What is a valve

What is a valve?

In fluid piping system, valve is the control element, its main function is to isolate equipment and piping system, regulate flow, prevent backflow, regulate and discharge pressure.
The valve can be used to control the flow of air, water, steam, various corrosive media, mud, oil, liquid metal and radioactive media. Because it is very important to select the most suitable valve for the piping system, it is very important to understand the characteristics of the valve and the steps and basis for selecting the valve.

20210402155331 21853 - What is a valve

Classification of valves

There are two main types of valves:
1. Automatic valve
Depending on the medium (liquid, gas) itself and the ability to act on its own valve.
Such as check valve, safety valve, regulating valve, drain valve, pressure reducing valve, etc.
2. Drive the valve
With the help of manual, electric, hydraulic, pneumatic to operate the action of the valve.
Such as gate valve, globe valve, throttle valve, butterfly valve, ball valve, plug valve, etc.
According to the structural characteristics and the moving direction of the closing part relative to the valve seat, it can be divided into three parts:

  • Gate shape: the closing part moves along the center of the valve seat;
  • Gate shape: the closing part moves along the center of the vertical valve seat;
  • Cock and ball: the closing part is a plunger or ball, which rotates around its own center line;
  • Swing shape: the closing part rotates around the axis outside the valve seat;
  • Disc shape: the disc of the closing part rotates around the axis in the valve seat;
  • Slide valve shape: the closing piece slides in the direction perpendicular to the channel.

According to the different uses of the valve, it can be divided into:

  • Cut off: used to connect or cut off pipeline medium, such as stop valve, gate valve, ball valve, butterfly valve, etc.
  • Check: used to prevent medium backflow, such as check valve.
  • Regulating: used to regulate the pressure and flow of medium, such as regulating valve and pressure reducing valve.
  • Distribution: used to change the flow direction of media, distribution media, such as three-way cock, distribution valve, slide valve, etc.
  • Safety valve: when the medium pressure exceeds the specified value, it is used to discharge excess medium to ensure the safety of pipeline system and equipment, such as safety valve and emergency valve.
  • Other special uses: such as drain valve, vent valve, drain valve, etc.

According to different driving modes, it can be divided into three types:

  • Manual: with the help of hand wheel, handle, lever or chain wheel, etc., it is driven by manpower. When driving large torque, there are worm gear, gear and other reduction devices.
  • Electric: driven by a motor or other electrical device.
  • Hydraulic: driven by (water, oil).
  • Pneumatic: driven by compressed air.

According to the pressure, according to the nominal pressure of the valve can be divided into:

  • Vacuum valve: the absolute pressure is less than 0.1MPa, that is, 760mmhg, which is usually expressed by mmHg or mmh2o.
  • Low pressure valve: valve with nominal pressure PN ≤ 1.6Mpa (including steel valve with PN ≤ 1.6Mpa).
  • Medium pressure valve: the valve with nominal pressure of pn2.5-6.4mpa.
  • High pressure valve: the valve with nominal pressure of pn10.0-80.0mpa.
  • Ultra high pressure valve: the valve with nominal pressure PN ≥ 100.0mpa.

According to the temperature of the medium, according to the working temperature of the valve medium can be divided into:

  • Common valve: suitable for medium temperature – 40 ℃ ~ 425 ℃.
  • High temperature valve: suitable for the valve with medium temperature of 425 ℃ ~ 600 ℃.
  • Heat resistant valve: suitable for valves with medium temperature above 600 ℃.
  • Low temperature valve: suitable for medium temperature – 150 ℃ ~ – 40 ℃.
  • Ultra low temperature valve: suitable for the valve with medium temperature below – 150 ℃.

According to the nominal diameter, according to the nominal diameter of the valve can be divided into:

  • Small diameter valve: the valve with nominal diameter DN < 40mm.
  • Medium diameter valve: the valve with nominal diameter of DN50 ~ 300mm.
  • Large diameter valve: dn350-1200mm.
  • Extra large diameter valve: the valve with nominal diameter DN ≥ 1400mm.

According to the connection mode with the pipeline, according to the connection mode between the valve and the pipeline, it can be divided into:

  • Flange connection valve: valve body with flange, flange connection with the pipeline.
  • Threaded valve: valve body with internal thread or external thread, and pipeline thread connection valve.
  • Welding connection valve: the valve body is equipped with a welding joint, which is connected with the pipeline by welding.
  • Clamp connection valve: valve with a clamp on the valve body and clamp connection with the pipeline.
  • Ferrule connected valve: the valve connected with pipe by ferrule.

Characteristics of valve

There are generally two kinds of valve characteristics, service characteristics and structural characteristics.

Characteristics of use

It determines the main performance and application range of the valve:

  • Types of valves (closed circuit valves, regulating valves, safety valves, etc.);
  • Product type (gate valve, globe valve, butterfly valve, ball valve, etc.);
  • Materials of main parts (valve body, valve cover, valve stem, valve disc and sealing surface) of the valve;
  • Valve transmission mode, etc.

