Foreign standard safety valves and our country* * What are the differences between standard safety valves?

Thread Content

With the reform and opening up, the technology and projects introduced in our country have increased, and the use of safety valves is very common. In this way, more and more foreign safety valves are used. The selection of foreign safety valves will be somewhat different from the domestic selection. In order to better understand and use safety valves, please discuss various standard safety valves to learn from each other.* . Let’s start with some ideas: Preliminary comparison of API standard safety valves and national standard safety valves: 1. Preface API is the abbreviation of American Petroleum Institute. Safety valves using API standards are widely used in the chemical industry. Moreover, this standard has been adopted by various * * Recognized and implemented standard safety valve products are also found all over the world. For decades, domestic safety valves in our country have been designed and manufactured in accordance with the standards of the Ministry of Machinery (JB). This type of safety valve is commonly known as "national standard valve". After the 1970s, with the reform and opening up of our country, the national economy developed rapidly, and a large number of petrochemical, chemical, metallurgical, textile, electric power and other devices and equipment were introduced from abroad. At the same time, a large number of foreign safety valves were randomly imported. Quite a few of these safety valves are designed, manufactured, tested and accepted in accordance with API standards. This type of safety valve is commonly known as "American Standard Valve". Now in China, the application of API standard safety valves is becoming more and more common, and it has replaced JB and GB standard safety valves in major industrial fields. In order to meet the needs of the above-mentioned devices and equipment for the localization of safety valves, API standard safety valves must be discussed and studied. 2. Characteristics of API standards (ASME specifications) 1. Terminology Pressure relief device - A pressure relief device prevents a process system or pressure vessel from exceeding a predetermined pressure value due to an increase in fluid pressure during accidents or abnormal working conditions. Common ones include: Spring direct load pressure relief valve, pilot pressure relief valve, bursting disc, etc. Pressure relief valve (pressure relief valve) - is a pressure relief device. When the system is overpressured, the valve opens and when the system pressure returns to normal, the valve closes to prevent the medium from continuing to flow out. Common ones include: Safety valve, relief valve, safety relief valve, pilot pressure relief valve, etc. Safety valve - Safety valve is a pressure relief valve driven by inlet static pressure and characterized by rapid opening or sudden jump. It is applied to compressible fluids. Relief valve (relief valve) - A relief valve is a pressure relief valve driven by inlet static pressure. Its opening is usually proportional to the pressure increase above the pressure. It applies to incompressible fluids. Safety relief valve - A safety relief valve is a pressure relief valve that functions as a safety valve or relief valve depending on the application. 2. API standards (ASME specifications) Characteristics of safety valves (1) There are complete and comprehensive basic requirements for the structural design, displacement calculation, specifications, pressure levels, materials, manufacturing, testing, transportation, installation, use, inspection and maintenance of safety valves. API Standards (ASME Code) Catalog: l API RP 520 Sizing, Selection and Installation of Refinery Pressure Relief Devices Part I Sizing and Selection (7th Edition, January 2000) Part II Installation (4th Edition, December 1994) l API Std 526 Steel Flanged Pressure Relief Valves (5th Edition, June 2002) l API Std 527 Seat tightness of pressure relief valves (3rd edition July 1991 revalidated May 1996) l API RP 576 Inspection of Pressure Relief Devices (2nd edition December 2000) l ASME Boiler and Pressure Vessel Code (2001) Volume I Power Boilers Volume VIII Volume 1 Pressure Vessel Pressure Relief Devices l ASME PTC 25 Pressure Relief Devices—Performance Test Specification (2001) API RP 520 and 576 introduce the structure and characteristics of various types of safety valves. API RP 520 Part I lists the calculation methods for the required flow channel area under the displacement of various fluid media (gas, water vapor, liquid and two-phase flow), and gives the relevant correction coefficients. API Std 526 stipulates various specifications, pressure levels and basic material requirements for 14 types of flow channel diameters (orifice diameters) at different temperatures and pressures, and stipulates that the corresponding manufacturing and testing requirements of ASME specifications must be implemented. API Std 527 specifies the tightness test methods and requirements for pressure relief valves. API RP 520 Part II lists some requirements for installation. API RP 576 lists inspection and repair requirements for pressure relief devices. ASME PTC 25 provides standards for test apparatus, test methods, and test reports for pressure relief devices. The contents of JB and GB standards are relatively simple. (2) There are many specifications and varieties of safety valves. spring loaded: 1″~ 8″ (DN25~DN200), 14 types (D~T) flow channel diameter (d0=9.5~146), 25 specifications, 6 pressure levels (150~2500 lb), temperature range: -268℃~+538℃. Pilot type: 1″~8″, 14 kinds (D~T) flow channel diameter (d0=9.5~146), 21 kinds of specifications, 6 kinds of pressure levels (150~2500 lb), -268℃~+260℃ temperature range. API has many standard specifications and varieties, making it easy to choose safety valves with reasonable specifications. There is no standard for pilot valves in the JB and GB standards, and the spring-loaded type has fewer specifications and varieties, ranging from DN32 to DN200 and 8 types of flow channel diameters (d0=20~125). (3) The temperature-pressure level of the safety valve must be clearly selected. After the flow channel area is determined through the calculation of the required displacement, the appropriate pressure level can be determined according to the temperature range in the flow channel area table and is lower than the maximum allowable set pressure. There are no clear provisions in the JB and GB standards. (4) Basic material requirements for safety valves. The valve disc, valve seat and guide parts should be made of corrosion-resistant materials and should be able to meet the temperature and usage conditions. The materials of the valve body and bonnet can be different, but they must meet the minimum pressure-temperature requirements. Specifies the minimum grade of valve body and valve