Valve is the basic element used to control the flow direction, pressure and flow of fluid in pipeline. The valve is the control component in the pipeline fluid delivery system, which is used to change the section of the access and the medium flow direction, and has the functions of diversion, cut-off, adjustment, throttling, check-back, diversion or overflow pressure relief, etc. The valve used for fluid control, from the simplest stop valve to various valves used in extremely complex automatic control system, has many varieties and specifications, the nominal diameter of the valve is from the extremely tiny instrument valve to the industrial pipeline valve with a diameter of 10m.
Classification of valves
Classification by use and role
- (1) block valves are mainly used to cut off or connect medium flow. Including gate valve, stop valve, diaphragm valve, ball valve, plug valve, butterfly valve, plunger valve, ball plug valve, needle type instrument valve, etc.
- (2) control valve is mainly used to adjust medium flow, pressure, etc. Including regulating valve, throttle valve, pressure reducing valve, etc.
- (3) check valves are used to prevent dielectric from flowing backwards. Including check valves of various structures.
- (4) shunt valve type is used to separate, distribute or mix media. It includes the distribution valve and the trap valve of various structures.
- (5) safety valves are used for safety protection when medium overpressures. Including various types of safety valves.
Classification by main parameters
Classification by pressure
- (1) vacuum valve working pressure is lower than the standard atmospheric pressure of the valve.
- (2) the low pressure valve nominal pressure pn≤1. 6 mpa.
- (3) the valve whose nominal pressure PN of medium pressure valve is 2.5, 4.0 or 6.4 MPa.
- (4) high Pressure Valve nominal pressure PN is 10.0~80.0 MPa valve.
- (5) super High Pressure Valve nominal pressure pn≥100 MPa.
Classification by medium temperature
- (1) high temperature valve is used for the valve with medium working temperature t> 450℃.
- (2) medium temperature valve is used for medium working temperature 120 ℃ valve.
- (3) normal temperature valve is used for valves with medium working temperature of-40℃ ≤t≤120℃.
- (4) low temperature valve is used for the valve of medium working temperature-100℃ ≤t≤40 ℃.
- (5) ultra low temperature valve is used for the valve with medium working temperature t <-100℃.
Classification by valve body material
- (1) non-metallic material valves: such as ceramic valves, FRP valves, plastic valves.
- (2) metal material valves: such as copper alloy valve, aluminum alloy valve, lead alloy valve, hastelloy alloy valve, monel alloy valve, cast iron valve, carbon steel valve, stainless steel valve, low alloy steel valve, high alloy steel valve.
- (3) metal valve body lining valve: such as lead lining valve, plastic lining valve and enamel lining valve.
This kind of classification method is divided according to both the principle and function, which is currently the most commonly used classification method at home and abroad. It is generally divided into gate valve, stop valve, throttle valve, instrument valve, plunger valve, diaphragm valve, plug valve, ball valve, butterfly valve,globe valves,check valve, reducing valve, safety valve, trap valve, bottom valve, filter, drain valve, etc.
Characteristics of valve
It determines the main performance and scope of use of the valve. The use characteristics of valves include: Valve category (closed-circuit valve, regulating valve, safety valve, etc.); Product Type (gate valve, stop valve, butterfly valve, ball valve, etc.); Main parts of valve (valve body, material of bonnet, valve stem, disc and sealing surface); Valve transmission mode, etc.
It determines some structural characteristics of Valve installation, maintenance, maintenance and other methods. Belonging to the structural characteristics: the structural length and overall height of the valve, and the connection form with the pipeline (flange connection, threaded connection, clamp connection, external threaded connection, welding end connection, etc.); the form of sealing surface (insert ring, thread ring, overlaying, spray welding, valve body); Structure of valve stem rotating rod, lifting rod) and so on.
Steps and basis of valve selection
Steps for selecting valves
- (1) specify the use of the valve in the equipment or device, determine the working conditions of the valve: applicable medium, working pressure, working temperature, etc.
