Quick understanding of safety valve
Usually, the ordinary valve only plays the role of a switch, while the safety valve can also play the role of protecting equipment and preventing accidents in addition to the role of a switch, it is a very important safety attachment in special design. Today, yaang will take everyone to learn about safety valves.
Classification and characteristics of safety valves
Classified by overall structure and loading mechanism
The safety valve can be divided into three types according to its overall structure and loading mechanism: hammer lever type, pulse type and spring type. The spring safety valve is commonly used.
①. Hammer lever safety valve
The weight lever safety valve uses the weight Hammer and the lever to balance the force acting on the valve flap. According to the lever principle, it can use a heavy hammer with a small weight to obtain a larger acting force through the increasing effect of the lever, and move the position of the heavy hammer (or change the mass of the heavy hammer) to adjust the opening pressure of the safety valve.
Advantages: The weight lever safety valve has simple structure, easy adjustment and more accurate, and the load added will not increase greatly due to the rise of the valve flap. It is suitable for occasions with higher temperature, in the past, it was widely used, especially in boilers and pressure vessels with higher temperature.
Disadvantages: but the structure of the heavy hammer lever safety valve is relatively heavy, the loading mechanism is easy to vibrate, and often leaks due to vibration; Its back-to-seat pressure is low, and it is not easy to close and keep tight after opening.
②. Pulse type safety valve
The pulse type safety valve is composed of the main valve and the auxiliary valve. The action of the main valve is driven by the pulse effect of the auxiliary valve.
③. Spring type safety valve
Spring safety valve uses the force of compression spring to balance the force acting on the valve flap. The compression amount of spiral ring spring can be adjusted by rotating the adjustment nut on it, and this structure can be used to correct the opening (setting) pressure of the safety valve as needed.
Advantages: The spring safety valve has a light and compact structure, high sensitivity, unlimited installation position, and can be used on a mobile pressure vessel because of its low sensitivity to vibration.
Disadvantages: the additional load will change with the opening of the valve, that is, with the increase of the valve flap, the compression amount of the spring will increase, and the force acting on the valve flap will also increase. This is unfavorable to the rapid opening of the safety valve. In addition, the spring on the valve will reduce its elasticity due to the long-term influence of high temperature. When used in containers with higher temperature, the heat insulation or heat dissipation of the spring is often considered, thus complicating the structure.
According to the ratio of the maximum height of the valve flap and the runner.
According to the ratio of the maximum open height of the valve flap to the diameter of the flow channel of the safety valve, the safety valve is mainly divided into full lift safety valve and micro start safety valve.
①. full lift safety valve
The opening height of the full lift safety valve is greater than or equal to 1/4 of the diameter of the runner. The discharge area of the full lift safety valve is the minimum sectional area of the throat of the valve seat. Its action process belongs to two-section action type, which requires the help of a force lift mechanism to achieve full opening. The full lift safety valve is mainly used in gas medium occasions.
②. Micro-start safety valve
The opening height of the micro starting safety valve is less than 1/4 of the diameter of the flow tube, usually 1/40 of the diameter of the flow tube. Its working process is in a proportional mode, mainly used in liquid occasions, and sometimes in gas occasions with small emissions.
Classification according to media emission methods
According to the different discharge modes of media, safety valves can be divided into three types: open type, semi-closed type and complete type.
①. Open safety valve
The cover of the open safety valve is open, so that the spring chamber is connected to the atmosphere, which is conducive to reducing the temperature of the spring, mainly suitable for the medium of steam, and the high temperature gas vessels that do not produce pollution to the atmosphere.
②. Semi-closed safety valve
Some of the gas emitted by the semi-enclosed safety valve passes through the exhaust pipe and some leaks out from the gap between the valve cover and the valve stem. It is mostly used for gas containers that will not pollute the environment.
③. Fully enclosed safety valve
When the fully enclosed safety valve is exhausting, all the gas is discharged through the exhaust pipe, and the medium cannot be leaked outward, which is mainly used for containers where the medium is toxic and flammable gas.
