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Overview of safety valves: Safety valves represent a rather special category within the family of valves. What makes them special is that, unlike other valves which merely serve to open and close, safety valves are designed primarily to protect the safety of equipment. A safety valve is a special type of valve that remains in a closed state when its moving parts are under external force. When the pressure of the medium inside a device or pipeline rises above the specified value, it releases the medium outside the system in order to prevent the pressure from exceeding that specified level. Safety valves belong to the category of automatic valves and are primarily used in boilers, pressure vessels, and pipelines to ensure that pressure does not exceed specified levels, thereby playing a vital role in protecting both human safety and the proper operation of equipment. Safety valves must undergo pressure testing before they can be used. With the rapid development of China’s economic construction, there are an increasing number of equipment projects that involve pressure control. Given the need to relieve pressure from the equipment, safety valves play a crucial role in protecting it. By 2013, there were over 700 manufacturers and traders of various types of safety valves, with more than 200 inquiries for transactions each day, indicating a clear increase in demand and supply in the safety valve market. Safety valves are widely used in: steam boilers, liquefied petroleum gas road tankers or railway tank cars, oil wells, high-pressure bypass systems in steam power generation equipment, pressure pipelines, pressure vessels, and more. Types of safety valves: The diameter of safety valves is generally not large; those commonly used range from DN15mm to DN80mm. Those with a diameter exceeding 150mm are usually referred to as large-diameter safety valves. Safety valves are divided into fully open safety valves and slightly open safety valves. Based on their overall structure and loading mechanism, safety valves can be divided into four types: gravity-type, lever-type, spring-type, and pilot-type. Depending on the method of connection to the pipeline, they can be further divided into threaded safety valves and flanged safety valves. A lever type is more suitable when the operating temperature is high but the pressure is not very high ； Spring-type types are recommended for high-voltage equipment ； Spring-loaded safety valves are commonly used. Installation method: As part of the commissioning of the entire equipment system, the valve seat is often damaged when the safety valve opens for the first time, due to the presence of dirt and debris in the system. To ensure that foreign contaminants do not enter the safety valve, the entire system should be thoroughly flushed before the safety valve is installed. The safety valve should be installed in a location where dirt, rust, and debris cannot accumulate. In steam applications, it is also important to reduce the tendency to leakage by installing safety valves; such valves should be installed in such a way that condensate does not accumulate upstream of the valve disc. As shown in the figure below, this can be achieved by installing the safety valve above the steam pipeline. When the safety valve is properly installed above the steam pipeline, its inlet pipe can serve to drain water automatically. The temperature safety valve should be installed on the top or side of the water heater. Precautions for installation: (1) A new safety valve should come with a product certification; it must be calibrated before installation, sealed with lead, and a calibration certificate for the safety valve should be issued. (2) The safety valve shall be installed vertically at the gas-phase interface of the container or pipeline. (3) The outlet of the safety valve should have no resistance to flow, in order to avoid backpressure. If a drain pipe is installed, its inner diameter must be larger than that of the safety valve’s outlet. Care must be taken to prevent freezing at the valve’s discharge point. For containers holding flammable or toxic, highly toxic substances, the drain pipe must lead directly to a safe location outside or to a facility where such substances can be properly handled; no valves shall be installed on the drain pipe. (4) No valves shall be installed between the pressure-bearing equipment and the safety valve. For containers holding flammable, explosive, toxic, or viscous media, a globe valve may be installed to facilitate replacement and cleaning; its structure and diameter shall not interfere with the proper operation of the safety valve. During normal operation, the stop valve must be fully open and sealed with a lead seal. (5) For pressure vessels containing flammable, explosive, or toxic media, the medium discharged by the safety valve must be equipped with safety devices and a recovery system. Lever-type safety valves must be installed in a vertical position, and spring safety valves should also be installed vertically to avoid affecting their operation. During installation, attention should also be paid to coordination, the coaxiality of the parts, and ensuring that each bolt is subjected to even stress. The cross-sectional area of the connecting pipe between the safety valve and the boiler or pressure vessel must not be smaller than the flow cross-sectional area of the safety valve. When all safety valves are installed on a single pipe, the cross-sectional area of that pipe should be at least 1.25 times the total flow cross-sectional area of those safety valves. Collection of installation experiences (1): When there are both gas and liquid phases in the container, the safety valve should be installed in the gas phase portion. (2) When a safety valve is used to vent flammable liquids, its outlet shall be connected to the accident storage tank. When the material being discharged is a high-temperature flammable substance, the receiving container should be equipped with appropriate protective measures. (3) Ordinary safety valves can be vented on-site; the vent outlet should be at least 1 meter above the operators, and it must not face any open flames, spark-producing areas, or high-temperature equipment within a radius of 15 meters. The vent openings for the safety valves of indoor equipment and containers should lead outside the roof, at a height of more than 2 meters above it. (4) When there is a isolation valve at the inlet of the safety valve, it should be in an open position and sealed with lead to prevent errors. (5) The cross-sectional area of the connection pipes and fittings between the safety valve and