7 replies to this topic

[alirah]

Hi everyone.
I have a gas turbine with situations below :

input heat: 18kW
natural gas fuel
exhaust tempreture: 511 C
mass flow rate: 21.74 kg/hr
allowable pressure drop at exhaust duct: 2.5 kPa

Guaranteed emissions: NOX: 100 ppm , CO: 50 ppm

 

how can I determine exhaust stack height?

please help me. emergency situation.

Edited by alirah, 16 July 2014 - 02:22 AM.

[dnrakesh2006]

alirah,

 

Fix the size of exhaust duct based on outlet of gas turbine exhaust. do the pressure drop calculation based on mass flow rate. consider pressure drop in the silencer based on vendor recommendations(normally 2-4 in H2O). overall you should not exceed the allowable pressure drop i.e 2.5 kpa (10 inch H2O)as recommended by gas turbine vendor.

 

Ask your piping engineer to route the duct to the safe location and get the length from the isometrics. again recalculate the pressure drop with exact length including bends equivalent length. reconfirm that pressure drop in duct is not exceed the allowable pressure drop limits. in worst case if it exceeds allowable limit try to increase duct size or check with Gas turbine vendor to re confirm the power output based on new pressure drop, if Gas turbine can produce the required power then issues.

Edited by dnrakesh2006, 14 July 2014 - 03:18 AM.

[alirah]

"Fix the size of exhaust duct based on outlet of gas turbine exhaust." please explain this part a little more. should I use air dispersion models as for boiler stacks? Is simple gausian model OK? what else should I consider?
and about pressure drop, am I allowed to consider the stack effect as a pressure recovery part? to compensate for a part of preesure drop?

I appriciate that.

Edited by alirah, 14 July 2014 - 09:56 PM.

[alirah]

dnrakesh2006

I deterimined the stack height as 35 meters based on simple Gaussian dispersion model. (and neglecting the plume rise) can you tell me if this assumption correct? And is my determined value (35m) acceptable based on experience for a 6 MW gas turbine?

Edited by alirah, 16 July 2014 - 11:58 PM.

[ankur2061]

alirah,

 

The virtual stack height calculated from the gaussian dispersion model is the sum of the actual flare height plus the momentum plume rise. The momentum plume rise is calculated as follows:

 

ΔH = 3*d*V_ex / U

 

where:

ΔH = plume rise due to gas exit velocity, m

d = stack diameter at exit, m

V_ex = Exit velocity from stack tip, m/s

U = Wind Speed, m/s

 

H' = H + ΔH

where:

H' = Virtual stack height, m

H = Actual stack height, m

ΔH = momentum plume rise, m

 

I am attaching an excel spreadsheet for calculating the virtual stack height uisng the "Gaussian" dispersion model

 

Based on the information provided you can now calcualte the actual stack height.

 

Regards,

Ankur

 

 

 

 

 

 

Attached Files

•  AirMod.xls   58KB   101 downloads

Edited by ankur2061, 16 July 2014 - 09:34 AM.

[alirah]

Ankur;

 

the value of pulme rise based on your formula (ΔH) is approximately equal to the value of virtual height (H'). does this mean that  2 or 3 meter of stack above turbine structure level is adequate? how  reliable is this formula for calculating plume rise?

 

[ankur2061]

Virtual stack height is a function of the permissible point concentration of the air pollutant. The point concentration is decided by the user based on local laws and regulations and health hazards of the point concentration of  the air pollutant exceeding a prescribed limit.

 

 

Once you know the virtual stack height and the momentum plume rise, the actual stack height is just the virtual stack height minus the momentum plume rise.

 

As far as the formula for momentum plume rise is concerned, the following is the reference:

 

"Handbook of Chemical Engineering Calculations", 3rd Edition, Nicholas P. Chopey

 

Regards,

Ankur.

[alirah]

Ankur,
 

thank you so much. I think I got the idea.

 

Best
Ali