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Sealing grade and selection of valves

DataTime:[2021-7-6]

Valve is an important component in the piping system of petrochemical plant. It is one of the main leakage sources in the plant with many kinds and large quantity. Therefore, the leakage requirement of valve is very important. Valve sealing performance refers to the ability of each sealing part of the valve to prevent medium leakage.

Sealing grade and selection of valves

The main sealing parts of the valve are: the matching 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 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 of the last 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 the safe production, causes the loss of working medium and enterprise economic loss, environmental pollution, and even causes production accidents in serious cases. Especially for high temperature and high pressure, flammable and 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 its use conditions on the leakage.

1. Classification standard of valve sealing grade in China

At present, there are two common classification standards of valve sealing grade in China.
  • 1.1 Classification of valve sealing grade in Chinese national standard GB / T 13927 pressure test for industrial valves.
  • 1.2 Classification of valve sealing grade in China mechanical industry standard JB / T 9092 inspection and test of valves.

2. International valve seal classification standard

At present, there are five commonly used classification standards of valve sealing grade.
2.1 Classification of valve sealing grade in former Soviet Union
In order to select the products according to the sealing degree and the specified use of the valves, the valves are classified according to the sealing degree.
2.2 ISO classification of valve sealing grade
ISO 5208 pressure testing of industrial valves and metal valves.
2.3 American Petroleum Institute (APL) classification of valve sealing grade American Petroleum Institute standard API 598-2004 inspection and testing of valves.
2.4 Classification of valve sealing class by MSS the allowable valve leakage requirements of MSS SP61 are as follows:
(1) If one sealing surface of the valve sealing pair is made of plastic or rubber, there shall be no visible leakage during the duration of the sealing test.
(2) The maximum allowable leakage on each side when closing shall be: the liquid is 0.4ml/mm and 0.4ml/h of nominal size (DN); The gas is nominal size (DN) 120 ml per millimeter per hour.
(3) The allowable leakage of check valve can be increased by 4 times.
2.5 Classification of control valve sealing class in ANSI / FCI
ANSI / FCI 70-2 (ASME b16.104) control valve seat leakage.
2.6 Classification of valve sealing grade in European standard
European standard EN 12266-1 testing of industrial valves Part 1. Pressure tests, test methods and acceptance criteria - mandatory requirements.

