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Flare Stack Sizing Worst Case Peak Flow Rate

flare stack sizing

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#1 hafiz akhtar

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Posted 16 April 2014 - 08:00 AM

Dear Sirs, i am sizing the flare stack for compressor station. i calculated the volume of isolated sections and done the depresuring dynamic model and calculated the peak flow rates.

 

The following sections  i have considered which shall be depressurized. The case 1 shall be depressurized on staggered mode. But i am not sure about case 2 blocked discharge flow rate depressuring. 

 

 Case 1

  • Inlet header peak flow rate 13000 kg/hr due to low pressure
  • Each compressor train after settle out conditions peak flow rate 11000 kg/hr
  • Outlet header peak flow rate 80000 kg/hr due to high pressure
  • Gas pipeline 16 km section  peak flow rate 50000 kg/hr

  Case 2.

 

  • Each Gas Compressor  blocked discharge flow rate 70000 kg/hr
  • Four Gas Compressor  blocked discharge flow rate 280000 kg/hr

Please guide me about the worst scenario. so that i may take the flow rate to size the stack diameter and sterile area for 4.73 kw/m2 radiations. This is urgent sir

 

Thanks 

 

Hafiz


Edited by hafiz akhtar, 16 April 2014 - 08:03 AM.


#2 fallah

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Posted 16 April 2014 - 10:19 AM

Hafiz,

 

Lack of adequate info and a sketch of the system...anyway:

 

If the sections in case 1 would be sequentially depressurized and the compressors valving arrangement is such that the blocked outlet cannot be happened for all compressors simultaneously, appears the stack to be sized for handling maximum flowrate of 80000 kg/hr i.e. for outlet header depressurizing as worst scenario...


Edited by fallah, 16 April 2014 - 10:24 AM.


#3 flarenuf

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Posted 16 April 2014 - 11:25 AM

hi

as Fallah says the stack looks like it should be able to handle 80K peak based on teh info you have provided.

Blocked outlet of all 4 compressors is double jeopardy as the valving should not allow this.

 

Remember the initial dp flow is only for a very short time so depending on the physical amount of piping you have in the flare system you will not see this peak flow  at the tip due to packing.

 

This will also affect your radiation levels in worst case weather scenario.

 

best regards

flarenuf



#4 Bobby Strain

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Posted 16 April 2014 - 04:36 PM

I won't give advice with so little information available. And, you should not accept advice until those advising have a detail knowledge of your installation.

 

Bobby



#5 hafiz akhtar

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Posted 17 April 2014 - 12:29 AM

 I am thankful to Mr Falah and Flarenuf.  I got 80k peak flow rate for sizing of stack and sterile area based on radiation levels 4.73kw/m2. I am satisfied with your answers.

My Colleague is considering blocked discharge flow rate 70k kg/hr per train and 280k kg/hr for all four compressors together depressurization. He is asking that during blocked discharge the PSV shall open and transfer incoming flow rate to stack via KO Drum. I think it is double jeopardy for short time. I could not convince to transfer the all four compressors gas flow rate to stack during upset conditions. It is increasing the sterile area and stack height.

I told him that if block discharge situation shall come then compressor shall minimize the speed to attain the discharge pressure and anti surge line shall open. If yet the blocked discharge situation remain then ONE compressor shall trip. If it happen for all four compressors then the station shall be down, means it shall be ESD. Now we shall wait for a while to achieve the settle out pressure for each train. Then we shall depressurize the all trains together to release the 50% pressure within 15 minutes to maintain the required radiation level. Then to ATM in next time.

Now we can depressurize the outlet header and then inlet header on staggered mode. You are requested to give further technical details so that I may convince him for outlet header peak flow rate 80k instead of blocked discharge four compressors flow rate 280k

Once again thanks in advance



#6 fallah

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Posted 17 April 2014 - 12:57 AM

Hafiz,

 

You fail to provide more info and upload the sketch. Anyway...:

 

To take 80,000 kg/hr as the relief load of the worst scenario, you should be ensured there is no possibility of simultaneous relieving from four compressors due to blocked discharge scenario. Indeed, you cannot take any credit for antisurge valve in PSV sizing for blocked discharge case while you should take maximum suction pressure and maximum speed into account.



