Valve leakage has become one of the main leakage sources in the device, so it is very important to improve the valve’s anti-leakage capability. To prevent valve leakage, it is necessary to master the basic knowledge of valve sealing parts to prevent medium leakage-valve sealing, this is the most important thing.
Principle of valve sealing
4 main points
Sealing is to prevent leakage, so the principle of valve sealing is also studied from the prevention of leakage.
There are two main factors that cause leakage. One is the most important factor that affects the sealing performance, that is, there is a gap between the seal pairs, and the other is the pressure difference between the two sides of the seal pair.
The principle of valve sealing is also analyzed from four aspects: liquid sealing, gas sealing, leaking channel sealing principle and valve sealing pair.
1. Liquid tightness
The tightness of the liquid is performed by the viscosity and surface tension of the liquid. When the capillary leaked by the valve is filled with gas, the surface tension may repel the liquid or introduce the liquid into the capillary. This forms a tangent angle. When the tangent angle is less than 90°, the liquid will be injected into the capillary, which will cause leakage.
The reason for the leakage is the different nature of the medium. Experiments with different media will give different results under the same conditions. Water, air or kerosene can be used. When the tangent angle is greater than 90°, leakage will also occur.
Because it is related to the grease or waxy film on the metal surface. Once the film on these surfaces is dissolved, the characteristics of the metal surface change, and the originally repelled liquid will wet the surface and cause leakage. In view of the above situation, according to Poisson’s formula, the purpose of preventing leakage or reducing the amount of leakage can be achieved when the diameter of the capillary is reduced and the viscosity of the medium is large.
2. Gas tightness
According to Poisson’s formula, the air tightness of a gas is related to the gas molecules and the viscosity of the gas. Leakage is inversely proportional to the length of the capillary tube and the viscosity of the gas, and proportional to the diameter of the capillary tube and the driving force. When the diameter of the capillary is the same as the average degree of freedom of the gas molecules, the gas molecules will flow into the capillary with free thermal motion.
Therefore, when we are doing the valve sealing test, the medium must be water to play a sealing role, and air or gas cannot play a sealing role.
Even if we use plastic deformation to reduce the diameter of the capillary below the gas molecule, it still cannot stop the flow of gas. The reason is that the gas can still diffuse through the metal wall. Therefore, when we do the gas test, we must be more stringent than the liquid test.
3. The sealing principle of the leakage channel
The valve seal is composed of the roughness of the unevenness scattered on the wave surface and the waviness of the distance between the wave crests. In the case that most of the metal materials in our country have low elastic strain force, if the sealing state is to be achieved, the compression force of the metal material needs to be higher, that is, the compression force of the material must exceed its elasticity.
Therefore, when designing the valve, the sealing pair is matched with a certain hardness difference.
4. Valve vice-sealing
The valve vice-sealing is the part that closes when the valve seat and the closing member are in contact with each other. The metal sealing surface is susceptible to damage by the entrapped medium, medium corrosion, wear particles, cavitation and erosion during use.
For example, wear particles, if the unevenness of the wear particles is smaller than the surface unevenness, the surface accuracy will be improved when the sealing surface is running in, but will not deteriorate.
On the contrary, it will deteriorate the surface accuracy. Therefore, when selecting wear particles, factors such as their materials, working conditions, lubricity and corrosion to the sealing surface must be considered comprehensively. Like wear particles, when selecting seals, we must comprehensively consider various factors that affect their performance to prevent leakage.
Therefore, we must choose those materials that are resistant to corrosion, abrasion and erosion. Otherwise, the lack of any requirement will greatly reduce its sealing performance.
The main factors affecting valve sealing
4 elements
There are many factors that affect valve sealing, mainly as follows:
1. Sealing structure
Under the change of temperature or sealing force, the structure of the vice-sealing part will change. Moreover, this change will affect and change the force between the vice-sealing part, thereby reducing the performance of the valve sealing.
Therefore, when choosing a seal, be sure to choose a seal with elastic deformation. At the same time, pay attention to the width of the sealing surface. The reason is that the contact surfaces of the sealing pair cannot be completely matched. When the width of the sealing surface increases, the force required for sealing must be increased.
2. Specific pressure of sealing surface
The specific pressure of the sealing surface affects the sealing performance of the valve and the service life of the valve.
