Handbook of Turbomachinery Second Edition Revised - Ventech!

to the continuous extension of a highly stressed material subjected to
elevated temperatures for long periods of time. Creep is a function of
material properties, the level of stress, the temperature, and its duration.
Depending on the part, the normal design philosophy is to limit the average
stress to a level such that the strain due to creep is below 1% to 2% over the
expected life of the component. Stress-rupture [3] is the ultimate failure of a
part due to creep. Different materials will undergo different amounts of
creep elongation before rupture. Stress-rupture may occur with almost no
noticeable creep or with considerable elongation depending on the material,
the stress, and the temperature. Blades and blade roots or attachments at the
disk are subject to stress-rupture. Improper design procedure and
nonisotropic material properties have often been responsible for these
failures. It is necessary to have a good knowledge of all the expected stresses
(thermal and mechanical) and material properties if such failures are to be
avoided.
Factors that are somewhat difficult to control are (1) corrosion and
erosion resulting in damage to parts along the flow path, especially the
leading edges of vanes and blades, (2) damage due to foreign objects like
birds hitting fan or compressor blades during flight, and (3) control
malfunction. Unfortunately, the effects of corrosion depend strongly on the
pollutants in the atmosphere as well as on the fuel, which may contain sulfur
or compounds of alkaline materials. These are hard to control and may
require surface coatings that prevent contact between the hot gases and the
surface (see discussion at the end of section on thermal/stress analysis and
life prediction). The impact of foreign objects like birds, tire treads, gravel,
and ice or hail on fan blades and the immediately following compressor
stages can be severe. It is necessary to ensure that the engine can withstand
such types of impact without failure. (See discussion related to bird
ingestion or foreign object damage tests near the end of this chapter.) To
reduce the probability of control malfunction, it is necessary to build a
certain amount of redundancy in the system, so that if one of the controls
fails for some reason, there is another control that will perform the required
operation. Experience is the guide in determining the degree of redundancy
necessary to minimize the probability of failure.
Thermal and Mechanical Design Approach
The design of a gas turbine consists of four main phases. In the first, a
marketing study determines the need of the customer for a proposed engine
and a conceptual study is performed to assess the feasibility that an engine
can profitably be designed and developed to satisfy this need. The second is
the preliminary design phase. In preliminary design, projections from
Copyright © 2003 Marcel Dekker, Inc.