Steps and basis for selecting a valve
1. Defining the purpose of the valve in the equipment or device, determining 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 to 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.
4. Determine the material of the casing and internal parts of the selected valve according to the medium, working pressure and working temperature delivered by the pipeline: gray cast iron, malleable cast iron, ductile 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. Determine the parameters of the valve: For the automatic valve, determine the allowable flow resistance, discharge capacity, back pressure, etc. according to different needs, and then determine the nominal diameter of the pipe and the diameter of the seat hole.
8. Determine the geometric parameters of the selected valve: the length of the structure, the form and size of the flange connection, the dimension of the height direction of the valve after opening and closing, the size and number of bolt holes to be connected, and the dimensions of the entire valve.
9. Use the existing information: valve catalogue, valve product samples, etc. to select the appropriate valve products.
The basis for selecting the valve:
While understanding the steps to master the valve, you should further understand the basis for selecting the valve.
1. The purpose of the selected valve, the working conditions and the control method.
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, the viscosity of the medium and so on.
3, the requirements of the valve fluid characteristics: flow resistance, discharge capacity, flow characteristics, sealing level and so on.
4, installation size and dimensions requirements: nominal diameter, connection with the pipe and connection size, size or weight restrictions.
5. Additional requirements for the reliability, service life and explosion-proof performance of the valve.
Note when selecting parameters:
If the valve is to be used for control purposes, the following additional parameters must be determined: method of operation, maximum and minimum flow requirements, pressure drop for normal flow, pressure drop at shutdown, maximum and minimum inlet pressure for the valve.
According to the above-mentioned basis and steps of selecting the valve, the internal structure of each type of valve must be well understood in order to properly and correctly select the valve, so that the correct choice can be made for the preferred valve.
The final control of the pipe is the valve. The valve opening and closing member controls the flow pattern of the medium in the pipe. The shape of the valve flow path allows the valve to have a certain flow characteristic, which must be taken into account when selecting the most suitable valve for the pipe system.
The principle that should be followed when selecting a valve
1. Valves for closing and opening media
The flow path is a straight-through valve with a small flow resistance and is usually selected as a valve for cut-off and open media. Down-closed valves (shut-off valves, plunger valves) are less preferred because of their tortuous path and higher flow resistance than other valves. Where a higher flow resistance is allowed, a closed valve can be used.
2. Valves for controlling flow
A valve that is easy to adjust the flow rate is usually selected as the control flow. A down-closed valve (such as a shut-off valve) is suitable for this purpose because its seat size is proportional to the stroke of the closure. Rotary valves (plug valves, butterfly valves, ball valves) and flexing valve body valves (clamping valves, diaphragm valves) can also be used for throttling control, but are usually only available in a limited range of valve sizes. The gate valve is a cross-cut movement of the circular valve seat with a disc-shaped shutter. It can control the flow better only when it is close to the closed position, so it is usually not used for flow control.
3. Valve for commutating diversion
This valve can have three or more passages depending on the need for a commutation split. Plug valves and ball valves are more suitable for this purpose, so most of the valves used for commutating splits select one of these types of valves. However, in some cases, other types of valves may be used for commutation splitting by simply interconnecting two or more valves.
4. Valves for media with suspended particles
When the medium has suspended particles, it is most suitable to use a valve with a wiping action of the sliding member along the sealing surface. If the closing movement of the closure to the valve seat is vertical, it is possible to hold the particles, so that the valve is only suitable for substantially clean media unless the sealing surface material allows for the incorporation of particles. The ball valve and the plug valve have a wiping action on the sealing surface during the opening and closing process, so it is suitable for use in a medium with suspended particles.
Valve selection instructions
1. Selection of gate valve
In general, the gate valve should be preferred. In addition to suitable for steam, oil and other media, the gate valve is also suitable for media containing granular solids and high viscosity, and is suitable for valves for venting and low vacuum systems. For media with solid particles, the gate valve body should have one or two purge holes. For low temperature media, low temperature special gate valves should be used.
