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Residence Time Dependence On Vessel Outlet Nozzle


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#1 lavlysuresh@outlook.com

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Posted 22 March 2023 - 02:22 PM

I have a query regarding Vessel/Tank purging time calculation with air blower 800m3/min for 12 cycles/volumes of purging required to make it safe for vessel entry... Can someone guide... For example Blower is fixed at 18" Manway of a separator with 20m3 volume and outlet nozzle available is 6"... Whether varying outlet nozzle size, increases or decreases the time required to finish 12 cycles of purging??

#2 Pilesar

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Posted 22 March 2023 - 08:39 PM

The air flow rate is what matters. With a vessel that has 20 m3 volume, it would take 18 seconds to purge it with 12 vessel volumes of air if the air flow rate is 800 m3/min. Nozzle size would only matter if it affected the air flow rate.



#3 Bobby Strain

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Posted 22 March 2023 - 10:28 PM

When I was roaming around plants during shutdown maintenance, we were using eductors, not blowers.

 

Bobby



#4 breizh

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Posted 22 March 2023 - 11:55 PM

Hi,

To me this is wrong, take the necessary time to purge your vessel and check the O2 content with a calibrated sensor prior to enter the vessel.

Breizh 



#5 james.corrobro@gmail.com

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Posted 24 March 2023 - 03:05 PM

Agree with breizh's comment 



#6 lavlysuresh@outlook.com

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Posted 27 March 2023 - 02:58 AM

API RP 2016 Explains about purge cycles calculation. It says 5 purge volumes of vessel is sufficient until any specific requirement. Solved examples given in it. A table in page 24 shows air CFM requirement based on outlet nozzle sizes. Can someone help me with the calculation of CFM and SCFM in it.

Edited by lavlysuresh@outlook.com, 27 March 2023 - 03:01 AM.


#7 breizh

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Posted 27 March 2023 - 04:33 AM

Hi,

I will not change one word on my previous reply to this thread.

You need to refer to your confined space entry procedure and confined space entry permit you must have in place before thinking to have someone entering a vessel. Consult your SHE engineer /manager who is supposed to approve the work prior to start. In other words, your vessel must be fully isolated with relevant means (blinds), the vessel must be purged with the necessary volume of air and the atmosphere controlled to ensure you don't put the personnel at risk during the intervention. A permit with all relevant signatures must be issued. The O2 content must be monitored continuously from start to completion of the work with a calibrated sensor and someone from outside the vessel must witness the progress of the work and the safety of the operation.

O2 must be always between 19 and 21 % 

Please don't rely on a standard and calculation, life could be at risk.  

 

 

Breizh 

 

 



#8 lavlysuresh@outlook.com

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Posted 02 April 2023 - 04:13 AM

Thank you Mr.Breizh for your insights. I do agree 100% with your statements and that's the industrial practice we follow for Confined Space Entry. There will be no calculation valid until required concentration of oxygen 19-21% and no toxics present inside any vessel before entry. Thank you for the attachment on purging.
But the initial question which I asked araised from an audit, where the audit requirement says us to calculate the time required to complete 12nos air purge cycles with respect to outlet flange size for all vessels. We calculated time for 12cycles for individual vessel volumes but they asked us to calculate based on outlet flanges or manways open while using 800m3/min blower. We got stuck with it and referring calculations. Pls help in any chance.

#9 latexman

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Posted 02 April 2023 - 04:56 AM

This is a fluid flow problem.  You have a system - inlet, blower, 18" manway, 20 m3 separator, 6" outlet nozzle, and exit. I recommend you study/consult Crane's Technical Paper No. 410.  If you do not own a copy, buy one.  Crane's TP 410 will show you how to characterize your system, construct a system curve onto the blower's flow curve, and find the resulting flow.

 

Quick calcs:

 

Mach 1 = 343 m/sec.

 

800 m3/min = 13.3 m3/sec and 6" nozzle = 0.0182 m2 flow area, so 13.3 m3/sec / 0.0182 m2 flow area = 731 m/sec.  731/343 = 2.13.  Mach 2.13 is impossible!

 

The resulting flow will be much, much less than 800 m3/min through a 6" outlet nozzle, which is the most restictive  component of your system, by far.  So, what would be the flow across just a 6" tank nozzle (inlet loss, pipe friction of length = to the extension of your nozzle, and an exit loss) with pressure drop = the deadhead (no flow) pressure of the blower?  This should be a good approximation of the flow for your system.



#10 breizh

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Posted 02 April 2023 - 07:01 AM   Best Answer

Hi ,

To support your work on unit conversion take a look at the doc attached: ACFM vs SCFM

 

Latexman is absolutely right about his comment on Flow vs  Nozzle size .

 

https://www.grc.nasa...ane/mflchk.html

Edit: 

With a air pressure of 5 bars at 20 C, I got a max flow rate of 21 kg/s for a 6 " nozzle. The flow rate should be somewhere between 15 to 20 kg/s.

Good luck

Breizh



#11 lavlysuresh@outlook.com

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Posted 08 April 2023 - 08:55 PM

Thank you @Latexman and @Briezh for your inputs. I will take up required calculations and get back if I have further queries.

Edited by lavlysuresh@outlook.com, 08 April 2023 - 09:01 PM.





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