Valve is an important component in the pipeline system of petrochemical plant. It is one of the main leakage sources in the plant because of its variety and quantity.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 anastomosing surface between the opening and closing parts and the valve seat, the fit between the packing and the stem and the packing box, the connection between the body and the bonnet.The first leakage 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 leakage is called leakage, that is, the medium from the valve leakage to the outside of the valve, it directly affects the safety of production, resulting in the loss of working medium and enterprise economic losses, environmental pollution, serious will cause production accidents.Especially for high temperature and high pressure, flammable and explosive, toxic or corrosive medium, the valve leakage is not allowed at all, because the consequences of its more serious than internal leakage, so the valve must have a reliable sealing performance, to meet the requirements of its operating conditions for leakage.

1. Classification standard of China's valve seal class

At present our country compares commonly used valve seal class classification standard mainly has the following two kinds.

National standard GB/T 13927 industrial valve pressure test.

Mechanical industry standard JB/T 9092 inspection and test of valves.

2.International classification standards for valve seals

At present, there are five kinds of commonly used valve seal classification standards.

2.1 classification of valve seal grades in the former Soviet union

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

2.2 classification of valve seal grades by iso

ISO5208 pressure test of metal valves for industrial valves.

API 598- 2004 "inspection and testing of valves".

2.4 classification of valve seal grades by the national institute for standardization of valve and pipe fitting industry manufacturers (MSS) the requirements for allowable valve leakage in MSS SP61 are as follows:

(1) in the case that a sealing surface in the valve sealing pair is made of plastic or rubber, no leakage shall be observed during the duration of the sealing test.

(2) when closed, the maximum allowable leakage amount on each side shall be: the nominal size of the liquid (DN) is 0.4ml per mm and per hour;Gases have nominal dimensions (DN) of 120 ml per mm per hour.

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

2.5 ANSI/FCI classification of control valve seal class

ANSI/fci70-2 (ASME B16.104) control valve seat leakage.

2.6 classification of valve seal class by eu standard

EN 12266-1, test for industrial valves, part l.Pressure tests, test methods and acceptance criteria - mandatory requirements.

3. Selection of valve seal grade

3.1 selection of domestic valve seal 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.Suitable for inspection and pressure test of industrial metal valves, including gate valves, globe valves, check valves, plug valves, ball valves and butterfly valves.The classification and maximum allowable leakage for seal tests are the same as those specified in ISO 5208.The standard is a revision of GB/T13927(general valve pressure test), which adds AA, CC, E, EE, F and G to GB/T13927.The new version of the standard stipulates that "the selection of the leak level shall be a strict requirement of the relevant valve product standard or order contract.If the product standard or the order contract does not specify, the non-metallic elastic sealing valve according to the requirements of class A, metal sealing auxiliary valve according to the requirements of class D..."Generally, class D is suitable for general valves. The key valves should be above class D.

(2) mechanical industry standard JB/T 9092 valve inspection and test is a revision of ZB J16006.The maximum allowable leakage for a seal test is in accordance with the American petroleum institute standard API

598--1996.Suitable for inspection and pressure testing of valves for the petroleum industry, including gate, globe, plug, ball, check and butterfly valves with metal seal pairs, elastic seal pairs and non-metallic seal pairs (such as ceramic).GB/T 9092 is currently under revision.

(3) it should be noted in the engineering design that the national standard GB/T19672(technical conditions for pipeline valves) is formulated with reference to the European standard ISO 14313 and the American petroleum institute standard API 6D.The national standard GB/T 20173 pipeline valves for oil and natural gas industry pipeline transportation system was formulated with reference to the European standard ISO 14313.The GB/T 19672 and GB/T 20173 acceptance criteria for valve leakage are the same as ISO 5208 class A and D requirements.Therefore, if there are leakage requirements higher than the standard in the engineering design, they should be given in the order contract.

3.2 selection of foreign valve seal grades

(1) the classification of valve seals in the former Soviet union was mainly applied in the 1950s. With the collapse of the former Soviet union, most countries do not choose this classification, but choose the European and American standards Seal classification.EN 12266-1 seal classification is in accordance with ISO 5208, but is short of AA, CC and EE.Compared with the 1999 edition, ISO 5208 has six new grades: AA, CC, E, EE, F and G.ISO 5208 gives a comparison of several seal grades with API 598 and EN 12266.Other nominal size seal class comparison can be calculated according to the diameter of the leakage.

