7 Types of Dynamic Seals Commonly Used in Mechanical Design and Manufacturing

GUIDE

This article mainly explains the 7 types of dynamic seal forms commonly used in mechanical design. They are packing seal, mechanical seal, dry gas seal, labyrinth seal, oil seal, power seal and spiral seal.

The problem of sealing of moving equipment is always present along with the operation of the equipment. Today, we have deliberately sorted out various types of seal forms commonly used on mobile equipment, as well as the scope of use and characteristics, so that everyone can have a deeper understanding of the sealing problem.

1. Packing Seal

• 1-bottom bush
• 2- stuffing box
• 3-sealing ring
• 4-filler
• 5-gland
• 6-gland bolt

According to its structural characteristics, packing seal can be divided into:

• Soft packing seal
• Hard packing seal
• Shaped packing seal

1). Soft packing seal

Type: packing

Packing is usually woven from softer threads and filled in a sealed cavity through a strip with a square cross-section. The pressure is generated by the gland, and the packing is compressed, forcing the packing to be pressed against the sealing surface (Outer surface of shaft and seal cavity). Thus, a radial force with a sealing effect is generated, and a sealing effect is exerted.

Application of soft packing:

The material selected for packing determines the sealing effect of the packing. Generally speaking, packing materials are limited by the temperature, pressure and pH of the working medium. In addition, the surface roughness, eccentricity, and linear velocity of the mechanical equipment that the packing works on will also have requirements on the material selection of the packing.

Graphite packing can withstand high temperature and high pressure. It is one of the most effective products to solve the problem of high temperature and high pressure sealing. It has corrosion resistance, excellent sealing performance, as well as stable and reliable function.

PH range of applicable medium	0-14 usually used for dynamic sealing parts
Graphite packing temperature	-100 ℃ -850 ℃
Graphite packing pressure	0-30.0MPa

Aramid packing is a kind of high-strength organic fiber. Then the braided packing is impregnated with PTFE latex and lubricant.

PH range of applicable medium	2-13 Suitable for dynamic sealing parts containing solid particles
Graphite packing temperature	260 ℃
Graphite packing pressure	10.0MPa

PTFE packing is made of pure PTFE dispersion resin as raw material. The raw material film is first made, and then twisted, braided and woven into packing. It can be widely used in food, pharmaceutical, paper-making chemical fiber and other valves and pumps with high cleanliness requirements and strong corrosive media.

2). Hard packing seal

There are two types of hard packing seals: opening loop and split loop.

Opening loop ring, metal self-tensioning seal ring, used in piston machines as piston rings, and used in rotary machines as expansion rings

Split ring, cylindrical surface contact dynamic seal. It can be used as rotary dynamic seal for steam turbine and aero engine. It can also be used as a reciprocating seal for steam engines, internal combustion engines and piston compressors (seal between piston rod and cylinder).

Main stuffing box structure

• Arranged between the compressor cylinder and the middle body
• Sealing pressure from vacuum to 3000bar
• Lubrication, cooling, protective gas and leak recovery can be set as required

Classification of packing rings

• Sealing ring-seal
• Choke ring-used in high temperature and high pressure occasions to prevent deformation and loss of non-metallic sealing rings
• Decompression ring-achieve the purpose of decompression by blocking flow

2. Mechanical Seal

The mechanical seal always consists of two parts: the rotating part (yellow part) and the stationary part (orange part). The two parts move relative to each other, and the static ring surface becomes the main sealing surface of the seal.

Mechanical seals are also known as face seals. According to the relevant national standards, it is defined as: It is kind of device that consists of at least one pair of end faces perpendicular to the rotation axis, which keeps posted and slides relative under the cooperation of the fluid pressure and the elastic force (or magnetic force) of the compensation mechanism as well as the auxiliary seal. Prevents fluid leakage.

The basic components that make up a mechanical seal are:

1. Static ring
2. Dynamic ring
3. Gland
4. Push ring
5. Spring
6. Positioning ring
7. Shaft sleeve
8. Dynamic ring seal
9. Static ring seal ring and other components

3. Dry Gas Seal

Dry gas seal is a new type of shaft end seal using slotted seal technology for gas seal, which belongs to non-contact seal.

