What are the valves commonly used in chemical pipelines?

The device used to control the flow of fluid in a pipeline is called a valve. Its main functions are:

Opening and closing function - cutting off or communicating the fluid flow in the pipeline;

Adjustment function - adjust the flow rate and flow rate of fluid in the pipeline;

Throttling effect - after the fluid flows through the valve, a large pressure drop occurs.

Classification of valves:

According to the different functions of valves in the pipeline, they can be divided into cut-off valves (also called globe valves), throttle valves, check valves, safety valves, etc.;

According to the different structural forms of valves, they can be divided into gate valves, ball valves, butterfly valves, diaphragm valves, lining valves, etc.

In addition, according to the materials used to make valves, they are divided into stainless steel valves, cast steel valves, cast iron valves, plastic valves, ceramic valves, etc.

The selection of various valves can be found in relevant manuals and samples. Here we only introduce the most common valves.

Globe valve

Due to its simple structure and convenient manufacturing and maintenance, it is widely used in medium and low pressure pipelines. It uses a circular valve disc (valve head) installed under the valve stem to cooperate with the flange part (valve seat) of the valve body to achieve the purpose of cutting off the flow of fluid.

The valve stem can be raised and lowered by the thread to adjust the opening degree of the valve, which plays a certain regulating role. Since the shutoff function of the valve relies on the flat contact seal between the valve head and the valve seat, it is not suitable for use on pipelines containing fluids containing solid particles.

The stop valve can use appropriate valve head, valve seat, and shell materials according to the characteristics of the medium used. For valves that have poor sealing or damaged valve heads, valve seats and other parts during use, methods such as light knife, grinding, and surfacing welding can be used to repair and use them to extend the service life of the valve.

 

Gate Valve

The gate valve relies on one or two flat plates perpendicular to the flow direction of the medium to cooperate with the sealing surface of the valve body to achieve the purpose of sealing. The rising of the valve plate opens the valve.

The plate rises and falls with the rotation of the valve stem, and the fluid flow is adjusted by the opening size. This kind of valve has small resistance, good sealing performance, and labor-saving opening and closing. It is especially suitable for large-diameter pipelines. However, the structure of the gate valve is relatively complex and there are many types.

According to the different structure of the valve stem, it can be divided into open stem and concealed stem; according to the structural form of the valve plate, it can be divided into wedge type, parallel type, etc.

Generally, the wedge valve plate is a single valve plate, while the parallel valve plate usually uses two valve plates. The parallel type is easier to manufacture and repair than the wedge type, and is not easily deformed during use. However, it is not suitable for transporting fluid pipelines containing impurities. It is mostly used for transporting water, clean gas, oil and other pipelines.

 

Plug valve

The cock, commonly known as cock, uses a conical plug with a hole in the center inserted into the valve body to open and close the pipeline.

According to different sealing forms, cocks can be divided into stuffed cocks, oil-sealed cocks and non-packed cocks. The cock has a simple structure, small dimensions, quick opening and closing, easy operation, and small fluid resistance, making it easy to make a three-way or four-way distribution or switching valve.

The sealing surface of the cock is large and easy to wear. It is laborious to open and close, and it is difficult to adjust the flow rate, but it can be cut off quickly. The cock can be used in fluid pipelines with low pressure and temperature or with solid particles in the medium, but it is not suitable for use in fluid pipelines with high pressure, high temperature or steam pipelines.

Throttle valve

Throttle valve is a type of stop valve. The shape of the valve head is conical or streamlined, which can better control and regulate the flow of fluid or perform throttling and pressure regulation. The valve requires high manufacturing precision and good sealing performance.

It is mainly used in pipelines such as instrument control or sampling, but it is not suitable for pipelines with high viscosity and media containing solid particles.

Ball valve

Ball valve, also known as ball valve, is a type of valve that has developed rapidly in recent years. It uses a ball with a hole in the middle as the valve core, and relies on the rotation of the ball to control the opening or closing of the valve.

It is similar to a cock, but has a smaller sealing surface than a cock, a compact structure, labor-saving opening and closing, and is much more widely used than a cock.

