Numerous developments around seal technology have surfaced in the last few years. Seal performance is especially critical in turbomachinery operating under high pressure and high speed conditions. The type of seal (configuration) can influence its rotordynamics behavior and therefore affect the overall system stability. The dynamic phenomena induced by interactions between rotor and seal fluid flow in turbomachines may lead to severe lateral vibrations of their rotors. Hence, these effects must be carefully evaluated and factored in during the design of the seal system to ensure their safe operation. In general, they fall into two categories: contacting seals and non-contacting seals.
- – Contacting seals cannot be used due to metallurgical limitations for sealing in locations where the temperature and/or the pressure are very high, and when the machine rotates at high speed. Therefore, noncontact seals are usually used in fluid machines requiring high performance.
- – Noncontact seals are used extensively in high-speed turbomachinery and have good mechanical reliability. They are not positive sealing which means they allow a small amount of internal leakages as a tradeoff to prevent rubbing.
Radial seals (labyrinth or honeycomb) separate regions of high pressure and low pressure in rotating machinery and their function is to minimize the leakage and improve the overall efficiency of a rotating machine by ensuring that as much of the flow as possible goes through the blade channels. To provide a better understanding, the comparison of different types of seal configurations (honeycomb and brush seal) are described below.
The occurrence of self-excited rotordynamics instability is of significant importance in modem high performance turbomachinery, particularly with the present trend towards higher speeds and loading conditions. Labyrinth seals are good in restricting the flow but do not respond well to dynamics and often lead to turbomachine instabilities. These rotordynamic instabilities are due to aerodynamic excitations from the gas circulating in the narrow annular cavities on rotor-stator seals. Seals control turbomachinery leakages, coolant flows and contribute to the overall system rotordynamic stability. Such instability can lead to destructive levels of vibration. It is therefore strongly desirable that turbomachines are designed to minimize the possible occurrence of such rotordynamics instability.