Structural characteristics

It determines some structural characteristics of valve installation, repair, maintenance and other methods:

  • The structural length and overall height of the valve, and the connection form with the pipeline (flange connection, thread connection, clamp connection, external thread connection, welding end connection, etc.);
  • Form of sealing surface (insert ring, threaded ring, surfacing, spray welding, valve body);
  • Stem structure (rotating rod, lifting rod), etc.

Steps and basis of valve selection

Selection steps

  • (1) The purpose of the valve in the equipment or device is clear, and the working conditions of the valve are determined: applicable medium, working pressure, working temperature, etc.
  • (2) Determine the nominal diameter and connection mode of the pipeline connected with the valve: flange, thread, welding, etc.
  • (3) Determine the way to operate the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electrical linkage or electro-hydraulic linkage, etc.
  • (4) According to the transmission medium, working pressure and working temperature of the pipeline, the materials of the shell and internals of the selected valve are determined: gray cast iron, malleable cast iron, nodular cast iron, carbon steel, alloy steel, stainless acid resistant steel, copper alloy, etc.
  • (5) Select the type of valve: closed circuit valve, regulating valve, safety valve, etc.
  • (6) Determine the type of valve: gate valve, globe valve, ball valve, butterfly valve, throttle valve, safety valve, pressure reducing valve, steam trap, etc.
  • (7) Determination of valve parameters: for automatic valves, the allowable flow resistance, discharge capacity, back pressure, etc. shall be determined according to different needs, and then the nominal diameter of the pipeline and the diameter of the valve seat hole shall be determined.
  • (8) Determine the geometric parameters of the selected valve: structure length, flange connection form and size, valve height direction after opening and closing, bolt hole size and number of connection, overall valve size, etc.
  • (9) Use the existing information: valve product catalog, valve product samples, etc. to select the appropriate valve products.

Basis of valve selection

At the same time, we should further understand the basis of valve selection.

  • (1) The purpose, working condition and operation control mode of the selected valve.
  • (2) The nature of the working medium: working pressure, working temperature, corrosion performance, whether it contains solid particles, whether the medium is toxic, whether it is flammable, explosive medium, viscosity of the medium, etc.
  • (3) Requirements for valve fluid characteristics: flow resistance, discharge capacity, flow characteristics, sealing grade, etc.
  • (4) Installation dimension and overall dimension requirements: nominal diameter, connection mode and connection dimension with pipeline, overall dimension or weight limit, etc.
  • (5) Additional requirements for reliability, service life of valve products and explosion-proof performance of electric device.

Attention should be paid when selecting parameters

If the valve is to be used for control purposes, the following additional parameters must be determined: operating method, maximum and minimum flow requirements, pressure drop for normal flow, pressure drop when closing, maximum and minimum inlet pressure of the valve.
According to the above basis and steps of valve selection, it is necessary to have a detailed understanding of the internal structure of various types of valves when reasonably and correctly selecting valves, so as to make a correct choice of preferred valves.
The ultimate control of the pipeline is the valve. The opening and closing parts of the valve control the flow pattern of the medium in the pipeline. The shape of the valve passage makes the valve have certain flow characteristics. This must be considered when selecting the most suitable valve for the pipeline system.

The selection of valves should follow the following principles:

Valves for closing and opening media

The flow channel is straight through valve, and its flow resistance is small. It is usually selected as the valve for cut-off and open medium. Downward closing valve (stop valve, plunger valve) is less used because of its tortuous flow channel and higher flow resistance than other valves. Closed type valve can be selected when higher flow resistance is allowed.

Valves for flow control

Usually choose easy to adjust the flow of the valve as control flow. A downward closing valve, such as a globe valve, is suitable for this purpose because its seat size is proportional to the stroke of the closing element. Rotary valve (plug valve, butterfly valve, ball valve) and flex body valve (clamp valve, diaphragm valve) can also be used for throttling control, but usually only in a limited range of valve diameter. Gate valve is a disc-shaped gate to cross cut the round valve seat. It can control the flow only when it is close to the closed position, so it is not usually used for flow control.

Valve for reversing and diverting

The valve can have three or more channels according to the need of directional diversion. Plug valve and ball valve are more suitable for this purpose, therefore, most of the valves used for directional diversion are selected as one of these valves. But in some cases, other types of valves, as long as two or more valves are properly connected with each other, can also be used for diversion.

Valves for media with suspended particles

When there are suspended particles in the medium, it is most suitable to use the valve whose closing part slides along the sealing surface and has the function of wiping. If the back and forth movement of the closing part to the seat is vertical, particles may be trapped. Therefore, this kind of valve is only suitable for basically clean medium unless the sealing surface material can allow particles to be embedded. Ball valve and plug valve can wipe the sealing surface in the process of opening and closing, so they are suitable for medium with suspended particles.

