Pressurized double-end face seal and unpressurized double-end face seal
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A pressurized face-to-face mechanical seal has two pairs of friction pairs; it is complex in structure and requires an external sealing fluid supply system. An isolation fluid with a pressure higher than that of the medium (0.5–1.5) x 105 Pa is introduced into the sealing chamber to serve functions such as sealing and lubrication. The isolation fluid acts as a lubricant for the inner seal. The pressure-free double-face mechanical seal (i.e., face-to-back tandem mechanical seal) features two pairs of friction pairs; it has a complex structure and requires an external sealing fluid supply system. The buffer fluid within the sealing chamber is not under pressure, and the process medium acts as a lubricant for the inner seal. The use of pressure-free double-face mechanical seals is on the rise year by year, and these seals can be widely applied to volatile and hazardous media such as vinyl chloride, carbon monoxide, and light hydrocarbons. The inner seal of the pressure-free double-face mechanical seal, known as the \"first seal,\" serves as the primary seal and functions like a single-face internal seal. Its lubricant is provided by the medium being sealed, and the seal chamber is filled with liquid from a fluid reservoir; no pressure is applied. Should the inner seal fail and cause an increase in pressure within the seal chamber, this can be detected, recorded, or triggered by an alarm via the pressure gauge on the fluid reservoir. Meanwhile, the outer seal acts to provide sealing and to contain any leaked liquid until repairs can be carried out.
The difference between pressurized and non-pressurized dual sealing lies in the properties of the medium being pumped. Pressurized dual sealing is used with media that pose a high level of risk; the pressure in the second sealing chamber must be higher than that in the first sealing chamber (it’s something like 0.5 to 0.7 Mpa, I’m not sure exactly). Moreover, the sealing fluid used in the dual sealing system must be compatible with the medium actually pumped by the pump, so that in the event of a leak in the first seal, no leakage will occur into the atmosphere.
In simple terms, a double-end face seal functions as having two sealing chambers: the first one contains the sealing medium, while the second contains a sealing isolation fluid or buffer. In a pressurized double-end face seal, the pressure of the isolation fluid/buffer is higher than that of the medium, so that the medium does not leak out in case of a seal failure. In a pressure-free double-end face seal, the pressure of the isolation fluid/buffer is lower than that of the medium; when the mechanical seal leaks, the medium leaks out.