Therefore, the specific pressure of the sealing surface is also a very important factor. Under the same conditions, too much specific pressure will cause damage to the valve, but too little wine will cause valve leakage. Therefore, we need to fully consider the appropriateness of the specific pressure when designing.
3. The physical properties of the medium
The physical properties of the medium also affect the sealing performance of the valve. These physical properties include temperature, viscosity and surface hydrophilicity.
The temperature change not only affects the slackness of the vice-sealing part and changes in the size of the parts, but also has an inseparable relationship with the viscosity of the gas. The gas viscosity increases or decreases as the temperature increases or decreases.
Therefore, in order to reduce the influence of temperature on the sealing performance of the valve, when we design the sealing pair, we must design it into a valve with thermal compensation such as an elastic valve seat.
4. The quality of the vice-sealing pair
The quality of the vice-seal part mainly refers to the selection, matching, and manufacturing accuracy of materials. For example, the valve clack and the sealing surface of the valve seat are in good agreement, which can improve the sealing performance. The characteristic of many hoop waviness is its good labyrinth sealing performance.
Valve leakage is very common in life and production. In the light of it, it will cause waste or bring danger to life. For example, the leakage of tap water valve will lead to serious consequences, such as poisonous, harmful, flammable, explosive and Corrosive media leakage, etc., serious threats to personal safety, property safety, and environmental pollution accidents.
Valve sealing Type
2 Types
The seal is also a very critical component in the valve. The sealing performance of the valve refers to the ability of each sealing part of the valve to prevent the leakage of the medium. It is the most important technical performance index of the valve.
There are three sealing parts of the valve:
❶ The contact between the opening and closing parts and the two sealing surfaces of the valve seat;
❷ The fit between the packing and the valve stem and stuffing box;
❸ The connection between the valve body and the valve cover.
The previous leakage is called internal leakage, which is commonly referred to as lax closure, which will affect the ability of the valve to cut off the medium. For shut-off valves, internal leakage is not allowed.
The latter two leaks are called external leakage, that is, the medium leaks from the inside of the valve to the outside of the valve. Leakage will cause material loss, pollute the environment, and even cause accidents in severe cases. For flammable, explosive, toxic or radioactive media, leakage is not allowed, so the valve must have reliable sealing performance.
Solving the sealing problem is not a trivial matter. Most of the problems occur here because the valve runs, leaks, drips, and leaks. Below we will look at the dynamic sealing and static sealing of the valve.
NO1: Dynamic seal
Valve dynamic seal, mainly refers to the valve stem seal. Preventing the medium in the valve from leaking with the movement of the valve stem is the central issue of the valve dynamic sealing.
1. Stuffing box form
The valve is dynamically sealed, mainly based on the stuffing box. The basic form of stuffing box is:
❶ Gland type
This is the most used form. The unified form can be distinguished in many details. For example, from the perspective of compression bolts, it can be divided into T-shaped bolts (used for low pressure valves with pressure ≤ 16 kg/cm²), stud bolts, and swivel bolts. From the gland, it can be divided into integral type and combined type.
❷ Compression nut type
This type of form has small dimensions, but the pressing force is limited, so it is only used for small valves.
2. Packing
In the stuffing box, the stuffing is in direct contact with the valve stem and is covered with the stuffing box to prevent the medium from leaking. The filler has the following requirements:
❶ Good sealing
❷ Corrosion resistance
❸ Small friction coefficient
❹ Comply with medium temperature and pressure
Frequently used fillers are:
❶ Asbestos packing: Asbestos packing has good temperature resistance and corrosion resistance, but when used separately, the sealing effect is not safe, and it is always impregnated or added with other materials. Oil-impregnated asbestos packing: There are two basic structural forms, one is twisted, and the other is braided. It can be divided into round and square.
❷ PTFE braided packing: the PTFE tape is braided into packing, which has excellent corrosion resistance and can be used for cryogenic media.
❸ Rubber O-ring: Excellent sealing effect under low pressure. The use temperature is restricted, such as natural rubber can only be used at 60°C.
❹ Plastic molding filler: Generally, it is made into three pieces, but it can also be made into other shapes. The plastics used are mostly polytetrafluoroethylene.
