Valve sealing level and selection

- May 11, 2018-

Valves are an important component of the pipeline system of petrochemical plants. They have many types and quantities and are one of the major sources of leakage in the installations. Therefore, the requirements for leakage of valves are of utmost importance. Valve sealing performance refers to the ability of the valve seals to prevent media leakage.


    The main sealing parts of the valve are: the matching surface between the opening and closing part and the valve seat, the fitting of the packing and the valve stem and the stuffing box, and the connection between the valve body and the valve cover. The leak at the first place is called inner leakage. It 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 leakage, that is, the leakage of the medium from the valve to the outside of the valve, which directly affects the safety of production, resulting in loss of working media and economic losses, environmental pollution, and in severe cases, production accidents. Especially for high-temperature, high-pressure, flammable, 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 a reliable sealing performance to meet its Use conditions for the amount of leakage requirements.


China's Valve Sealing Rating Classification Standard


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


1.1 China National Standard Classification of Valve Sealing Class National Standard GB/T 13927 "Industrial Valve Pressure Test".


1.2 Classification of Valve Sealing Classes by China Machinery Industry Standards Machinery industry standard JB/T 9092 "Valve Inspection and Testing".


International Valve Sealing Rating Classification Standard


At present, the commonly used classification criteria for valve sealing grades on the fence are mainly the following five types.


2.1 Classification of Valve Sealing Levels in the Former Soviet Union

In order to select the product according to the degree of sealing of the valve and the intended use, the valve is classified according to the degree of sealing.


2.2 Classification of Valve Sealing Levels by the International Organization for Standardization

ISO standard ISO5208 "Pressure test of industrial valve metal valves".


2.3 Classification of Valve Seal Classes by the American Petroleum Institute (APl) American Petroleum Institute Standard API 598--2004 "Checking and Testing of Valves".


2.4 US Valves and Fittings Manufacturers' Standardization Association (MSS) Classification of Valve Sealing Grades The American Valves and Fittings Industry Manufacturers' Standards Association Standard "Pressure Testing of Steel Valves" MSS SP61 allows valve leakage to be as follows:


(1) In the case of a plastic or rubber seal face in the valve seal pair, there shall be no visible leakage for the duration of the seal test.


(2) The maximum allowable leakage on each side when closed shall be: the liquid shall have a nominal size (DN) per millimetre, 0.4 ml per hour; the gas shall have a nominal size (DN) per millimetre and shall be 120 millilitres per hour.


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


2.5 Classification of Control Valve Sealing Levels by American National Standards/ANSI Standards (ANSI/FCI)


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


2.6 European Union Classification of Valve Sealing Ratings

European Standard EN 12266-1 "Testing of valves for industrial valves", Part 1. Pressure test, test method and acceptance criteria - mandatory requirements.


Selection of valve sealing level


3.1 Selection of Domestic Valve Sealing Grade


(1) The national standard GB/T13927 implemented on July 1, 2009 ((Industrial Valve Pressure Test) is based on the European standard ISO 5208. Suitable for industrial metal valves, including gate valves, globe valves, check valves, and cocks Valve, ball valve, butterfly valve inspection and pressure test.The classification and maximum allowable leakage of the sealing test are the same as those of ISO 5208. This standard is a revision of GB/T 13927 (General Valve Pressure Test), compared with GB/T 13927. Added AA, CC, E, EE, F and G levels.


The new version of the standard stipulates that “the choice of leakage level should be one of the requirements stipulated in the relevant valve product standards or in the ordering contract. If there is no special provision in the product standard or ordering contract, the non-metallic elastic sealing valve shall comply with Class A requirements and metal sealing Valves are required to meet Class D requirements.” Normally, Class D applies to general valves, and more critical valves should use Class D or higher leakage.


(2) The mechanical industry standard JB/T 9092 "Valve Inspection and Testing" is a revision of ZB J16006. The maximum allowable leakage of the seal test was established with reference to the American Petroleum Institute Standard API 598-1996. Applicable to petroleum industry valves, including metal seals, elastic seals and non-metal seals (such as ceramics) gate valves, globe valves, plug valves, ball valves, check valves, and butterfly valve inspections and pressure tests. The current GB/T 9092 is being revised.


