Safety valves are safety accessories for boilers, pressure vessels and other pressurized equipment that can effectively prevent an explosion caused by overpressure of the production plant. Therefore, the safety valve in the chemical plant is very important. To thoroughly understand the safety valve, you have to start from its classification, characteristics, selection and installation requirements.
Classification and features safety valve
1. According to the overall structure and loading agencies
Safety valve according to their overall structure and loading mechanism can be divided into heavy hammer lever, spring and pulse three.
Three types of safety valve, the more common is the use of spring-loaded safety valve.
① hammer lever safety valve
Hammer lever safety valve is the use of weights and levers to balance the role of the valve flap force.
According to the principle of leverage, it can use the weight of the smaller hammer by increasing the lever to obtain greater force, and by moving the weight of the position (or change the mass of the hammer) to adjust the relief valve opening pressure.
advantage:
Hammer lever safety valve structure is simple, easy to adjust and more accurate, the added load will not increase due to the valve has a greater increase, suitable for high temperature applications, used to be more common in the past, especially Is used in boilers and high temperature pressure vessels.
Disadvantages:
However, heavy hammer leveraged safety valve structure is relatively bulky, loading mechanism is easy to vibration, and often due to vibration and leakage; its back seat pressure is low, not easy to close after opening and maintain tight.
② spring-type safety valve
Spring-loaded safety valve is the use of compression spring force to balance the role of the valve flap force.
The amount of compression of the helical coil spring can be adjusted by turning the adjusting nut above it, with which it is possible to correct the opening (setting) pressure of the safety valve as required.
advantage:
The spring-loaded safety valve is lightweight and compact, with high sensitivity and unlimited mounting position. It is also suitable for mobile pressure vessels because of its low sensitivity to vibration.
Disadvantages:
The added load changes with the opening of the valve, that is, as the valve flap increases, the amount of compression of the spring increases, and the force acting on the valve flap also increases. This is a detriment to the quick opening of the safety valve.
In addition, the spring on the valve will reduce the spring force due to the long-term high temperature. When used on a container with a high temperature, the structure is complicated by the fact that the problem of heat insulation or heat dissipation of the spring is often taken into consideration.
③ pulse safety valve
The pulse safety valve is composed of a main valve and a sub-valve. The main valve is driven by the pulse of the sub-valve. The structure of the valve is complicated and usually only applies to boilers and pressure vessels with large safety discharge volume.
2 according to the different media emissions
In accordance with the different media discharge, safety valve can be divided into fully enclosed, semi-closed and open three.
① fully enclosed safety valve
Fully enclosed safety valve exhaust, the gas emissions through the exhaust pipe, the media can not leak outside, mainly for the media for toxic, flammable gas containers.
② semi-closed safety valve
Part of the gas discharged from the semi-enclosed safety valve through the exhaust pipe, but also part of the leakage from the gap between the valve cover and stem, and more for the medium is not polluting the environment of the gas container.
③ open safety valve
Open valve cover is open, so that the spring chamber with the atmosphere, which will help reduce the temperature of the spring, mainly for the medium is steam, and the atmosphere does not produce pollution of hot gas containers.
3. Press the valve to open the maximum height and flow ratio
Divided according to the ratio of the maximum height of the valve disc opening and the diameter of the safety valve runner, the safety valve is mainly divided into two types of safety valve and the fully open relief valve.
① micro-type safety valve
Micro-Kai safety valve opening height is less than 1/4 of the diameter of the runner, usually 1/40 a runner diameter 1/20.
Micro-Kai-type safety valve action is proportional action type, mainly for liquid applications, and sometimes also for small emissions gas occasions.
② Kai-type safety valve
Full-opening safety valve opening height greater than or equal to the diameter of the channel 1/4. The full relief valve discharge area is the smallest cross-sectional area of the valve seat throat.
The action process belongs to two sections of action type, must be resorted to a lift mechanism to achieve full open, all-purpose safety valve is mainly used for gas media occasions.
According to the principle of action
According to the principle of action can be divided into direct-acting safety valve and non-direct-acting safety valve.
① direct-acting safety valve
Direct-acting safety valve is open in the direct role of the working medium, that is, rely on the role of the working medium pressure to overcome the loading mechanism imposed on the valve disc mechanical load, the valve open.
This safety valve has the advantages of simple structure, quick action and good reliability. But because of relying on the structure of the load, the load size is limited, can not be used for high pressure, large diameter of the occasion.
② non-direct-acting safety valve
This type of safety valve can be divided into pilot-operated safety valve, safety valve with power-assisted device.
Pilot-operated safety valve:
Is driven or controlled by the medium exiting the pilot valve, which itself is a direct-acting safety valve and sometimes other types of valves.
Pilot-type safety valve for high pressure, large diameter occasions. The main valve of the pilot-operated safety valve can also be designed to seal against the working medium or to apply a much greater mechanical load to the valve disc than the direct-acting safety valve, thus providing a good seal.
At the same time, its movements are rarely affected by back pressure. The disadvantage of this type of safety valve is that its reliability is related to the main valve and the pilot valve. The operation is not as fast, reliable and complex as the direct acting safety valve.
Safety valve with power assist:
With the help of a power-assisted device, the safety valve is forced to open below normal cracking pressure.
This safety valve is suitable for opening pressure is very close to the working pressure of the occasion, or need to open the safety valve on a regular basis to check or blow off the adhesive, frozen media occasions. At the same time, a means of forcibly opening the safety valve in an emergency situation is also provided.
Selection instructions Safety valve
1. Safety valve to determine the various parameters
① determine the nominal pressure relief valve.
