W= CV2
Where: w-discharge energy, j;
C-equivalent capacitance between conductors, f;
Potential difference between conductors.
From the above formula, it can be clearly seen that the spark generated by electrostatic discharge cannot be based solely on the potential, and it is uncertain how many kilovolts of electrostatic charge will generate the spark when the equivalent capacitance is uncertain.
At present, there is no clear data about the minimum discharge energy that can produce sparks. However, the following data are of great reference value to practical work:
1. When the energy of single pulse discharge of induced corona is less than 20μJ, sometimes sound and light can be generated, and the ignition ability is very small;
2. The discharge energy that can produce moderate ignition ability generally does not exceed 4 MJ;
3. Spark discharge between charged metal conductors close to each other has strong ignition ability because of the concentration of released energy;
4. When the potential difference between conductor electrodes is lower than 1.5kV, the paraffin oil vapor with minimum ignition energy of 0.25mJ or more will not be ignited by electrostatic discharge;
5. Inductive corona in local space such as grounding needle tip will not ignite combustible gas, and the minimum ignition energy is greater than 0.2 MJ.
6. When light oil is loaded, the oil level potential should be lower than 12kV.
How to effectively prevent it? (2)
National standards and industry standards such as General Guidelines for Preventing Electrostatic Accidents (GB 12 158-90) and Electrostatic Safety Inspection Regulations for Chemical Enterprises (HG/T23003-92) have made relatively standardized and perfect provisions, and these standards have been included in the Practical Work Manual for Chemical Safety edited and published this year. Considering that you may not have bought it, how can chemical enterprises prevent the harm caused by static electricity? The simple answer is as follows:
1. All metal devices, equipment, pipelines and storage tanks must be grounded according to the standard. It is not allowed to use metal equipment or metal parts insulated from the ground. Sub-conductors or non-conductors should be indirectly grounded, or electrostatic shielding should be adopted, and the shielding body must be reliably grounded.
(1) The total leakage resistance of each production device system or device unit shall be below/kloc-0 /×106Ω, and the specially designed electrostatic grounding resistance shall not be greater than100Ω;
(2) Metal equipment and equipment, pipelines and pipelines, if connected with metal flanges, do not connect jumpers, but there must be more than two bolts; The total leakage resistance must be lower than1×106 Ω;
(3) When loading and unloading flammable liquids, the tankers that can't be grounded at ordinary times must be grounded at the preset place according to the requirements of operating procedures, especially the materials used must be materials that will not generate sparks when they are hit;
(4) Large metal devices with a diameter greater than 2.5m or a volume greater than 50m3 should have more than two grounding points, and long pipelines should have 1 grounding points every 80- 100m.
2. Control the stirring speed of combustible liquid in the reactor according to the operation procedures.
3. When handling and transporting flammable liquids, prevent static electricity.
(1) When filling, the liquid should enter from the bottom of a large container such as a tanker, or the injection pipe should extend into the bottom of the container;
(2) control the flow rate of liquid:
Answer: When the railway tank car is filled, the allowable velocity of liquid in the crane pipe is calculated according to the following formula:
VD≤0.8
Where: v-hydrocarbon liquid velocity, m/s;
D—— internal diameter of crane pipeline, m.
The speed of the discharge port of the crane pipe can exceed the calculated value of the above formula, but not more than 5 m/s. ..
B: When the tank car is filled, the allowable velocity of liquid in the crane pipe is calculated as follows:
VD≤0.5
Where: v-hydrocarbon liquid velocity, m/s;
D—— internal diameter of crane pipeline, m.
(3) In the process of transportation and filling, the liquid should be prevented from scattering and splashing, and the end of the refueling pipe that enters the tank from the bottom or the upper part should be designed into an inverted T-shape or other shapes that are not easy to scatter the liquid, or a deflector should be added; Or when the upper part is full, the liquid slowly flows down along the side wall;
(4) When filling hydrocarbon liquid into large containers such as oil tankers, it is advisable to feed oil from the bottom. If the top oil inlet is used as the last resort, the oil filler pipe should extend into the oil tank and be no more than 200mm away from the bottom of the oil tank. Before the oil filler tube invades the liquid, its flow rate should be limited to1m/s. ..
(5) Hydrocarbon liquid should avoid mixing with other incompatible second-phase impurities such as water, and try to reduce and eliminate the accumulated water at the bottom of the tanker and in the pipeline. When the second phase obviously exists in the pipeline, its velocity should be limited to1m/s;
(6) In large containers such as tanks and tankers, there are no ungrounded conductive floating objects on the surface of flammable liquids;
(7) When controlling the flow cannot reduce the static accumulation, a liquid static eliminator can be installed at the end of the pipeline;
(8) When using a small portable container to hold flammable insulating liquid, it is advisable to use a metal or electrostatic conductive container, and avoid using a non-conductive container with electrostatic conductivity. Metal containers and metal funnels should be bridged and grounded.
(9) In the process of filling, mixing or circulating in the equipment, it is forbidden to carry out on-site operations such as measuring ruler, sampling and measuring temperature;
(10) When filling, mixing or circulation stops, it shall be allowed to stand for a period of time according to the operation procedures before proceeding to the next working procedure.
4. Non-metallic pipes should not be used to transport flammable liquids. If it must be used, it is advisable to use a conductive tube or a tube with a wire mesh inside. One end of the wire mesh is reliably grounded or shielded from static electricity.