Boiler-system efficiency is one of the most documented and published topics for plant engineers and utility managers in commercial and industrial facilities. Understandably, with gas and oil costs at $6 to $10 per million British thermal units of fuel energy value, boiler fuel can represent 30 to 50 percent of total energy budgets in plants with large thermal loads. Typically, fuel consumption accounts for 80 percent of the operating costs for a gas- or oil-fired steam plant. Labor, depreciation, maintenance, and water treatment account for the other 20 percent. Fuel optimization and stack-emissions reduction are crucial.

The U.S. Department of Energy and the American Boiler Manufacturers Association (ABMA) report more than 200,000 firetube boilers and 25,000 watertube boilers are in service in the United States. Moreover, nearly half of those units are estimated to be at least 25 years old, with many operating 5- to 10-percent below original design-performance levels.

IMPACTS ON BOILER EFFICIENCY

Many factors can impact boiler efficiency, such as poor water treatment, high flue-gas temperature, low-quality fuel, too much excess air (i.e., high oxygen [O₂]), low feed-water supply temperature, low combustion-air supply temperature, radiant-heat loss, poor combustion, conduction-heat loss (e.g., fouled tubes), operation at low or cyclic loads, and poor controls/instrumentation.

Two of every three steam-plant boilers my company inspects are in need of operational improvements, many of which include major cost-saving opportunities. Facilities with more “mature” steam plants may be using excess fuel, emitting higher levels of nitrogen oxide (NOx) and carbon dioxide (CO₂), and impacting manufacturing-process reliability.

A small 350-hp firetube boiler can consume nearly $700,000 in fuel per year, while a large 60,000-pph watertube boiler can consume about $4 million worth. (These figures are based on a fuel cost of $8 per million British thermal units of fuel energy value, 90 percent load, and 24-hr-per-day/seven-days-per-week/350-days-per-year [24/7/350] operation). Even minor performance issues can be costly. Therefore, it is important to be tuned in to steam-plant operating conditions.

The economic downturn has caused many companies to reduce operating costs. Technical staff and maintenance people were some of the first to be affected. Steam-plant oversight frequently is delegated to hourly employees, and many commercial and industrial boilers have been licensed for “unattended” operation. A boiler technician may spend 1 or 2 hr per day in a steam plant conducting routine water tests, collecting data, and performing minor maintenance checks. Over the weekend, a maintenance-staff member may substitute as a boiler operator. Therefore, it is crucial that boiler-system controls and instrument equipment be in optimal condition.

Although mandated safety checks may be ongoing, what about operational effectiveness? Left unattended, the performance of sophisticated process controls will degrade over time. Most industrial mechanical utility systems are not equipped with artificial intelligence. Critical unsafe conditions are monitored, alarmed, and tripped, but anomalies are not artificially repaired online. Boiler efficiency may not be monitored electronically at all.

Because of these complex and conflicting conditions, steam-plant-system managers should utilize real-time performance data to monitor not only legally required American Society of Mechanical Engineers (ASME) safety devices, but equipment efficiency and reliability standards. For example, mechanical-vibration monitoring data can alert a maintenance manager to shut down a fan or pump when velocity readings exceed 0.3 in. per second (approaching imminent failure). Similarly, when boiler fuel usage increases to a preset value (per unit of steam production), it may be time to implement contingency plans, including verifying the cost impact of actual performance.

Efficiency tests should be conducted annually in facilities with boiler fuel costs exceeding $400,000, semi-annually in facilities with costs exceeding $800,000, and quarterly in facilities with costs exceeding $2 million.

STEAM-DISTRIBUTION-SYSTEM LOSSES

Consider the daily or weekly economic impact of a 2-, 4-, or 6-percent degradation in boiler efficiency. Losses in a larger facility's steam-distribution system often exceed losses in boiler efficiency (e.g., from excessive fuel use). Causes of steam-distribution-system losses include steam/condensate leaks, poor or missing insulation, direct steam use, steam venting, defective steam traps, traps blowing to atmosphere, the dumping of condensate, the use of condensate for process-chemical-tank makedown, and frequent system startups.

WAR STORIES

Boiler fuel consumption can be reduced in several ways, producing annual cost savings. Consider these examples:

Too many cooks in the kitchen

A client requested an evaluation of three 25,000-pph gas-fired boilers at a manufacturing plant because of operating-personnel concerns. Three crews had been “empowered” to manage separate units. Minimal centerlining, benchmarking, and oversight were found. All three units were being “trip-tested” while under heavy load at the start of every shift, but the mandated “safety” practice was causing myriad problems. Air-fuel control was poor, and excess-O₂ levels were reaching 5 to 9 percent. After a couple of days of testing, recharacterization, and operator training, fuel savings averaged $700 per day (with a 5-percent efficiency improvement and fuel costs of $9 per million British thermal units of fuel energy value).

Pull the plug

A 3-year-old, 80,000-pph gas-fired boiler mysteriously began consuming more fuel. The operators were at a loss as to the cause of the problem. The furnace chamber looked like a tire fire. The generating bank had heavy soot with black deposits, and exit-gas temperatures were 100°F higher than normal. It eventually was discovered that the airflow pitot-tube in the forced-draft fan plenum was severely plugged with fiber dust.

Neglecting maintenance costs money

Two 90,000-pph, No. 6 oil-fired boilers were experiencing high exit-gas temperatures, and 4.5- to 5.5-percent excess O₂ at three-fourths of a load. Casing deterioration was extensive. Forced-draft-fan inlet louvers and fan rotors were heavily fouled. Routine maintenance obviously had been neglected. Testing revealed that a 3-percent efficiency improvement would save approximately $180,000 per year based on a fuel cost of $6.10 per million British thermal units of fuel energy value and 24/7/350 operation. Heavy oil-fired boilers must incur extensive mechanical cleaning and repair. Fuel-costs saving cannot be achieved without regular maintenance.

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