(3) Engineering design should pay attention to: National Standard GB/T19672 (Technical Conditions for Pipeline Valves) is based on European Standard ISO 14313 and American Petroleum Institute Standard API 6D. National Standard GB/T 20173 Oil and Gas Industry Pipeline Transportation System “Pipeline Valves” was developed in accordance with the European Standard ISO 14313. Both GB/T 19672 and GB/T 20173 standards have the same acceptance criteria for valve leakage as ISO 5208 Class A and Class D. Therefore, there are high levels of engineering design. In the case of its standard leakage requirements, it should be given in the order contract.


3.2 Selection of foreign valve sealing grade


(1) The former Soviet Union classified the level of valve seals mainly in the 1950s. With the disintegration of the former Soviet Union, most countries do not use this classification of seal levels, but instead use European and American standards for seal classification. The European standard EN 12266-1 classifies the sealing class according to ISO 5208, but lacks AA, CC and EE.


Compared with the 1999 version, ISO 5208 has added six levels of AA, CC, E, EE, F, and G. The ISO 5208 standard gives several comparisons with the API 598 and EN 12266 standards. 4. The comparison of other nominal size seal grades can be calculated by calculating the leak size.


(2) The 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 the inspection of "fully open" and "fully closed" steel valves, but not for control valves. American Standard Valves usually do not use the MSS SP61 test. API 598 is suitable for valve sealing performance tests manufactured by the following API standards:


Flanged, lug, clip-on and butt welded connection check valve API 594 Flanged, threaded and butt welded connection metal stopcock API 599, oil and gas industry DNl00 and up steel valve made of globe valve and check Valve API 602 Corrosion-resistant flanges and butt-welded connections Bolted-bonnet gate valves API 603 Flanged, threaded and butt-welded metal ball valves API 608 Double-flanged, lug and wafer-type butterfly valves APl609 : Compared with the 1996 edition, API 598--2004 cancels the inspection and pressure test of API 600 (Steel Valve for Bolted Bonnet Steel Valves for the Oil and Gas Industry).


The API 600-2001 (ISO 10434--1998) standard stipulates that the sealing performance test of a valve is in accordance with ISO 5208, but the leakage amounts in Tables 17 and 18 of the standard are in accordance with the provisions of API 598--1996, rather than the sealing of ISO 5208. Hierarchical classification.


The contradiction in the 2001 edition was corrected in the API 600 standard implemented on September 1, 2009. The seal performance test of the valve was in accordance with API 598, but no version was specified. This contradicts API 598-2004.


Therefore, when selecting the API 600 and its sealing performance test API 598 standard in the engineering design, the standard version must be clearly defined to ensure the uniformity of the standard content.


(3) The Acceptance Criteria for the leakage of the valve by the American Petroleum Institute Standard API 6D (ISO 14313) "A pipeline valve for a pipeline transportation system in the oil and gas industry" is: "The leakage of the soft seal valve and the oil seal plug valve must not exceed ISO 5208 A Grade (without visible leakage), the leakage of metal valve seats shall not exceed ISO Class 5208 (1993) Class D, but the leakage test shall not be greater than Class D2 of ISO 5208 (1993) according to the sealing test described in B.4. Times, unless otherwise specified.” Note in the standard: “Specific applications may require less than the ISO 5208 (1993) D class rating.”


Therefore, in engineering design, there is a leakage rate higher than its standard, which should be given in the order contract. The additional test requirements of API 6D--2008, Annex B, stipulate the additional test requirements for the valve f J. to be made by the manufacturer at the time 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 upper sealing test and the liquid sealing test.


According to the type of valve, diameter and pressure level to select the sealing test, refer to the ISO 5208 standard. For the valves on the long-distance transmission pipelines GAl and industrial pipelines, the low-pressure sealing test is recommended to improve the qualified rate of the valves.


When using high-pressure sealing test, attention should be paid to the sealing performance of the elastic sealing valve after high-pressure sealing test, which may reduce its performance in low-pressure working conditions. Should be based on the actual conditions of the media use conditions, a reasonable choice of valve sealing test requirements, can effectively reduce the valve's production costs.