- (2) determine the nominal diameter and connection method of the pipe connected with the valve: flange, thread, welding, etc.
- (3) determine the mode of operating the valve: manual, electric, electromagnetic, pneumatic or hydraulic, electrical linkage or electro-hydraulic linkage, etc.
- (4) determine the materials of the shell and internals of the selected valve, gray cast iron, malleable cast iron, ductile iron, carbon steel, alloy steel, stainless acid-resistant steel, copper alloy, etc.
- (5) select the types of valves: Closed-circuit valves, regulating valve, safety valves, etc.
- (6) determine the form of the valve: gate valve, stop valve, ball valve, butterfly valve, throttle valve, safety valve, pressure reducing valve, steam trap valve, etc.
- (7) determine the parameters of the valve: for the automatic valve, according to different needs, first determine the allowable flow resistance, discharge capacity, back pressure, etc., and then determine the nominal diameter of the pipeline and the diameter of the valve seat hole.
- (8) determine the geometric parameter structure length of the selected valve, flange connection form and size, size of valve height direction after opening and closing, size and quantity of connected bolt holes, external dimensions of the whole valve, etc.
- (9) use the existing information, such as product catalog of valves and samples of valve products, to select appropriate valve products.
Basis of valve selection
- (1) the purpose, working condition and control mode of the selected valve. For example, the choice of the pump house water pump outlet valve, first need to meet the water pump can close the valve start and stop to reduce the start current and stop the impact on the water pump; Second need to be equipped with a check valve, when the pump unit is accidentally shut down, the valve can be quickly closed to prevent the pump from being reversed for a long time; Thirdly, the valve to eliminate the water hammer should be equipped to ensure the operation safety of the pump unit. After understanding the purpose of the selected valve, it is necessary to understand the installation site and use conditions to correctly select the valve. If electric butterfly valve + micro stop check valve is selected, electric butterfly valve can meet the requirements of start-up and shutdown of the closed valve. Micro retarding closed check valve is used to prevent water pump reversal and water hammer. Due to its relatively simple structure, the failure rate is lower, but the installation length is longer and the water resistance is larger, so it is suitable for large installation space, the pump station which does not have high energy consumption requirements, such as the use of small pump station; The choice of the hydraulic slowly-closing type check valve can meet the three necessary functions of the pump outlet valve at the same time, and the installation length can be very small, and the water resistance is, however, due to the complicated structure, a set of high-pressure hydraulic system is needed, so the failure rate is high and the maintenance is difficult. Therefore, it is suitable to be installed in the large pumping station where there are many spare units; multi-function pump control valve is also available for pump outlet, and its installation length is shorter than that of electric butterfly valve + micro stop closed check valve, so there is no need for electric or hydraulic system vacancy, with the simplest structure and the lowest failure rate, and the water hammer has the best removal, however, the water resistance is the highest, which is suitable for the use of water hammer in severe or unattended pumping stations.
- (2) properties of working medium working pressure, working temperature, corrosion performance, whether it contains solid particle medium, whether it is toxic, whether it is the viscosity of flammable and explosive medium, etc.
- (3) requirements for valve fluid characteristics: flow resistance, discharge capacity, flow characteristics, sealing grade, etc.
- (4) installation dimension and outer dimension requirements: nominal diameter, connection method and connection dimension of the pipe, outer dimension or weight limitation, etc.
- (5) additional requirements for the reliability, service life and explosion-proof performance of electric devices of valve products. Attention should be paid to when selecting parameters: if the valve is to be used for control purpose, the following additional parameters must be determined: Operation method, maximum and minimum flow requirements, normal flow pressure drop, pressure Drop when closed, maximum and minimum inlet pressure of valve.
According to the above-mentioned basis and steps for selecting valves, the internal structure of various types of valves must also be understood in detail when selecting valves reasonably and correctly, so as to make the right choice for the preferred valves.
The final control of the pipeline is the valve. The valve hoist controls the medium in the pipeline flow way the shape of the valve passage makes the valve have certain flow characteristics, which must be taken into account when choosing the most suitable valve for installation in the pipeline system.
