0
Select Articles

How Many Turbine Stages? PUBLIC ACCESS

[+] Author Notes
Brent A. Gregory

Creative Power Solutions

Mechanical Engineering 139(05), 56-57 (May 01, 2017) (2 pages) Paper No: ME-17-MAY5; doi: 10.1115/1.2017-May-5

Abstract

This article discusses various stages of turbines and the importance of having more stages in turbine design. The article also highlights reasons that determine the designer’s choice to select the number of turbine stages for a given design of gas turbine. The highest performance turbines are defined by lower work requirements and slower velocities in the gas path. The fundamental factors determining performance might be relegated to only two factors: demand on the turbine and axial velocity. Aircraft engine technologies drive new initiatives because of the need to increase firing temperature and dramatically improve efficiency for substantially less weight. Also, the expansion across each stage determined the annulus area so that the optimums implied by the Pearson chart were largely ignored in the article. Developments in aircraft engine gas turbines have forced heavy frame gas turbines’ original equipment manufacturers to rethink many historical paradigms.

Turbine blades all come in the same usual shape with the only seeming variation being varying sizes and either having a rotating shroud or not. However, for similar size Gas Turbines there are sometimes quite dramatic (if not subtle) differences. A good example, that affects the owner/ operator directly, is the number of stages in the gas turbine hot section, even though they have similar ratings of MW capacity. Some units produced by various OEM's (Original Equipment Manufacturer) have three (such as GE) stages of turbine, and for a similar output some have four (such as Siemens). Occasionally one might see five stages (as in the case of Alstom). This may be of some cost concern when considering hot gas path replacement or performance-related issues. Other concerns rarely bother the operator, except if clearances or cooling flows are monitored in the form of more performance-related bookkeeping of this particular variable.

The question we wish to ask here is - what determines the designer's choice to select the number of turbine stages for a given design of gas turbine? A reasonable follow up question is who, how or what decides the number of blades in a particular blade row.

Consider this chart:

Looking at the co-ordinates, the shaft work is akin to the “loading” or work performed on the shaft or sometimes by each stage of turbine. The annulus height (a function of the length of the turbine blades), in the case of the Smith chart, is in turn a function of the normalized axial velocity. The chart represents the relationship of the amount of work extracted from the hot gas to that of the blade height (for a given mass flow).

This chart, by Rolls-Royce, shows that the fundamental variables strongly correlate with turbine performance as the independent variable. This can be seen by the red contours in the figure, which illustrate lines of constant efficiency (increasing to the lower left). The highest performance turbines are defined by lower work requirements and slower velocities in the gas path. The fundamental factors determining performance might be relegated to only two factors:

  1. Demand on the turbine (shaft work)

  2. Axial Velocity (blade height)

It can be seen, therefore, that the blue line represents the peak efficiency line (and hence only one solution) for any given design of turbine.

The chart is significant because:

Given that work is fixed (demand of the compressor and generator [or fan]), and has fixed wheel speed (generally 3,000 rpm or 3,600 rpm for gas turbines in power generation) by the requirement to be synchronous or for aircraft engines the gas generator core speed or the limits of the fan size. Then, since the work is fixed (since the work =Mass Flow * Temperature Drop * Cp), the only variables left are the mass flow required to achieve that work and the temperature drop across the turbine.

Then the only variable not fixed is the axial through flow velocity (labeled “Turbine Annulus Height”) which, from the physics of flow (continuity equation), prescribes the turbine annulus height. This means the designer has few options with which to manipulate a turbine gas path: deviations away from the blue line have a consequence on the blade shape and blade numbers.

Consider an example, the point on the Y-axis at “A” which denotes the required work (given the duty of the compressor and the generator) by the turbine, then the resulting highest performing turbine is prescribed as the point B on the X-axis. Given the annulus area is “fixed” at point B, the designer can fix the maximum diameter, and the annulus area is deduced from simple math (annulus area is a function of the tip and hub diameters).

