Boilers: Saving Energy, Contributing to 'Green'
A comprehensive boiler maintenance program that addresses organizations needs can help achieve big-picture goals
The cost of fuel oil and natural gas continues to rise, making the energy dollar that much more precious. Inefficient operation of a boiler plant can waste energy and increase the bottom-line cost to heat the facilities, and undermine organizations’ efforts to become greener. Leaks, uninsulated piping, dirt build-up, inoperable controls, and other overlooked maintenance basics can translate directly into added energy costs.
To prevent these problems, maintenance and engineering managers need to ensure their departments’ boiler maintenance activities and priorities address the needs of the equipment and help the organization achieve its goals for energy efficiency and green operations.
Cleaning a boiler is more important than most maintenance and engineering managers realize. The byproducts of burning fuel oil are soot and ash, which need to be cleaned from combustion-chamber surfaces to maintain proper heat-transfer efficiency. A layer of soot and ash that hardly looks like a candidate for removal can reduce efficiency by 10 percent. Letting that build-up continue over a period of as little as five years can result in the loss of 15-20 percent of a boiler’s efficiency.
The most overlooked component of a boiler plant is the barometric damper, which is typically discovered inoperable and in the closed position. It is a passive draft-control device located at the base of the chimney. A tall chimney is an asset if when burning wood or coal, as it provides good draft to maintain combustion.
Oil- and gas-fired systems only need enough draft to remove the flue gases. Too much draft, and unburned fuel is dragged up the chimney, along with energy dollars. The barometric damper is similar to a regulator. It constantly adjusts to assure proper draft, regardless of weather conditions or flue-gas temperature.
Technicians normally perform burner adjustments when installing a boiler, but managers must make sure they check combustion conditions regularly. Combustion-air volume needs to be correct for the fuel volume, or it will push unburned fuel up the chimney.
Assuring boilers are clean, allowing for best heat transfer and keeping an eye on the barometric damper are ongoing activities that managers and boiler technicians and operators cannot dismiss. In support of such activities, technicians should fire-test boilers periodically using flue-gas analysis to properly adjust the flue draft and combustion air input to optimize efficiency.
This work typically includes: efficiency testing, which requires checking: the carbon dioxide and oxygen content of the flue gas; stack temperature; burner and barometric damper adjustment; and controls and safeties.
Keep in mind the typical, newly installed, steel fire-tube boiler has a combustion efficiency of about 86 percent. Let the boiler maintenance go unattended, and that efficiency can slip to as low as 54 percent.
Fuel oil presents technicians with a different set of problems. Even when there is enough consumption, water traveling with the fuel oil can collect on the tank bottom, accelerating corrosion that eventually will perforate the bottom. Routine maintenance of the fuel-oil storage tank should include using additives to the fuel oil to help dissipate water collection. Typically, the fuel additive of choice also stabilizes the cetane rating of the fuel and prevents paraffin precipitation and microorganism growth.
A central heating system that is designed, installed, and maintained properly will have minimal corrosion problems. Studies have shown that numerous failures to building heating systems result from excessive infiltration of oxygen through minor leaks in pipe fittings or malfunctioning steam traps.
Oxygen also is present in the domestic make-up water used in closed-loop — hot-water heating system — and open-loop — steam-heating system — building heating configurations. Corrosion problems can occur when oxygen enters the water circulation loop, which is often a direct result of improper design or improper installation or operating practices, such as unacceptable or no water treatment. Also, an important consideration with hot-water boilers is keeping the water temperature above 165 degrees. Lower temperatures support high-surplus oxygen content that leads to pitting.
Corrosion issues normally occur only in closed-loop, hot-water boiler plants on initial start-up or after draining the system and refilling with domestic water, when testing of the boiler water is necessary. Should testing indicate the need for chemical treatment of the water, a one-time treatment should be sufficient.
Nitrite is an oxygen scavenger and a chemical-treatment product used primarily in closed-loop boilers. Testing for nitrite levels occurs during start-up to gauge the level of treatment, after servicing that involves draining, or an ongoing leak is discovered. Another possible use of nitrite testing is when technicians suspect a leak exists. Operators then will need to adjust nitrite levels to control the oxygen level because of the introduction of fresh make-up water.
