Many conditions can lead to sudden and unexpected failure of the boiler’s pressure vessel

Many conditions can lead to sudden and unexpected failure of the boiler’s pressure vessel, often requiring complete dismantling and replacement of the boiler. These situations can be avoided if preventive procedures and systems are in place and strictly followed. However, this is not always the case.
All boiler failures discussed here involve failure of the pressure vessel/boiler heat exchanger (these terms are often used interchangeably) either due to corrosion of the vessel material or mechanical failure due to thermal stress resulting in cracks or separation of components. There are usually no noticeable symptoms during normal operation. Failure can take years, or it can happen quickly due to sudden changes in conditions. Regular maintenance checks are the key to preventing unpleasant surprises. Heat exchanger failure often requires replacement of the entire unit, but for smaller and newer boilers, repair or replacement of just the pressure vessel may be a reasonable option.
1. Severe corrosion on the water side: Poor quality of the original feed water will result in some corrosion, but improper control and adjustment of chemical treatments can lead to a serious pH imbalance that can quickly damage the boiler. The pressure vessel material will actually dissolve and the damage will be extensive – repair is usually not possible. A water quality/chemical treatment specialist who understands local water conditions and can help with preventive measures should be consulted. They must take into account many nuances, since the design features of various heat exchangers dictate a different chemical composition of the liquid. Traditional cast iron and black steel vessels require different handling than copper, stainless steel or aluminum heat exchangers. High capacity fire tube boilers are handled somewhat differently than small water tube boilers. Steam boilers usually require special attention due to higher temperatures and a greater need for make-up water. Boiler manufacturers must provide a specification detailing the water quality parameters required for their product, including acceptable cleaning and treatment chemicals. This information is sometimes difficult to obtain, but since acceptable water quality is always a matter of guarantee, designers and maintainers should request this information before placing a purchase order. Engineers should check the specifications of all other system components, including pump and valve seals, to ensure they are compatible with proposed chemicals. Under the supervision of a technologist, the system must be cleaned, flushed and passivated before the final filling of the system. Fill fluids must be tested and then treated to meet boiler specifications. The sieves and filters should be removed, inspected and dated for cleaning. There should be a monitoring and correction program in place, with maintenance personnel trained in proper procedures and then supervised by process technicians until they are satisfied with the results. It is recommended to hire a chemical processing specialist for ongoing fluid analysis and process qualification.
Boilers are designed for closed systems and, if properly handled, the initial charge can take forever. However, undetected water and steam leaks can cause untreated water to continually enter closed systems, allow dissolved oxygen and minerals to enter the system, and dilute treatment chemicals, rendering them ineffective. Installing water meters in the filling lines of pressurized municipal or well systems boilers is a simple strategy for detecting even small leaks. Another option is to install chemical/glycol supply tanks where the boiler fill is isolated from the potable water system. Both settings can be visually monitored by service personnel or connected to a BAS for automatic detection of fluid leaks. Periodic analysis of the fluid should also identify problems and provide the information needed to correct chemistry levels.
2. Severe fouling/calcification on the water side: The continuous introduction of fresh make-up water due to water or steam leaks can quickly lead to the formation of a hard layer of scale on the water side heat exchanger components, which will cause the metal of the insulating layer to overheat, resulting in cracks under voltage. Some water sources may contain sufficient dissolved minerals such that even the initial filling of the bulk system can cause mineral buildup and failure of the heat exchanger hot spot. In addition, failure to properly clean and flush new and existing systems, and failure to filter solids from the fill water can result in coil fouling and fouling. Often (but not always) these conditions cause the boiler to become noisy during burner operation, alerting maintenance personnel to the problem. The good news is that if internal surface calcification is detected early enough, a cleaning program can be performed to restore the heat exchanger to near new condition. All of the points in the previous point about engaging water quality experts in the first place have effectively prevented these problems from occurring.
3. Severe corrosion on the ignition side: acidic condensate from any fuel will form on heat exchanger surfaces when the surface temperature is below the dew point of the specific fuel. Boilers designed for condensing operation use acid-resistant materials such as stainless steel and aluminum in heat exchangers and are designed to drain condensate. Boilers not designed for condensing operation require flue gases to be constantly above the dew point, so condensation will not form at all or will evaporate quickly after a short warm-up period. Steam boilers are largely immune to this problem as they typically operate at temperatures well above the dew point. The introduction of weather-sensitive outdoor discharge controls, low-temperature cycling, and night-time shutdown strategies contributed to the development of warm water condensing boilers. Unfortunately, operators who do not understand the implications of adding these features to an existing high temperature system are dooming many traditional hot water boilers to early failure – a lesson learned. Developers use devices such as mixing valves and separating pumps as well as control strategies to protect high temperature boilers during low temperature system operation. Care must be taken to ensure that these devices are in good working order and that the controls are adjusted correctly to prevent condensation from forming in the boiler. This is the initial responsibility of the designer and commissioning agent, followed by a routine maintenance program. It is important to note that low temperature limiters and alarms are often used with protective equipment as insurance. Operators must be trained on how to avoid errors in the adjustment of the control system that could trigger these safety devices.