3. Selection of valve sealing grade

3.1 Selection of domestic valve sealing grade
(1) The national standard GB / T 13927 (pressure test for industrial valves) implemented on July 1, 2009 is formulated with reference to the European standard ISO 5208. It is suitable for inspection and pressure test of industrial metal valves, including gate valve, globe valve, check valve, plug valve, ball valve and butterfly valve. The classification and maximum allowable leakage of sealing test are the same as those specified in ISO 5208. This standard is a revision of GB / T 13927 (pressure test for general purpose valves). Compared with GB / t13927, it adds AA, CC, e, EE, F and G grades. The new version of the standard stipulates that "the selection of leakage grade shall be one of the strict requirements in the relevant valve product standard or order contract. If there is no special provision in the product standard or order contract, the non-metallic elastic sealing valve shall meet the requirements of class A, and the metal sealing auxiliary valve shall meet the requirements of class D. " Generally, class D is suitable for general valves, and the leakage class above class D should be selected for key valves.
(2) Mechanical industry standard JB / t9092 "inspection and test of valves" is a revision of zbj16006. The maximum allowable leakage of sealing test is made according to API 598-1996. It is suitable for inspection and pressure test of gate valve, globe valve, plug valve, ball valve, check valve and butterfly valve used in petroleum industry, including metal sealing pair, elastic sealing pair and non-metal sealing pair (such as ceramic). At present, GB / T 9092 is being revised.
(3) It should be noted in engineering design that the national standard GB / T 19672 (technical specifications for pipeline valves) is formulated with reference to European standard ISO 14313 and American Petroleum Institute standard API 6D. The national standard GB / T 20173 "pipeline valves for petroleum and natural gas industries" is formulated with reference to the European standard ISO 14313. Both GB / T 19672 and GB / T 20173 have the same acceptance criteria for valve leakage as ISO 5208 class A and D. Therefore, the leakage requirement higher than the standard in the engineering design should be given in the order contract.
3.2 Selection of foreign valve sealing grade
(1) The former Soviet Union mainly used the classification of valve sealing grade in the 1950s. With the disintegration of the former Soviet Union, most countries do not use this classification of valve sealing grade, but use European and American standards seal grade classification. European standard EN 12266-1 seal grade classification conforms to ISO 5208, but lacks AA, CC and EE. Compared with the 1999 edition, the ISO 5208 has added AA, CC, e, EE, F and G grades. ISO 5208 standard gives the comparison of several seal grades with API 598 and EN 12266 standards.
The comparison of other nominal size seal grades can be obtained by calculating the leakage according to the diameter.
(2) 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 the inspection of "fully open" and "fully closed" steel valves, but it is not applicable
Not suitable for control valves. Msssp61 is not usually used for American Standard valves. API598 is applicable to the sealing performance test of valves manufactured according to the following API standards:
  • Flange type, lug type, clip type and butt welded check valve API 594;
  • Flange, thread and butt welding metal plug valve API 599;
  • DN l00 and below steel gate valve, globe valve and check valve API 602 for petroleum and natural gas industry;
  • Flange and butt welded corrosion-resistant bolted bonnet gate valve API 603;
  • Flange, thread and butt welding metal ball valve API 608;
  • Double flange, lug and wafer butterfly valve API 609.
Attention should be paid in engineering design: API 598-2004 cancels the inspection and pressure test of API 600 (bolted bonnet steel gate valve for petroleum and natural gas industry) compared with 1996 edition. API 600-2001 (ISO 10434-1998) stipulates that the sealing performance test of valves shall refer to ISO 5208, but the leakage in Table 17 and table 18 of the standard is the same as that of API 598-1996, instead of ISO 5208. API 600 standard implemented in September, 2009 corrected this contradiction in 2001 edition, which stipulated that the sealing performance test of valves should be carried out in accordance with API 598, but there was no specified edition, which was in contradiction with API 598-2004. Therefore, when selecting API 600 and API 598 standard for sealing performance test in engineering design, the version of the standard must be clear to ensure the consistency of the content of the standard.
(3) American Petroleum Institute standard API 6D (ISO 14313) Petroleum and natural gas industries - pipeline transportation systems - pipeline valves: the acceptance criteria for valve leakage are as follows: "the leakage of soft seal valve and oil seal plug valve shall not exceed ISO 5208A (no visible leakage), and the leakage of metal seat valve shall not exceed ISO 5208 (1993) d, but according to the sealing test described in b.4, The leakage shall not be more than twice of 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, the leakage requirement higher than the standard in engineering design should be given in the order contract. API 6D-2008 Appendix B additional test requirements specifies the additional test requirements of valve FJ to be conducted by the manufacturer when specified by the buyer. Seal test includes low pressure and high pressure gas seal test. High pressure seal test with inert gas as test medium will replace liquid seal test and liquid seal test. According to the type, diameter and pressure level of the valve, the selection of sealing test can refer to the provisions of ISO 5208 standard. It is suggested that low pressure sealing test should be used for valves on gal and GCL of long distance pipeline to improve the qualified rate of valves. When selecting high pressure sealing test, it should be noted that the sealing performance of elastic sealing valve under low pressure condition may be reduced after high pressure sealing test. The production cost of valve can be effectively reduced by reasonably selecting the valve sealing test requirements according to the actual working conditions of medium.
(4) American national standard ANSI / FCI 70-2 (ASME B16.104) is applicable to control valve sealing class. In engineering design, metal elastic seal or metal seal should be selected according to the characteristics of medium and the opening frequency of valve. The sealing grade of metal sealing control valve shall be specified in the order contract. According to experience, for metal seal control valve, the requirements of grade I, II and III are relatively low, and the selection in engineering design is relatively small. Generally, grade IV is the lowest for metal seal control valve, and grade V or VI is the most critical control valve. The control valve of the flare system of an ethylene plant is designed to meet the requirements of grade IV metal seal, which runs well.
(5) In addition, attention should be paid in engineering design: API 6D stipulates that the chloride ion content of water used in austenitic stainless steel valve sealing test shall not exceed 30ug/g, and ISO 5208 and API 598 both stipulate that the chloride ion content of water used in austenitic stainless steel valve sealing test shall not exceed 100ug/g. Due to the different requirements of each standard, it is suggested that the chloride ion content of water used in sealing test should be specified in the valve order contract.

4. Classification standard for sealing grade of low leakage valve

Low leakage valve refers to the valve actual leakage is very small, rely on conventional water pressure, air pressure seal test has been unable to determine, need to use more advanced means and instruments to detect the small leakage. The tiny leakage of the valve to the external environment is called low leakage. At present, there are three standards commonly used to detect low leakage of valves in the world
(1) EPA method 21, leakage detection of volatile organic components.
(2) ISO 15848 (industrial valves: low leakage measurement, testing and qualification procedures).
(3) Shell MESC SPE 77 / 312 "industrial valves: low leakage measurement, classification system, qualification procedures and type approval and product testing of on-off and control valves".
EPA method 21 standard of the United States Environmental Protection Agency only specifies the detection method, but does not divide the leakage level. It belongs to the local standard regulations, and is rarely used. According to ISO 15848 and shell MESC SPE 77 / 312, the performance of valves is evaluated from three aspects: tightness grade, durability grade and temperature grade. According to the leakage of valve stem and valve body seal, the tightness grade is divided into a, B and C. for iso15848 standard, the leakage requirement of valve body seal is ≤ 50 em3/m3, while the leakage of valve stem is calculated according to the diameter of valve stem.
The sealing grade of ISO 15848A is the highest, and that of B and C is the same as shell MESC SPE 77/312. Generally, the sealing grade of low leakage valve is lower than grade B, while the sealing grade of bellows seal valve is lower than grade a due to the metal bellows seal used in the sealing part of valve stem.

