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Psv Orifice Selection

psv orifice selection

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#1 ChemiFreak

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Posted 30 July 2014 - 11:56 PM

Dear all,

 

In PSV sizing, the required orifice area that I calculated is 1.282 square inches . Could I go with "J" orifice which has an area of 1.287 square inches according to API-526 or should I go with "K" orifice since the ratio between required orifice area (1.282 square inches) and selected orifice area (1.287 square inches) is more than 99%. Please clarify me.


Edited by ChemiFreak, 30 July 2014 - 11:56 PM.


#2 fallah

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Posted 31 July 2014 - 12:47 AM

ChemFreak,

 

You can use "J" orifice as preliminary orifice selection. But after actual orifice selection from selected vendor you should ensure 0.975*1.287 (KAPI* "J"Area) to be equal to or lower than Kvendor*Areavendor. It is to be done for checking if actual K*A would handle the required relief load...



#3 breizh

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Posted 31 July 2014 - 12:49 AM

Hi ,

"The actual discharge area should be equal or greater that the calculated required effective area " quote from Selection and sizing of pressure relief valve from  Randolph Whitesides , P.E . This makes sense to me .

 

Breizh



#4 ChemiFreak

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Posted 31 July 2014 - 01:06 AM

Thanks Fallah and Breizh for the clarification.



#5 flarenuf

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Posted 31 July 2014 - 07:11 AM

Just to put things into perspective here as well in regard to the " i am at 99% capacity concern"

 

1) if the relief case is a full flow blocked outlet then being at capacity means that if there is a possibility of the "full flow" increasing in the future ( due to an increase in throughput of the plant) this PSV will be too small and will need upgrading. so that could be  areason to put in a larger valve now.

 

2) if this is a fire case then unless the composition of the contents change dramatically in the future  the relief load is stable. So the valve is OK.

 

3) When you model fire cases  using dynamic simulation , say Hysys or VMg et al  you  size the PSV based on the peak pressure
and calculate say a 32.55mm diam  port required . Now a J is 32.50 , so thats too small according to the code!
next up is a K orifice at 38.80

if we use a J orifice  what happens ?  the valve cannot pass the required load at the relief pressure and the vessel pressure rises a little above the 121% MAAP,  to 121.5%  maybe just for a few seconds.  Is that unsafe ?
Don't get me wrong here guys  I am not advocating that engineers regularly recommend the installation of PSV's that are too small !!

Its just that in this day and age of computers and 8 decimal places and, I feel, a lack of practical thinking  engineers adhere to the recommended values too much.

recommended BP on a conventional PSV valve 10%  i calculate 10.6%  at peak flow for 5 seconds is that a problem?

the depressuring takes 16 minutes , API says 15 mins , we need to redesign...  do we

as far as i am aware the 15 mins is an arbitrary number that was originally a guideline by API to use.

It now seems to be a standard number to stick to or perish..

 

ramblings of an old engineer maybe but a concern i think that the  old " ..thats near enough to 1 decimal place" philosophy is dying

 

critical thoughts from others welcome :-)



#6 Marc-Andre Leblanc

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Posted 31 July 2014 - 03:53 PM

Hello,

To the comments of flarenuf, I would add that most pressure vessel are required to be hydrotested at 30% overpressure, or even more depending on design temperature and material of construction as per ASME sec. VIII code.

 

Also, to avoid chattering and other problems, API recommand that pressure drop in the inlet piping of a conventionnal PSV should be kept bellow 3% of the set pressure (API 520 , part II, sect 4.2.2). This pressure drop is calculated at the rated capacity of the installed PSVA K orifice would have significantly larger rated capacity than a J orifice, and to respect this pressure drop limit, could require larger pipe diameter or even the use of a "Piloted" PSV with sence line to the pressure vessel.

 

Regards,

Marc-Andre


Edited by Marc-Andre Leblanc, 31 July 2014 - 04:12 PM.


#7 ChemiFreak

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Posted 31 July 2014 - 10:36 PM

Thanks Flarenuf and Marc-Andre Leblanc for enlightening me on this topic.

 

The PSV which I am concerned is sized for Reverse flow case (which is the governing case) for a compressor package.

 

Thanks again to everyone for the contribution.



#8 fallah

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Posted 01 August 2014 - 12:27 AM

ChemFreak,

 

Actually, to get an effective result about the ability of "J" orifice to handle the required relief load, you should compare Keff*Acalculated (0.975*API 526 orifice Area Designation) with the KActual*AActual (from selected vendor) rather than comparing the area alone. It might the retio of Keff*Acalculated to KActual*AActual be lesser enough than 0.99; then using "J" orifice would be OK, otherwise if the ratio is close to one and if there would be the concern of an increase in throughput it's better using "K" orifice but in modulated pilot operarted valve due to possibility of chattering...



#9 ChemiFreak

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Posted 01 August 2014 - 03:19 AM

Thanks Fallah.

 

Ill check once the vendor data is available.



#10 Mrinmayee Marathe

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Posted 19 December 2015 - 09:59 AM

Dear ChemiFreak,

 

As everyone has already said, please check for required orifice area with actual coefficient of discharge and actual area of "J" and "K".

 

However, please also check if the client requirement allows the use of actual Kd and actual area.

 

Some clients specifically ask that the PSV sizing and selection shall be done with effective coefficient of discharge and effective area as per API 526.

 

In such case, it is advisable to go for "K" orifice since slight change in any process parameter may land you in requirement of higher orifice designation.



#11 S.AHMAD

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Posted 24 December 2015 - 05:05 AM

Hi there,

After giving a deep thought of your problem, I believe the proposal by Breizh makes sense that you should considered.  In addition consider following points before making any decision:

1. In sizing the PSV we are using the recommended 10% over pressure. However, in real practice, we are allowed to operate 1.5xMAWP or the hydrotest pressure whichever is lower, for a short duration. One company that I have experienced with, giving a guideline that 30% higher than MAWP is allowed for a short duration no more than 10 hours. Before deciding on this kind of option, please check the latest ASME code of pressure vessel.

2. Based on actual observation on actual operating plant, popping of PSV does not happened frequently. Majority of PSVs installed never pop until the end of the plant life. This is due to the fact that PSV is the 5th layer of protection in any LOPA that we normally considered. The other 4 Layers as below:

1st layer - Process design

2nd layer - basic process control

rd layer - alarms and human intervention

4th layer - safety instrumented system/function

5th layer - physical type such as PSV.

 

We may conclude that PSV is the LAST line of defense

 

Hope the above sort comments help.

 

s.ahmad

.



#12 Bobby Strain

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Posted 24 December 2015 - 02:04 PM

You should let the valve vendor choose the proper valve according to your requirements. Unless the relief situation is unusual, like 2-phase or flashing flow. You should never expect any action to avoid relief. Presume it is your only protection.

 

Bobby






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