According to the valve material, the working temperature and the maximum working pressure selected nominal pressure.
② determine the working pressure of safety valve rating.
According to the design pressure of the pressure vessel and the design temperature of the selected working pressure level, the working pressure of the safety valve and the working pressure of the spring have different meanings.
Safety valve working pressure refers to the safety valve in normal operation before the valve withstand the static pressure, it is the protection of the system or equipment working pressure is the same.
The working pressure level of a spring refers to the working pressure range that a certain spring allows, in which the relief valve opening pressure (ie, the setting pressure) can be adjusted by changing the preload of the spring.
The same nominal pressure relief valve, according to the spring design requirements, can be divided into a variety of different working pressure levels.
Selection of safety valve, should be based on the required opening pressure value to determine the working pressure of the valve.
③ determine the safety valve discharge pressure Pd
Relief valve discharge pressure is generally 1.1 times the setting pressure (cracking pressure), steam boiler relief valve discharge pressure of 1.03 times the set pressure.
④ determine the safety valve diameter
According to the required emissions to determine the safety valve discharge capacity ≥ required emissions.
The amount of emissions required for a protected system is the amount that must be excluded in order to prevent overpressure when abnormal overpressure occurs in the system. It is determined by factors such as working conditions and capacity of the system or equipment, as well as possible overpressure.
⑤ material to determine
Selection of safety valve material should consider the medium temperature, working pressure, media properties and material technology, economy and other factors.
2. Safety valve to determine the special structure
① For safety valve with opening pressure of more than 3MPa steam or gas valve with medium temperature exceeding 320 ℃, safety valve with radiator (fin) should be used.
② To bear the additional back pressure of the safety valve, and its back pressure changes more than 10% of the set pressure should be selected bellows safety valve.
In addition to the use of corrosive media safety valve, in order to prevent the spring and the steering mechanism by medium corrosion, bellows safety valve should also be used.
③ For flammable, extremely toxic or highly hazardous media must use a closed safety valve, if you need to use a lifting mechanism, you should use a closed safety valve with a wrench.
④ on the air, hot water above 60 ℃ or steam and other non-hazardous media, safety valve with a wrench should be used.
⑤ liquefied tank (tank) car, should be built-in safety valve.
⑥ for the large amount of discharge conditions, the whole Kai-style should be used; for the working pressure is stable, the amount of small discharges should be used micro-Kai-style; for high pressure, large amount of discharge conditions, should use non-direct start, Such as pulse relief valve; for the container length of more than 6m should be set two or more safety valve.
⑦ low pressure fixed containers, static weight can be used (pressure cooker) or lever heavy hammer safety valve. Mobile devices should be spring-loaded safety valves.
⑧ for the medium thick and easy to plug, should choose a safety valve and bursting disc series combination of the release device.
3 safety valve use
Installation location and requirements safety valve
1. Installation location?
① vertical upward installation.
② installation location, as far as possible to be protected equipment or pipelines.
③ installed in easy to repair and adjustment of the place, there is enough space around.
④ pressure vessel safety valve, installed in the container above the liquid level gas space, can also be installed in the connection point is located in the pressure vessel gas space pipe.
⑤ Contain flammable, toxic, viscous medium containers, equipment, can be installed in the safety valve before the closing valve, but the closing area of the valve shall not be less than the minimum flow area of the safety valve, and the installation of seals to ensure that the globe valve In full, normally open state.
⑥ safety valve to be installed with the reverse valve reverse to reduce the impact of gravity from the valve to avoid stress fatigue and emissions vibration fatigue.
⑦ For the safety valve which may be clogged or corroded by material, a rupture disc shall be provided in front of the entrance of the safety valve and a check valve shall be installed between the safety valve and the rupture disc to adopt anti-purge, heat tracing or heat preservation etc. on its inlet pipe Anti-blocking measures.
⑧ installed in the pipeline safety valve should be set in the fluid pressure is relatively stable and a certain distance from the source of fluctuations in place, can not be installed at the dead ends of the horizontal pipe.
⑨ For liquid medium pipes, heat exchangers or pressure vessels, etc., when the valve is closed may appear thermal expansion, resulting in pressure holding back the problem, the set safety valve can be installed horizontally, direct discharge of liquid down.
⑩ air emergency discharge, nozzle should be smooth, not sharp, no glitches to prevent electrostatic discharge.
⑪ safety valve installation point, the safety valve can not withstand excessive back pressure value, should be within the allowable range.
⑫ safety valve body, to support a smooth.
⑬ The medium is a container of extreme and highly hazardous or flammable and explosive media. The outlet of the safety valve should be brought to a safe place and disposed of properly.
When two or more safety valves share a single discharge pipe, the discharge pipe shall have a cross-sectional area not less than the sum of the cross-sectional areas of all safety valve outlets, but oxygen or combustible gas and other two gases that can chemically react with each other may not be shared Root discharge pipe.
2. Installation requirements?
① For corrosive media, the safety valve should be installed in combination with the rupture disk.
② For highly toxic media, to choose a good seal of the safety valve type.
③ for high temperature medium safety valve, high temperature on the spring greater impact, should avoid the use of spring-loaded safety valve.
④ For important safety valve, or a safety valve susceptible to fire, to set the spray protection.
⑤ For spherical tank safety valve, to install double safety valve, and any one of the relief valve pressure relief capability, can meet the safety requirements of the spherical tank pressure relief.
⑥ safety valve to set the static line across.
⑦ In order to prevent repeated opening and closing safety valve, chatter, damage the valve, to reduce the pressure relief valve inlet pipe, which increases the inlet diameter, shorten the inlet pipe.
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