9 replies to this topic

[chemks2012]

Dear all,

 

I wish to design a pressure relief valve for a reactor having independent bursting disc both discharging to a vessel having open vent.

 

• The valve is designed for vapour release for external fire case.


• Set pressure is 2barg and design pressure of vessel is 6barg


• Overpressure allowed is 10% of set pressure


• For external fire case, the vapour release rate is 5000kg/hr and for this release rate, minimum area required is 1225mm2 [I have calculated this area using equation Kd A =  Q /(Po. C. SQRT(M/(Z.To)))]


• From API standard, I have selected orifice ‘L’ which equates the vent area of 1841mm2. I have back calculated the maximum capacity with this area and is about 7000kg/hr

 

I have few queries as below please.

 

1. I believe, the overpressure allowance should be 21% as the valve is designed for fire case. Do you think, we can design a valve with 10% overpressure for external fire case?


2. If I allow 21% overpressure, I believe, I need to limit the back pressure to 21% and not 10%. Please correct me if I am wrong.


3. Also to check the valve discharge line back pressure, I believe, I need to calculate pressure drop for the flowrate of 7000kg/hr and NOT 5000kg/hr. Please comment/correct.


4. What would be the total back pressure [i.e. superimposed + built up back pressure] ? For example the calculated pressure drop at the valve discharge line for the release rat of 7000kg/hr is 0.2bar and as the line is open to vessel with open vent, can I say the back pressure is only 0.2barg and is OK as it’s 10% of 2barg? Please comment.

 

Thanks in advance.

[chemks2012]

Hello Engineers!!!

 

Any help on this please?

[Bobby Strain]

Be patient. You can have a relieving pressure up to 21% greater than the vessel design pressure. Any pressure less is, therefore, OK. Seems like your objective is to prevent disk from bursting. So, you have lots of lattitude. It's like the old contractor joke: "cut to suit, beat to fit, paint to match".

 

Bobby

[fallah]

Hi,

 

My comments are the red color statements as follows:

 

 

Dear all,

 

I wish to design a pressure relief valve for a reactor having independent bursting disc both discharging to a vessel having open vent.

 

• The valve is designed for vapour release for external fire case.
• 
  
• Set pressure is 2barg and design pressure of vessel is 6barg
• 
  
• Overpressure allowed is 10% of set pressure
• 
  
• For external fire case, the vapour release rate is 5000kg/hr and for this release rate, minimum area required is 1225mm2 [I have calculated this area using equation Kd A =  Q /(Po. C. SQRT(M/(Z.To)))]
• 
  
• From API standard, I have selected orifice ‘L’ which equates the vent area of 1841mm2. I have back calculated the maximum capacity with this area and is about 7000kg/hr
• 
  

 

I have few queries as below please.

 

1. I believe, the overpressure allowance should be 21% as the valve is designed for fire case. Do you think, we can design a valve with 10% overpressure for external fire case? You can, but you are allowed to design based on 21% overpressure...
2. 
   
3. If I allow 21% overpressure, I believe, I need to limit the back pressure to 21% and not 10%. Please correct me if I am wrong. In your case you can limit the back pressure to 21%...
4. 
   
5. Also to check the valve discharge line back pressure, I believe, I need to calculate pressure drop for the flowrate of 7000kg/hr and NOT 5000kg/hr. Please comment/correct. If the valve isn't modulating POSV, you are right...
6. 
   
7. What would be the total back pressure [i.e. superimposed + built up back pressure] ? For example the calculated pressure drop at the valve discharge line for the release rat of 7000kg/hr is 0.2bar and as the line is open to vessel with open vent, can I say the back pressure is only 0.2barg and is OK as it’s 10% of 2barg? Please comment. Back pressure is OK...
8. 
   

 

Thanks in advance.

[latexman]

Can you describe this "open vent"?  From my experience, it could be anything from an open top tank to a 2" gooseneck vent.

[chemks2012]

 

Hi,

 

My comments are the red color statements as follows:

 

 

Dear all,

 

I wish to design a pressure relief valve for a reactor having independent bursting disc both discharging to a vessel having open vent.

