## Atmosphere

B. Flow of heat

FIGURE 4 Processes involved at contacting seal faces

In a contacting seal, as the shaft begins to rotate, a small fluid film develops, along with frictional heat from the surfaces in sliding contact. These processes occurring at the seal faces are shown in Figure 4. The amount of heat developed at the seal faces must be removed to prevent the liquid being sealed from flashing or beginning to carbonize. Seal heat can be removed with a seal flush located at the seal faces. To analyze the performance of a seal and determine amount of cooling, the following calculations can be made.

Seal Balance The greatest concern to the seal user is the dynamic contact between the mating seal surfaces. The performance of this contact determines the effectiveness of the seal. If the load at the seal faces is too high, the liquid at the seal faces will vaporize or carbonize and the seal faces can wear out. Damage to the seal faces can occur due to unstable conditions. A high wear rate from solid contact and leakage can occur if the bearing limits of the materials are exceeded. Seal balancing is a feature that is used to avoid these conditions and provide for a more efficient installation.

The pressure in any seal chamber acts equally in all directions and forces the primary ring against the mating ring. Pressure acts only on the annular area ac (see Figure 5a), so that the force in pounds (Newtons) on the seal face is as follows:

Fc = Pac where p = seal chamber pressure, lb/in2 (N/m2) and ac = hydraulic closing area, in2 (m2)

The pressure in lb/in2 (N/m2) between the primary ring and mating ring is p, = Fc = pa, f a a where ao = hydraulic opening area (seal face area), in2(m2).

To relieve the pressure at the seal faces, the relationship between the opening and closing forces can be controlled. If ao is held constant and ac is decreased by a shoulder on a sleeve or seal hardware, the seal face pressure can be lowered (see Figure 5b). This is called seal balancing. A seal without a shoulder in the design is referred to as an unbalanced seal. A balanced seal is designed to operate with a shoulder.

The ratio of the hydraulic closing area to the face area is defined as seal balance b:

Seals can be balanced for pressure at the outside diameter of the seal faces, as shown in Figure 5b. This is typical for a seal mounted inside the seal chamber. Seals installed outside the seal chamber can be balanced for pressure at the inside diameter of the seal faces. In special cases, seals can be double-balanced for pressure at both the outside and inside diameters of the seal. Seal balances can range from 0.65 to 1.35, depending on operating conditions.

Face Pressure As relative motion takes place between the seal planes, a liquid film develops. The generation of this film is believed to be the result of surface waviness in the individual sealing planes. Pressure and thermal distortion, as well as anti-rotation devices such as drive pins, keys, or dents used in the seal design, have an influence on surface waviness and on how the film develops between the sliding surfaces. Hydraulic pressure develops in the seal face, which tends to separate the sealing planes. The pressure distribution, referred to as a pressure wedge, shown in Figure 6, can be considered as linear, concave, or convex. The actual face pressure Pf in lb/in2 (N/m2) is the sum of the hydraulic pressure ph and the spring pressure Psp designed into the mechanical seal. The face pressure Pf is a further refinement of P'f, which does not take into account the liquid film pressure or the mechanical load of the seal:

FIGURE 5 Hydraulic pressure acting on the primary ring: a) unbalanced, b) balanced

FIGURE 5 Hydraulic pressure acting on the primary ring: a) unbalanced, b) balanced

FIGURE 6 The pressure distribution can be considered linear, concave, or convex.

Ap = pressure differential across seal face, lb/in2 (N/m2)

k = pressure gradient factor b = seal balance

The mechanical pressure for a seal design is

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