What are the conventional safety accessories for pressure vessels?

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Overview: During operation, pressure vessels may, for various reasons, experience a pressure inside them that exceeds their maximum allowable pressure (usually the design pressure). To prevent overpressure and ensure the safe operation of pressure vessels, safety relief devices are generally installed to automatically and quickly discharge the medium inside the vessel, keeping its internal pressure below its maximum allowable level. So, here is an overview of the conventional safety accessories for pressure vessels. Keywords: pressure vessel, safety valve, explosion relief disc, explosion proof cap, pressure gauge, level gauge. The common safety pressure relief devices for pressure vessels include the following: (1) Safety valve. When a pressure vessel is operating at its normal working pressure, the safety valve remains airtight and leak-free ； When the pressure exceeds the set value, the safety valve opens automatically under the influence of that pressure, allowing the container to release pressure and thus preventing damage to the container or pipelines ； When the container pressure drops to normal levels, it closes on its own and stops releasing gas. 1. Types of safety valves Safety valves are classified according to their overall structure and the type of loading mechanism; the commonly used types are lever-type and spring-type. Spring-loaded safety valves are widely used in petrochemical plants. The loading mechanism of a spring-type safety valve is a spring; by adjusting the nut, it is possible to change the degree of compression of the spring, thereby adjusting the force with which the valve disc presses against the valve seat and determining the value of its opening pressure. The spring-type safety valve is compact in size, small in volume, and responds quickly; it is not very sensitive to vibrations. It can be installed on mobile containers. The downside is that the elasticity of the spring inside the valve decreases when exposed to high temperatures. 2. Selection of safety valves: The Regulations on Safety Supervision of Pressure Vessels stipulate that the manufacturer of safety valves must possess a manufacturing license issued by the Ministry of Labor in order to produce such valves. The product should come with a certificate of conformity upon leaving the factory; this certificate must bear the seal of the quality inspection department as well as the date of inspection. The selection of a safety valve should take into account the process conditions of the container and the properties of the working medium, considering aspects such as the safe discharge capacity of the safety valve, the loading mechanism, the sealing mechanism, the gas discharge method, and the operating pressure range. The discharge capacity of the safety valve is a key factor in selecting it; its discharge capacity must be no less than the safe discharge capacity of the container. In terms of gas emission methods, closed safety valves should be used for containers holding toxic, flammable, or environmentally harmful substances. When selecting a safety valve, attention should be paid to its operating pressure range, which must match that of the pressure vessel. 3. Installation of the safety valve: The safety valve should be installed vertically upward in the gas phase space above the liquid level of the pressure vessel itself, or it should be connected to a pipe that is attached to the gas phase space of the pressure vessel. It is indeed inconvenient to install a safety valve directly on the container body; when connecting it to the container using a short pipe, the diameter of this pipe must be larger than the inlet diameter of the safety valve. Valves or other outlet pipes are generally not allowed to be installed on such pipes. When several safety valves are installed on a connection port of a pressure vessel, the area at the inlet of that connection port shall be at least equal to the total area of those safety valves. Generally, it is not advisable to install an intermediate stop valve between the pressure vessel and the safety valve. For vessels containing flammable substances, or those with extremely high, high, or medium-high toxicity levels, or viscous media, a stop valve can be installed to facilitate the replacement and cleaning of the safety valve. However, the flow area of this stop valve must not be smaller than the minimum flow area of the safety valve, and reliable measures and strict procedures must be in place to ensure that the stop valve remains fully open during operation and is sealed with lead. When selecting the installation location, consideration should be given to the convenience of daily inspection, maintenance, and repair of the safety valve. Safety valves installed in outdoor areas must have reliable measures to prevent water inside the valve from freezing in winter. The exhaust pipe equipped with a safety valve must have a minimum cross-sectional area that is larger than the outlet cross-sectional area of the safety valve. The exhaust pipe should be as short and straight as possible, and no valves shall be installed on it. When installing a lever-type safety valve, its valve stem must be kept in a vertical position. The bolts of all the inlet and outlet pipe connection flanges must be tightened evenly to prevent additional stress from being applied to the valve body, which could disrupt its concentricity and affect the proper operation of the safety valve. 