Valve is an important component in the pipeline system of petrochemical plant. It has many types and large quantities. It is one of the main leakage sources in the plant. Therefore, the leakage requirements of the valve are very important. The sealing performance of the valve refers to the ability of the sealing parts of the valve to prevent the leakage of the medium.

The main sealing parts of the valve are: the matching surface between the opening and closing parts and the valve seat, the cooperation between the packing and the valve stem and the stuffing box, and the connection between the valve body and the valve cover. The first leakage is called internal leakage, which directly affects the ability of the valve to cut off the medium and the normal operation of the equipment. The leakage in the latter two places is called external leakage, that is, the medium leaks from the inside of the valve to the outside of the valve, which directly affects safe production, causing loss of working medium, economic loss of the enterprise, environmental pollution, and production accidents in serious cases. Especially for high temperature and high pressure, flammable, explosive, toxic or corrosive media, the external leakage of the valve is not allowed at all, because the consequences are more serious than the internal leakage, so the valve must have reliable sealing performance to meet its requirements. The requirements for leakage in the working conditions.

1 Classification standard of valve sealing grade in my country

At present, the more commonly used valve sealing grade classification standards in my country mainly include the following two.

1.1 Classification of valve sealing grades by Chinese national standards The national standard GB/T 13927 "Pressure Test for Industrial Valves".

1.2 Classification of valve sealing grades by Chinese machinery industry standards Machinery industry standard JB/T 9092 "Valve Inspection and Test".

2 International classification standards for valve sealing grades

At present, the commonly used valve sealing grade classification standards are mainly the following five.

2.1 Classification of valve sealing grades in the former Soviet Union

In order to select products according to the degree of sealing of the valve and the specified use, the valves are classified according to the degree of sealing.

2.2 Classification of valve sealing grades by the International Organization for Standardization

International Organization for Standardization Standard ISO5208 "Pressure Test of Metal Valves for Industrial Valves".

2.3 American Petroleum Institute (APl) classification of valve sealing grade American Petroleum Institute standard API 598--2004 "Valve Inspection and Test".

2.4 The American Valve and Fittings Industry Manufacturers Standardization Society (MSS) classification of valve sealing grades The American Valve and Fittings Industry Manufacturers Standardization Society standard "Pressure Test of Steel Valves" MSS SP61 allows the valve leakage requirements as follows:

(1) In the case where plastic or rubber is used for one of the sealing surfaces of the valve sealing pair, there should be no visible leakage for the duration of the sealing test.

(2) The maximum allowable leakage on each side when closed should be: liquid is nominal size (DN) per mm, 0.4 ml per hour; gas is nominal size (DN) per mm, 120 ml per hour.

(3) The allowable leakage of the check valve can be increased by 4 times.

2.5 American National Standard/American Instrument Institute Standard (ANSI/FCI) classification of control valve sealing grade

American National Standard American Instrument Association Standard ANSI/FCI70-2 (ASME B16.104) "Control Valve Seat Leakage".

2.6 Classification of valve sealing grades by EU standards

European Standard EN 12266-1 "Testing of Industrial Valves" Part l. Pressure Testing, Test Methods and Acceptance Criteria - Mandatory Requirements.

3 Selection of valve sealing grades

3.1 Selection of domestic valve sealing grades

(1) The national standard GB/T13927 ((Industrial Valve Pressure Test) implemented on July 1, 2009 is formulated with reference to the European standard ISO 5208. It is suitable for industrial metal valves, including gate valves, globe valves, check valves, cocks Inspection and pressure test of valves, ball valves and butterfly valves. The classification and maximum allowable leakage of the sealing test are the same as those specified in ISO 5208. This standard is a revision of GB/T13927 (General Valve Pressure Test, compared with GB/T13927, Six new grades AA, CC, E, EE, F and G are added. The new version of the standard stipulates that "the choice of leakage grade should be one of the stricter requirements in the relevant valve product standard or order contract. If the product standard or order contract requires If there is no special regulation, non-metallic elastic sealing valve shall be in accordance with A-level requirements, and metal-sealed auxiliary valves shall be in accordance with D-level requirements." Usually, D-level valves are suitable for general valves, and more critical valves should use D-level leakage levels or above.