In addition, for example, in a steam valve at 250°C, asbestos packing and lead rings are alternately stacked to reduce steam leakage; there are valves and the medium is frequently changed, such as the use of asbestos packing and PTFE tape. , The sealing effect is better. In order to reduce friction on the valve stem, where there are places, molybdenum disulfide (MOS2) or other lubricants can be added.
Requests are being made for novel fillers. For example, polyacrylonitrile fiber is impregnated with polytetrafluoroethylene emulsion, and after pre-oxidation, it is sintered and pressed in a mold to form a packing with excellent sealing performance; another example is a corrugated packing made of stainless steel sheet and asbestos, which can withstand high temperature and high pressure And erosion.
3. Bellows seal
The rapid growth of the chemical industry and the nuclear energy industry, the increase in flammable, explosive, highly toxic and radioactive materials, has imposed more stringent requirements on valve sealing. In some places, it is no longer possible to use packing seals, and a new form of sealing-bellows seals has occurred. This type of seal does not require packing, and is also called non-packing seal.
Both ends of the bellows are welded to other parts. When the valve stem is raised and lowered, the bellows expands and contracts, and the bellows does not leak by itself, and the medium cannot escape. To be safe, double seals with bellows and packing are often used.
NO2: Static seal
What is static sealing?
Static seal usually refers to the seal between two static surfaces. The sealing method is mainly to use gaskets.
1. Gasket material
❶ Non-metallic materials: such as paper, hemp, cowhide, asbestos products, plastics, rubber, etc.
Paper, hemp, cowhide, etc., have pores and are easy to permeate, so they must be impregnated with oil, wax or other impermeable materials. Generally, valves are rarely used.
Asbestos products, there are asbestos belts, ropes, plates and asbestos rubber sheets. Among them, the asbestos rubber sheet has a compact structure, good pressure resistance, and good temperature resistance. The valve itself and the connection between the valve and the pipe flange are extremely common.
Plastic products have good corrosion resistance and are commonly used. Varieties include polyethylene, polypropylene, soft polyvinyl chloride, polytetrafluoroethylene, nylon 66, nylon 1010 and so on.
Rubber products are soft in nature, and various types of rubber have certain acid, alkali, oil and sea water resistance. Varieties include natural rubber, styrene butadiene rubber, nitrile rubber, neoprene rubber, isobutyl rubber, polyurethane rubber, fluorine rubber and so on.
❷ Metal materials: Generally speaking, metal materials have high strength and strong temperature resistance. But lead is not like this, it only takes the characteristics of its resistance to dilute sulfuric acid. Frequently used varieties are brass, copper, aluminum, low carbon steel, stainless steel, Monel, silver, nickel, etc.
❸ Composite materials: such as metal sheath (internal asbestos) washers, combined wave washers, surrounding entangled washers, etc.
2. Comm use of gasket performance
When using a valve, it is often the case that the original gasket is replaced. Common gaskets are: rubber flat washers, rubber O-rings, plastic flat washers, PTFE-covered washers, asbestos rubber washers, metal flat washers, metal special-shaped washers, metal wrap washers, wave washers, surrounding entangled washers, etc.
❶ Rubber flat washers: easy to deform and not difficult to compress, but they have poor pressure and temperature resistance. They are only used for low pressure and low temperature. Natural rubber has certain acid and alkali resistance, and the use temperature should not exceed 60°C; chloroprene rubber can also be resistant to certain acids and alkalis, and the use temperature is 80°C; nitrile rubber is oil-resistant and can be used up to 80°C; fluororubber has good corrosion resistance and resistance The temperature performance is also stronger than that of general rubber, and it can be used in 150℃ medium.
❷ Rubber O-shaped gaskets: the cross-sectional shape is a perfect circle, has a certain self-tightening effect, the sealing effect is better than flat gaskets, and the pressing force is smaller.
❸Plastic flat gaskets: The most important feature of plastics is good corrosion resistance, and most plastics have poor temperature resistance. PTFE is the crown of plastics, has excellent corrosion resistance, wide temperature resistance range, and can be used for a long time within -180℃～+200℃.
❹ Teflon-wrapped gaskets: to fully demonstrate the advantages of PTFE, and at the same time make up for its poor elasticity, and make Teflon-wrapped rubber or asbestos rubber gaskets. In this way, it has the same corrosion resistance as the PTFE flat gasket, but also has excellent elasticity, which enhances the sealing effect and reduces the pressing force.
