Selection of mechanical seal categories, seal types, and installation methods

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The API 682 standard issued by the American Petroleum Institute classifies mechanical seals based on their categories, types, installation methods, and flushing schemes; in engineering applications, the selection of mechanical seals should also take these factors into consideration during analysis. 　　Furthermore, the pV value of mechanical seals is an important parameter for selecting, using, and designing them; 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 seals. (Related reading: Analysis of typical mechanical seal selection for pumps) 1. Selection of mechanical seal types API 682 specifies three types of seals. One type is intended for seal chambers that do not conform to API 610 dimensions, where the chamber temperature ranges from -40 to 260 °C and the chamber pressure is ≤ 2.2 MPa(A). Since most pumps used in the petrochemical industry comply with the API 610 standard, it is not recommended to use this type of seal. 　　Type II seals are used in seal chambers that meet the dimensional requirements of API 610, where the chamber temperature ranges from -40 to 400 °C and the chamber pressure is ≤ 4.2 MPa(A). This type of seal is widely used in oil refineries; it is suitable for most hydrocarbon media. 　　The temperature and pressure ranges for these three types of seals are the same as those for Type 2 seals, but the certification tests required for these seals, as well as the requirements regarding technical drawings, are more stringent. Additionally, these three types of seals require a porous and uniform flushing system, and the throttle bushings must be equipped with floating graphite rings. At present, domestic sealing manufacturers do not possess the capability to produce seals of Category 3 in a strict sense; therefore, it is recommended to use imported seals only in situations where such seals are required due to extremely high performance demands. 　　2. Seal type selection: Type A seal uses a multi-spring, thrust ring-based compensation mechanism; the compensation device is rotary, and the auxiliary sealing element is an elastic O-ring. Type B seal also employs a bellows-based compensation mechanism without a thrust ring; its compensation device is rotary as well, with an elastic O-ring serving as the auxiliary sealing element. Type C seal features a bellows-based compensation mechanism without a thrust ring, but its compensation device is stationary, while flexible graphite is used as the auxiliary sealing material. 　　In addition to the above 3 standard sealing types, API 682 also specifies 4 alternative seals, namely the static Type A(B) seal, the single-spring Type A seal, and the rotary Type C seal. 　　(1) Selection between stationary and rotary mechanical seals At the same shaft diameter, the radial size of a stationary seal is one size class larger than that of a rotary seal; therefore, rotary seals feature smaller radial dimensions and easier installation, and they are generally used in such cases. When the linear speed at the sealed end face is 23 m/s or higher, the centrifugal force generated by the springs and other rotating components is significant, which imposes high requirements on dynamic balance. Additionally, it consumes mixing power and generates heat from mixing; in such cases, a static seal should be used. 　　(2) Selection between single-spring and multi-spring mechanical seals The single-spring mechanical seal has a simple structure and is generally used for shafts with smaller diameters. 　　Multi-spring mechanical seals are suitable for larger shaft diameters; they have a small axial dimension and uniform axial force, making them appropriate for high-speed mechanical seals. However, the wire diameter of multi-spring types is small, and corrosion or the accumulation of crystalline particles can easily lead to spring failure or sticking. In such cases, a single-spring type seal should be used, or a proper arrangement should be adopted to prevent the springs from coming into contact with the medium. When using a single-spring mechanical seal, measures should be taken to ensure a more even distribution of spring force, thereby avoiding failure of the seal itself due to uneven force distribution. 　　(3) Selection between spring-type and bellows-type mechanical seals: Spring-type mechanical seals offer good follow-up ability and compensation capability, as well as the capacity to function in high-pressure environments; therefore, they are more commonly used in general applications. However, due to the temperature limitation of the auxiliary sealing ring in spring-type seals, they cannot be used in environments with high temperatures; generally, the temperature should be below 200 °C. 　　Bellows-type mechanical seals can be divided into three main categories: rubber bellows, polytetrafluoroethylene bellows, and welded metal bellows. The first two types of bellows require auxiliary springs due to their low elasticity, which limits their use; metal bellows are more commonly used in practical applications, as they exhibit good resistance to low and high temperatures, with a temperature range of -40 to 400 °C. (Related reading: Advantages of bellows mechanical seals, common materials, and applications) 3. Selection of arrangement method Arrangement 1 features a pair of friction pairs in each modular seal set, available in two configurations: with a fixed throttle sleeve (1CW-FX) and with a floating throttle sleeve (1CW-FL). In Arrangement 2, each modular seal contains two pairs of friction pairs, and the pressure of the buffer fluid is lower than that in the sealing chamber. It is available in three configurations: double-contact wet seal (2CWCW), contact wet internal seal (2CW-CS), and non-contact internal seal (2NC-CS). In Arrangement 3, each modular seal contains two pairs of friction pairs, and the pressure of the isolation fluid is higher than that in the sealing chamber; there are six configurations: face-to-face (3CW-FF/3NC-FF), face-to-back (3CW-FB/3NC-FB), and back-to-back (3CW-BB/3NC-BB). 　　Layout 1 is mainly suitable for media at normal temperature that are non-toxic and harmless; Layout 2 is suitable for light hydrocarbons and easily vaporizable media, and it is not recommended for use when the media are toxic; Layout 3 is suitable for high-temperature, toxic, and harmful media, as well as in situations where medium leakage is not permitted.