Valve seal classification? How to choose?

Valve is an important component of the piping system of petrochemical plant. Due to its large number and large quantity, the valve is one of the main leakage sources in the installation. Therefore, the valve leakage is of vital importance.

Valve sealing performance refers to the valve sealing parts of the media to prevent the ability of leakage. The main sealing parts are: the opening and closing pieces and the seat between the anastomosis, packing and the stem and stuffing box with the valve body and the valve cover at the junction. The first 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. The latter two leaks are called external leakage, that is, the medium leaks from the valve to the outside of the valve, which has a direct impact on safety production, resulting in loss of working medium and economic loss of the enterprise, environmental pollution and serious production accidents. Especially for high temperature and pressure, flammable, toxic or corrosive media, the valve leakage is not allowed at all, because the consequences than the internal leakage is more serious, so the valve must have a reliable sealing performance to meet its Usage conditions on the amount of leakage requirements.

Valve seal classification? How to choose?

1, China's valve seal classification standards

At present, China's more commonly used valve sealing level classification criteria are the following two.

1.1 China's national standards on the classification of the valve seal National Standard GB / T 13927--2008 "industrial valve pressure test."

1.2 China Machinery Industry Standard classification of the valve sealing machinery industry standards JB / T 9092--1999 "valve inspection and testing."

2, the international valve seal classification standards

At present, the more commonly used on the valve sealing level classification criteria are the following five.

2.1 The former Soviet Union on the classification of the valve seal

In order to seal the valve and the provisions of the use of the selected products, according to the degree of sealing the valve has been classified, the basic principles of the classification in Table 3 and Table 4.

2.2 International Organization for Standardization of the classification of the valve seal

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

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

2.4 US Valve and Fitting Industry Manufacturers Association for Standardization (MSS) classification of the valve seal American Valve and Fitting Industry Manufacturers Standardization Association standard "pressure test of steel valves" MSS SP61 --- 2003 allowable valve leakage requirements are as follows :

In the case of a sealing face of a valve seal with plastic or rubber, there should 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 millimeter, 0.4 milliliters per hour; gas is nominal size (DN) per millimeter, 120 milliliters per hour. Check valve allows the leakage can be increased 4 times. 2.5 American National Standard / American Society of Instrument Standards (ANSI / FCI) Classification of Control Valve Sealing

American National Standard Bin American Instrument Association Standard ANSI / FCI70-2 (ASME B16.104) -2006 "control valve seat leakage."

2.6 EU standards on the classification of the valve seal

European Standard EN 12266-1-2003 "Part 1 of the Test for Industrial Valves. Pressure tests, test methods and acceptance criteria - Mandatory requirements.

3, the choice of sealing the valve

3.1 Domestic valve sealing level selection

(1) The national standard GB / T13927-2008 (Industrial valve pressure test), which was implemented on July 1, 2009, is formulated with reference to the European standard ISO 5208-2008 and is applicable to industrial metal valves including gate valves, globe valves , Check valves, plug valves, ball valves, butterfly valves and pressure test classification seal test the maximum allowable leakage and the provisions of ISO 5208--2008 The standard is GB / T13927--1992 (general valve pressure Test "revision, compared with the GB / T13927--1992, the new AA, CC, E, EE, F and G six new levels of standards stipulated that" the choice of leakage levels should be related to the product standard valve requirements or Ordering contract in a strict one .If the product standard or contract is not specified in the contract, non-metallic flexible sealing valve according to the requirements of Class A, metal sealing deputy valve according to D requirements. "Usually Class D for the general valve, The more critical valve should be used above grade D leak level.

(2) Machinery Industry Standard JB / T 9092--1999 "valve inspection and testing" is a revision of ZB J16006 --- 90. The maximum allowable leak test seal developed with reference to the American Petroleum Institute standards API598--1996 developed. It is suitable for the inspection and pressure test of valves for petroleum industry, including gate valves, globe valves, plug valves, ball valves, check valves and butterfly valves with metal sealing pair, flexible sealing pair and nonmetal sealing pair (such as ceramics). Currently GB / T 9092--1999 is being revised.

(3) The engineering design should pay attention to: National Standard GB / T19672-2005 (Technical Specifications of Pipeline Valves is formulated according to the European Standard ISO and American Petroleum Institute Standard API 6D- 2002. National Standard GB / T Oil and gas pipeline plumbing system piping valves are formulated with reference to the European standard ISO 14313--1999 GB / T 19672_2005 and GB / T these two standards on the valve leakage criteria for the acceptance Are required to meet the requirements of ISO 5208 Class A and Class D. Therefore, engineering design requirements higher than their standard shall be given in the order contract.