The maximum diameter is, however, usually chosen at a point where the mechanical designer is comfortable with the maximum stresses on the turbine attachments and the blade length. This is typically the last stage blade. The aerodynamicist divides the work required on the shaft into several “parts” (where each “part” represents a stage of turbine. If the work A is divided between two stages then the corresponding point on the axis is lowered and the optimum performance curve (the blue line) describes a new point on the Y axis (annulus area). If the work is split among three stages then the points making up the sum of A's value are further lowered and hence also the corresponding points on the X-axis. Note that as the designer invokes this option, each stage of turbine performance increases.

What are the key factors that drive the designer's choice that determine the designer's choice of 2, 3, 4 or even 5 stages of turbine?

Each OEM then has an optimum set of curves of efficiency, and it is certain they do not line up with one another, consistent with their own particular brand of design. These “brand” characteristics are determined by such things as empiricism derived from previous designs and the results of performance criteria derived from specific tests based on years of research in the academic world and by emerging technologies such as Computational Fluid Dynamics (CFD).

OEM's, typically producing Heavy Frame Gas Turbines (HFGT) together with Aircraft Engines, have a significant advantage when it comes to making technology decisions regarding the components of a Frame Engine, because it increases the range of experience for a given variable—hence, 3-stage designs may even trump the 4-stage design in terms of performance.

Aircraft engine technologies drive new initiatives because of the need to increase firing temperature and dramatically improve efficiency for substantially less weight. Also, the expansion across each stage determined the annulus area so that the optimums implied by the Pearson chart were largely ignored. Developments in aircraft engine gas turbines have forced HFGT OEMS to rethink many historical paradigms.

Below is a typical 3-stage HFGT turbine, and below it is what an equivalent 4 stage would like.

Considered on the Smith chart the stage characteristics for the above turbine schematics are represented in the following chart:

Note the unique features that have forced the OEM's to rethink:

  1. Lower “through flow” allows the expansion of each stage to be incorporated in the same diameter as a 3-stage allowing for a retrofit of a 4-stage unit into a similar area.

  2. Shorter chords as a result of highly loaded airfoil (more work per blade) technology.

  3. Higher loading, reducing airfoil count

  4. Attachment areas are refined based on aircraft engine technology.

  5. Lower through flow allows for optimized efficiency.

  6. Four stage designs do allow for increased performance with larger capacity.

References

“A Simple Correlation of Turbine Efficiency” S. F. Smith, Journal of Royal Aeronautical Society, Vol 69, July 1965
Improvements to the Ainley-Mathieson Method of Turbine Performance Prediction J. Dunham and P. M. Came. J. Eng. Gas Turbines Power 92 (3), 252– 256 (Jul 01, 1970) (5 pages) doi:10.1115/1.3445349 [CrossRef]
Craig, H. R. M., and Cox, H. J. A., “Performance Estimation of Axial Flow Turbines,”Proceedings of the Institution of Mechanical Engineers, Vol. 185, No. 18, pp 407– 424 1971. [CrossRef]
Copyright © 2017 by ASME
View article in PDF format.

References

“A Simple Correlation of Turbine Efficiency” S. F. Smith, Journal of Royal Aeronautical Society, Vol 69, July 1965
Improvements to the Ainley-Mathieson Method of Turbine Performance Prediction J. Dunham and P. M. Came. J. Eng. Gas Turbines Power 92 (3), 252– 256 (Jul 01, 1970) (5 pages) doi:10.1115/1.3445349 [CrossRef]
Craig, H. R. M., and Cox, H. J. A., “Performance Estimation of Axial Flow Turbines,”Proceedings of the Institution of Mechanical Engineers, Vol. 185, No. 18, pp 407– 424 1971. [CrossRef]

Figures

Tables

Errata

Discussions

Some tools below are only available to our subscribers or users with an online account.

Related Content

Customize your page view by dragging and repositioning the boxes below.

Related Journal Articles
Related eBook Content
Topic Collections

Sorry! You do not have access to this content. For assistance or to subscribe, please contact us:

  • TELEPHONE: 1-800-843-2763 (Toll-free in the USA)
  • EMAIL: asmedigitalcollection@asme.org
Sign In