Vacuum valves refer to valves with an inner cavity in a vacuum state, usually called negative pressure, where the inner cavity pressure is lower than the atmospheric pressure. They are used in vacuum pipeline systems and include vacuum ball valves, vacuum butterfly valves, vacuum diaphragm valves, etc. The medium is of high purity.

Ball valves, gate valves, globe valves, etc. can all be produced into vacuum valves and ordinary conventional valves. There is no difference in appearance, but there are differences in the design of the valve connection parts. It's just a matter of details.

Vacuum valves have the following characteristics compared with positive pressure valves:

The pressure is lower than the atmospheric pressure, and the pressure drop on the valve disc cannot exceed 1 kilogram force per square centimeter. The working temperature of the medium depends on the technological process of the device in use. The temperature generally does not exceed the range of -70 to +150℃.

The most fundamental requirement for this type of valve is to ensure the high degree of sealing of the connection and the compactness of the structure and gasket material.

Vacuum valves can be classified into four groups according to the

medium pressure:

Low vacuum valve: Medium pressure p=760 to 1 MMHG;

2) Medium vacuum valve: p=1×10-3 MMHG.

3) High vacuum valves: p=1×10-4 to 1×10-7 MMHG;

4) Ultra-high vacuum valve: p≤1×10-8 MMHG.

The vacuum bellows stop valve with a linear movement of the valve stem is widely used in vacuum valves. Gate valves are subject to greater restrictions, but for large diameters, ball valves, plunger valves and butterfly valves are also available.

There are numerous types of vacuum valves, which play a significant role in vacuum systems. They are typically classified based on their functions, structural types, driving methods, materials, and applications. There are five common connection forms for vacuum valves as follows.

1. Flange connection

This is the most commonly used connection form in valves. According to the shape of the joint surface, it can be further classified into the following types:

1) Smooth type: Used for valves with relatively low pressure. It is relatively convenient to process.

2) concave-convex type: It has a relatively high working pressure and can use medium-hard gaskets

3) Tenon and groove type: It can use gaskets with relatively large plastic deformation and is widely used in corrosive media, providing better sealing performance.

4) Trapezoidal groove type: It uses an elliptical metal ring as a gasket and is suitable for valves with a working pressure of ≥64 kilograms per square centimeter or high-temperature valves.

5) Lens type: The gasket is in the shape of a lens and made of metal. High-pressure valves or high-temperature valves with a working pressure of ≥100 kilograms per square centimeter.

6) O-ring type: This is a relatively new form of flange connection. It has developed with the emergence of various rubber O-rings and is more reliable in sealing effect than ordinary flat gaskets.

2. Threaded connection

This is a simple connection method, often used for small valves. It is further divided into two situations:

1) Direct sealing: The internal and external threads directly serve the sealing function. To ensure that the connection does not leak, lead oil, hemp and polytetrafluoroethylene raw tape are often used for filling. Among them, polytetrafluoroethylene raw tape is increasingly widely used. This material has excellent corrosion resistance and outstanding sealing effect. It is convenient to use and store. When disassembled, it can be removed completely because it is a non-sticky film, which is much superior to lead oil and hemp.

2) Indirect sealing: The force of tightening the thread is transmitted to the gasket between the two planes, allowing the gasket to perform a sealing function.

3. Ferrule connection

Ferrule connection has only developed in China in recent years. Its connection and sealing principle is that when the nut is tightened, the ferrule is subjected to pressure, causing its cutting edge to bite into the outer wall of the pipe. The outer conical surface of the ferrule then fits tightly with the conical surface inside the joint body under pressure, thus reliably preventing leakage.

The advantages of this connection form are:

1）It is small in size, light in weight, simple in structure and easy to disassemble and assemble.

2) It has strong connection force, a wide range of applications, and can withstand high pressure (1000 kilograms per square centimeter), high temperature (650℃), and shock and vibration.

3) A variety of materials can be selected, which are suitable for anti-corrosion.

4) The processing accuracy requirement is not high;

5) It is convenient for installation at heights.

At present, the ferrule connection form has been adopted in some small-diameter valve products in our country.

4. Clamp connection

This is a quick connection method that only requires two bolts and is suitable for low-pressure valves that are frequently disassembled.

5. Internal self-tightening connection

All the above-mentioned connection forms utilize external forces to counteract the pressure of the medium and achieve sealing. The following introduces the connection form that uses the pressure of the medium for self-tightening. Its sealing ring is installed at the inner cone, forming a certain Angle with the side facing the medium. The medium pressure is transmitted to the inner cone and then to the sealing ring. On the conical surface at a certain Angle, two component forces are generated, one parallel to the centerline of the valve body and outward, and the other presses against the inner wall of the valve body. The component force behind is the self-tightening force. The greater the medium pressure is, the greater the self-tightening force will be. So this connection form is suitable for high-pressure valves. It saves a lot of materials and labor compared with flange connection, but it also requires a certain preload to ensure reliable use when the pressure inside the valve is not high.

Valves made based on the principle of self-sealing are generally high-pressure valves.

There are many forms of valve connection. For example, some small valves that do not need to be removed are welded together with pipes. Some non-metallic valves are connected by socket and spigot, etc. Valve users should handle the situation specifically according to the specific circumstances.