Selection of safety valves

It should be selected based on the device’s maximum voltage tolerance, choosing a value that is 10% to 20% lower than that tolerance. If the maximum pressure tolerance of the equipment is 100 atmospheres, a safety valve rated at 80 to 90 atmospheres should be selected.

The equipment that the safety valve is designed to protect, the setting parameters of the safety valve, the properties of the medium used, and the material from which the safety valve is made

1. Overview: Safety valves are important safety accessories on boilers, pressure vessels, and other pressurized equipment. The reliability of its operation and its performance have a direct impact on the safety of equipment and people, and are closely related to energy conservation and environmental protection. And some users and design departments always choose the wrong model when making selections. To this end, this paper analyzes the selection of safety valves. II. Definition: In a broad sense, a safety valve includes relief valves. From the perspective of regulatory requirements, it is a valve that must be installed directly on steam boilers or pressure vessels of category 1, and it is necessary for such a valve to be approved by the technical supervision authorities. In a narrow sense, it is referred to as a safety valve; other similar valves are generally called relief valves. Safety valves and relief valves are very similar in structure and performance; both discharge the fluid inside automatically when the set pressure is exceeded, in order to ensure the safety of the production equipment. Due to this fundamental similarity, people often confuse the two when using them; furthermore, some manufacturing equipment also allows either one to be used according to the regulations. Therefore, the differences between the two are often overlooked. As a result, many problems have arisen. If a clearer definition for both is to be provided, it can be understood in accordance with the definition outlined in Part 1 of the ASME Boiler and Pressure Vessel Code: (l) Safety Valve – an automatic pressure relief device driven by the static pressure of the medium in front of the valve. It is characterized by a sudden, full-open starting action. For use with gases or vapors, as shown in Figure 1. (2) Relief Valve, also known as a overflow valve, is an automatic pressure-relief device driven by the static pressure of the medium in front of the valve. It opens proportionally as the pressure exceeds the opening force. It is mainly used in fluid applications. As shown in Figure 2. (3) Safety relief valve, also known as a safety overflow valve, is an automatic pressure-relief device driven by the pressure of the medium. It can be used as either a safety valve or a relief valve, depending on the application. Taking Japan as an example, there are few clear definitions for safety valves and relief valves. Devices used as safety mechanisms in large pressure vessels such as boilers are generally referred to as safety valves, while those installed on pipes or other facilities are called relief valves. However, according to the \"Technical Standards for Thermal Power Generation\" set by Japan’s Ministry of International Trade and Industry, safety valves are specified as an essential component for ensuring safety in equipment such as boilers, superheaters, and reheaters. Where the lower side of the pressure relief valve needs to be connected to a boiler and a turbine, a drain valve or safety valve must be installed. From this perspective, safety valves are required to be more reliable than relief valves. Furthermore, based on the regulations regarding high-pressure gas management set by Japan’s Ministry of Labor, as well as the rules issued by the Ministry of Transport and various ship associations concerning the determination of safe discharge levels, we refer to valves that ensure such discharge levels as safety valves, while those that do not guarantee such discharge levels are called relief valves. In China, both fully open and slightly open types are collectively referred to as safety valves. III. Selection 1. Classification Currently, the safety valves produced in large quantities fall into two main categories: spring-type and rod-type. There are also impulse-type safety valves, pilot-operated safety valves, safety switching valves, safety pressure relief valves, gravity-type safety valves, and others. Spring-loaded safety valves operate primarily by the force of a spring. There are two types of such valves: those with a sealed design and those without a sealed design. For media that are flammable, explosive, or toxic, a sealed design is generally preferred, while for steam or inert gases, an unsealed design can be used. Additionally, there are spring-loaded safety valves equipped with a wrench and those that do not. The main purpose of the wrench is to check the flexibility of the valve disc; it can also be used for manual emergency pressure relief, as shown in Figure 