Steam Turbine Seal Leakage Calculation in Design

Steam turbine seals are parts inserted between moving and stationary components, to reduce and prevent steam leakage and air leaking into the low pressure areas. The leakage can happen through vane, gland, and shaft, etc. To reduce leakage from those parts while guaranteeing smooth operation of a steam turbine, engineers have to design these seals, taking into account not only efficiency, but also mechanical strength, vibration and cost.

As an example, steam turbine flow path seals improve overall efficiency installing various types of shrouds, diaphragms, and end seals which prevent idle leaks of working steam in the cylinders. In steam turbines, labyrinth seals are widely used. Some labyrinth seals are also used with honeycomb inserts. It is believed that the use of such seals makes it possible to achieve a certain gain due to smaller leaks of working fluid and more reliable operation of the system under the conditions in which the rotor’s rotating parts may rub against the stator elements. However, we can only consider it as a successful design if the structures are compliant with the manufacturing capabilities and have good vibration stability. [1] Furthermore, seal leakage can significantly affect efficiencies. Better seals increase efficiencies but add extra cost to both manufacturing and maintenance, so the design needs to be done with the turbine flow path design. Although modeling the seals in 3D CFD is theoretically possible, the calculation resources and time are extremely demanding.

This important task can be completed very easily with AxSTREAM NETTM. AxSTREAM NETTM provides a flexible method to represent fluid path and solid structure as a set of 1D elements, which can be connected to each other to form a thermal-fluid network. For each fluid path section, the program calculates fluid flow parameters for inlet and outlet cross-sections, like velocity, density, temperature, mass flow rate, etc. Therefore, the leakage from the whole system can be modeled in this network, as shown in Figure 1.

Steam Turbine Seal Leakage Calculation with AxSTREAM NET
Figure 1. Steam Turbine Seal Leakage Calculation with AxSTREAM NET™

AxSTREAM NET™ is capable of doing:

  1. Choice of seal design at the stage of the steam/gas turbine preliminary design.
  2. Calculation of balances of pressures and mass flow rate to correctly account for the efficiency of the steam/gas turbine.
  3. Calculation of seals fluid flow parameters on the startup mode to estimate the thermal expansion of rotor and casing element.
  4. Calculation of thermal boundary conditions for thermo stresses estimation.


In practice for multi cylinders units with high, intermediate and low pressure turbines each gland seal section is connected to different part of turbine, such as:

  1. Turbine flow path stages;
  2. Pressurized (equalization) pipe; and
  3. Feed water heaters.

 

The leakages balance analysis of such situations can be done as an iterative process into AxSTREAM® (responsible for flow path calculation) and AxSTREAM NET™ (responsible for gland seal calculation). While AxSTREAM ION™ is responsible for integration between these two process to determine leakages balance. This collaboration is shown in Figure 2 below.

Steam Turbine and Cycle Design with Leakage Calculation within AxSTREAM IO Platform
Figure 2. Steam Turbine and Cycle Design with Leakage Calculation within AxSTREAM ION™ Platform

Using these three programs together saves substantial engineering time while providing the client with the best possible seals at the lowest possible cost for production and maintenance.

References:

[1]. Steam Turbine Flow Path Seals, Neuimin, V.M. Therm. Eng. (2018) 65: 125. https://doi.org/10.1134/S0040601518030059

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