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Fully open safety valves are mainly used in applications with gaseous media. Slightly opening safety valves are mainly used in liquid applications, and sometimes in gas applications with very low discharge volumes. I. Classification and Characteristics of Safety Valves 1. Classification by Overall Structure and Loading Mechanism Safety valves can be divided into three types based on their overall structure and loading mechanism: weight-lever type, pulse type, and spring type. Spring-loaded safety valves are more commonly used. ①Weight-lever type safety valve: A weight-lever type safety valve uses a weight and a lever to balance the force acting on the valve disc. Based on the principle of levers, it can use a weight with a smaller mass to generate a greater force through the amplifying effect of the lever, and adjust the opening pressure of the safety valve by changing the position of the weight (or its mass). Advantages: The weight-lever type safety valve has a simple structure, is easy and relatively accurate to adjust, and the load applied does not increase significantly as the valve disc rises. It is suitable for high-temperature environments and was widely used in the past, especially in boilers and pressure vessels operating at high temperatures. Disadvantages: However, the structure of the weight-lever type safety valve is relatively bulky; the loading mechanism tends to vibrate, and leaks often occur as a result of these vibrations ； It has a low return pressure, and once opened, it is difficult to close and maintain a tight seal. ②Pulse safety valve: A pulse safety valve consists of a main valve and an auxiliary valve; the pulse action generated by the auxiliary valve drives the main valve to operate. It has a complex structure and is typically used only in boilers and pressure vessels that require a large amount of safety venting capacity. ③Spring-loaded safety valve: A spring-loaded safety valve uses the force of a compressed spring to counteract the force acting on the valve disc. The compression amount of the helical coil spring can be adjusted by turning the adjustment nut on it; this mechanism allows the opening (setting) pressure of the safety valve to be corrected as needed. Advantages: The spring-type safety valve is lightweight and compact, has high sensitivity, and its installation location is not restricted. Additionally, due to its low sensitivity to vibrations, it can be used in mobile pressure vessels. Disadvantage: The applied load changes as the valve opens; that is, as the valve disc rises, the amount of compression of the spring increases, and the force acting on the valve disc also increases accordingly. This is detrimental to the rapid opening of the safety valve. Additionally, the spring on the valve may lose its elasticity due to prolonged exposure to high temperatures. When used on containers with high temperatures, the insulation or heat dissipation of the spring often needs to be considered, which complicates the design. 2. Classification based on the ratio of the maximum opening height of the valve disc to the flow channel: Safety valves are primarily divided into full-opening safety valves and slightly-opening safety valves, according to the ratio of the maximum opening height of the valve disc to the diameter of the safety valve’s flow channel. ① Full-open safety valve: The opening height of a full-open safety valve is greater than or equal to 1/4 of the diameter of the flow channel. The discharge area of a fully open safety valve is the minimum cross-sectional area at the valve seat throat. Its operating mechanism is of two-stage type; a lifting mechanism is required to achieve full opening. Full-opening safety valves are mainly used in applications involving gaseous media. ② Slightly opening safety valve: The opening height of a slightly opening safety valve is less than 1/4 of the diameter of the flow channel, usually ranging from 1/40 to 1/20 of the diameter of the flow channel. Its operating mechanism is proportional, and it is mainly used in liquid applications; sometimes it is also used in applications involving gases in very small quantities. 3. Classification by medium discharge method: Depending on the way in which the medium is discharged, safety valves can be divided into three types: open-type, semi-closed type, and fully closed type. ①Open-type safety valve: The valve cover of an open-type safety valve is open, allowing the spring chamber to be in contact with the atmosphere; this helps to reduce the temperature of the spring. It is mainly suitable for containers containing steam or other high-temperature gases that do not cause pollution to the atmosphere. ②Semi-enclosed safety valve: The gas discharged by a semi-enclosed safety valve passes partly through the exhaust pipe, while some of it leaks out through the gap between the valve cover and the valve stem; it is commonly used in containers containing gases that do not pose a risk of environmental contamination. ③Fully enclosed safety valve: When a fully enclosed safety valve releases gas, all of the gas is discharged through the exhaust pipe, preventing any leakage of the medium; it is mainly used in containers containing toxic or flammable gases. 