Features:

It has the advantages of good sealing performance, long service life, no sealing oil system required, low power consumption, simple operation and low maintenance costs. Dry gas seal, as a maintenance-free sealing system that does not require any sealing end surface cooling and lubricating oil, is replacing the floating ring seal and labyrinth seal to become the main body seal of the high speed centrifugal compressor shaft seal in the petrochemical industry.

Applications:

Centrifugal compressors and other high-speed fluid machinery are suitable for working conditions where a small amount of process gas leaks into the atmosphere without harm, such as air compressors and nitrogen compressors.

4. Labyrinth Seal

The labyrinth seal is a series of annular seal teeth arranged in turn around the rotating shaft, and a series of interception gaps and expansion cavities are formed between the teeth. The labyrinth seal is a series of annular seal teeth arranged in turn around the rotating shaft, and a series of interception gaps and expansion cavities are formed between the teeth.

Labyrinth seals are the most basic form of sealing between stages and shaft ends of centrifugal compressor. According to the different structural characteristics, it can be divided into four types: smooth type, zigzag type, step type and honeycomb type.

1). Smooth labyrinth seal

The smooth labyrinth seal has two structures: integral and panel. Its structure is simple and easy to manufacture, but its sealing effect is poor.

2). Zigzag labyrinth seal

The zigzag labyrinth seal is also divided into two structures: integral and panel. The zigzag labyrinth seal is also divided into two structures: integral and panel.

The structure characteristic of this labyrinth seal is that the protruding height of the sealing teeth is different, and the high and low teeth are arranged alternately.

The matching shaft surface is a special concave-convex groove. This kind of structure of high and low teeth matching the concave and convex grooves makes the smooth sealing gap into a zigzag type.

Therefore, the flow resistance is increased and the sealing efficiency is improved. However, it can only be used in a cylinder or partition with a horizontal split surface, and the sealing body must also be made into a horizontal split type.

3). Step labyrinth seal

Step labyrinth seal is structurally similar to a smooth labyrinth seal, but the sealing effect is similar to a zigzag labyrinth seal. It is often used at the impeller cover and balance plate.

4). Honeycomb labyrinth seal 

The honeycomb labyrinth seal teeth are welded into a honeycomb shape to form a complex-shaped expansion chamber.

Its sealing performance is better than the general sealing form, and it is suitable for occasions with large pressure difference, such as the balance plate seal of centrifugal compressor.

The honeycomb labyrinth seal has a complicated manufacturing process, high strength of the sealing sheet and good sealing effect.

5. Oil Seal

Oil seal is a self-tightening lip seal with simple structure, small size, low cost, convenient maintenance, and low resistance torque. It can prevent the leakage of the medium and the invasion of external dust and other harmful substances, and also has a certain ability to compensate for wear, but it is not resistant to high pressure.

Therefore, it is generally used on chemical pumps in low pressure applications.

6. Power Seal

When the chemical pump is running, the pressure head produced by the auxiliary impeller balances the high-pressure liquid at the outlet of the main impeller, thereby achieving sealing. When stopping, the auxiliary impeller does not work, so it must be equipped with a parking seal to solve the chemical pump leakage that may occur during parking.

The auxiliary impeller has a simple sealing structure, a reliable seal, and a long service life. Water can be leak-proof during the operation of the chemical pump. Therefore, it is often used on chemical pumps that transport impurity-containing media.

7. Spiral Seal

Spiral seals are also a form of power seal. A spiral groove is machined on a rotating shaft or a shaft containing sleeve, and a sealing medium is filled between the shaft and the sleeve.

The rotation of the shaft causes the spiral groove to have a pump-like transport effect, thereby preventing leakage of the sealing fluid. Its sealing capacity is related to the spiral angle, pitch, tooth width, tooth height, tooth length, and the gap between the shaft and the sleeve.

Because friction does not occur between the seals, so it has a long life, but due to the limitation of structural space, its spiral length is generally short, so its sealing ability is also limited. When the pump is used at a reduced speed, its sealing effect will be greatly reduced.