With the improvement of ball valve manufacturing precision, ball valves are not only used in medium and low pressure pipelines, but also in high pressure pipelines. However, due to the limitations of sealing materials, it is not currently suitable for use in high-temperature pipelines.

 

 

Diaphragm valve

Common ones include membrane valves. This kind of valve is opened and closed by a special rubber diaphragm. The diaphragm is sandwiched between the valve body and the valve cover. When closing, the disc under the valve stem presses the diaphragm against the valve body to achieve a seal.

This kind of valve has simple structure, reliable sealing, easy maintenance and low fluid resistance. It is suitable for transporting acidic media and fluid pipelines with suspended solids, but is generally not suitable for pipelines with higher pressures or temperatures above 60°C, and is not suitable for pipelines transporting organic solvents and strongly oxidizing media.

Check valve

Check valve is also called check valve or one-way valve. Installed in the pipeline, the fluid can only flow in one direction and no reverse flow is allowed.

It is an automatic closing valve with a disc or rocking plate in the valve body. When the medium flows forward, the fluid automatically pushes the valve disc open; when the fluid flows backward, the fluid (or spring force) automatically closes the valve disc. According to the different structures of check valves, they are divided into lift type and swing type.

The lifting check valve disc moves vertically to the valve body channel and is generally used on horizontal or vertical pipelines; the disc of the swing check valve is often called a rocking plate. One side of the rocking plate is connected to the shaft, and the rocking plate Rotating around the axis, swing check valves are generally installed on horizontal pipelines. For small diameter ones, they can also be installed on vertical pipelines, but be careful not to allow the flow rate to be too large.

Check valves are generally suitable for clean medium pipelines, but should not be used in medium pipelines containing solid particles and high viscosity. The sealing performance of the lift-type check valve is better than that of the swing-type check valve, but the fluid resistance of the swing-type check valve is smaller than that of the lift-type check valve. Generally, swing check valves are suitable for large-diameter pipelines.

 

Butterfly valve

The butterfly valve relies on a rotatable disk (or elliptical disk) in the pipe to control the opening and closing of the pipeline. It has a simple structure and small dimensions.

Due to sealing structure and material problems, the valve has poor sealing performance and is only suitable for adjustment in low-pressure, large-diameter pipelines. It is commonly used in pipelines transporting water, air, gas and other media.

Pressure reducing valve

The pressure reducing valve is an automatic valve that reduces the medium pressure to a certain value. Generally, the pressure behind the valve is less than 50% of the pressure in front of the valve. It mainly relies on diaphragms, springs, pistons and other parts to use the pressure difference of the medium to control the disc and valve. The gap between the seats achieves the purpose of pressure reduction.

There are many types of pressure reducing valves, the most common ones are piston type and diaphragm type.

 

 

Lined valve

In order to prevent corrosion from the medium, some valves need to be lined with corrosion-resistant materials (such as lead, rubber, enamel, etc.) on 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 lined valves are made of right-angle or straight-flow type.

Safety valve

In order to ensure the safety of chemical production, safety devices are often installed in pressure pipeline systems, that is, metal sheets of a certain thickness are selected and installed at the end of the pipeline or on the tee interface like a blind plate.

When the pressure in the pipeline increases, the sheet is broken to achieve the purpose of pressure relief. Blasting plates are generally used in low-pressure, large-diameter pipelines, but safety valves are used in most chemical pipelines. There are many types of safety valves, which can be roughly divided into two categories, namely spring type and lever type.

Spring-type safety valves mainly rely on the force of springs to achieve sealing. When the pressure in the pipe exceeds the elastic force of the spring, the valve is pushed open by the medium, and the fluid in the pipe is discharged, reducing the pressure.

Once the pressure in the pipe drops below the spring force, the valve closes again. The lever type safety valve mainly relies on the force of the weight on the lever to achieve sealing, and its working principle is the same as that of the spring type. The selection of safety valves 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 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 safety valve's tripping pressure, testing and acceptance, which are regularly checked and sealed by the safety department. They must not be adjusted arbitrarily during use to ensure safety.