Pressure vessels are widely used in various industrial, civilian, and military fields as sealed containers, responsible for safely storing or transmitting gases and liquids under internal and external pressure. As modern industry increasingly demands high efficiency and safety, the use of pressure vessels has become more common. Safety valves, as crucial safety devices, play a key role in ensuring the safe operation of these vessels. This article will provide a detailed guide to the use of pressure vessel safety valves, covering their types, selection principles, discharge capacity calculation, installation requirements, and regular maintenance to ensure the safe operation of pressure vessels.
Types of Safety Valves and Selection Principles
Safety valves are essential components of pressure vessels, primarily designed to prevent accidents like explosions caused by excessive pressure. Depending on the working principle and application environment, safety valves come in various types and designs. Choosing the right safety valve not only ensures equipment safety but also extends its service life.
1. Spring-Type Safety Valve
The spring-type safety valve is the most commonly used type, widely applied to various pressure vessels, particularly in high-temperature or high-pressure environments. Its working principle involves adjusting the spring's tightness to set the opening and closing pressure. These valves have a compact structure, responsive operation, and high reliability. However, the spring may be affected by temperature, and its elasticity may decrease in high temperatures. Therefore, when used in high-temperature environments, it is essential to select appropriate materials and designs to ensure the spring's stability and reliability at elevated temperatures.
2. Lever-Type Safety Valve
The lever-type safety valve has a simple structure, making it suitable for vessels with lower pressure and no vibration. It operates on the principle of a lever, adjusting pressure by altering the lever's length and pivot point. This type of valve is typically used in situations that do not require high sensitivity, offering good durability for simple processes. Due to its straightforward design, maintenance is relatively easy, but it may not meet performance requirements for high-pressure or high-precision applications.
3. Closed-Type Safety Valve
Closed-type safety valves are typically used in vessels containing hazardous substances, such as toxic gases, flammable liquids, or refrigerants. The design of these valves effectively prevents the leakage of harmful gases, ensuring the safety of personnel and the environment. Unlike open or semi-open safety valves, closed-type valves prevent the release of contaminants, making them ideal for use in hazardous environments. For example, in the chemical, petroleum, and natural gas industries, closed-type safety valves are the preferred choice for vessels storing toxic or flammable gases.
4. Full Lift and Low Lift Safety Valves
Full lift and low lift safety valves are primarily chosen based on the vessel's pressure rating and discharge capacity requirements. Full lift safety valves are suitable for high-pressure vessels or those requiring large discharge capacities, enabling rapid operation when the pressure exceeds the set value, releasing gas through a large opening. Low lift safety valves, on the other hand, are used for vessels with lower pressures and require more precise opening and closing movements, making them suitable for precise flow control applications. The selection of full lift or low lift valves should consider the vessel's wall thickness, operating pressure, and discharge needs to enhance safety and efficiency.
Discharge Capacity and Size Calculation
The selection of safety valves is not only based on pressure but also on their discharge capacity. Discharge capacity refers to the amount of gas or liquid that a safety valve can release when opened. The discharge capacity must exceed the vessel's safe discharge volume to ensure that the valve opens in time when the internal pressure exceeds the set value, preventing catastrophic events such as explosions.
1. Discharge Capacity Calculation
Discharge capacity calculation involves several factors, including the medium's properties inside the vessel, the vessel's capacity, working conditions, and potential changes in the process. The maximum pressure of the vessel and the design discharge capacity must be determined first. Then, factors such as the temperature, density, and gas state of the medium are considered to calculate the safety valve's discharge capacity. To avoid calculation errors, it is recommended to use standard calculation formulas and professional engineering software to ensure accuracy.
2. Size Selection
The size selection is closely related to the vessel's size and discharge requirements. Generally, the larger the valve size, the greater its discharge capacity. Choosing the right size not only ensures the required discharge but also improves the valve's sensitivity. Under high-pressure conditions, selecting a larger size can effectively reduce the load on the safety valve, ensuring stable operation.