3. Lever-type safety valves operate primarily by the force of a lever and weight, but their large size often limits their range of application. Use a safety valve with a radiator at higher temperatures. The main parameter of a safety valve is its discharge capacity, which is determined by the diameter of the valve seat and the opening height of the valve disc. Depending on this opening height, they are classified into two types: slightly open type and fully open type. Slightly open type refers to a valve disc opening height of 1/40 to 1/20 of the valve seat throat diameter. Full opening means that the opening height of the valve disc is 1/4 of the throat diameter of the valve seat. 2. Selection of safety valves: The nominal pressure of the safety valve is determined by the operating pressure, while its operating temperature range is determined by the operating temperature. The pressure setting range for the spring or lever is determined based on the calculated pressure setting value of the safety valve. The material and structural type of the safety valve are chosen according to the medium being used, and the throat diameter of the safety valve is calculated based on its discharge capacity. The following are the general rules for selecting safety valves. (l) Hot water boilers generally use unenclosed, wrench-operated slightly-opening safety valves. (2) Steam boilers or steam pipes generally use unenclosed, full-opening safety valves with levers. (3) Incompressible media such as water generally use closed micro-tilt safety valves, or safety relief valves. (4) Closed full-opening safety valves are generally used for high-pressure feed water, such as in high-pressure feed water heaters and heat exchangers. (5) Compressible media such as gases generally use closed full-opening safety valves, such as in gas storage tanks and gas pipelines. (6) Class E steam boilers generally use gravity-type safety valves. (7) Pulse-type safety valves are generally used in large-diameter, high-displacement, and high-pressure systems, such as temperature and pressure reduction devices and power station boilers, as shown in Figure 8. (8) Tank cars, road tankers, and storage tanks used for transporting liquefied gas generally employ internal safety valves, as shown in Figure 4. (9) Hydraulic safety valves are generally used at the top of oil tanks, and they need to be used in conjunction with breather valves. (10) Pilot-operated safety valves are generally used for underground drainage or gas pipelines, as shown in Figure 6. (11) A safety reflux valve is generally used on the liquid-phase reflux pipeline at the outlet of the pump in LPG station tanks. (12) Systems that may experience negative pressure or generate negative pressure during operation generally use vacuum negative pressure safety valves. (13) Bellows safety valves are generally used for systems with large backpressure fluctuations and for containers or piping systems that are toxic or flammable. (14) For systems with a low freezing point of the medium, insulated jacketed safety valves are generally used, as shown in Figure 7. 3. Comparison of major domestic manufacturers and selection of connection dimensions: There are many manufacturers in China that produce safety valves, and their connection dimensions are often not standardized. It is mainly divided into the following categories: (1) The general category, based on JB/T2203–1999 \"Structural Length of Spring-Operated Safety Valves\". At present, most domestic safety valve manufacturers design and produce in accordance with this standard. Such as Shanghai Yiwei Fluid Control Technology Co., Ltd., Luofu Boiler Accessories Factory, Hangzhou Valve Factory, Jiangsu Wujiang Valve Tools Factory, Shanghai Valve Factory, Kaifeng High-Pressure Valve Factory, Hai’an Valve Factory, and others. However, this standard is not perfect either; its specifications are incomplete. The maximum nominal diameter for slightly opening safety valves is DN100, while that for fully opening safety valves is DN200. The specifications DN65 and DN125 are missing in between. Based on the specifications of safety valves produced by our factory and the information available to us, the maximum nominal diameter for micro-opening safety valves is currently DN250, while that for full-opening safety valves is DN400. After my own verification, the connection dimensions provided by different manufacturers are not uniform; for example, for DN150 full-opening safety valves, those offered by Shanghai Yiwei Fluid Control Technology Co., Ltd., Luofu Boiler Accessories Factory, Shanghai Valve Factory, and Jiangsu Wujiang Valve Tools Factory all differ from each other. To establish a unified standard that allows components of the same specification to be interchanged during selection and installation, it is recommended that Hefei General Machinery Research Institute revise JB/T 2203–1999 \"Structural Length of Spring-Operated Safety Valves\". It is recommended that design institutes and users