Valve selection instructions

1. Selection of gate valve
In general, gate valves should be preferred. The gate valve is not only suitable for steam, oil and other media, but also suitable for media with granular solids and high viscosity, as well as valves for venting and low vacuum systems. For media with solid particles, the valve body should have one or two purge holes. For low temperature medium, low temperature special gate valve should be selected.
2. Selection of stop valve
The stop valve is suitable for the pipeline which does not have strict requirements on fluid resistance, that is, the pressure loss is not taken into account, and the pipeline or device with high temperature and high pressure medium is suitable for the pipeline with DN < 200 mm and other medium; the small valve can choose the stop valve, such as needle valve, instrument valve, sampling valve, pressure gauge valve, etc.; the stop valve has flow regulation or pressure regulation, but the regulation accuracy is not required For highly toxic media, the stop valve with bellows seal should be selected; however, the stop valve should not be used for media with high viscosity and media with particles that are easy to precipitate, or for vent valve and low vacuum system.
3. Selection of ball valve
Ball valve is suitable for low temperature, high pressure, high viscosity medium. Most ball valves can be used in the medium with suspended solid particles, and can also be used in powdery and granular medium according to the sealing material requirements; full channel ball valves are not suitable for flow regulation, but are suitable for occasions requiring quick opening and closing, so as to realize emergency cut-off in case of accidents; they are usually used in strict sealing performance, wear, necking channel, quick opening and closing action, and high pressure cut-off (large pressure difference) It is recommended to use ball valve in the pipeline with low noise, gasification phenomenon, small operating torque and small fluid resistance; ball valve is suitable for light structure, low pressure cut-off and corrosive medium; ball valve is also the most ideal valve for low temperature and cryogenic medium; low temperature ball valve with valve cover should be selected for pipeline system and installation with low temperature medium; when floating ball valve is selected, its seat material should be To meet the load of ball body and working medium, large diameter ball valve needs larger force during operation, and worm gear drive type shall be adopted for ball valve with DN ≥ 200 mm; fixed ball valve is suitable for larger diameter and higher pressure occasions; in addition, ball valve used for process highly toxic materials and combustible medium pipeline shall have fireproof and antistatic structure.
4. Selection of throttle valve
The throttle valve is suitable for the occasions with low medium temperature and high pressure, and it is suitable for the parts where the flow and pressure need to be adjusted. It is not suitable for the medium with high viscosity and solid particles, and it is not suitable to be used as a block valve.
5. Description of plug valve selection
The plug valve is suitable for the occasions requiring quick opening and closing. It is generally not suitable for steam and medium with high temperature. It is also suitable for medium with low temperature and high viscosity and medium with suspended particles.
6. Selection of butterfly valve
Butterfly valve is suitable for large diameter (such as DN > 600mm) and short structure length requirements, as well as the need for flow regulation and fast opening and closing requirements. It is generally used for water, oil, compressed air and other media with temperature ≤ 80 ℃ and pressure ≤ 1.0MPa. Because the pressure loss of butterfly valve is relatively large compared with gate valve and ball valve, butterfly valve is suitable for pipeline system with lax pressure loss requirements.
7. Check valve selection
Check valve is generally suitable for clean media, not for media containing solid particles and high viscosity.
When DN ≤ 40mm, lifting check valve should be used (only allowed to be installed on horizontal pipeline);
When DN = 50-400mm, the swing type lifting check valve should be used (it can be installed on both horizontal and vertical pipelines, if it is installed on vertical pipelines, the flow direction of medium should be from bottom to top);
When DN ≥ 450mm, buffer check valve should be used;
When DN = 100 ~ 400mm, the double clip check valve can also be selected;
Swing check valve can be made into a very high working pressure, PN can reach 42MPa, according to the different materials of shell and seal, it can be applied to any working medium and any working temperature range. The medium is water, steam, gas, corrosive medium, oil, medicine, etc. The working temperature range of the medium is – 196 ~ 800 ℃.
8. Description of diaphragm valve selection
Diaphragm valve is suitable for oil, water, acid medium and medium containing suspended solids with working temperature less than 200 ℃ and pressure less than 1.0MPa, but not suitable for organic solvent and strong oxidant medium; weir diaphragm valve should be selected for abrasive granular medium, and its flow characteristic table should be referred to; straight through diaphragm valve should be selected for viscous fluid, cement slurry and sedimentary medium, except for special features Diaphragm valve should not be used in vacuum pipeline and vacuum equipment.
Valve application, operation frequency and service are ever-changing. To control or eliminate even low leakage, the most important and critical equipment is valve. It is very important to learn how to choose the right valve.

Requirement

Installation requirements

  • 1. Before the valve is installed, the model and specification of the valve used shall be carefully checked to see if it is in conformity with the design;
  • 2. Check whether the valve can be used under the required conditions according to the valve model and factory instruction;
  • 3. When the valve is hoisted, the rope shall be tied to the flange connection between the valve body and the valve cover, and it shall not be tied to the handwheel or stem to avoid damaging the valve stem and handwheel;
  • 4. When installing valve on horizontal pipe, valve stem shall be vertical upward, and valve stem shall not be installed downward;
  • 5. When installing the valve, it is not allowed to use the forced butt joint method of raw pulling to avoid damage due to uneven force;
  • 6. The open stem gate valve should not be installed in the wet place underground to avoid the corrosion of the valve stem.

