Valves are an important component in the pipeline system of petrochemical plants. They are of many types and large numbers. They are one of the main sources of leakage in the plant. Therefore, the requirements for valve leakage are very important. Valve sealing performance refers to the ability of each sealing part of the valve to prevent medium leakage.

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

1 my country's valve seal classification standards

At present, there are two main classification standards for valve sealing grades commonly used in my country.

1.1 The Chinese standard GB/T 13927 "Industrial Valve Pressure Test" for the classification of valve sealing grades.

1.2 Classification of valve sealing grades in China Machinery Industry Standard Machinery Industry Standard JB/T 9092 "Valve Inspection and Testing."

2 International classification standards for valve sealing grades

At present, there are mainly the following five classification standards for valve sealing grades commonly used in the surrounding area.

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 Industrial Valves and Metal Valves".

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

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

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

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

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

2.5 American Standards/American Instrument Institute Standards (ANSI/FCI) classification of control valve sealing grades

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

2.6 Classification of valve sealing grades according to EU standards

European Standard EN 12266-1 "Industrial Valves and Valves Tests" Part 1. Pressure test, test method and acceptance criteria-mandatory requirements.

3 Selection of valve sealing grade

3.1 Selection of domestic valve sealing grade

(1) The national standard GB/T13927 ((Industrial Valve Pressure Test) implemented on July 1, 2009 was formulated with reference to the European standard ISO 5208. It is suitable for industrial metal valves, including gate valves, globe valves, check valves, and plugs 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 levels of AA, CC, E, EE, F, and G have been added. The new version of the standard stipulates that "the choice of leakage level should be one of the strict requirements of the relevant valve product standard or order contract. If the product standard or order contract When there are no special regulations, non-metallic elastic sealing valves shall comply with the requirements of Class A, and the metal-sealed secondary valves shall comply with the requirements of Class D..." Generally, Class D is suitable for general valves, and the more critical valves should use leakage grades above Class D.

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

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

3.2 Selection of foreign valve sealing grades

(1) The former Soviet Union's classification of valve sealing grades was mainly applied in the 1950s. With the disintegration of the former Soviet Union, most countries now choose not to use this sealing grade classification, but choose the European and American standard sealing grade classification. The European standard EN 12266-1 sealing grade classification complies with the requirements of the International Organization for Standardization standard ISO 5208, but lacks the three grades of AA, CC and EE. Compared with the 1999 edition, ISO 5208 has added six grades AA, CC, E, EE, F and G. The ISO 5208 standard gives a comparison of several sealing grades with API 598 and EN 12266 standards.

The comparison of other nominal sizes and sealing grades can be obtained by calculating the leakage according to the caliber.

(2) The American Petroleum Institute standard API 598 is a commonly used inspection and pressure test standard for American standard valves. The manufacturer's standard MSSSP61 is often used for inspections of "fully open" and "fully closed" steel valves, but it does not apply to control valves. MSS SP61 inspection is usually not used for American standard valves. API 598 is suitable for the sealing performance test of valves manufactured in the following API standards:

Flange, lug, wafer and butt weld connection check valve API 594

Metal plug valve API 599 with flange, thread and butt welding connection

Steel gate valve and check valve of DN100 and below for oil and gas industry API 602

Corrosion-resistant bolted bonnet gate valve with flange and butt welding API 603

Flange, thread and butt welded metal ball valve API 608

Double flange type, lug and wafer butterfly valve APl609

Attention should be paid to engineering design: API 598-2004 compared with the 1996 edition, canceled the inspection and pressure test of API 600 ((Steel gate valve with bolted bonnet for oil and natural gas industry). API 600-2001 (ISO 10434 --1998) The standard stipulates that the sealing performance test of the valve should refer to ISO 5208, but the leakage amount in Table 17 and Table 18 in the standard is the same as that in the API 598-1996 standard, instead of adopting the ISO5208 sealing grade classification method. September 2009 The API 600 standard implemented on January 13 corrected this contradiction in the 2001 version, stipulating that the sealing performance test of the valve is in accordance with API 598, but there is no prescribed version, which is in contradiction with API 598-2004. Therefore, it is selected in the engineering design. When API 600 and its sealing performance test API 598 standard, the standard version must be clarified to ensure the unity of standard content.

