Author: Luofu Valve Group Co., Ltd. - Yan Zhengwei

Abstract: Starting from the definition, the difference and selection of safety valves and relief valves are introduced in detail. It also analyzes the main domestic safety valve series and lists the general rules for selecting safety valves.

Keywords: safety valve, relief valve, valve selection, safety valve purchase

Overview

Safety valves are used on boilers, pressure vessels and other pressure equipment Important safety accessory. Its action reliability and performance are directly related to the safety of equipment and people, and are closely related to energy saving and environmental protection. Some users and design departments always choose the wrong model when selecting models. For this reason, this article analyzes the selection of safety valves.

2. Definition

The so-called safety valve broadly speaking includes relief valves. From the perspective of management rules, they are directly installed on steam boilers or first-class pressure vessels. The necessary conditions are: Valves that must be approved by the technical supervision department are called safety valves in a narrow sense, and others are generally called relief valves. Safety valves and relief valves are very similar in structure and performance. Both of them automatically discharge the internal medium when the opening pressure is exceeded to ensure the safety of the production device. Due to this essential similarity, people often confuse the two when using them. In addition, some production equipment also stipulates in the rules that either type can be used. Therefore, the differences between the two are often overlooked. As a result, many problems have arisen. If you want to make a clearer definition of the two, you can understand it according to the definition stated in Chapter 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 protruding full opening action. Used in gas or steam applications, as shown in Figure 1.

(2) Relief Valve, also known as 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. Mainly used in fluid applications. As shown in Figure 2.

(3) Safety relief valve (Safet Relief Valve), also known as safety relief valve, is an automatic pressure relief device driven by medium pressure. It can be used as either a safety valve or a relief valve depending on the use occasion. Taking Japan as an example, there are relatively few clear definitions of safety valves and relief valves. Safety devices generally used for large energy storage pressure vessels such as boilers are called safety valves, and safety valves installed on pipelines or other facilities are called safety valves. It's a relief valve. However, according to the provisions of the "Technical Standards for Thermal Power Generation" of the Ministry of International Trade and Industry of Japan, safety valves are designated for important parts of the safety protection of the equipment, such as boilers, superheaters, reheaters, etc. When the lower side of the pressure reducing valve needs to be connected to the boiler and turbine, a relief valve or safety valve needs to be installed. Looking at it this way, safety valves are required to be more reliable than relief valves. In addition, judging from the recognition and regulations of safe emissions in the high-pressure gas management regulations of the Ministry of Labor, the Ministry of Transport, and the rules of shipping associations at all levels, we call those that guarantee the emissions a safety valve, but do not guarantee the emissions. The valve that releases the volume is called a relief valve. In China, whether fully opened or slightly opened, they are collectively called safety valves.

3. Selection

1. Classification

Currently mass-produced safety valves fall into two categories: spring type and rod type. In addition, there are impulse safety valves, pilot safety valves, safety switching valves, safety relief valves, dead weight safety valves, etc. Spring-type safety valves mainly rely on the force of springs to work. There are closed and unclosed spring-type safety valves. Generally, the closed type should be selected for flammable, explosive or toxic media, and the unclosed type can be selected for steam or inert gases. There are also spring-type safety valves with and without wrenches. The main function of the wrench is to check the flexibility of the valve disc. Sometimes it can also be used for manual emergency pressure relief, as shown in Figure 3. The lever safety valve mainly relies on the force of the lever weight to work. However, due to the large size of the lever safety valve, the range of selection is often limited. When the temperature is high, use a safety valve with a radiator.

The main parameter of the safety valve is displacement. This displacement is determined by the diameter of the valve seat and the opening height of the valve disc. Depending on the opening height, it is divided into two types: micro-opening type and full-opening type. The micro-opening type means that the opening height of the valve disc is 1/40~l/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 valve

The nominal pressure of the safety valve is determined by the operating pressure, the operating temperature range of the safety valve is determined by the operating temperature, and the constant pressure range of the spring or lever is determined by the calculated constant pressure value of the safety valve. , then determine the material and structural type of the safety valve according to the medium used, and then calculate the throat diameter of the safety valve based on the discharge volume of the safety valve. The following are general rules for selecting safety valves.

(l) Hot water boilers generally use unsealed slightly opened safety valves with a wrench.

(2) Steam boilers or steam pipelines generally use unsealed full-open safety valves with a wrench.

(3) Liquid incompressible media such as water generally use closed slightly opened safety valves or safety relief valves.

(4) High-pressure water supply generally uses closed full-open safety valves, such as high-pressure water supply heaters, heat exchangers, etc.

(5) Gas and other compressible media generally use closed full-open safety valves, such as gas storage tanks, gas pipelines, etc.

(6) E-class steam boilers generally use dead weight safety valves.

(7) Pulse safety valves are generally used in large-diameter, large-displacement and high-pressure systems, such as temperature and pressure reduction devices, power station boilers, etc., as shown in Figure 8.

(8) Train tankers, automobile tankers, storage tanks, etc. that transport liquefied gas generally use built-in safety valves, as shown in Figure 4.

(9) A hydraulic safety valve is generally used on the top of the oil tank and needs to be used in conjunction with the breathing valve.

(10) Underground drainage or natural gas pipelines generally use pilot-operated safety valves, as shown in Figure 6.

(11) A safety return valve is generally used on the liquid phase return pipeline at the outlet of the liquefied petroleum gas station tank pump.

(12) Systems with negative pressure or that may generate negative pressure during operation generally use vacuum negative pressure safety valves.

(13) Bellows safety valves are generally used for containers or piping systems with large back pressure fluctuations and toxic and flammable ones.

