Boiler Operation and Maintenance Basics
Boilers have been at the heart of the industrial revolution since the invention of the steam engine by James Watt back in the 18th century. Essentially like a giant tea kettle, they are vessels in which water is heated, flashed to steam, and then extracted to perform useful work. However, instead of using steam to drive a piston, turn a generator, or perform any other kind of mechanical work, a building’s boiler system uses the heat created by the steam it generates to provide heat and a comfortable working environment for the building’s inhabitants.
The boilers themselves need extensive maintenance to operate properly and they need repairs when necessary. It’s important to note that repair and maintenance are not the same thing at all. Since “an ounce of prevention equals a pound of cure,” money spent on maintenance is actually a cost savings. It is a repair that is a true cost, since it can put the entire operation on hold or even at risk from a major repair problem or safety incident. Repairs happen unexpectedly, throwing operating schedules off and resulting in magnified costs down the line. Maintenance, on the other hand, is planned and proactive, performed at regular intervals and for planned durations so as to not disrupt scheduled operations. The question then becomes, what are the best practices to guide building boiler maintenance and what steps are taken by managers in the field?
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Boiler Operations in a Building Environment
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Smaller, more modern residential units tend to use forced hot air heating systems with heated air pushed by a blower throughout the house by means of ductwork and vents. Boiler-based heating systems that transfer heat via hot water or steam, on the other hand, were used by older residential homes and are typically reserved for larger apartment complexes and commercial buildings. The steam and hot water created by the boiler is carried through the building by a series of pipes to baseboards or free-standing radiators located in each room or apartment. The fluids enter the radiator, become hot, and then radiate the heat into the room.
Credit: Parker Boiler Co.
A boiler’s energy efficiency is measured by an Annual Fuel Utilization Efficiency rating (AFUE).
The rate of heating is measured in British Thermal Units (BTU) per hour (BTU/hr). One BTU is equal to the amount of heat necessary to raise 1 pound (0.454 kg) of water 1°F (0.55°C). Each living and working space has a defined number of BTUs per square foot (BTU/sf) needed to achieve required ambient air temperature. Building codes and industrial specifications have resulted in standard measurements to achieve the required amount of heat. According to the US Department of Energy’s (DOE) office of Energy Credit:Efficiency and Renewable Energy (EERE), industrial and commercial boilers in the US are categorized as follows:
“The combined inventory of industrial and commercial boilers includes about 163,000 boilers with an aggregate capacity of 2.7 million MMBTU/hour (fuel input basis)…
“The inventory includes 43,000 industrial boilers with a total capacity of 1.6 million MMBTU/hour and 120,000 commercial boilers with a total capacity of 1.1 million MMBTU/hour. Industrial boilers tend to be larger than commercial units. There are 19,500 industrial boilers larger than 10 MMBTU/hour, including more than 1,300 larger than 250 MMBTU/hour. Commercial facilities have 26,000 boilers larger than 10 MMBTU/hour but only about 130 larger than 250 MMBTU/hour. The vast majority of commercial boilers are smaller than 10 MMBTU/hour. Overall, the size of the average industrial boiler is 36 MMBTU/hour, compared to 9.6 MMBTU/hour for the average commercial boiler.
“In addition to the industrial boilers included in the primary analysis, EEA estimates that there are approximately 16,000 industrial boilers in the nonmanufacturing sector with an aggregate capacity of 260,000 MMBTU/hour. Because these units are not well characterized, they were not included in the industrial boiler results listed in the analysis.
“Over 70% of the boiler units are less than 10 MMBTU/hour heat input, mostly in the commercial sector. These boilers account for only 15% of boiler capacity and typically have lower utilization than the larger industrial boilers. Due to the small size of these boilers and the lack of detailed information on them, this report focuses primarily on boilers larger than 10 MMBTU/hour.” (Source: EERE, “Characterization of the US Industrial/Commercial Boiler Population,” May 2005.)
For non-industrial residential and commercial operations both large and small, there are several general rules of thumb for determining the required heating (measured in BTU/hr) per floor space area (measured in square feet): about 50 BTU per square foot of interior space in a cold climate, approximately 35 BTU per square foot in a moderate climate, and at least 20 BTU per square foot in a hot climate.