cover materials. The material grade of JB and GB standard safety valves is generally lower than API standard products. The spring material should be corrosion-resistant or have a corrosion-resistant coating, and should meet a series of performance requirements. JB standard safety valves have much fewer spring requirements. (5) There are basic requirements for the manufacture and testing of safety valves. The basic conditions of the manufacturing plant - process equipment, test equipment and quality control procedures can ensure the performance consistency of the safety valves produced. Safety valve performance test conditions—The test device and container should be of sufficient size and capacity. The capacity of the container in the test device must be sufficient, which is critical for correctly testing the performance of the safety valve. There are no provisions in this regard in the JB and GB standards. (6) Transportation and installation of safety valves Safety valves should be transported with protective plugs (plugs). The pressure loss of the safety valve inlet pipe ΔP≯3%Ps. There are no provisions in this regard in the JB and GB standards. (7) Use, inspection and maintenance of safety valves. Therefore, the API standard safety valve has been closely related to our country for decades. * Compared with the JB and GB standard safety valves produced by China, it has significant features and advancement. Therefore, in recent years, some companies have explicitly required safety valves, safety protection devices for their important devices and equipment, to be manufactured in accordance with API standards. For example, it is clearly stated in Sinopec’s “Technical Regulations for Safety Valve Installation” that: “Safety valves installed on equipment and pipelines that handle flammable, explosive, harmful, and toxic media should use API standard safety valves." In order to adapt to market demand, some domestic safety valve manufacturers have begun to design and manufacture API standard safety valves. However, due to limitations in technical level and equipment conditions, API standards cannot be fully and comprehensively implemented, and the actual results of different manufacturers vary greatly. Many manufacturers only use API standards for docking dimensions, while actual design, manufacturing, and testing still use JB and GB standards. Mainly manifested in: (1) The material is of low grade. The main reason is that the valve disc, valve seat and guide parts have poor corrosion resistance. After the safety valve was put online, due to the long-term influence of the working environment and atmosphere, the corrosion of parts caused the safety valve to not work properly when it needed to take off. (2) The performance of the spring does not meet the requirements of API standards (ASME specifications), and premature tripping or leakage failure may occur after the safety valve is put online. (3) Without testing equipment that meets the requirements of API standards (ASME specifications), product performance, such as displacement performance, is difficult to guarantee. In addition to problems in implementing API standards, domestic safety valve products also have a common problem of low displacement coefficient compared with foreign products. Different manufacturers and different product series have different displacement coefficients. Therefore, when selecting and calculating the safety valve, the calculation must be reviewed according to the displacement coefficient value provided by the manufacturer. 3. Common safety valve quality problems What we are talking about here is the quality problem of the safety valve itself. 　　⑴Open in advance. Not long after the safety valve went online, it opened in advance when the online pressure was lower than the set pressure. The main reason was that the spring force was weakened. The reasons for the weakened spring force include design factors and material selection issues, but more importantly, process stability processing. One of the important technical requirements for spring manufacturing is the amount of permanent deformation. The ASME specification implemented by API has clear requirements. After pressing three times and staying for 10 minutes, the permanent deformation amount shall not exceed 0.5% of the free height. 　　⑵He is stuck and does not return to his seat. The safety valve is stuck after opening and the valve disc cannot return to its seat. The main reason is that the safety valve's guide mechanism design and material selection are inappropriate. The safety valve does not operate online for a long time, and the maximum duration can be up to one year or even an overhaul period. However, it should be opened immediately as long as there is overpressure. The online environmental conditions of the safety valve are poor, exposed to the sun, rain, and the surrounding atmosphere. Therefore, it is necessary to select guide materials with high corrosion resistance according to different environmental conditions. 　　⑶Poor sealing performance. Safety valves generally do not operate online for a long time, but their sealing requirements are high. Since the safety valve is an automatic valve, the sealing load of the spring direct load safety valve is very small (the sealing load is generated by the difference between the setting pressure and the operating pressure). Therefore, special attention should be paid to the structural design, material selection and spring shape and position requirements of the safety valve. 　　⑷Performance is off the charts. Safety valves are different from other gate valves, globe valves and other valves in that they have performance requirements, and their performance needs to be accurately measured on test equipment with sufficient size and capacity, and most domestic manufacturers do not have this condition. Therefore, its allowable overpressure and reseating pressure are difficult to guarantee. Another reason is that the allowable range of domestic safety valve springs is too wide, and often overpressure and seat return cannot be taken into consideration. In addition, most manufacturers of rated displacement (rated displacement coefficient), which is crucial to safety valves, have not been certified, which is also one of the hidden dangers. 　　⑸Cannot be opened normally. Due to improper material selection and poor guidance, it often happens that the safety valve cannot open under normal pressure. A more prominent example is a compressor failure in a factory in Tianjin last year. The reason was that the safety valve that should be opened at 4.2MPa could not be opened until the inlet pressure reached 7.3MPa. 　In summary, the basic reasons for the five common quality failures of safety valves are:: 　　①Structural design and material selection ② Spring design and process ③ Manufacturing level ④ Performance testing conditions This post was last edited by dongf123 on 2009-2-4 15:15 ]