❺ Asbestos rubber gasket: cut from asbestos rubber sheet. Its components are 60-80% asbestos and 10-20% rubber, filler, vulcanizing agent, etc. It has good heat resistance, cold resistance, chemical stability, abundant supply and low price. When used, the pressing force does not need to be large. It can adhere to metal, and it is best to be coated with a layer of graphite powder to avoid laborious disassembly.
❻Metal flat hot ring: lead, temperature resistance 100℃; aluminum 430℃; copper 315℃; low carbon steel 550℃; silver 650℃; nickel 810℃; Monel (nickel-copper) alloy 810℃, stainless steel 870℃. Among them, lead has poor pressure resistance, aluminum can withstand 64 kg/cm², and other materials can withstand high pressure.
❼ Metal anisotropic washer
Lens gasket: self-tightening, used for high-pressure valves.
Oval washers: also belong to high-pressure self-tightening washers.
Cone double washer: used for self-seal sealing under high pressure.
In addition, there are square, diamond, triangle, tooth, dovetail, B, C, etc., which are generally only used in high and medium pressure valves.
❽Metal-clad gasket: metal has excellent temperature and pressure resistance, and excellent elasticity. The covering materials are aluminum, copper, low carbon steel, stainless steel, Monel alloy and so on. The filling materials include asbestos, polytetrafluoroethylene, glass fiber, etc.
❾ Wave washer: It has the characteristics of small pressing force and good sealing effect. The combination of metal and non-metal is often used.
➓ Surrounding entangled gasket: It is a thin metal belt and a non-metallic belt that are closely attached together, and the winding is entangled into a multi-layer circular shape, with a wave-like cross section, which has good elasticity and airtightness. The metal belt can be made of 08 steel, 0Cr13, 1Cr13, 2Cr13, 1Cr18Ni9Ti, copper, aluminum, titanium, Monel, etc. Non-metallic belt materials include asbestos, polytetrafluoroethylene and so on.
Above, when describing the performance of sealing gaskets, some figures are listed. It must be stated that these numbers are closely related to the flange form, media conditions and installation and repair technology. They can be crossed and not reached. Pressure resistance and temperature resistance are also mutually converted. For example, when the temperature is high, the pressure resistance often decreases. , These subtle issues can only be experienced in practice.
Valve sealing material
10 kinds of materials
The valve sealing material is an important part of the valve seal, and it serves as the direct contact surface of the valve seal. What are the valve sealing materials?
Here is a brief introduction to the conditions of use of various sealing materials, as well as the commonly used valve types.
1. Synthetic rubber
Synthetic rubber is better than natural rubber in its comprehensive properties such as oil resistance, temperature resistance and corrosion resistance. Generally, the use temperature of synthetic rubber is t≤150℃, natural rubber t≤60℃, and rubber is used to seal shut-off valves, gate valves, diaphragm valves, butterfly valves, check valves, pinch valves and other valves with a nominal pressure of PN≤1MPa.
2. Nylon
Nylon has the characteristics of low friction coefficient and good corrosion resistance. Nylon is mostly used for ball valves and globe valves with temperature t≤90℃ and nominal pressure PN≤32MPa.
- PTFE
Polytetrafluoroethylene is mostly used for stop valves, gate valves, ball valves, etc. with temperature t≤232℃ and nominal pressure PN≤6.4MPa.
4. Cast iron
Cast iron is used for gate valves, stop valves, and plug valves for temperature t≤100℃, nominal pressure PN≤1.6MPa, gas and oil.
5. Babbitt alloy
Babbitt alloy is used for ammonia stop valve with temperature t-70~150℃ and nominal pressure PN≤2.5MPa.
6. Copper alloy
Commonly used copper alloy materials include 6-6-3 tin bronze and 58-2-2 manganese brass. Copper alloy has good wear resistance and is suitable for water and steam with temperature t≤200℃ and nominal pressure PN≤1.6MPa. It is often used in gate valves, globe valves, check valves, plug valves, etc.