In-house technicians can perform testing with an easy-to-use test kit. Nitrites typically are not used in steam boiler treatment schemes. But if a loss of water in the closed-system points to a design problem or leak, it might be necessary to introduce make-up water. Operators then will need to adjust nitrite levels to control the oxygen level of make-up water.
Steel fire-tube and water-tube steam boilers — open-loop systems — require annual water testing because of ongoing water use. Results of boiler water tests determine surplus oxygen content, a major cause of deterioration due to pitting and iron content and, in turn, a major cause of scale build-up and clogging. Technicians should install a flow meter on the make-up water line of a hot-water boiler. An indication of water flow suggests water loss somewhere in the system and the need to monitor water treatment more closely.
Treating the Problem
The constant supply of make-up water introduced to a steam system means technicians must treat the water. Water treatment is necessary in heating systems to prevent the restriction of water circulation and heat output, prevent bi-metallic corrosion by galvanic action, and prevent pitting corrosion, which creates pinhole pipe leaks.
Improper or non-existent boiler make-up water treatment is a major factor in system failures, which ultimately results in boiler downtime and costly repairs. All fresh water available from natural sources requires varying degrees of treatment before use in a boiler. Solids in the form of minerals, chemicals and organic material are all found in fresh water and have a different effect on the internal surfaces of a boiler.
But it is important to note that manufacturers of cast-iron boilers recommend not using chemical treatment in their boilers because of possible interaction with the section seals. Chemical treatment has been known to deteriorate those seals, causing leaks.
Beyond cleaning, technicians should test the low-water cutouts and lift safety valves periodically. A licensed engineer must check an operating boiler plant at least once every 24 hours. Daily checks should include a look at the settings and performance of the operating controls and the high-limit controls.
Technicians should perform hydrotesting and inspections on boilers to assess their overall condition and uncover hidden deficiencies. The work includes opening and inspecting the fireside and watersides of the boilers, along with pressure testing.
Maintenance targeting leaks or pipe deterioration can control the amount of domestic make-up water entering the system. Adequate chemical treatment for a system’s make-up water is necessary to prevent deposits, remove dissolved gases — free oxygen — and prevent corrosion
It is imperative managers remain aware of the basic precautions in the maintenance of boilers and their heating systems. Failure to implement a maintenance program can result in excessive damage to the boiler and piping, which becomes costly to repair and return to operating condition.
Managers should develop preventive maintenance for a building’s heating system in conjunction with an operations and maintenance plan, which includes the necessary tasks and associated labor. Good maintenance and operating practices of a building’s heating system can prolong equipment service life and ensure energy efficiency. Fine-tuning, cleaning, and conducting proper water management costs money; but improved boiler efficiency will return that modest investment with important savings in fuel consumption that far exceed the outlay.
Walter M. D’Ascenzo is a senior project manager with Facilities Engineering Associates. Inc. — www.feapc.com — specializing in building environmental and infrastructure systems. Rebecca A. Gutierrez has been involved in the mechanical, electrical and plumbing aspects of facility-condition assessments and reserve studies.
Water Replenishment: Potential Problems
Should continual boiler water replenishment be necessary for a building’s heating system, some of the following problems might occur:
Restricted water circulation: Corrosion of steel heat exchangers as found in boilers will result in the build-up of black or brown oxide powder in the bottom of the tubes or the collection of nitrogen and hydrogen in the top of the tubes. Both conditions can restrict water circulation and heat output.
Bi-metallic corrosion: Where dissimilar metals are exposed in oxygenated water, galvanic action will corrode the less noble metal. In mixed metal systems, the metals most likely to be targeted are aluminum, steel and iron. If the less noble metal has a large exposed surface, as in a boiler, it can tolerate a small amount of corrosion.
Pitting corrosion: Where excessive flux residues are present, such as those found in copper tubes, it is possible for localized corrosion to develop in the form of pits, which eventually can penetrate the tubes and create pinholes. Technicians should follow manufacturers’ recommendations for neutralization and removal of flux residues.
— Walter M. D’Ascenzo and Rebecca A. Gutierrez