3.2 foreign valve sealing grade selection

(1) The classification of the valve seal of the former Soviet Union was mainly applied in the 1950s. With the disintegration of the former Soviet Union, most countries do not select this seal classification, but adopt the European and American standard seal classification. European Standard EN 12266-1_2003 Sealing level classification in line with the provisions of the International Organization for Standardization ISO 5208--2008, but the lack of AA, CC and EE three levels. ISO 5208-2008 compared with the 1999 version, the new AA, CC, E, EE, F and G six levels. The ISO 5208--2008 standard gives a comparison with several sealing classes of the API 598 and EN 12266 standards. Other nominal size seal level comparison can be calculated by caliber leakage.

(2) American Petroleum Institute Standard API 598 is the most commonly used test and pressure test standard for American Standard valves. The manufacturer's standard MSSSP61 is often tested for "fully open" and "fully closed" steel valves, but not for control valves. American standard valve is usually not used MSS SP61 inspection. API 598--2004 Valve seal performance test for the following API standards:

Flanges, Lugs, Clip and Butt Weld Check Valves API 594 Metal Plug Valves for Flanges, Threaded and Butt Weld Connections API 599 Steel Gate Valves Globe Check Valves and Check Valves for Oil and Gas Industry DNl00 and Below API 602 Corrosion-resistant bolted bonnet valve for flange and butt weld connection API 603 Metal ball valve for flange, thread and butt weld connection API 608 Double Flange, Lug and Clip Butterfly Valves APl609 In engineering design, please note: API 598--2004 Compared with the 1996 version, the inspection and pressure test on API 600 (Bolted bonnet steel gate valves for the petroleum and natural gas industries) was abolished as required by API 600 2001 (ISO) The sealing performance of the valve is tested in accordance with ISO 5208. However, the amount of leakage in Table 17 and Table 18 in the standard is in accordance with API 598-1996, rather than in accordance with ISO 5208. September 2009 13 APIs implemented 600--2009 standard to correct this conflict in the 2001 version, the provisions of the valve sealing performance test in accordance with API 598, but no version, which in turn contradicts API 598--2004 Therefore, the engineering design selection API 600 and Its sealing performance test API 598 standard time To clear the standard version, to ensure the unity of standard content.

(3) American Petroleum Institute Standard API 6D - 2008 (ISO14313--2007) "Pipeline Valves for One Pipeline Conveyor System in Oil and Gas Industry" The acceptance criteria for valve leakage are: "Leakage of soft seal valves and oil seal plug valves The amount may not exceed ISO 5208 Class A (no visible leakage), metal valve seat valve leakage shall not exceed the level of ISO 5208 (1993) D, but according to the sealing test B.4, the leakage can not be greater than ISO 5208 ( 1993) Class D, unless otherwise specified. "Standard Note:" Special applications may require less than ISO 5208 (1993) Class D leakage. "Therefore, there are higher than the standard Leakage requirements, should be given in the order contract. API 6D - 2008 Annex B Additional Test Requirements specifies the additional test requirements for valve f J to be made by the manufacturer at the time of purchase. Seal test points low-pressure and high-pressure gas seal test, with inert gas as the test medium will replace the high-pressure seal test liquid seal and liquid seal test. According to the type of valve, caliber and pressure level selection seal test, refer to the provisions of the ISO 5208 standard. For long-distance pipeline GAl, industrial pipeline valve recommended on the GCl low-pressure seal test, you can improve the valve qualified rate. Selection of high pressure seal test should pay attention to the elastic seal valve after the high pressure seal test, it may reduce its sealing performance under low pressure conditions. Should be based on the actual conditions of the use of media conditions, a reasonable choice of valve seal test requirements, can effectively reduce the valve production costs.

(4) American National Standard American Society of Instrumentation Standards ANSI / FCI 70-2 (ASME B16.104) -2006 for control valve sealing level requirements. 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. Metal seal control valve sealing level should be specified in the order contract. According to experience, for the metal seal control valve, I, Ⅱ, Ⅲ lower requirements, engineering design used less, usually the general choice of metal seal control valve minimum Ⅳ, the more critical control valve selection V or Ⅵ level. A vinyl plant torch system control valve design, the choice of metal sealing Ⅳ requirements, operating well.

(5) In addition, the engineering design should pay attention to: API 6D provides austenitic stainless steel valve seal test using water chloride content of 30ug / g, ISO 5208 and API 598 are provided austenitic stainless steel valve seal test The chloride content of water used shall not exceed 100ug / g. Due to the different requirements of each standard, it is suggested that the valve ordering contract best clarify the chloride ion content of the water used in the sealing test.

4, low leakage valve seal classification standards

Low leakage valve means the actual amount of leakage valve is very small, by conventional water pressure, pressure seal test can not determine, you need to use more advanced means and instruments to detect tiny external leakage. This tiny valve leak to the outside world is called a low leak. At present, the more commonly used standards for the detection of low leakage valve in the world are the following three:

(1) United States 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 MESC SPE 77/312 "Industrial Valves: Low Leakage Measurement, Grading Systems, Qualification Procedures and Type Approval and Product Testing of Switching Valves and Control Valves."

