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Psv Relief Sizing For Tank With N2 Blanketing System

psv hot oil cyclopentane

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#1 isidore

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Posted 16 February 2017 - 02:50 AM

Dear All, I am looking for pointers to design for PSV for Hot Oil Drain Tank with N2 Blanketing System. I am designing the PSV based on Fire case and Wetted vessel scenario, and I have made the following basis and would like to know if they are correct. During External Fire Scenario, hot oil tank and its contents (both Hot Oil and N2) that would be subjected to heat. How should I approach the sizing scenario for the PSV.

 

I) Should I size the PSV based on a wetted vessel (as it contains HOT Oil), and take into account the ONLY hot oil vapour generated due to heat, how about the N2 Gas Expansion?

 

II) I attempted to size it based on unwetted vessel , only taking into account of the N2 relieving, but I however think that this might not be appropriate and also I found that the Relieving Temperature exceeds the specified Wall Vessel Temperature (1100F) due to big difference between the N2 Blanket Operating pressure(1.08 BarA) and High PSV Set Point (4.5 BarG).

 

Or I should size for both scenario and take the worst case scenario? Please advise. I have summarized my key findings for both cases with the suggested orifice area for reference. Thank you Colin

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Edited by isidore, 16 February 2017 - 02:54 AM.


#2 farid.k

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Posted 16 February 2017 - 07:49 PM

Personally you need to check based on all cedible cases, full oil, full n2 and half oil.

#3 isidore

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Posted 16 February 2017 - 09:09 PM

Dear Farid,

 

I have performed 2 scenarios:

For Wetted Vessel Case (i.e. Hot Oil vaporization case)

I performed flash using Hysys to get the boiling point of Hot Oil 419 C at the relieving pressure i.e. 12.5 =barA [ PSV set P of 9.5 barG with 21% accumulation. ]

Relieving Temp is below Wall Vessel Temp, TW which is assumed to be 1100 F (593 C) at this point. 

For Unwetted Vessel Case (i.e. Nitrogen gas expansion case)

I have 1 major concern over the Unwetted Case calculations.

a) Exposed surface area of Vessel, A' - In reality my vessel is a wetted vessel with a column of vapour space of N2 sitting on top of the Hot Oil. Should I estimate the exposed surface area only of the N2 portion for my calculation, which is say the top half portion of the vessel only? 


For both cases, I get surprisingly similar orifice size which is 2H3 (with very different relief loads of course, one is N2 and the other being Hot oil vapour.

Thanks
Isidore



#4 shahidulislam48

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Posted 17 February 2017 - 05:22 AM

Dear isidore,

 

I am also facing same sort of problem.

 

The point farid had mentioned is quite helpful to pick up the worst case.

 

One thing I do not get is that is it allowable to change the set pressure of PSV if it is fixed one time?

Is PSV's set pressure is 4.5 barg or 9.5 barg?

 

As far as I can remember, Naser Fallah had replied in a post regarding unwetted area having higher relieving temperature than wall temperature. He had recommended to take the difference of wall T and relieving T as 0.01 if relieving T is higher than wall T. But that will lead to very low orifice area (in my case).

I would like you to look at that thread if you can (unfortunately I could not provide you the link as I had not collected it)

 

Thanks

Shahidul


Edited by shahidulislam48, 18 February 2017 - 02:57 AM.


#5 isidore

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Posted 17 February 2017 - 09:32 PM

Dear Shahidul,

 

Thanks for your interest.

 

I apologies for the confusion over the SetP, I was sizing for a multiple tanks here, just let's say Let's just assume 9.5 barG is the one for further discussion.

 

I have another query though, which would normally be the worst case scenario,

a) Wetted Vessel - Vaporization of Liquid or

B) Unwetterd - gas expansion.

 

My calculation tend to suggest Case B where Nitrogen Gas Expansion Case would require larger orifice albeit a lower relief load than that of Case A Hot Oil Evaporation .

 

This case even apply for my other calculation for  Cyclopentane and N2 System too, whereby Cyclopentane has an even lower bubble point than Hot Oil, where the rate of evaporation of Cyclopentane should be higher than Hot Oil - I know there' s a whole lot of factors contributing to this, just looking for a thumb rule.

