The four types of valves introduced here are the most commonly used shut-off valves in the chemical industry. Their main function is to connect and cut off the flow of medium in the pipeline. Although some of them can adjust the flow rate, they cannot be used entirely as regulating valves.

1、 Ball valve

This is a comprehensive valve that is improved on the rotation control opening and closing of the plug valve and the center ball of the valve body. It has flexible action, small volume, light weight, low medium flow resistance, and good sealing effect.

Ball valves are divided into floating and fixed types, with the main difference being the support method of the middle ball. Floating ball valves have no support shaft on the lower side of the ball and are supported by the valve seats at both ends of the ball. When the ball is under pressure, it will slightly lean towards the outlet valve seat. If the pressure is too high, it will aggravate the wear of the sealing layer. The fixed ball valve ball is fixed to the valve stem, and the other end of the valve stem is connected to a bearing. After working under pressure, the ball will not deviate and is supported by the bearing seat. The sealing surface of the valve seat is not highly compressed, and the fixed internal structure is more complex and relatively expensive.

Floating ball valves are generally used for medium and low pressure pipelines below DN200, while fixed ball valves are preferred for high pressure and large pipe diameters. Although ball valves can play a partial regulating role in use, it is generally not recommended to use ordinary ball valves to regulate flow. If the ball valve is partially opened for a long time, it will greatly reduce the service life of the ball valve. The impact of the medium will damage the edge of the inner hole of the ball and the sealing layer of the valve seat, causing internal leakage.

2、 Gate valve

As one of the most commonly used valves in the chemical industry, it is also the oldest valve with a wide range of applications. The closing key of the gate valve is the gate valve, which moves vertically in the direction of the medium flow in the pipeline and interrupts the medium flow in the pipeline by closing with the valve seat. The gate valve has a simple structure, and the flow direction of the medium does not change when passing through it. It has low fluid resistance, easy opening and closing, a wide range of applications, stable and reliable sealing, and durability.

But it has a large stroke, long opening and closing time, and needs to be fully opened or fully closed when opening and closing. Its external dimensions are high and require a certain amount of space. Gate valves are divided into wedge gate valves and flat gate valves, and their differences mainly lie in whether the two sealing surfaces of the gate and valve body are in a parallel state or a wedge-shaped state with a certain angle.

The clamping force of the gate plate of the flat gate valve against the valve seat is provided by the medium pressure and the supporting mechanism between the double gate plates. The sealing surface is always in a fitting state during the opening and closing process, which can avoid the influence of particles and suspended solids in the medium on the sealing surface and achieve self-cleaning. The medium pressure will affect the downstream sealing surface stress level, and high pressure and high flow can also seal well.

The sealing principle of wedge-shaped gate valves relies on the combination of the wedge-shaped gate plate and the sealing surface on the valve body to achieve sealing. The pressure on the sealing surface comes from the downward pressure of the medium and the gate plate. Because the gate plate of wedge-shaped gate valves has a slope, the sealing surface can separate faster during the opening process, with less wear, but it is more likely to adhere to sediment. Impurities such as hard particles can damage the sealing surface, and the cleanliness of the medium directly affects the service life of the valve.

Gate valves have almost no regulating function and, like ball valves, can only be used as fully open or fully closed. When the gate is partially open, significant vibration and impact will occur, damaging the valve seat seal and reducing its service life.

3、 Globe valve

The opening and closing keys of the globe valve are plug shaped valve discs. The sealing surface is flat or conical, and the up and down movement of the valve disc changes the distance between the valve disc and the valve seat, just like our water bottle stopper, which seals the medium flow path. The valve disc has a short stroke, quick and wear-resistant switch, long service life, single sealing surface, and easy maintenance. The medium in the valve will cause changes in flow velocity and direction, resulting in high fluid resistance. The opening and closing process will counteract the pressure of the medium, making it difficult to open and close with large diameters and high pressures.

Globe valves have directionality, and the flow of medium from bottom to top can facilitate opening, reducing leakage at the packing point. They are generally used in medium and low pressure pipelines. The flow of the medium from top to bottom will facilitate closure, improve valve disc sealing, and is suitable for high-pressure pipelines.

The commonly used types of globe valves are straight through and direct flow, and their main difference is the flow direction of the medium in the valve. Direct current type will reduce the pressure loss of the medium.

Straight through type is the most common model, with an internal structure similar to a Z-shape. They can partially regulate the flow rate during use. Compared to ball valves and gate valves, they have less wear on the sealing surface during adjustment and are suitable for small pipe diameters.

The direct current type has a relatively small impact on the flow direction of the medium and is generally used when there are requirements for the pressure of the medium.

4、 Butterfly valve

Butterfly valves achieve the function of opening and closing fluid channels by rotating with the valve stem through a central circular butterfly plate. Butterfly valves have a simple structure, small size, light weight, large diameter, and flexible and fast operation. Ordinary butterfly valves can achieve good sealing under medium and low pressure conditions. Compared with the other three types of valves, the stability and durability of the seal are slightly lower, but the adjustability is better.

There are many types of butterfly valves, but I generally only classify them into center butterfly valves and eccentric butterfly valves. One is an adaptive seal based on the elasticity of the sealing surface material, and the other is a strengthened seal that can assist in applying external force, similar to the relationship between parallel gate valves and wedge gate valves.

There are single eccentricity, double eccentricity, and triple eccentricity in the eccentric butterfly valve, which are the axial eccentricity of the sealing surface center and the valve disc rotation center, the radial eccentricity of the valve flow to the center and the valve disc rotation center, and the sealing surface angle eccentricity. These are all design improvements aimed at improving sealing performance and reducing wear.

Summary:

1. Ball valves have a wide range of applicability, with floating type for low pressure and fixed type for high pressure. They should not be used to regulate flow unless they are specialized regulating or V-shaped ball valves.

2. Gate valves are durable and widely applicable, but they have a long operating time. When operating, do not use force levers indiscriminately, as damage to the sealing surface is irreversible. Also, like ball valves, do not use them to regulate flow.

3. Globe valves have high fluid resistance, so it is recommended to use them with small diameters without any requirement for medium pressure loss.

4. Butterfly valves are actually quite comprehensive valves. With the development of technology, the sealing, durability, and applicability of butterfly valves have been greatly improved. They are flexible and adjustable in operation and are commonly used in low-pressure large-diameter pipelines.