Depending on the working conditions and the properties of the medium, mechanical seals include high temperature-resistant, low-temperature-resistant mechanical seals, high-pressure and corrosion-resistant mechanical seals, particle-resistant mechanical seals, and mechanical seals adapted to easily vaporized light hydrocarbon media, etc. The use of mechanical seals of different structural types and materials is selected. The main parameters for selection are: sealed cavity pressure (MPA), fluid temperature (°C), working speed (M/S), fluid characteristics and effective space for sealing installation.

The basic principles of selection are:

1. According to the pressure of the sealing cavity, the sealing structure is determined to be balanced or unbalanced, single-end or double-end, etc.

2. According to the working speed, it is determined to use rotary or static, hydrodynamic or non-contact.

3. According to the temperature and fluid properties, determine the friction pair and auxiliary sealing materials, and correctly select the mechanical seal circulation protection system such as lubrication, flushing, heat preservation, cooling, etc.

4. According to the effective space of the installation seal, it is determined to use multiple springs or single springs or wave springs, built-in or external. The mechanical seal with the static ring mounted on the inside of the gland is called the built-in mechanical seal: the mechanical seal with the static ring mounted on the outside of the gland is called the external mechanical seal.

A mechanical seal with a spring placed in the sealing fluid is called an internal mechanical seal; a mechanical seal with a spring placed outside the sealing fluid is called an external mechanical seal.

The spring and moving ring of the built-in (built-in) mechanical seal are installed in the sealing box to contact with the medium, and can be sealed by the medium pressure in the box. Its advantages are: good stress state, small leakage (because the direction of leakage is opposite to the direction of centrifugal force), good cooling and lubrication conditions, and wide working range

The spring and moving ring of the external (external) mechanical seal are installed outside the sealing box and do not contact with the medium. They are suitable for strong corrosion, high viscosity and easy crystallization medium. Because the direction of leakage is the same as the direction of centrifugal force, the amount of leakage is large, and its working pressure is low.
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