"Bleed valve" is another one of those poor American English terms for a valve which is meant to siphon off a portion of the flow from another system.
A GE-design heavy-duty gas turbine employs compressor bleed valves to protect the axial compressor during start-up and shutdown against stalling and surging conditions. These valves are typically only open during acceleration to rated speed and deceleration from rated speed. Newer heavy-duty gas turbine with complex combustion system arrangements designed to limit emission of harmful gases also employ the compressor bleed valves during very light loading to limit the amount of air entering the combustor.
Compressor bleed valves discharge into the gas turbine exhaust duct. Compressor bleed valves are generally closed during normal operation (with the previous exception) since "dumping" axial compressor discharge into the exhaust reduces efficiency (but it's MANDATORY during start-up and acceleration to protect the compressor!). Compressor bleed valves are generally either fully open or fully closed; they are not modulated.
GE-design heavy-duty gas turbines also employ inlet bleed heat valves to recirculate a portion of the axial compressor discharge back to the inlet of the axial compressor for a variety of functions. One application uses them to heat the inlet air to prevent icing on the compressor inlet (appropriately called anti-icing protection); another application uses inlet bleed heat to protect the axial compressor when the IGVs (axial compressor Inlet Guide Vanes) are closed below the "normal" operating conditions ("normal" being before complicated combustion systems designed to limit exhaust emissions were invented).
Inlet bleed heat control valves are modulated to control the amount of air recirculated back to the axial compressor inlet. Recirculating axial compressor discharge back to the inlet is also inefficient, but is generally limited to approximately 5% of rated air flow through the compressor--and necessary to protect the compressor at low IGV angles. Inlet bleed heat valves are generally closed before the unit reaches rated load (except for anti-icing protection) and should be closed during rated power output (Base Load) operation (except for anti-icing protection).
If you require more information, your question isn't exactly clear--are you asking about compressor bleed valves or inlet bleed heat valves or?.?.? Or just bleed valves in general?
I was asking about compressor bleed valve. In our case, its a 21 MW mark v controlled GE gas turbine.
Compressor bleed valve is a preventive measure of compressor soaking and compressor surging. For further details, please search for compressor soaking on google.com.
With respect to aircraft gas turbines, during take-off & landing up to an altitude where bird ingestion is possible - are the bleed valves open or closed?
Need more information on compressor bleed value. what is the use of bleed air in IGV? and when it was developed and its main application?
Need more information about the turbine you are referring to. Different manufacturers have different schemes and to properly reply to your query you need to tell us what turbine you are referring to.
Also, if you're referring to GE-design heavy duty gas turbines there are several good answers above.
> What is the significance of Bleed Valve operation from
> start up to full loading a Gas Turbine?
GE usually provides a schedule of how far open/closed they would be during operation compared to compressor speed, with a separate transient schedule for positioning during quick speed changes.
As MarkVGuy said, once you're up to speed you pretty much want them closed for efficiency reasons.
I would also add you want to keep them closed during a purge.
> I would also add you want to keep them closed during a purge.
What's your definition of purging? When does it occur?
>What's your definition of purging? When does it occur?
Either a simple compressor purge or HRSG purge prior to light off. My thinking was that it would be best if all the compressor air was going through the turbine and or HRSG purging rather than out the bleed valves.
>My thinking was that it would be best if all
>the compressor air was going through the turbine and or HRSG
>purging rather than out the bleed valves.
Not bad thinking, but the compressor bleed valves "exhaust" in the GT exhaust, so it goes into the HRSG, just probably at a slightly lower pressure. This is shown on the P&IDs.
>> My thinking was that it would be best if all
>> the compressor air was going through the turbine and or HRSG
>> purging rather than out the bleed valves.
> Not bad thinking, but the compressor bleed valves "exhaust"
> in the GT exhaust, so it goes into the HRSG, just probably
> at a slightly lower pressure. This is shown on the P&IDs.
I guess that would depend on site installation, as the site I'm at now the VBVs do not exhaust in the HRSG. They have a separate vent/silencer.
And this conversation was about bleed valves, not variable bleed valves. And bleed valves on GE-design heavy duty gas turbines in particular.