4. Classified by principle of operation, they can be divided into non-direct-acting safety valves and direct-acting safety valves. ①Safety valves of the non-direct-acting type can be divided into safety valves with power assist devices and pilot-operated safety valves. Safety valve with power assist device: A power assist device is used to force the safety valve to open at a pressure lower than the normal opening pressure. It is suitable for situations where the opening pressure is very close to the operating pressure, or where the safety valve needs to be opened regularly for inspection or to remove adhered or frozen media. At the same time, it also provides a means to force the safety valve to open in emergency situations. Pilot-operated safety valve: It is driven or controlled by the medium discharged from the pilot valve, which itself is a direct-acting safety valve; sometimes other types of valves are also used. Suitable for high-pressure and large-diameter applications. The main valve of a pilot-operated safety valve can also be designed to be sealed by the working medium, or a much greater mechanical load can be applied to the valve disc than in a directly-acting safety valve, thereby achieving excellent sealing performance. At the same time, its operation is little affected by back pressure. The disadvantage of this type of safety valve is that its reliability depends on the main valve and pilot valve; it does not operate as quickly or reliably as directly-acting safety valves, and its structure is more complex. ②Direct-acting safety valve: A direct-acting safety valve opens under the direct action of the working medium; that is, it relies on the pressure of the working medium to overcome the mechanical load applied to the valve disc by the loading mechanism, thereby causing the valve to open. This type of safety valve has advantages such as a simple structure, rapid operation, and good reliability. However, due to reliance on structural loading, its load capacity is limited, making it unsuitable for high-pressure and large-diameter applications. II. Guidelines for Selecting Safety Valves 1. Determination of various parameters of safety valves ① Determine the nominal pressure of the safety valve: The nominal pressure is selected based on the operating temperature, the material of the valve, and the maximum operating pressure. ②The operating pressure class of the safety valve is determined based on the design temperature and design pressure of the pressure vessel; the operating pressure class of the spring has a different meaning from that of the safety valve’s operating pressure. The operating pressure range of a spring refers to the range of working pressures for which a particular spring can be used; within this range, the opening pressure of the safety valve (i.e., the set pressure) can be adjusted by changing the degree of pre-compression of the spring. The operating pressure of a safety valve refers to the static pressure acting on it before the valve during normal operation; this pressure is identical to the operating pressure of the system or equipment being protected. Safety valves with the same nominal pressure can be divided into various operating pressure classes depending on the requirements of their spring design. When selecting a safety valve, the operating pressure class of the valve should be determined based on the required opening pressure value. ③Determine the discharge pressure Pd of the safety valve: For steam boilers, the discharge pressure of the safety valve is 1.03 times the set pressure, while generally, it is 1.1 times the set pressure (opening pressure). ④When determining the material for a safety valve, various factors such as the operating temperature and pressure of the medium, the properties of the medium itself, the processability of the material, and cost considerations must be taken into account. ⑤The diameter of the safety valve is determined based on the required discharge volume, with the discharge capacity of the safety valve being ≥ the required discharge volume. The discharge volume necessary for the protected system refers to the amount that must be removed in order to prevent overpressure when the system experiences abnormal pressure levels; it is determined by factors such as the operating conditions and capacity of the system or equipment, as well as those that could cause overpressure. 2. Determination of the special structure of safety valves: ① For safety valves used with steam at a discharge pressure greater than 3 MPa, or for safety valves used with gases at temperatures exceeding 320°C, safety valves equipped with radiators (fins) should be selected. ②For flammable media with extremely or highly hazardous toxicity, closed safety valves must be used; if a valve equipped with a lifting mechanism is required, a closed safety valve with a wrench should be employed. ③For safety valves subjected to additional back pressure, and when the change in this back pressure exceeds 10% of the set pressure, diaphragm safety valves should be used. Furthermore, for safety valves used with corrosive media, in order to prevent the springs and guiding mechanisms from being corroded by the medium, diaphragm safety valves should