Boiler Seasonal Maintenance And Operational Standards
The safe use of boilers is extremely important since they are considered special equipment.
To maintain the normal operation of boilers, avoid accidents, and extend their service life, it is necessary to adhere to the standard operating procedures and perform regular boiler seasonal maintenance.
The present paper systematically discusses the main aspects of daily boiler maintenance operations from five areas: operator’s qualifications, water quality management, water level gauge maintenance, blowdown operations, and shutdown/startup preservation.
Qualified Operators Must Operate Boilers
Boiler operation requires a lot of technical knowledge and involves hazards. The professional competence of the boiler operator is the primary barrier to the safe operation of boilers. According to the regulations of the Administration of Quality and Technical Supervision, boiler operators must obtain a boiler operator’s license issued by the municipal Administration of Quality and Technical Supervision before being authorized to operate a boiler.
This regulation was emphasized by inspectors from the Administration of Quality and Technical Supervision during their inspection of the boiler before authorizing its operation.
All users must strictly comply with this regulation. The licensed operation system ensures that the operators have received systematic theoretical training and practical skills evaluations and have correctly understood the operating procedures for boiler startup, operation, and shutdown.
Furthermore, they are capable of recognizing abnormal conditions and responding to emergencies. Users should create management files for boiler operators, provide regular retraining sessions, and prohibit unauthorized personnel from operating the boilers. By doing so, users can eliminate accidents caused by human operating errors.
Enhanced Water Quality Management
During the operation of a steam boiler, continuous evaporation increases the concentration of dissolved salts in the boiler water.
Scale forms on the heating surfaces. Scale has extremely poor thermal conductivity, making heat transfer difficult.
The metal wall temperature rises excessively, reducing material strength.
This can result in bulging of the furnace tubes or even explosions.
Additionally, high concentrations of boiler water can cause foaming and priming, causing misleading readings on the water level gauges.
Therefore, the operator cannot accurately determine the actual water level within the boiler. In extreme cases, this can lead to dry firing of the boiler, resulting in a serious accident.
To prevent these issues, water quality management must be carried out from two aspects:
A. Management of Water Treatment Equipment
The user unit should choose suitable water treatment equipment based on raw water quality and boiler type. The treated water should be regularly tested to ensure that the boiler feed water meets the requirements of GB1576 “Water Quality for Industrial Boilers”.
Operations such as regeneration, backwashing, and cleaning of the steam boiler water treatment should be conducted according to the required timetable. Records should be kept detailing each operation, and any problems should be immediately identified and resolved.
B. Management of Boiler Water
Boiler water samples should be taken regularly for testing to verify that all water quality parameters meet the requirements of GB1576 “Water Quality for Industrial Boilers“. Based on variations in boiler water quality, blowdown should be scheduled to remove sludge in a timely manner and maintain clean boiler water. All water quality management operations and testing results should be accurately documented as an essential part of the boiler operation record.
Daily Usage And Cleaning Of Water Level Gauges
Water level gauges act as the “eyes” for operators to monitor the boiler water level. The accuracy of the water level gauge readings directly affects the safety of boiler operations.
Water level gauges should always remain clean to prevent contaminants from obstructing observation of the water level. Even more critically, if scale or rust sludge builds up on the inside surfaces of the steam connection pipe and water connection pipe, it may obstruct the pipes.
This can result in a false water level; i.e., the water level displayed on the gauge does not accurately reflect the actual water level within the boiler. False water levels represent an extremely hazardous condition that can easily lead to a boiler accident.
In order to prevent false water levels, water level gauges should be cleaned and checked daily.
Cleaning should be performed with boiler pressure at 0.1-0.2 MPa.
Cleaning should be performed following these steps:
Step 1: Open the drain valve on the water level gauge. Both the steam connection pipe and water connection pipe will be flushed with steam and water simultaneously, initially removing impurities from the pipes and gauge.
Step 2: Close the water connection pipe valve. Only the steam connection pipe and gauge itself will be flushed with steam. High temperature and high-velocity steam will remove scale that has adhered to the pipe walls.
Step 3: Open the water connection pipe valve while closing the steam connection pipe valve. Only the water connection pipe will be flushed with water to ensure unobstructed flow through the pipe.
