Key words:Safety valve relief valve selection
I. Overview
Safety valves are important safety accessories on boilers, pressure vessels and other pressure equipment.Its operational reliability and performance are directly related to equipment and personal safety, and are closely related to energy conservation and environmental protection.However, some users and design departments always choose the wrong model when selecting a model.For this reason, this article analyzes the selection of safety valves.
XNUMX. Definition
In a broad sense, the so-called safety valve includes a relief valve. From the perspective of management rules, it is directly installed on a steam boiler or a type of pressure vessel. The necessary condition is that the valve must be approved by the technical supervision department. In a narrow sense, it is called a safety valve. Others are generally called relief valves.The safety valve and the relief valve 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.Because of this inherent similarity, people often confuse the two when using them. In addition, some production devices also stipulate that either can be used in the rules.Therefore, the difference between the two is often overlooked.Therefore, many questions have arisen.If you want to make a clearer definition of the two, you can understand it according to the definition set out in the first article of the "ASME Boiler and Pressure Vessel Code": (l) Safety Valve is a kind of static pressure of the medium in front of the valve. Driven automatic pressure relief device.It is characterized by a sudden full opening action.Used in gas or steam applications.
(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.
(3) Safet Relief Valve (Safet Relief Valve), also known as a safety relief valve, is an automatic pressure relief device driven by medium pressure.It can be used as both a safety valve and a relief valve according to different applications.Take Japan as an example. There are relatively few clear definitions for safety valves and relief valves. Generally, safety devices used for large energy storage pressure vessels such as boilers are called safety valves, which are installed on pipelines or other facilities. It is a relief valve.However, according to the "Technical Standards for Thermal Power Generation" issued by the Ministry of International Trade and Industry of Japan, the important parts of equipment safety protection are designated to use safety valves, such as boilers, superheaters, and reheaters.Where the lower side of the pressure reducing valve needs to be connected to the boiler and the turbine, a relief valve or safety valve needs to be installed.In this way, the safety valve is required to be more reliable than the relief valve.In addition, from the Japanese Ministry of Labor’s high-pressure gas management rules, the Ministry of Transport, and the rules of the shipping associations at all levels, we refer to the safety valve that guarantees the discharge as a safety valve, and we do not guarantee it. The discharge valve is called a relief valve.Regardless of whether the full lift type or the slightly lifted type is collectively referred to as a safety valve in China.
Three, selection
1.classification
At present, the mass-produced safety valves are divided into two categories: spring type and rod type.There are also impulse safety valves, pilot safety valves, safety switching valves, safety decompression valves, deadweight safety valves, etc.The spring-loaded safety valve mainly relies on the force of the spring to work. There are closed and unclosed spring-loaded safety valves. Generally, flammable, explosive or toxic media should be closed. Steam or inert gas can be unclosed. In the spring-loaded safety valve, there are also those with a wrench and without a wrench.The function of the wrench is mainly to check the flexibility of the valve disc, and sometimes it can also be used for manual emergency pressure relief, as shown in Figure 3.The lever type safety valve mainly relies on the force of the lever hammer to work, but the large size of the lever type safety valve often limits the scope of selection.Choose a safety valve with a radiator when the temperature is high.
The main parameter of the safety valve is the displacement. This displacement is determined by the diameter of the valve seat and the opening height of the valve clack. It is divided into two types: micro-opening and full-opening according to the opening height.Micro-opening means that the opening height of the valve flap is 1/40～l/20 of the throat diameter of the valve seat.Full-open type means that the opening height of the valve flap is 1/4 of the throat diameter of the valve seat.
2.Selection of safety valve
The operating pressure determines the nominal pressure of the safety valve, the operating temperature determines the operating temperature range of the safety valve, the calculated constant pressure value of the safety valve determines the constant pressure range of the spring or lever, and then the material and the safety valve are determined according to the medium used. Structure type, and then calculate the throat diameter of the safety valve based on the relief valve discharge volume.The following are general rules for the selection of safety valves. (L) Hot water boilers generally use open safety valves with wrenches. (2) Steam boilers or steam pipes generally use unclosed full-open safety valves with wrenches. (3) Liquid incompressible media such as water generally use a closed micro-open safety valve or a safety relief valve. (4) High-pressure water supply generally uses closed full-open safety valves, such as high-pressure water heaters, heat exchangers, etc. (5) Compressible media such as gas are generally closed and fully lifted safety valves, such as gas storage tanks, gas pipelines, etc. (6) E-class steam boilers generally use deadweight safety valves. (7) Large-diameter, large-displacement and high-pressure systems generally use pulsed safety valves, such as temperature and pressure reduction devices, power station boilers, etc., as shown in Figure 8. (8) Train tank cars, automobile tank cars, storage tanks, etc. transporting liquefied gas generally use built-in safety valves, as shown in Figure 4. (9) Hydraulic safety valves are generally used on the top of oil tanks, which need to be used in conjunction with breathing valves. (10) Pilot safety valves are generally used for underground drainage or natural gas pipelines, as shown in Figure 6. (11) A safety return valve is generally used on the liquid phase return pipeline at the outlet of the tank pump of the LPG station. (12) Vacuum negative pressure safety valve is generally used in systems with negative pressure or negative pressure during operation. (13) Bellows safety valves are generally used for containers or piping systems with large back pressure fluctuations and toxic and flammable. (14) The system with a low freezing point of the medium generally selects the insulation jacket type safety valve.
