The device used to control the flow of fluid in the pipeline is called a valve. Its main function is:
- On-off – cut off or connect the fluid flow in the pipeline；
- Regulating – regulating the flow rate and flow rate of the fluid in the pipeline
- According to the different function of the valve in the pipeline, it can be divided into cut-off valve (also known as globe valve), throttle valve, check valve, safety valve, etc.
- According to the different structure of the valve, it can be divided into gate valve, plug cock(It is often called Cork), ball valve, butterfly valve, diaphragm valve, lining valve, etc.
In addition, according to the material of the valve, it is divided into stainless steel valve, cast steel valve, cast iron valve, plastic valve, ceramic valve, etc.
The selection of various valves can be found in the relevant manuals and samples. Only the most common types of valves are introduced here.
Because of its simple structure and convenient manufacture and maintenance, it is widely used in middle and low pressure pipelines. It uses the round valve disc (valve head) installed under the valve stem to cooperate with the flange part (valve seat) in the valve body to achieve the purpose of shutting off the fluid flow.
The valve stem can adjust the opening status of the valve by the thread lifting, and plays a certain adjusting role. Because the valve’s shut-off function relies on the sealing between the valve head and the valve seat, it is not suitable for pipelines containing solid particles.
For the globe valve, the material of the valve head, valve seat and body can be selected according to the characteristics of the medium used. For valves that are damaged due to poor sealing or damaged parts such as valve heads and valve seats, you can use light knife, grinding, surfacing and other methods to repair them to extend the service life of the valve.
The gate valve is based on one or two flat plates perpendicular to the flow direction of the medium, and cooperates with the sealing surface of the valve body to achieve the purpose of turning off. The valve plate is raised to turn on the valve.
The plate moves up and down with the rotation of the valve stem, and the status of the opening is used to adjust the flow rate of the fluid. This kind of valve has low resistance, good sealing performance, and labor-saving switching. It is especially suitable for large-diameter pipelines, but the structure of the gate valve is relatively complex and there are many types.
According to the different structure of the valve stem, there are outside rod and non-rising rod; according to the structure of the valve plate, it is divided into wedge type and parallel type.
Generally, the wedge valve plate is a single valve plate, and the parallel type uses two valve plates. The parallel type is easier to manufacture than the wedge type, easy to repair, and not easy to deform during use, but it is not suitable for conveying fluid pipelines containing impurities, and is mostly used for conveying water, clean gas, oil and other pipelines.
Plug valve was commonly known as cock, It uses a tapered plug with a hole in the center of the valve body to turning on and off the pipeline.
Plug valve can be divided into packing plug valve, oil-sealing plug valve, and packing-free plug valve according to different sealing forms. It is simple in structure, small in shape, quick in opening and closing, convenient in operation, low in fluid resistance, and convenient in making three-way or four-way distribution or switching valves.
The sealing surface is large so it is easy to be worn, on-off is laborious, it is not easy to regulate the flow, but it can be cut quickly. The plug valve can be used in fluid lines with low pressure and temperature or solid particles in the medium, but it is not suitable for high pressure, high temperature or steam lines.
It is a kind of globe valve. The shape of the valve head is conical or streamlined, which can better control and adjust the flow of fluid or adjust the pressure. This valve requires high precision and good sealing performance.
It is mainly used in pipelines such as instrument control or sampling, but should not be used in pipelines with high viscosity and medium containing solid particles.
It is also known as ball core valve, it is a kind of valve that has developed rapidly in recent years. It uses a sphere with a hole in the middle as the valve core and relies on the rotation of the sphere to control the on-off of the valve.
It is similar to the plug valve, but has a smaller sealing surface than the plug valve, compact structure, and labor-saving switch, which is far more widely used than the plug valve.
With the improvement of manufacturing precision of ball valves, ball valves are not only used in medium and low pressure pipelines, but also in high pressure pipelines. However, due to the limitation of sealing materials, it is not suitable for high temperature pipelines at present. Such as fully welded gas ball valves and heating ball valves.
