Outlet static pressure of a Gas Turbine 1

[MechMad1]

I have a manufacturer that states that its turbine has a Outlet static pressure of, let's say, 200 mmH2O.
How can I translate this information into the outlet pressure in bars?

I get the idea that the static pressure is the pressure of a fluid when it is not moving, like the air inside a car tire.
But I can't figure out what the manufacturer is meaning here.

Thank you.

 

[IRstuff]

I would guess that what they are describing is what a pressure gage would measure, were it placed right at the outlet. This pressure is essentially from the exhaust mass flow impinging at its mean velocity against a surface, not that different than aerodynamic force, and calculated the same way.

TTFN
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[MechMad1]

" This pressure is essentially from the exhaust mass flow impinging at its mean velocity against a surface, not that different than aerodynamic force, and calculated the same way. "
But that would be the dynamic pressure, instead of the static pressure, right?

BTW, what type of pressure is measure in a gauge? Total, static or dynamic?

 

[IRstuff]

Pressure is pressure; a gauge essentially measures the force applied to a known area, hence pressure. The only question is whether the gauge is measuring absolute or relative (to atmospheric) pressure.

What you are calling "static" pressure is defined EXACTLY the same way, "mass flow impinging at its mean velocity against a surface" Even in a closed box, the air molecules that hit the sides of the box have a mean velocity that's due to the thermal energy of the gas. The box, as a whole, has a mean velocity of zero, but the individual molecules do not. That's why pressure decreases when temperature decreases.

TTFN
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[chicopee]

Remember Bernoulli's equation about the static and velocity pressure components. The static pressure would be measured perpendicular to the gas flow and the velocity pressure would be measured in line to the gas flow. What I wonder is the 200 mmH2O positive gauge pressure, negative(partial vacuum) gauge pressure, or is it an absolute pressure?.

 

[Lou Scannon]

Wouldn't it be better to ask your question directly to the manufacturer?

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[MisterDonut]

The pressure at the outlet is dependent on the configuration of the stack, etc., on the discharge. A simple stack has a lower pressure drop across it that a heat recovery steam generator. What the manufacturer probably means is that the performance listed is based on 200 mmH20 static pressure at the outlet. More pressure drop across the outlet stack/HRSG means lower performance in the gas turbine.

 

[MechMad1]

Yeah, I agree. This data may be referenced for performance. The outlet pressure will be determined by the HRSG downstream the turbine (the losses in the HRSG).

But I still don't understand something. The total pressure in the exhaust is the sum of the static and the dynamic pressure. The flow of gases surely has a dynamic pressure (due to its velocity), so to calculate the total pressure in the exhaust we should know also the dynamic pressure.

 

[Timelord]

My two cents worth:

"How can I translate this information into the outlet pressure in bars?" To answer the OP's question directly, 1 Bar = 760 mm Hg at STP. The next question was correctly asked by Chicopee, gauge or absolute?

Timelord

 

[MikeHalloran]

Static pressure in this context means the gage pressure measured by a gage or manometer connected to a radial port just downstream of the exhaust nozzle, hopefully in a position where the gas velocity neither adds to nor subtracts from the pressure reading. I.e., not a pitot pressure.

Gas turbine and other engine manufacturers specify such a pressure (but rarely provide a designated port for measuring it) so as to specify a limit to the downstream restriction ("backpressure") under which the engine warranty will be effective. For gas turbines and large Diesels, the pressure limit usually amounts to a few tens of millibars at WOT.

Diesels operating with more than the specified backpressure are sometimes warranted for a derated power rating and so blessed for operation.

I have been led to believe that gas turbines operating with more than the specified backpressure tend to fail in expensive ways that are not covered by warranty.



Mike Halloran
Pembroke Pines, FL, USA

 

[drawoh]

MechMad1,

200mm[ ]H₂O is the reading of a manometer with water in it. Standard pressure is something like 10m of water. I am too lazy to look it up. 760mm is for a manometer with mercury in it.

Manometers normally show gauge pressure.

--
JHG

 

[Timelord]

You are right, I misread H2O into Hg. The actual conversion is: 200 millimeter of water [4°C] = 0.0196133 bar. I believe the conversion is taken at 4 deg C because that is where water's density vs temp is a local max.

Timelord

 

[MechMad1]

All clear now. Thank you all.

 

[racookpe1978]

Yeah, I agree. This data may be referenced for performance. The outlet pressure will be determined by the HRSG downstream the turbine (the losses in the HRSG).

But I still don't understand something. The total pressure in the exhaust is the sum of the static and the dynamic pressure. The flow of gases surely has a dynamic pressure (due to its velocity), so to calculate the total pressure in the exhaust we should know also the dynamic pressure.

Don't know if you've ever stood in the GT exhaust areas, but any (and all!) of the different definitions "could" apply - but at different locations in the "exhaust". Your answer IS going to be VERY, very different in each area.

Right after the last blade, still inside the circular exhaust shell - usually of Hastex in the machines I weld in: Very high velocity gas, very high temperature, constrained by the circular exhaust metal that is attached to the end of the GT.

After the circular exhaust section, outside of the directed gas flow. Still high temperature exhaust gas, but here it is circulating & swirling around. Now at high temperatures still, but static pressure. This is full of eddies and is very turbulent flow , so you'd expect vibrations and "shuddering" of the outside steel walls and their insulating liner. Outside the GT, this is tan expanding section of regular carbon steel (no longer Hastex/Hastalloy) and the cross-section is rectangular always expanding towards the rectangular HRSG (boiler) section downstream. The bigger the cross-sectional area in this transition section, the lower the velocity and the higher the static pressure. Nothing much to cool the gas though, sop it stays pretty high.

Through the Heat Recovery Steam Generator HRSG) if present. Usually a constant cross-section, but of relatively low speed compared to upstream values. The finned tubes slow the gasses down through friction losses, but cool it a great deal: Net? Cooler gas is more dense, and a slightly slower velocity. Higher static pressure again, but not much higher than in the transition section. (The transition area is filled many thousand cubic feet of "nothing" ... )

After the rectangular HRSG the gas flow horizontally to a vertical exhaust tube. This is a smaller area than the HRSG, so velocity increases substantially, and static pressure goes down. Flow "turns" from horizontal through the "nest" of finned HRSG tubes to a simple round tower exhaust into the air. This "turning" creates a constantly changing (vibrational) turbulence so the wall thicknesses are larger, or are reinforced with outside ribs and stiffeners on the tower section.