Classification according to action principle
Can be divided into non direct action type safety valve and direct action type safety valve.
①. Non-direct acting safety valve
This kind of safety valve can be divided into safety valve with power auxiliary device, pilot safety valve.
Safety valve with power auxiliary device: With the help of a power auxiliary device, the safety valve is forced to open under the normal opening pressure. It is suitable for occasions where the opening pressure is very close to the working pressure, or occasions where the safety valve needs to be opened regularly to check or blow off the adhesive or frozen medium. At the same time, it also provides a means to forcibly open the safety valve in an emergency.
Pilot-operated safety valve: It depends on the media discharged from the pilot valve to drive or control, while the pilot valve itself is a direct action safety valve, and sometimes other forms of valve. Suitable for high pressure and large diameter occasions. The main valve of pilot-operated safety valve can also be designed to rely on the working medium to seal the form, or it can impose a much larger mechanical load on the valve flap than the direct action safety valve, so it has a good sealing performance. At the same time, its movement is rarely affected by back pressure. The disadvantage of this safety valve is that its reliability is related to the main valve and guide valve. The action is not as fast and reliable as the direct action safety valve, and the structure is more complex.
②. Direct acting safety valve
Direct acting safety valve is opened under the direct action of working medium, that is, relying on the action of working medium pressure to overcome the mechanical load of loading mechanism added to the valve flap and make the valve open.
This safety valve has the advantages of simple structure, rapid operation and good reliability. But because it depends on the structure to load, the load size is limited, and it can not be used in the occasions of high pressure and large diameter.
Instructions for safety valve selection
Determination of various parameters of safety valve
①. Determine the nominal pressure of safety valve
Nominal pressure is selected according to working temperature, valve material and maximum working pressure.
②. Determine the working pressure grade of the safety valve
The working pressure grade is selected according to the design temperature and design pressure of the pressure vessel. The working pressure grade of the spring has different meanings from the working pressure of the safety valve. The working pressure level of the spring refers to the working pressure range allowed by a certain spring. Within this pressure range, the opening pressure (I .e. the set pressure) of the safety valve it can be adjusted by changing the pretightening shrinkage of the spring. The working pressure of the safety valve refers to the static pressure borne by the valve before the normal operation of the safety valve, which is the same as the working pressure of the protected system or equipment. The safety valve of the same nominal pressure, according to the spring design requirements, can be divided into a variety of different working pressure levels. When the safety valve is selected, the working pressure level of the valve should be determined according to the required opening pressure value.
③. Determine the discharge pressure of the safety valve Pd
The discharge pressure of the steam boiler safety valve is 1.03 times of the set pressure, and the discharge pressure of the safety valve is generally 1.1 times of the set pressure (opening pressure).
④. Determination of material
Select the material of the safety valve should consider the working temperature of the medium, working pressure, medium performance and material process economy and other factors.
⑤. Determine the diameter of the safety valve
According to the necessary emissions to determine, the discharge capacity of the safety valve ≥ necessary emissions. The necessary emission of the protected system refers to the amount that must be eliminated to prevent the system from being over-pressed when abnormal overpressures occur. It is determined by the working conditions of the system or equipment, capacity and possible factors such as causing overpressure.
Determination of special structure of safety valve
- ①. The safety valve with a radiator (FIN) should be selected for the safety valve for steam or the safety valve for gas with medium temperature exceeding 320℃.
- ②. For medium which is extremely flammable and toxic or highly harmful, closed safety valve must be adopted. If it is needed to adopt the safety valve with lifting mechanism, closed safety valve with wrench should be adopted.
- ③. To the safety valve bearing the additional back pressure, and when the change of the back pressure exceeds 10% of the set pressure, Bellows safety valve should be used. In addition to the use of corrosive media safety valve, in order to prevent the spring and guide mechanism from medium corrosion, also should use bellows safety valve.