#7 hafiz akhtar

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Posted 17 April 2014 - 02:46 AM

Hi Sir,

 

The simultaneous relieving from four compressors due to blocked discharge scenario is 280k. The block discharge can or can not be happened for all four compressors simultaneously or separately. But we have to depressurize the all four trains together to release the 50% pressure within 15 minutes to maintain the required radiation level.

The question is BLOCK DISCHARGE IS THE WORST CASE or depressurizarion of Compressor trains,inlet header ,outlet header and pipeline section 16 km upstream and downstream of compressor station. Can anyone refer the worst case scenario for the attached sketch? I have not considered the blocked discharge worst case in my past projects due to short process upset. 

 

The sketch is attached.

 

Hafiz

Attached Files



#8 Malek

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Posted 18 April 2014 - 05:55 PM

To me, I think the probability of inadvertent valve closure of the ESD valve at the common discharge header is the same as the ones of the individual compression trains, so the 280 k scenario shouldn't be a double jeopardy here and you can consider it.
 

 


Edited by malek, 18 April 2014 - 06:19 PM.


#9 hafiz akhtar

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Posted 22 April 2014 - 03:24 AM

Dear Sir,

 

Sorry I could not convince. It means that we have to waste completely the 280k kg/hr gas during block discharge. But block discharge shall be for even short time. The vent header, KO drum, stack diameter, stack height including sterile area seems oversize.

Normally we have to depressurize the PEAK FLOW RATE for piping and equipments based on VOLUME during shutdown to safe the system instead to take total compressor station incoming gas flow rate and consider the discharge blocked discharge case and release it.

 You are requested to please share any guideline or code sections which highlight the blocked discharge case if any. Thanks

 

Regards,

 

Hafiz



#10 Bobby Strain

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Posted 22 April 2014 - 12:48 PM

Based on your sketch, nobody is going to suggest that the maxium relieving is less than the total. So, if you already have a flare that won't handle this load, then you need to look for HIPPS instrumentation to mitigate the load.

 

Bobby



#11 Malek

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Posted 22 April 2014 - 04:34 PM

Normally we have to depressurize the PEAK FLOW RATE for piping and equipments based on VOLUME during shutdown to safe the system instead to take total compressor station incoming gas flow rate and consider the discharge blocked discharge case and release it.

 
Hafiz,
 
http://speedy.sh/Bp4wx/Sketch.pdf    

       safety critical systems eg. flare systems are not designed to recover the normal or the more frequent upsets but to recover the worst "peak flow rate" upsets.
 
on 2010, BG rashpetco in Egypt, a 36'' HP flare header on 3000 bbl, 18 bcfpd slug catcher has ruptured when the 7 BDV's staggered blowdown control system had failed (vales had opened simultaneously) and then caused a complete rupture for the downstream flare header, fortunately no body had injured but loss of containment, asset and production loss were the gains.
 
It was a bad day, a learnt lesson will be ever kept in mind. 
 
Again, I think you have to consider the peak flow even if you are forced to spend more money.
 
Best regards
 



#12 flarenuf

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Posted 24 April 2014 - 09:19 AM

Hafiz,

The main question still seems to be if you need to consider a blocked outlet for all four compressors or just one.
Once that one is settled, you get to the question of 70k versus 80k and in order to answer definitively this you really need to do a dynamic simulation. . A blocked outlet would probably result in the compressor tripping and hence that flow peak wouldn't last long . Remember there will be an inherent lag in flows to the tip if you have significant volume in the flare system



#13 Propacket

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Posted 01 May 2014 - 05:46 PM

Certainly, all compressors can be exposed to a blocked outlet situation due to inadvertant closure of a valve in outlet header. So you must consider all compressors for blocked outlet. Someone can say that its not logical to see all the PSVs popping at exactly same moment because of slight differences in discharge pressure, discharge piping and PSV inlet piping. There may be some time lag between PSVs popping. But this has to be looked into very carefully.

Both blocked outlet and depressurization are credible scenarios for flare designing. One with the highest flow should be taken into account.




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