(2) The mechanical industry standard JB/T 9092 "Valve Inspection and Test" is a revision to ZB J16006. The maximum allowable leakage of the seal test is based on the American Petroleum Institute standard API598--1996. It is suitable for inspection and pressure testing of valves for petroleum industry, including metal sealing pairs, elastic sealing pairs and non-metallic sealing pairs (such as ceramics), gate valves, globe valves, plug valves, ball valves, check valves and butterfly valves. Currently GB/T 9092 is being revised.

(3) Attention should be paid to the engineering design: the national standard GB/T19672 (Technical Conditions for Pipeline Valves) is formulated with reference to the European standard ISO 14313 and the American Petroleum Institute standard API 6D. "Pipeline Valve" is formulated with reference to the European standard ISO 14313. The acceptance criteria for valve leakage in GB/T 19672 and GB/T 20173 are the same as ISO 5208 Class A and Class D requirements. Therefore, there is a high level of engineering design. When the standard leakage is required, it should be given in the order contract.

3.2 Selection of foreign valve sealing grades

(1) The classification of valve sealing grades in the former Soviet Union was mainly used in the 1950s. With the disintegration of the former Soviet Union, most countries do not use this classification of sealing grades, but use European and American standard seals.

Seal class classification. The European standard EN 12266-1 sealing grade classification conforms to the provisions of the International Organization for Standardization standard ISO 5208, but lacks three grades AA, CC and EE. Compared with the 1999 edition, ISO 5208 has added six new grades: AA, CC, E, EE, F and G. The ISO 5208 standard gives a comparison of several seal classes with the API 598 and EN 12266 standards.

④. The comparison of sealing grades of other nominal sizes can be obtained by calculating leakage by caliber.

(2) American Petroleum Institute standard API 598 is the most commonly used inspection and pressure test standard for American Standard valves. The manufacturer's standard MSSSP61 is often used for inspection of "fully open" and "fully closed" steel valves, but

Not suitable for control valves. American Standard valves usually do not use MSS SP61 inspection. API 598 is applicable to the sealing performance test of valves manufactured to the following API standards:

Flanged, Lug, Wafer and Butt Welded Check Valves API 594

Metal Plug Valves with Flanged, Threaded and Butt Welded Connections API 599

Steel Gate Valves Globe and Check Valves of DNl00 and Below for Oil and Gas Industry API 602

Corrosion Resistant Bolted Bonnet Gate Valves with Flanged and Butt Welded Connections API 603

Metal Ball Valves with Flanged, Threaded and Butt Welded Connections API 608

Double Flanged, Lug and Wafer Butterfly Valve APl609

It should be noted in engineering design: API 598--2004, compared with the 1996 edition, cancels the inspection and pressure test of API 600 ((bolted bonnet steel gate valve for oil and gas industry). API 600-2001 (ISO 10434 --1998) standard stipulates that the sealing performance test of the valve refers to ISO 5208, but the leakage in Table 17 and Table 18 in the standard is the same as that specified in the API 598--1996 standard, rather than the classification method of the sealing level of ISO5208. September 2009 The API 600 standard implemented on January 13 corrected this contradiction in the 2001 edition, which stipulated that the sealing performance test of the valve was in accordance with API 598, but there was no specified version, which again contradicted API 598--2004. Therefore, the engineering design selected API 600 and its sealing performance test API 598 must specify the version of the standard to ensure the uniformity of the standard content.

