Safety device is a device to protect the safe operation of equipment or production device and prevent explosion under abnormal conditions. All production devices with the risk of combustion and explosion shall be equipped with corresponding safety devices.

1、 Explosion proof pressure relief device

Pressure relief device: it is an overpressure protection device.

Function: when the container operates under normal working pressure, it shall be kept tight without leakage. Once the pressure in the container exceeds the requirements, it can automatically, rapidly and sufficiently discharge the gas inside the container, so that the pressure in the container is always kept within the maximum allowable pressure range. At the same time, it also has the function of automatic alarm to make the operator take measures.

Type of pressure relief device

According to the structure, it can be divided into valve type, fracture type, melting type and combination type.

1

Valve type safety relief device -- common safety valve

Characteristic:

·Only discharge the pressure higher than the specified part in the container, when the pressure in the container drops to the normal operating pressure, it will automatically close.

·In this way, the production interruption and waste caused by the overpressure of the container which must discharge all the gas can be avoided, so it is widely used.

Disadvantages:

·Poor sealing;

·Because of the inertia action of spring, the opening of valve often lags behind;

·When used for some unclean gas, the valve port may be blocked or the valve disc may be stuck.

Bursting discs are used for medium and low pressure vessels; explosion-proof caps are mostly used for high and ultra-high pressure vessels.

Characteristic:

·Good sealing performance;

·The pressure relief reaction is fast;

·The pollution in the gas has little influence on it.

Disadvantages:

·After pressure relief, it can't be used any more, and the container must be stopped and replaced;

·Generally, it is only used on vessels with less possibility of overpressure and unsuitable to install valve type safety relief devices.

Features: it is through the melting of fusible alloy, so that the gas of the container is discharged from the hole originally filled with fusible alloy and the pressure is discharged.

Application: it is mainly used to prevent overpressure of container due to temperature rise. It is generally used for liquefied gas cylinder.

4

Combined safety relief device

·Combination of valve type and fracture type; it has the advantages of valve type and fracture type, that is, it can prevent the leakage of valve type safety relief device, and it can make the container continue to operate after discharging too high pressure. When the vessel is overpressured, the bursting disc is broken and the safety valve is opened for exhaust. When the pressure drops to the normal pressure, the safety valve is closed and the vessel continues to operate.

·Combination of valve type and melting type. Pressure relief device.

Introduction to pressure relief facilities

Pressure relief equipment includes safety valve, bursting disc, explosion-proof cap, explosion-proof door, explosion-proof ball valve, vent valve, etc.

1

Safety valve

Safety valve can be divided into static weight type, lever type and spring type according to its structure and action principle.

The selection of safety valve shall be determined according to the working pressure, temperature and medium characteristics of pressure vessel. During selection, in addition to correct selection, pay attention to its pressure range and its exhaust volume.

The installation of safety valve shall pay attention to the following points:

① The newly installed safety valve shall be attached with the product qualification certificate. Before installation, it shall be recalibrated, lead sealed and the safety valve calibration report shall be issued;

② The safety valve shall be installed vertically on the gas-phase interface of the vessel or pipeline;

③ The outlet of the safety valve shall be free of resistance to avoid back pressure. If a discharge pipe is installed, its inner diameter shall be larger than the outlet diameter of the safety valve. Attention shall be paid to antifreeze at the outlet of the safety valve. For containers containing inflammable, toxic and highly toxic media, the discharge pipe shall be directly connected to the outdoor safe place or have proper treatment facilities. It is not allowed to install any valve in case of discharge;

④ No valve shall be installed between the pressure equipment and the safety valve. For containers containing flammable, explosive, toxic or viscous media, in order to facilitate the replacement and cleaning, the block valve can be installed, and its structure and size shall not hinder the normal operation of the safety valve. During normal operation, the stop valve must be fully opened and sealed with lead;

⑤ The cross section of the short connecting pipe between the safety valve and the boiler pressure vessel shall not be less than the flow cross section of the safety valve. Several safety valves shall be on one connecting pipe at the same time, and the cross section of the connecting pipe shall not be less than 1.25 times of the total cross section of the safety valve.

2

Bursting disc (bursting disc, bursting disc)

Function: it is mainly used to discharge the pressure generated by chemical explosion of gas, steam or dust in the equipment. It is generally used in the following situations:

① The spring type safety valve is not suitable for the occasion of explosion or abnormal reaction which makes the pressure rise rapidly;

② Do not allow any leakage of medium, and the safety valve generally has a small amount of leakage;

③ A large number of precipitates or adherents are produced in the production process, which hinders the normal operation of the safety valve.

3

The explosion-proof door (window) is actually an explosion-proof flap.

It is generally installed on the outer wall of combustion furnace (chamber). The total area of the explosion-proof door is generally no less than 250cm2 / m3 (calculated according to the net volume of the combustion chamber). In order to prevent people from being burned or injured by the opened cover when the combustion gas is ejected, the explosion-proof door shall be installed at the place where people do not often go, and the height shall not be less than 2m.4

There are many styles of explosion-proof cap. Its main component is a thick wall short pipe with one end closed and a weak section in the middle. When the pressure in the vessel exceeds the specified value, so that the tensile stress on the weak section reaches the strength limit of the material, the explosion-proof cap will break from this point, and the gas will be discharged from the pipe hole.