Boiler System Design, Construction, and Operation
A boiler’s design, manufacture, construction, and operation are, in principle, very simple. Essentially, a boiler is just an enclosed vessel or tank containing water which is heated by an external fuel source or electrical heat. Its containment walls, joints, fixtures, and appurtenances are engineered and manufactured to withstand the anticipated pressures that will build up once the water heating begins. When the water is heated, the boiler will produce either hot water or steam. These act as working fluids to transmit heat throughout the building. Smaller boiler units can utilize electric heating coils, but larger units typically require the burning of a combustible fuel.
To perform this heat transfer, a boiler utilizes the following parts: a containment vessel or tank with a refractory liner, several fuel burners (depending on size), an ignition mechanism, a heat exchanger transferring the heat of combustion to the water inside the boiler tank, a circulating pump to move water in and out of the boiler tank, an expansion tank, a boiler water pressure regulator with safety relief valves, and a radiator to transmit heat to the surrounding air. Boiler control systems include a thermostat which regulates the amount of heat delivered to the building as a whole, an individual room, or living space. The thermostat controls an information feedback loop back to the boiler, either signaling the boiler to produce more heat should it get too cold or shut off if it gets too hot.
Beyond the boiler, there is a system for distributing heat throughout the building envelope. This system consists of a feed water supply to make up for any water lost in the system along with a pump to supply the water to the boiler; a series of pipes that form a loop throughout the building, starting at the boiler outlet and returning to its inlet; a fuel storage tank or hookup to a local utility; pressure-reducing valve (emergency relief valve); air purge vent; and a backflow preventer. The distribution pipe forms an overall loop, taking water or steam from the boiler and returning it after it radiates heat and cools down for reheating. Each radiator point is typically part of a branching pipe loop that diverges off the main circulation loop with an inlet and outlet pipe attached to the radiator.
A typical temperature for the water being circulated by a hot water radiant heating system is a constant 180°F. Should the thermostat temperature in a room fall below a set comfort level, the thermostat sends a signal that turns on the pump that circulates the water to the house or room. Because it forces water through a pipe system with circulating pumps, a hot water heating system delivers more even heat distribution through the building. However, it takes a hot water boiler system longer to deliver this heat, especially in larger buildings. As such, hot water heat systems are not typically used for buildings higher than six floors given the excessive static head the circulating pump would have to overcome. But their better overall efficiency makes them preferable for moderately tall buildings.
A steam heating system operates a little differently. As with the hot water system, the temperature of the water is kept at approximately 180°F minimum. However, should the thermometer determine that room temperature has fallen below a set temperature, it will signal the burner to turn on and “flash-steam” the hot water in the boiler to a temperature above 212°F and send it as steam through the pipe distribution system to the radiators. Given the increased pressure from steam formation, a steam heating system does not require circulating pumps. The burner cycles on and off throughout the day and night to match the building’s heating and cooling cycle, or simply to keep the water in the boiler at or above the minimum required 180°F. Steam delivers heat more rapidly but not as evenly. It also typically requires larger radiators to efficiently extract the heat, but can be effectively used in taller buildings.
The burner itself is fed by a fuel pump that draws liquid fuel (heating oil) through a filter to remove impurities and ensure a clean burn, or via a valve pressure regulator connected to the local gas utility pipeline to deliver gaseous fuel (natural gas). Liquid fuel is atomized into a high-efficiency combustible mist by the fuel injector ports, then mixed with forced air from a blower, and ignited by a spark assembly. A continuous feed of either kind of fuel ensures a stable flame and a continuous level of heating. Refractory bricks surrounding the flame deflect it towards a heat exchanger. This is a series of connected metal tubes containing water as a working fluid for the heat transfer. The pipe manifold surrounds the combustion chamber and transmits the heat into the tank holding the boiler water. The pipes are somewhat oversized to provide room for steam when it bubbles out of the water working fluid. Exhaust fumes from the combustion are vented via an exhaust stack which discharges at the building’s roof.