4. Adjustment, maintenance, and inspection of safety valves: Before installation, safety valves should undergo hydrostatic and airtightness tests by qualified professionals; after passing these tests, they are adjusted and calibrated. The opening pressure of the safety valve shall not exceed the design pressure of the container. The calibrated and adjusted safety valve should be sealed with lead. To ensure that the safety valve operates sensitively and reliably with good sealing performance, regular maintenance checks must be carried out. Safety valves should be kept clean at all times to prevent the valve springs and other components from becoming clogged with oil residue or rust. It is also necessary to regularly check whether the lead seal of the safety valve is intact, to determine if freezing is possible in low temperatures, and to check for any leaks in the safety valve. For lever-type safety valves, it is necessary to check whether the weight is loose or has been moved. If any defects are found, they should be corrected or replaced promptly. The Regulations on Safety Inspection of Pressure Vessels stipulate that safety valves must be inspected regularly, with at least one calibration per year. Regular inspection work includes cleaning, grinding, testing, and calibration. (II) Explosion vent discs: Also known as explosion relief membranes or explosion vent plates, these are fracture-type safety pressure relief devices. Explosion-proof discs have advantages such as good sealing performance, fast response, and resistance to the influence of contaminants in the medium. But it releases pressure by breaking the diaphragm, so it cannot be used after pressure release, and the container is forced to stop operating. Therefore, it is only used on pressure vessels where it is not appropriate to install a safety valve. The structure of the explosion-proof disc is relatively simple. Its main component is a very thin metal plate, which is held in place by a special set of pipe flanges and inserted into the outlet stub of the container; in some cases, the diaphragm is placed directly together with the sealing gasket in the pipe flange. When the container is operating normally, although the explosion vent may experience significant deformation, it remains airtight and leak-free. When the container is overpressured, the diaphragm ruptures to release the medium, preventing the container from exploding due to excessive pressure. The designed burst pressure of an explosion-proof disc is generally 1.25 times the operating pressure; for containers with large pressure fluctuations, the designed burst pressure must be even higher. However, under any circumstances, the burst pressure of the explosion vent must not be greater than the design pressure of the container. (III) Explosion-proof cap: The explosion-proof cap, also known as a blast cap, is another type of fracture-type safety pressure relief device. It comes in many styles, but the basic principle of operation remains the same. Its main component is a thick-walled short tube that is closed at one end and has a thin weak section in the middle. When the pressure inside the container exceeds the specified level, the explosion-proof cap breaks at its weak point, and the gas is released through the openings in the cap. To prevent the explosion-proof cap from flying out and causing injury in case it breaks, a protective device should be installed on its outside. (IV) Pressure gauge A pressure gauge is a measuring instrument used to detect the pressure of the medium inside pressure vessels. There are many types of pressure gauges, which can be divided into four main categories: liquid column type, elastic element type, piston type, and electrical type. Pressure vessels mostly use single-bellows pressure gauges with elastic elements. For pressure gauges installed on pressure vessels, the maximum scale value on the gauge should be 1.5 to 3 times the vessel’s maximum operating pressure; 2 times is ideal. The larger the range of a pressure gauge, the greater the absolute value of the allowable error, and the greater the visual error as well. When selecting a pressure gauge, it is also necessary to consider the pressure rating of the container and the operational requirements. According to the pressure class requirements for containers, low-pressure containers should generally be at least class 2.5, while medium-pressure and high-pressure containers should be at least class 1.5. To enable operators to see the pressure indication clearly and accurately, the diameter of the pressure gauge dial must not be too small. Under normal circumstances, the dial diameter should not be less than 100 mm. If the pressure gauge is located far from the point of observation, its dial diameter should be increased; when the distance exceeds 2 meters, the dial diameter should ideally be no less than 150 mm ； When the distance exceeds 5m, it should not be less than 250mm. The dial diameter of the pressure gauge for ultra-high pressure vessels should be no less than 150 mm. When installing a pressure gauge, it should be placed in a location that is easily visible to the operator, with adequate lighting; at the same time, care must be taken to protect it from radiant heat, low temperatures, and vibrations. Pressure gauges installed at high positions should be tilted slightly forward, but the tilt angle should not exceed 300. The pressure gauge connection should be directly connected