(2) American petroleum institute standard API 598 is the most commonly used inspection and pressure test standard for American standard valves.Manufacturer's standard MSSSP61 is often used for the inspection of "fully open" and "fully closed" steel valves, but

Not suitable for control valves.MSS SP61 test is not usually used for American standard valves.API 598 is applicable to the following API standard manufacturing valve seal performance test:

Flanged, lug, folio and butt welded joint check valves API 594

Flanged, threaded and butt welded metal plug valves API 599

Steel gate, globe and check valves DNl00 and below API 602 for the oil and gas industry

Corrosion resistant bolted bonnet gate valve API 603 flange and butt welded connection

Flanged, threaded and butt welded metal ball valves API 608

Double flange, lug and wafer butterfly valve APl609

Note in engineering design: API 598--2004 the inspection and pressure test of API 600(bolted bonnet steel gate valves for the oil and gas industry) is cancelled as compared to the 1996 edition.The API 600-2001 (ISO 10434--1998) standard requires that valve seal performance tests be conducted in accordance with ISO5208, but the leakage rates in tables 17 and 18 of the standard are the same as those in the API 598--1996 standard, instead of the ISO5208 seal classification.This contradiction in the 2001 version was corrected in the API 600 standard implemented on September 13, 2009, which stipulated that the valve seal performance test was in accordance with API 598, but there was no specified version, which again contradicted API 598--2004.Therefore, when selecting API 600 and its sealing performance test API 598 standard in engineering design, the version of the standard must be clearly defined to ensure the unity of the standard content.

(3) the American petroleum institute API 6 D standard (ISO14313) of the oil and gas industry A pipeline system is A pipeline valve, the valve leakage acceptance criteria is: "soft seal valves and oil seal cock valve leakage shall not exceed the ISO 5208 A grade (no visible leakage), metal seated valve leakage shall not exceed the ISO 5208 (1993) D, but according to b. sealing test of 4, the leakage is not greater than the ISO 5208 (1993) D two times, unless otherwise specified."Standard note: "special applications may require less than ISO 5208(1993) class D" J ".Therefore, the engineering design has higher than its standard leakage requirements, should be given in the order contract.API 6D--2008 appendix B additional test requirements specify the f J additional test requirements 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 test, and the high pressure sealing test with inert gas as the test medium will replace the liquid sealing test and liquid sealing test.According to the type, caliber and pressure level of the valve to choose the seal test, refer to the ISO 5208 standard.It is recommended to choose low pressure seal test for the valves on the long pipeline GAl and industrial pipeline GCl, which can improve the qualified product rate of the valves.When choosing high pressure seal test, it should pay attention to the elastic seal valve after high pressure seal test, may reduce its sealing performance in low pressure condition.According to the actual working conditions of the medium, reasonable selection of the valve seal test requirements, can effectively reduce the production cost of the valve.

(4) ANSI/FCI 70-2(ASME B16.104) is applicable to the control valve seal class.Engineering design should be based on the characteristics of the medium and valve

The opening frequency of the door and other factors to consider the choice of metal elastic seal or metal seal.The sealing class of the metal seal control valve shall be specified in the order contract.According to experience, for the metal sealing valve, the I, Ⅱ, Ⅲ level requirement is low, less in use in the engineering design, general metal sealing valve usually choose Ⅳ level minimum, is key to choose V or Ⅵ level control valve.An ethylene plant torch system control valve design, choose the metal sealing Ⅳ level requirement, as well.

(5) in addition to the engineering design should be noted: API 6D stipulates that the chlorine 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 chlorine ion content of the water used in the sealing test of austenitic stainless steel valves shall not exceed 100ug/g.Due to the different standard requirements, it is recommended that the valve order contract should specify the chlorine ion content of the water used in the seal test.

4.Classification standards for low leakage valve seals

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

(1) U.S. environmental protection agency EPA method 21 "volatile organic component leakage detection".

(2) ISO 15848(industrial valves: procedures for low leakage measurement, testing and qualification).

(3) SHELL MESC SPE 77/312, "industrial valves: low leakage measurements, grading systems, qualification procedures and type approval and product testing of switch and control valves", SHELL MESC SPE 77/312.

EPA Method 21 of the United States environmental protection agency only stipulates the test Method without the classification of leakage level, which is a local standard regulation and seldom applied.ISO 15848 and SHELL MESC SPE 77/312 are standards for the performance of valves in terms of tightness rating, durability rating and temperature rating.Tight class for stem and body seal leakage, divided into A, B, C three grades, for ISO 15848 standard valve seal leakage requirements ≤50 em3 / m3, and stem leakage at both standards are calculated according to the diameter of the stem.