Assembly requirements

Clean parts must be sealed and kept for installation.
The requirements for installation process are as follows:

  • 1. The installation workshop must ensure that it is clean or temporary clean area should be built, such as using newly purchased color strip cloth or plastic film to prevent dust from entering during installation.
  • 2. The assembly workers must wear clean cotton work clothes, wear pure cotton caps on their heads, and keep their hair from leaking. Wear clean shoes on their feet, and wear plastic gloves in their hands to degrease.
  • 3. The tools used for assembly must be degreased and cleaned before assembly to ensure cleanliness.

Acceptance requirements

Acceptance shall be in accordance with HG 20202-2000 code for construction and acceptance of degreasing engineering. Before assembly, each part shall be wiped with clean precision filter paper, and dead angle of components shall be selected. The filter paper shall be qualified without change color.

Valve specification requirements

  • 1. The model of the valve shall be indicated with the requirements of the national standard number. If the enterprise standard is concerned, the relevant description of the model shall be indicated.
  • 2. The working pressure of the valve shall be ≥ the working pressure of the pipeline. Under the premise of not affecting the price, the working pressure that the valve can bear shall be greater than the actual working pressure of the pipeline.
  • 3. The valve manufacturing standard shall indicate the national standard number according to. If the enterprise standard is adopted, the enterprise documents shall be attached to the procurement contract.

Performance testing requirements

  • 1. When a valve is manufactured in batch, the official organization shall be entrusted to perform the following performance tests: ① the opening and closing moment of the valve under the working pressure condition; ② the detection of the flow resistance coefficient of the valve under the condition of pipeline water conveyance.
  • 2. The following tests shall be carried out before the valve leaves the factory: ① under the open condition, the valve body shall bear the internal pressure test with twice the working pressure value of the valve; ② under the closed condition, the valve shall bear 11 times the working pressure value of the valve respectively without leakage; however, the leakage value of the metal sealed butterfly valve shall not exceed the relevant requirements.

Sealing requirements

The sealing performance of valve refers to the ability of sealing parts of the valve to prevent the leakage of medium, which is the most important technical performance index of the valve. There are three sealing parts of the valve: the contact between the sealing surfaces of the opening and closing parts and the valve seat; the matching place between the packing and the valve stem and the packing box; and the connection between the valve body and the valve cover. The previous leak is called internal leakage, which is generally said to be not tight, which will affect the valve’s ability to block the medium. For block valve classes, internal leakage is not allowed. The second two leaks are called external leakage, i.e. medium leakage from the inside to outside of the valve. Leakage will cause material loss, environmental pollution and serious accidents. For flammable, explosive, toxic or radioactive medium, leakage is not allowed, so the valve must have reliable sealing performance.

Other requirements

  • 1. The assembled valve shall be purged with nitrogen for at least 1 minute.
  • 2. The air tightness test must be conducted with pure nitrogen.
  • 3. After the air tightness test is qualified, the sealing shall be carried out, and the clean polyethylene cap shall be used for sealing. Before the polyethylene cap is used, it shall be soaked in organic solvent and wiped clean.
  • 4. Then seal with a vacuum bag.
  • 5. Finally, packing.
  • 6. Measures shall be taken to ensure that the package seal is not damaged during transportation.

Cleaning steps

The parts of the valve must go through the following process before assembly:

  • 1. If there is burr, it can’t be polished according to the requirement;
  • 2. Degrease all parts;
  • 3. After degreasing, pickling and passivation are carried out, and the cleaning agent does not contain phosphorus;
  • 4. After pickling and purification, clean it with pure water, and there is no pesticide residue. This step is omitted for carbon steel parts;
  • 5. Dry the parts one by one with non-woven cloth, do not leave the surface of parts such as wool, or blow dry with clean nitrogen;
  • 6. Wipe the parts one by one with non-woven fabric or precision filter paper dipped with analytical pure alcohol until there is no dirty color.

Daily maintenance

  • 1. The valve should be stored in a dry and ventilated room, and both ends of the channel should be blocked.
  • 2. The valve shall be inspected regularly, the dirt on it shall be removed, and the antirust oil shall be applied on its surface.
  • 3. After the installation of the valve, it should be regularly overhauled to ensure its normal work.
  • 4. Check whether the valve sealing surface is worn, and repair or replace it according to the situation.
  • 5. Check the trapezoidal thread wear of valve stem and valve stem nut, check whether the packing is out of date and failure, and replace it if necessary.
  • 6. The sealing performance of the valve shall be tested to ensure its performance.
  • 7. The valve in operation shall be in good condition, the bolts on the flange and bracket shall be complete, the thread shall not be damaged, and there shall be no looseness.
  • 8. If the hand wheel is lost, it should be matched in time instead of being replaced by a spanner.
  • 9. The packing gland is not allowed to be skewed or has no pre tightening clearance.
  • 10. If the valve is used in a harsh environment and is easily contaminated by rain, snow, dust, sand and other dirt, a protective cover should be installed for the valve stem.
  • 11. The scale on the valve shall be complete, accurate and clear, and the valve shall be sealed and capped.
  • 12. The insulation jacket shall be free of depression and crack.
  • 13. For the valve in operation, avoid knocking or supporting heavy objects.

