Case Study | Reason Analysis and Countermeasures of Styrene Storage Tank Sag Deformation

Analysis of Deformation Accident of Metal Atmospheric Pressure Fixed Top Storage Tank

Metal atmospheric pressure fixed top storage tank is a widely used storage tank in the chemical industry, mainly used to store various chemical raw materials and products. During use, if the safety facilities of the storage tank are not complete or the operation is improper, it is easy to have equipment accidents such as tank roof and tank body depression deformation. Therefore, analyzing the causes of tank depression deformation and proposing preventive measures are of great significance to the safe production of atmospheric pressure storage tanks.

Accident process

The accident enterprise is a private composite material manufacturer in Nanpu Development Zone, Tangshan City. It owns a production device of 20,000 tons/year unsaturated polyester resin and 3.60 million square meters/year lighting board.

The accident happened at around 8:00 in the morning. The operator started the styrene transfer pump to deliver styrene to the dilution kettle. At this time, the storage tank stored styrene about 40 cubic meters. During the transportation process, the styrene storage tank suddenly made a loud noise and deformed. The operator took measures to stop the pump and stop the delivery of styrene. Since there was only one styrene storage tank, the accident caused the enterprise to be unable to continue production, causing considerable economic losses to the enterprise.

The effective volume of the accident storage tank is 100 cubic meters. It is made of stainless steel and is equipped with a liquid level gauge and a temperature gauge. The top of the tank is equipped with a flame arrester, a breathing valve and a spray water cooling facility. The temperature of the storage tank is around 30 ° C.

Incident analysis

On-site inspection found that the inlet and outlet metal mesh flame arrester of the breathing valve at the top of the tank were severely blocked, and in addition, the positive pressure valve disc of the breathing valve was glued to the valve seat and could not be switched on and off.

During the delivery of styrene on the day of the incident, the liquid level dropped, and negative pressure was formed in the tank, requiring a large amount of inhaled air. Due to the serious blockage of the inlet and outlet metal mesh flame arrester of the breathing valve, the amount of inhaled air could not be replenished in time, and the negative pressure of the storage tank continued to increase, eventually causing the storage tank to sag and deform.

Through the on-site inspection of the accident storage tank, the company's styrene storage tank has the following problems in terms of safety facilities:

• The temperature control facilities do not meet the requirements, and only use the method of spraying water on the top of the tank to control the temperature. The tank is not kept cold, and no refrigerated circulating water cooling device is installed. The temperature of styrene in the tank is controlled at 30 ° C, which is much higher than the suitable storage temperature of styrene at 18 ° C.
• Nitrogen protection settings are not set to prevent air from entering the storage tank during external delivery, creating conditions for styrene polymerization.
• The tank pressure gauge is not set, and the operator cannot know the tank pressure during the process of entering and exiting the tank, which is also one of the important reasons for the depression deformation of the tank.

Reasons for the formation of styrene polymers and prevention methods

Check that the blockage on the metal mesh flame arrester of the breathing valve is styrene polymer. The main cause of this accident is that the styrene gas has polymerized in the flame arrester and breathing valve, which blocks the metal mesh flame arrester of the breathing valve, causing the respiratory effect of the storage tank to fail.

The conditions for polymerization of styrene during storage and transportation and the measures to be taken to prevent polymerization are analyzed.

Properties of styrene

Styrene, molecular formula C8H8, molecular weight 104.14, is a colorless and transparent liquid with a strong unpleasant odor, flammable, specific gravity 0.909, boiling point 145.2 ° C, flash point 31 ° C (open cup method), ignition point 490 ° C, explosion limit 1.1% -6.1% (volume). Styrene is an unsaturated aromatic hydrocarbon, and under the action of oxygen in the air, a slow thermally excited polymerization reaction can occur at room temperature.

Method for controlling styrene polymerization

• Increase the polymerization inhibitor. The commonly used polymerization inhibitor is tert-butyl-o-bisphenol (TBC), which is generally added in an amount of 10-50 ppm.
• Control the storage temperature. The appropriate temperature for styrene storage is 18 ° C. To control the temperature, the styrene storage tank should be kept cold and a refrigerated circulating water device should be installed.
• Install a nitrogen protection device. The nitrogen protection of the styrene storage tank can effectively prevent the oxygen in the air from contacting the styrene in the storage tank, prevent the accumulation of styrene condensation and polymerization in the breathing valve and flame arrester, and cause blockage. At the same time, the nitrogen seal can also reduce the evaporation of styrene vapor and avoid polluting the environment.
• Reduce the influence of iron ions. Rust has a greater impact on the amount of polymer formed in styrene products, because in the presence of oxygen, styrene generates peroxides through the catalytic action of Fe2 + ions, which promote the formation of polymers with increasing temperature.

Measures to prevent styrene storage tank depression

Analyze a number of tank depression deformation accidents that have occurred in recent years. Most of them are related to the properties of the materials in the tank. Among them, materials with high viscosity, easy condensation and easy polymerization are most likely to cause blockage or failure to open the breathing valve. In addition, it is also greatly related to the sudden drop in material storage temperature and the influence of weather. In order to prevent the depression deformation of atmospheric pressure storage tanks, the following safety measures are recommended:

1. Set up corresponding safety facilities according to the nature of the material and safety requirements, so that the material can be stored under suitable temperature and pressure operating conditions.
2. To reduce the possibility of material polymerization, a nitrogen sealing device is added to the storage tank to prevent the polymerization of styrene on the one hand, and to reduce the risk of fire and explosion in the storage tank on the other hand.
3. Increasing the pressure detection setting of the storage tank can detect the abnormality of the pressure of the storage tank in time and take prompt measures; increasing the interlocking of the pump with low and low pressure stops to avoid the occurrence of tank depression deformation accidents.
4. Strictly implement the regular inspection system of respiratory valves, increase the inspection frequency for respiratory valves prone to condensation and polymer materials, and regularly conduct disassembly inspections of respiratory valves.
5. Strictly control the transportation speed of materials to prevent excessive changes in liquid level from causing overpressure (negative pressure) in the storage tank.