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Blowdown Calculations

blowdown

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#1 Pulsater555

Pulsater555

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Posted 28 September 2021 - 02:02 PM

Hello All, 

 

I am hoping you can help me confirm a relatively simple question on blowdown release calculations for a reciprocating gas compressor.

I have built a tool that calculates the systems total volume, settle out temperature, and settle out pressure.  These calculations are based on the suction and discharge pressures, temperatures, and volumes for each stage of compression.

 

What I am currently trying to calculate is when blowing down the compressor to atmosphere or a flare stack what is the correct calculation to use?

I believe is should be using the adiabatic compression for changes of state but the number seems low to me. Please let me know if I am doing this correctly or if there is another calculation method I should be using.

 

(Pa) Atmospheric Pressure                          = 13.44psia

(V1) System Volume                                    = FT^3

Settle Out Temperature (not used)               = 174.41°F

(P1) Settle Out Pressure                              = 643.69psia

Average Comp. Factor (Z) (not used)          = 0.92

Average Specific Heat Ratio (k)                   = 1.2741  (average based on compressor program calculations) 

(P2) Flareheader Pressure                          = 18.44psia

Flareheader Temperature (not used)           = 68°F

 

(V2) Volume of released gas                        = X scf

 

Based on Adiabatic Change of State formula P2/P1 = (V1/V2)^k

To solve for V2, the re-arranged formula would be; V2= V1 / ((P2/P1)^(1/k))

Based on my calculations I am coming up with 4317.69FT^3.

Is this correct? Do I need to do anything further to get it to standard cubic feet (scf)?

 

Would you also be able to let me know how to correctly calculate the Specific Heat Ratio for each stage of compressor? Currently my compressor sizing programming calculates this but I would also like to understand how to get this value as well.

 



#2 Pilesar

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Posted 28 September 2021 - 07:18 PM

I admit I may be somewhat confused by your simple question. You have a known fixed volume of gas at a known temperature and pressure. You should be able to calculate the moles of gas in that volume. Then you release some of the gas to get a new temperature and pressure in the same fixed volume. You should be able to calculate the remaining moles of gas at the new conditions. Moles of gas can be directly converted to standard cubic feet. That is how I have approached similar problems in the past.



#3 breizh

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Posted 28 September 2021 - 08:04 PM

Hi,

PV= ZRT should be sufficient .

Breizh 






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