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Time：2026/3/8 16:54:02 Category：TECHNICAL ARTICLE Source： Click：
Threaded safety valves are widely used in industrial piping systems where overpressure protection is required but flange connections are unnecessary or space is limited. Compared with flanged safety valves, threaded safety valves offer easier installation, lower cost, and convenient maintenance. They are commonly installed on small-diameter pipelines, pressure vessels, compressors, boilers, and auxiliary equipment.
In the Chinese valve industry, several threaded safety valve models are widely used, including A11, A21, A22, A27, and A28. Among them, A21, A27, and A28 are standard national models, while A11 and A22 are usually non-standard customized typesdesigned for special working conditions. This article introduces the characteristics, applications, and structural features of these threaded safety valve models to help users better understand and select the appropriate valve for their systems.
A threaded safety valveis a pressure relief device connected to pipelines or equipment through threaded connections (usually NPT, BSPT, or metric threads). Its primary function is to automatically release excess pressure when system pressure exceeds the preset limit, ensuring the safety of equipment and personnel.
Key advantages of threaded safety valves include:
Compact structure and lightweight design
Easy installation and replacement
Suitable for small and medium pipelines
Lower installation cost compared to flanged valves
Widely used in air, steam, gas, and liquid systems
Threaded safety valves are commonly used in industries such as:
Air compressor systems
Petrochemical equipment
Boiler auxiliary systems
Pressure vessels
Water treatment systems
Gas pipelines
Among the many models available, the A-series threaded safety valves are among the most widely used in China and international markets.
The A11 threaded safety valveis typically a non-standard customized safety valvedesigned according to specific customer requirements. Unlike standardized models, the A11 type may vary in structure, materials, pressure range, and sealing form depending on the application.
The A11 safety valve usually adopts a spring-loaded structure, where the spring force keeps the valve closed during normal operation. When system pressure exceeds the preset set pressure, the valve disc lifts and releases excess pressure.
Common design features include:
Threaded inlet and outlet connections
Compact body suitable for small equipment
Adjustable set pressure
Spring-loaded direct acting mechanism
Because it is a customized model, the A11 threaded safety valve is often used in:
Special industrial equipment
Experimental or laboratory pressure systems
Customized machinery
Small pressure vessels
The materials can also be customized according to media conditions, including carbon steel, stainless steel, or brass.
The A21 threaded safety valveis a Chinese national standard micro-opening safety valve, commonly used in liquid systems.
The A21 valve belongs to the spring micro-opening type safety valve, meaning that the valve opening height is relatively small when pressure relief occurs. This design allows precise pressure control and stable operation.
Key features include:
Spring loaded pressure control
Threaded connection for easy installation
Compact body structure
Stable opening and closing performance
A21 threaded safety valves are mainly used for liquid media, including:
Water systems
Oil pipelines
Chemical liquids
Hydraulic equipment
Because of its reliable structure and standardized design, the A21 model is widely used in industrial equipment where liquid overpressure protection is required.
The A22 threaded safety valveis another customized model, often designed based on the A21 structure but modified for special operating conditions.
Since A22 valves are usually non-standard products, they can be designed according to customer specifications, including:
Customized pressure settings
Special material requirements
Unique sealing structures
Modified connection sizes
The valve can also be designed for different working media such as oil, chemical liquids, or gas mixtures.
A22 threaded safety valves are often used in:
Customized chemical equipment
Industrial machinery protection systems
Special pressure containers
OEM equipment manufacturing
Due to its flexible design capability, the A22 model is suitable for engineering projects requiring tailored safety protection solutions.
The A27 threaded safety valveis a spring micro-opening safety valve designed mainly for gas or air systems.
The A27 model has a similar structure to the A21 valve but is optimized for gas media. It provides stable pressure release and reliable sealing performance.
Key characteristics include:
Spring-loaded direct acting design
Threaded inlet and outlet connections
High reliability for gas pressure protection
Compact structure suitable for small pipelines
A27 threaded safety valves are available in multiple materials depending on operating conditions:
Carbon steel (WCB)
Stainless steel 304
Stainless steel 316
Stainless steel 316L
Stainless steel models are particularly suitable for corrosive gases or harsh environments.
The A27 valve is commonly used in:
Air compressor pipelines
Gas pressure vessels
Industrial air systems
Natural gas equipment
Because of its reliable performance and flexible material selection, the A27 threaded safety valve is widely used in many industrial sectors.
The A28 threaded safety valveis one of the most widely used gas safety valvesin compressor and air system applications.
The A28 safety valve is designed specifically for gas and air pressure systems. It offers fast response and reliable sealing performance.
Key features include:
Spring loaded direct acting valve structure
Threaded connection for quick installation
Sensitive pressure response
Durable sealing components
Similar to the A27 model, the A28 threaded safety valve can be manufactured in different materials:
Carbon steel (WCB)
Stainless steel 304
Stainless steel 316
Stainless steel 316L
These material options allow the valve to be used in both standard and corrosive working environments.
One of the most common variants is the A28X-16T threaded safety valve, which is specially designed for air compressors.
Features of A28X-16T include:
Optimized for compressed air systems
Stable pressure relief performance
Reliable sealing under frequent pressure cycles
Widely used in industrial air compressors
Because of its reliability and simple installation, the A28X-16T model is widely installed on air compressor tanks and pressure vessels.
Selecting the correct threaded safety valve requires considering several factors:
Different models are suitable for different media:
A21– liquid systems
A27 / A28– gas or air systems
The valve set pressure must match the maximum allowable pressure of the system.
Choose appropriate materials based on working conditions:
Carbon steel (WCB)– general industrial environments
Stainless steel 304– moderate corrosion resistance
Stainless steel 316 / 316L– strong corrosion resistance
If the system has special structural or pressure requirements, non-standard models like A11 or A22may be more suitable.
Threaded safety valves play an essential role in protecting industrial equipment and pressure systems from dangerous overpressure conditions. Among the commonly used models, A21, A27, and A28 threaded safety valves are standard designs widely used in liquid, gas, and air systems, while A11 and A22 models are typically customized valves for special applications.
With flexible material options such as WCB, stainless steel 304, 316, and 316L, and specialized models like A28X-16T for air compressors, threaded safety valves provide reliable protection for a wide range of industrial systems.
When selecting a threaded safety valve, engineers should carefully consider medium type, pressure rating, material compatibility, and installation requirementsto ensure safe and efficient operation of the entire system.
If properly selected and installed, these valves will provide long-term pressure protection and operational safety for industrial equipment and pipelines.