From the boiler, the heated water or steam is pumped via a series of pipes to radiators that are located throughout the building at strategic locations. These radiators can be located in a room’s ceiling, along the baseboard of the wall, in a series of tubes running through the floor, or, more traditionally, to free-standing radiators. After radiating heat to the surrounding air, the now-cooled water is returned to the boiler by the return piping of the distribution pipe loop. This return cycle through the rest of the pipe distribution network completes the boiler water loop. Since steam is lighter than air, it will self-rise via natural buoyancy through the pipe distribution network to its radiator points and then drain as a liquid via gravity flow back to the boiler. Hot water heating systems, on the other hand, require circulating pumps to move the hot water through the distribution network.
Credit: Harsco Industrial Patterson-Kelley
A technician adjusts the gas valve on a Patterson-Kelley Sonic boiler
The individual room radiators are often equipped with manual shut-off valves along with their thermostat controls. A room’s occupants can then manually shut off the hot water or steam feed at the branch pipeline servicing the room if desired or required. This prevents hot water or steam from entering the radiator. Valves can be designed to cut off flow completely or to regulate the flow rate in part. Either way, it puts a human in the decision cycle as to how to most effectively utilize available heat while also providing a shut-off backup system in case of emergencies.
The boiler’s overall energy efficiency is measured by an Annual Fuel Utilization Efficiency rating (AFUE) developed by the DOE. It’s a measurement of the percentage of heat from the fuel combustion that is actually used to heat a building. For example, a boiler unit with an AFUE of 75% will have 75% of the energy contained in each gallon or cubic foot of fuel transferred to the heat exchange system to heat the contents of the boiler. The remaining 25% of the heat of combustion will escape up the flume with the exhaust gases. The more efficient the boiler system, the higher the AFUE rating and the less fuel it will require to deliver the same amount of heat to the building’s occupants. In other words, boilers with higher AFUE ratings are cheaper to operate. Newer boiler models have AFUE ratings higher than 95%.
Onsite fuel storage and supply is typically required for liquid fuel operated boiler systems. Enclosed storage tanks are used to store fuel oil and can be located underground, aboveground and outside the building, or inside the building. Residential aboveground oil storage tanks are typically about 300 gallons in capacity, with aboveground tanks ranging from 500 gallons to 1,000 gallons. Oil pumps are used to extract oil from the storage tanks and deliver it to the boiler combustion assembly. On the way, fuel is forced through a filter by pump pressure to remove impurities.
Boiler Maintenance and Repair—So What Could Go Wrong?
The more complex the system and the more moving parts, the more wear and tear is incurred, and the greater the probability of repairs along with the need for preventative maintenance. Even the most basic boiler and radiant heat system has a plentitude of parts, moving or otherwise. The following is just a short list of some of the things that could go wrong and the preventative maintenance required to keep that from happening:
- Water levels in both the boiler and the supply tank as well as the heat exchanger should be checked daily. Loss of water results in an inability to transmit or dissipate heat since it is your only practical working fluid. Loss of water can lead to overheating of individual pipes or the whole system and is usually indicative of a serious leak in the pipe system. Keep a running record of water amounts added to the system to maintain operating levels. The cumulative totals will indicate if there is any problem in the system.
- In addition to water levels, water temperatures should be checked daily if not continuously in order to ensure that the minimum required system temperature of 180°F is being maintained. Simultaneously with temperature, the related measurement of system pressure should be made.
- Like water, fuel oil levels should be checked daily for those boilers that utilize liquid fuel from storage tanks. Given the vagaries of weather and climate, fuel usage rates can vary considerably from day to day; if not checked regularly, the system’s fuel oil could run out before the next available oil tanker truck delivery date.
- Lubrication oil should also be checked daily in any compressor or pump used by the system. A burned-out pump can shut down the entire system and, unless spare pumps are available onsite, could lead to extended shutdowns while awaiting delivery. Oil pressure in the fuel delivered by the pumps needs to be monitored as well.
- All pressure gauges and temperature readings (stack thermometers, water thermometers, etc.) be checked for accuracy on at least a weekly basis.
- The combustion flame should be visually inspected on a daily basis to determine the length of the flame and complete burning of all of the combustion feeds. Also, it’s important to inspect the oil preheater if the system uses one.
- General weekly maintenance should include testing of the low-water cut off safety switches, performing blow-downs of the various valves (including surface valves, bottom emptying valves, and automatic water feeders), cleaning the strainers and oil filters, and replacing the oil filters if necessary.