(4) American National Standard American Instrument Association Standard ANSI/FCI 70-2 (ASME B16.104) applies to the control valve seal rating. The engineering design should be based on the characteristics of the medium and the valve

The door opening frequency and other factors consider the choice of metal elastic seal or metal seal. Metal seal control valve seal level should be specified in the order contract.


According to experience, for metal-sealed control valves, Class I, II, and III requirements are relatively low, and the number of engineering designs used is relatively small. Normally, the general metal-sealed control valve is the lowest grade IV, and the more critical control valve is V or VI. The design of the control valve of a certain ethylene device torch system was selected to meet the IV requirements of the metal seal and it worked well.


(5) In other engineering design, attention should be paid: API 6D stipulates that the chloride ion content of the water used in austenitic stainless steel valve sealing test shall not exceed 30ug/g. Both ISO 5208 and API 598 stipulate the austenitic stainless steel valve seal test. The water used must have a chlorine ion content of no more than 100 μg/g.


Due to different requirements for each standard, it is recommended that the valve ordering contract be able to clearly define the chloride ion content of the water used in the sealing test.


Low leak valve seal rating standard


    Low-leakage valve means that the actual leakage of the valve is very small. It cannot be judged by conventional water pressure and air pressure sealing tests, and it needs a more advanced means and instrument to detect the minor leakage. This small leak of the valve to the outside environment is called a low leakage. At present, the most commonly used international standards for the detection of low leakage valves are the following three:


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


(2) International Standardization Organization 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, grading systems, qualification procedures, and type approval and product testing of on-off valves and control valves."


The US Environmental Protection Agency's EPA Method 21 standard only stipulates the detection method without the classification of leakage levels. It belongs to local standards and regulations and has few applications. Both ISO 15848 and SHELL MESC SPE 77/312, both of the ISO standards, assess valve performance in terms of tightness, durability, and temperature ratings.


The tightness class is based on the leakage of the valve stem and the valve body seal and is divided into three grades: A, B and C. For the ISO 15848 standard, the leakage of the valve body seal is ≤50 em3/m3, and the leakage at the valve stem is of two standards. Are calculated according to the diameter of the stem.


ISO 15848A has the highest seal rating, and Class B and C seal ratings are equivalent to the SHELLMESC SPE 77/312 standard. Normally, the low-leakage valve has a seal rating lower than that of Class B, while the bellows seal valve has a seal rating lower than Class A due to the metal bellows seal at the stem seal.


Low leakage valve selection


    Bellows sealed valves are one of the low-leakage valves. In the past, the bellows-sealed valves were generally used for valves with special requirements for leakage levels. However, because bellows-sealing valves f-JJm are difficult and technically demanding, their bellows materials cannot be fully domesticated and the cost is too high. , Which restricts its extensive use in the petrochemical industry.


    At present, as people's safety and environmental awareness continue to increase, with the increase of foreign technical cooperation, and the continuous strengthening of the domestic valve manufacturers' own technical strength, the awareness of low-leakage valves by domestic technical personnel has been continuously improved, and its application scope has been improved. Constantly expanding.


    If the valves used in petrochemical enterprises that are inflammable, explosive and toxic media can meet the low-leakage standards, this will undoubtedly greatly reduce the emissions of poisonous, flammable and explosive media in the device and avoid fire, explosion and poisoning caused by leakage of the valves. Accidents that endanger life safety. Low-leakage valves that meet ISO15848 and SHELL MESC SPE 77/31 standards are simpler in structure and easier to manufacture than bellows valves. The 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 levels, the leakage of tight-grade B-level valves can generally meet the low-leakage requirements of certain special working conditions, the machining 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, low-leakage valves have practical significance for oil and gas field purification systems with high hydrogen sulfide content.


    Because hydrogen sulfide is a highly toxic and flammable gas that is heavier than air and can accumulate in low-lying areas, inhalation of certain concentrations of hydrogen sulfide can harm the body and even cause death. Therefore, the leakage requirements for such natural gas purification facilities are more stringent.




    When selecting the sealing level and the specified allowable leakage, it should be noted that leakage of the medium between the sealing surfaces in the high pressure valve can cause surface erosion. If the corrosive medium leaks, the metal at the leak will be corroded. With the increase of the leak gap, the leakage will increase rapidly and the valve will be scrapped.