EPA Method 21 of the US Environmental Protection Agency provides only the detection method without the classification of leaks, is a local standard and regulations, less used. International Organization for Standardization ISO 15848 and Shell Oil Corporation SHELL MESC SPE 77/312 these two standards, the valve performance, from the close level, durability level and temperature rating of three aspects of assessment. The tightness level is for leaks at the stem and body seals and is divided into three levels, A, B and C, ≤50 em3 / m3 for ISO 15848 standard body seals and two for stem leakage All are calculated on the stem diameter.

ISO 15848A grade seal the highest level, B, c-level seal with SHELLMESC SPE 77/312 standard. Low-leakage valves typically have a seal rating of less than Class B, while bellows-sealed valves have a lower seal rating than Class A due to the use of metal bellows seals at the stem seals.

5, low leakage valve selection

Bellows sealed valve is one of the low leakage valve. In the past on the valve leakage levels have special requirements of the working conditions of the bellows seal valve generally used, but the bellows seal valve f-JJm workers difficult, high technical requirements, the bellows material can not be fully realized, the cost is too high , Thus restricting its extensive use in the petrochemical industry. At present, as people's safety, environmental protection and consciousness continue to increase, with the increase in foreign technical cooperation, as well as the domestic valve manufacturer's own technical capabilities continue to strengthen, domestic technicians for low-leakage valve is also increasing awareness of the scope of its application Continue to expand. For petrochemical enterprises, flammable and explosive, toxic medium selection of valves if they meet the low leakage standards, which will undoubtedly greatly reduce the device of toxic, flammable and explosive media emissions to avoid leakage caused by the valve fire, explosion, poisoning, etc. Accidents that endanger life and safety. The low leak valves, which meet ISO15848 and SHELL MESC SPE 77/31 standards, are simple in construction and easy to manufacture compared to bellows valves and cost about 10% to 20% more than conventional valves. According to the analysis and comparison of these two standard sealing levels, the leakage of the tightness class B valve can generally meet the requirements of low leakage in some special working conditions, the processing precision requirements are relatively easy to be achieved, and the manufacturing cost is not increased much , Which can replace part of the use of bellows valves. At present, low-leakage valves are more practical for oil and gas purification systems with high hydrogen sulfide content. Because hydrogen sulfide is a highly toxic and combustible gas that is heavier than air and can accumulate in low-lying areas, the absorption of certain concentrations of hydrogen sulfide can injure the body and even lead to death, thus requiring more stringent leaks for such natural gas purification facilities.

6, concluding remarks

When selecting the sealing grade and the specified allowable leakage, it should be noted that the leakage of the medium between the sealing surfaces in the high-pressure valve may cause surface erosion. If there is leakage of corrosive media, the metal in the leak will be corroded, with the leakage gap increases, the amount of leakage will quickly increase, so that the valve scrapped. Therefore, for high pressure or corrosive media working conditions of the valve, to ensure that the sealing should be put forward higher requirements. In the transport of flammable and toxic medium pipeline, the valve sealing surface between the media leakage may cause personal harm, economic losses and even accidents. Therefore, for the delivery of flammable and toxic valve medium, the level of danger should be reasonable to put forward the requirements of the tightness.

Any seal sometimes allows a slight amount of leakage, which can be considered a seal if the amount of leakage is not practical. The technical standards for the manufacture of valves usually require that metal to metal seals be permitted to have a certain amount of leakage when tested for sealing performance in a closed condition. In order to ensure the high sealing performance of the valve, it is necessary to grind the sealing surface particularly carefully to increase the specific pressure on the sealing surface, but less than the allowable specific pressure of the sealing surface material. At the same time, the rigidity of the structure should be increased. The experience with the use of valves has shown that in many cases it is not necessary to place excessive demands on the sealing performance of the valve because some conditions allow a slight leakage of the medium completely and the amount of leakage is not sufficient to affect the use of the valve. In contrast, increasing the sealing performance of these valves complicates the manufacturing process, increases costs and causes unnecessary waste. The structural design of the valve itself and the manufacturing and processing of the most obvious impact on their external leakage, low leakage valve valve body, stem and stuffing box and other key components of the design and manufacturing requirements are more stringent, for example:

(1) Body and bonnet quality, especially in the forging or casting should avoid folding, slag, porosity, tissue evacuation, hidden cracks and other defects and uneven composition.

(2) the processing quality of the parts such as 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 valve cover filler hole, and the machining precision.

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

In summary, the type of valve used in addition to meet the process conditions and standards, but also give full consideration to a variety of working conditions, engineering design should be selected to meet the selection of the valve sealing level to meet the safety, reasonable and economical principles.