7. Chrome stainless steel
Commonly used grades of chromium stainless steel are 2Cr13 and 3Cr13 after quenching and tempering treatment, which has good corrosion resistance. It is commonly used on valves of water, steam, petroleum and other media with temperature t≤450℃ and nominal pressure PN≤32MPa.
8. Chromium nickel titanium stainless steel
The commonly used grade of chromium-nickel-titanium stainless steel is 1Cr18Ni9ti, which has good corrosion resistance, erosion resistance and heat resistance. It is suitable for steam, nitric acid and other media with temperature t≤600℃ and nominal pressure PN≤6.4MPa. It is used for stop valves, ball valves, etc.
9. Nitrided steel
The commonly used grade of nitriding steel is 38CrMoAlA, which has good corrosion resistance and scratch resistance after carburizing treatment. Commonly used for power station gate valves with temperature t≤540℃ and nominal pressure PN≤10MPa.
10. Boronizing
Boronizing is directly processed from the valve body or valve clack body material to form the sealing surface, and then boronizing surface treatment, the sealing surface has good wear resistance. Used for power station blowdown valve.
Valve sealing grade
From classification standard to selection
Valve sealing level is very important. From classification standards to selection, how should one choose for domestic and foreign countries?
1. Classification standard of valve sealing grade
Domestic
At present, there are two main classification standards for valve sealing grades commonly used in my country:
Chinese national standard GB/T 13927 “Industrial Valve Pressure Test”.
Classification of valve sealing grades according to China Machinery Industry Standard Machinery Industry Standard JB/T 9092 “Valve Inspection and Testing”.
Internatioanal
At present, there are mainly the following six types of valve seal classification standards commonly used in the world:
1. Former Soviet Union
In order to select products according to the degree of sealing of the valve and the specified use, the valves are classified according to the degree of sealing.
2. International Organization for Standardization
International Organization for Standardization Standard ISO5208 “Pressure Test of Industrial Valves and Metal Valves”.
3. American Petroleum Institute (APl)
American Petroleum Institute Standard API 598-2004 “Inspection and Testing of Valves”.
4. The American Valve and Fitting Industry Manufacturers Standardization Society (MSS)
The standard “Pressure Test of Steel Valves” MSS SP61 allows the valve leakage requirements of the American Valve and Pipe Fitting Industry Manufacturers Standardization Association standard as follows:
In the case of a sealing surface of the valve sealing pair using plastic or rubber, there should be no visible leakage during the duration of the sealing test.
The maximum allowable leakage on each side when closed should be: liquid is nominal size (DN) per millimeter, 0.4 ml per hour; gas is nominal size (DN) per millimeter, 120 ml per hour.
The allowable leakage of the check valve can be increased by 4 times.
5. American National Standards/American Instrument Institute Standards (ANSI/FCI) for control valves
American National Standard Bin American Instrument Association Standard ANSI/FCI70-2 (ASME B16.104) “Control Valve Seat Leakage”.
2. Selection of valve sealing grade
Domestic
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.
It is suitable for the 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 of the sealing test are the same as those specified in ISO 5208.
This standard is a revision of GB/T13927 (General Valve Pressure Test”. Compared with GB/T13927, six grades AA, CC, E, EE, F and G have been added. The new version of the standard stipulates “the choice of leakage grades” It should be one of the strict requirements in the relevant valve product standard or order contract. If there is no special requirement in the product standard or order contract, the non-metal elastic sealing valve shall meet the requirements of Class A, and the metal sealing valve shall meet the requirements of Class D…” Usually Class D is suitable for general valves, and the more critical valves should have a leakage level above Class D.
2. Machinery industry standard JB/T 9092 “Valve Inspection and Test” is a revision of ZB J16006.
The maximum allowable leakage of the seal test is formulated with reference to the American Petroleum Institute standard API598-1996. It is suitable for the inspection and pressure test of valves for the petroleum industry, including gate valves, globe valves, plug valves, ball valves, check valves and butterfly valves of metal seal pairs, elastic seal pairs and non-metal seal pairs (such as ceramic). Currently GB/T 9092 is under revision.
3. Attention should be paid in engineering design: the national standard GB/T19672 ((Pipeline Valve Technical Conditions) 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 Pipeline Transportation Systems in the Petroleum and Natural Gas Industry” is formulated with reference to the European standard ISO 14313. The two standards GB/T 19672 and GB/T 20173 have the same acceptance criteria for valve leakage as the requirements of ISO 5208 Class A and Class D. Therefore, when there is a leakage requirement higher than its standard in the engineering design, it should be given in the order contract.
