Principle and main features of dry gas seals for pumps

January 15, 2023

1. Working principle

Pump dry gas seal is a new type of high-performance, long-life seal type. It is structurally different from ordinary mechanical seals in that the moving and static ring seals have a wide end face; they are machined on the moving ring or static ring end face. Specially shaped fluid dynamic pressure grooves, such as spiral grooves, generally have a groove depth of between 3 and 10 pm. When the moving ring rotates at a high speed, the spiral groove on the moving ring or the end face of the stationary ring pumps the high pressure gas at the outer diameter downward between the sealing end faces, the gas flows from the outer radial center, and the sealing dam throttles the gas to the center, so the gas The compression causes an increase in pressure to form a high pressure zone at the root of the tank. The end face film pressure is formed to form an opening force. When the seal is stably operated, the opening force is balanced with the closing force formed by the gas pressure and the spring force acting on the back of the compensating ring, and the seal maintains non-contact, wear-free operation, and the film thickness thereof. Generally maintained at 2-3pm. If some disturbance factor occurs, the sealing gap is reduced, causing the opening force to decrease, while the closing force is constant, the sealing gap will be reduced, and the seal will soon return to equilibrium again. This ability of the dry gas seal to resist changes in the air gap is called film stiffness. Although the gas film gap of the pump is small, the film stiffness is very large, and the film stiffness is much larger than that of the liquid film lubrication mechanical seal.

2. Main advantages

Compared with traditional contact mechanical seals, the use of dry gas seals in centrifugal pumps has several advantages:

(1) Low frictional power consumption

Since the two sealed end faces of the dry gas seal are separated by a thin layer of stable gas film and the gas chamber is a low viscosity in the sealed cavity, the frictional power consumption of the dry gas seal end face and the friction friction loss of the moving ring assembly are better than the liquid lubrication. The frictional power consumption of the sealing device is much smaller, and the ratio of power consumed by the two is generally about 1:10-20.

(2) No wear and long service life

For dry gas sealing, since the two oppositely rotating end faces are non-contact, under normal conditions of use, generally more than 3 years.

(3) No sealing system, can achieve zero leakage or zero overflow of pumped medium

Sealing systems are often complex and expensive, and there is an inevitable risk of failure. The external leakage of the pumped medium and the sealing and sealing of the liquid are all dependent on the perfection of the sealing system. Dry gas sealing avoids all these complications. It uses a dry and clean nitrogen source as the sealing gas. It is easy to achieve zero leakage or zero overflow of the pumping medium, no pollution to the pumping medium, and the system is relatively simple and reliable. high.

3. Main technical difficulties

Compared with dry gas compressors for high-speed turbine compressors, there are three difficulties in using dry gas seals for centrifugal pumps:

(1) The structural size is limited. The radial space of the centrifugal pump packing box is relatively small, especially for the modification of some old pump shaft seals, the problem is more prominent. In this way, the sealing surface of the general dry gas seal of the pump is relatively narrow. Since the sealing surface must include the grooved area for pumping downward and the dam area for sealing, the dry end sealing structure parameters of the pump are used. The design (including the geometry of the flow cell) is not the same as the dry gas seal of the turbo compressor, which is more difficult.

(2) The rotation speed is low and the dynamic pressure effect is small. Due to the low speed of the centrifugal pump spindle, in order to maximize the film stiffness under the working film thickness to enhance the reliability of the sealing work, special consideration must be made in the selection of the end face groove type and the optimization of its structural parameters.

(3) The performance price ratio is high. Since the value of the centrifugal pump machine itself is much smaller than that of the turbo compressor, the price of the dry gas seal for the pump is much lower than that of the dry seal of the turbo compressor, which requires performance price. The ratio must be very high. Therefore, when designing the dry gas seal and its control system for the pump, under the premise of ensuring the sealing and system performance, it must also fully consider its economy, so as to facilitate the popularization and application of the pump dry gas seal, which increases the seal and system. Difficulties in design and production.

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