- The following components should be maintained at least monthly: check the combustion chamber for any built-up oil residue and clean it when necessary, thoroughly clean the burner assembly, test the safety pressure relief valve, and check the associated pressure gauge. Pressure reducing/regulating and relief valves should be inspected for proper operation. Other steam control valves should be calibrated in accordance with manufacturers’ specifications and instructions.
- In addition to the daily check of water levels, water quality should also be tested. Pipe corrosion, rust, and other impurities may impact water quality and with it the ability to properly transmit heat through the system. Maintaining a proper pH level and chemical balance within the water will go far to preventing corrosion in the first place.
- In addition to the above items, the combustion air supply should be checked monthly. This includes air inlets to both the boiler room (to ensure safe operations and prevent the buildup of poisonous exhaust gases) and to the boiler itself (to ensure sufficient oxygen is being delivered to the combustion assembly). Blowers should also be checked, especially their driving belts to ensure that there is no slack, slippage, wear, or tear.
- Similarly, the boiler system should be checked monthly for air leaks around access openings and the combustion assembly. This includes testing the seals on all gaskets to ensure a tight seal.
- The inside and outside of the boiler tank should be checked for corrosion and the wearing away of insulation to prevent the development of hot spots on the tank walls.
- Annual maintenance efforts are as follows: complete burner overhaul including removal and reassembly, perform an efficiency test to determine the boiler’s AFUE rating, clean all vent stack/chimneys and outlets, remove or recondition in place the relief valve, clean and inspect the fuel oil storage tank, steam clean the oil feeding the boiler’s combustion unit, clean and recondition the feed water pumps, clean condensate receivers and purge the deaeration system, and clean and recondition system pumps and appurtenances (filters, pilot, oil preheaters, oil storage tanks, etc.).
- Cleaning of all exposed surfaces should be done annually. This is not just some sort of “spring cleaning” performed for aesthetic purposes. Surface dirt, grime, and dust can adversely affect exposed joints, vents, intakes, etc. Surfaces are defined as either “water-side” or “fire-side,” depending on their location relative to the combustion assembly. Another surface requiring attention is the refractory brick lining. This needs to be inspected annually as well and replaced or repaired as needed.
Flow Control Industries manufactures valves and appurtenance to control liquid flows in variable temperature regimes. One of these is their new patented DeltaP SmartValve. By adding advanced instrumentation to the precision control performance of their standard DeltaPValve, they can optimize comfort and system performance. This new technology allows for flow measurement, differential temperature and pressure measurements, and calculation of BTU, all in real time. Furthermore, it serves as a precise control valve. The collected data is easily accessed via a BACnet interface over TCP/IP. Its controls are pressure independent and allow for a 100:1 turndown ratio. It continuously monitors seven different performance variables including supply and return water pressure, inlet and outlet pressure, and thermal energy expended. In doing so, it provides nonstop troubleshooting and reporting of heating coil performance.
The Parker Boiler Co. was founded in 1946 and is a world-renowned manufacturer of industrial and commercial steam boilers from 1.5 to 150 horsepower with pressures to 250 psi capable of delivering dry steam in under 10 minutes. They manufacture industrial and commercial direct-fired hot water boilers (available as gas, oil, propane, or combination-fired)—ranging from 300,000 to 6,800,000 BTU, with operating temperatures up to 400°F and pressures to 300 psi—and condensing boilers from 399,000 BTU to 5,433,000 BTU and up to 180°F and 80 psi. Their indirect-fired water heaters (available as gas or propane-fired) range from 300,000 to 3,000,000 BTU input, with operating temperatures to 190°F degrees and pressures to 150 PSI. They manufacture high-temperature thermal liquid heaters (available as gas, oil, propane, or combination-fired) for process heating up to 650°F degrees, ranging from 126,000 to 6,250,000 BTU input. They also are the makers of low-NOx steam and hot water boilers and burner systems. Most models are UL or ETL listed. In addition to manufacturing their products, Parker Boiler is in the business of providing expertise and support to their customers.
Their boilers are constructed and furnished in accordance with the American Society of Mechanical Engineers (ASME) Sect. I & IV Power Boiler Code and the National Board of Boiler Pressure Vessel Inspectors. Emphasizing safety, their boilers can generate full steam in less than 10 minutes after a cold start.