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What is the classification of valve seal class? How to choose?


Release time:

2018-10-11

Valve sealing performance refers to the ability of each sealing part of the valve to prevent leakage of the medium. The main sealing parts of the valve are: the joint surface between the opening and closing parts and the valve seat, the matching of the packing and the valve stem and the stuffing box, and the connection between the valve body and the valve cover. The first part of the leak is called internal leakage, which directly affects the ability of the valve to cut off the medium and the normal operation of the equipment. After the leakage of the two called leakage, that is, the leakage of media from the valve to the valve outside, it directly affects the safety of production, resulting in the loss of working media and economic losses, environmental pollution, serious when it will cause production accidents. Especially for high temperature and high pressure, flammable and explosive, toxic or corrosive media, the leakage of the valve is basically

Valve sealing performance refers to the ability of each sealing part of the valve to prevent leakage of the medium. The main sealing parts of the valve are: the joint surface between the opening and closing parts and the valve seat, the matching of the packing and the valve stem and the stuffing box, and the connection between the valve body and the valve cover. The first part of the leak is called internal leakage, which directly affects the ability of the valve to cut off the medium and the normal operation of the equipment. After the leakage of the two called leakage, that is, the leakage of media from the valve to the valve outside, it directly affects the safety of production, resulting in the loss of working media and economic losses, environmental pollution, serious when it will cause production accidents. Especially for high temperature and high pressure, flammable, explosive, toxic or corrosive media, the leakage of the valve is not allowed, because the consequences are more serious than the internal leakage, so the valve must have reliable sealing performance to meet the requirements of the leakage of its operating conditions.

What is the classification of valve seal class? How to choose?

1, China's valve seal grade classification standards

At present, China's more commonly used valve seal grade classification standards mainly have the following two.

1.1 China's national standards for the classification of valve sealing class national standards GB/T 13927-2008 "industrial valve pressure test".

1.2 China Machinery Industry Standard Classification of Valve Sealing Class Machinery Industry Standard JB/T 9092-1999 "Inspection and Testing of Valves".

2. International valve sealing grade classification standard

At present, the more commonly used on the surrounding international valve seal grade classification standards mainly have the following five.

2.1 the classification of the valve sealing class of the former Soviet Union

In order to select the product according to the sealing degree of the valve and the specified use, the valve is classified according to the sealing degree. The basic principles of the classification are shown in Table 3 and Table 4.

2.2 International Organization for Standardization classification of valve sealing class

International Organization for Standardization standard ISO5208--2008 "pressure test of industrial valves metal valves".

2.3 American Petroleum Institute (APl) classification of valve seal class American Petroleum Institute standard API 598-2004 "valve inspection and testing."

2.4 American valve and pipe fitting industry manufacturers standardization association (MSS) classification of valve sealing grade American valve and pipe fitting industry manufacturers standardization association standard "pressure test of steel valves" MSS SP61---2003 allowable valve leakage requirements are as follows:

Where plastic or rubber is used for one of the sealing faces in the valve sealing pair, there shall be no visible leakage for the duration of the sealing test. The maximum allowable leakage on each side when closed shall be: liquid is nominal size (DN) per mm, 0.4 ml per hour; gas is nominal size (DN) per mm, 120 ml per hour. The amount of leakage allowed by the check valve can be increased by 4 times. 2.5 American National Standard/American Instrument Association Standard (ANSI/FCI) Classification of Control Valve Sealing Class

American National Standard ANSI/FCI70-2(ASME B16.104)-2006 "Control Valve Seat Leakage".

Classification of valve sealing class 2.6 to EU standards

European Standard EN 12266-1-2003 Tests for industrial valves-Part l. Pressure tests, test methods and acceptance criteria-mandatory requirements.