Connection mode

Flange connection

This is the most commonly used connection in valves. According to the shape of the joint surface, it can be divided into the following types:

  • 1. Smooth type: for low pressure valves. Processing is convenient.
  • 2. Concave convex type: high working pressure, medium hard washer can be used.
  • 3. Mortise and groove type: the washer with large plastic deformation can be used, which is widely used in corrosive media and has good sealing effect.
  • 4. Trapezoidal groove type: oval metal ring as gasket, used for valves with working pressure ≥ 64 kg / cm2 or high temperature valves.
  • 5. Lens type: washer is lens shape, made of metal. Used for high pressure valve with working pressure ≥ 100 kg / cm2 or high temperature valve.
  • 6. O-ring type: This is a relatively new form of flange connection, it is developed with the emergence of various rubber O-rings, it is in the form of sealing effect connection.

Clamp connection

The valve and two end pipes are directly clamped together with bolts.

Butt welding connection

A connection directly welded to a pipe.

Threaded connection

This is a simple connection method, commonly used in small valves. There are two cases

  • 1. Direct sealing: internal and external threads directly play a sealing role. In order to ensure that the joint does not leak, it is often filled with lead oil, thread hemp and polytetrafluoroethylene raw material belt; among them, polytetrafluoroethylene raw material belt is widely used; this material has good corrosion resistance, excellent sealing effect, convenient use and preservation, and can be completely removed when dismantling, because it is a non sticky film, which is much better than lead oil and thread hemp.
  • 2. Indirect sealing: the force of thread tightening is transmitted to the washer between two planes, which makes the washer play a sealing role.

Ferrule connection

The connection and sealing principle of the ferrule connection is that when the nut is tightened, the ferrule is under pressure, so that its edge bit into the outer wall of the pipe, and the outer cone surface of the ferrule is close to the inner cone surface of the joint under pressure, thus the leakage can be reliably prevented.
The advantages of this connection form are as follows:

  • 1. Small size, light weight, simple structure, easy disassembly;
  • 2. Strong connection force, wide application range, high pressure (1000 kg / cm2), high temperature (650 ℃) and shock vibration resistance
  • 3. A variety of materials can be selected for corrosion protection;
  • 4. The machining accuracy is not high, and it is easy to install at high altitude.

Ferrule connection has been used in some small diameter valves in China.

Clamp connection

This is a quick connection method. It only needs two bolts and is suitable for low pressure valves that are often disassembled.

Internal self tightening connection

All the above connection forms are sealed by using external force to offset the medium pressure. The following describes the use of medium pressure for self tightening connection. Its sealing ring is installed at the inner cone and forms a certain angle with the opposite side of the medium. The medium pressure is transmitted to the inner cone and the sealing ring. On the cone with a certain angle, two components are generated, one is parallel to the center line of the valve body and the other is pressed to the inner wall of the valve body. The latter component is the self tightening force. The greater the medium pressure is, the greater the self tightening force is. Therefore, this connection is suitable for high pressure valves. Compared with flange connection, it saves a lot of materials and manpower, but it also needs a certain preload, so that it can be used reliably when the pressure in the valve is not high. The valve made of self tightening sealing principle is generally high pressure valve.
There are many forms of valve connection, for example, some small valves that do not need to be removed are welded together with pipes; some non-metallic valves use socket connection, and so on. Valve users should be treated according to the specific conditions.

Related accessories

There are valves and pipe fittings, which are used in the pipeline connection or control system. Valves and pipe fittings can not exist independently and complement each other. Valve fittings are carbon steel and stainless steel, and PVC, or other materials, commonly used is the first two, in recent years, with the improvement of people’s living standards, the demand for non-staple food has also increased. Therefore, it led to the rapid development of food machinery, so the production of stainless steel sanitary valve and pipe fittings is booming. People usually say that the most commonly used valve and pipe fittings are stainless steel sanitary ones.

Valve maintenance

Requirement
1. The valve shall be stored in a dry and ventilated room, and both ends of the passage shall be blocked.
2. The valve stored for a long time should be inspected regularly, the dirt should be removed, and antirust oil should be applied on the processing surface.
3. After installation, regular inspection shall be carried out. The main inspection items are as follows:

  • (1) Wear condition of sealing surface.
  • (2) Trapezoidal thread wear of stem and stem nut.
  • (3) Whether the packing is out of date and invalid, and if it is damaged, it should be replaced in time.
  • (4) After valve maintenance and assembly, sealing performance test shall be conducted.

All kinds of valves in operation shall be complete and intact. The bolts on the flange and bracket are indispensable, and the thread shall be intact without looseness. If the fastening nut on the hand wheel is loose, it should be tightened in time to avoid wearing the connection or losing the hand wheel and nameplate. If the handwheel is lost, it is not allowed to replace it with a spanner, and it should be matched in time. The packing gland is not allowed to be skewed or has no pre tightening clearance. For the valve in the environment easily contaminated by rain, snow, dust, sand and other pollutants, the valve stem shall be installed with a protective cover. The scale on the valve shall be complete, accurate and clear. The lead seal, cap and pneumatic accessories of the valve shall be complete and in good condition. The insulation jacket shall be free of depression and crack. It is not allowed to knock, stand or support heavy objects on the valve in operation, especially for non-metallic valve and cast iron valve.