International
1. The former Soviet Union’s classification of valve sealing grades was mainly applied in the 1950s. With the disintegration of the former Soviet Union, most countries now choose not to use this sealing grade classification, but to choose the European and American standard sealing grade classification.
The European standard EN 12266-1 sealing grade classification conforms to the requirements of the International Organization for Standardization standard ISO 5208, but lacks the three grades of AA, CC and EE. Compared with the 1999 edition, ISO 5208 has added six grades: AA, CC, E, EE, F and G. The ISO 5208 standard gives a comparison of several sealing grades with API 598 and EN 12266 standards. The comparison of other nominal sizes and sealing grades can be obtained by calculating the leakage amount according to the caliber.
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 it does not apply to control valves. MSS SP61 inspection is usually not used for American standard valves.
API 598 is applicable to the sealing performance test of valves manufactured in the following API standards:
Flange type, lug, wafer type and butt welding connection check valve API 594
Metal plug valve with flange, thread and butt welding connection API 599
Steel gate valves and check valves of DN100 and below for the oil and natural gas industry API 602
Corrosion-resistant bolted bonnet gate valve with flange and butt welding connection API 603
Metal ball valve with flange, thread and butt welding connection API 608
Double flange type, lug and wafer butterfly valve APl609
Attention should be paid to engineering design: API 598-2004 has cancelled the inspection and pressure test of API 600 ((Steel gate valve with bolted bonnet for oil and natural gas industries) compared with the 1996 version. API 600-2001 (ISO 10434 –1998) The standard stipulates that the sealing performance test of the valve should refer to ISO 5208, but the leakage amount in Table 17 and Table 18 in the standard is the same as that in the API 598-1996 standard, instead of adopting the ISO5208 sealing grade classification method.
The API 600 standard implemented in September 2009 corrected this contradiction in the 2001 version, stipulating that the sealing performance test of the valve is in accordance with API 598, but there is no prescribed version, which again contradicts API 598-2004. Therefore, when selecting API 600 and its sealing performance test API 598 standard in engineering design, the standard version must be clarified to ensure the unity of standard content.
3. The American Petroleum Institute standard API 6D (ISO14313) “Oil and Natural Gas Industry-Pipeline Transportation System-Pipeline Valve”, the acceptance criteria for valve leakage are:
“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 the seal described in B.4 In the test, the leakage cannot be greater than twice the level of ISO 5208 (1993) 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.
API 6D-2008 Appendix B Additional Test Requirements specifies the additional test requirements for valves f J that the manufacturer shall make when 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 sealing test and the liquid sealing test.
According to the valve type, caliber and pressure level to choose the sealing test, please refer to the ISO 5208 standard. For the valves on the long-distance pipeline GAl and the industrial pipeline GCl, it is recommended to use the low-pressure sealing test, which can improve the qualified product rate of the valve.
When selecting high-pressure sealing test, it should be noted that after the high-pressure sealing test of the elastic sealing valve, its sealing performance under low-pressure working conditions may be reduced. The valve sealing test requirements should be selected reasonably according to the actual operating conditions of the medium, which can effectively reduce the production cost of the valve.
4. The American National Standards Image American Instrument Association standard ANSI/FCI 70-2 (ASME B16.104) is applicable to the regulations of the sealing level of the control valve.
In engineering design, metal elastic seals or metal seals should be selected according to factors such as the characteristics of the medium and the opening frequency of the valve. The sealing level of the metal-sealed control valve shall be specified in the order contract. According to experience, for metal-sealed control valves, the requirements for grades I, Ⅱ, and Ⅲ are relatively low, and there are fewer selections in engineering design. Generally, metal-sealed control valves should be at least grade Ⅳ, and more critical control valves should be grade V or Ⅵ. The design of the control valve of the flare system of an ethylene plant has adopted the metal seal grade IV requirement, and it is running well.
5. In addition, attention should be paid 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. 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 30ug/g. 100ug/g. Due to the different requirements of various standards, it is recommended that the chlorine ion content of the water used in the seal test should be specified in the valve order contract.
