BASIC KNOWLEDGE OF PUMP SEALS (5)

The gland packings of chemical pumps is to put the packing with rich compressibility and resilience into the packing box, and the axial compression force of the gland is converted into the radial sealing force, thereby playing a sealing role. This sealing method is called gland packings. 

Due to its simple structure, easy replacement, low price, wide range of speed, pressure and medium, the gland packings are widely used in the design of chemical pumps seals.

1. The working principle of gland packings

In the machinery industry, gland packings are mainly used as dynamic seals, and are often used as rotary shaft seals for centrifugal pumps, compressors, vacuum pumps, and mixers. After the packing is loaded into the packing cavity, it is compressed axially by the gland screw.

When there is relative movement between the shaft and the packing, due to the plasticity of the packing, it generates radial force and is in close contact with the shaft. At the same time, the lubricant impregnated in the filler is squeezed out, forming an oil film between the contact surfaces. Since the contact state is not particularly uniform, the contact part appears.

The boundary lubrication state is called the "Bearing Effect". The uncontacted recesses form a small oil groove with a thick oil film, and the contact parts and non-contact parts form an irregular labyrinth, which plays a role in preventing liquid leakage. This is called “Labyrinth Effect”. This is the working principle of gland packings. A good seal is to maintain the "Bearing Effect" and “Labyrinth Effect”. That is to say, to maintain good lubrication and proper compression, if the lubrication is poor or over-pressed, the oil film will be interrupted, causing dry friction between the filler and the shaft, and finally causing the shaft to burn and seriously wear.

Therefore, we often need to adjust the packing degree of the gland packings. After the lubricant in the packing has been lost for a period of time, we should squeeze out some lubricant, and at the same time, compensate for the loosening of the packing due to the volume change. Of course, pressing the packings frequently in this way will eventually deplete the impregnating agent, so regular replacement is necessary. In addition, in order to maintain the liquid film and the frictional heat, it is also necessary to intentionally allow a small amount of leakage at the packings.

2. The problems of gland packings in the use of chemical pumps

The shaft seal of chemical pumps generally adopts the packing with advantages of wear resistance, heat resistance, good flexibility and high strength. We are using gland packing and find that it has the following disadvantages:

1) The surface of the packing is rough, the friction coefficient is large, and there is leakage.

2) The shaft seal of newly repaired equipment is in good condition when it starts to run, but it will not use long and the leakage will continue to increase. The work of adjusting the gland and replacing the packing is also frequent. The shaft sleeve is worn out after less than one cycle operation. In serious cases, the shaft sleeve may be worn out, and the packing that cannot be replaced behind the water seal ring has rotted and cannot play a sealing role. 

3) The packing is in direct contact with the shaft and rotates relatively, causing wear on the shaft and the sleeve, so the sleeve must be replaced regularly or irregularly.

4) In order to dissipate the frictional heat generated between the packing and the shaft in time, the gland packings seals must maintain a certain amount of leakage. 

5) The friction between the packing and the shaft causes the motor to reduce power and consume electrical energy.

3. Failures and reasons of gland packings

The main failures and reasons are as follows:

1) The outer surface of the packing is flawed, and the outside of the packing gland leaks. This is because the outer diameter of the packing is too small. 

2) When the packing is squeezed into the gap between the shaft and the retaining ring or the shaft and the gland, it is mostly because the designed gap is too large or eccentric.

3) The medium leaks along the packing gland, this is because the packing is not assembled properly or the retaining ring is damaged. 

4) The leakage is too large and cannot be adjusted, this is because the packing expands or breaks, the packing is cut too short or the assembly is wrong.

In addition to gland packings, there are also mechanical seals, such as spring seals, elastomer bellow seals, etc.