Function and working principle of safety valve

Function and working principle of safety valve:

The function and working principle of the relief valve abstract: relief valve is a kind of static imports from open automatic pressure protection device, it is one of the pressure vessel for important safety accessories, its function is: when the container pressure exceeds a certain value, depending on the medium itself pressure automatically open the valve, quick discharge amount of medium. When the pressure inside the container to the allowable values, valve automatically shut down again, the container pressure is always lower than allowed 2002: machine tools, the wto is a challenge but also opportunity experts point out that to increase the intensity of research and development of CNC machine tools in our country, quicken the development of affordable nc machine tools, service competition in the machine tool manufacturing industry in China China milling machine and machining center status quo and outlook of the market, technical level and development trend of domestic and international lathe, machining center production, demand and development trend of the world, the development trend of machine tools at home and abroad, the development trend of numerical control system, machining technology and the development of CNC machine tools, product technology development trend of modern machine tools, Our country present situation and development trend of high speed machining technology, the use of advanced manufacturing technology improve car piston, level of processing on the cap and bottle mould special thread CNC, turning deep hole thread processing and CNC lathe cutting method analysis and application of grinding worm grinding wheel dressing, a new method of grinding precision parts more steps to track the grinding technology of crankshaft connecting rod journal.

　　Air compressor safety valve is a kind of automatic pressure relief protection device opened by imported static pressure, it is one of the important safety accessories of pressure vessel, its function is: when the pressure in the vessel exceeds a certain value, the pressure of the medium itself automatically open the valve, quickly discharge a certain amount of medium. When the pressure in the container falls to the allowable value, the valve will automatically close, making the pressure in the container always lower than the upper limit of the allowable pressure and automatically preventing accidents that may occur due to overpressure. Therefore, the safety valve is also known as the Z final protection device of the pressure container. For LPG storage tank safety valve used in spring type air compressor valve, its working principle is: the spring force is greater than the medium ACTS on the valve core of normal pressure (pressure value of the selected beforehand), valve core is closed, as the tank pressure exceed the permitted pressure medium, the spring is compressed, the valve core to leave the seat, the valve automatically open, medium from releasing, decompression; When the pressure returns to normal, the spring pressure pushes the spool back to the seat and the valve closes automatically.　

Structural characteristics of safety valve

Pressure vessel generally refers to the airtight vessel used in industrial production to complete the process of reaction, mass transfer, heat transfer, separation and storage, and can withstand pressure. It is widely used in petroleum, chemical, energy, metallurgy, machinery, textile, medicine, national defense and other industrial fields.

1. Basic knowledge of pressure vessels

1) operating conditions of pressure vessels

(1) pressure. Pressure in a pressure vessel may come from two sources, one from outside the pressure vessel, and the other from inside the pressure vessel.

The Z high working pressure of pressure vessel refers to the Z high pressure which may appear on the top of the pressure vessel in the normal use of the pressure vessel if the pressure vessel is under internal pressure. For the pressure vessel under external pressure, it refers to the Z high pressure that may appear on the top of the jacket in the normal use of the pressure vessel. The design pressure of the pressure vessel refers to the pressure used to determine the shell thickness of the vessel at the corresponding design temperature, that is, the design pressure of the vessel marked on the nameplate, whose value shall not be less than the working pressure of Z. When the static pressure of the liquid column in each part of the container or the pressurized element is up to 5% of the design pressure, the sum of the design pressure and the static pressure of the liquid column shall be taken for the design calculation of the part or element. For pressure vessels equipped with safety relief devices, the design pressure shall not be lower than the opening pressure or blasting pressure of the safety relief devices. The design pressure of the container shall be determined in accordance with the corresponding provisions of GB 150.　

(2) temperature. Metal temperature refers to the average temperature of the pressurized container elements along the thickness of the section. Under no circumstances shall the surface temperature of the element metal exceed the allowable temperature of the steel.

Design temperature refers to the metal temperature of the pressurized element set under the corresponding design pressure under normal operation of the container, and its value shall not be lower than the high metal temperature of the element metal which may be reached; For metal temperatures below 0~C, the design temperature shall not be higher than the Z low metal temperature that the element metal may reach. The design temperature of the container (that is, the design medium temperature marked on the vessel nameplate) refers to the design temperature of the shell.