5. Selection of low leakage valve

Bellows seal valve is one of the low leakage valves. In the past, bellows sealing valve was generally used in the working conditions with special requirements for valve leakage grade. However, due to the great difficulty and high technical requirements of bellows sealing valve f-jjm, its bellows material can not be completely localized and the cost is too high, which restricts its extensive application in 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, domestic technical personnel's understanding of low leakage valves is also constantly improving, so that its application scope is constantly expanding. If the valves selected for inflammable, explosive and toxic media in petrochemical enterprises can meet the low leakage standard, it will undoubtedly greatly reduce the emission of toxic, combustible and explosive media in the device, and avoid fire, explosion, poisoning and other accidents endangering life safety caused by valve leakage. Compared with bellows valves, low leakage valves meeting ISO 15848 and shell MESC SPE 77/31 standards have simple structure and easy manufacturing, and their cost is about 10% - 20% higher than that of general valves. According to the previous analysis and comparison of the two standard sealing grades, the leakage of tight grade B valve can generally meet the low leakage requirements of some special working conditions, the machining accuracy requirements are relatively easy to achieve, and the manufacturing cost is not much increased. It can replace part of the bellows valve. At present, low leakage valve has more practical significance for purification system of oil and gas field with high content of hydrogen sulfide. Because hydrogen sulfide is a highly toxic and combustible gas, heavier than air, and can gather in low-lying areas. Inhaling a certain concentration of hydrogen sulfide will harm the body and even lead to death. Therefore, the requirements for leakage of such natural gas purification facilities are more stringent.

6. Conclusion

When selecting the sealing grade and the specified allowable leakage, it should be noted that the leakage of medium between the sealing surfaces of high-pressure valves will cause surface erosion. If there is leakage of corrosive medium, the metal at the leakage place will be corroded. With the increase of leakage gap, the leakage volume will also increase rapidly, so that the valve will be scrapped. Therefore, for the high pressure or corrosive medium working condition of the valve, in ensuring the sealing should put forward higher requirements. In the pipeline conveying flammable, explosive and toxic media, the leakage of media between valve sealing surfaces may cause personal harm, economic loss and even accidents. Therefore, for the valves conveying flammable, explosive and toxic media, the requirements for sealing should be reasonably put forward according to the dangerous level of the media.
Any seal sometimes allows a small amount of leakage, if the leakage does not play a practical role, it can be considered as sealed. The technical standard of valve manufacturing usually stipulates that metal to metal seal is allowed to have a certain amount of leakage when sealing performance test is carried out under closed state. In order to ensure the high sealing performance of the valve, it is necessary to grind the sealing surface carefully, increase the specific pressure on the sealing surface, but it should be less than the allowable specific pressure of the sealing surface material, and improve the structural stiffness. The application experience of valves shows that in many cases, it is unnecessary to put forward too high requirements on the sealing performance of valves, because some working conditions completely allow a small amount of medium leakage, because the leakage is not enough to affect the use of valves. 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 process of the valve itself have the most obvious impact on its external leakage. For low leakage valves, the design, manufacturing and processing requirements of key components such as valve body, stem and stuffing box are more stringent, such as:
(1) The quality of valve body and cover, especially in forging or casting, should avoid folding, slag inclusion, porosity, microstructure evacuation, hidden cracks and other defects and uneven composition.
(2) The processing quality of the parts at the connection of valve stem and valve body, especially the roughness of valve stem and stuffing box, the straightness of valve stem, the verticality of valve cover stuffing box hole and the processing accuracy.
(3) The structure selection of the valve stuffing box, because the seal at the valve stem is dynamic seal, the packing is easy to wear during the rotation or sliding of the valve stem. It is necessary to select a special low leakage packing seal and packing seal combination, and strictly control the gap between the packing and the valve stem, and between the packing and the packing box.
To sum up, the selection of valve type should not only meet the process conditions and standards, but also fully consider various working conditions. In the engineering design, the valve sealing grade should meet the principles of safety, rationality and economy.
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)
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