ISO 15848A has the highest seals, B and c are the same as the SHELLMESC SPE 77/312.Generally, low leakage valves have A seal class below B, while bellows seal valves have A seal class below A due to their metal bellows seal on the stem seal.

5 selection of low leakage valve

Bellows seal valves are one of the low leakage valves.In the past, bellows seal valve is generally selected for the conditions with special requirements for the external leakage level of the valve. However, due to the great difficulty and high technical requirements of the bellows seal valve f-jjm, the bellows material can not be fully localized and the cost is too high, which restricts its extensive promotion in the petrochemical industry.At present, with the people's safety, environmental awareness to enhance, and the increase of technical cooperation with foreign countries, as well as the domestic valve manufacturer's own technical force is strengthening, the domestic technical personnel for low leakage valve awareness is also improving, so that its application scope is expanding.If the valve selected in petrochemical enterprises can meet the low leakage standard, it will undoubtedly greatly reduce the discharge of toxic, flammable and explosive medium in the device, to avoid the fire, explosion, poisoning and other hazards to life safety accidents caused by valve leakage.Compared with bellows valves, low leakage valves that meet the standards of ISO15848 and SHELL MESC SPE 77/31 are simple in structure, easy to process and manufacture, and their cost is about 10% ~ 20% higher than general valves.According to the previous analysis and comparison of these two standard seal grades, the leakage volume of class B valves of tight grade can generally meet the low leakage requirements of some special working conditions, the machining accuracy requirements are easy to meet, the manufacturing cost is not increased much, and it can replace the use of part of the bellows valve.At present, the low leakage valve has more practical significance for oil and gas purification system with high hydrogen sulfide content.Because hydrogen sulfide is a highly toxic, flammable gas heavier than air that can accumulate in low-lying areas, breathing in certain concentrations of hydrogen sulfide can harm the body and even lead to death, so the requirements for such natural gas purification facilities are more stringent.

6 the conclusion

When selecting the seal class and the specified allowable leakage amount, it should be noted that the leakage of the medium between the sealing surface in the high pressure valve will cause surface erosion.If there is a corrosive medium leakage, the metal at the leakage will be corroded, with the increase of leakage gap, leakage will soon increase, even make the valve scrap.Therefore, for high pressure or corrosive medium working conditions of the valve, in order to ensure the sealing should be higher requirements.In the pipeline transporting inflammable and explosive and toxic medium, the leakage of the medium between the sealing surface of the valve may cause personal harm, economic loss and even accidents.Therefore, for the transport of inflammable and explosive and toxic medium of the valve, should be in accordance with the medium of the danger level of reasonable requirements for tightness.

Any seal may sometimes allow a small amount of leakage, and may be considered sealed if the leakage amount is not of practical use.The technical standard for the manufacture of valves usually states that a metal to metal seal is allowed to allow a certain amount of leakage during seal performance tests in the closed state.In order to ensure the high sealing performance of the valve, it is necessary to carefully grind the sealing surface to increase the specific pressure of the sealing surface, but less than the allowable specific pressure of the sealing surface material, at the same time to improve the stiffness of the structure.Experience with valves shows that, in many cases, it is unnecessary to make excessive demands on the sealing performance of the valve, because some conditions fully allow the medium to have a small amount of leakage, because the leakage volume 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 have the most obvious influence on its external leakage. Low leakage valves have more stringent requirements on the design, manufacturing and processing of key parts such as valve body, stem and packing box, etc., such as:

(1) the quality of the valve body and bonnet, especially in forging or casting, shall avoid defects such as folding, slag inclusion, porosity, tissue evacuation, hidden cracks and uneven composition.

(2) machining quality of stem and valve body joints, etc., especially stem and packing box roughness, stem straightness, cover packing box hole verticality and machining accuracy.

(3) the structure of the valve stuffing box selection, because the valve stem seal for the dynamic seal, the valve stem in the process of rotation or sliding packing easy wear, need to choose special low leakage packing seal and packing seal combination, and strictly control the gap between the packing and the valve stem, the gap between the packing and the packing box.

To sum up, the choice of valve type in addition to meet the technical conditions and standards, but also should fully consider a variety of operating conditions, engineering design should strive to choose the valve seal level to meet the safety, reasonable and economic principles.