(3) medium. There are many kinds of process media involved in the production process, and there are many classification methods. According to the state of matter, there are gases, liquids, liquefied gases, elemental substances and mixtures; According to the classification of chemical characteristics, there are combustible, flammable, inert and combustion four; According to their degree of human poisoning, can divide again extremely (I), the hazards (Ⅱ) (Ⅲ), mild, moderate, harm (Ⅳ) level.

Flammable medium: it refers to the gas with the lower limit of explosion less than 10% mixed with air, or the difference between the upper limit and lower limit of explosion is greater than or equal to 20%, such as methylamine, ethane, ethylene, etc.

Toxic medium: the pressure vessel safety technical supervision regulation (hereinafter referred to as "content regulation") for the medium

The classification of sexual degree refers to GB 5044 "classification of occupational exposure to toxic substances", which is divided into four levels. The high allowable Z concentrations were: extreme hazard (class I)< 0.1 mg/m3; High hazard (Ⅱ) 0.1 ~ < 1.0 mg/m3; 3. O ~ <10mg/m3; 3. ≥10mg/m3. < p = "" >

When the medium in the pressure vessel is a mixed substance, it shall be determined by the composition of the medium and according to the classification principle of the degree of toxicity or inflammable medium. The degree of toxicity of the medium shall be determined by the process design department of the design unit or the production technology department of the using unit.

Corrosive medium: the petrochemical medium is required to be corrosion-resistant to the materials used for pressure vessels. Sometimes due to impurities in the medium, so that the corrosion dramatically increased. The types and properties of corrosive media are different, and the corrosive properties of the media are different due to the different process conditions. This requires that in the selection of pressure vessel materials, in addition to meeting the requirements of mechanical properties under the conditions of use, but also have enough corrosion resistance, when necessary, but also to take certain anti-corrosion measures.　　

2) classification of pressure vessels.

There are many classification methods for pressure vessels. In order to facilitate the safety technical supervision and management, the pressure vessels are divided into three categories in the code of contents.

(1) one of the following situations is category iii pressure vessel: high pressure vessel; Medium pressure vessels (only toxic and highly hazardous media); Medium pressure storage container (flammable or moderately toxic medium only, with pV greater than or equal to 10MPa) M3); Medium pressure reaction vessel (flammable or moderately toxic medium only, with pV greater than or equal to o.5 mpa) M3); Low pressure vessel (only for media with extreme and high degree of toxicity and with pV greater than or equal to o.2 mpa? M3); High pressure, medium pressure tube shell type waste heat boiler; Medium pressure vessel lined with glass; Pressure vessels made of materials of a higher strength grade (i.e. the lower limit of tensile strength specified in the corresponding standard is greater than or equal to 540MPa); Mobile pressure vessels include railway tank trucks (whose medium is liquefied gas and cryogenic liquid), tank trucks (whose medium is liquefied gas and cryogenic liquid) and tank containers (whose medium is liquefied gas and cryogenic liquid). Spherical storage tank (volume greater than or equal to 50m3); Cryogenic liquid storage container (volume > 5m3).

(2) one of the following situations is the second type of pressure vessel: medium pressure vessel; Low-pressure vessels (only toxic and highly hazardous media); Low-pressure reaction vessels and low-pressure storage vessels (only flammable or moderately toxic media); Low pressure shell type waste heat boiler; The low pressure vessel is lined with glass.

(3) low-pressure vessels are the first type of pressure vessels.　

2. Secure attachment

1) relief valve

Safety overflow valve is an automatic pressure relief valve opened by inlet static pressure. It depends on the pressure of the medium itself to discharge a certain amount of fluid, in order to prevent the pressure in the container or the system from exceeding the predetermined safety value; When the pressure in the container returns to normal, the valve closes itself and prevents the medium from continuing to drain. Safety valve is divided into full - opening safety valve and micro - opening safety valve. According to the overall structure and loading method of the safety valve can be divided into four types: static weight type, lever type, spring type and pilot type.

2) blasting

The bursting disc device is a kind of non-heavy closed pressure relief device, which is used to release the medium when the bursting disc is compressed by the static pressure at the entrance, so as to prevent the pressure in the container or system from exceeding the predetermined safety value.

Blasting disc, also known as blasting film or explosion-proof film, is a kind of fracture type safety release device. Compared with the safety valve, it has the characteristics of simple structure, fast response to pressure relief, good sealing performance and strong adaptability.