3.Comparison of major domestic manufacturers and choice of connection size
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-purpose category based on JB/T2203-1999 "Structural Length of Spring Type Safety Valves".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 not complete. The maximum nominal diameter of the micro-lift safety valve is D NI 00, and the maximum nominal diameter of the full-lift safety valve is DN200. The two specifications of DN65 and DN125 are missing.According to the safety valve specifications produced by our factory and the data we have, the maximum nominal diameter of the micro-lift safety valve reaches DN250, and the nominal diameter of the full-lift safety valve reaches DN400.I have verified that the connection dimensions of various manufacturers are not uniform, such as DN150 full lift safety valve, Zhejiang Luofu Boiler Accessory Factory, Shanghai Valve Factory and Jiangsu Wujiang Valve Tool Factory are all different.In order to have a unified standard, the user can interchange the same specifications when selecting and installing, it is recommended that Hefei General Machinery Research Institute revise JB/T2203-1999 "Spring Type Safety Valve Structure Length".It is recommended that the design institute and users choose according to the standard, and the safety valve manufacturer designs and manufactures according to the standard.
(2) The American standard system based on API526 "Flanged Steel Safety Relief Valve" (Flanged Steel Safety Relief Valve).The safety valve connection size of domestic imported chemical equipment is generally in accordance 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 (9~5mm).This standard is relatively scientific and standardized, and comprehensively considers pressure, materials, temperature, and throat diameter.According to the throat diameter to determine the specifications, the same throat diameter can have several specifications, on the contrary, the same specification may have several throat diameters to choose from.For example, DN146~DN100 (150”~4”) have four choices of throat diameter: L, M, N, and P.With the continuous advancement of international trade and the localization of imported equipment, this standard will be greatly promoted in China.At present, the standard has not been converted into a national standard.
(3) Pilot Operated Pressure Relief Valves (Pilot Operated Pressure Relief Valves) series based on Anderson Greenwood & Co., which has a relatively large international influence.It is generally called a pilot safety valve in China. As shown in Figure 6, the pilot safety valve is composed of a main valve and a pilot valve, and the pilot valve operates the opening and closing of the main valve.This kind of valve has a large displacement; it is not affected by back pressure; it can operate without leakage at very close to the opening pressure; it has advantages such as small opening and closing pressure difference.Generally applicable to natural gas pipelines, etc.At present, there is no pilot safety valve standard and connection size standard in China.And this type of valve has just been developed and has not been widely promoted.According to our factory's experience and the information we have, most domestic manufacturers design and manufacture according to the company's data, such as Zhejiang Luofu Boiler Accessory Factory, Aerospace Eleven Research Institute, etc.It is recommended that Hefei General Machinery Research Institute draft and release pilot safety valve standards as soon as possible.
(4) The safety valve designed and developed by the 1th Research Institute of China Aerospace Industry Corporation forms a system of its own.There are many varieties of HT series safety valves developed by Aerospace XI, including HTO ordinary safety valves (as shown in Figure XNUMX), 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-operated safety valve, etc., and have good performance.However, except for the HTXD series of pilot-operated safety valves, which have the same connection size with Anderson Greenwood, the rest are different from the American standard and the national standard.Please pay attention to this when choosing.
（5）以兰州炼油厂设计研制开发的A型、TA型封闭全开启弹簧式安全阀自成一个体系。该系列口径DN25~DN150 （1”~6’），公称压力l．6~4.0MPa，喉径D~R（9．5－115mm）。该体系连接尺寸与美标及国标均不相同，为兰炼专用。
(6) A series of impulse safety valves (as shown in Figure 8) for boilers, power station equipment, and temperature and pressure reducing devices.Such as Harbin Boiler Factory, Dongfang Boiler Factory, Wuhan Boiler Factory, Qingdao Power Station Auxiliary Equipment Factory and other supporting special safety valves.The structure and connection dimensions of this series of valves are generally different from each manufacturer, and some of them may be the same.Be sure to pay attention to the difference between valve throat diameter and connection size when selecting.
4.Calculation of throat diameter
The calculation of the throat diameter is generally in accordance with the formula listed in Annex 520 of the "Boiler and Pressure Vessel Safety Technical Code".Or in accordance with the formula listed in "Part One Design of the Recommended Implementation Method for Design and Installation of APIRPXNUMX Refinery Pressure Relief System".The calculation results of the above two formulas are basically the same or not much different.
Fourth, the 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.The above lists are some summary of our factory's experience, due to space limitations, may not be complete, for your reference.