Classification Of Seals By Design

There are four seal classification groups:

• Unbalanced or balanced

• Rotating or stationary seal head

• Single-spring or multiple-spring construction

• Pusher or nonpusher secondary seal design

The selection of an unbalanced or balanced seal is determined by the pressure in the seal chamber. Balance is a way of controlling the contact pressure between the seal faces and power generated by the seal. When the percentage of balance b (the ratio of hydraulic closing area to seal face area) is 100 percent or greater, the seal is referred to as unbalanced. When the percentage of balance for a seal is less than 100 (1.0), the seal is balanced. Figure 18 illustrates common unbalanced and balanced seals.

The selection of a rotating or stationary seal is determined by the speed of the pump shaft. A seal that rotates with the shaft is a rotating seal assembly. Typical rotating seals are shown in Figures 17, 21 and 22. When the mating ring rotates with the shaft, the seal is stationary (see Figure 19). Rotating seal heads are common in the industry for normal pump shaft speeds. As a rule of thumb, when the shaft speed exceeds 5,000 ft/min (25.4 m/s), stationary seals are required. Higher speed applications require a rotating mating ring to keep unbalanced forces, which may result in seal vibration, to a minimum. A stationary seal should be considered for all split case pumps. This will eliminate seal problems that occur when the top and bottom halves of the pump casing do not line up. The pressure in the pump can cause a misalignment of these parts that creates an out-of-square condition at the seal faces.

The selection of a single-spring or multiple-spring seal head construction is determined by the space limits and the liquid sealed. Single-spring seals are most often used with bellows seals to load the seal faces (see Figure 20a). The advantage of this type of construction is that the openness of design makes the spring a nonclogging component of the seal assembly. The coils are made of a large diameter spring wire and therefore can withstand a great deal of corrosion.

Multiple-spring seals require a shorter axial space. Face loading is accomplished by a combination of springs placed about the circumference of the shaft (refer to Figure 1 and see Figure 20b). Most multiple-spring designs are used with assemblies having O-rings or wedges as secondary seals.

Pusher-type seals are defined as seal assemblies in which the secondary seal is moved along the shaft by the mechanical load of the seal and the hydraulic pressure in the seal chamber. The designation applies to seals that use an O-ring, wedge, or V-ring. A typical construction is illustrated in Figure 21.

The primary ring, with a hardened metal surface, rotates with the shaft and is held against the stationary ring by the compression ring through loading of the O-ring. The

FIGURE 18 Common unbalanced and balanced seals

compression ring supports a nest of springs that is connected at the opposite end by a collar, which is fixed to the shaft. The primary ring is flexibly mounted to take up any shaft deflection or equipment vibration. The collar is fixed to the shaft by setscrews.

Another pusher-type seal is illustrated in Figure 22. When elastomers cannot be used in the product, a wedge made of TFE must be considered. A metal retainer locked to the shaft by (A) provides a positive drive through the shaft and to the primary ring (F) through drive dents (D), which fit corresponding grooves. The seal between the primary ring and shaft or sleeve is made by a wedge (E), which is preloaded by multiple springs (B). The spring load is distributed uniformly by a metal disc (C). The primary ring (G) contacts the mating ring (H) to form the dynamic seal.

Pusher seals also come in split designs. Illustrated in Figure 23 is a split seal design for an ANSI pump. This is a fully split seal design with all of the basic parts fitted outside the seal chamber. The gland plate is fully split and provides easy access to other seal components. A finger spring located on the atmospheric side of the seal provides an axial load and drive to the stationary primary ring. Since it is located on the atmospheric side of the seal, it will not be clogged from material in the pumpage. This is suited for a variety of applications, including paper stock, sewage, slurries, and river water. Two flush ports in the gland plate provide for a seal flush for cooling.

FIGURE 19 Stationary seal with a rotating mating ring (John Crane Inc.)
FIGURE 20 Comparison of a) single-spring and b) multiple-spring seals
FIGURE 21 O-ring type mechanical seal (Flowserve Corp.)
Nonpusher Type Mechanical Seal
FIGURE 22 Wedge-type mechanical seal (John Crane Inc.)

FIGURE 23 A split seal design for an ANSI FIGURE 24 A half-convolution bellows seal (John pump (John Crane Inc.) Crane Inc.)

FIGURE 23 A split seal design for an ANSI FIGURE 24 A half-convolution bellows seal (John pump (John Crane Inc.) Crane Inc.)

Nonpusher seals are defined as seal assemblies in which the secondary seal is not forced along the shaft by the mechanical load or hydraulic pressure in the seal chamber. Instead, all movement is taken up by the bellows convolution. This definition applies to those seals that use half-, full-, and multiple-convolution bellows as a secondary seal.

The half-convolution bellows seals are always made of an elastomer (see Figure 24). The tail of the bellows is held to the shaft by a drive band. This squeeze fit seals the shaft and enables the unit to rotate with the shaft. Positive drive is accomplished through the drive band, retainer, and primary ring by a series of slots and dents. A static seal is created at the back of the primary ring and at the front of the bellows. This type of seal is used for light-duty service conditions. The amount of axial travel along the shaft is half that of a full convolution bellows.

The full-convolution bellows seal is illustrated in Figure 25. The tail of the bellows is held to the shaft by a drive band. The squeeze fit seals the shaft and enables the unit to rotate with the shaft. The drive for the seal assembly is similar to that of the half-convolution seal. Static sealing is accomplished at the front of the bellows and the back of the primary ring. The heavier full-convolution bellows design can tolerate greater shaft motion and runout to pressures of 1200 lb/in2 (8.3 bar).

Multiple-convolution bellows seals are necessary to add flexibility to those secondary seal materials that cannot be used in any other shape. The mechanical characteristics of TFE and metals require multiple-convolution designs.

FIGURE 25 A full-convolution bellows seal (John Crane Inc.)

A TFE bellows assembly is illustrated in Figure 26. Because of the large cross-sectional area, these types of seals are mounted outside the seal chamber. Pressure at the inside diameter of the seal helps keep the faces closed. Small springs on the atmosphere side of the seal supply the mechanical load to keep the seal faces closed initially.

Multiple convolution metal bellows seals come in various designs and are discussed in the following section.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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