Step 4: Open the steam connection pipe valve, close the drain valve, completing the cleaning process and returning the water level gauge to normal operation.
After cleaning, observe whether the water level can rapidly return to normal fluctuations, thus confirming that the gauge and connecting pipes are unobstructed. If the water level returns slowly or displays abnormal behavior, identify and rectify the problem prior to resuming operation.
Daily Blowdown Operation
Scale, sludge, and other contaminants collect at the bottom of the drum and inside the blowdown system during boiler operation. If not removed in a timely manner, these contaminants may obstruct blowdown pipes and valves. This can render blowdown capabilities ineffective and impair control over boiler water quality.
Therefore, blowdown must be performed at least once daily to maintain normal functioning of the blowdown system. Blowdown should be performed based upon “frequent, small, and even blowdown.” The specific procedure is outlined below: Before initiating blowdown, adjust boiler water level to the normal high water level and maintain boiler pressure between 0.1-0.2 MPa.
Under these conditions, initiate blowdown using a short, intermittent technique — open the blowdown valve quickly and then immediately close it; after a brief pause, open it again and immediately close it again.
Continue repeating this process until the boiler water level reaches the normal low water level. Short-intermittent blowdown is more effective than lengthy continuous blowdown. Short-intermittent blowdown utilizes instantaneous pressure differentials to forcefully eject sludge from the drum. During blowdown, operators must continuously monitor water level variations and regulate both the frequency of blowdown cycles and length of valve openings. Operators must absolutely not discharge the entire volume of water from the boiler.
Complete discharge of boiler water exposes furnace tubes directly to high temperature flue gases. Direct exposure of furnace tubes to high temperature flue gases can easily result in overheating, bulging or even burning-through of furnace tubes, leading to a catastrophic failure. Upon completion of blowdown, confirm that the blowdown valve is fully closed and free from leaks and document blowdown operation in logbook.
Boiler Blowdown Process
Boiler blowdown is the process of removing some of the concentrated boiler water from a boiler in order to lower the total dissolved solids (TDS) and eliminate sediment from the bottom of the boiler. This is an essential step to ensure that the boiler water remains suitable for its intended use. There are two common methods of performing blowdown: continuous blowdown and intermittent manual blowdown.
Continuous blowdown continuously removes high TDS boiler water from the upper portion of the boiler through a series of valves. Intermittent manual blowdown is usually performed on a regularly scheduled basis by opening the bottom drain valve for a short period of time to remove sludge and other solid particles that have settled at the bottom of the boiler.
Frequency And Rate Of Blowdown
The frequency and amount of blowdown required depend upon the operating conditions of the boiler and the quality of the feedwater being used. It is generally recommended that a manual blowdown should occur at least once every 8 hours. The amount of blowdown that will be required is typically between 2% and 10% of the total feedwater flowing into the boiler.
However, it may be necessary to increase this percentage if the recovery of condensate is insufficient or if the quality of the make-up water is unacceptable. When performing a blowdown, open the bottom drain valve for a few seconds to allow highly concentrated boiler water to exit the boiler. Replenish with lower TDS make-up water to dilute the boiler water so it meets the established specifications.
Shutdown & Startup Preservation
Oxygen corrosion occurring on internal surfaces of boiler due to air is a significant factor limiting service life of boilers. To mitigate corrosion damage resulting from air entering the boiler during shutdown & startup, operations must be conducted strictly according to established procedures.
A. Shutdown Procedure:
At the conclusion of boiler operation, cut off power at normal pressure. Promptly close main steam valve & boiler feed water valve to prevent entry of air into the boiler via these pathways.
Maintaining a closed state internally within boiler limits contact between oxygen & internal metallic surfaces & significantly limits oxygen corrosion during the shutdown period. B. Startup Procedure: At the commencement of startup, open the boiler feed water valve to refill the boiler to the normal water level.
Next activate the main power supply to the boiler & open the main steam valve to a small opening to vent & pressurize the boiler. This allows any residual air within the boiler to gradually be discharged along with steam, preventing air that entered during shutdown from remaining within the boiler, causing corrosion as pressure increases.
