It's the flash point of heat transfer oil that's to blame again!

Case 1: Asphalt pool mixed with heat-conducting oil bursts into flames at a carbon company in Jinan

At about 9:30 on November 12, 2018, an asphalt pool explosion caused by heat-conducting oil leakage occurred in the forming workshop of the old plant of a Jinan carbon company, resulting in 6 deaths and 5 injuries and direct economic losses of 11.45 million yuan. Post-incident investigation determined that the direct cause: high-temperature heat transfer oil leakage and asphalt in the closed asphalt pool mixed, the site staff illegal fire operations, triggering the asphalt pool heat transfer oil (determined by the test, heat transfer oil flash point of 66 ℃), asphalt combustion caused by fire.

Case 2: fire in the pump room of a petrochemical company in Daqing

At 16:35 on February 27, 2016, a fire broke out in the pump room of Daqing a petrochemical company limited. After receiving the fire alarm, Daqing Oilfield Fire Brigade rushed to the scene and the fire was extinguished at 17:58. The accident caused one death and one minor injury. Accident analysis report shows that: the low flash point of the heat-conducting oil contact sparks generated fire.

Case 3: Flash fire in a heat-conducting oil pump room of a factory in Fujian

In November 2015, the thermal oil pump room of a factory in Fujian caught fire, and the flames could be seen hundreds of meters away. After the analysis, it was also caused by the low flash point of the heat transfer oil!

As a heat transfer oil worker, every time I see similar news, I feel extremely sad. Why do fires occur frequently, and even endanger the lives of workers? The safety issue of heat transfer oil cannot be ignored!

The operating temperature of heat transfer oil is generally above 200°C, but its spontaneous ignition point is generally above 340°C, so why does heat transfer oil still catch fire?

1. Definition of flash point

First, introduce the concept of flash point to everyone. The flash point refers to the lowest temperature at which a petroleum product can flash when its vapor and air mixture touches the flame under heated conditions . There are two types of flash point: open flash point and closed flash point.

2. Effect of heat transfer oil structure on flash point

1. In homologues, the flash point increases with the increase of carbon atoms.

2. The normal form has a higher flash point than the isomer.

3. The flash point of saturated hydrocarbons is higher than that of unsaturated hydrocarbons.

4. The smaller the specific gravity of mineral heat transfer oil, the lower the flash point.

3. Analysis of the cause of flash fire

1. Equipment problem

In the production of actual working conditions, due to the cut corners of welders and instrument manufacturers, etc., the whole production system is often caused to leak, emit, drip, and leak. According to years of experience, customers rarely weld blind plates on site, smoke from instrument panels, and weld joints in pipes. Trachoma abounds, and what's more, low-viscosity and low-boiling products leak into the heat transfer oil and directly cause flash fires.

2. Operation problems

In addition to equipment problems, system operation problems are also a key part. When a customer is slowing down, two out of ten production lines have been opened. At this time, when the load of the hot section becomes smaller, it is necessary to open the bypass supply regulating valve to supply the flow rate to ensure the flow rate of the heat transfer oil in the furnace tube (there is quite a manual valve set by the factory, which needs to be manually adjusted); the customer is in a power outage Finally, it is necessary to open the cold oil replacement valve to replace the cold oil in the high tank with the hot oil in the boiler (customers without emergency generators). If the cold oil replacement system is not opened in time, the flow rate of the heat transfer oil will be too slow, exceeding the film temperature, and the high level The trough emits a large amount of smoke.

3. Oil problem

In addition to ensuring equipment and personnel, the selection of the heat transfer oil itself is also very important. Mineral oil has complex components and many long-chain structures. It is easy to crack at high temperature and has poor temperature resistance, resulting in a lower flash point. The synthetic heat transfer oil has a single component, and its oxidative cracking rate is significantly better than that of mineral oil under high temperature conditions.

4. How to prevent the occurrence of fire accidents in the heat transfer oil heating system

1. The location of the fire:

(1) Flanges, valves, etc.: Flanges are screwed off, valve packing is faulty, and gasket material and size selection are not correct, resulting in oil leakage.

(2) Trachoma at the welding joint and the pipeline: poor welding, trachoma flaw detection not found; the material selection of the heat transfer oil pipeline is incorrect.

(3) Radiant section: The radiant section corrodes the pipe wall over-temperature; the coking and carbon deposits of the heat transfer oil seriously lead to poor heat exchange, and the coil burns through the heat transfer oil and leaks.

(4) High-level tank: The oil temperature in the high-level tank is too high due to excessive low boiling substances and low liquid level; the high-temperature oil gas is oxidized in contact with air, which leads to intensified thermal cracking, resulting in fire or even explosion.

2. Avoid fires related to heat transfer oil:

(1) The selection and design of the heat transfer oil must be in accordance with the national regulations, and the heat transfer oil exceeding 300 °C must be sealed.

(2) The selected heat transfer oil must have a corresponding type report and be operated at the temperature specified in the type report.

