BRUSH SEALS

BENEFITS, LIMITATIONS AND FUTURE DEVELOPMENTS IN TURBOMACHINERY APPLICATIONS

by Dr. Stephan Pröstler

Unlike “impasto,” the brush stroke technique that the artist Van Gogh is famous for, brush seals do not stick out. Their whole point is to make operations run as smoothly, efficiently and with as little leakage as possible.

Brush seals can be found in airplane engines, gas and steam turbines, compressors and many other mechanical engineering applications. The technology was developed in the 1980s, initially for military and aero engine applications, before making an entry into the power industry in the 1990s.

The move into the energy industry came after years of development, improvement and refinement according to lessons learned in aerospace. Reducing leakage in engines and turbines was the primary motivating force behind this development.

Essentially, brush seals are made up of thousands of thin bristles fixed together using core wire and a clamping tube to form a flexible seal (Figure). Incoming gases press this wire pack against a supporting ring, compressing it further. The seal continuously adapts to the moving surface being sealed and eliminates up to 90% of leakage. Brush seals are used, for instance, in the bearing chamber, shaft, interstage, balance piston, impeller and static seals of turbines and compressors.

Brush seals are compact and comprise two casings and the seal itself. This design requires less space than labyrinth seals and provides additional options for the design of rotors and casings.

The sealing element includes a core wire, a wire or bristle pack and a clamping tube. Each individual bristle is mechanically bent around the core wire and fixed in place by the clamping tube. This eliminates the need for welding (a method patented by MTU Aero Engines).

The wires or bristles are normally arranged at an angle of 45° or 20° to the rotor’s direction of rotation. With their elasticity, they compensate for all rotor excursions, invariably returning to their original position.

The design ensures safe positive retention and enables the use of non-metallic bristles, such as Aramid (a class of heat-resistant and strong synthetic fibers) and Polymer. Their sealing effect is better than that of metallic wires.

The housing comprises a cover ring to protect the wires or bristles and a support ring to prop them up when under gas pressure, preventing the bristle pack from bending in the axial direction. The outer housing can be designed according to the industrial application.

A pressure relief chamber sits between the housing and the seal. The pressure in-side it is nearly the same as that upstream of the seal. This relieves pressure acting on the upper portion of the bristle pack and, in turn, enhances the function and service life of the seal.

Increased efficiency

A reduction in leakage equates to an increase in efficiency of around a third com-pared to conventional labyrinth seals. For instance, they boost power generation by about 400 kW in a 20 MW steam turbine. Additionally, every increase in efficiency results in lower fuel consumption, and reduced CO2 and NOx emissions.

Other benefits include product loss reduction. About 500 kg/h in product loss can be prevented for a 590 kW geared air separation compressor. Additionally, this reduces the compressor’s energy consumption by 60 kW.

The overall length of the shaft can also be reduced to improve rotor dynamics. Empirical data demonstrates that brush seals wear more slowly than other seals. Therefore, they require less maintenance, and can be replaced in a couple of hours.

However, brush seals are currently limited in their applications according to the temperatures and pressures of operating environments. Seals can be used in temperatures up to 700°C and under pressure up to 70 bar. Research and development is ongoing to increase these limits and broaden the number of applications.

Recent brush seal developments include seals for Organic Rankine Cycle (ORC) turbines. As these turbines do not use water steam, they can be run at significantly lower temperatures than traditional steam turbines, making them more efficient and increasingly popular. Brush seals reduce leakage and eliminate areas of inefficiency in ORC technology.

Furthermore, brush seals are finding increased use in subsea compressors, which must operate autonomously without maintenance for long periods.

The incorporation of additive manufacturing is bringing further brush seal enhancements. Many projects are underway to develop and refine prototypes with 3-D printing being implemented in the casings. These developments will standardize manufacturing beyond the scope of manufacturing processes today.

Author: Dr. Stephan Pröstler is Brush Seals Project Manager at MTU Aero

Engines. The company is a German engine manufacturer and has been producing brush seals since 1983, when the technology was first implemented

in military applications. Brush Seals from MTU Aero Engines can be found in

GE, MAN, Siemens, Alstom, Ansaldo and other designs. www.mtu.de