The Importance of Conductivity
Conductivity is one of the important parameters of the physical and chemical properties of the solution. It characterizes the conductivity of the solution and is widely used in quality control in the fields of petroleum and petrochemical, food, medicine, environmental protection, military, and national defense. The conductivity of oil reflects the ability of the accumulated charge in petroleum products to be exported and released and is one of the important parameters to eliminate the electrostatic hazard of oil.
Necessity of oil conductivity measurement
With the continuous development of the petrochemical industry, the production, storage, and transportation capacity of oil products continue to increase, and the resulting safety hazards are also increasing. Oil products are flammable and explosive liquids. When the energy generated by electrostatic discharge exceeds the minimum ignition energy of oil vapor, the oil products will be ignited and detonated, resulting in fire or explosion. According to relevant information, about 10% of fire and explosion accidents are electrostatic accidents.
Therefore, during the loading, unloading, storage, and transportation of oil products, the generation and accumulation of static electricity should be minimized, and the static electricity generated during operation should be exported and released in time through anti-static measures. The release speed of static electricity in oil products is closely related to its electrical conductivity. The greater the electrical conductivity, the less likely it is to accumulate static electricity and facilitate the discharge of static electricity, which can effectively improve the safety of oil products.
Adding a small amount of antistatic agent to the oil can increase the conductivity of the oil. However, if the amount added is too small, the electrostatic export efficiency of the oil production will be low, which will affect the safety of the oil product; if the amount added is too much, the water separation index of the oil production will be affected, and the quality of the oil production will be affected. Therefore, it is necessary to accurately measure the conductivity value of the oil to control the number of antistatic additives to ensure the safety and quality of the oil during production, storage, and use.
Influencing factors and correct operation methods
The accuracy of oil conductivity measurement results is affected by many factors, it is not only related to the technical level of measuring instruments, measuring methods, and operators, but also related to the test environment. Based on years of experience in the measurement of oil conductivity and the measurement of oil conductivity meters, and consulting a large number of literature, the author puts forward some opinions and suggestions for the problems in the process of oil conductivity measurement. By analyzing various factors that affect the measurement of oil conductivity These factors regulate the method of operation during the measurement process.
Operators
The oil conductivity measurement personnel and other relevant operators must be educated on anti-static hazard safety, and the corresponding training content should be arranged in the business training. The rules and regulations and equipment inspection must have specific anti-static content.
Oil conductivity measurement personnel and other relevant operators must be proficient in GB 12158-2006 "General Guidelines for Preventing Electrostatic Accidents", GB 6950-2001 "Safe Static Conductivity of Light Oil Products", GB 13348-2009 "Static Electricity of Liquid Petroleum Products Safety Regulations", GB/T 4756-2015 "Petroleum Liquid Manual Sampling Method", GB 6539-1997 "Aviation Fuel and Distillate Fuel Conductivity Measurement Method" and other related technical guidelines, and strictly follow the relevant provisions of the specification for actual operation or other Activities to prevent static electricity safety accidents. When measuring fuel conductivity in the laboratory, testers should wear anti-static gloves as much as possible to avoid the interference of human static electricity on the measurement results.
Measuring instruments
Sample container
The container containing the oil sample should be selected as a metal medium as much as possible because the metal storage medium has weak adsorption of the conductive active ingredients in the antistatic agent, which can effectively reduce the attenuation of the sample conductivity; the volume of the sample container should not be less than 1 L; the sample container Before use, it must be fully cleaned with a cleaning solvent, the number of cleanings must be at least three times, and it must be dried with a dry air stream.
Measuring instruments
The measuring instrument refers to the oil conductivity meter (oil conductivity tester). Whether its value is accurate or not directly affects the accuracy of the oil measurement results, so it must be verified regularly, and the general verification cycle is 1 year.
Visual inspection of the instrument
Observe whether the measuring instrument is damaged by bumps or corrosion, and report for repair in time if any; check whether the conductivity cell is dirty, and clean it if necessary.
Battery Judgment
After starting up, first, check whether the battery can support normal work. If the battery displays a warning message and the displayed value keeps changing, the battery needs to be replaced.
Instrument Status Confirmation
At present, the commonly used oil conductivity measuring instruments can be divided into the following types: from the power-on state, it can be divided into those that start to measure after the power-on self-check and those that do not need self-test; from the data lock function, it can be divided into There are 3 s lock function and no 3 s lock function.
① For the power-on self-test instrument, during the power-on self-test, do not plug or unplug the conductivity cell or contact the socket of the conductivity cell with other objects, keep the state stable for a period of time, and perform other operations after the self-test is completed; otherwise, the instrument will automatically The inspection process is greatly disturbed by the outside world, and the self-inspection cannot pass or the instrument cannot be reset to zero after the self-inspection.
