Maintaining High-Efficiency Finned Copper-Tube Boilers

CSE's February Roundtable focused on specifying boilers, and this "Deep Link" from Lochinvar Corporation offers insight on their maintenance.

02/15/2002


A basic inspection and cleaning program can increase the service life of a boiler and reduce the chance of a "no heat" service call. To ensure continued operation at peak levels of performance, manufacturers recommend a preventive maintenance program for boilers.

Boiler maintenance is not limited to the boiler, but must include all related components used to install and operate it. More than 85% of operational problems are related to the installation. The first step in a boiler maintenance program is to review the installation, including the piping, gas supply, venting, pumps and the physical environment of the mechanical room.

The mechanical room

The mechanical room can be an influence on the reliability and operational life of the boiler. Proper operation requires adequate combustion and ventilation air supply to the mechanical room. The National Fuel Gas Code and local codes specify the size and location of combustion and ventilation air openings. Inadequate supplies of combustion air can cause operational problems with the burners and combustion processes, resulting in venting problems.

Partially or completely blocking the combustion air openings to the mechanical room in an effort to prevent cold weather damage to related piping can result in incomplete combustion, operational problems and possible spillage of flue products into occupied living space. If there are concerns about cold weather, motorized combustion air louvers can be installed. If motorized louvers are installed, they also need to be inspected for proper operation.

The mechanical room often becomes a storeroom for items that may be detrimental to proper boiler operation:

  • Chemicals, solvents, salts or cleaning materials can give off fumes that cause damage to the boiler and vent system when combined with combustion air.

  • Chillers, air-conditioning equipment and other machinery can leak refrigerants into the ambient air of the mechanical room. The chlorine contained in many refrigerants breaks free when drawn into the boiler's combustion process and may combine with water vapor in the flue products to attack the boiler and vent system.

  • Dust, dirt and airborne particulate matter can collect in combustion air fans, screens, grills and burner ports and cause operational problems.

Piping systems

Leaks present in the piping, fittings or system components must be repaired. The constant addition of make-up water to a boiler loop can result in scale and fouling. Pipes and fittings should be adequately sized for the required system flow. Velocities more than 5 feet per second cause system noise which can radiate throughout the building, while flow rates more than 8 feet per second result in system leaks due to erosion damage. High flow rates physically wear away the pipe and fittings.

Piping configuration must also be inspected. Reduction in flow to the boiler because of improper piping configurations will result in short cycles of operation, increasing fuel consumption and reducing service life. Short cycles can also produce condensation and reduce the service life of the vent system.

Inspect the boiler pressure relief valve and verify that it is properly sized per code requirements. Manually check operation of the boiler pressure relief valve by lifting the handle. Use extreme care if testing the relief valve on an operating system, as the water discharge from the valve will be hot and may cause scald injuries. Also, ensure that the discharge from the relief valve is piped to an adequate drain.

Water treatment

Alkalinity and pH should be properly controlled in the boiler loop. A high pH will result in scale and fouling. Is the system properly charged with glycol if required? Glycol is denser than water and may affect pump performance and boiler heat transfer.

Pumps

The pumps need to provide adequate water flow rates to the boiler. Low flow to a high-efficiency, low-mass boiler results in noise at the point of heat transfer. If the flow rate is not sufficient to carry the available heat to the system, noise ranging from a low-frequency moan to a hammering knock as the water flashes to steam will occur.

Review the electrical service to the boiler and pumps. Ensure that all wiring and breakers are properly sized and check that the safety controls, such as flow switches and low water cut off, are properly installed and will shut down boiler operation on a control sensed malfunction.

Gas supply

Properly sized pipe, regulators and meters for the gas supply are essential. Check for any loose fittings or possible gas leaks. Electronic leak detectors work well, but don't forget that a soap solution over suspect fittings makes an excellent leak indicator. Gas supply components are typically rated in cubic feet per hour. Multiplying cubic feet by 1,000 gives an approximate Btu/hr capacity.

When the boiler is initially fired, check the gas supply pressure, which should not drop below the manufacturer's specified minimum inlet pressure. If supply pressure is less than the manufacturer's recommended minimum, review the gas line sizing and ratings of all components. Once proper supply pressure is verified, check the boiler manifold pressure while the unit is firing. Adjust the boiler's manifold pressure to the inches water column setting specified on the unit's rating plate.

The venting system

The flue pipe should be inspected for rust, leaks or other damage and should be free of water marks or white streaks at joints and seams. Exterior streaks or rust on a vent pipe indicate that the flue products are condensing before they exit the building. Flue gas condensate is acidic and will damage the flue and boiler. Any damage to vent pipe or system components must be corrected to ensure safe operation. If draft hoods or barometric dampers are required, ensure that they control draft within the manufacturer's specifications with a draft gauge. Vent system failures can result in property damage, personal injury or loss of life.

The routing of the vent pipe is extremely important. The vent pipe should be installed as directly as possible to the chimney. If it is a horizontal vent, horizontal runs should never exceed 11/2' for every inch of vent diameter in a negative draft vent system. Consult the National Fuel Gas Code venting tables to verify that all Category I negative draft vents are properly sized. Category II, III or IV vents must be sized per the boiler manufacturer's recommendations. Also, ensure that the correct vent pipe materials are used and are properly supported.