Once pressure slowly increases to 0.1-0.15 MPa, close the main steam valve & continue increasing pressure to normal operating pressure.
Although venting may appear trivial & inconsequential, it plays an important role in preventing corrosion resulting from air that entered the boiler during shutdown & therefore must not be omitted.
Steam Boiler Water Treatment
Improperly treating or failing to treat boiler feedwater will result in many problems. These problems include scaling, corrosion, and foaming, which will all threaten the safety of the boiler. To ensure proper boiler operation, key parameters to consider during the treatment process include Total Dissolved Solids (TDS), pH Value, Alkalinity, and Dissolved Oxygen.
TDS should meet the requirements set forth by the equipment manufacturer. TDS should typically not exceed 3000-3500 μS (microSiemens). The pH Value should remain in the range of 10.5-12 to minimize acidic corrosion. Dissolved Oxygen causes pitting corrosion on boiler tubing.
Therefore, Dissolved Oxygen in boiler feedwater for low and medium-pressure boilers should be maintained below 0.007 mg/L. There are two ways to remove dissolved oxygen from the feedwater. One way is through deaeration. Deaeration involves heating the feedwater to about 180°F, then venting off the gases.
Another way is to use chemical oxygen scavengers such as Sulfite. Sulfite converts the dissolved oxygen to non-toxic sulfate salts. Regular monitoring of boiler water Conductivity, pH Value, and Alkalinity is essential to ensuring reliable and safe operation of the boiler.
Additionally, regular adjustments to blowdown frequencies and chemical dosages are required based on monitoring data.
Boiler Burner Maintenance
Regular servicing and calibration of your boiler burner will help maintain high levels of combustion efficiency, as well as reduce pollutants emitted by the boiler.
In addition to regular servicing, you should also clean the inside of the burner to eliminate any buildup of ash, examine and clean the fuel nozzle(s), air filter(s), and control systems.
When performing routine servicing, verify the functionality of all control and safety systems, test for leaks in the fuel lines, document the combustion parameters, and if needed, recalibrate the burner to ensure that the combustion process is completed without emitting excessive amounts of hazardous gases into the atmosphere.
Additionally, inspect the physical state of the combustion chamber and explosion door. If you find bends, cracks, or other deformities in either component, they should be repaired immediately. Most regulatory agencies require that a full commissioning be performed on each boiler’s burner at least once annually.
When a boiler has an oxygen trim system installed, the frequency of commissioning can be extended to five years. All other types of boilers must have a full commissioning performed on their burners every 25 months. After completing a commissioning, a written report documenting the results of the commissioning (including combustion performance data), repairs made during commissioning, and adjustments made to optimize combustion performance should be provided to the customer.
Boiler Low Water Cutoff Test
The Low Water Cutoff (LWCO) is a critical safety feature of all boilers. It helps protect the boiler from “dry” firing or explosions due to extremely low water levels. Different LWCOs have varying test methods. Float Type LWCOs: Blow down the boiler by opening the blowdown valve.
While blowing down the boiler, observe the drop in water level in the water glass. When the water level falls below the established cutoff point, the burner should shut off. Probe Type LWCOs: Push the test button for approximately 6 seconds when the boiler is operating. The burner should shut off instantly after pushing the test button. The burner should not automatically relight. The National Board of Boiler and Pressure Vessel Inspectors recommends that all LWCO devices used on commercial steam boilers be tested daily. Float-type units should be blown down at least once per month during the entire heating season. Probe-type units should be disassembled for annual inspections, and the probe element should be replaced every 10 years. Upon completion of testing, fill the boiler with water to the typical operating water level. Confirm that the burner lights correctly to complete the test.
Conclusion
Daily maintenance of boilers is a systematic & detailed process incorporating operator certification, water quality management, instrument maintenance, blowdown management & shutdown/startup preservation.
Each regulation stems from empirical data & lessons learned from accidents. User units & boiler operators must strictly comply with all operating regulations outlined above & implement daily maintenance operations on a day-to-day basis during every operation, ensuring safe & cost-effective operation while maximizing service life of equipment & providing reliable safety assurance for production & daily life
