In industrial production, the safe operation of pressure vessels, pipelines, and various equipment is crucial. When overpressure occurs, it can not only damage equipment but also trigger serious safety accidents. As a key safety protection device, a safety valve can quickly discharge excess fluid under overpressure conditions, thereby reducing pressure and ensuring the safety of equipment and personnel. This article provides a detailed introduction to the characteristics of different types of safety valves and key selection points, helping you choose the right safety valve for your equipment.
Before delving into the key points of safety valve selection, it is necessary to have a clear understanding of the different types of safety valves and their characteristics. Understanding these basics will help us more accurately match the equipment requirements with the safety valve features during the selection process. Let us now explore the types and characteristics of safety valves.
A spring-loaded safety valve controls the opening pressure by adjusting the spring's pre-compression. When system pressure exceeds the set value, the spring loses control, and the valve opens to release fluid, thereby reducing pressure. Its advantages include a lightweight, compact structure, high sensitivity, unrestricted installation location, and low sensitivity to vibration, making it suitable for mobile pressure vessels. However, as the valve disc rises, the spring compression increases, and the force on the valve disc also increases, which may affect the rapid opening of the valve. Additionally, in high-temperature environments, the spring's elasticity may decrease due to prolonged heat exposure, so insulation or heat dissipation for the spring should be considered.
The screw-type safety valve controls the opening pressure by adjusting a screw mechanism. Its structure is relatively simple, easy to operate and maintain, and suitable for small- to medium-sized equipment. This type of valve is intuitive to operate, with opening pressure controlled by simple screw adjustment, making it suitable for situations where precise opening pressure is not critical.
The quick-opening safety valve, also known as a rapid-opening valve, opens instantly through self-weight or spring force to quickly discharge overpressure fluid. This valve has the advantages of rapid response and high reliability, and is typically applied to large equipment and systems under high-pressure conditions. In scenarios requiring quick action to prevent equipment damage or safety accidents, the quick-opening safety valve plays a critical role.
The weighted lever safety valve uses a weight and lever to balance the force acting on the valve disc. According to lever principles, the opening pressure can be adjusted by moving the weight position or changing its mass. This type of valve has a simple structure, easy and accurate adjustment, and the applied load does not significantly increase with valve lift. It is suitable for high-temperature conditions and was commonly used in boilers and high-temperature pressure vessels in the past. However, it is bulky, the loading mechanism is prone to vibration, often causing leakage, and its reseating pressure is low, making it difficult to close tightly after opening.
The pulse safety valve consists of a main valve and an auxiliary valve, where the auxiliary valve triggers the main valve through a pulse action. Its structure is relatively complex and is usually only suitable for boilers and pressure vessels with large safety discharge requirements. In applications that require handling large fluid discharge, the pulse safety valve can effectively control the release process and ensure system safety.
Safety valves can be classified according to the ratio of maximum valve lift to the flow passage diameter:
Micro-Lift Safety Valve: The lift is less than 1/4 of the flow passage diameter, usually 1/40–1/20. Its action is proportional and is mainly used for liquids, and sometimes for gases with very small discharge.
Full-Lift Safety Valve: The lift is greater than or equal to 1/4 of the flow passage diameter, and the discharge area is the minimum cross-sectional area of the valve seat throat. Its action is two-stage and requires a lifting mechanism to achieve full opening, mainly used for gas media.
Next, we need to explore the key factors in safety valve selection. These factors directly affect the valve's performance and reliability, determining whether it can effectively protect equipment and personnel. Multiple aspects must be considered comprehensively to ensure the selected valve perfectly matches the equipment's requirements. The key factors are as follows:
The set pressure of a safety valve should be slightly higher than the design working pressure of the vessel or pipeline to ensure timely opening under overpressure conditions. Too high a set pressure may prevent timely activation, while too low may cause frequent misoperation, affecting normal equipment operation. Set pressure should be determined by considering design pressure, actual working pressure, and potential overpressure scenarios to ensure the valve functions when needed.
Choose a valve diameter based on the system flow requirements to ensure sufficient flow capacity. An undersized valve may not discharge enough fluid during overpressure, while an oversized valve increases cost and installation space. Diameter selection should be calculated according to maximum system flow, medium characteristics, and valve discharge capability.
