An Operations & Maintenance Focus Lowers Cost, Increases Efficiency
November 30, 2004
There are numerous measures that may improve fuel-use efficiency and reduce energy consumption—and the accompanying cost—at federal facilities:
Check and adjust combustion efficiency of natural gas-fired equipment. Inspect furnaces, space heaters, and water heaters. Tune and adjust natural gas burners to achieve proper excess air settings and uniform, efficient combustion. Performing this maintenance can often save from 2 to 12 percent of annual fuel use. Contact the local natural gas utility company for assistance if necessary.
Lower thermostat settings, particularly in large heated spaces during the coldest winter days. A common rule-of-thumb is that for each degree the thermostat setting can be lowered, a 3 percent reduction in fuel consumption can be achieved. Implement dress code changes to allow the use of warmer functional clothing.
Lower setback temperatures in buildings during unoccupied periods. For a typical building, a 10 percent reduction in annual fuel consumption can be achieved if the thermostat setting is lowered 10 degrees an average of 8 hours each day. Isolate unoccupied building areas to further reduce space temperatures and provide only minimum freeze protection.
Optimize morning warmup and night setback controls. Programmable temperature controls, particularly energy management and control systems (EMCS) at large installations, are oftentimes not adjusted to coincide with building occupancy schedules as they change. Heating is needlessly activated when the buildings are not in use. Fuel savings can be achieved by updating warmup and setback control schedules to coincide with current occupancy periods in affected buildings for each heating zone and weekday.
Reduce and eliminate major sources of infiltration. Leakage of outside air into heated spaces during the coldest winter days can be the largest single contributor to the heating load in some buildings. Keep large overhead doors tightly closed in warehouses, hangars, and industrial buildings. Check and repair overhead door seals which are often deficient and can allow significant leakage. Shut off exhaust fans when not needed.
Minimize use of outside air for process ventilation. Many large installations use 100 percent outside air to ventilate hazardous areas, meaning that none of the heated air is recirculated. The heating requirement associated with these kinds of systems can be substantial. It is estimated that the fuel cost that will be incurred this year to heat one facility at one DOE site with this type of ventilation system will be about $250,000, if gas prices increase as anticipated. Verify with facility managers the cost implications of outside air ventilation in view of the higher fuel prices expected this winter, and that all available opportunities have been taken to minimize the impact.
Modify work activities to reduce heating requirements without affecting productivity. During the coldest part of the heating season, implement a 4-day work schedule for buildings that are least energy efficient. Large industrial shops having minimal insulation and high infiltration would be good candidates for this initiative. Where possible, temporarily relocate work activities from larger, less energy-efficient buildings to smaller, more efficient ones.
Minimize the use of natural gas-fired refrigeration equipment. Several federal installations operate gas-fired refrigeration equipment (including absorption refrigeration and steam turbine-driven centrifugal machines) to provide space cooling in one or more buildings. Natural gas-fired refrigeration equipment is typically more expensive to operate during the heating season than electric-driven equipment. The use of natural gas-fired refrigeration should be minimized during the winter if mechanical refrigeration is required and electric-driven equipment is available.
In Central Heating Plants
Conduct boiler efficiency tests. Boiler efficiency tests are often the only reliable way of revealing deficiencies in a heating plant and identifying problem areas that can impact fuel consumption. Boiler efficiency tests should be conducted for the largest site boilers if such testing has not been completed within the last several years.
Optimize combustion efficiency. It is important that the correct air-to-fuel ratio be maintained in boilers and that sufficient excess air is used to assure complete combustion. Maintaining too much excess air is a common occurrence and unnecessarily wastes fuel. With well designed natural gas-fired boilers, an excess air level of 10 percent is usually attainable. Excess air levels should be continuously monitored by utility personnel and corrected if necessary. An often stated rule-of-thumb is that fuel costs can be reduced by 1 percent, if the amount of excess air is reduced by 15 percent.
Perform boiler maintenance. Stack temperature more than 150 degrees Fahrenheit above steam temperature often indicates the presence of excessive water-side scaling, which can reduce heat transfer and increase fuel consumption by as much as 10 percent. If stack temperatures are excessive, heat transfer surfaces should be cleaned to remove scaling.
Minimize boiler blowdown. Reliable steam plant operation requires that a portion of the boiler water be discharged to drain in order to maintain acceptable solids concentrations. Blowdown rates are often excessive and waste fuel. Plant personnel should continuously monitor boiler blowdown to minimize energy losses.
Optimize steam plant heat balance. Many large steam plants use a combination of electric motors and steam turbines to drive auxiliary plant equipment. Continuous venting of large amounts of steam at a steam plant usually indicates that these drives are not optimally balanced, which can be costly when fuel prices are high and electric rates are low. Plant personnel should immediately correct these imbalances when they occur.
Minimize deaerator steam venting. Excessive steam losses in a steam plant can often be attributed to deaeration, a corrosion control process that removes air and gases from boiler feedwater. Plant personnel should keep deaerator venting to the minimum acceptable level.
Optimize boiler loading to coordinate the operation of multiple boilers and ensure that all load conditions are met in the most efficient manner. Selected boilers should be shut down during the low load periods so that the remaining boilers can operate at higher, more efficient firing rates.
With Thermal Distribution
Inspect/replace steam traps. Steam traps are mechanical devices that remove condensate from steam piping and equipment. Hundreds of steam traps may be in service in a typical system, and it is not uncommon to find 15 to 20 percent not functioning properly. Collectively, trap losses can be significant. A single failed trap, which might cost $400 to replace, will increase fuel costs by about $2,000 this year if gas prices increase as expected. In systems with a scheduled maintenance program, leaking traps should account for no more than 5 percent of the total trap population.
Inspect/repair condensate return equipment. Inoperative condensate return equipment, like steam traps, often go unnoticed because collected condensate can be wasted to drain, while the steam system continues to function. Condensate contains useful thermal energy that can be recovered to offset fuel costs. If condensate is returned to a steam plant, fuel costs will typically be reduced by about 10 percent.
Locate/repair steam leaks. Steam leaks can also be significant. A continuous steam leak with a visible plume only a few feet in length will likely cost about $8,000 in additional gas purchases this year if no corrective action is taken. Steam leaks can also pose a significant safety hazard.
Repair insulation. Up to one-quarter of total heating system fuel costs can be attributed to the thermal losses from distribution piping, valves, and equipment. Deteriorated or missing insulation from a 10-foot section of a 6-inch steam line, for example, will increase gas costs by about $1,000 this year if left unrepaired. An uninsulated 6-inch steam valve will cost about $300 in additional natural gas purchases. Thermographic instruments and infrared pyrometers can be helpful in surveying steam lines and identifying areas needing repair.
Isolate non-essential distribution piping. Changing missions have reduced the steam requirements at many sites. Steam distribution systems may no longer be optimally configured to serve facility loads. Opportunities may exist to discontinue operation of major sections of a distribution system originally designed to supply much larger loads, allowing existing loads to be served by other more efficient means. The avoided distribution losses can be substantial. Fuel purchases attributable to thermal losses from a typical 6-inch steam line 1,000-feet in length, for example, will cost about $12,000 this year at the anticipated higher natural gas price.
Reduce distribution pressure. Load reductions that have resulted from changing missions and energy conservation measures may also afford the opportunity to lower steam pressures in existing distribution systems to achieve a corresponding reduction in thermal losses. For example, lowering the average distribution pressure in 1,000 feet of 6-inch steam line from 120 to 80 psig would reduce distribution losses by about 10 percent, saving about $1,200 in natural gas purchases this year.
For more information on O&M, please visit the Operations and Maintenance Web site.