Water level control is a fundamental element for the safe operation of industrial boilers. Once thermal energy is out of effective control, it presents extremely high risks, potentially leading to equipment damage or even catastrophic accidents. This article aims to provide operators with methods for identifying boiler dry-firing and help them master risk mitigation techniques to strengthen the safety defenses of industrial production.
Boiler dry-firing often stems from system failures or human operational errors. Malfunctions of automated feedwater control systems and mechanical water level sensors are common causes. Sensor signal distortion or control system malfunctions can lead to interruptions in feedwater supply, failing to respond to the need for water level replenishment. Feedwater pump failures or feedwater pipeline blockages directly cut off the feedwater path, making it difficult to maintain stable water levels in the boiler even if the control system is functioning normally.
Leakage of the blowdown valve or pipe rupture can cause a sudden loss of water in the boiler. This sudden water loss is often difficult to predict quickly and can trigger a low water level crisis in a short time. When manually managing the boiler, operator negligence or errors should not be overlooked, such as forgetting to monitor the water level or misoperating the blowdown valve, which can disrupt the water level balance and gradually lead to a dry-firing state.
Industrial boiler dry-firing is a typical high-risk condition in industrial production. Its formation process, resulting hazards, and response logic need to be analyzed in depth in conjunction with technical principles to provide accurate guidance for on-site operations.
Boiler dry-firing does not happen instantaneously, but rather evolves gradually from a low water level state. As the water level in the boiler continues to drop, the heating surface gradually becomes uncovered by water, and the normal heat exchange process fails. Water, as the main cooling medium, can no longer absorb the heat transferred from the heating surface, and heat continuously accumulates on the metal wall.
As heat continues to accumulate, the metal temperature rises rapidly, exceeding the boiler's design safety limits. Typically, the safe operating temperature of industrial boiler steel does not exceed 450 degrees Celsius. When dry-firing continues, the metal temperature can rise to over 600 degrees Celsius within 30 minutes, causing fundamental changes in the steel's properties.
Extreme high temperatures can cause metal degradation. Steel gradually loses strength at high temperatures, exhibiting softening, bulging, and other deformations, ultimately losing its structural load-bearing capacity. This damage is irreversible; even with subsequent repairs, the original structural strength cannot be restored, significantly shortening the boiler's service life.
The most serious hazard of dry firing is the risk of explosion. When the superheated metal surface comes into contact with feedwater, the water instantly vaporizes, expanding hundreds of times in volume in a very short time, generating a huge pressure shock wave and causing a catastrophic steam explosion. Such explosions are extremely powerful and can directly destroy the boiler body, causing fatal injuries to surrounding equipment and personnel.
Equipment losses are also significant. Dry firing can lead to deformation and rupture of boiler tubes, bulging and deformation of the boiler drum, and spalling and cracking of the refractory lining due to high-temperature thermal shock. Repairing such damage requires replacing a large number of core components, and the repair cost usually accounts for 30% to 50% of the boiler's original value, with a repair period of several weeks, seriously affecting the continuity of industrial production.
When signs of dry firing occur, emergency boiler shutdown procedures must be immediately implemented. Fuel supply and combustion air supply must be stopped immediately to cut off the heat source and prevent further temperature increases. The shutdown operation must be quick and decisive; even a one-second delay can exacerbate the extent of equipment damage.
Strict adherence to the "no water replenishment" instruction is mandatory. The superheated boiler metal walls will undergo violent thermal expansion and contraction upon contact with water, which not only easily triggers an explosion but also causes cracking of the metal walls. Even if there appears to be a return to normal water level inside the boiler, any form of water replenishment is strictly prohibited. The probability of an explosion caused by unauthorized water replenishment is over 90%.
After shutdown, natural cooling methods should be used to allow the system to slowly cool down to ambient temperature. The cooling process usually takes 24 to 48 hours, depending on the boiler capacity and the duration of the dry firing. During cooling, opening inspection ports or performing any maintenance operations is strictly prohibited to avoid personnel burns or secondary equipment damage.
The water level gauge is a device for visually monitoring the water level. When the water level gauge reading falls below the minimum safe level, or a low water level indicator appears, the cause of the water shortage must be investigated immediately. The water level gauge should be kept clean during daily operation to prevent debris from obstructing the reading.