Greasing maintenance

The professional maintenance of valves before and after welding plays an important role in serving the production and operation of valves. Correct, orderly and effective maintenance will protect the valves, make them function normally and prolong their service life. Valve maintenance seems simple, but it is not. There are often neglected aspects in work.

  • First, when the valve is greased, the amount of grease is often ignored. After the grease gun is filled with oil, the operator selects the valve and grease connection mode to carry out the grease injection operation. There are two situations: on the one hand, the amount of grease injection is less, and the sealing surface is worn faster due to the lack of lubricant. On the other hand, excessive fat injection causes waste. The reason is that there is no accurate calculation for the sealing capacity of different valves according to the valve type. The sealing capacity can be calculated according to the valve size and type, and then a reasonable amount of grease can be injected.
  • Second, when the valve is greased, the pressure problem is often ignored. During the operation of fat injection, the pressure of fat injection changed regularly from peak to valley. If the pressure is too low, the seal leaks or fails, if the pressure is too high, the grease injection port is blocked, the grease in the seal is hardened, or the seal ring is locked with the valve ball and valve plate. Usually, when the grease injection pressure is too low, the injected grease flows into the bottom of the valve chamber, which usually occurs in small gate valves. If the grease injection pressure is too high, on the one hand, check the grease injection nozzle and replace it if the grease hole is blocked; on the other hand, if the grease hardens, use cleaning fluid to soften the failed sealing grease repeatedly and inject new grease for replacement. In addition, seal type and seal material also affect the grease injection pressure. Different seal forms have different grease injection pressure. Generally, the grease injection pressure of hard seal is higher than that of soft seal.
  • Third, when greasing the valve, pay attention to the valve in the on-off position. Ball valve maintenance is generally in the open state, special circumstances choose to close maintenance. Other valves can not be treated as open. The gate valve must be closed during maintenance to ensure that the grease fills the sealing groove along the sealing ring. If it is open, the grease will directly fall into the flow channel or valve cavity, causing waste.
  • Fourth, when the valve is greased, the effect of greasing is often ignored. During the operation, the pressure, amount and switch position were normal. However, in order to ensure the effect of valve greasing, sometimes it is necessary to open or close the valve, check the lubrication effect, and confirm that the surface of valve ball or ram is evenly lubricated.
  • Fifthly, attention should be paid to the blowdown of valve body and the pressure relief of screw plug during grease injection. After the pressure test of the valve, the gas and water in the valve chamber of the sealing chamber will be increased due to the rise of the ambient temperature, and the blowdown and pressure relief should be carried out before the grease injection, so as to facilitate the smooth progress of the grease injection. The air and water in the sealing cavity are fully replaced after grease injection. The pressure of the valve chamber is released in time, which ensures the safety of the valve. After grease injection, the drain and pressure relief plugs must be tightened to prevent accidents.
  • Sixth, when injecting fat, we should pay attention to the problem of even fat. During normal fat injection, the nearest fat hole to the fat injection port first gets fat, then goes to the low point, finally to the high point, and then gets fat one by one. If not in accordance with the law or not out of fat, prove the existence of blockage, timely clearing treatment.
  • Seventh, when greasing, also observe that the valve diameter is flush with the sealing ring seat. For example, for ball valve, if there is open position interference, the open position limiter can be adjusted inward, and it can be locked after confirming that the diameter is straight. The adjustment of the limit should not only pursue the position of the opening or closing side, but should be considered as a whole. If the opening position is flush and the closing position is not in place, the valve will not be closed tightly. In the same way, the corresponding adjustment of the opening position should also be considered when adjusting the closing position. Ensure right angle travel of the valve.
  • Eighth, after the injection, the injection port must be sealed. Avoid impurities entering or lipid oxidation at the grease injection port. The cover shall be coated with anti rust grease to avoid rusting. So that it can be applied in the next operation.
  • Ninth, when greasing, we should also consider the specific treatment of specific problems in the future oil sequential transportation. In view of the different quality of diesel and gasoline, the scouring and decomposition capacity of gasoline should be considered. In the later valve operation, in case of gasoline section operation, timely add grease to prevent wear.
  • Tenth, do not ignore the greasing of the valve stem when greasing. There is sliding sleeve or packing on the valve shaft, which also needs to be lubricated to reduce the friction resistance during operation. If the lubrication can not be ensured, the torque will be increased during electric operation, and the worn parts will be worn, and the switch will be laborious during manual operation.
  • Eleventh, some ball valve body marked with arrow, if not with English fiow handwriting, it is the direction of action of the seal seat, not as a reference to the flow direction of the medium, the direction of the valve leakage is opposite. In general, double seat sealed ball valve has two-way flow direction.
  • Twelfth, valve maintenance, but also pay attention to the problem of water in the electric head and its transmission mechanism. Especially in rainy season. One is to rust the transmission mechanism or shaft sleeve, the other is to freeze in winter. When the electric valve is operated, the torque is too large, and the transmission parts will be damaged, which will make the motor no-load or over torque protection jump off, and the electric operation cannot be realized. The transmission parts are damaged, and manual operation cannot be carried out. After the over torque protection action, manual operation is also unable to switch, if forced operation, the internal alloy parts will be damaged.

