Function and working principle of safety valves

The safety valve serves as a safety protection device in the system. When the system pressure exceeds the specified value, the safety valve opens, releasing some of the gas in the system into the atmosphere so that the pressure remains below the allowable level, thereby preventing accidents caused by excessive pressure. A safety valve is also known as a relief valve. The diagram shows several typical structural forms of safety valves. Figure a shows a piston-type safety valve, whose valve core is a flat plate. The air source pressure acts on piston A; when this pressure exceeds the safe value determined by the spring force, piston A is pushed aside, and a portion of the compressed air is discharged into the atmosphere through the valve ； When the gas source pressure is below the safe value, the spring drives the piston downward to close the valve opening. Figures b and c show the ball-valve type and diaphragm type safety valves respectively, whose working principle is exactly the same as that of the piston type. All three types of safety valves use springs to provide the controlling force; by adjusting the pre-tension of the spring, it is possible to change the value of the safety threshold, which is why they are called direct-acting safety valves. Figure d shows a pilot-operated safety valve, in which a small direct-acting valve is used to apply control pressure to the diaphragm; the solid core on the diaphragm serves as the valve element, which presses against the valve seat. When the air source pressure A is greater than the safety pressure, the valve core opens, and compressed air is discharged into the atmosphere through the outlet on the left side. Diaphragm safety valves and pilot-operated safety valves have good pressure characteristics and are sensitive in operation ； However, the maximum opening force is relatively low, meaning the flow characteristics are poor. In practical applications, the type of safety valve should be selected based on actual needs, and its diameter should be chosen according to the maximum exhaust volume.

The opening height of a fully open safety valve is ≤0.25d0 (minimum discharge throat diameter). Its working principle is as follows: the spring force is greater than the normal pressure exerted by the medium on the valve core (a pre-determined pressure value), so the valve core remains in a closed position. When the pressure of the medium inside the tank exceeds the allowable level, the spring is compressed, causing the valve core to move away from the valve seat and the valve to open automatically, allowing the medium to flow out and thus reducing the pressure ； When the pressure returns to normal, the spring force pushes the valve core back against the valve seat, causing the valve to close automatically. The function of safety valves is to prevent the pressure of the medium in pipelines or devices from exceeding specified values, thereby achieving safety protection. A safety valve is a valve used for safety protection; its operating element remains in a closed position under external forces. When the pressure of the medium inside a device or pipeline rises above the specified value, the valve opens automatically, allowing the medium to be discharged outside the system thereby preventing the pressure from exceeding the set limit. The main parameter of a safety valve is its discharge capacity, which is determined by the diameter of the valve seat and the opening height of the valve disc. Depending on this opening height, they are classified into two types: slightly open type and fully open type. Slightly open type refers to a valve disc opening height of 1/40 to 1/20 of the valve seat throat diameter. Full opening means that the opening height of the valve disc is 1/4 of the throat diameter of the valve seat. The selection of a safety valve is determined by the operating pressure, which determines the nominal pressure of the valve; the operating temperature determines the temperature range in which the valve can be used. The set pressure value of the valve, as calculated, determines the set pressure range for the spring or lever. The material and structural design of the valve are chosen based on the medium being handled, and the throat diameter of the valve is calculated based on its discharge capacity. The following are the general rules for selecting safety valves. (l) Hot water boilers generally use unenclosed, wrench-operated slightly-opening safety valves. (2) Steam boilers or steam pipes generally use unenclosed, wrench-operated full-opening safety valves. (3) Incompressible media such as water generally use closed micro-tilt safety valves, or safety relief valves. (4) Closed full-open safety valves are generally used for high-pressure feed water, such as in high-pressure feed water heaters and heat exchangers. (5) Compressible media such as gases generally use closed full-opening safety valves, such as in gas storage tanks and gas pipelines. (6) Class E steam boilers generally use gravity-type safety valves. (7) Pulse-type safety valves are generally used in large-diameter, high-displacement, and high-pressure systems, such as temperature and pressure reduction devices and power station boilers. (8) Tank cars, road tankers, and storage tanks used for transporting liquefied gas generally employ internal safety valves. (9) Hydraulic safety valves are generally used at the top of oil tanks, and they need to be used in conjunction with breather valves. (10) Pilot-operated safety valves are generally used for underground drainage or gas pipelines.

