3.3 Dynamic Seals *

Dynamic seals are used where sealing takes place between two surfaces having relative movement viz. rotary, reciprocating, and oscillating. The main focus of the present section is on rotary seals. It has wide variety of applications in the high-speed, high-pressure and cryogenic temperature conditions of aviation and space industries such as in turbine stages, turbo-pumps, compressors, gear boxes, etc.

3.3.1 Classification of seals

Seals are broadly classified as liquid and gas seals according to the working fluid used in the system. The most common working fluids are water, air, nitrogen, Triflurobromomethane (CBrF ), liquid oxygen, liquid hydrogen etc. In addition, they can be categorized as the static and dynamic seals. Static seals are used where the two surfaces do not move relative to one another. Gasket-type seals are static seals (Fig. 3.31). Rotary seals can be subdivided into two main categories as clearance seals and contact seals. Clearance seals are circumferential non-contacting seals (Fig. 3.32(a)). In contact seals, the contact is formed by positive pressure, while in the case of clearance seals; they operate with positive clearance (no rubbing contact). The most commonly used material for dynamic seals (especially for rotary seals) are stainless steel, bronze, aluminum, nickel-based alloys, polytetrafluroethane etc. Fig. 3.32(a) shows a typical rotary seal with the clearance exaggerated. Rotary seals based on geometry can be classified as

1. Ungrooved plain seals (or Smooth annular seals): (a) Straight (Fig. 3.32(b)), (b) Tapered (Fig. 3.32c), and (c) Stepped (Fig. 3.32(d)). In geometry they are similar to journal bearings but the clearance/radius ratio is as low as two times and as high as ten times (or more) large to avoid rotor/stator contact.
2.  Grooved/Roughened surface seals: (a) Porous surface seals, (b) Labyrinth seals (Figs. 3.33(a-d)), (c) Helically grooved / Screw seals, (d) Circular holes or triangular patterns seals, and (e) Honeycomb patterns seals (Fig. 3.34). These seals are used in the centrifugal and axial compressors and pumps and in turbines. Different internal surface patterns of seals are shown in Fig. 3.35.
3. Contact seals: (a) Brush seals (Fig. 3.36(a)), (b) Face seals, and (c) Lip seals (Fig. 3.36(b)). Because of rubbing, these seals are used commonly in low speed pumps, or where the working fluid can act as a coolant. Contact seals provide much lower leakage rates than either of non-contact seals (Adams, 1987), however, the latter can operate at very high speed and pressure conditions.
4. Floating-ring oil seals: The ring whirls or vibrates with the rotor in the lubricating oil, but does not spin. They are used in high-pressure multi-stage centrifugal compressors.

It is an advanced topic and it can be omitted.