> And this conversation was about bleed valves, not variable
> bleed valves. And bleed valves on GE-design heavy duty gas
> turbines in particular.
Sorry CSA, didn't mean to offend you. To be clear original question said nothing about "Heavy Duty" or "GE-design", they simply asked the significance of operating bleed valves on "a gas turbine" and I was attempting to contribute my limited knowledge on the subset of gas turbines I work with. The later post specified 21MW (probably a 2500?) which I assumed was an aeroderivative, as I haven't seen anything GE's Heavy Duty below the 6B at 44MW, and they specified "compressor" bleed valves, which are variable, even if you only use them as on/off.
Can anyone explain how bleed valves actually prevent compressor from surging during startup or shutdown? Also can high inlet vacuum in gas turbine/compressor affects compressor surge margin?
Axial compressor bleed valves allow a low-pressure path for air to flow "out" of the axial compressor when it might otherwise cause stalling and/or surging (which are exactly what they sound like--air can't flow from one stage of compression to the next and the axial compressor wants to stop turning (very destructive) and if the flow does start it can stop again just as quickly (also very bad if allowed to continue unabated)).
If someone put an extremely restrictive mask over your nose and mouth while you were performing very vigorous work or exercise, how would your body react? Compressors don't do well under those circumstances, either. And axial compressors are not like centrifugal or reciprocating compressors and respond differently (poorly) to flow restrictions when running at rated speeds. They just don't become inefficient, and stall/surge conditions can occur even at rated speed under the right conditions.
Further, high vacuum in the inlet duct work can cause it to collapse, which would be likely disastrous. Inlet duct work isn't designed for higher vacuums (that would unnecessarily increase costs).
Finally, if inlet air filters get so "choked" they collapse a LOT of dirt--and debris--can be very quickly ingested into the axial compressor inlet, also with disastrous results.
Hope this helps!!!
thanks for your positive response..
is high rotor cooling air and high disc cavity temperatures has any effect on gas turbine power output?
Since rotor cooling air generally comes from some area of the axial compressor discharge, then it's likely that too little air is entering or being allowed to flow through the rotor--and that means overall unit efficiency won't likely suffer much. But the rotor can experience problems because of the lack of cooling.
High "disc cavity" (I presume you do not have a GE-design heavy duty gas turbine) temperatures mean that some of the hot combustion gases are making their way into the "disc cavity" instead of flowing through the turbine and being expanded to produce work (torque)--OR too little cooling air is entering the "disc cavity." If too much hot combustion gas enters the "disc cavity" this will have a small effect on overall unit efficiency. And if too much hot combustion gas flows into the "disc cavity" then the turbine wheels (discs) are going to experience thermal stresses which could lead to premature wheel (disc) failure, and catastophic damage.
Also, depending on the air flow paths through the rotor and "disc cavities" it could be that excessive hot combustion gases are flowing into the "disc cavities" and preventing proper circulation of cooling air through the rotor and "disc cavities"--which isn't good for the shaft or the discs (wheels).
Has the unit just returned to service from a maintenance outage? Are you certain the proper orifices were re-installed in the cooling and sealing air lines? Or, that valves in the cooling and sealing air lines are in their proper positions (isolation valves open; drain valves closed)?
Has the shaft moved axially recently?
Please write back to let us know what you find!
Actually we have two westinghouse Siemens gas turbines of 50MW capacity. There is load difference of 2~3MW between two turbines from last few years. Now days, both turbines has same inlet temperatures, compressor discharge temperature/pressure, blade path path/exhaust temperatures. However there is difference of disc cavity- 2,3 and rotor cooling air temperatures (20~30 degree F higher for turbine having lower load). We have taken all preventive measures like verification of temp/pressure measurements, blades clearances, replacement of fuel valves, bleed valves and turbine/compressor rotor etc but no positive result.... can anyone help out cause of lower power output for one turbine as compared to other?
this is normal, as every machine had different behavior, this is related t the efficiency of each machine? You try to calculate it as per your machine OEM .
you did not mention about other measurements such as inlet air mass flow in m3/hr, you measured pressure and temperature pnly which is not enough.
One factor may affect the efficiency the mass flow is the inlet filter pressure drop (delta p) you check this point as well.
Also both compressors cleanness is a major factor .