Principles that must be followed when choosing valves
Valves for cut-off and open media
The channel is a through-through valve, commonly used butterfly valve, gate valve, etc., whose flow resistance is relatively small, usually choose as the cut-off and open medium valve. The downward closed valve (stop valve, plunger valve) is seldom used because its channel is tortuous and its flow resistance is higher than other valves. In the occasion allowing a higher flow resistance, such as the transport medium is gas, you can choose to use the downward closed valve. When choosing butterfly valve, when there are more impurities in the transport medium, such as raw water or sewage, horizontal butterfly valve should be selected, because its valve shaft is horizontal, the bottom of the flow channel is not easy to accumulate debris and scale, which is conducive to the protection of the valve plate seal ring. Choose the type of gate valve that has no groove at the bottom of the valve runner to avoid valve leakage due to the accumulation of sundries in the groove.
The valve used for flow control
Generally, a valve that is easy to adjust the flow is selected to control the flow. A downward closed valve (such as a stop valve) is suitable for this purpose because its valve seat size is proportional to the stroke of the closing piece. Rotary Valve Rotary plug valve, butterfly valve, ball valve) and flexible valve body type 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 disc disc to make a cross-cutting movement of the circular valve seat. It can control the flow well only when it is close to the closed position, so it is usually not used for flow control.
Reversing shunt valve
According to the need of reversing shunt, this valve can have three or more channels. Plug valves and ball valves are more suitable for this purpose. Therefore, most of the valves used for reversing and shunt are one of these valves. However, in some cases, other types of valves, as long as two or more valves are properly connected to each other, can also be used for reversing diversion.
Valve for medium with suspended particles
When the medium has suspended particles, it is most suitable to use its closing parts along the sealing surface to slide the valve with wiping effect. If the back and forth movement of the closing piece to the valve seat is vertical, it is possible to clamp the particles. Therefore, this valve is only suitable for basic clean media unless the sealing surface material can allow embedded particles. Ball valve and plug valve in the opening and closing process on the sealing surface are wiping effect, so it is suitable to be used in the medium with suspended particles.
The installation of the valve should be carried out in accordance with the valve instruction and relevant provisions. During the construction process, it should be carefully checked and carefully constructed. Before installing the valve, install it after the pressure is tested to be qualified, carefully check whether the specification and model of the valve are consistent with the drawings, check whether the parts of the valve are intact, and whether the opening and closing valve can rotate flexibly, whether the sealing surface is damaged or not can be installed after confirmation.
When installing the valve, the operation mechanism of the valve should be at about 1.2m away from the operation ground and be in conformity with the chest. When the center of the valve and the hand wheel are more than 1.8m away from the operating ground, an operating platform should be set for valves and safety valves that are frequently operated. For pipelines with more valves, the valve should be concentrated on the platform as far as possible for easy operation.
For a single valve exceeding 1.8m and not frequently operated, Sprocket, extension rod, movable platform, movable ladder and other equipment can be used. When the valve is installed under the operation surface, stretching rod should be set, and the ground valve should be set up in the ground well. For the sake of safety, the ground well should be covered.
For the valve stem of the valve on the horizontal pipe, it is best to vertically up, and it is not suitable to install the valve stem down. The valve stem is installed downward, which is inconvenient to operate, inconvenient to repair, and easy to corrode the valve. The floor valve should not be installed in a crooked way to avoid inconvenience.
Valves on the pipelines which are side by side should have vacancy for operation, maintenance and disassembling, and the net distance between the hand wheels should not be less than 100mm. If the distance between the pipes is narrow, the valves should be staggered.
For those valves which have big opening force, low strength, high brittleness and heavy weight, the valve frame should be set to support the valve before installation to reduce starting stress.
When installing the valve, the pipe near the valve uses the pipe wrench, while the valve itself uses a common wrench. At the same time, during installation, the valve should be semi-closed to prevent valve rotation and deformation.