The high and low water level monitoring system will emit an audible alarm signal when the water level is abnormal. Operators must be familiar with the characteristics of the alarm sound to ensure a response within 30 seconds of hearing the alarm. The sensitivity of the alarm system should also be tested regularly to prevent alarm failure due to circuit malfunctions.
A surge in flue gas temperature is an important technical early warning signal of water shortage. Under normal operating conditions, there is a fixed difference between the flue gas temperature and the feedwater temperature. When a water shortage occurs, heat cannot be absorbed by the water, and the flue gas temperature will rise by more than 150 degrees Celsius within 10 minutes. This change can be captured in real time through flue gas monitoring instruments.
The low water cut-off device is a core safety component for preventing dry burning. When the water level drops to a preset minimum value, the device will automatically cut off the fuel supply, preventing the heating surface from continuing to heat up. The response time of this device directly affects safety. High-quality products have a response delay of no more than 2 seconds.
Float-type and electrode-type water level sensors each have their advantages and disadvantages. Float-type sensors are simple in structure and lower in cost, but are susceptible to scale buildup and require monthly cleaning and maintenance; electrode-type sensors are more sensitive and suitable for high-temperature and high-pressure environments, but their purchase cost is about 40% higher than float-type sensors, and they have stricter requirements on water quality.
A secondary auxiliary low water cut-off device provides redundant safety protection. It can be activated promptly if the main device fails, preventing dry burning. Equipping a secondary device increases the initial investment in equipment, but it can reduce the risk of dry burning by more than 80%, effectively reducing accident losses in the long run.
Daily flushing procedures are necessary maintenance measures. Regular flushing removes impurities and sediment from the water column, preventing sensor blockage and malfunction. Flushing requires a certain amount of water, with each flush losing approximately 5% to 8% of the water in the boiler. This should be done during production breaks to avoid affecting normal steam supply. Regular testing of the burner's response to low water level signals is required, with at least one simulated low water level test conducted monthly to check if the burner can shut off the fuel supply promptly. The testing process will briefly interrupt boiler operation, with each test taking approximately 30 minutes. Testing times should be planned accordingly.
Electrical wiring and solenoid valves should be inspected weekly, focusing on checking for signs of corrosion, looseness, or failure. Severely corroded components should be replaced promptly; replacement costs vary from hundreds to thousands of yuan depending on the component type, but neglecting inspections can lead to serious safety accidents.
Scale buildup is a significant indirect factor in inducing dry burning. Scale adhering to the sensor surface can cause signal failure, resulting in false water levels and misleading operators. For every 1 mm increase in scale thickness, sensor sensitivity decreases by 15%, and prolonged neglect can lead to complete failure.
Boiler foaming can create false water levels in the water level gauge. Foam occupies the space in the water level gauge, causing the displayed water level to be higher than the actual level, misleading operators. Foaming is often caused by excessive oil content in the water or improper chemical additive ratios and needs to be detected promptly through water quality testing.
Continuous chemical water treatment effectively protects safety hardware by adding scale inhibitors, corrosion inhibitors, and other chemicals to control water hardness and impurity content. Chemical water treatment requires continuous investment in chemical costs, with monthly chemical costs accounting for 2% to 3% of boiler operating costs, but it can significantly extend the service life of safety components and reduce the risk of dry burning.
Operators must have the authority to shut down the boiler when the water level is unclear, regardless of the urgency of production tasks. If the water level cannot be accurately determined, the boiler must be shut down immediately for inspection; risky operation is strictly prohibited. Clearly defining this decision-making authority can prevent delays in handling due to hierarchical reporting.
Operators must comply with international safety standards, such as ASME Section I and other relevant standards. These standards provide detailed requirements for boiler water level control and emergency operations. Strict adherence to these standards reduces the risk of accidents caused by improper operation. Familiarity with these standards requires time for training, and companies must bear the corresponding training costs.
Regular emergency drills for dry burning scenarios are crucial. At least one drill should be conducted quarterly to ensure operators are proficient in emergency procedures. The drills will take up production time and consume certain materials, but they can effectively shorten response time during actual accidents and reduce losses.
Boiler dry firing can have devastating effects on industrial productivity and safety, leading not only to equipment failure and production interruptions, but also potentially causing casualties and inflicting heavy economic losses and reputational damage on the company. Controlling the risk of dry firing is a top priority in industrial boiler operation management.