② After the instrument is in a stable state (some need to be turned on and warmed up for 2 to 3 minutes), observe the display value of the instrument, and the displayed value is 0 (within ±2 pS/m); if the deviation is too large, it needs to be calibrated according to the instrument manual.
③ Use a conductor to short-circuit the core of the conductivity cell socket, and the instrument will display an over-range state. After canceling the short-circuit, the instrument will display 0 (within ±2 pS/m) again in a short time.
Judging by the above operations, it can be confirmed that the electrical part of the instrument is in good condition.
Connect the conductivity cell to the electric meter of the instrument, repeat ② and ③, if it can pass, it can confirm that the instrument is in good condition; if there is an abnormality, it can be judged that the conductivity cell may be polluted, and it is necessary to use isopropanol first and then toluene Clean, after cleaning, blow dry with dry air, and then perform operations ② and ③ again.
After the instrument has passed the confirmation of the supporting status, it can start the measurement work; if it cannot pass the status confirmation, it is necessary to contact the relevant department for maintenance in time.
Follow-up work of the instrument
① Separate the electrometer of the instrument from the conductivity cell, turn off the electrometer part, and place it in the instrument package.
② Wipe the conductivity cell clean with neutral filter paper, then wrap it with filter paper and place it in the package.
Measurement method and process
Measurement method
The conductivity of oil products is generally measured by DC low voltage, and the positive and negative ions in the oil accumulate near the negative and positive electrodes respectively, resulting in a difference between the concentration of conductive ions near the electrodes and the concentration of the main liquid; at the same time, the accumulated conductive The ions react chemically on the two poles, causing the actual measured voltage to change, thereby affecting the accuracy of the measurement. Therefore, the measurement should be completed in a very short time, generally 3 s.
Test process
① Before starting to measure the sample, the conductivity cell must be cleaned with the test sample to remove other residual oil on the conductivity cell.
② Immerse the conductivity cell into the test sample until the liquid level reaches the upper line of the conductivity cell (or the middle of the overflow hole), keep the conductivity cell stable, observe the instrument indication, and record the highest reading after the initial stabilization, which should be completed within 3 s (directly read the lock value with lock function), and record the sample temperature at the same time. During operation, the conductivity cell should not be in contact with the wall or bottom of the measuring container, so as not to cause reading errors.
③ Take out the conductivity cell, wait for the sample in the conductivity cell to flow out, make sure that there is no oil drop in the conductivity cell to cause connection (oil hanging), press the instrument measurement button, and the instrument display should be 0 (within ±2 pS/m). If the deviation is too large, the measurement result is invalid, and it is necessary to re-adjust the instrument state and measure again.
Environmental conditions
Ambient temperature influence
As the temperature rises, the dipole ion pairs in the antistatic agent will accelerate their alignment, the positively charged ions will accelerate to the positive pole of the external electrostatic field, and the negatively charged ions will accelerate to the negative pole of the external electrostatic field, forming a "polarized electric field" The strength is enhanced, thereby reducing the original external electric field strength. Macroscopically, the original electrostatic field strength is reduced, and the electrical conductivity of the oil is increased. According to calculations based on relevant data, the temperature coefficient of electrical conductivity of oil added with domestic antistatic additive T1502 or foreign antistatic additive S450 is about 2%/°C [3]. Therefore, when measuring the conductivity of oil, the influence of temperature must be considered, and it is generally measured at 20°C.
Storage time
The sample to be tested is a newly added antistatic additive, and it needs to be placed for 24 hours before it can be measured. Since the antistatic agent is an organic substance, the interaction between molecules is slow, and the sample needs to be shaken thoroughly and placed for a period of time before the antistatic additive molecules can be evenly distributed in the oil.
Other samples to be tested should complete the sample conductivity measurement within 24 hours to prevent impurities and moisture in the ambient air from entering the sample, resulting in excessive differences between the test results and the original sample.
When measuring the conductivity of jet fuel, it should be carried out in a dark environment.
Impact of Environmental Electromagnetic Interference
When measuring the conductivity of oil products, it should be far away from obvious electromagnetic interference sources, such as motors, air compressors, high-voltage alternating current, etc., to avoid the influence of environmental electromagnetic interference on the measurement results; in addition, the measurement laboratory should be well ventilated to avoid oil and gas content in the air. If the temperature is too high, the test personnel will be poisoned by inhalation or cause other safety hazards.
Conclusion
There are many factors that affect the measurement results of oil conductivity. In the work, we must first understand various influencing factors and adopt the correct operation method for each influencing factor to eliminate or reduce its influence as much as possible, reduce measurement errors, and ensure measurement accuracy. The accuracy of the results improves the safety of oil products in production, storage, and use.
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