The correct installation of the valve should make the internal structure form conform to the flow direction of the medium, and the installation form conform to the special requirements and operation requirements of the valve structure. In particular, pay attention to the valve with medium flow requirements should be installed according to the requirements of the process pipeline. The arrangement of the valve should be convenient and reasonable, and it is easy for the operator to get access to the valve. For the lift stem type valve, the operating space should be reserved.
Installation of valve connection surface
When installing the valve connected with thread at the end, the thread should be screwed into the valve at a proper depth, and the thread should be screwed into a too deep and compressed valve seat, which will affect the good coordination between the valve seat and the Ram, and screwed into the shallow, it will affect the sealing reliability of the joint and easily introduce leakage. At the same time, the thread sealing material should use tetrafluoroethylene green tape or sealant.
For the valve connected by the end of the flange, first find the connection surface of the positive flange, the cover is perpendicular to the pipeline, and the bolt holes should be aligned. The valve flange should be parallel to the pipe flange, the flange clearance is moderate, and the phenomena such as misopening and tilting should not occur. The center gasket between the flanges should be placed in the middle, without deflection, and the bolts should be tightened symmetrically and evenly. Prevent the tightening of the forced connection during the installation of the valve and produce an additional residual force.
Before installation, it is necessary to thoroughly remove the dirt from the inner wall of the pipe and the external thread; Remove the burr and foreign matter that block the flow of the medium and may affect the operation of the equipment, and blow out the dirt in the pipe before connecting the pipe, slag and other sundries. To prevent damage to the sealing surface of the valve or blockage of the valve.
When installing the welding end connection valve, the two ends of the valve should be opened after spot welding, and then the weld is welded according to the welding process card, and the appearance of the weld and the quality of the inner weld are inspected after welding, make sure that there are no pores, slag, cracks, etc., and the welding lines should be checked by ray or override when necessary.
Heavy valve installation
When installing heavier valve (DN>100), lifting tools or equipment should be used. The lifting rope should be tied to the flange or bracket of the valve, and should not be tied to the handle-type valve stem of the valve, to avoid damaging the valve.
What are the general requirements for valve installation?
The general requirements of valve installation, the most suitable installation height, the direction of the valve and the valve stem on the horizontal pipe are as follows:
- The valve should be set in a place that is easy to access, easy to operate and maintain. Valves on rows of pipes (such as pipes for receiving and discharging devices) should be arranged in a centralized manner, and the setting of operation platform and ladder should be considered. The central line of the valve on the pipe in parallel layout should be taken as close as possible. The net distance between the hand wheels should not be less than 100mm. In order to reduce the pipeline spacing, the valves can be staggered;
- The installation position of frequently operated valve should be easy to operate, and the most suitable installation height should be 1.2m or so away from the operation surface. When the height of the center of the valve hand wheel exceeds 2m of the operation surface, platform should be set for the central arrangement of the valves, the frequently operated individual valves and the safety valve, appropriate measures should also be taken for individual valves that are not frequently operated (e.g. sprocket, extension rod, movable platform and movable ladder, etc.). The chain of the sprocket should not hinder traffic. Pipelines of the dangerous media and valves of the equipment should not be installed within the height range of the head to avoid bumping and damaging the head or directly damaging people's face due to valve leakage;
- The valve used for partition equipment should be directly connected with or close to the nozzle of equipment. The valve connecting the pipe with extremely harmful and highly harmful toxic medium should be directly connected with the equipment port, and the valve should not be operated by chain wheel;
- Accident handling valves such as fire water valve and fire steam valve should be arranged in separate places, and the safe operation in the accident should be considered. This kind of valve should be arranged in the control room. There are safety zones behind the safety wall, outside the factory gate, or within certain distance from the accident site; So that the operators can operate safely when there is a fire accident;
- Except for special requirements of craft, valves on the bottom pipelines of tower, reactor, vertical vessel and so on should not be arranged in skirt;
- The cut-off valve of the horizontal branch that leads out from the main pipe should be set on the horizontal pipe section near the root;
- Lifting check valves should be installed on the horizontal pipe, and vertical lifting check valves should be installed on the vertical pipe where media flows from bottom to top. Swing check valve should be preferentially installed on the horizontal pipe, or installed on the vertical pipe in which medium flows from bottom to top; Bottom valve should be installed on the installation height of centrifugal pump suction butterfly check valve can be selected; when the diameter of pump outlet is different from that of the connected pipe, reducing check valve can be used;
- The center distance between the hand wheel of the valve arranged around the operation platform and the edge of the operation platform should be no larger than 450mm. When the valve stem and the hand wheel stretch into the above of the platform with the height less than 2 meters, it should not affect the operation and transportation of the operator;
- The valve of underground pipeline should be set in the trench or valve well, and the extension rod of the valve should be set when necessary. Fire valve well should have obvious sign;
- For the valve on the horizontal pipe, the direction of the valve stem can be determined according to the following order: vertically up; Horizontally; Up to 45 °; Down to 45 °; Not vertically down;
- The stem is horizontally installed open-lever valve. When the valve is open, the stem shall not affect the passage.