3. Selection of valve sealing grade

Selection of 3.1 Domestic Valve Sealing Grade

(1) The national standard GB/T13927--2008 implemented on July 1, 2009 (((Industrial Valve Pressure Test "is formulated with reference to European standard ISO 5208-2008. Suitable for industrial metal valves, including gate valves, globe valves, check valves, plug valves, ball valves, butterfly valves inspection and pressure testing. The classification of the seal test and the maximum allowable leakage shall be the same as in ISO 5208-2008. This standard is a revision of GB/T13927--1992 (General Valve Pressure Test ". Compared with GB/T13927--1992, AA, CC, E, EE, F and G are added. The new version of the standard stipulates that "the choice of leakage level should be the relevant valve product standards or order contract requirements of a strict one. If there are no special provisions in the product standard or order contract, the non-metallic elastic sealing valve shall be in accordance with the requirements of Class A, and the metal sealing valve shall be in accordance with the requirements of Class D." Generally, Class D is suitable for general valves, and leakage grade above Class D should be selected for key valves.

(2) Machinery industry standard JB/T 9092-1999 "inspection and testing of valves" is a revision of ZB J16006---90. The maximum allowable leakage for the seal test is based on the American Petroleum Institute standard API598--1996. It is suitable for the inspection and pressure testing of valves used in the petroleum industry, including gate valves, globe valves, plug valves, ball valves, check valves and butterfly valves of metal sealing pairs, elastic sealing pairs and non-metal sealing pairs (such as ceramics).

(3) It should be noted in engineering design that the national standard GB/T19672--2005((Technical Conditions for Pipeline Valves is formulated with reference to European Standard ISO 14313-1999 and American Petroleum Institute Standard API 6 D--2002. The national standard GB/T 20173-2006 "Petroleum and Natural Gas Industry Pipeline Transportation System Pipeline Valves" is based on the European standard ISO 14313-1999. GB/T 19672_2005 and GB/T 20173-2006 These two standards for valve leakage acceptance criteria are the same as ISO 5208 A and D requirements. Therefore, if there is a leakage requirement higher than its standard in the engineering design, it should be given in the order contract.

3.2 Selection of Foreign Valve Sealing Grade

(1) the former Soviet Union on the valve seal grade classification is mainly applied in the 1950 s, with the disintegration of the former Soviet Union, most countries now do not choose this seal grade classification, but choose the European and American standard seal grade classification. The European standard EN 12266-1_2003 seal class classification conforms to the provisions of the International Organization for Standardization standard ISO 5208-2008, but lacks the three classes AA, CC and EE. Compared with the 1999 version, ISO 5208-2008 has added six new grades AA, CC, E, EE, F and G. The ISO 5208-2008 standard gives comparisons with API 598 and EN 12266 standards for several seal classes. The comparison of other nominal size seal grades can be obtained by calculating the leakage according to the caliber.

(2) American Petroleum Institute standard API 598 is the most commonly used valve inspection and pressure test standards. The manufacturer's standard MSSSP61 is often used for "fully open" and "fully closed" steel valves, but does not apply to control valves. American standard valves usually do not use MSS SP61 inspection. API 598-2004 is applicable to the following API standard manufacturing valve sealing performance test:

Flanged, Lug, Wafer and Butt-welded Check Valve API 594 Flanged, Threaded and Butt-welded Metal Plug Valve API 599 Steel Gate Valve Globe Valve and Check Valve API 602 Flanged and Butt-welded Corrosion Resistant Bolted Bonnet Gate Valve API 603 Flanged, Threaded and Butt-welded Metal Ball Valve API 608 Double Flanged, Lug and Butterfly Valve APl609 Engineering Design Note: API 598-2004 Compared with the 1996 version, the inspection and pressure test of API 600((Bolted Bonnet Steel Gate Valve for Petroleum and Natural Gas Industry "has been canceled. API 600-2001(ISO 10434-1998) standard requires that the sealing performance test of valves refer to ISO 5208, but the leakage in Table 17 and Table 18 of the standard is the same as API 598-1996 standard, instead of using the ISO5208 sealing grade classification method. The API 600-2009 standard implemented on September 13, 2009 corrected this contradiction in the 2001 version, stipulating that the sealing performance test of valves should be 598 according to API, but there is no specified version, which is in contradiction with API 598-2004. Therefore, when selecting API 600 and its sealing performance test API 598 standard in engineering design, the standard version must be clearly defined to ensure the unity of the standard content.

