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Initial Condition For Blowdown Calcualtion


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#1 youji257

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Posted 21 October 2012 - 05:29 AM

Hello Every body!

I'm now designing blowdown calcultion for a system with operating condition are listed as below:

-Operating Presure: from 100 barg to 137.5 barg
-Design Pressure: 150 barg
-Operating temperature: 21 degree C to 23 degree C
-Minimum ambient temperature: 20.2 degree C

I use HYSYS-depressuring dynamic - utility to calculate blowdown.
The input data as below:

For Fire Case:
1) I use initial pressure = PAHH (95% of design pressure = 142.5 barg)
2) Initial temperature = temperature related to PAHH. I use this formular to calculate:
P operating / T operating = P blowdown initial / T blowdown initial
with P operating = Maximum operating pressure = 137.5 barg
T operating = Maximum operating temperature = (23 + 273) = 296 degree K
P blowdown initial = PAHH = 142.5 barg.
---> T initial blowdown = 33.76 C.
3) I used model " Fire API521" with equation Q = 43.2 F Aw^0.82
The problem occure, since my system has only gas, wetted area = 0, so no heat input ??? Do I understand right? This means that this equation can not be applied for my case?

4) PV work contribution = 0%

5) I fix final pressure = 6.9 barg, blowdown time = 15 minute.

6) with these input data, I find out flowrate and RO size.



For Adiabatic Case:

1) initial pressure also equal to PAHH = 142.5 barg
2) initial temperature equal to 21 degree C (because my minimum operating temperature is lower than minimum ambient temperature). If minimum operating temperature higher than minimum ambient temperature, initial temperature for blowdown will be minimum ambient temperauter and initial pressure still keep at PAHH?


3) I used model " Adiabatic"
4) PV work contribution = 100%
5) I used the RO calculated in fire case and define time for depressuring to 0 barg.



Are the steps for blowdown calculation above right? I'm so suspicious about the initial temperature and pressure.

The strange thing is that in the final result, the flowrate in adiabatic case is higher than Fire Case, while the concept is maximum flowrate will be in fire case.


Thanks in advance!!!!!!!!!!!!!

#2 paulhorth

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Posted 22 October 2012 - 10:34 AM

Youji,
Fire Case
I agree with your steps 1,2, 4, 5 and 6. For Step 3, you should not use this equation, this is valid for heat in put for wetted surface only, as you have realised.For a dry wall vessel with gas only, you need to use the HYSYS input option which allows you to define a rate of heat input. If you look into API RP 521 section 5.15.1.2.2. you will find some guidance on heat input from fire to a dry wall vessel. The figures given are for a pool fire. If the vessel can be exposed to a jet fire, the rate of heat input can be several times higher.

Non-fire case
I agree with your steps, in particular I agree with using the minimum ambient temperature and PAHH in Step 2. The PV efficiency of 100% will yield the lowest gas temperature which is what you want to establish - later you can check the sensitivity of using say 50%. The flowrate should be lower than the fire case (for the same orifice size) when you have re-worked the fire case to include a heat input.

Paul

#3 chemsac2

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Posted 23 October 2012 - 11:08 AM

youji,

As suggested by Paul, you need to use "Applied duty" option in heat flux-operating mode button. Specify constant C2 to correspond to pool/jet fire heat flux corresponding to vessel area within 25 ft with rest of the constants 0.

Adiabatic mode can have peak flowrate more than fire case for low molecular weight gases and no liquid present.

As temperature decreases, density increases with corresponding increase in flowrate. Refer valve equation in HYSYS equation help.

In fact I have observed that fire case peak flowrate with minimum operating as starting temperature is higher than for maximum operating temperature as starting temperature. This is due to increased number of initial moles at lower starting temperature (P*V = n*R*T) which need to be evacuated in 15 minutes and due to effect on density.

Logic that higher starting temperature is more conservative is probably more to do with depressurization rate than peak flowrate. With higher the starting temperature, vessel would reach failure stress faster. Hence, higher starting temperature may call for faster depressurization e.g. PAHH to 7 barg in say 13 minutes.

Regards,

Sachin

#4 Propacket

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Posted 23 October 2012 - 01:18 PM

Guys,
Regarding higher flow in adiabatic case, note that depressuring in adiabatic case is to lower pressure (o Psig) compared to fire case (100 psig). This may contribute to higher flow rate as well.

#5 youji257

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Posted 27 October 2012 - 11:34 PM

Dear all!
thank you so much for comments!
About heat flux for unwetted vessel. I read API 521 section 5.15.1.2.2 as Mr. Paulhorth advised. So could I used "Fire" mode in Hysys. The equation of "Fire" mode is Q= C1+C2*Time + C3(C4-T vessel) + C5 * Liquid volume(time t)/ Liquid volume (time 0). I will set C2, C3, C4,C5 =0 and C1 = 100. A with:
100 is 100 kW /m2 as adivsed by API 521 section 5.15.1.2.2
A : total area of Vessel.
Am i right?

#6 BramHysys

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Posted 23 August 2013 - 02:47 AM

Hi,

I would like to know how the blowdown valve can be sized if the final pressure Pf is higher than the operating P0.
Example: operating pressure 4 barg and design pressure of 14 barg. According to the API the blowdown is from operating P0 to design presser /2 Pf=PD/2 in 15 min. Please correct me if I am wrong.

Thank you.