Technical requirements for valve installation
- Direction. On the valve body of general valve, the direction indicated by the arrow is the direction in which the gas flows forward. Special attention must be paid to the fact that it must not be reversed. Because there are various valves requiring one-way gas circulation, such as safety valve, pressure reducing valve, check valve, throttle valve, etc. For the stop valve, in order to facilitate opening and maintenance, gas is also required to pass through the valve seat from bottom to top, but the gate valve, when installing the cock, it is not limited by the flow direction.
- Installation location. The long-term operation and maintenance of the valve should be considered, making it as convenient as possible to operate and maintain, and at the same time, attention should be paid to the beautiful appearance when assembling.
- The valve handle can be turned vertically to the above direction, and can also be tilted to a certain angle or placed horizontally, but the hand wheel can not be downward to avoid the operation of looking up; The hand wheel of the landing valve should be best close to the chest to facilitate the opening and closing; the open bar gate valve cannot be used underground to prevent the valve stem from being corroded.
- The installation position of some valves has special requirements, such as the pressure reducing valve is required to be installed vertically on the horizontal pipe and cannot be tilted. The lift check valve requires that the valve clack is vertical swing check valve and the pin shaft is required to be horizontal. In short, the installation position should be determined according to the principle of the valve, otherwise the valve will not work effectively or even will not work.
- Installation of the cock. Check the specifications and models, identify whether there is any damage, clean the sealing cap of the threaded opening and excessive grease and sundries in the thread, and check the sealing performance of the cock. Proper strength must be applied when installing the gas cock. According to the size of the cock, different specifications of pipe tongs or spanners should be chosen to install.
- When installing the valve with screw thread, the valve company should ensure that the thread is intact and intact; When the wrench can be used, do not use the pipe wrench to avoid damage to the appearance of the valve body.
- The installation of flange valve must ensure that the two flange sections are parallel to each other and on the same axis. When tightening the bolt, it should be carried out in a cross way to make the valve end face evenly stressed.
- The valve connected by flange and thread should be installed in the closed state. When the butt welding valve is connected with the pipe, the bottom weld should be argon welded to ensure the internal clean welding, and the valve should not be closed to prevent heating deformation.
- Under normal circumstances, the valve is directly connected with the corrugated expansion joint, so the pipes on both sides of the valve and the corrugated expansion joint should be cut off according to the size of the valve and the corrugated expansion joint as well as the flange and the gasket to set aside the installation position.
- When the valve is hoisted, the rope should not be tied to the first wheel or valve stem to prevent damage. When placed on the pier, according to the requirements of the elevation pad is stable and balanced. There must be a solid pier or bracket under the valve to hold the valve, and the valve is not allowed to generate stress.
Pipe valve installation 35 professional tips
- 1. Pay attention to the direction of medium flow during installation.
