40. Briefly describe the structure and working principle of dry gas seals

When the rotating dynamic ring, which has hydrodynamic grooves (2.5–10 μm) on its outer surface, rotates, these grooves pump the high-pressure gas on the outer diameter side (referred to as the upstream side) into the space between the sealing surfaces. The pressure of the air film increases from the outer diameter toward the groove diameter, while it decreases from the groove diameter toward the inner diameter. The increase in surface pressure at the sealing surfaces results in a force that overcomes the closing force acting on the sealing ring, thereby creating a very thin air film (1–3 μm) between the frictioning surfaces, allowing the seal to function in a non-contact manner. The formed gas film completely blocks the leakage path for the relatively low-pressure sealing medium, achieving zero leakage or zero escape of the sealing medium. 　　The dry gas seal has a balanced modular structure similar to that of ordinary mechanical seals, but its end face design is different; it features grooves several micrometers to a dozen micrometers deep on its surface, and the end face is relatively wide. Unlike conventional lubricated mechanical seals, dry gas seals create a stable air film between the two sealing surfaces. This gas film possesses sufficient stiffness to keep the two sealed surfaces completely separated and to maintain a certain sealing gap; this gap should not be too large, usually being a few micrometers. If the sealing gap is too large, it will lead to an increase in leakage and a poor sealing effect; on the other hand, if the sealing gap is too small, it is easy for the two sealing surfaces to come into contact. Since the frictional heat generated by the dry gas seal cannot be dissipated promptly, and there is no lubrication at the point of contact between the surfaces, this will quickly cause the seal to deform and the surfaces to overheat, resulting in seal failure. The presence of this gas film not only effectively separates the end faces but also cools the two end faces that are in relative motion. Since the end faces do not come into contact, friction and wear are reduced**, which gives the seal a long service life and thus extends the lifespan of the main unit. 　　The slotted sealing surface is divided into two functional zones: an outer zone and an inner zone. When gas enters the outer zone of the slots, these slots compress the incoming gas, creating a local high-pressure area at the base of the slots. This leads to the separation of the end faces and the formation of a gas film of a certain thickness. To achieve the necessary pumping effect, the dynamic pressure slots must be located on the high-pressure side. An increase in pressure within the grooved sealing gap is crucial for the proper functioning of the dry gas seal; it ensures that a stable gas film is maintained, preventing the seal from being damaged even under high axial loads. The sealed inner area (i.e., the dam area) is planar, and its throttling effect limits the discharge rate. During sealing operation, the opening force generated by the air film on the end faces is in balance with the closing force produced by the spring and the forces exerted by the medium, thereby enabling non-contact operation. The spring force of the dry gas seal is very small. The main purpose is to ensure that the seal remains closed when it is not under pressure or not in use, thereby preventing accidents.

A typical dry gas seal structure includes components such as a static ring, a dynamic ring assembly (rotating ring), secondary sealing O-rings, a static seal, springs, and spring seats (cavities). The static ring is located within a stainless steel spring seat and is sealed by a secondary sealing O-ring. The spring brings the stationary ring into alignment with the moving ring assembly fixed to the rotor under sealed, unloaded conditions. As the rotor rotates, the gas is pumped inward toward the root of the helical groove; the groove-free area outside the root is known as the sealing dam. The sealing dam exerts a resistive force on gas flow, increasing the gas film pressure. The pressure between the mating surfaces causes the static ring surface to separate from the dynamic ring assembly, maintaining a very small gap, typically around 3 micrometers. Nitrogen is introduced between the two sealing layers as a blocking gas, thereby creating a reliable blocking seal system. The pressure of the nitrogen is controlled to remain at a level 0.2–0.3 MPa higher than the pressure of the process gas; this ensures that any leakage from the sealing gas occurs in the direction of the process gas and the atmosphere, thus preventing the process gas from leaking into the atmosphere.