also be selected. ④Liquefied gas tankers should be equipped with built-in safety valves. ⑤For non-hazardous media such as air, hot water above 60°C, or steam, safety valves equipped with wrenches should be used. ⑥For fixed containers with low operating pressure, gravity-type (pressure cookers) or lever-weight type safety valves can be used. Mobile devices should be equipped with spring-loaded safety valves. ⑦For conditions with high discharge rates, a fully open type should be selected ； For operating conditions with stable working pressure and low discharge volume, a slightly open type is advisable ； For containers with a length of over 6 meters, two or more safety valves should be installed ； For high-pressure conditions with large discharge volumes, a non-direct-acting type, such as a pulse safety valve, is recommended. ⑧For media that are thick and prone to clogging, a discharge device consisting of a safety valve in series with a burst disc is recommended. III. Guidelines for Selecting Safety Valves 1. Installation requirements ① For highly toxic media, safety valve types with good sealing performance should be selected. ②For corrosive media, the safety valve should be installed in combination with a burst disc. ③Spray protection should be provided for important safety valves, or those that are susceptible to fire. ④For safety valves used with high-temperature media, high temperatures have a significant impact on the springs; therefore, spring-type safety valves should be avoided. ⑤An anti-static bridge wire should be installed on the safety valve. ⑥To prevent the safety valve from opening and closing repeatedly, which can cause fluttering and damage to the valve, it is necessary to reduce the pressure drop in the pipeline leading to the safety valve; this is achieved by increasing the diameter of the inlet pipe or shortening the length of the inlet section. ⑦For safety valves used on spherical tanks, two safety valves should be installed, and the pressure relief capacity of either of these valves must be sufficient to meet the tank’s safety pressure relief requirements. 2. Installation location ①: The installation site should be as close as possible to the equipment or pipeline that is to be protected. ②Install vertically upward. ③It should be installed in a location that is easy to maintain and adjust, with sufficient space around it. ④For containers and equipment used to hold flammable, toxic, or viscous substances, a globe valve can be installed in front of the safety valve; however, the flow area of this globe valve must not be smaller than the minimum flow area of the safety valve. It should also be sealed with lead to ensure that it remains in an open position at all times. ⑤The safety valve of a pressure vessel is installed in the gas phase space above the liquid level within the vessel body, or it can be installed on a pipe whose connection point is located in the gas phase space of the pressure vessel. ⑥For safety valves that may be blocked or corroded by materials, a burst disc is installed in front of their inlet, and a check valve is placed between the safety valve and the burst disc. Anti-clogging measures such as backwashing, heating, or insulation are applied to the inlet pipes. ⑦The safety valve should be installed in the opposite direction to the stop valve, in order to reduce the influence of the valve’s own weight and prevent stress fatigue as well as vibration-induced fatigue during discharge. ⑧Safety valves installed on pipelines should be placed in areas where the fluid pressure is relatively stable and at a certain distance from sources of pressure fluctuations; they must not be installed in dead corners of horizontal pipelines. ⑨For emergency air discharge, the pipe outlet must be smooth and not sharp, with no burrs, to prevent static discharge. ⑩For pipelines, heat exchangers, or pressure vessels that use liquid media, when valves are closed thermal expansion may occur, leading to high pressure buildup; in such cases, the safety valve can be installed horizontally to allow the liquid to flow out directly downward. ⑪The valve body of the safety valve must be supported stably. ⑫The installation point of the safety valve must not subject it to excessive back pressure; it should remain within the specified allowable range. ⑬For containers containing extremely hazardous or highly hazardous, as well as flammable and explosive substances, the outlet of the safety valve should be led to a safe location and properly managed. When two or more safety valves share a single discharge pipe, the cross-sectional area of that pipe must be at least equal to the sum of the cross-sectional areas of the outlets of all the safety valves. However, oxygen or flammable gases, as well as any two gases that can react with each other chemically, cannot share a single discharge pipe. IV. Failure modes of safety valves and countermeasures: Abnormal opening, which occurs at a pressure higher than the set value – in other words, the valve opens only when the pressure exceeds the maximum allowable operating pressure of the equipment. This results in the loss of the overload protection function provided by the safety valve. The reasons for this situation and the corresponding countermeasures