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# Fire Case Wetted Area Calculation For Vessel

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### #1 shahidulislam48

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Posted 13 September 2017 - 12:47 AM

Hello,

I have already seen some methods from previous posts for vertical vessel wetted area calculation in Fire Case for PSV relief load calculation.

But when I have gone through "Liptak, Bela G.-Instrument Engineers' Handbook, 4th Edition, Vol.1"- Chapter 7, Section 7.15, I have found different formula for wetted area calculation. Which leads to higher wetted area comparing to the methods that have been mentioned in this forum.

Could anyone help me by suggesting which one should I consider and what is the basis for wetted area calculation mentioned in Liptak, Bela G.-Instrument Engineers' Handbook?

I have attached a screenshot for your clarification.

Wetted Area.jpg   172.42KB   23 downloads

Thanks in advance.

Shahidul

### #2 fallah

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Posted 13 September 2017 - 06:38 AM

But when I have gone through "Liptak, Bela G.-Instrument Engineers' Handbook, 4th Edition, Vol.1"- Chapter 7, Section 7.15, I have found different formula for wetted area calculation. Which leads to higher wetted area comparing to the methods that have been mentioned in this forum.

Could anyone help me by suggesting which one should I consider and what is the basis for wetted area calculation mentioned in Liptak, Bela G.-Instrument Engineers' Handbook?

I have attached a screenshot for your clarification.

Shahidul,

There is no substantial difference between the methods you mentioned. In fact, the basis for wetted area calculation in attached method is adding the area of bottom elliptical head to the area of wetted shell side to obtain total wetted area.

Nevertheless if there would be a little bit difference between methods; the subsequent effect, if any, on relevant relief load won't result in any anxiety.

The difference between NFPA and API methods is due to higher degree of conservation in NFPA method and the owner of the relevant equipment had been already accepted the consequence(s) of selected method at the stage of the applicable codes and standards selection.

Edited by fallah, 13 September 2017 - 07:05 AM.

### #3 shahidulislam48

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Posted 14 September 2017 - 03:39 AM

But when I have gone through "Liptak, Bela G.-Instrument Engineers' Handbook, 4th Edition, Vol.1"- Chapter 7, Section 7.15, I have found different formula for wetted area calculation. Which leads to higher wetted area comparing to the methods that have been mentioned in this forum.

Could anyone help me by suggesting which one should I consider and what is the basis for wetted area calculation mentioned in Liptak, Bela G.-Instrument Engineers' Handbook?

I have attached a screenshot for your clarification.

Shahidul,

There is no substantial difference between the methods you mentioned. In fact, the basis for wetted area calculation in attached method is adding the area of bottom elliptical head to the area of wetted shell side to obtain total wetted area.

Nevertheless if there would be a little bit difference between methods; the subsequent effect, if any, on relevant relief load won't result in any anxiety.

The difference between NFPA and API methods is due to higher degree of conservation in NFPA method and the owner of the relevant equipment had been already accepted the consequence(s) of selected method at the stage of the applicable codes and standards selection.

Thank you Sir for your concern.

But whenever I have calculated wetted area by using attached method for a same vessel, it leads to higher wetted area comparing to the frequently used method. Hence we get higher relief load and bigger PSV.

### #4 latexman

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Posted 14 September 2017 - 08:03 AM

Notice the article starts off, If NFPA or API.  If you cannot logically, reasonably, and legally determine which applies to the specific application in it's local jurisdiction, simply drill up into your companies pertinent management structure (start with your supervisor), and ask which standard your company follows in that case.  If no answer, use your engineering judgment and document the basis.

### #5 fallah

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Posted 14 September 2017 - 11:16 AM

Thank you Sir for your concern.

But whenever I have calculated wetted area by using attached method for a same vessel, it leads to higher wetted area comparing to the frequently used method. Hence we get higher relief load and bigger PSV.

Wetted area in either method cannot be so different and what could be the source of difference is the effective part of wetted area which arises from the difference in effective height of fire from flame source in API (25 ft) and NFPA (30 ft)...

### #6 rakesh jain

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Posted 15 September 2017 - 12:42 AM

Can any one suggest me how to calculate nonwetted surface area of different vessels?

