16/01/2019
Why are bleed valves used in GT compressor and of what benefit is it to a gas turbine in terms of performance?
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Asked 3 years ago
Samuel Oliver Ef om
9.44 · Cran eld University
Why are bleed valves used in GT compressor and of what bene t is it to a gas turbine in terms of
performance?
For both Indutrial gas turbines and Jet engines
Gas Turbines
Aeronautical Engineering
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Martin George Rose
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3 years ago
Compressors are unstable machines. If the mass ow is too low the aerofoils can stall, this creates a
phenomenon called rotating stall. Stall cells form and rotate inside the machine, performance is
compromised, pressure rise drops and ef ciency falls. If the pressure rise is high there is a risk that
this process causes surge of the whole engine: this can result in ames comming out tof the fan. If
this happens the engine has to be shut down: frightening if you are in ight! The bleed valves avoid
these dire consequences by blowing air out and increasing the air ow through the compressor. The
performance effect is very negative, SFC increases and a lot of noise is generated, but the engine still
can run which is the whole idea.
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T.Karthikeya Sharma
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3 years ago
dear friend,
hope this will help you http://control.com/thread/1026230271
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16/01/2019
Why are bleed valves used in GT compressor and of what benefit is it to a gas turbine in terms of performance?
Martin George Rose
Added an answer
3 years ago
Compressors are unstable machines. If the mass ow is too low the aerofoils can stall,
this creates a phenomenon called rotating stall. Stall cells form and rotate inside the
machine, performance is compromised, pressure rise drops and ef ciency falls. If the
pressure rise is high there is a risk that this process causes surge of the whole engine:
this can result in ames comming out tof the fan. If this happens the engi … Read more
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Mohammed Mohammed Said
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3 years ago
Besides, Martin clari cation you may distinct between two cases where the bleed-off
valve functions are critical.
During start up condition:
Among the most critical parameters during engine starting are the compressor stall
margin, and the required torque. In such cases, the conditions of low ow, and low
speed off-design are ful lled. The positive OR negative angle of attacks MAY BE
revealed in startup due to design incapability to satisfy optimum angle of attack for both
front and rear stages. As surge margin is the difference between the ow of both
equilibrium running line and the surge line, bleed-off valve is used to improve the surge
margin by accommodating high ow at compressor front stages and low swallowing
capacity of the rear stages.
During shutdown and part load conditions:
When the engine speed is reduced, the ow in the rear stages of an axial compressor is
accelerated due to the density decrease and thus may cause choking of the rear blade
passages. Increasing the axial velocity decreases the angle of attack and the ow
separates from the pressure side of the blade; this is known as negative stall surge.
However, for low axial inlet velocities, the angle of attack is increased and the ow
separates on the suction side of the blade. Opening the bleed valve moves the
equilibrium running line down and away from the surge line on the compressor
characteristic map, as it permits high mass ow at the front stages, while less ow is
passed through the rear stages.
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Samuel Oliver Ef om
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3 years ago
Tanx Sharma, Martins & Said.
Your answers have been so helpful.
Cheers.
… Read more
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Ioannis Roumeliotis
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3 years ago
Citing Saravanamuttoo: “The incidence could be maintained at the design value by
increasing the speed of the last stage and decreasing the speed of the rst stage. These
con icting requirements can be met by splitting the compressor into two or more
sections, each being driven by a separate turbine.” So yes two spool con gurations is a
solution to avoid stability issues during low power operation, as is compressor stators
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16/01/2019
Why are bleed valves used in GT compressor and of what benefit is it to a gas turbine in terms of performance?
variable geometry (VIGVs and VSVs) and Bleed Off Valves. There are engines that utilize
only one method for ensuring stability (e.g. RR Olympus: two spool three shaft engine,
no BOV no variable geometry, GE LM2500: twin shaft engine only variable geometry
stators and VIGVs), there are engines that utilize two methods (e.g. RR Tyne: two spool
three shaft engine and an intercompressor BOV) and engines that utilize three methods
(e.g. LM6000, which is a two spool engine with intercompressor VBV and HPC VSVs).
The decision has to do with several parameters, since applying only one method may
not give suf cient Surge Margin during transients or may result to signi cant part load
performance penalty; additionally for really high OPRs a single shaft solution is not
viable due to torque limitations. As mentioned in the insightful replies BOV is a bad
solution in terms of overall engine performance, but it is a simple solution. I would
recommend reading “Gas Turbine Theory” by Saravanamuttoo (Ch. 5: Axial Flow
Compressor sec. 5.12) Walsh and Fletcher “Gas Turbine Performance” (Ch. 5: Gas
Turbine Engine Components, sec. 5.2.7) and Rolls-Royce “The Jet Engine” (Ch. 3, sec.
Air ow control”) as an introduction to the subject. I hope this helps.
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Marek Mazur
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3 years ago
Another point is that bleed air is also used for cabin air pressurization in aircraft. See the
corresponding article (and especially the articles in the sources section):
https://en.wikipedia.org/wiki/Cabin_pressurization
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Nisha Patel
Cran eld University
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