(3) The American Petroleum Institute standard API 6D (ISO14313) "Oil and Gas Industry-Pipeline Transmission System-Pipeline Valves" accepts the valve leakage as follows: "The leakage of soft-sealed valves and oil-sealed plug valves shall not exceed ISO 5208 A Class D (no visible leakage), the leakage of metal seat valves shall not exceed ISO 5208 (1993) Class D, but according to the sealing test described in B.4, the leakage shall not be greater than ISO 5208 (1993) Class D. times, unless otherwise specified." Note in the standard: "Special applications may require leakage less than ISO 5208 (1993) Class D ¨J." Therefore, if there is a leakage requirement higher than the standard in engineering design, it should be given in the order contract. API 6D--2008 Appendix B Additional Test Requirements specifies the additional test requirements for valve f J to be performed by the manufacturer when specified by the purchaser. The sealing test is divided into low-pressure and high-pressure gas sealing test. The high-pressure sealing test using inert gas as the test medium will replace the liquid-up sealing test and the liquid-sealing test. According to the type, diameter and pressure level of the valve, the sealing test can be selected, and the provisions of the ISO 5208 standard can be referred to. For the valves on the long-distance pipeline GAl and the industrial pipeline GCl, it is recommended to use the low-pressure sealing test, which can improve the qualified product rate of the valve. When selecting the high-pressure sealing test, it should be noted that after the high-pressure sealing test of the elastic sealing valve, its sealing performance under low-pressure conditions may be reduced. The valve sealing test requirements should be selected reasonably according to the actual working conditions of the medium, which can effectively reduce the production cost of the valve.

4) The American National Standard of the American Instrument Association Standard ANSI/FCI 70-2 (ASME B16.104) is applicable to the regulation of the sealing level of the control valve. Engineering design should be based on the characteristics of the medium and the valve

Factors such as the opening frequency of the door should consider choosing a metal elastic seal or a metal seal. The sealing grade of the metal-sealed control valve shall be specified in the order contract. According to experience, for metal-sealed control valves, the requirements for grades I, II, and III are relatively low, and they are less used in engineering design. Generally, metal-sealed control valves are generally at least grade IV, and more critical control valves use grade V or VI. The control valve design of the torch system of an ethylene plant adopts the requirements of metal seal grade IV and runs well.

(4) In addition, attention should be paid to the engineering design: API 6D stipulates that the chloride ion content of the water used in the sealing test of austenitic stainless steel valves shall not exceed 30ug/g, and both ISO 5208 and API 598 stipulate that the sealing test of austenitic stainless steel valves The chloride ion content of the water used shall not exceed 100ug/g. Due to the different requirements of each standard, it is suggested that the chloride ion content of the water used in the sealing test should be clearly specified in the valve order contract.

4 Classification standard of low leakage valve sealing grade

Low leakage valve refers to the small leakage of the valve, which cannot be determined by conventional water pressure and air pressure sealing tests, and needs to be detected by more advanced means and instruments. This small leakage of the valve to the external environment is called low leakage. At present, the commonly used standards for detecting low leakage of valves in the world mainly include the following three:

(1) U.S. Environmental Protection Agency EPA method 21 "Leak Detection of Volatile Organic Components".

(2) International Organization for Standardization ISO 15848 (Industrial Valves: Low Leakage Measurement, Testing and Qualification Procedures."

(3) Shell Oil Company SHELL MESC SPE 77/312 "Industrial Valves: Low Leakage Measurement, Classification System, Qualification Procedure and Type Approval and Product Testing of On-Off Valves and Control Valves".

The EPA Method 21 standard of the U.S. Environmental Protection Agency only specifies the detection method without the classification of leakage grades. It is a local standard and regulation and is less used. The two standards of the International Organization for Standardization ISO 15848 and SHELL MESC SPE 77/312 of the Shell Oil Company evaluate the performance of the valve from three aspects: tightness level, durability level and temperature level. The tightness grade is divided into three grades: A, B, and C for the leakage of the valve stem and the valve body seal. The leakage requirement of the ISO 15848 standard valve body seal is ≤50 em3/m3, and the leakage at the valve stem has two standards. All are calculated according to the diameter of the valve stem.

