Thermal oil boilers are widely used in textile printing and dyeing, plastic and rubber, petrochemical, paper making, wood processing, food processing, pharmaceutical and other industries. Thermal oil as the heat transfer medium of thermal oil boilers, has inherent characteristics that distinguish it from hot water and steam, which determines that when we conduct regular internal and external inspections of thermal oil boilers, in addition to the requirements of the "Rules for Periodic Inspection of Boilers", we must also refine the parts and inspection items that should be focused on from the inherent physical and chemical properties of thermal oil.

Thermal Oil Characteristics Analysis

Thermal oil is an excellent heat transfer medium with low pressure and high temperature. Compared with steam and hot water medium, it has the characteristics of high heat transfer efficiency, uniform temperature field, easy temperature control, recyclable and low operation cost. Thermal oil can be divided into mineral oil type and synthetic type according to its chemical composition.

Thermal oil is chemically stable, its main characteristics are: low toxicity and tasteless; in the permitted temperature range, thermal stability is good; thermal conductivity, fluidity and pumpable performance is good; when the oil temperature is higher than 70 ℃, its contact with air will produce strong oxidation reaction, so the work system is best sealed; overheating will occur cracking or condensation, and coking or carbon accumulation in the container, pipeline; its flash point, ignition point and self-ignition point are High.

The physical properties of Thermal oil, such as density, specific heat capacity, thermal conductivity, kinematic viscosity, vapor pressure, boiling range, flash point, ignition point and self-ignition point, etc., affect its performance. Viscosity and heat transfer efficiency are related, the oil viscosity is small, the oil flows fast, the heat transfer efficiency is high, the appropriate viscosity can provide higher circulation efficiency; higher specific heat capacity and thermal conductivity can effectively transfer heat and improve the boiler operating efficiency; the higher the boiling range, the higher the maximum use temperature of the oil; the use temperature of Thermal oil should be higher than the flash point, but lower than the self-ignition point; Thermal oil volume expansion after heat is significant.

Based on the above physical and chemical properties, Thermal oil in the boiler as a heat transfer medium, should ensure that there is a certain flow rate and shall not leak; and need to consider the amount of expansion when heated, but not superheated use, so as not to cause coking carbon; high temperature Thermal oil can not be exposed to the air, so as not to oxidation degradation. Therefore, in the internal and external inspection of the Thermal oil boiler, the focus of the inspection should be considered from the prevention of the above safety hazards.

Points to Be Paid Attention to External Inspection of Thermal Oil Boiler 

At present, the basic structure of the Thermal oil boiler to coil type and tube frame type, small capacity of the Thermal oil boiler is generally used coil type, capacity of 2.8Mw above the general use of tube frame type. External inspection of boilers, in addition to the provisions of the "rules for periodic inspection of boilers", should also be based on the characteristics of the thermal oil, the external inspection of its focus on attention is:

(1) Examine the leakage points and the corrosion and leakage of the equipment to confirm whether there is any leakage in the Thermal oil system. Once the Thermal oil leaks, it is easy to cause a fire, so you should check whether the boiler room is equipped with fire-fighting equipment.

(2) Since Thermal oil tends to corrode rubber asbestos gaskets, Thermal oil boilers must use oil-resistant, pressure-resistant and high-temperature-resistant graphite gaskets reinforced with metal mesh or graphite composite gaskets with metal winding, instead of rubber asbestos gaskets. Except for the threaded connection for instruments and meters, threaded connection should not be used in the system.

(3) To ensure that the thermal oil circulates well at high temperature, prevent the flow rate of Thermal oil from being too low or the circulation from stagnating, causing the Thermal oil to over-temperature and overheat causing the oil to deteriorate. The export oil pressure of the boiler and the import oil pressure should have a certain pressure difference, so the boiler should be checked whether it is equipped with sensitive and reliable differential pressure alarm and interlock device.

(4) To prevent oxidation of high-temperature heat-conducting oil, the system should be checked to see if a closed expansion tank is used. Closed expansion tank should be equipped with safety valve, high and low level alarm device and over pressure alarm device. If the open system is used, the Thermal oil in the expansion tank should be maintained at a certain level and equipped with a low level alarm to ensure that the temperature of the Thermal oil in the expansion tank of the open system is lower than 60℃ as appropriate.

(5) In order to ensure that the high expansion tank can accommodate the thermal expansion of the system thermal oil, and can be timely replenishment of cold oil to the system, the expansion tank and the boiler body connected to the expansion tube shall not be added to the valve; check the expansion tank level gauge for leakage.

(6) Boiler safety accessories inspection, mainly to check the system on the pressure gauge, thermometer is normal and sensitive, whether in the calibration period.

(7) Circulation pump and backup circulation pump is intact and normal; in order to prevent power outage or circulation pump failure, the Thermal oil stops flowing and the local overheating, the boiler room should be checked to see if it is equipped with a backup circulation pump and self-powered.

Points to Be Paid Attention to Internal Inspection of Thermal Oil Boiler

In accordance with the requirements of the "Rules for Periodic Inspection of Boilers", the internal inspection of thermal oil boilers is mainly to check whether the boiler pressure-bearing parts have cracks, grooves, overheating, deformation, leakage, corrosion, wear and other defects affecting the safety of the boiler during operation. In addition, according to the physical and chemical characteristics of the thermal oil, attention must be paid to the following unique internal inspection of thermal oil boilers projects:

(1) combustion chamber coil, convection tube is a Thermal oil boiler should focus on checking the high-temperature parts, check whether the coil, convection tube has serious deformation, expansion coarse or leakage traces; tube butt weld and heat-affected zone has no cracks and other defects; whether the tube surface has serious corrosion or wear thinning. Quantitative measurement of deformation should be made when necessary.

(2) When the annual inspection stops the furnace, the filter should be removed for cleaning; check whether there are cracks or signs of leakage on the exterior of the filter, expansion joint, oil and gas separator and each valve body in the system to prevent major safety accidents such as fire and casualties caused by Thermal oil leakage when the boiler is running.

(3) the system should be set up at least one thermal oil sampling cooler. When the boiler is shut down for internal inspection, no less than 2 liters of thermal oil samples must be taken from the sampling cooler within the system. Note that when taking samples, the stagnant oil in the sampler should be released first, and take a uniform sample that can represent the oil currently in use in the system.

(4) The thermal oil of the boiler in use is sampled and tested at least once a year. By testing the residual carbon, acid value, viscosity, flash point, melting point and other physical and chemical indicators of heat-conducting oil, to keep abreast of its quality changes, analyze the reasons for the changes, in order to prevent the thermal oil deterioration caused by coking, carbon, thermal oil boiler metal corrosion or corrosion damage under the scale, in serious cases, will make the furnace tube metal cracks or corrosion pits, causing overheating burst tube.

Conclusion

The unique physical and chemical properties of Thermal oil determine the use characteristics of Thermal oil boilers with it as the medium and its safety characteristics. The duty of boiler inspectors is to find out the possible defects or safety hazards of boilers through regular inspection to ensure safe operation of boilers. Therefore, we must be from the characteristics of the thermal oil, to deeply understand the focus of the thermal oil boiler inspection and attention, only then, our regular inspection work will be more focused and accurate, but also to ensure the quality of inspection work and the use of thermal oil boiler safety.

If you have any questions about the use and testing of Thermal oil boilers, please contact EPCB Boiler.

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