 

• The valve is designed for vapour release for external fire case.
• 
  
• Set pressure is 2barg and design pressure of vessel is 6barg
• 
  
• Overpressure allowed is 10% of set pressure
• 
  
• For external fire case, the vapour release rate is 5000kg/hr and for this release rate, minimum area required is 1225mm2 [I have calculated this area using equation Kd A =  Q /(Po. C. SQRT(M/(Z.To)))]
• 
  
• From API standard, I have selected orifice ‘L’ which equates the vent area of 1841mm2. I have back calculated the maximum capacity with this area and is about 7000kg/hr
• 
  

 

I have few queries as below please.

 

1. I believe, the overpressure allowance should be 21% as the valve is designed for fire case. Do you think, we can design a valve with 10% overpressure for external fire case? You can, but you are allowed to design based on 21% overpressure... Thanks very much
2. 
   
3. If I allow 21% overpressure, I believe, I need to limit the back pressure to 21% and not 10%. Please correct me if I am wrong. In your case you can limit the back pressure to 21%...Did you mean - though I have designed the valve for 10% overpressure and can allow back pressure up to 21%? If so, will it not cause the valve to chatter? I am asking this because, I believe if I allow overpressure of 10%, I have to limit back pressure up to 10% and if I allow overpressure of 21%, I have to limit back pressure up to 21%.
4. 
   
5. Also to check the valve discharge line back pressure, I believe, I need to calculate pressure drop for the flowrate of 7000kg/hr and NOT 5000kg/hr. Please comment/correct. If the valve isn't modulating POSV, you are right...Thanks but it would be great if you please explain the logic of modulating POSV? I did not get what did you say here?
6. 
   
7. What would be the total back pressure [i.e. superimposed + built up back pressure] ? For example the calculated pressure drop at the valve discharge line for the release rat of 7000kg/hr is 0.2bar and as the line is open to vessel with open vent, can I say the back pressure is only 0.2barg and is OK as it’s 10% of 2barg? Please comment. Back pressure is OK... So back pressure is nothing but pressure drop in the discharge line. Please correct me if I am wrong. Now what if my total back pressure is 40% of set pressure? Is balanced bellows type relief valve is the solution? I guess balance bellows relief valve would also have some kind of limitation on back pressure but not sure on that.
8. 
   

 

Thanks in advance.

Hi Fallah,

 

Thanks very much for your reply and let me tell you - you are a gem and an asset to this forum.

I have commented in blue text against yours . It would we great if you comment/clarify some of doubts please.

 

Regards

Edited by chemks2012, 15 February 2014 - 04:13 AM.

[fallah]

Hi,

 

My second comments again in red color are as follows:

 

 

 

1. I believe, the overpressure allowance should be 21% as the valve is designed for fire case. Do you think, we can design a valve with 10% overpressure for external fire case? You can, but you are allowed to design based on 21% overpressure... Thanks very much
2.  
3. If I allow 21% overpressure, I believe, I need to limit the back pressure to 21% and not 10%. Please correct me if I am wrong. In your case you can limit the back pressure to 21%...Did you mean - though I have designed the valve for 10% overpressure and can allow back pressure up to 21%? If so, will it not cause the valve to chatter? I am asking this because, I believe if I allow overpressure of 10%, I have to limit back pressure up to 10% and if I allow overpressure of 21%, I have to limit back pressure up to 21%. Why are you allowed for 10% overpressure in fire case? Does the reactor designed per PD5500?
4.  
5. Also to check the valve discharge line back pressure, I believe, I need to calculate pressure drop for the flowrate of 7000kg/hr and NOT 5000kg/hr. Please comment/correct. If the valve isn't modulating POSV, you are right...Thanks but it would be great if you please explain the logic of modulating POSV? I did not get what did you say here? Supposing 5000 kg/hr is required relief rate, because a modulating POSV can open as required and doesn't pop up, 5000 kg/hr would be the base of pressure drop calculation...
6.  
7. What would be the total back pressure [i.e. superimposed + built up back pressure] ? For example the calculated pressure drop at the valve discharge line for the release rat of 7000kg/hr is 0.2bar and as the line is open to vessel with open vent, can I say the back pressure is only 0.2barg and is OK as it’s 10% of 2barg? Please comment. Back pressure is OK... So back pressure is nothing but pressure drop in the discharge line. Please correct me if I am wrong. Now what if my total back pressure is 40% of set pressure? Is balanced bellows type relief valve is the solution? I guess balance bellows relief valve would also have some kind of limitation on back pressure but not sure on that. If the total back pressure would be 40% of the set pressure a ballanced bellows PSV should be used. A ballanced bellows can withstand the back pressure almost up to 50% of the set pressure but with considering back pressure correction factor for PSV capacity...
8.  