are as follows: (1) The set pressure is incorrect; it has been set too high. Countermeasure: Readjust the compression amount of the spring. (2) The valve disc is stuck to the valve seat. The safety valve failed to operate during prolonged operation, and rust formed due to a lack of regular inspection and maintenance, which caused the valve disc to stick to the valve seat. Countermeasures: Regularly perform manual venting or water release tests on the safety valves, and be careful to prevent moisture and humidity during use. (3) Internal foreign objects get stuck, preventing the valve disc from rising over the obstacle, which results in an increased opening pressure or an inability to open the valve. Countermeasure: Check and remove foreign objects. (4) Blockage by external objects: External forces applied manually or other factors prevent the valve disc from opening properly in the case of gravity-operated or lever-type safety valves. Countermeasure: Check and remove foreign objects. (5) A decrease in operating temperature leads to an increase in the opening pressure. Countermeasure: When adjusted for normal temperature use in low-temperature conditions, the set pressure value at normal temperature should be slightly lower than the required opening pressure value. It opens below the set pressure, that is, it activates before reaching the set pressure. Mainly: (1) The set pressure is inaccurate. Countermeasure: Readjust the compression amount of the spring ； (2) The elasticity of the spring decreases due to aging. Due to long-term use or medium corrosion, the spring ages, resulting in a decrease in its elasticity. Countermeasure: Tighten the adjustment screw appropriately or replace the spring. (3) An increase in operating temperature leads to a decrease in the opening pressure. Countermeasure: When adjusted at normal temperature for use at high temperatures, the set pressure value at normal temperature should be slightly higher than the required opening pressure value. (4) Due to leaks or other reasons, the operating environment of the safety valve is above the allowable working temperature of the spring, resulting in a decrease in the spring’s elasticity. Countermeasures: Grind the sealing surface promptly to eliminate leaks and replace it with a new spring. (5) The safety valve is tilted, due to installation or being pressed and touched, causing it to tilt. Countermeasures: Safety valves, especially weight-type and lever-type safety valves, must be installed vertically, with a sturdy protective ring in place. A safety valve leak occurs when, at the normal operating pressure of the equipment, there is a leakage between the valve disc and the valve seat sealing surface that exceeds the allowable level. (1) There are impurities between the valve disc and the valve seat sealing surface. Countermeasure: Use a lifting wrench to open the valve several times in order to flush out impurities or debris ； (2) Damage to the sealing surface. Due to factors such as the material and hardness of the sealing surface as well as the way it is assembled, the deformation or misalignment of the sealing surface that occurs during the opening and closing cycle of the safety valve, or the expansion and deformation of the sealing surface caused by changes in the temperature of the sealing medium, can all lead to fluid leakage. Countermeasures: Select a safety valve made of an appropriate material based on the properties of the sealing medium and possible temperature changes. Defects or damages to the sealing surface of the safety valve should be repaired by grinding or by grinding after turning, depending on the situation; attention should also be paid to the grinding method to prevent the formation of grooves. (3) Damaged gasket. The gasket may be damaged due to impurities or hard objects that get stuck in it, squeeze it, or press on it; replace it with a qualified gasket. Also, make sure to clean the installation site and the objects onto which the gasket will be installed before placing it there ； (4) The valve stem is bent or tilted, causing the valve disc to be out of alignment with the valve seat. Countermeasure: Reassemble or replace ； (5) The spring loses its elasticity or its resilience decreases, as the spring is corroded by the medium or softens due to an increase in the medium’s temperature; as a result, the pre-tightening force declines and leakage occurs. Countermeasures: Strengthen rust prevention and heat insulation measures for the springs, and replace the springs in a timely manner or readjust the opening pressure. (6) Spring breakage. Remedial action: Replace the spring again. (7) Improper assembly or inappropriate dimensions of related parts; during assembly, the valve core and seat are not fully aligned, or there is light penetration at the mating surface, as the sealing surfaces of the valve core and seat being too wide hinders proper sealing. Countermeasure: Check the size and uniformity of the clearance around the valve core to ensure that the tip hole of the valve core is aligned properly with the sealing surface ； Check that the clearances in all areas do not allow the valve core to lift ； The drawings require the width of the sealing surface to be appropriately reduced to