(3) The flow velocity of the heat transfer oil in the convection section must be greater than 1.5m/s, and the flow velocity in the radiation section must be greater than 2m/s, so as to avoid cracking of the heat transfer oil caused by local overtemperature, resulting in light components and lowering of the flash point.

(4) Regularly sample and test the heat transfer oil during use to diagnose whether it is used normally.

(5) For textile and similar enterprises, the coils around the heat exchanger need to be cleaned regularly every year to avoid the attachment of flammable materials.

5. Detection of flash point of heat transfer oil

The detection of the open flash point of heat transfer oil follows the national standard GB/T 3536-2008, and the accuracy of the experiment is restricted by various conditions. In the detection of flash point, there are various factors that affect the accurate measurement of flash point, mainly including: sample volume, air flow, flame size, thermometer height, residual oil accumulation and so on.

1. Sample capacity

According to the test standard, the oil product is required to be loaded into the specified scale line of the test cup, and the liquid level of the oil product should just reach the scale line. This amount can lead to a deviation of the flash point.

To solve this problem, you can stop pouring oil every time you approach the scale line, and use a higher-precision dropper to add the amount.

2. Air flow

When testing the open flash point, air flow must be avoided within the close range of the oil cup. The air flow caused above the cup mouth will cause dilution of the oil vapor concentration, and then make the mixing ratio of oil vapor and air deviate from the flash point during the test. The ideal value of the fire directly leads to the deviation of the temperature of the flash point.

To solve this problem, the entire test can be carried out in a closed environment, and people should be avoided as much as possible to move around during the test, thereby reducing air flow.

3. Flame size

There is a detail in the flash point test that can best test the operating skills of a test operator, which is the flame state of the igniter. The size and shape of the ignition flame and the fuel used for ignition will have a direct impact on the detection results.

Different ignition fuels have different calorific values, which also determines the difference in the heat released by different ignition fuels when they are completely burned, and thus directly affects the size of the required flame. If the flame is too small, the oil-air mixture is not easy to ignite, and the flame may go out during the sweeping process, resulting in high detection results or missing the timing of flash fire; if the flame is too large, the ignition energy will be high and premature flash will occur. fire, resulting in low flash point results.

To solve this problem, a calibration test is performed with known flash point substances to find the proper flame size.

4. Thermometer height adjustment

For the measurement of oil products, the final measurement result of the open flash point will also change with the distance between the bottom of the thermometer and the bottom of the oil cup. The law that can be detected in the results again and again can be clearly known: as the distance between the bottom of the thermometer and the bottom of the oil cup increases, the results of the flash point test are continuously reduced.

To solve this problem, the distance between the lower end of the standard thermometer and the bottom surface of the oil cup must be strictly followed.

5. Residual oil accumulation

The residual oil in the test cup will affect the next test result. Different oil products have different volatilities at the same temperature, resulting in differences in light and heavy fractions in the mixed oil sample. If light fractions are mixed in, the sample will cause early ignition and the test result will be low. And the continuous volatilization of oil vapor will leave oil vapor on the inner wall of the test cup, which will also directly affect the test results.

To solve this problem, clean the test cup with petroleum ether after each test, dry it, and rinse it with the sample to be tested at least twice before adding the test sample.

The closed flash point of the used oil is mainly tested, while the test of the closed flash point of the heat transfer oil follows the national standard GB/T 261-2008. The accuracy of the closed flash point is restricted by the water content of the oil, the atmospheric pressure, the preset temperature, and the stirring and heating rate.

1. Moisture content

If the sample contains undissolved water, the free water should be removed by physical methods. The presence of water will obviously increase the flash point of the oil.

2. Atmospheric pressure

Since the actual measurement is the flash point under the ambient atmospheric pressure, it needs to be corrected to the flash point of the standard atmospheric pressure, so the accuracy of the ambient atmospheric pressure is also closely related to the accuracy of the flash point measurement.

3. Preset flash point temperature

When the preset flash point is higher than 110°C, start the ignition at 23°C below the expected flash point, and ignite once every time the sample rises by 2°C, and stop stirring when igniting. If the preset flash point is too high, it will cause direct flash fire and miss the real time. If the preset flash point is too low, the heating time will be too long, light components will escape, and the flash point will be high.

4. Stirring heating rate

The speed of stirring and heating also affects the flash point detection. Too fast will miss the occurrence of flash fire phenomenon, and too slow will cause light components to escape and cause the flash point to rise.

To sum up, in order to measure the flash point more accurately, in addition to strict control requirements on the above five points, a sample can be measured twice consecutively. If the error does not exceed 1°C, the result is relatively accurate; if the result is different If it is larger, you need to check the influencing factors and re-measure.

6. Write at the end

With a heavy heart, I write this article and hope that friends from related industries can circulate it so that you can pay attention to the danger of flash point reduction from the smallest details and avoid safety accidents from yourself. The director of Changzhou Special Equipment Testing Institute, Mr. Hu Yuexin, gave a correction to this article and expressed his heartfelt thanks for it.