Influence of Seals on Rotordynamic Stability

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.

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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.

Honeycomb seals supply large amounts of damping compared to labyrinth seals. This large increase in direct damping is the major factor that ensures stable operation when this type of seals is used although the largely increased cross-coupled stiffness does contribute to this effect as well. The damping available from honeycomb seals is not so high that it can always control subsynchronous vibrations without a shunt hole system from the diffuser. In honeycomb seals, large seal forces in the radial direction significantly influence the rotordynamics. Honeycomb seals have several mechanical advantages over labyrinth seals: (1) A honeycomb balance piston can reduce leakage by as much as 60 percent and in most cases results in a lower thrust load. (2) They are capable of sustaining higher temperatures while conventional labyrinth seals are prone to fail at high compressor discharge temperatures. (3) Honeycomb seals have the capability to control rotor stability.

Brush seal designs offer improved leakage control over conventional labyrinth seals by applying a compliant bristle with very tight clearance over the rotating shaft. Brush seals are the first simple, practical alternative to the finned labyrinth seal that offers extensive performance improvements.

Benefits of brush seals over labyrinth seals include 1) Reduced leakage (upwards of 50 percent possible); 2) Possibility to accommodate shaft excursions due to stop/start operations and other transient conditions. Labyrinth seals often incur permanent clearance increases under such conditions, degrading seal and machine performance over time; 3) Require significantly less axial space than labyrinth seal; and 4) More stable leakage characteristics over long operating periods.

Let’s explore how to predict these instabilities in rotor systems using the AxSTREAM Bearings™ and AxSTREAM Rotordynamics™ software.

Rotordynamic Instability in Labyrinth Seals:

The labyrinth seal lateral rotordynamic instability is the unstable motion that is often caused by variation of fluid dynamic pressure around the circumference of a rotor component. These destabilizing rotordynamic forces are load dependent. The destabilizing cross-coupled stiffness coefficient arises primarily because of the circumferential velocity within the seal. From a rotordynamics viewpoint this is dangerous for the stability of the rotor. To estimate the influences of seals, labyrinth seals (such as shaft end seals, eye seals and inter-stage shaft seals) were modelled and analyzed in AxSTREAM Bearings™ as shown in Figure 1 (a) and the calculated results are presented in Figure 1 (b).

Labyrinth seal
Figure 1 Labyrinth seal, (a) Geometry settings window, (b) Results in AxSTREAM Bearings

The stability calculation of a rotor system is an essential part of rotordynamics analysis. In rotating equipment, the most serious excitations are the occurrence of instabilities caused by bearing instability, aero cross coupling, and seals. To accurately predict the turbomachinery behavior, the complete rotor-bearing-seal modelling is essential. For example, the multi-stage centrifugal compressor rotor model used in the present analysis is shown in Figure 2. The AxSTREAM RotorDynamics™ program used is based on the finite element method.

2 Rotor-bearing-seal system model in AxSTREAM Rotordynamics
Figure 2 Rotor-bearing-seal system model in AxSTREAM Rotordynamics™

The rotor stability analysis was performed for the rotor with and without seals as well as with some pre-swirl. Figure 3 shows the stability map results for the case without seal while Figure 4 and Figure 5 show them when seals are accounted for.

Figure 3 Stability map of the rotor-bearing system without seals in AxSTREAM Rotordynamics
Figure 3 Stability map of the rotor-bearing system without seals in AxSTREAM Rotordynamics™

The rotor stability analysis was performed for the rotor with a low (0.05 swirl ratio) and high (0.5 swirl ratio) pre-swirl labyrinth seal. The stability map results presented in Figure 4 and Figure 5 show that for the low pre-swirl case the first mode may become unstable at high rotating speeds with it occurring earlier as pre-swirl is considered (~380 vs. ~990 rps in the 5% and 50% pre-swirl cases, respectively). These result confirm that pre-swirl has a significant influence on the rotordynamic coefficients of the labyrinth seal and thus strongly affects the rotordynamics behavior of the system which may lead to instabilities. Looking at Figure 3 it can be seen that in the absence of seals in the model the rotor would be mistakenly thought to be stable which strengthens the fact that seal effects need to be accounted for to accurately represents the dynamics behavior of the rotor.

Figure 4 Stability map of the rotor-bearing system with 5 pre-swirl labyrinth seal results in AxSTREAM Rotordynamics
Figure 4 Stability map of the rotor-bearing system with 5% pre-swirl labyrinth seal results in AxSTREAM Rotordynamics™
Figure 5 Stability map of the rotor system with 50 pre-swirl labyrinth seal results in AxSTREAM Rotordynamics
Figure 5 Stability map of the rotor system with 50% pre-swirl labyrinth seal results in AxSTREAM Rotordynamics™

One way to overcome the stability issues in seals is therefore to eliminate the circumferential flow in the chambers of the seal (swirl). The main goal of this is to provide the lowest cross-coupling stiffness coefficients and to increase direct damping coefficients. To reduce this swirl entering the seals, arrangements of anti-swirl devices and swirl brakes have been the two main approaches used over the past decades. The swirl brakes reduce the pre-swirl and, as a consequence, the cross-coupled stiffness is reduced, and additionally they even increase their direct damping. This can reduce the fluid-induced force of labyrinth seals, especially for short labyrinth seals, and improve the system efficiency and stability which are related to many factors, such as pressure ratio, flow rate, rotational speed, etc.

In recent years, numerical tools have been developed to predict more accurately the rotordynamic, bearing and seal dynamic characteristics. The entire rotor-bearing-seals system analyses can be done using computer programs such as AxSTREAM Bearing™ and AxSTREAM Rotordynamics™ developed by SoftInWay.

Interested in learning more? Contact us at info@SoftInway.Com  or leave a comment below. We’d love to hear from you!

 

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