achieve effective sealing. (8) Leakage at the valve body joint surface; insufficient tightening force of the bolts used to secure the joint, or uneven tightening, resulting in poor sealing at that joint surface ； The seal gasket at the valve body joint does not meet the standards. Countermeasures: Adjust the bolt tension; when tightening the bolts, do so in a diagonal pattern. It is best to measure the gaps at various points while tightening, and keep tightening until the bolts can no longer be moved further, ensuring that the gaps across the joint surfaces are uniform. Additionally, standard-compliant gaskets should be used. (9) Rust at the contact point between the valve stem and the valve disc causes the valve disc to lose its ability to return to its proper position; when the exposed valve stem is disturbed, the valve disc shifts, resulting in a failure of the seal and leakage. Countermeasures: Regularly inspect the contact groove between the valve stem and the valve disc, and take appropriate anti-rust measures. (10) If the set pressure is too close to the operating pressure of the equipment, fluctuations in the operating pressure that are close to or exceed the sealing pressure of the safety valve may cause leakage. Countermeasures: Select an appropriate safety valve and set its setting pressure reasonably. The safety valve disc is unstable, with frequent jumping or fluttering of the disc. Frequency jumping refers to the phenomenon where, after the safety valve returns to its seated position, it opens again once the pressure rises slightly; this happens several times in a row, and such behavior is known as \"frequency jumping\" of the safety valve. Frequent jumping is extremely detrimental to the seal of a safety valve, as it can easily lead to leakage at the sealing surface. The oscillation that occurs in a safety valve during discharge is known as valve flutter. This phenomenon can easily lead to metal fatigue, reducing the mechanical properties of the safety valve and creating serious risks for the equipment. Its instability is mainly due to: (1) the spring stiffness being too high, which prevents the valve disc from reaching the opening height; as a result, the discharge volume is too low. Once the safety valve opens, the pressure continues to rise, causing it to open again. Replace the countermeasure with a spring of appropriate stiffness ； (2) If the adjusting ring is not set properly and the channel area is too large, once the safety valve opens, the pressure drops sharply below the operating pressure. The valve then closes as the valve disc strikes the valve seat violently. Since the cause of the pressure increase has not been eliminated, the valve disc opens again, resulting in frequent opening and closing. Countermeasure: Readjust the position of the adjustment ring. (3) The resistance in the discharge pipeline is too high, causing the momentum of the flowing medium to decrease sharply and resulting in excessive discharge backpressure. Countermeasures: Reduce the resistance in the exhaust pipeline and remove obstacles from it. (4) Improper use of the valve; a valve with an excessive discharge capacity was selected. Countermeasure: Ensure that the rated discharge capacity of the selected valve is as close as possible to the discharge requirement of the equipment. After the safety valve operates, it does not return to its normal position, and the pressure continues to rise after venting. This is mainly due to: (1) the discharge capacity of the selected safety valve being less than the equipment’s maximum allowable discharge volume; as a result, when the safety valve opens, there isn’t enough time to release the excess pressure, and the pressure inside the pressure vessel continues to rise. Countermeasure: Select a suitable safety valve again. (2) An incorrect centerline of the valve stem or rusted springs prevent the valve disc from rising to the proper height, resulting in a reduced discharge volume of the safety valve. Countermeasures: Reassemble the valve stem or replace the spring, and pay attention to anti-rust measures for the spring ； (3) The cross-sectional area of the exhaust pipe is insufficient, resulting in the discharge capacity of the safety valve being less than the equipment’s safe discharge capacity. Countermeasure: Use an exhaust pipe that meets the requirements for safe discharge area. (4) The spring grade does not meet the requirements; due to its low stiffness, excessive compression of the spring occurs during adjustment, resulting in an insufficient opening height. Countermeasure: Replace the spring of the appropriate grade, ensuring that its stiffness is suitable. The valve disc does not return to its original position after discharge. This is mainly due to: (1) the spring bending the valve stem, or the valve disc being installed in the wrong position or stuck. Countermeasures: It should be reassembled, and the moving parts should be carefully inspected for defects such as dents, scuffing, rust, or burrs, which must be removed promptly. (2) The safety valve spring breaks during use; after it opens, the broken part shifts out of place, preventing the valve disc from returning to its proper position. Countermeasure: Replace with a new spring promptly.