to the container body. To facilitate the removal and replacement of, as well as the calibration of, the pressure gauge, a three-way valve should be installed between the pressure gauge and the container. The stopcock should be installed on a vertical pipe section and equipped with an opening indicator for verification and replacement. A trap should be installed between the pressure gauge and the steam container. For containers holding high-temperature, highly corrosive, and condensable media, an isolation buffer device should be installed between the pressure gauge and the container. For pressure gauges in use, a warning red line should be marked on their dials according to the maximum operating pressure of the equipment; however, this marking should not be made on the dial glass, to avoid creating illusions for the operator due to the movement of the glass and thereby preventing accidents. Pressure gauges that have not passed inspection and do not have lead seals shall not be installed or used. The pressure gauge should be kept clean, and the glass on its dial must be clear and transparent, so that the pressure value indicated by the pointer inside can be seen clearly. The connections of the pressure gauge should be blown out regularly. During the operation of the container, if it is found that the pressure gauge is not functioning properly, its scale is unclear, the dial glass is broken, the pointer does not return to zero after pressure release, or the seal is damaged, it should be corrected or replaced immediately. The maintenance and calibration of pressure gauges shall comply with the relevant regulations of the **metrology authorities. The pressure gauge should have calibration marks indicating the date of the next calibration or the validity period of the calibration. The calibrated pressure gauge should be sealed with a lead seal. (5) Level gauge: The level gauge is a safety accessory for pressure vessels. Glass plate level gauges are commonly used to display the liquid level in general pressure vessels. For pressure vessels in petrochemical plants, such as those used for storing various liquefied petroleum gases, level indicating instruments with different principles of operation, structures, and performance characteristics are selected. For pressure vessels whose medium is powdered material, radioactive isotope level gauges are commonly used to indicate the level height of the powder. Regardless of the type of level gauge or instrument chosen, it must meet the safety requirements specified in the \"Regulations on Safety Supervision of Pressure Vessels.\" These requirements mainly include the following: 1. The appropriate gauge should be selected based on the medium contained within the pressure vessel, as well as its maximum operating pressure and temperature. 2. Before installation and use, low- and medium-pressure vessel level gauges shall undergo a hydrostatic test at 1.5 times the nominal pressure of the gauge ； High-pressure vessel level gauges shall undergo a hydrostatic test at 1.25 times the nominal pressure of the gauge. 3. Pressure vessels containing media at temperatures below 0°C should be equipped with frost-proof level gauges. 4. For level gauges used outdoors in cold areas, those with a jacketed or insulated design should be selected. 5. Pressure vessels for liquefied gases that are flammable and possess an extremely high level of toxicity, constituting a high hazard, shall be equipped with plate-type or automatic level indicators, as well as protection devices to prevent leaks. 6. For applications requiring a stable liquid level indication, float-type level gauges should not be used. 7. The level gauge should be installed in a location where it is easy to observe. If the installation location of the level gauge is not convenient for observation, other auxiliary facilities should be added. Large pressure vessels should also be equipped with centralized control facilities and alarm devices. The highest and lowest safe liquid levels of the level gauge should be clearly marked. 8. Pressure vessel operators should strengthen the maintenance of liquid level indicators, ensuring they remain in good condition and clear at all times. A regular maintenance system should be implemented for level gauges, and the using units may specify details in their management systems based on the actual operating conditions. 9. The level gauge should be discontinued from use if it is in one of the following conditions: ① The inspection period has expired ； ②The glass plate (tube) has cracks or is broken ； ③Valve component stuck ； ④False levels often occur. 10. When using radioactive isotope level detection instruments, it is necessary to strictly comply with the provisions of the \"Regulations on Radiation Protection for Radioactive Isotopes and Radiation Devices\" issued by the State Council, and take effective protective measures to prevent radiation hazards at the site of use. Furthermore, in the petrochemical production process, some reaction pressure vessels and storage pressure vessels are equipped with level detection alarms, temperature detection alarms, pressure detection alarms, and interlocks. These devices serve both as instruments for production monitoring and as safety accessories for pressure vessels; therefore, their management must be strengthened in accordance with relevant regulations. These safety accessories are not only used in common boilers but also widely applied in most liquefied gas transport vehicles.