 

[chemks2012]

 

Hi,

 

My second comments again in red color are as follows:

 

 

 

1. I believe, the overpressure allowance should be 21% as the valve is designed for fire case. Do you think, we can design a valve with 10% overpressure for external fire case? You can, but you are allowed to design based on 21% overpressure... Thanks very much
2.  
3. If I allow 21% overpressure, I believe, I need to limit the back pressure to 21% and not 10%. Please correct me if I am wrong. In your case you can limit the back pressure to 21%...Did you mean - though I have designed the valve for 10% overpressure and can allow back pressure up to 21%? If so, will it not cause the valve to chatter? I am asking this because, I believe if I allow overpressure of 10%, I have to limit back pressure up to 10% and if I allow overpressure of 21%, I have to limit back pressure up to 21%. Why are you allowed for 10% overpressure in fire case? Does the reactor designed per PD5500? Yes, it PD5500 vessel. Please let me know what is the significance and relation between PD5500 vessel and overpressure 10% relationship.
4.  
5. Also to check the valve discharge line back pressure, I believe, I need to calculate pressure drop for the flowrate of 7000kg/hr and NOT 5000kg/hr. Please comment/correct. If the valve isn't modulating POSV, you are right...Thanks but it would be great if you please explain the logic of modulating POSV? I did not get what did you say here? Supposing 5000 kg/hr is required relief rate, because a modulating POSV can open as required and doesn't pop up, 5000 kg/hr would be the base of pressure drop calculation... Thanks. It does make sense. But just out of curiosity, in what case you would see/recommend POSV?
6.  
7. What would be the total back pressure [i.e. superimposed + built up back pressure] ? For example the calculated pressure drop at the valve discharge line for the release rat of 7000kg/hr is 0.2bar and as the line is open to vessel with open vent, can I say the back pressure is only 0.2barg and is OK as it’s 10% of 2barg? Please comment. Back pressure is OK... So back pressure is nothing but pressure drop in the discharge line. Please correct me if I am wrong. Now what if my total back pressure is 40% of set pressure? Is balanced bellows type relief valve is the solution? I guess balance bellows relief valve would also have some kind of limitation on back pressure but not sure on that. If the total back pressure would be 40% of the set pressure a ballanced bellows PSV should be used. A ballanced bellows can withstand the back pressure almost up to 50% of the set pressure but with considering back pressure correction factor for PSV capacity... Thanks.
8.  

 

 

Thanks Fallah,

 

Sorry, my queries/comments in blue again.

 

Regards

[fallah]

Hi,

 

My comments in red color are as follows:

 

 

 

 

 

1.  
2. If I allow 21% overpressure, I believe, I need to limit the back pressure to 21% and not 10%. Please correct me if I am wrong. In your case you can limit the back pressure to 21%...Did you mean - though I have designed the valve for 10% overpressure and can allow back pressure up to 21%? If so, will it not cause the valve to chatter? I am asking this because, I believe if I allow overpressure of 10%, I have to limit back pressure up to 10% and if I allow overpressure of 21%, I have to limit back pressure up to 21%. Why are you allowed for 10% overpressure in fire case? Does the reactor designed per PD5500? Yes, it PD5500 vessel. Please let me know what is the significance and relation between PD5500 vessel and overpressure 10% relationship. As far as i know due to lower safety factor in PD5500 the maximum overpressure of the fire case for the vessels under such code, should be 10% of the set pressure...
3.  
4. Also to check the valve discharge line back pressure, I believe, I need to calculate pressure drop for the flowrate of 7000kg/hr and NOT 5000kg/hr. Please comment/correct. If the valve isn't modulating POSV, you are right...Thanks but it would be great if you please explain the logic of modulating POSV? I did not get what did you say here? Supposing 5000 kg/hr is required relief rate, because a modulating POSV can open as required and doesn't pop up, 5000 kg/hr would be the base of pressure drop calculation... Thanks. It does make sense. But just out of curiosity, in what case you would see/recommend POSV? Mostly for the cases in which there is a vast range of the releif loads for credible scenarios leading to significant difference between minimum and maximum loads...

 

 

 

 

[chemks2012]

Hi Fallah,

 

Thank you so much for all your help and efforts.

 

Regards, KS