There are common membrane valves. The opening and closing of this valve is a special rubber diaphragm. The diaphragm is sandwiched between the valve body and the bonnet. When it closed, the disc under the valve stem presses the diaphragm on the valve body to seal.
This valve has a simple structure, reliable sealing, easy maintenance, and low fluid resistance. It is suitable for conveying acidic medium and fluid pipeline with suspended matter, but it is generally not suitable for pipelines with higher pressure or temperature higher than 60℃, and it is not suitable for pipelines for conveying organic solvents and strong oxidizing media.
It is also called non-return valve or one-way valve. It was installed in the pipeline so that the fluid can only flow in one direction, not allowing reverse flow.
It is an automatic shut-off valve with a flap or swing plate in the valve body. When the medium flows downstream, the valve flap is automatically opened by the fluid; when the fluid flows backward, the valve flap is closed automatically by the fluid (or spring force). According to the different structure of the check valve, it is divided into two types: lifting type and swing type.
The lifting check valve flap is lifting perpendicular to the valve body channel, and it is generally used on horizontal or vertical pipelines; the valve flap of the swing check valve is often called a swing plate, and the side of the swing plate is connected to the shaft and the swing plate It can rotate around the axis, and the swing check valve is generally installed on horizontal pipelines. For small diameter ones, it can also be installed on vertical pipelines, but it should be noted that the flow rate should not be too large.
Check valves are generally suitable for cleaning medium pipelines, and should not be used for pipelines containing solid particles and high viscosity media. The lifting type check valve has better sealing performance than the swing type, but the fluid resistance of the swing type check valve is smaller than that of the swing type. Generally, the swing check valve is suitable for large diameter pipelines.
The butterfly valve relies on a rotating disk (or elliptical disk) in the pipe to control the on-off of the pipeline. It is simple in structure and small in size.
Due to the sealing structure and material problems, the valve has poor sealing performance and it is only suitable for the adjustment of low-pressure, large-diameter pipelines. It is commonly used in pipelines that transport water, air, gas and other media.
It is an automatic valve that reduces the medium pressure to a certain value. Generally, the pressure behind the valve is 50% less than the pressure before the valve. It mainly depends on the diaphragm, spring, piston and other parts to use the pressure difference of the medium to control the gap between the valve flap and the valve seat to achieve the purpose of decompression.
There are many types of relief valves, common ones are piston type and membrane type.
In order to prevent the corrosion of the medium, some valves need to be lined with corrosion-resistant materials (such as lead, rubber, enamel, etc.) in the valve body and valve head. The lining material should be selected according to the nature of the medium.
For the convenience of lining, most of the lining valves are made with 90°angle or straight way type.
In order to ensure the safety of chemical production, there is often a safety device in the pressure pipeline system, that is, a metal foil with a certain thickness is selected, and it is installed on the end of the pipeline or the three-way interface like a blind plate.
When the pressure in the pipeline increases, the slice is broken to achieve the purpose of pressure relief. Bursting plates are generally used in low-pressure, large-diameter pipelines, but safety valves are used in chemical pipelines mostly. There are many types of safety valves, but it can be roughly divided into two categories, spring type and lever type.
The spring type safety valve mainly depends on the force of the spring to achieve the seal. When the pressure in the tube exceeds the elastic force of the spring, the valve is pushed open by the medium, and when the fluid in the tube is discharged, the pressure is reduced.
Once the pressure in the tube drops down below the spring force, the valve closes again. The lever type safety valve is mainly sealed by the force of the heavy hammer on the lever, and the principle of action is the same as the spring type. The selection of the safety valve is based on the working pressure and working temperature to determine the nominal pressure level, and its caliber size can be calculated and determined with reference to the relevant regulations.
The structural type of the safety valve and the material of the valve should be selected according to the nature of the medium and working conditions. There are special regulations on the open pressure, test and acceptance of the safety valve, which shall be checked and printed regularly by the safety department, and shall not be adjusted arbitrarily in use to ensure safety.