- ④. Liquefying tank (tank) truck should adopt built-in safety valve.
- ⑤ for air, hot water above 60℃ or steam and other non-hazardous Media, safety valve with wrench should be used.
- ⑥. Static type (Pressure Cooker) or lever hammer type safety valve can be used for fixed containers with low working pressure. Mobile equipment should use spring safety valve.
- ⑦. For the condition with large discharge capacity, the full-start type should be selected; For the condition with stable working pressure and small discharge capacity, the micro-start type should be selected; For the vessel length over 6m, two or more safety valves should be set; for working conditions with high pressure and large discharge capacity, non-direct starting type, such as pulse type safety valve, should be selected.
- ⑧. As for the medium which is thicker and easy to be blocked, it is appropriate to choose the series combined relief device of relief valve and rupture disk.
Instructions for safety valve selection
- ①. For highly toxic media, the safety valve type with good sealing performance should be selected.
- ②. For corrosive medium, the safety valve should be installed in combination with the blasting plate.
- ③. Spray protection should be set for important safety valves or safety valves easily affected by fire.
- ④. For high temperature dielectric safety valve, high temperature has great influence on spring, so spring safety valve should be avoided.
- ⑤. The safety valve should be set with static cross-line.
- ⑥. In order to prevent the safety valve from repeatedly starting and closing, causing flutter and damaging the valve, the pressure drop of the safety valve inlet pipeline should be reduced, that is, the inlet diameter should be enlarged and the inlet pipeline should be shortened.
- ⑦. For the safety valve used for the spherical tank, double safety valves should be installed, and the pressure relief capacity of any of the safety valves can meet the safety relief requirements of the spherical tank.
- ①. The installation location should be as close as possible to the protected equipment or pipeline.
- ②. Vertical upward installation.
- ③. It is installed in a place where it is easy to repair and adjust, and there is enough space around it.
- ④. For containers and equipment containing flammable, toxic and viscous media, a stop valve can be installed in front of the safety valve, but the circulation area of the stop valve shall not be less than the minimum circulation area of the safety valve, and install the lead seal to ensure that the stop valve is in the state of all open and normally open.
- ⑤ the safety valve of the pressure vessel is installed in the gas phase space above the liquid level of the container itself, or it can be installed on the pipeline at the connection point located in the gas phase space of the pressure vessel.
- ⑥. For the safety valve which may be blocked or corroded by materials, a rupture disk should be set before the entrance, and a check valve should be installed between the safety valve and the rupture disk, and the reverse blowing should be adopted on the Inlet Pipeline, anti-blocking measures such as heat tracing or heat preservation.
- ⑦. The safety valve should be installed reversely with the stop valve to reduce the influence of valve gravity and avoid stress fatigue and vibration fatigue.
- ⑧. The safety valve installed on the pipeline should be set at the place where the fluid pressure is relatively stable and there is a certain distance from the fluctuation source, but not at the dead angle of the horizontal pipeline.
- ⑨. For emergency discharge, the nozzle should be flat and not sharp, without burrs to prevent electrostatic discharge.
- ⑩. For pipelines, heat exchangers or pressure vessels of liquid media, when the valve is closed, the thermal expansion may occur, which may cause the problem of high pressure suppression, the set safety valve can be installed horizontally to discharge the liquid directly downward.
- ⑪. The valve body of safety valve should be supported steadily.
- ⑫. The safety valve installation point, can not make the safety valve to withstand too much back pressure value, should be within the prescribed allowable range.
- ⑬. The medium is the container of extremely and highly hazardous or flammable and explosive medium, and the outlet of the safety valve should be led to a safe place and be properly handled.
When more than two safety valves share the same drain pipe, the cross-sectional area of the drain pipe should not be less than the total cross-sectional area of all the safety valves, but oxygen or combustible gas and other two kinds of gas can produce chemical reaction with each other can not share a discharge pipe.
Source: China Safety Valves Manufacturer – Yaang Pipe Industry (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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