 

 

With respect to Fallah's post about the Twall - Yes, I agree. I came across the following points ad would like to verify them after going through lots of online postings. 

 

a) The relieving temperature is a lot higher than wall vessel, however there is possibility of overheating of wall vessel beyond 1100 F since flame temperature could reach 1500 -2000 F.

 

B) However, we don't design vessel (or in this case the Design temp of PSV, I assume) for Fire Case Relieving Temperature because there should be the system like fire fighting, depressurizing to prevent the vessel from reaching such temperature.

 

c) There is no need to reduce the set P for the PSV just to accommodate the F' Value. 

 

 

Thanks

 

Isidore


Edited by isidore, 17 February 2017 - 09:33 PM.


#6 shahidulislam48

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Posted 18 February 2017 - 01:16 AM

Dear isidore,

 

According to your last reply what should be the provision for selecting PSV; larger area or larger relief load?

 

In PSV's data sheet there is no provision for wetted or unwetted vessel. All you need is to mention the relief load and the phase of relieving fluid to calculate the size of orifice. Of course, lower relief load results in lower PSV otherwise it is mentioned that process fluid in the vessel is in gaseous phase and sizing basis is fire case. That would be an clear indication for selecting unwettwd vessel equation. Then there will be no significance of relief load in the case of unwetted vessel. It is important to recheck the rated capacity of vendor's PSV for the validity of our required relief.

 

But major problem arises when there is N2 and process liquid in the vessel. If N2 blanketing layer is too thin (assume 90% height of vessel is liquid and 10% is N2) then colossal amount during fire case would be contributed by the vaporization of fluid rather than expansion of N2.

 

Best Regards,

Shahidul



#7 shahidulislam48

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Posted 18 February 2017 - 03:29 AM

Dear isidore,

 

In your uploaded doc, why Relieving P for  "Case A" is different from "Case B"? Shouldn't they be the same?

 

I really want to give a hand on the calculation.

 

Best Regards,

Shahidul



#8 isidore

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Posted 19 February 2017 - 03:03 AM

Dear Shahidul,

 

My opinion is that the larger PSV area would have priority, however please feel free to comment if I have missed out anything. 

 

In almost  all of my calculations, it seems that N2 Expansion would give the worst case i.e. Larger area however a lower relief load than that of Hot Oil or Cyclopentane (due to MW difference I think). 

 

NOTE: Only in the mentioned example above and you can see in my revised document that, surprisingly both cases share the same PSV size (which has already been questioned by some in other forums). I wonder if the difference in relief load and of different chemicals would have any impact on the PSV selection criteria. 


 

In PSV's data sheet there is no provision for wetted or unwetted vessel. All you need is to mention the relief load and the phase of relieving fluid to calculate the size of orifice. Of course, lower relief load results in lower PSV otherwise it is mentioned that process fluid in the vessel is in gaseous phase and sizing basis is fire case. That would be an clear indication for selecting unwettwd vessel equation. Then there will be no significance of relief load in the case of unwetted vessel. It is important to recheck the rated capacity of vendor's PSV for the validity of our required relief.

 

 

Agreed with the above statement. 

 

But major problem arises when there is N2 and process liquid in the vessel. If N2 blanketing layer is too thin (assume 90% height of vessel is liquid and 10% is N2) then colossal amount during fire case would be contributed by the vaporization of fluid rather than expansion of N2.

 

Yes, I have doubts over that too. There seems to be no in between method (2 phase is not considered in this scenario, I think). I have seen some perform dynamics over this situation, which I think is a great way to fully understand the actual scenario. I am however less inclined to go to that extent at this stage. The fundamental question is during pressure relieving scenario, what actually is happening. We are treating them as separate cases, but in reality I would more inclined to think that Expansion P of N2 would suppress the other liquid e.g. Hot Oil/ Cyclopentane from bubbling , and hence N2 Expansion might take precedence. 

 

Another point to ponder, the N2 compartment on top of the liquid. How do you related the exposed surface area/volume of it for expansion cases. Do you take only the top portion surface area? 

 

Please also find the revised document for your reference, regarding the different relieving Pressure for Case B. It should be the same for both case, and it was my original mistake to lover the set P in order to avoid relieving Temp from exceeding Wall Temp.

 

Thanks again,

isidore

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