(3) American Petroleum Institute Standard API 6D-2008 (ISO14313--2007) "Petroleum and Natural Gas Industry-Pipeline Transportation System-Pipeline Valve" has acceptance criteria for valve leakage: "The leakage of soft seal valves and oil seal plug valves shall not exceed ISO 5208 Class A (no visible leakage), and 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 twice ISO 5208(1993) Class D, 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, which should be given in the order contract. Additional test requirements for valves f J to be performed by the manufacturer when specified by the purchaser are specified in API 6 D--2008 Appendix B Additional Test Requirements. The sealing test is divided into low pressure and high pressure gas sealing test. The high pressure sealing test with inert gas as the test medium will replace the liquid upper sealing test and liquid sealing test. According to the valve type, caliber and pressure level selection of sealing test, can refer to the provisions of ISO 5208 standard. For the valves on long-distance pipeline GAl and industrial pipeline GCl, it is recommended to use low-pressure sealing test, which can improve the qualified product rate of the valve. When selecting high-pressure sealing test, attention should be paid to the elastic sealing valve after high-pressure sealing test, which may reduce its sealing performance under low-pressure conditions. Should be based on the actual conditions of the use of the medium, the reasonable selection of valve sealing test requirements, can effectively reduce the production cost of the valve.

(4) The American National Instrument Association standard ANSI/FCI 70-2(ASME B16.104)-2006 applies to the control valve sealing level. Engineering design should be based on the characteristics of the medium and the valve opening frequency and other factors to consider the choice of metal elastic seal or metal seal. The sealing grade of metal seal control valve shall be specified in the order contract. According to experience, for the metal seal control valve, I, II, III level requirements are low, the engineering design of the selection of less, usually the general metal seal control valve selection of the lowest IV level, more critical control valve selection of V or VI level. The control valve design of the flare system of an ethylene plant selects the metal seal level IV requirements and runs well.

(5) In addition, it should be noted 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, and ISO 5208 and API 598 both stipulate that the chloride ion content of the water used in the sealing test of austenitic stainless steel valves shall not exceed 100ug/g. Due to the different requirements of each standard, it is recommended that the valve order contract should be able to clarify the chloride ion content of the water used in the sealing test.

4. Classification standard for sealing grade of low leakage valve

Low leakage valve refers to the actual leakage of the valve is very small, by conventional water pressure, air pressure seal test can not be determined, the need to use more advanced means and instruments to detect the small external leakage. This small leakage of the valve to the external environment is called low leakage. At present, there are three main standards for detecting low leakage of valves commonly used in the world:

(1) U.S. Environmental Protection Agency EPA Method 21, Volatile Organic Component Leak Detection.

(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 Systems, Qualification Procedures, Type Approval and Product Testing of On-Off and Control Valves.

The EPA Method 21 standard of the United States Environmental Protection Agency only stipulates the detection method without the division of leakage level, which belongs to the local standard regulation and is less applied. The International Organization for Standardization ISO 15848 and Shell Oil Company SHELL MESC SPE 77/312 these two standards, for the performance of the valve, from the tight grade, durability grade and temperature grade three aspects of assessment. The tightness level is divided into three levels: A,B and C for the leakage of the valve stem and the valve body seal. The leakage of the valve body seal of ISO 15848 standard is ≤ 50em3/m3, and the leakage of the valve stem is calculated according to the diameter of the valve stem.

ISO 15848A has the highest sealing grade, and the sealing grade of B and C is the same as SHELLMESC SPE 77/312 standard. Usually low leakage valve sealing grade is lower than the B level, and the bellows seal valve due to the valve stem sealing parts using metal bellows seal, the sealing grade is lower than the level.

