This article will discuss various types of valves and their applications.
The JIS standard defines valves as follows:
A movable device that allows, prevents, or controls the flow of fluid by opening or closing a passage. Based on the structure and characteristics of the valve, they can be divided into the following categories:
1. Valves that influence flow by rotating the valve element within the passage: For example, ball valves, butterfly valves
2. Valves that influence flow by the valve element acting as a "seal or plug" within the passage: For example, globe valves
Valves that influence flow by "inserting" the valve element into the passage: For example, gate valves
In the cylindrical passage of the butterfly valve body, a disc-shaped butterfly plate rotates around an axis, primarily controlling flow by rotating the disc 90°.
Gate Valve
The structure of a gate valve is similar to a floodgate. The main feature of this valve is that it has very low pressure loss when fully open. However, the valve must be completely lifted from the passage to be fully open, which means the handwheel needs to be turned many times. The opening size of the butterfly valve is very small, and it is characterized by very low pressure loss. This type of valve is usually used for water and air.
Let's discuss in detail the ball valves and globe valves, which are more commonly used in steam systems.
Ball Valve Ball valves have excellent shutoff capability and can be closed by rotating the handle 90°, making them very convenient to use. The structure of the ball valve can be full bore, meaning the opening of the valve is the same size as the internal diameter of the pipe, resulting in very low pressure loss. Another main feature is that it reduces the possibility of packing leakage because the valve stem only needs to rotate 90°.
However, it must be noted that this type of valve can only be used in fully open or fully closed positions. It cannot be used as a partially open valve to control flow for any purpose.
Ball valves typically use annular soft seats. If the valve is used in a partially open state, the pressure acts on the local seat, which may cause deformation of the seat. Once the seat is deformed, the sealing performance will deteriorate, leading to leakage.
Globe Valve Globe valves are suitable for various applications, from flow control to fluid shutoff.
When the valve plug and seat are in close contact, the valve is closed. When the plug leaves the seat, the valve opens. Therefore, flow control is determined not by the opening of the seat but by the lift of the plug (the distance between the plug and the seat). The characteristic of this type of valve is that even in a partially open state, the damage to the seat and plug from the fluid is very minimal. In some applications, when precise flow control is required, needle globe valves are often used.
However, it must be noted that because the flow passage of this type of valve is S-shaped, its pressure loss is higher than others. In addition, the valve stem must be turned many times to open or close the valve, which may cause packing leakage. Moreover, because the valve stem must be turned many times until the plug fits tightly against the seat to close the valve, it is difficult to determine when the valve is fully closed. Many cases have shown that inadvertently over-tightening the valve stem can damage the sealing surface.
Additional Information Diaphragm valves control flow by "cutting off" the passage from the outside and are mainly used in liquid systems, but sometimes similar-named valves are found in steam systems. This is an automatic valve with a diaphragm actuator, commonly referred to as a "diaphragm valve." Therefore, when mentioning this name, it is important to distinguish it accurately.