The principle and main characteristics of dry gas seal for pump

1. Working principle The dry gas seal for pumps is a new type of seal with high performance and long life. The structure of the seal factory is obviously different from the usual mechanical seals: the end faces of the dynamic and static ring seals are wider; The end face of the ring or static ring is machined with a hydrodynamic groove of a special shape, such as a spiral groove, and the groove depth is usually between 3-10pm. When the mechanical seal of the moving ring high-speed pump rotates, the spiral groove on the end face of the moving ring or the static ring will pump the high-pressure gas mechanical seal at the outer diameter down into the sealing end face, the gas flows from the outer radial base, and the seal The dam restricts the flow of gas to the base, so the gas is compressed and the pressure rises, forming a high pressure area at the root of the groove. The air film pressure on the end face constitutes the opening force. When the seal works stably, the opening force is balanced with the closing force composed of the air pressure and the spring force acting on the back of the compensation ring. The seal maintains non-touch and wear-free operation. The thickness is usually maintained at 2-3pm. If there are some disturbance factors that reduce the sealing gap, resulting in a decrease in the opening force, while the closing force is unchanged, the sealing gap will be reduced, and the seal will quickly return to equilibrium again. The ability of the dry gas seal to resist changes in the air gap is called the air film stiffness. Although the air gap of the dry air seal of the pump is small, the air film stiffness is very large, which is much greater than that of the liquid film smooth mechanical seal.

2. The main advantages Compared with the traditional touch-type mechanical seal, the use of dry gas seals in centrifugal pumps has the following advantages: (1) Low friction and power consumption because the two sealing end faces of the dry gas seal are stabilized by a thin layer. The gas film is blocked by the gas film and the sealing cavity is a low-viscosity gas medium, so the friction power consumption of the end face of the dry gas seal and the loss of the stirring friction of the moving ring assembly are much smaller than the friction power consumption of the liquid smooth sealing device, and usually both cost The power ratio is about 1:10-20. (2) Non-wearing work, long service life and dry gas seal, because the two oppositely rotating end faces touch wrongly, under normal operating conditions, they can usually reach over 3 years. (3) The system without sealing liquid, which can achieve zero leakage or zero overflow of the pumping medium, is often complicated and expensive, and there is an unavoidable risk of failure. The external leakage of the pumping medium and the cooling and sealing of the sealing liquid all depend on the improvement of the sealing liquid system. The dry gas seal avoids all these complicated elements. It uses a dry and clean nitrogen source as the sealing gas, which can easily achieve zero leakage or zero overflow of the pumped medium, without any pollution to the pumped medium, and the system is relatively simple and very reliable. high.

3. Main technical difficulties Compared with dry gas seals for high-speed turbo compressors, dry gas seals for centrifugal pumps have three difficulties: (1) The structural scale is limited. The radial space of the centrifugal pump stuffing box is relatively small, especially for the transformation of some old pump shaft seals, the problem is even more prominent. In this way, the sealing surface of the dry gas seal of the pump is usually relatively narrow, because the sealing surface must include the groove area for the downward pumping effect and the dam area for the sealing effect, so the end surface structure of the dry gas seal of the pump is used. The planning of parameters (including the geometric parameters of the runner) is not the same as the dry gas seal of the turbocompressor, which is more difficult. (2) The speed is low and the dynamic pressure effect is small. Because the rotational speed of the main shaft of the centrifugal pump is not high, in order to improve the air film rigidity of the seal under the working film thickness as much as possible to enhance the reliability of the sealing operation, 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. Because the value of the centrifugal pump machine itself is much smaller than that of the turbo compressor, the quotation of the dry gas seal for the pump is much lower than that of the dry gas seal for the turbo compressor, which requires its performance quotation much higher than necessary. Therefore, when planning the dry gas seal for pumps and its control system, on the premise of ensuring the performance of the seal and the system, it is necessary to fully consider its economy, so as to facilitate the popularization and application of dry gas seals for pumps, which increases the sealing and The difficulty of system planning and production.