5. Selection of low leakage valve

The bellows seal valve is one of the low leakage valves. In the past, bellows sealed valves were generally used for working conditions with special requirements for external leakage levels of valves. However, due to the difficulty in f-JJm and high technical requirements of bellows sealed valves, the bellows materials cannot be fully localized and the cost is too high, thus restricting their large-scale promotion and use in the petrochemical industry. At present, as people's safety and environmental awareness continues to increase, with the increase in foreign technical cooperation, and domestic valve manufacturers continue to strengthen their own technical strength, domestic technical personnel for low-leakage valve awareness is also constantly improving, so that its scope of application Continuously expand. If the valves selected for flammable, explosive and toxic media in petrochemical enterprises can meet the low leakage standards, this will undoubtedly greatly reduce the emissions of toxic, flammable and explosive media in the device, and avoid fire, explosion, poisoning and other life-threatening accidents caused by valve leakage. Meet the ISO15848, SHELL MESC SPE 77/31 standard low leakage valve and bellows valve compared to simple structure, easy manufacturing, its cost compared with the general valve about 10% to 20%. According to the analysis and comparison of the two standard sealing levels before, the leakage of the tight class B valve 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 is not increased much. It can replace the use of some bellows valves. At present, the low leakage valve has more practical significance for the oil and gas field purification system with high hydrogen sulfide content. Because hydrogen sulfide is a highly toxic and combustible gas, heavier than air, it can accumulate in low-lying areas. Inhaling a certain concentration of hydrogen sulfide will harm the body and even cause death. Therefore, the leakage requirements of such natural gas purification facilities are more stringent.

6. Concluding remarks

When selecting the sealing grade and the allowable leakage, attention should be paid to the leakage of the medium between the tight cover in the high-pressure valve will cause surface erosion. If there is leakage of corrosive media, the metal at the leakage will be corroded. With the increase of the leakage gap, the leakage will increase quickly, so that the valve will be scrapped. Therefore, for high pressure or corrosive medium working conditions of the valve, to ensure sealing should be put forward higher requirements. In the pipeline conveying flammable, explosive and toxic media, the leakage of the medium between the valve sealing surface may cause personal harm, economic loss and even accidents. Therefore, for the transportation of flammable, explosive and toxic media valves, the requirements for sealing should be reasonably put forward according to the dangerous level of the medium.

Any seal is sometimes allowed to have a small amount of leakage, if the leakage does not play an actual role, it can be considered to be sealed. The technical standards for valve manufacturing usually stipulate that the metal-to-metal seal allows a certain amount of leakage when the sealing performance test is carried out in the closed state. In order to ensure the high sealing performance of the valve, it is necessary to grind the sealing surface carefully and 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 improve the rigidity of the structure. Through the use of the valve experience shows that in many cases, the valve sealing performance requirements are not necessary, because some conditions completely allow the medium has a trace leakage, because the leakage is not enough to affect the use of the valve. On the contrary, improving the sealing performance of these valves will complicate the manufacturing process, increase the cost, and cause unnecessary waste. The structural design and manufacturing of the valve itself has the most obvious impact on its external leakage. Low leakage valves have stricter requirements on the design, manufacturing and processing of key components such as valve body, valve stem and stuffing box, for example:

(1) the quality of the valve body and valve cover, especially in the forging or casting should avoid its folding, slag, porosity, organization evacuation, hidden cracks and other defects and uneven composition.

(2) the stem and valve body at the junction of parts such as processing quality, especially the stem and stuffing box roughness, stem straightness, valve cover stuffing box hole verticality and processing accuracy.

(3) The structure of the valve stuffing box is selected. As the seal at the valve stem is a dynamic seal, the packing is easy to wear during rotation or sliding of the valve stem. Special low-leakage packing seal and packing seal combination are required, and the clearance between the packing and the valve stem, and the clearance between the packing and the stuffing box are strictly controlled.

In summary, the selection of the valve type in addition to meet the process conditions and standards, but also should fully consider the use of a variety of conditions, engineering design should strive to choose the valve seal level to meet the safety, reasonable and economic principles.