5 Selection of low leakage valve

Bellows-sealed valves are one type of low-leakage valve. In the past, the bellows seal valve was generally used in the working conditions that had special requirements for the leakage level of the valve. However, due to the difficult work and high technical requirements of the bellows seal valve, the bellows material could not be fully localized, and the cost was too high. , thus restricting its widespread use in the petrochemical industry. At present, with the continuous enhancement of people's awareness of safety and environmental protection, the increase of technical cooperation with foreign countries, and the continuous strengthening of domestic valve manufacturers' own technical strength, the domestic technical personnel's understanding of low-leakage valves has also been continuously improved, making its application range in constantly expanding. If the valves selected for flammable, explosive and toxic media in petrochemical enterprises can meet the low leakage standard, this will undoubtedly greatly reduce the discharge of toxic, flammable and explosive media in the device, and avoid fire, explosion, poisoning, etc. caused by valve leakage. A life-threatening accident occurs. Compared with the bellows valve, the low-leakage valve that meets the ISO15848 and SHELL MESC SPE 77/31 standards has a simpler structure and is easier to manufacture, and its cost is about 10% to 20% higher than that of general-purpose valves.

6 Conclusion

When selecting the sealing level and the specified allowable leakage, it should be noted that the leakage of the medium between the sealing surfaces in the high-pressure valve will cause surface erosion. If there is leakage of corrosive medium, the metal at the leakage will be corroded. With the increase of the leakage gap, the leakage will also increase rapidly, and the valve will be scrapped. Therefore, for valves working in high pressure or corrosive medium conditions, higher requirements should be put forward in terms of ensuring tightness. In pipelines transporting flammable, explosive and toxic media, the leakage of media between valve sealing surfaces may cause personal harm, economic losses and even accidents. Therefore, for valves that transport flammable, explosive and toxic media, the sealing requirements should be reasonably put forward according to the hazard level of the media.

Any seal can sometimes allow a small amount of leakage, and if this leakage does not actually work, it can be considered a tight seal. The technical standard of valve manufacturing usually stipulates that a certain amount of leakage is allowed when the metal-to-metal seal is tested for sealing performance in the closed state. In order to ensure the high sealing performance of the valve, the sealing surface must be ground carefully to increase the specific pressure on the sealing surface, but it is less than the allowable specific pressure of the sealing surface material, and at the same time, the rigidity of the structure must be improved. The experience of using the valve shows that in many cases, it is unnecessary to put too high requirements on the sealing performance of the valve, because some working conditions completely allow a small amount of leakage of the medium, because this leakage is not enough to affect the use of the valve. Conversely, improving the sealing performance of these valves complicates the manufacturing process, increases costs, and creates unnecessary waste. The structural design and manufacturing of the valve itself have the most obvious impact on external leakage. Low-leakage valves have stricter requirements on the design, manufacture and processing of key components such as the valve body, valve stem and stuffing box, such as:

(1) The quality of the valve body and valve cover, especially when forging or casting, should avoid defects such as folding, slag inclusion, pores, tissue evacuation, hidden cracks and uneven composition.

(2) The processing quality of the parts at the connection between the valve stem and the valve body, especially the roughness of the valve stem and the stuffing box, the straightness of the valve stem, the verticality of the bonnet stuffing box hole and the machining accuracy.

(3) The structure of the valve stuffing box is selected. Since the seal at the valve stem is a dynamic seal, the packing is easy to wear during the rotation or sliding process of the valve stem. Special low-leakage packing seals and packing seal combinations should be selected, and the packing and the packing should be strictly controlled. Stem clearance, packing and stuffing box clearance.

In summary, the selection of valve types should not only meet the process conditions and standard specifications, but also fully consider various operating conditions. In the engineering design, the valve sealing grade should be selected to meet the principles of safety, rationality and economy.