A fouled firebox heat exchanger can also lead to destructive corrosion. Pollutants come from only two sources: fuel or combustion air. Potential fuel contamination, especially fuel oil and LPG, should be investigated, although gas supplies have occasionally been affected. “Bad” fuel contains sulfur and other pollutants above the acceptable level. Modern standards are designed to ensure the purity of the fuel supply, but substandard fuel can still get into the boiler room. The fuel itself is difficult to control and analyze, but frequent campfire inspections can reveal issues with pollutant deposition before serious damage occurs. These contaminants can be very acidic and should be cleaned and flushed out of the heat exchanger immediately if detected. Continuous check intervals should be established. The fuel supplier should be consulted.
Combustion air pollution is more common and can be very aggressive. There are many commonly used chemicals that form strongly acidic compounds when combined with air, fuel, and heat from combustion processes. Some notorious compounds include vapors from dry cleaning fluids, paints and paint removers, various fluorocarbons, chlorine, and more. Even exhaust from seemingly harmless substances, such as water softener salt, can cause problems. The concentrations of these chemicals do not have to be high to cause damage, and their presence is often undetectable without specialized equipment. Building operators should strive to eliminate sources of chemicals in and around the boiler room, as well as contaminants that may be introduced from an external source of combustion air. Chemicals that should not be stored in the boiler room, such as storage detergents, must be moved to another location.
4. Thermal shock/load: The design, material and size of the boiler body determines how sensitive the boiler is to thermal shock and load. Thermal stress can be defined as the continued flexing of the pressure vessel material during typical combustion chamber operation, either due to operating temperature differences or wider temperature changes during start-up or recovery from stagnation. In both cases, the boiler gradually heats up or cools down, maintaining a constant temperature difference (delta T) between the supply and return lines of the pressure vessel. The boiler is designed for a maximum delta T and there should be no damage during heating or cooling unless this value is exceeded. A higher Delta T value will cause the vessel material to bend beyond design parameters and metal fatigue will begin to damage the material. Continued abuse over time will cause cracking and leakage. Other problems may arise with components sealed with gaskets, which may begin to leak or even fall apart. The boiler manufacturer must have a specification for the maximum allowable Delta T value, providing the designer with the information necessary to ensure adequate fluid flow at all times. Large fire tube boilers are very sensitive to delta-T and must be tightly controlled to prevent uneven expansion and buckling of the pressurized shell, which can damage the seals on the tube sheets. The severity of the condition directly affects the life of the heat exchanger, but if the operator has a way to control the Delta T, the problem can often be corrected before serious damage is caused. It is best to configure the BAS so that it issues a warning when the maximum Delta T value is exceeded.
Thermal shock is a more serious problem and can destroy heat exchangers instantly. Many tragic stories can be told from the first day of upgrading the nighttime energy saving system. Some boilers are maintained at the hot operating point during the cooling period while the system’s main control valve is closed to allow the building, all plumbing components and radiators to cool down. At the appointed time, the control valve opens, allowing room temperature water to be flushed back into the very hot boiler. Many of these boilers did not survive the first thermal shock. Operators quickly realized that the same protections used to prevent condensation can also protect against thermal shock if properly managed. Thermal shock has nothing to do with the temperature of the boiler, it occurs when the temperature changes abruptly and abruptly. Some condensing boilers operate quite successfully at high heat, while an antifreeze fluid circulates through their heat exchangers. When allowed to heat and cool at a controlled temperature difference, these boilers can directly supply snowmelt systems or swimming pool heat exchangers without intermediate mixing devices and without side effects. However, it is very important to obtain approval from each boiler manufacturer before using them in such extreme conditions.
Roy Kollver has over 40 years of experience in the HVAC industry. He specializes in hydropower, focusing on boiler technology, gas control and combustion. In addition to writing articles and teaching on HVAC related topics, he works in construction management for engineering companies.