(14) Systems with a low medium freezing point generally use thermal insulation jacketed safety valves, as shown in Figure 7.

3. Comparison of major domestic manufacturers and selection of connection sizes

There are many domestic manufacturers of safety valves, and most of the connection sizes are not uniform. Mainly divided into the following categories:

(1) General category based on JB/T2203-1999 "Structural Length of Spring Type Safety Valve". At present, most domestic safety valve manufacturers design and produce according to this standard. Such as Zhejiang Luofu Boiler Accessories Factory, Hangzhou Valve Factory, Jiangsu Wujiang Valve Tool Factory, Shanghai Valve Factory, Kaifeng High Pressure Valve Factory, Haian Valve Factory, etc. However, this standard is not perfect, and the specifications are incomplete. The maximum nominal diameter of the slight lift safety valve is D NI 00, and the maximum nominal diameter of the full lift safety valve is DN200. There are two specifications of DN65 and DN125 missing in between. According to the specifications of the safety valves produced by our factory and the information we have, at present, the nominal diameter of the micro-lift safety valve reaches a maximum of DN250, and the nominal diameter of the full-lift safety valve reaches DN400. According to my research, the connection sizes of various manufacturers are not uniform. For example, the DN150 full-open safety valve has different sizes from Zhejiang Luofu Boiler Accessories Factory, Shanghai Valve Factory and Jiangsu Wujiang Valve Tool Factory. In order to have a unified standard and allow users to interchange the same specifications during selection and installation, it is recommended that Hefei General Machinery Research Institute revise JB/T2203-1999 "Structural Length of Spring Type Safety Valve". It is recommended that design institutes and users select according to standards, and safety valve manufacturers design and manufacture according to standards.

(2) American standard system based on API526 "Flanged Steel Safety Relief Valve". The connection dimensions of safety valves equipped with domestically imported chemical equipment, etc. generally comply with this standard, as shown in Figure 5. The nominal diameter of this standard is DN25~DN200 (l”~8”), the nominal pressure is 2~42MPa, and the throat diameter is from D-T (9.5~146mm). This standard is relatively scientific and standardized, taking into account pressure, materials, temperature, throat diameter, etc. The specifications are determined according to the orifice diameter. The same orifice diameter can have several specifications. On the contrary, the same specification may have several orifice diameters to choose from. For example, there are four throat diameters of DN100~DN150 (4”~6”) to choose from: L, M, N, and P. With the continuous advancement of international trade and the localization of imported equipment, this standard will be greatly promoted domestically. At present, this standard has not been converted into a national standard.

(3) Pilot Operated Pressure Relief Valves series based on Anderson Greenwood & Co., which has a relatively large influence in the world. In China, it is generally called a pilot-operated safety valve. As shown in Figure 6, the pilot-operated safety valve consists of a main valve and a pilot valve. The pilot valve operates the opening and closing of the main valve. This kind of valve has the advantages of large displacement; it is not affected by back pressure; it can operate without leakage at very close to the opening pressure; and it has small opening and closing pressure difference. Generally suitable for natural gas pipelines, etc. At present, there are no pilot safety valve standards and connection size standards in China. And this type of valve has just been developed and has not yet been widely promoted. According to the experience of our factory and the information we have, most domestic manufacturers design and manufacture according to the company's data, such as Zhejiang Luofu Boiler Accessories Factory, Aerospace 11th Research Institute, etc. It is recommended that Hefei General Machinery Research Institute draft and publish standards for pilot-operated safety valves as soon as possible.

(4) The safety valve designed and developed by the 11th Research Institute of China Aerospace Industry Corporation forms a system of its own. The HT series safety valves developed by Aerospace 11th Institute include HTO ordinary safety valves (as shown in Figure 1), HTB balanced bellows safety valves, HTR relief valves, HTN special safety valves, and HTGS high-performance steam safety valves. . HTXY liquid pressure relief valve, HTXD pilot safety valve, etc., with good performance. However, except that the HTXD series pilot-operated safety valves have the same connection dimensions as those of Anderson Greenwood Company, the rest are different from American standards and national standards. Please pay attention to this when choosing.

(5) The A-type and TA-type closed fully-open spring-type safety valves designed and developed by Lanzhou Refinery form a system of their own. The diameter of this series is DN25~DN150 (1"~6'), the nominal pressure is 1.6~4.0MPa, and the throat diameter is D~R (9.5-115mm). The connection dimensions of this system are different from the American standard and the national standard. Specially used for Lanlian.

(6) Impulse safety valve series for boilers, power station equipment, and temperature and pressure reduction devices (as shown in Figure 8). Such as Harbin Boiler Factory, Dongfang Boiler Factory, and Wuhan. Special safety valves for boiler plants, Qingdao power station auxiliary equipment plants, etc. The structure and connection dimensions of this series of valves are generally different from each manufacturer, and may be partially the same. When selecting, be sure to pay attention to the differences in valve throat diameter and connection dimensions.

4. Calculation of throat diameter

The calculation of throat diameter is generally based on the formula listed in Annex 5 of the "Safety Technical Specifications for Boilers and Pressure Vessels" or in accordance with APIRP520 refinery pressure relief system design and installation. The calculation results of the above two formulas are basically the same or have little difference.

IV. Conclusion

The selection of safety valves is. A relatively important job, the appropriate selection will directly affect the safety of the equipment. The above are some experience summaries of our factory. Due to space limitations, they may not be complete and are provided for your reference.

References 1. Compilation of foreign standards for safety valves (M) Hefei General Machinery Research Institute, 1985