The selection
The operating pressure decides the nominal pressure of safety valve, operating temperature determines the operating temperature range, the calculated set pressure value decides the spring or lever set pressure range, and then according to the used media to decide material and structural type, and then by displacement to calculated the throat diameter. The followings are general rules of safety valve selection.
（l）Hot water boiler generally uses not closed low lift type safety valve with wrench.
（2）Steam boiler or steam pipe generally uses not closed fall lift type safety valve with wrench.
（3）Water and other liquids incompressible media generally use enclosed low lift type safety valve, or safety relief valve.
（4）High-pressure feedwater generally adopts closed fall lift-type safety valve, such as the high-pressure feedwater heater, heat exchanger, etc..
（5）Gas and other compressible medium generally uses closed fall lift-type safety valve, such as the gas tank, gas pipelines, etc.
（6）E-level steam boiler safety valve usually use static weight safety valve.
（7）Large-diameter, large displacement and high pressure systems are generally used impulse safety valve, such as temperature and pressure reduction devices, boiler and so on.
（8）Transporting liquefied gas train tankers, car tankers, storage tanks and others generally use built-in safety valve.
（9）The top of oil tank generally use hydraulic safety valve, should be used in conjunction with the breather valve.
（10）Underground drainage or natural gas pipelines generally use the pilot safety valve.
（11）Liquefied petroleum gas station tank liquid pump outlet refulx pipes use safety reflux valve.
（12）Negative pressure or the operating process may produce negative pressure systems typically use vacuum negative pressure safety valve.
（13）Big backpressure volatile and toxic flammable container or piping systems generally use bellows safety valve.
（14）Low medium freezing point system generally uses in insulation jacket safety valve. The domestic safety valve manufacturers are more about the comparison of major domestic manufacturers and connection dimensions selections, the connecting dimensions are mostly non-uniform size. It mainly consists the following broad categories:
（1）The main common used according to JB/T2203-1999 "spring type safety valve face to face dimensions". Recently the design and manufacture of most of the domestic safety valve manufacturers are according to this standard. Such as Zhejiang Louvre Boiler Accessories Factory, Hangzhou Valve Factory, Jiangsu Wujiang Valve Tool Factory, Shanghai Valve Factory, Kaifeng High Pressure Valve Factory, MSC valve factory. However, this standard is also not perfect, specifications incomplete, the largest nominal diameter of low lift type safety valve is DN 100, the largest nominal diameter of fall lift type safety valve is DN200, the middle lack of DN65, DN125 specifications.
Currently the largest nominal diameter of low lift safety valve can reach DN250, the largest nominal diameter of fall lift safety valve reach DN400. After my research, the connecting dimensions of various manufacturers are not quite uniform, such as the fall lift-type safety valve DN150, Zhejiang Louvre Boiler Accessories Factory, Shanghai Valve Factory and Jiangsu Wujiang Valve Tools Factory are all different. In order to have a unified standard, the user can swap the selection and installation the same size, it is recommended Hefei General Machinery Research Institute to amend the JB/T2203-1999 " spring type safety valve face to face dimensions ". and recommend Design Institute and the user to select according to the standard, safety valve manufacturer design and manufacture according to this standard..
（2）According to API526 "Flanged Steel Safety Relief Valve" American Standard system. China imported chemical equipment, etc. equipped safety valve connection dimensions generally in accordance with this standard, as shown in Figure 5. this standard nominal diameter is DN25 ~ DN200 (l "~ 8"), nominal pressure is 2 ~ 42MPa, throat diameter is D-T (9.5 ~ 146mm). This standard more scientific and norms, with a overall consideration of pressure, material, temperature, throat diameter and so on. According to throat diameter to determine specifications, the same throat diameter can have several specifications, on the opposite, the same size can select sereval throat diameter. For example, DN100 ~ DN150 (4 "~ 6") the throat diameter can be chose L, M, N, P kinds. As international trade and continued promotion of imports equipment, the standard will be expanded greatly in our country. At present the standard has not yet been translated into national standard.
（3）Piston Type Pilot Operated Safety Pressure Relief Valve series which according to the greatest impact international firm of Anderson, Greenwood (Anderson Greenwood & Co.). Domestic commonly known as pilot-type safety valve, as shown in Figure 6, the pilot-type safety valve is composed by main valve and pilot valve, pilot valve operates the opening and closing of the main valve. This valve displacement is large; without affecting by back pressure; can operate without leakage under the very close to the opening pressure; small hoist pressure and other advantages. It is generally applicable to natural gas pipelines. Currently there is no pilot safety valve standards and connecting dimensions standard. And this type of valve has just been developed, there is no widely. According to our factory experience and the available statistics show that most domestic manufacturers design and manufacture in accordance with the company's data, such as Zhejiang Louvre Boiler Accessories Factory, Space Research Institute 11. Hefei General Machinery Research Institute is suggested to prepare and publish pilot safety valve standards as early as possible.
（4）The 11th of China's Institute of Aerospace Industry Corporation designed and developed safety valve has formed a self-contained system. The HT series of safety valves developed by Space 11 are varieties, there are HTO general safety valve (shown in Figure 1), HTB balanced bellows safety valve, HTR Discharge valve, HTN special safety valve, HTGS high-performance Steam Safety Relief Valve. HTXY liquid pressure relief valve, HTXD pilot safety valve and so on, and with good performance. But apart from HTXD series of pilot safety valve are same sizes with the Anderson Greenwood's, the others are all different to the U.S. standard, national standard. This is point must be attentioned when selection.
（5）A-type, TA-type fall lift spring type safety valve designed and manufactured by Lanzhou refinery forms independent system. The series diameter is DN25 ~ DN150 (1 "~ 6 '), nominal pressure is l. 6 ~ 4.0MPa, throat diameter is D ~ R (9.5-115mm). this system connecting dimensions and all different to US Standard and the national standard, it is Lanzhou refine special.
（6）The impulse-type safety valve series are for the boiler, power station equipment, temperature and pressure reduction equipments. Such as the Harbin Boiler Works, Dongfang Boiler Factory, Wuhan Boiler Factory, Qingdao Power Station Auxiliary Equipment Factory and other supporting special safety valve. Such series of valve structure and connection sizes are not the same of the manufacturers, there may be some of the same. Please pay more attention to the difference of valve throat diameter and connecting dimensions when selection.
4、Throat diameter calculations are normally based on the formula of Annex 5 of "Boiler and Pressure Vessel Safety Technical Specification". Or according to the formula of the first design part of recommended implementation of APIRP520 refinery system. The results of these two formula is basically the same, or more or less.