Select valve materials and sealing designs suitable for the working temperature of the medium. Different materials perform differently at various temperatures; some may lose strength or deform at high temperatures, while others may become brittle at low temperatures. Choosing the right material and sealing design ensures reliable operation under all temperature conditions.
Select materials that can withstand corrosion, high temperature, and other environmental factors. For acidic or alkaline media, corrosion-resistant materials are necessary. High-temperature environments require attention to material heat resistance and spring insulation or cooling. Other environmental factors such as vibration and dust must also be considered as they can affect valve performance and lifespan.
Ensure the safety valve meets applicable international or industry standards to guarantee performance and reliability. Standard-compliant valves undergo rigorous testing and certification, providing higher safety assurance. Check relevant standards, including national or regional pressure vessel and safety valve manufacturing standards.
Determine the nominal pressure based on valve material, working temperature, and maximum operating pressure. The working pressure is the static pressure the valve faces during normal operation, equal to the system or equipment working pressure. The spring's working pressure range allows adjustment of opening pressure by changing spring pre-compression. Select a valve based on the required opening pressure.
The discharge pressure is usually 1.1× the set pressure (1.03× for steam boiler valves). The valve's discharge capacity should meet or exceed the required relief volume, which depends on system operating conditions, capacity, and potential overpressure. Accurately calculate required discharge and ensure the selected valve meets this requirement.
Valve material selection should consider medium temperature, pressure, properties, manufacturability, and cost. Different materials offer different performance and cost. Examples include:
Hot water boilers: non-enclosed micro-lift valves.
Steam boilers/pipelines: non-enclosed full-lift valves.
Incompressible liquids like water: enclosed micro-lift or relief valves.
High-pressure feedwater systems: enclosed full-lift valves.
Compressed gases: enclosed full-lift valves.
E-class steam boilers: deadweight valves.
Large diameter, high discharge, high-pressure systems: pulse valves.
LPG transport: internal valves; oil tank tops: hydraulic valves with breathers.
Underground drainage or gas pipelines: pilot valves.
Liquefied petroleum gas station pump outlet: safety return valves.
Systems prone to negative pressure: vacuum/negative-pressure valves.
Systems with large backpressure fluctuations or toxic/flammable media: bellows valves.
Systems with low freezing-point media: insulated jacket safety valves.
After understanding valve types and key selection factors, the following steps ensure a systematic selection process.
Clarify Equipment Requirements: Determine design pressure, working pressure, temperature, medium type, and flow requirements. Accurate knowledge of equipment parameters is the basis for selection.
Determine Valve Type: Select the type based on equipment, medium, and environment. For example, quick-opening valves for high-pressure rapid-response systems, weighted lever or open valves for high-temperature applications, fully enclosed valves for toxic or flammable gases.
Calculate Set Pressure and Discharge Volume: Determine set pressure based on design and operating pressures. Calculate required discharge volume based on flow requirements and potential overpressure, ensuring effective protection under worst-case conditions.
Choose Appropriate Materials: Select materials according to medium temperature, pressure, and corrosiveness, balancing safety and cost.
Check Standards Compliance: Ensure the valve meets applicable standards and obtain certification or test reports from the supplier.
Determine Nominal and Working Pressure Ratings: Set nominal and working pressure based on design pressure, temperature, and material limits, considering long-term stability and reliability.
Consider Installation and Maintenance: Ensure the valve is easy to install, observe, and maintain, and that it is securely mounted to avoid vibration issues. Consider maintenance cycles and costs.
Safety valves play a vital role in industrial production, providing strong protection for pressure vessels, pipelines, and equipment. Selection requires considering equipment requirements, medium properties, and environmental conditions, including valve type, set pressure, diameter, material, and standards compliance. Regular maintenance ensures performance and reliability. Proper selection and use of safety valves can effectively reduce equipment damage and safety risks, ensuring smooth industrial operations.
In short, safety valve selection is a complex yet critical process, requiring comprehensive analysis according to specific applications. This guide aims to provide useful reference and guidance to help select the appropriate safety valve for safe equipment operation.