### #7 latexman

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Posted 15 September 2017 - 06:42 AM

rakesh,

Please don't "hijack" another post.  Make a new one with a title describing your question.

### #8 shahidulislam48

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Posted 15 September 2017 - 11:42 PM

Thank you Sir for your concern.

But whenever I have calculated wetted area by using attached method for a same vessel, it leads to higher wetted area comparing to the frequently used method. Hence we get higher relief load and bigger PSV.

Wetted area in either method cannot be so different and what could be the source of difference is the effective part of wetted area which arises from the difference in effective height of fire from flame source in API (25 ft) and NFPA (30 ft)...

I have performed wetted area calculation for vessel using both the wetted area methods including my attached one. Later method provides higher surface area.

Please have a look on the attached file.

V-1.jpg   37.09KB   7 downloads

### #9 shahidulislam48

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Posted 15 September 2017 - 11:44 PM

Notice the article starts off, If NFPA or API.  If you cannot logically, reasonably, and legally determine which applies to the specific application in it's local jurisdiction, simply drill up into your companies pertinent management structure (start with your supervisor), and ask which standard your company follows in that case.  If no answer, use your engineering judgment and document the basis.

Does it matter whether I use NFPA or API on the wetted area calculation except fire height?

### #10 flarenuf

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Posted 16 September 2017 - 08:42 AM

hi

This link might help you in your decision

http://www.eng-tips.....cfm?qid=303990

also note that some companies do not take account of the bottom head wetted area on the basis that the vessel skirt will prevent fire impingement on the head

You need to base your decision on your company/client standards

### #11 Bobby Strain

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Posted 16 September 2017 - 12:05 PM

Your second formula is incorrect. You need to apply a bit of understanding and not use formulas that are not correct. There is probably some fine print that you failed to read or comprehend. Surely you know how to calculate the area of a cylinder with given diameter and height. It's that simple.

Bobby

### #12 breizh

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Posted 17 September 2017 - 04:44 AM

Hi ,

Chapter seven should support your query .

Good luck

Breizh

### #13 shahidulislam48

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Posted 19 September 2017 - 01:06 AM

Hi ,

Chapter seven should support your query .

Good luck

Breizh

Thank you Sir for  your concern.

Although this procedure, I have studied this before, provide pretty good idea but it does not contain calculation for Vertical/Horizontal vessel with elliptical head.

### #14 shahidulislam48

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Posted 19 September 2017 - 01:11 AM

Your second formula is incorrect. You need to apply a bit of understanding and not use formulas that are not correct. There is probably some fine print that you failed to read or comprehend. Surely you know how to calculate the area of a cylinder with given diameter and height. It's that simple.

Bobby

Thank you Sir for your concern.

Yes, it is pretty easy to calculate wetted area. But whenever I have got different method for the same calculation, it is natural to get confused.

I don't know too much how the second formula come about or it is incorrect, but I wonder, Liptak, Bela G.-Instrument Engineers' Handbook could contain such kind of error!

### #15 shahidulislam48

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Posted 19 September 2017 - 01:12 AM

hi

This link might help you in your decision

http://www.eng-tips.....cfm?qid=303990

also note that some companies do not take account of the bottom head wetted area on the basis that the vessel skirt will prevent fire impingement on the head

You need to base your decision on your company/client standards

pdf link doesn't work. Do you have another link?

### #16 breizh

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Posted 19 September 2017 - 09:44 AM

Hi,

https://checalc.com/...fireLiqPSV.html

you may find what you are looking for there .

Breizh

Note : It seems to me that no one is willing to do the math , just press a button to get a result , Engineering is about hard work !

### #17 shahidulislam48

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Posted 19 September 2017 - 10:29 PM

Hi,

https://checalc.com/...fireLiqPSV.html

you may find what you are looking for there .

Breizh

Note : It seems to me that no one is willing to do the math , just press a button to get a result , Engineering is about hard work !

Thank you Sir.

I would try to compare different values of wetted area by different methods.

### #18 breizh

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Posted 20 September 2017 - 05:54 AM

additional info .

Breizh