Installation Requirements for Safety Valves
The installation position and method of safety valves directly affect their performance. Incorrect installation may prevent the valve from opening correctly and even damage the vessel. Therefore, proper installation is essential to ensure the valve functions as designed.
1. Installation Location
Safety valves should be installed on the pressure vessel itself, particularly in liquefied gas storage tanks, where the safety valve should be positioned in the gas phase. Since the vessel's medium may contain volatile gases, the installation location should be chosen to ensure smooth gas discharge to enhance efficiency. In outdoor installations, the safety valve must be equipped with anti-freezing features to prevent freezing in low temperatures, which could affect the valve's performance.
2. Vertical Installation
For proper functioning, spring-type and lever-type safety valves must be installed vertically. If the valve is not installed as required, its opening and closing performance may be compromised, leading to malfunction. This is particularly important under high-pressure or high-temperature conditions, where correct installation angles and positions are crucial.
3. Vibration Interference
For spring-type safety valves, all possible vibration sources should be minimized. Vibration can cause deviations in the opening and reseating pressures, affecting the valve's performance. To prevent interference, dampers should be installed, or valves with stronger vibration resistance should be selected to reduce the impact of vibration.
4. Reaction Force Issues
For high-discharge safety valves, special attention should be paid to the reaction force generated during exhaust. The reaction force may impose additional loads on the pipeline and the valve's connection. Therefore, the connecting piping must be able to withstand sufficient stress to avoid pipeline damage or valve failure due to excessive reaction force.
Pressure Tests and Regular Calibration of Safety Valves
To ensure that safety valves possess sufficient strength and reliability, pressure tests are required before use. These tests are divided into two parts: testing the lower and upper parts of the valve body. During the test, the safety valve is subjected to pressure higher than its working pressure to ensure its sealing and structural strength can withstand the vessel's pressure fluctuations.
1. Pressure Test
Lower part of the valve body: The pressure should be 1.15 times the working pressure to ensure that the lower sealing surface does not leak or deform.
Upper part of the valve body and valve cover: The pressure should not exceed the working pressure but must ensure that the sealing of these parts is intact to prevent leaks.
2. Regular Calibration
The opening and closing pressures of safety valves should be regularly calibrated to ensure their performance is within the design specifications. It is recommended to use a pressure gauge with a precision of at least 110 to ensure accurate measurement. Regular calibration helps identify performance deviations and make necessary adjustments to prevent safety accidents due to valve failure.
Maintenance and Common Issues with Safety Valves
The maintenance of safety valves is crucial for their long-term stable operation. Regular inspections and timely replacement of components can effectively extend their service life and ensure functionality. Common maintenance issues include spring replacement and seal face grinding, which are discussed in detail below.
1. Spring Replacement and Seal Treatment
If leakage is found in the valve seat or valve mouth, the spring should be promptly checked and replaced. Additionally, the seal faces of the valve seat and valve core should be ground to ensure sealing effectiveness. It is essential not to use spring shims as a repair method, as this can cause safety hazards and potentially lead to valve failure.
2. Improper Maintenance Methods
Some improper maintenance methods can reduce the safety valve's performance. For example, adding a sleeve between the valve seat and valve core to address leakage is prohibited, as it not only fails to solve the problem but can also result in more severe leakage or valve damage.
3. Lever-Type Safety Valve Maintenance
Lever-type safety valves require regular inspection to ensure that the device preventing the weight from moving automatically is functioning correctly. The guide rails and weight components of the lever should be free to move, preventing failure caused by wear or jamming of the components.
Conclusion
Safety valves are indispensable safety devices in pressure vessels. Proper selection, installation, and maintenance are essential to ensuring the safe operation of pressure vessels under high pressure and complex conditions. By choosing the right safety valve, accurately calculating the discharge capacity, ensuring correct installation, and conducting regular pressure tests and calibrations, the safety and stability of pressure vessels can be significantly improved. Regular inspections and maintenance will also effectively extend the service life of safety valves, reduce the likelihood of accidents, and ensure smooth industrial production.
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