Laboratory Safety-Electrostatic Protection (3)

Fourth, the prevention of static electricity

The most serious danger of static electricity is to cause explosion or fire.

In the chemical laboratory, fire or explosion accidents caused by static electricity of materials (mainly solvents) are mainly related to the polarity and flow rate of the materials (including pipeline transportation speed, stirring speed, and the degree of friction between vapor molecules and the condenser tube during heating and reflux, etc.

Electrostatic discharge sparks can only occur when static electricity and electrostatic charge are accumulated to a certain degree to form a high potential

The conditions for the amount of static charge and discharge of electric sparks are met, and the explosion limit of the explosive mixture must be met to cause an explosion or fire.

In addition to the harm caused by the static electricity of the solvent and the electrostatic accumulation and discharge, the static electricity and discharge of the human body can also cause explosion accidents in the explosive limit environment

Continuously and promptly remove the generated static electricity, and take other protective measures can also prevent fire or explosion caused by static electricity and discharge

The main electrostatic prevention measures in chemical laboratories are as follows:

(1) Environmental hazard control

(2) Antistatic agent

(3) Process control

In order to limit the generation of dangerous static electricity, when hydrocarbons flow in large pipelines, the flow rate and pipe diameter should satisfy the following relationship:

V ² D≤0.

In the formula, V is the flow velocity, m/s; D is the pipe diameter, m

The faster the flow rate, the greater the electrostatic charging, so it is necessary to limit the flow rate of the fluid in the pipeline

For small pipe diameters, the above formula is not appropriate

In order to prevent electrostatic discharge, sampling, testing or temperature measurement operations are not allowed during the liquid filling process

In the pilot laboratory or production workshop, if a non-polar hydrocarbon is used as a solvent, the reaction tag must be carried out in a reactor with a metal inner wall (stainless steel, ordinary carbon steel), and an enameled glass kettle must not be used

(4) Grounding of instruments, equipment and pipelines
.
The main function of grounding is to eliminate static electricity on the conductor, and the metal conductor should be directly grounded
.
In order to prevent spark discharge, the gap where spark discharge may occur should be bridged and grounded
.
Grounding of instruments, equipment, and pipelines is the simplest, most commonly used, and most basic anti-static measure, as shown in Figure 9-21
.
But it should be noted that it can only eliminate static electricity on the conductor, but not on the insulator
.

Figure 9-21 Anti-static measures

Also need to pay attention: ① The anti-static system must have an independent and reliable grounding device (equipotential bonding device), the grounding resistance should generally be less than 12Ω, and the embedding and testing methods should comply with the "Cement Concrete Pavement Construction and Acceptance Specification" (GBJ97-1987) Requirements; ②The anti-static ground wire must not be connected to the neutral wire of the power supply, and must not be shared with the lightning protection wire: ③The three-phase five-wire system is used for power supply, and the ground wire can be used as an anti-static ground wire, but the neutral wire and the ground wire must not be mixed ④The cross-sectional area of ​​the main grounding line should be no less than 100mm², and the cross-sectional area of ​​the branch trunk line should be no less than 6mm².
The grounding wires of equipment and workbenches should use multi-stranded plastic-coated wires with a cross-sectional area no less than 1.
25mm².
The color of the grounding wires should be yellow-green.
Appropriate
.

(5) Humidification
.
In order to prevent a large amount of static electricity, the relative humidity of the air should be above 50%; in order to improve the effect of static elimination, the relative humidity should be increased to 65% to 70%
.
When the relative humidity of the air is greater than 70%, an extremely thin water film is often formed on the surface of the object
.
The water film can dissolve carbon dioxide in the air, greatly reduce the surface resistivity, and accelerate the dissipation of static electricity.
However, it should be noted that the method of humidification is not suitable for preventing static electricity on the insulator in a high-temperature environment
.

(6) Anti-static work clothing and shoes
.
For flammable and explosive workplaces, staff should wear anti-static work clothes and shoes uniformly, and have strict requirements on all aspects of anti-static clothing and shoes and performance requirements
.
Anti-static clothing and shoes can eliminate hidden dangers of static electricity during the wearing process and avoid production accidents
.

(7) Static eliminator
.
Before entering the environment where the explosion limit may exist, or before touching the equipment and facilities, you need to remove the static electricity of the human body.
You can install a human body static eliminator at the door and touch it to fully discharge the human body, as shown in Figure 9-22 and Figure 9- 23 shown
.

Figure 9-22 Logo

Figure 9-23 Human body static eliminator

Not only should the equipment be grounded to remove static electricity, the human body should also be grounded statically
.
If only one side of the equipment is grounded in a certain environment, when the electrically charged human body (or object) in an insulated state approaches or touches the grounded body, a discharge spark will be generated; on the contrary, the grounded human body (or object) approaches or touches the isolated static electricity When it is a conductor, it will also produce spark discharge
.

(8) Thunder and lightning
.
For example, lightning, transportation thunder and ground thunder in the air are actually electrostatic discharge phenomena in nature, and the harm is also great
.
Lightning protection needs to be paid attention to when building design and implemented in accordance with relevant design and construction standards: At the highest part of the building, such as around the parapet wall, standard 25mm×4mm lightning-proof flat steel should be used, and the connecting wire needs to be grounded.
Over 2m, including the outdoor facilities such as exhaust fans on the roof of each laboratory, must be firmly grounded for lightning protection
.
Special attention should be paid to the anti-static system grounding wire of the instruments, equipment and pipelines in the laboratory, which can be connected to the leakage-proof grounding wire and connected to the equipotential bonding terminal box, but not to the lightning-proof grounding network wire
.