Common problem

Why can’t the control valve be used as a shut-off valve?
The advantage of double seat valve core is the force balance structure, allowing large pressure difference, and its outstanding disadvantage is that the two sealing surfaces can not be in good contact at the same time, resulting in large leakage. If it is used artificially and compulsorily in the cutting off situation, the effect is obviously not good. Even if many improvements have been made to it (such as double seal sleeve valve), it is not desirable.
Why is it easy to oscillate when the double seat valve works with small opening?
For single core, when the medium is flow open, the stability of the valve is good; when the medium is flow closed, the stability of the valve is poor. The double seat valve has two valve cores, the lower valve core is in the flow closed position, and the upper valve core is in the flow open position. In this way, the flow closed valve core is easy to cause the vibration of the valve when the small opening is working, which is the reason why the double seat valve can not be used for the small opening.
What kind of straight stroke control valve has poor anti blocking performance and angular stroke valve has good anti blocking performance?
The valve core of straight stroke valve is vertical throttling, while the medium flows in and out horizontally. The flow channel in the valve cavity must turn upside down, which makes the flow path of the valve quite complicated (such as inverted S-shape). In this way, there are many dead zones, which provide space for the medium precipitation. In the long run, it will cause blockage. The throttling direction of the angle stroke valve is the horizontal direction. The medium flows in and out horizontally, which is easy to take away the unclean medium. At the same time, the flow path is simple, and the space for medium precipitation is also small, so the angle stroke valve has good anti blocking performance.
Why is the cut-off pressure difference of angle stroke valve larger?
The larger cut-off pressure difference of angle stroke valve is because the resultant force generated by medium on the valve core or valve plate has very small torque on the rotating shaft, so it can bear larger pressure difference.
Why is the stem of straight stroke control valve thinner?
It involves a simple mechanical principle: large sliding friction and small rolling friction. Straight stroke valve stem up and down movement, packing a little bit tight, it will package the stem very tight, resulting in a larger backlash. As a result, PTFE packing is often used to reduce the friction coefficient of the valve stem, so as to reduce the service life of the valve stem. To solve this problem, the best way is to use the traveling valve stem, that is, the angle stroke control valve. Its stem is 2-3 times thicker than the straight stroke stem, and the graphite packing with long service life is selected. The stem stiffness is good, and the packing life is long. What is the hard seal?

The lower the leakage of the shut-off valve, the better. The leakage of the soft seal valve is the lowest. Of course, the shut-off effect is good, but it is not wear-resistant and has poor reliability. According to the double standards of small leakage and reliable sealing, soft sealing is not as good as hard sealing. For example, the full-function ultra light control valve is sealed and protected by wear-resistant alloy, with high reliability and leakage rate of 10 ~ 7, which can meet the requirements of shut-off valve.

Model establishment

Preface of valve model preparation

The valve model shall generally indicate the valve type, driving mode, connection type, structural features, sealing surface material, valve body material and nominal pressure, etc. The standardization of valve model provides convenience for valve design, selection and sales. Nowadays, there are more and more types and materials of valves, and the model establishment of valves is becoming more and more complicated. Although there is a unified standard for valve model establishment in China, it is increasingly unable to meet the needs of valve industry development. For new valves that can not be numbered according to the standard, each manufacturer can work out the model according to their own needs.
The standard of valve model preparation method is applicable to industrial pipeline Gate valve, throttle valve globe valvebutterfly valvediaphragm valveplunger valveplug valvecheck valvesafety valvepressure relief valvedrain valve. It includes the designation of the valve type and the designation of the valve.

