow these procedures and log all important events, including unsafe conditions, operating problems, and equipment issues.