A safety valve is a type of valve used for safety protection in closed systems. Its operating element remains in a closed position under the action of external forces; when the pressure of the fluid in the closed system rises and exceeds the value set by the safety valve, it opens automatically, allowing the fluid to be discharged outside the system in order to reduce the pressure and prevent damage caused by high pressures within the system. Safety valves belong to the category of automatic valves and are primarily used in boilers, pressure vessels, as well as in air conditioning and solar systems to ensure that pressure does not exceed specified levels, thereby playing a crucial role in protecting both human safety and the proper operation of equipment. 　　Terminology for safety valves: (1) Safety valve. An automatic valve. It relies on no external force, but rather uses the inherent properties of the medium itself to discharge a specified amount of fluid in order to prevent the pressure within the system from exceeding a predetermined safe level ； Once the pressure returns to normal, the valve closes again to prevent further flow of the medium. 　　(2) Direct-loaded safety valve. A safety valve that overcomes the force exerted by the pressure of the medium beneath the valve disc through directly applied mechanical loads, such as weights, lever-assisted weights, or springs. 　　(3) Safety valve with power assist device. This safety valve can be opened at a pressure lower than the normal operating pressure with the help of a power-assisting device. Even if this auxiliary device fails, such safety valves can still meet the standard requirements. 　　(4) Safety valve with supplementary load. This type of safety valve maintains a pressure with enhanced sealing at its inlet until the pressure reaches the opening pressure. This additional force (supplementary load) can be provided by an external energy source, and should be released reliably when the safety valve reaches its opening pressure. Its size should be set such that, assuming this additional force is not released, the safety valve can still achieve its rated discharge capacity as long as the inlet pressure does not exceed a certain percentage of the pressure at which it is specified to open according to regulations. 　　(5) Pilot-operated safety valve. A safety valve that relies on the discharge of medium from a pilot valve to be driven or controlled. The pilot valve itself should be a directly-loaded safety valve that meets standard requirements. 　　(6) Proportional safety valve. A safety valve that opens or closes proportionally over the entire range of opening heights, or over a fairly large range of opening heights. 　　(7) Full-open safety valve. A safety valve that opens proportionally only within a narrow range of slight opening, before suddenly opening to its full position. The opening height shall be not less than 1/4 of the channel diameter. 　　(8) Slightly opening safety valve. It is a direct-acting safety valve designed for use only in liquid media. The opening height is within the range of 1/40 to 1/20 of the channel diameter. 　　(9) Operating pressure (rated pressure). The inlet pressure at which the valve disc of a safety valve begins to rise under operating conditions; at this pressure, a measurable opening height is reached, and the medium is discharged continuously in a manner that can be detected visually or audibly. 　　(10) Discharge pressure. The inlet pressure when the valve disc reaches the specified opening height. The upper limit of the discharge pressure must comply with the requirements of **relevant standards or specifications. 　　(11) Exceeding pressure. The difference between the discharge pressure and the opening pressure is usually expressed as a percentage of the opening pressure. 　　(12) Seat-back pressure. It is the inlet pressure when the valve disc comes into contact with the valve seat again after discharge, that is, when the opening height becomes zero. 　　(13) Opening and closing pressure difference. The difference between the opening pressure and the closing pressure is usually expressed as a percentage of the closing pressure relative to the opening pressure; only when the opening pressure is very low is the difference between these two pressures used for representation. 　　(14) Back pressure. The pressure at the outlet of the safety valve. 　　(15) Rated discharge pressure. Standards specify the upper limit for the discharge pressure. 　　(16) Sealing test pressure. The inlet pressure used for the sealing test is the pressure at which the leakage rate through the sealing surface of the closing element is measured. 　　(17) Opening height. The actual lift of the valve disc from the closed position. 　　(18) Channel area. It refers to the minimum cross-sectional area of the flow path between the inlet end of the valve disc and the sealing surface of the closing element, and is used to calculate the theoretical displacement in the absence of any resistance effects. 　　(19) Channel diameter. The diameter applied to the flow channel area. 　　(20) Curtain area. The area of the cylindrical or conical passage formed between its sealing surfaces when the valve disc is above the valve seat. 　　(21) Emission area. The minimum cross-sectional area of the fluid passage when the valve is open. For fully disclosed safety valves, the discharge area is equal to the flow channel area ； For slightly opening safety valves, the discharge area is equal to the curtain area. 　　(22) Theoretical displacement. It is the calculated displacement of an ideal nozzle in which the cross-sectional area of the flow channel is equal to that of the safety valve’s flow channel. 　　(23) Displacement coefficient. The ratio of the actual displacement to the theoretical displacement. 　　(24) Rated displacement coefficient. The product of the displacement coefficient and the reduction coefficient (taken as 0.9). 　　(25) Rated displacement. It refers to the portion of the actual displacement that is permitted to be used as a basis for applying safety valve regulations. 　　(26) Equivalent calculated displacement. It refers to the calculated discharge capacity of a safety valve when conditions such as pressure, temperature, and medium properties are identical to those applicable to its rated discharge capacity. 　　(27) Frequency hopping. The safety valve disc moves back and forth rapidly and abnormally, coming into contact with the valve seat during this movement. 　　(28) Flutter. The safety valve disc moves back and forth rapidly and abnormally, without making contact with the valve seat during this movement.

Which foreign manufacturers produce both safety valves and breather valves?