Specific compilation method of valve model

The following is the sequence diagram of each code in the standard valve model preparation method:
Understanding the diagram on the left is the first step to understand various valve models. Here is an example to give you a general understanding:
Valve model: “z961y-100i DN 150” this is a complete gate valve model. The final “DN 150” is not included in the compilation, which means the valve diameter is 150 mm, which is easy to understand. The front part: “z961y-100i” is seated according to the sequence diagram above, as follows:
“Z” is 1 unit; “9” is 2 units; “6” is 3 units; “1” is 4 units; “Y” is 5 units; “100” is 6 units; “I” is 7 units
The meaning of this valve model is: gate valve, electric drive, welding connection, wedge single gate, cemented carbide seal, 10MPa pressure, chrome molybdenum steel valve body material.
Unit 1: valve type code
type
Safety valve
butterfly valve
Diaphragm valve
Check valve
(bottom valve)
Globe valve
throttle valve
Blowdown valve
globe valve
Drain valve
Plunger valve
Plug valve
Pressure relief valve
Gate valve
Code name
A
D
G
H
J
L
P
Q
S
U
X
Y
Z
For valves with other functions or special mechanisms, a Chinese character should be added before the valve type code.
Pinyin alphabet, as specified in the following table:
type
Thermal insulation
Low temperature type
Fireproof type
Slowly closed type
Deslagging type
Fast type
(stem seal) bellows type
Code name
B
D
F
H
P
Q
W
Unit 2: transmission mode
transmission
mode
Electromagnetism
Electromagnetic hydraulic
Electro hydraulic
Worm gear
Spur gear
Bevel gear
Pneumatic
Hydrodynamics
Pneumatic hydraulic
Electric
Handle
handwheel
Code name
0
1
2
3
4
5
6
7
8
9
No code
Three units: connection type
Connection mode
Internal thread
External thread
Two different connections
flange
welding
Counter clamp
Clamp
Ferrule
Code name
1
2
3
4
6
7
8
9
Unit 4: structural type
Structural type code of gate valve
structural style
Code name
Stem lifting type
(rising pole)
Wedge gate
Elastic RAM
0
Rigid gate
Single ram
1
Double ram
2
Parallel gate
Single ram
3
Double ram
4
Non lifting stem
(concealed rod)
Wedge gate
Single ram
5
Double ram
6
Parallel gate
Single ram
7
Double ram
8
Code of structure type of stop valve, throttle valve and plunger valve
structural style
Code name
structural style
Code name
Disc unbalanced
Straight channel
1
Disc balanced
Straight channel
6
Z-channel
2
Angle channel
7
Tee channel
3
Angle channel
4
Direct flow channel
5
Code of ball valve structure
structural style
Code name
structural style
Code name
Floating ball
Straight channel
1
Fixed ball
Straight channel
5
Y-shaped tee
2
Four way channel
6
L-shaped tee
4
T-shaped tee
8
T-shaped tee
5
L-shaped tee
9
Hemispherical straight through
0
Code of butterfly valve structure
structural style
Code name
structural style
Code name
Sealed type
Single eccentricity
0
Unsealed
Single eccentricity
5
Center vertical plate
1
Center vertical plate
6
Double eccentricity
2
Double eccentricity
7
Three eccentricities
3
Three eccentricities
8
Linkage mechanism
4
Linkage mechanism
9
Code of diaphragm valve structure
structural style
Code name
structural style
Code name
Ridge channel
1
Straight channel
6
Direct flow channel
5
Y-shaped angle channel
8
Code of plug valve structure
structural style
Code name
structural style
Code name
Packing seal
Straight channel
3
Oil seal
Straight channel
7
T-shaped tee
4
T-shaped tee
8
Four way channel
5
Code of check valve structure
structural style
Code name
structural style
Code name
Lifting disc
Straight channel
1
Swing disc
UNIVALVED structure
4
Vertical structure
2
Multilobed structure
5
Angle channel
3
Bicuspid structure
6
Butterfly check type
7
Code of safety valve structure
structural style
Code name
structural style
Code name
Spring loaded spring seal structure
Full open type with heat sink
0
Spring load spring not closed and with wrench structure
Micro lift, double valve
3
Micro open
1
Micro open
7
Full swing
2
Full swing
8
Full open type with wrench
4
Lever type
Single lever
2
Full open type with control mechanism
6
Double lever
4
Pulse type
9
Code of structure type of pressure reducing valve
structural style
Code name
structural style
Code name
Membrane type
1
Bellows type
4
Spring film type
2
Lever type
5
Piston type
3
Steam trap Code of structural form
structural style
Code name
structural style
Code name
Floating ball
1
Steam pressure type or capsule type
6
Floating bucket
3
Bimetallic chip
7
Liquid or solid expansion type
4
Pulse type
8
Bell float type
5
Disc type
9
Code of drain valve structure
structural style
Code name
structural style
Code name
Level connection drain
Cut off straight through
1
Intermittent discharge of liquid bottom
Cut off DC type
5
Cut off angle type
2
Cut off straight through
6
Cut off angle type
7
Floating gate type straight through type
8
Unit 5: material code of sealing surface and lining
Sealing surface or lining material
Tin based bearing alloy (Babbitt)
enamel
Nitriding steel
Fluoroplastics
ceramics
Cr13 series stainless steel
Rubber lining
Monel alloy
Code name
B
C
D
F
G
H
J
M
Sealing surface or lining material
Nylon plastics
Boronizing steel
Lead lining
austenitic stainless steel
Plastic
copper alloy
rubber
Cemented carbide
Code name
N
P
Q
R
S
T
X
Y
Unit 6: nominal pressure value
Expressed directly in Arabic numerals, the value is 10 times the value in MPA units.
Unit 7: valve body material
valve body
Material Science
Titanium and
titanium alloy
carbon steel
Cr13 series
stainless steel
Chromium molybdenum steel
malleable iron
aluminium alloy
18-8 series
stainless steel
Ductile iron
Mo2ti system
stainless steel
Plastic
Copper and
copper alloy
Cr Mo V steel
Gray cast iron
Code name
A
C
H
I
K
L
P
Q
R
S
T
V
Z
This item is omitted for gray cast iron low pressure valve and steel medium pressure valve.

Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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