If the heater is vented into a masonry chimney, inspect the overall condition of the chimney. A masonry chimney on the exterior of a building is more likely to excessively cool the flue products resulting in low draft and the formation of condensate that can damage a vent system. This could explain intermittent venting problems in the colder months. If a metallic liner is installed in the chimney, make sure it meets the manufacturer's requirements and complies with local code.

The National Fuel Gas Code and local codes provide detailed information to ensure proper location of a vent termination point. If the vent is terminated at the rooftop, make sure it is not adjacent to surfaces, walls or buildings. Sidewall termination must be properly installed using only components specified by the manufacturer. Check for proper clearance between the flue outlet and the combustion air inlet to the mechanical room. The flue must be a minimum of 4 feet from a gravity air inlet and 10 feet from a forced air inlet.

Venting is an area where good common sense can prove to be an invaluable tool. You can read and consult all of the manuals and codes for installation recommendations, but if a system looks like it may be a problem, it probably will.

The boiler

Once a review of the installation has been completed, the boiler should be carefully inspected, cleaned and adjusted to the manufacturer's specifications. Check the combustion air path into the boiler for any obstruction. Clean any screens, grills or filters. If the boiler is fan-assisted, check the blower(s) for dust or dirt fouling of the blower wheel.

The burners and heat exchanger Follow the manufacturer's instructions to access the gas train for removal and inspection of the burners, and clean any fouled or obstructed areas. Replace damaged, cracked or heat stressed burners with new original equipment manufacture (OEM) burners. The gaskets or seals removed to access the burner(s) need to be replaced.

Carefully inspect the combustion chamber while the burners are removed. Check the refractory lining of the combustion chamber for any breaks, cracks or deterioration, and replace as needed with OEM parts. Also, carefully check for soot or traces of incomplete combustion in the combustion chamber.

Inspect the exterior surface of the finned copper-tube heat exchanger for soot fouling, and check the heat transfer surface for fouling from condensate. The presence of condensate or the associated pitting or fouling indicates that a boiler needs repairs. Soot or fouling must be removed by the cleaning method recommended by the boiler manufacturer.

Complete removal of the heat exchanger may simplify the cleaning process and prevent possible damage to the combustion chamber. Carefully reassemble any component parts removed for cleaning. Check the boiler's ignition source (pilot, hot surface igniter, direct spark, etc.) and method of flame supervision. Ensure that the ignition source or flame sensor is not damaged or fouled. Inspect the ceramic insulators on flame sensors or spark igniters to make sure they are free of cracks or fouling. Carbon tracking or other fouling on a ceramic insulator can prevent boiler ignition or sustained operation.

The flue collector on top of the heat exchanger needs to be inspected to ensure there is no blockage, obstruction or traces of condensate formation or incomplete combustion. Baffles should be properly positioned on the heat transfer surface.

Test Firing

The boiler can now be test fired to ensure proper function of all safety and operating controls. As the boiler fires, carefully observe initial cycles of operation. Make sure that gas lines or connections that were removed in the inspection process are leak tested. If the combustion process is fan-assisted, ensure that the discharge from the fans is set per the manufacturer's recommendations. Check the setting of the operating temperature control and high limit control. Set the temperature control to desired boiler water setpoint, and adjust the boiler setpoint if an outdoor reset function is used.

After the temperature controls have been set, turn off the main gas supply and turn on the main power to the boiler. Check the circulating pumps to make sure that they are in operation. Cycle the boiler and allow it to attempt to fire and then lock out on the flame safety controls. This will prove operation of the controls before the first cycle of burner operation.

Reset the ignition safety controls as required, which generally entails pushing a manual reset. Turn on the main gas supply and turn on the main power to the boiler. Carefully observe start-up and main burner ignition. Observe the burners for proper flame pattern and check the temperature rise across the boiler.

Check the draft in the flue verifying that it is within the manufacturer's specified range for proper operation. If available, use a combustion analyzer to check CO 2 and CO levels in the flue products. If an analyzer is not available, ensure that combustion air and gas are adjusted per the manufacturer's specifications. All flue products discharged at the vent cap must be free of any odor, which is an indication that the gas and air mixture to the burner is not properly adjusted.

Listen for noise from the boiler. Noise during operation may be due to low water flow and a flashing to steam, and can be corrected by increasing flow to adequately carry away the heat. Increasing system pressure and the boiling point of the boiler water may also control low frequency noise. Use care not to exceed the relief valve setting or maximum system pressure when increasing system pressure.

Routine boiler maintenance should be performed at the beginning of each heating season or every six months for a boiler in continuous service. A basic inspection and cleaning program can increase the service life of the boiler and reduce maintenance costs.

References:
*National Fuel Gas Code, 1999 Edition, ANSI Z223.1-1999, NFPA 54-1999
*American National Standard for Gas-Fired Low Pressure Steam and Hot Water Boilers, ANSI Z21.13-2000
*2000 ASHRAE HVAC Systems and Equipment Handbook, Chapter 27, Boilers
*American Society of Mechanical Engineers, Controls and Safety Devices for Automatically Fired Boilers, ASME *CSD-1-1998, Part CM - Testing and Maintenance
*The NALCO Guide to Boiler Failure Analysis, Robert D. Port and Harvey M. Herro





No comments