- 2. Check valve should be installed before condensate water enters the recycling main pipe after steam trap to prevent condensate water from returning.
- 3. The open bar valve cannot be buried to prevent the stem from rusting. In the trench with cover plate, the valve should be installed in a convenient place for maintenance, inspection and operation.
- 4. For pipelines that require to be closed with small water hammer impact or no water hammer, it is best to choose slow-closing butterfly check valve and slow-closing swing check valve.
- 5. When installing the threaded valve, it is necessary to ensure that the thread is intact, and the filler is coated according to the different media. When tightening, it is necessary to use even force to avoid damage to the valve and valve parts.
- 6. When installing the socket-welded valve, the socket-welded valve should have a gap of 1-2m to prevent the thermal stress from exceeding the ambassador during welding.
- 7. When installing on the horizontal pipe, the valve stem should be vertically upward, or inclined to a certain angle, and the valve stem is not allowed to be installed downward.
- 8. The bottom layer of welding seam connecting the docking valve and pipe should adopt argon arc welding, and the valve should be opened during welding to prevent overheating and deformation.
- 9. Before installing the steam trap, the pipeline must be blown with pressure steam to remove sundries in the pipeline.
- 10. Do not install the steam trap in series.
- 11. Diaphragm type check valve is often used in pipelines which are easy to generate water hammer, because the diaphragm can well eliminate the water hammer generated when the medium is countercurrent, but it is limited by the temperature and pressure, generally used on low pressure normal temperature pipes.
- 12. Filter should be installed before the steam trap to ensure that the steam trap is not blocked by pipeline sundries, and the filter should be cleaned regularly.
- 13. The valve connected with flange and thread should be closed during installation.
- 14. The direction of condensate water should be consistent with the sign of the arrow installed on the trap.
- 15. The steam trap should be installed at the lowest place of the equipment outlet, and the condensed water should be discharged in time to avoid steam resistance in the pipeline.
- 16. When installing flange valve, it should be ensured that the two flange end faces are parallel and concentric with each other.
- 17. The valve should be installed before and after the trap, so that the trap can be repaired at any time.
- 18. The mechanical trap should be installed horizontally.
- 19. If the drain valve is found to run steam, it should be discharged in time and the filter should be cleaned according to the actual use situation, and repaired at any time in case of failure.
- 20. Do not make the check valve bear the weight in the pipeline. Large check valve should be supported independently, so that it is not affected by the pressure generated by the pipeline system.
- 21. After the drain valve, the condensate recovery main pipe cannot climb, which will increase the back pressure of the drain valve.
- 22. If there is no trap installed at the lowest point of the equipment, a reverse water bend should be added at the lowest point of the water outlet to raise the condensation water level before installing a trap to avoid steam resistance.
- 23. The outlet pipe of the trap should not be soaked in water.
- 24. If condensate is recycled after the steam trap, the outlet pipe of the drain valve should be connected to the main pipe from above the recovery main pipe to reduce back pressure and prevent backflow.
- 25. Each equipment should be installed with traps.
- 30. Lift type level flap check valve should be installed on the level pipe.
- 31. Install a drain valve on the steam pipe. In the main pipe, a condensate water-collecting well which is close to the radius of the main pipe should be set, and then use a small tube to lead to the drain valve.
- 32. If there is condensate recovery after the trap, it needs to be recovered separately by pipelines with different levels of pressure.
- 33. Lift-type vertical flap check valve needs to be installed vertically.
- 34. When the mechanical trap is not in use for a long time, it is necessary to remove the sewage screw and let the water inside go to prevent freezing.
- 35. Before the thermostatic type steam trap, a subcooled pipe that does not keep warm is needed for more than one meter. Other types of steam trap should be as close as possible to the equipment.
At present, whether in municipal water supply, petrochemical, or in other industries, the application, operating frequency and service of pipeline system valves are ever-changing, it is most important to control or eliminate even low leakage, the most critical equipment is the valve. The final control of the pipeline is that the service and reliable performance of the valve in various fields are unique.
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)