2, the description of the valve selection
The shut-off valve is suitable for pipelines with less demanding fluid resistance, that is, the pressure loss is not considered, and the pipeline or device of high temperature and high pressure medium is suitable for medium pipes such as steam with DN<200mm; Valves, such as needle valves, instrument valves, sampling valves, pressure gauge valves, etc.; shut-off valves have flow regulation or pressure regulation, but the adjustment accuracy is not high, and the pipeline diameter is relatively small, the use of shut-off valves or throttling should be used Valve; for highly toxic medium, bellows sealed shut-off valve should be used; but the shut-off valve should not be used for medium with high viscosity and medium containing particles, and should not be used as valve for venting valve and low vacuum system.
3, ball valve selection instructions
Ball valves are suitable for low temperature, high pressure, high viscosity media. Most ball valves can be used in media with suspended solid particles. They can also be used for powder and granular media depending on the material requirements of the seal. Full-channel ball valves are not suitable for flow regulation, but are suitable for applications requiring fast opening and closing. Accident emergency cut-off; usually in the pipeline with strict sealing performance, wear, shrinkage passage, rapid opening and closing action, high pressure cutoff (large differential pressure), low noise, gasification, small operating torque, and small fluid resistance. Ball valve; ball valve is suitable for light structure, low pressure cut-off, corrosive medium; ball valve is also the ideal valve for low temperature, cryogenic medium, low temperature medium piping system and device, should choose the low temperature ball valve with bonnet; When the floating ball valve is used, the valve seat material should bear the load of the ball and the working medium. The large-diameter ball valve needs a large force during operation. The ball valve with DN≥200mm should use the worm gear transmission form; The fixed ball valve is suitable for the larger diameter and In the case of high pressure; in addition, the ball valve used in the process of highly toxic materials and flammable medium pipes shall have a fireproof and antistatic structure.
4, throttle valve selection instructions
The throttle valve is suitable for applications where the medium temperature is low and the pressure is high. It is suitable for the parts where the flow rate and pressure need to be adjusted. It is not suitable for medium with high viscosity and solid particles. It is not suitable as a block valve.
5, plug valve selection instructions
Plug valves are suitable for applications requiring fast opening and closing. They are generally not suitable for steam and high temperature media. They are used for mediums with low temperature and high viscosity, and also for media with suspended particles.
6, butterfly valve selection instructions
The butterfly valve is suitable for occasions with large caliber (such as DN>600mm) and short structural length requirements, as well as need to adjust the flow rate and fast opening and closing requirements. It is generally used for water, oil and compression with temperature ≤80°C and pressure ≤1.0MPa. Air and other media; because the butterfly valve has a relatively large pressure loss relative to the gate valve and the ball valve, the butterfly valve is suitable for use in a piping system where pressure loss is not critical.
7, check valve selection instructions
Check valves are generally suitable for cleaning media and should not be used for media containing solid particles and high viscosity. When DN≤40mm, it is advisable to use lifting check valve (only allowed to be installed on horizontal pipe); when DN=50~400mm, it is advisable to use swing-type lifting check valve (can be installed on both horizontal and vertical pipes, If installed on a vertical pipeline, the flow direction of the medium should be from bottom to top; when DN≥450mm, buffer type check valve should be used; when DN=100~400mm, the clamp type check valve can also be used; The return valve can be made to a high working pressure, and the PN can reach 42 MPa. It can be applied to any working medium and any operating temperature range depending on the material of the housing and the seal. The medium is water, steam, gas, corrosive medium, oil, medicine, and the like. The media operating temperature range is between -196 and 800 °C.
8, diaphragm valve selection instructions
Diaphragm valve is suitable for oil, water, acid medium and suspended medium with working temperature less than 200 °C and pressure less than 1.0MPa. It is not suitable for organic solvent and strong oxidant medium. Grinding granular medium should be selected with diaphragm valve. For the diaphragm type diaphragm valve, refer to its flow characteristic table; for viscous fluids, cement slurries and sedimentary media, straight-through diaphragm valves should be used; diaphragm valves should not be used for vacuum lines and vacuum equipment except for specific requirements.
Valve applications, operating frequency and service are ever-changing. To control or eliminate even low leakage, the most important and critical equipment is the number of valves. It is important to learn to choose the right valve.