the boiler or pressure vessel must not be smaller than the inlet cross-sectional area of the safety valve ； When several safety valves share one inlet pipe, the cross-sectional area of the inlet pipe must be no smaller than the sum of the inlet cross-sectional areas of the safety valves. (6) Generally, no shut-off valves or steam extraction pipes shall be installed between the safety valve and the boiler’s drum or header. It is generally not advisable to install stop valves or other outlet pipes between the safety valve and the pressure vessel. For pressure vessels containing highly toxic, highly hazardous, moderately hazardous, flammable, corrosive, viscous media, or valuable media, a stop valve may be installed between the safety valve and the pressure vessel only with the approval of the technical responsible person in charge of pressure vessels at the user facility, and after reliable preventive measures have been established. During the normal operation of the pressure vessel, the stop valve must remain fully open and sealed or locked. The structure and bore size of the stop valve must not prevent the safe discharge of the safety valve. (7) Spring-loaded safety valves with threaded connections shall be connected to threaded short pipes, while the short pipes shall be welded to the cylinder and the header. (8) The safety valve must be equipped with a discharge pipe. The exhaust pipe should avoid bends and sharp turns as much as possible to minimize resistance. The exhaust pipe must lead to a safe location and have sufficient cross-sectional area to ensure smooth exhaust flow. Safety valves that can interact to undergo chemical reactions cannot share a single discharge pipe. When a safety valve is installed on equipment exposed to corrosive or flammable gases, anti-corrosion or fire and explosion prevention measures should also be taken during discharge ； When the equipment equipped with a safety valve contains toxic substances, and the vapor density of these substances is greater than that of air, the substances and vapor discharged from the safety valve must be directed into a closed system, where they can be recovered and reused in production processes. (9) The safety valve discharge pipe must be secured to prevent excessive additional stress on the safety valve or vibration. (10) Safety valves installed outdoors shall be equipped with reliable measures to prevent the water contained in the fluid inside the valve from freezing when the temperature drops below 0°C, thereby avoiding any impact on the valve’s ability to discharge. (11) When the crystallization temperature of the medium in the safety valve is higher than the lowest ambient temperature, the safety valve must be equipped with a thermal insulation jacket, along with insulation purge steam, to prevent the medium from crystallizing and blocking the safety valve, thereby avoiding disruptions to its proper functioning. The inlet and outlet pipes of the safety valve must also be designed with steam insulation jackets or additional insulated steam tracing to prevent the medium from crystallizing and blocking the pipes. (12) The safety valve shall be equipped with a device to prevent the weight from moving on its own and guide rails to prevent the lever from going out of place; spring-type safety valves shall have a lifting handle and a device to prevent the adjustment screws from being turned arbitrarily. (13) It should be installed vertically at the highest point of the boiler and the header. No steam extraction outlet pipes or valves shall be installed between the safety valve and the boiler drum or header. (14) For boilers with a rated steam pressure of 3.82 MPa or less, the throat diameter of the safety valve shall not be less than 25 mm ； For boilers with a rated steam pressure greater than 3.82 MPa, the throat diameter of the safety valve should not be less than 20 mm. (15) The safety valve of a pressure vessel should preferably be installed directly at the highest point of the vessel’s body. The safety valve of a liquefied gas storage tank must be installed in the gas phase section. Generally, a short pipe can be used to connect to the container; in such cases, the diameter of this safety valve’s short pipe should be no smaller than the valve diameter of the safety valve itself. (16) Boilers with a rated evaporation capacity of more than 0.5 t/h shall be equipped with at least two safety valves ； Boilers with a rated evaporation capacity of 0.5 t/h or less must be equipped with at least one safety valve. Safety valves must be installed at the outlet of the fractional economizer and at the outlet of the steam superheater. (17) The outlet of the safety valve should have no resistance to flow, in order to avoid backpressure. If a drain pipe is installed, its inner diameter must be larger than that of the safety valve’s outlet. Measures must be taken to prevent freezing at the valve’s discharge point. For containers containing flammable or toxic, highly toxic substances, the drain pipe must lead directly to a safe location outside or to a facility where such substances can be properly treated; no valves shall be installed on the drain pipe. Compilation of safety valve maintenance: Strengthen daily maintenance to keep it clean and prevent corrosion as well as blockages caused by oil, dirt, and debris ； Regularly check the lead seals to prevent others from moving the weight of the lever-type safety valve at will or turning the adjustment screws of the spring-type safety valve. To prevent the valve from sticking to its seat, a regular manual discharge procedure (by pulling or lifting) should be established based on the actual conditions of the pressure-bearing equipment. For example, the safety valve of a steam boiler’s drum should generally be manually discharged once a day, with the discharge pressure ideally being at least 80% of the specified maximum operating pressure ； If a leak is detected, it should be repaired or replaced promptly; it is strictly prohibited to attempt to eliminate the leak by increasing the load (such as moving the weight of a lever-type safety valve or over-tightening the adjustment screws of a spring-type safety valve). The safety valve must undergo a regular inspection at least once a year. The inspection includes dynamic testing and disassembly testing. The steps of dynamic testing are divided into assembly, pressurization (to operating pressure), pressure holding (no leakage for 3–5 minutes), pressurization activation (immediate action at the opening pressure), depressurization and return to rest, and further pressure holding (remaining at operating pressure for 3 minutes with no leakage after returning to rest).