3) combination of safety valve and bursting disc device

When the safety valve is combined with the bursting disc device in parallel, the calibrated bursting pressure of the bursting disc shall not exceed the design pressure of the vessel. The opening pressure of the safety valve should be slightly lower than the nominal bursting pressure of the bursting disc.

When the bursting disc device is installed in series between the safety valve inlet and the container, the following conditions shall be satisfied: the relief capacity of the combination of safety valve and bursting disc device shall meet the requirements; The release area of the bursting disc after rupture shall not be less than the area at the entrance of the safety valve, and the broken fragments of the bursting disc shall not affect the normal operation of the safety valve. A pressure gauge, cock, vent or alarm indicator should be installed between the bursting disc device and the safety valve to check whether the bursting disc is broken or leaking.

When the bursting disc device is installed in series at the outlet side of the relief valve, the following conditions shall be satisfied: the medium in the container shall be clean and free from sticking or blocking substances; The relief valve's releasing capacity should meet the requirements; When there is back pressure between the safety valve and the bursting disc, the valve can still be opened accurately under the opening pressure. The discharge area of the bursting disc shall not be less than the inlet area of the safety valve; An air discharge pipe or blowdown pipe shall be arranged between the safety valve and the bursting disc device to prevent pressure accumulation in this space.

4) blasting cap

The blasting cap is closed at one end and has a short tube with thick wall with weak section in the middle. The blasting pressure error is small and the discharge area is small. It is mostly used for ultra-high pressure vessels. When the tensile stress on the weak surface reaches the strength limit of the material, the fracture occurs. Due to the influence of temperature, it is generally made of high strength steel materials (such as 34crNi3Mo, etc.) with stable heat treatment performance and little change with temperature. The ratio of the bursting pressure to the material strength is generally 0.2~0.5.

5) fusible plug

Fusible plug belongs to the "melting type" (" temperature type ") safety relief device, its action depends on the temperature of the container wall, it is mainly used for small pressure vessels of medium and low pressure, and it is more widely used in cylinders containing liquefied gas.

6) emergency cut-off valve and pressure reducing valve　

Emergency cut-off valve is a special structure and special purpose valve, which is usually installed in series with the globe valve on the medium outlet pipe close to the container, in order to stop the leakage in case of a large number of pipeline leakage; In general, it also has the performance of overcurrent closure and overtemperature closure, and can be operated independently at short range and remote range. Emergency cut-off valve can be divided into mechanical (or manual) traction type, hydraulic control type, pneumatic control type and electric control type according to the different operating modes.

Pressure-reducing valve is the use of diaphragm, spring, piston and other sensitive elements to change the gap between the disc and the seat, when the medium through throttling, pressure drop and its decompression valve.

When adjusting bolt tightening down, spring is compressed, push the diaphragm downward, top drive pulse valve disc, part of the high side of medium enters after high-pressure channel, the pulse valve clearance between disc and seat into the annular channel into the cylinder, push the piston down and open the main valve disc, then the high side then medium from the main valve clearance between disc and seat through the throttling decompression. At the same time, the low voltage side part of the medium low pressure into the space below the diaphragm channels, as the medium pressure due to the high side pressure and increase the elasticity of the spring enough to offset the diaphragm upward push pulse valve disc gradually closed, reduces the medium into the cylinder, piston and the main valve disc moving up, the main valve down, thus reducing the flow to the low voltage side of the interface quality, make the pressure of the low voltage side is not due to the high side pressure, so as to achieve the goal of automatically adjust the pressure.

7) pressure gauge, thermometer and liquid level meter　

(1) pressure gauge. The pressure gauge is an instrument indicating the pressure of the medium in the pressure vessel and an important safety device for the pressure vessel. According to its structure and function principle, the pressure gauge can be divided into liquid column type, elastic element type, piston type and electricity type four categories. The piston type manometer is usually used as a standard instrument for calibration. The liquid column manometer is generally only used for measuring very low pressure. Pressure vessels are widely used with various types of elastic element manometers.

(2) liquid level meter. Liquid level gauge, also known as liquid level gauge, is used to observe and measure the change of liquid level position in the container. Especially for the container containing liquefied gas, the level gauge is an essential safety device.

(3) thermometer. Thermometer is used to measure the cold and hot degree of material instrument, can be used to measure the temperature of pressure vessel medium, for the need to control the wall temperature of the container, also must be installed to test the wall temperature thermometer.