The structure of explosion-proof cap is simple and easy to manufacture, and the error of explosion pressure is small, so it is easy to control.

It is suitable for ultra-high pressure vessels, because the safety relief device of ultra-high pressure vessels does not need a large discharge area, and the explosion pressure is high, the weak section of the explosion-proof cap can have a large thickness, which is easy to manufacture.

5

Vent pipe

In case of severe reaction of reaction materials, the equipment that can not prevent the occurrence of over temperature, over pressure, explosive agglomeration and decomposition explosion accident by strengthening cooling and reducing feeding shall be equipped with automatic or local manual emergency vent pipe.

Safety liquid seal

·Safety liquid seal is generally installed between gas pipeline and equipment with pressure lower than 0.02MPa (gauge pressure). The commonly used safety liquid seals are open type and closed type.

·Principle of liquid seal fire resistance: it is to use non combustion liquid to block.

·Water drilling is a kind of safety liquid seal, which is installed on the sewage pipe network with combustible gas, flammable liquid vapor or oil pollution in petrochemical enterprises to prevent combustion explosion from spreading along the sewage pipe network.

·The water seal pressure in the water seal well shall not be less than 2.45kpa.

In the high-temperature equipment, combustion chamber, high-temperature oxidation furnace and high-temperature oxidizer that are easy to cause combustion and explosion, between the pipelines conveying flammable gas and flammable liquid vapor, as well as on the containers, pipelines and exhaust pipes of the equipment for flammable liquid and flammable gas, flame arresters are often used to block the fire.

Type: corrugated metal flame arrester, gravel flame arrester, etc.

Fire extinguishing principle: when the flame passes through the narrow pores, it will be extinguished because the heat loss suddenly increases, making the combustion unable to continue.

The factors that affect the performance of flame arrester are the thickness of flame retardant layer and the size of pores or channels.

The critical diameters of some flammable gas and flammable liquid vapor fire orifices are as follows: methane: 0.4-0.5m; hydrogen and acetylene: 0.1-0.2m; gasoline and natural oil gas: 0.1-0.2m.

Sand, pebble, glass ball, iron chips and copper chips are used as filling materials to separate the space in the device into many small pores, which has good fire resistance effect, especially the effect of preventing carbon disulfide flame is better.

The diameter of gravel is 3-4mm, and it can also be used as glass ball or small clay ring filler and metal ring.

Fire retardant materials: generally aluminum, copper and stainless steel.

The size of the inner diameter of the flame arrester and the length of the shell are determined by the diameter of the pipe in which the flame arrester is installed.

Inner diameter of flame arrester: generally 4 times of the diameter of the pipeline where the flame arrester is installed.

4

Check valve

One way valve is also called check valve and check valve. In production, there are lift type, swing plate type and ball type check valves.

Function of one-way valve: it only allows the fluid to flow in a certain direction, and automatically closes in case of backflow, which can prevent the high pressure from flowing into the low-pressure system and causing pipeline, container and equipment explosion.

5

Fire damper

Fire stop gate: it is set to prevent the flame from spreading along the ventilation pipe. Under normal circumstances, the fire gate is open under the control of the fusible metal elements. Once the temperature rises and the fusible metal melts, the gate can automatically close to prevent the fire from spreading along the pipeline to the upper and lower layers or adjacent rooms.

Fusible elements: generally, lead, tin, lead, pickaxe, mercury and other metals are used with low-temperature fusible elements, and the temperature of saishuo, nylon, plastic and other materials when they lose strength can also be used as the control temperature of the action of the fire damper.

6

Mars extinguisher

Mars extinguisher is also called fire cap. Generally, it is installed on the equipment and devices that produce Mars or on the exhaust pipe of motor vehicles to prevent the flying Mars from igniting flammable and explosive substances.

Basic principles

·The smoke with Mars enters into large volume from small volume, resulting in pressure reduction and flow velocity slowing down, settling down the heavy Martian particles instead of flying out of the exhaust duct.

·Set obstacles, change the direction of smoke flow, increase the distance Mars takes, and make Mars go out or sink.

·Set grid, impeller, etc. to block or disperse the larger Mars, so as to speed up the extinction of Mars.

·Use water spray or steam to extinguish Mars, etc.

Boiler chimneys or other chimneys using blowers. Sparks flying out of the chimneys may cause a fire hazard. A spark extinguishing device can be installed on the chimneys.

7

Safety device management

Various safety devices shall be maintained and repaired after a certain period of operation. For example, the wire mesh in the flame arrester and Mars extinguisher is easy to be damaged and corroded, so it should be repaired in time; the operating pressure of the safety valve should be checked regularly to keep it in good condition.

There are many kinds of alarm and interlock devices used in petrochemical production, which can be summarized as follows:

1. Component automatic control interlocking;

2. Temperature control interlock;

3. Pressure control interlocking;

4. Liquid level self regulating interlock;

5. Ignition source cut-off interlock;

6. Automatic fire extinguishing interlock;

7. Automatic material cutting, automatic venting, automatic power cutting and automatic stop interlocking;

8. Automatic fire alarm and other audible and visual alarm.