Technical Principle and Application of Dry Gas Seal in Industrial Pumps

Heilongjiang Petroleum chemical plant workshop MEK heat medium pump P403, model CHZE100-250B, transmission medium for conducting oil. Since 1998 device to drive, we strive leak has been resolved, and domestic well-known bellows seal factory sealed cloning, North carbon seal, a seal being companies such as joint research, set up the best seal installed a continuous operation Month began to leak, the general 7,8 days began to smoke, a year down only a seal on the cost of a 160,000 yuan, plus relief pressure relief, the direct economic losses caused by leakage of more than 10 million, a serious threat MEK workshop safety and sanitation.

After the structural analysis of the seal, it was found that the main problem lies in that the process medium (heat conducting oil) deposits carbon between the two friction end faces, partially leaks and accumulates outside the stationary seal ring. As time goes by, Coking gradually increased, and finally static ring face contour, thus virtually changing the mechanical seal PV value caused by leakage.

Chengdu Yitong Technology Co., Ltd. dry gas seal technology in the successful application of industrial pumps to solve this problem. The seal is the use of hydrodynamic principles, through the open end of the seal face dynamic pressure seal groove to achieve the non-contact operation of the seal face, therefore, the seal friction material is not limited by the PV value, compared with the mechanical seal, dry gas seal advantages: (1) long service life, stable and reliable operation. (2) The sealing power consumption is small, only about 5% of the contact seal. (3) Compared with other non-contact seals, dry gas seal gas leakage is small. (4) can achieve zero overflow of the medium, is an environmentally friendly seal. (5) seal auxiliary system is simple, reliable, use does not require maintenance.

Dry gas seal structure principle shown in Figure 1, by the rotating ring, static ring, spring, ring, and spring seat and sleeve composition. Figure 2 is a dry gas seal rotating ring diagram.

Figure 1 dry gas seal structure diagram

Figure 2 dry gas seal dynamic pressure groove schematic

Rotating ring sealing surface after grinding, polishing, and processed in its special role on the hydrodynamic groove. Dry gas seal Rotating ring rotation, the seal gas is sucked into the dynamic pressure groove from the outer diameter towards the center of the radial direction toward the seal weir flow. Due to the throttling effect of the sealing weir, the gas entering the sealing surface is compressed and the pressure of the gas is increased, so that the sealing surface is pushed away by this pressure. The flowing gas forms a thin gas film between the two sealing surfaces It replaces the normal sealed liquid seal between the two films). According to the theory of gas dynamics, when the gap between two end faces of dry gas seal is 2 ~ 3mm, the gas flow layer through the gap is the most stable. Therefore, the thickness of gas film is generally selected to be about 3mm. When gas static pressure and spring force are formed When the closing force and the film reaction force are equal, the thickness of the film is very stable. Under normal conditions, the closing force (spring force and medium force) acting on the sealing surface is equal to the opening force (film reaction force), and the sealing work is in a stable working clearance. When subjected to external interference and the thickness of the gas film decreases, the gas film Reaction force increases, the opening force is greater than the closing force, forcing the sealing work to increase the gap back to normal. On the contrary, if the thickness of the seal film increases, the film reaction force decreases, the closing force is greater than the opening force, the sealing surface and interference return to normal, so long as within the design range, when the external interference disappears, the film thickness You can revert to the design value, so anti-pump ability is very strong. Therefore, the gas seal formed between the dry gas seal surface has a certain stiffness of the gas film, the greater the rigidity of the gas film, dry gas seal anti-pump (interference) the stronger the more stable and reliable seal operation.

Dry gas seal performance is mainly reflected in the stability of the seal operation (that is, life) and seal the contradiction between the amount of leakage. The direct factor that affects the amount of dry gas seal leakage is the dry gas seal film thickness that is the working clearance formed between the seal faces during dry gas seal operation.

1, seal structure parameters

(1) The shape of dynamic seal with air seal is the strongest. The hydrodynamic pressure effect produced by logarithmic spiral pattern is the strongest. The rigidity of the film is the highest with dry gas seal dynamic pressure groove, and the stability of dry seal is the best.

(2) When the depth of dynamic pressure groove dry gas seal and the dynamic pressure groove depth of dry gas seal are the same order of magnitude as the gas film thickness, the rigidity of the sealed gas film is the largest.

(3) the number of dynamic pressure sealed dry gas tank, dynamic pressure tank width, dynamic pressure tank length.

2, the operating parameters of the impact of seal leakage

(1) The larger the seal diameter, the higher the speed, the greater the seal ring line speed, the greater the dry gas seal leakage.

(2) In the case of certain sealed working gap, the higher the sealing gas pressure, the greater the gas leakage.

(3) The influence of the medium temperature on the seal leakage is due to the influence of temperature on the medium viscosity. The increase of the medium viscosity, the dynamic pressure effect and the increase of the film thickness increase the resistance to flow through the gap between the seal faces Temperature, medium viscosity on the seal leakage is not significant.

Since the P403 retrofit started with a dry gas seal in October 2002, the pump was operating in a very stable condition with no leaks. The pump movement before and after the movement in Table 1.

Table 1 before and after the transformation of the pump operating conditions

As can be seen from Table 1, saving electricity for only one item saves 300,000 yuan for the whole year, saving about 120,000 yuan. Although a higher clean nitrogen supply system is required to increase the cost (about 20,000 yuan), in contrast, Is negligible, part of the sealed nitrogen into the HTF system after venting, part of the direct discharge into the atmosphere, will not cause any pollution to the air, is a kind of environmentally friendly seal.

Because this kind of seal replaces the traditional "liquid blockage" with "gas blockage", it requires relatively high gas pressure, the dry gas pressure is 0.2-0.3MPa higher than the pressure in the sealed chamber and the gas contains impurities less than 3Î¼m , Sealed nitrogen is strictly prohibited interruption or less than the pressure in the sealed chamber and pump reverse operation, otherwise it will damage the sealing surface caused by leakage.

3, conclusion

The successful application of dry gas sealing technology on this pump provides a new way of high-quality and environment-friendly leakage prevention for petrochemical systems, especially those with high requirements of toxic and harmful medium leakage, which not only saves money, but also reduces the frequency of maintenance and workers The labor intensity is worth promoting vigorously new technology.