Differences in the operation of mechanical seals for liquids and gases
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This post was last edited by yxmqh on 2009-10-10 at 14:25. In dry gas seals, the end faces of the rotating and stationary rings are equipped with working grooves; these grooves are used to pump gas toward the seal barrier in order to create a pressurized sealing gas. Liquid mechanical seals, however, do not have such structures on their rotating and stationary ring end faces. So how does liquid get into the end faces of these seals? Additionally: at the beginning of operation, there is no medium on the end face, resulting in dry friction or boundary friction; can it be understood that operation under these conditions for a short period of time at the start is permissible?
Mechanical seals and packing seals are now commonly used for liquid sealing; packing seals have gradually been phased out, while for sealing gases, comb seal and dry gas seal are the most frequently used methods. 1. A mechanical seal pair should actually be referred to as a friction pair; wear is inevitable, but it is minimal and can be ignored. Mechanical seals must never be started without a medium present, as this will cause the sealing surfaces to heat up and wear out instantly. 2. Dry gas seals are commonly used for shaft end sealing in large-scale units; they can achieve zero wear during normal operation, and theoretically their lifespan can be indefinite. The principle of dry gas seals for pumps is the same as that for machinery; spiral grooves are provided on both the moving and stationary surfaces, with their direction being exactly opposite to the direction of rotation. Gas enters these spiral grooves, overcomes the pressure of the medium as well as the spring force, thereby pushing the sealing surfaces apart and creating a rigid air film between the moving and stationary rings. At this point, the opening force is exactly equal to the medium pressure and the spring force. Sealing ensures smooth operation. For a detailed explanation of dry gas seals, there are posters available at the station; please search for them!
1. Liquid mechanical seals belong to the category of contact mechanical seals, with the sealing surfaces in full contact. 2. Gas mechanical seals are non-contact mechanical seals; they achieve non-contact by utilizing micrometer-scale grooves on the rotating or stationary ring to create a pumping effect that pushes the two rings apart. This pressure is known as the end face specific pressure, and equilibrium is achieved when this pressure balances the closing force of the springs. This equilibrium is dynamic, and it is maintained through changes in the gap between the two sealing surfaces as well as the action of the springs； During the initial startup phase, dry friction occurs; therefore, dry gas seals have a critical rotation speed. Meanwhile, the sealing rings typically combine soft and hard materials, giving them a certain degree of self-lubrication that allows them to meet the requirements in a very short time