(3) The American Petroleum Institute standard API 6D (ISO14313) "Petroleum and Natural Gas Industry One Pipeline Transportation System One Pipeline Valve" accepts the valve leakage criteria as follows: "The leakage of soft-sealed valves and oil-sealed plug valves shall not exceed ISO 5208 A Class (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 cannot be greater than the second of 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, there is a leakage requirement higher than the standard in the engineering design. It is given in the order contract. API 6D-2008 Appendix B Additional Test Requirements specifies the additional test requirements for valves f J that the manufacturer shall make when specified by the purchaser. The sealing test is divided into low pressure and high pressure gas sealing tests. The high pressure sealing test using inert gas as the test medium will replace the liquid sealing test and the liquid sealing test. According to the valve type, caliber and pressure level to choose the sealing test, please refer to the ISO 5208 standard. 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 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 working conditions may be reduced. The valve sealing test requirements should be selected reasonably according to the actual operating conditions of the medium, which can effectively reduce the production cost of the valve.

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

Consider the selection of metal elastic seals or metal seals for factors such as the opening frequency of the door. The sealing level 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 fewer options are used in engineering design. Generally, metal-sealed control valves are lower than grade Ⅳ, and more critical control valves are graded V or Ⅵ. The design of the control valve of the flare system of an ethylene plant has adopted the metal seal grade IV requirement, which is operating well.

(5) In addition, attention should be paid in 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. ISO 5208 and API 598 both 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 recommended that the chloride ion content of the water used in the seal test should be specified in the valve order contract.

4 Classification standards for sealing grades of low-leakage valves

Low-leakage valve refers to the small leakage that the actual leakage of the valve is very small, which can no longer be determined by conventional water pressure and air pressure sealing test. It needs more advanced means and instruments to detect the tiny external leakage. The small leakage of the valve to the external environment is called low leakage. At present, there are mainly the following three standards for detecting low leakage of valves commonly used in the world:

(1) US Environmental Protection Agency EPA method 21 "Volatile Organic Compound Leakage Test".

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

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

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

ISO 15848 Class A sealing grade, B, C grade sealing grade are the same as SHELLMESC SPE 77/312 standard. Generally, the sealing grade of low-leakage valves is lower than Grade B, while the sealing grade of bellows-sealed valves is lower than Grade A because of the metal bellows seal at the stem sealing part.

5 Selection of low leakage valve

Bellows-sealed valves are one of the low-leakage valves. In the past, bellows-sealed valves were generally used for working conditions with special requirements for valve external leakage levels. However, due to the difficult work and high technical requirements of bellows-sealed valves f-JJm, the bellows materials cannot be fully localized and the cost is too high. , Thereby restricting its extensive use in the petrochemical industry. At present, with the increasing awareness of people’s safety and environmental protection, the increase in technical cooperation with foreign countries, and the continuous strengthening of the technical strength of domestic valve manufacturers, domestic technicians have also continuously improved their understanding of low-leakage valves, making their application scope in Keep 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 emission of toxic, flammable and explosive media in the device, and avoid fire, explosion, poisoning, etc. caused by valve leakage. An accident that endangers life safety occurs. Compared with bellows valves, low-leakage valves that meet ISO15848 and SHELL MESC SPE 77/31 standards have simple structure, easy processing and manufacturing, and their cost is about 10% to 20% higher than that of general-purpose valves. According to the previous analysis and comparison of these two standard sealing grades, the leakage of tight grade B valves can generally meet the low leakage requirements of some special working conditions, the processing accuracy requirements are relatively easy to achieve, and the manufacturing cost does not increase much. , It can replace part of the bellows valve. At present, low-leakage valves have more practical significance for oil and gas field purification systems with high hydrogen sulfide content. Because hydrogen sulfide is a highly toxic and combustible gas, which is heavier than air and can accumulate in low-lying areas. Inhaling a certain concentration of hydrogen sulfide can harm the body and even cause death. Therefore, the leakage requirements of such natural gas purification facilities are more stringent.

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 a corrosive medium leaks, the metal at the leak will be corroded. As the leakage gap increases, the amount of leakage will increase quickly, and the valve will be scrapped. Therefore, for valves that work under high pressure or corrosive medium conditions, higher requirements should be put forward in terms of ensuring tightness. In pipelines that transport flammable, explosive and toxic media, the leakage of the media between the valve sealing surfaces may cause personal hazards, economic losses and even accidents. therefore.