select products in accordance with the standards, while valve manufacturers should design and produce them in line with these standards. (2) The American standard system, based primarily on API526 \"Flanged Steel Safety Relief Valve\". The connection dimensions of safety valves used in domestically imported chemical equipment generally follow this standard, as shown in Figure 5. The nominal diameter of this standard ranges from DN25 to DN200 (1” to 8”), the nominal pressure is 2 to 42 MPa, and the throat diameter ranges from D-T (9.5 to 146 mm). This standard is relatively scientific and standardized, taking into account factors such as pressure, materials, temperature, and throat diameter as a whole. The specifications are determined based on the throat diameter; several specifications can exist for the same throat diameter, and conversely, several throat diameters may be available for the same specification. For throat diameters of DN100~DN150 (4”~6”), there are four options: L, M, N, and P. With the continuous advancement of international trade and the localization of imported equipment, this standard will be widely adopted in China. At present, this standard has not yet been converted into a national standard. (3) A series of pilot-operated pressure relief valves based on the internationally renowned Anderson Greenwood & Co., Ltd. In China, it is generally referred to as a pilot-operated safety valve. As shown in Figure 6, a pilot-operated safety valve consists of a main valve and a pilot valve, with the pilot valve controlling the opening and closing of the main valve. This type of valve has a large flow rate ； Unaffected by back pressure ； Leak-free operation can be carried out at a pressure very close to the operating pressure ； Advantages such as a small opening and closing pressure difference. It is generally applicable to natural gas pipelines, etc. Currently, there are no domestic standards for pilot-operated safety valves or connection dimensions. Moreover, this type of valve has just been developed and has not yet been widely adopted. Based on our factory’s experience and the information we have, most domestic manufacturers design and produce products according to that company’s specifications, such as Zhejiang Luofu Boiler Accessories Factory and the 11th Aerospace Research Institute. It is recommended that the Hefei General Machinery Research Institute draft and issue standards for pilot-operated safety valves as soon as possible. (4) The safety valves designed and developed by the 11th Research Institute of China National Space Administration form their own system. The HT series of safety valves developed by the 11th Aerospace Research Institute comes in a variety of types, including the HTO standard safety valve (as shown in Figure 1), the HTB balanced diaphragm safety valve, the HTR drain valve, the HTN special safety valve, and the HTGS high-performance steam safety valve. HTXY liquid pressure relief valves, HTXD pilot-operated safety valves, etc., all exhibit excellent performance. However, except that the pilot-operated safety valves of the HTXD series have the same connection dimensions as those of Anderson Greenwood Company, the rest do not conform to either American or Chinese standards. Please be sure to keep this in mind when making your choice. (5) The Type A and TA closed, fully open spring-loaded safety valves designed and developed by Lanzhou Refinery form their own system. This series has diameters ranging from DN25 to DN150 (1” to 6’), a nominal pressure of 1.6 to 4.0 MPa, and a throat diameter of D–R (9.5–115 mm). The connection dimensions of this system differ from both American and Chinese standards, and it is designed specifically for Lanzhou Refinery. (6) The impulse-type safety valve series designed for use with boilers, power plant equipment, and temperature and pressure reduction devices (as shown in Figure 8). Special safety valves designed for use with facilities such as Harbin Boiler Factory, Dongfang Boiler Factory, Wuhan Boiler Factory, and Qingdao Power Plant Auxiliary Equipment Factory. The structure and connection dimensions of such series of valves generally vary among different manufacturers, although some similarities may exist. When selecting, be sure to pay attention to the differences in the valve throat diameter and connection dimensions. 4. Calculation of throat diameter The calculation of throat diameter is generally carried out using the formula specified in Appendix 5 of the \"Safety Technical Specifications for Boilers and Pressure Vessels\". Or according to the formulas listed in Part 1: Design, of the Recommended Practice for the Design and Installation of Pressure Relief Systems in Refineries API RP 520. The calculation results of the above two formulas are basically the same or only slightly different. IV. Conclusion The selection of safety valves is a relatively important task, and whether the selection is appropriate or not will directly affect the safety of the equipment.