In the American Petroleum Association API 682 standard, the category, type, arrangement , flushing scheme of mechanical seals are classified. In the engineering application, the selection of mechanical seals should also pay attention to the above points.

　　In addition, the pV value of mechanical seal is an important parameter for the selection, use and design of mechanical seal, and the power consumption, wear and temperature rise are all related to this value. The higher the pV value, the higher the technical level and difficulty coefficient of mechanical sealing.(Related reading: Analysis of typical pump mechanical seals)

　　1、 Selection of mechanical seal category

　　Three sealing categories are specified in API 682. One seal is used for non-API 610 size sealing cavity, and sealing cavity temperature of-40~260℃ and sealing cavity pressure of 2.2 MPa (A). Most of the refining industry uses API 610 standard pumps, so such seals are not recommended.

　　Class II seal is used for a sealing cavity that meets the requirements of API 610 size and has the temperature of-40~400℃ and the sealing cavity pressure of 4.2 MPa (A). Such seals are widely used in refineries, including most hydrocarbon media.

　　The temperature and pressure range of the three seals are the same as that of Class 2, but the requirements for seal certification test and drawings are more stringent. The other three seals require porous uniform flushing system and throttle liner in the form of floating graphite liner. At present, the domestic sealing manufacturers do not have the strict sense of the three types of sealing production capacity, so it is only recommended to use imported seals and the application requirements are very high.

　　2、 Sealing type selection

　　Type A seal is multi-spring, thrust ring compensation structure, compensation device is rotating, auxiliary seal is elastic O-ring; B seal is endless ring, bellows compensation structure is rotating, auxiliary seal is elastic O-ring; C seal adopts endless ring, bellows compensation structure, compensation device is stationary, and auxiliary seal is flexible graphite.

　　In addition to the above three standard sealing forms, API 682 also provides four alternative seals, namely, static type A (B) seal, single spring type A seal, and rotary type C seal.

　　(1) The selection of static and rotary mechanical seals

　　Under the same shaft diameter, the radial size of the static seal is one grade larger than the rotary seal, so the rotary seal has the characteristics of small radial size and convenient installation. Generally, the rotary seal is used. When the sealing end line speed is not less than 23 m/s, the spring and other rotating parts produce large centrifugal force, high dynamic balance requirements, at the same time consume stirring power, stirring heat, at this time, static seal should be used.

　　(2)Single spring with multiple spring mechanical seal selection

　　Single spring type mechanical sealing structure is simple, generally used for smaller shaft diameter situation.

　　Multi-spring mechanical seal is suitable for larger axial diameter, with its small axial size, uniform axial force, and is suitable for high-speed mechanical seal. But more spring spring wire diameter is small, due to corrosion or crystalline particle accumulation easily cause spring failure or lag, should use single spring seal or adopt reasonable arrangement to avoid spring contact with the medium, when using single spring type mechanical sealing should take certain measures to make the spring force distribution more uniform, to avoid the failure of the sealing body caused by uneven distribution of spring force.

　　(3) Selection of spring type and bellows type mechanical seals

　　Spring mechanical seal follow ability and compensation ability, and can adapt to high pressure occasions, generally more applications. However, because the spring seal is limited by the temperature resistance of the auxiliary sealing ring, it can not be used for high temperature occasions, and generally should be lower than 200℃.

　　The bellows mechanical seal can be divided into three categories: rubber bellows, PVC bellows and welded metal bellows. Because of the small elasticity, the auxiliary spring is limited; there are many metal bellows, which have good low resistance and high temperature and temperature range from-40 to 400℃.(Related reading: Advantages of bellows mechanical seals, common materials and applications)

　　3、Selection of the arrangement mode

　　Layout 1 has a set of friction pairs, divided into two configurations with fixed throttle liner (1 CW-FX) and floating throttle liner (1 CW-FL). Arrange 2 each set of two friction pairs, and the buffer fluid pressure is lower than the sealing chamber pressure, divided into double contact wet seal (2 CWCW), contact wet internal seal (2 CW-CS) and non-contact inner seal (2NC-CS) three configurations. Each arrangement 3, and the isolation liquid pressure is higher than the sealing chamber pressure, is divided into six configurations: face to face (3 CW-FF / 3NC-FF), facing back (3 CW-FB / 3NC-FB) and back-to-back (3 CW-BB / 3NC-BB).

　　Layout 1 is mainly suitable for normal temperature, non-toxic and non-harmful medium; layout 2 is mainly suitable for light hydrocarbon and easy vaporization medium, which is not recommended when the medium is toxic; Arrangement 3 is mainly suitable for high temperature, toxic and harmful medium and medium leakage is not allowed.