#7 PaoloPemi

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Posted 23 August 2013 - 03:48 AM

in these cases I evaluate the depressuring rate criteria for the specific application,

in 12.3 API 521 gives different application cases with comments,

I have a different software (PRODE PROPERTIES) where I specify initial pressure, final pressure and time allowed.



#8 BramHysys

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Posted 23 August 2013 - 12:30 PM

Thank you PaoloPemi

I couldn’t find the paragraph that you are talking about, 12.3 in the API521.... The maximum I have is 7.4.3.5.2 from 5th edition, January 2007

.



#9 gegio1960

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Posted 24 August 2013 - 01:10 AM

could it be 5.20?



#10 PaoloPemi

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Posted 24 August 2013 - 02:17 AM

in 5th edition should be in chapter 5 (determination of individual relieving rates)

I have 6th edition (chapter 4.12...)



#11 BramHysys

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Posted 26 August 2013 - 07:59 PM

Thank you guys :)



#12 Manan_Joshi

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Posted 23 May 2014 - 12:21 PM

Just like to add one point.  Just keep the Vessel heater on when you have very less liquid into the system.  As the liquid level goes below 5% the HYSYS does not add any heat to the system.  Rest I also had a simillar system where the pressure was incresing.  For such cases they to check for both equations of API 521 C1 = 43500 and C2 = 70900. I hope this helps.



#13 runsaep

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Posted 20 May 2015 - 04:29 AM

hello youji 

 

Why use PAHH as initial pressure in fire calculations; what is the background for that? Because if the limit for utiiizing the flare capacity is bounded to the design pressure in the K.O Drum - then PAHH gets closer to this limit, than using normal operating pressure that gives a lower backpressure and then having a larger margin to K.O Drum design pressure.

 

And beside, using PAHH then the gas  inventory will be more heated up 8 until PSHH blows the vessel down; compare to starting to blow down from normal operating pressure. Starting from lower pressure; the orifice an be increased in size and the time to rupture will be longer

 

 

 

 

Hello Every body!

I'm now designing blowdown calcultion for a system with operating condition are listed as below:

-Operating Presure: from 100 barg to 137.5 barg
-Design Pressure: 150 barg
-Operating temperature: 21 degree C to 23 degree C
-Minimum ambient temperature: 20.2 degree C

I use HYSYS-depressuring dynamic - utility to calculate blowdown.
The input data as below:

For Fire Case:
1) I use initial pressure = PAHH (95% of design pressure = 142.5 barg)
2) Initial temperature = temperature related to PAHH. I use this formular to calculate:
P operating / T operating = P blowdown initial / T blowdown initial
with P operating = Maximum operating pressure = 137.5 barg
T operating = Maximum operating temperature = (23 + 273) = 296 degree K
P blowdown initial = PAHH = 142.5 barg.
---> T initial blowdown = 33.76 C.
3) I used model " Fire API521" with equation Q = 43.2 F Aw^0.82
The problem occure, since my system has only gas, wetted area = 0, so no heat input ??? Do I understand right? This means that this equation can not be applied for my case?

4) PV work contribution = 0%

5) I fix final pressure = 6.9 barg, blowdown time = 15 minute.

6) with these input data, I find out flowrate and RO size.



For Adiabatic Case:

1) initial pressure also equal to PAHH = 142.5 barg
2) initial temperature equal to 21 degree C (because my minimum operating temperature is lower than minimum ambient temperature). If minimum operating temperature higher than minimum ambient temperature, initial temperature for blowdown will be minimum ambient temperauter and initial pressure still keep at PAHH?


3) I used model " Adiabatic"
4) PV work contribution = 100%
5) I used the RO calculated in fire case and define time for depressuring to 0 barg.



Are the steps for blowdown calculation above right? I'm so suspicious about the initial temperature and pressure.

The strange thing is that in the final result, the flowrate in adiabatic case is higher than Fire Case, while the concept is maximum flowrate will be in fire case.


Thanks in advance!!!!!!!!!!!!!



#14 fallah

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Posted 21 May 2015 - 12:05 AM

 

hello youji 

 

Why use PAHH as initial pressure in fire calculations; what is the background for that? Because if the limit for utiiizing the flare capacity is bounded to the design pressure in the K.O Drum - then PAHH gets closer to this limit, than using normal operating pressure that gives a lower backpressure and then having a larger margin to K.O Drum design pressure.

 

And beside, using PAHH then the gas  inventory will be more heated up 8 until PSHH blows the vessel down; compare to starting to blow down from normal operating pressure. Starting from lower pressure; the orifice an be increased in size and the time to rupture will be longer

 

 

runsaep,

 

With a fixed final pressure and depressurizing time (15 min), higher initial pressure will lead to higher peak flow rate. Because the BDVs for depressurizing are normally activated by manual push buttons following an F&G command, supposing the operator might wouldn't activate the BDV on time, design pressure of the vessel is normally taken as initial pressure in most cases in which the maximum operating pressure is around 90% of the design pressure. Obviously, if the maximum operating pressure differs more than 15% from design pressure it's reasonable getting maximum operating pressure or PAHH as initial pressure.

 

Starting from lower initial pressure will result in lower peak flow rate which if actually being higher can exceed the flare capacity more than design value...

 

In designing the RO and tail pipe to flare a fixed superimposed back pressure is to be considered to which the build up one due to peak flow rate can be added... it's adequate the back pressure being in design range of the flare network and make sure not to prevent meeting the peak flow rate through the RO...
 


Edited by fallah, 21 May 2015 - 12:09 AM.





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