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Pressure Relief Atm Dispersion Of Flammable/toxic Gases - Mist Formati

psv dispersion toxicity safe relief api

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#1 Xenolithian

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Posted 22 November 2023 - 04:08 PM

Hello, 

I am working on developing a tool for evaluating "safe relief" conditions for relieving scenario and am seeking information about predicting misting conditions/ coalescing vapor droplets. In this particular case I am concerned with phenolics (Phenol, Bisphenol, Xylenols) and how they will behave during different relieving scenarios. For example a phenol relief during a fire case will be at ~T,P of the boiling point at the relieving pressure which should be well above amb conditions. Then the gas will be superheated at lower pressure (albeit slightly cooled by Joule Thompson effects) at PSV outlet and will then drop in pressure/ cool slightly more as it approaches PSV tailpipe exit. Upon exit the vapor will mix with ambient air and assuming jet velocity conditions prevail under circumstances outlined in API 521, air will be entrained into the relieving stream and rapidly dilute and cool the phenol vapors. I believe due to phenol's relatively high boiling/freezing points there is a likelihood of misting/ solidification that may occur at some point through this process. I am seeking guidance on sources used to predict this behavior, or guidance on how such a system would behave. I would like to be able to predict dispersion for evaluating toxicity/ flammability parameters.

 

I am open to any recommendations and can further clarify my question and/or provide additional information upon request. I realize this may be verbose, I am working on wrapping my head around the scenario in question myself.



#2 Pilesar

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Posted 22 November 2023 - 06:23 PM

Phenol is not a material I would want raining down on me. My first inclination would be to heat the entire relief route and send this to a scrubber.



#3 breizh

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Posted 22 November 2023 - 09:19 PM

Hi,

Regarding Phenol (10 years' experience) and freezing point # 40 C, I got a PVRV frozen during wintertime and a Storage tank collapsed due to lack of tracing on the safety valve combined with the very low temperature of N2 Blanketing.

Nasty product, severe burns.

Breizh 



#4 shvet1

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Posted 23 November 2023 - 12:11 AM

Some mist / solidification always present at some extent as relief system temperature is colder than relief temperature. Correct?

The question is - is relief system able to handle such or not? Describe your case in details.

See para. 5.8.2.3  API 521 for details.

 

API 521-2020

 

5.8.2.3 Mist Emission

...

Loudon [111] gives a method of calculating whether condensation of a discharge from a PRV can occur. These calculations indicate that most emissions do not condense, regardless of relative molecular mass, although relatively heavy molecular mass hydrocarbons can condense in the range previously noted (10 μm to 20 μm). This approach is supported by experience with refinery relief installations involving discharge to the atmosphere of vapor streams covering a wide range of conditions.

...

D. E. Loudon, “Requirements for Safe Discharge of Hydrocarbons to Atmosphere,” API Proceedings, Volume 43, 1963, pp. 418–433

 

CCPS' Guidelines for Pressure Relief and Effluent Handling Systems 2nd ed.

 

1.5.3 Problems Inherent in Pressure Relief and Effluent Handling Systems

There are many uncertainties involved in designing pressure relief and effluent handling systems to attain system reliability. Many pressure relief and effluent handling systems have been constructed on the basis of available information and technology. These have performed satisfactorily with few reported problems. Designers of pressure relief and effluent handling systems should be aware of the uncertainties involved and provide for back-up measures in preparation for contingencies in applications that involve highly hazardous materials. Specialists in the design of effluent handling equipment should be consulted. Experimental validation or testing the design on a large scale should be considered for critical applications.

...

The technological and experimental foundation for designing and predicting the performance of effluent handling equipment under emergency relief conditions is sometimes limited. High inlet velocity of flashing vapor-liquid mixtures can generate hard-to-separate small liquid droplets which may not be collected by a cyclone or gravity separator. If the effluent from the separator is going to a flare or some other treatment device, the discharge of smaller droplets may be of little consequence. If the effluent from the separator is hazardous, additional treatment will be required before discharge to the atmosphere.
...

Edited by shvet1, 23 November 2023 - 12:11 AM.


#5 Xenolithian

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Posted 27 November 2023 - 02:03 PM

Hello all,

Thank you for your replies I appreciate the time you took to answer.

 

Pilesar - I agree that phenol/ phenolics in general are nasty chemicals and you would definitely not want any droplets raining down on you. I agree with the recommendation for using a scrubber or other effluent processing. 

 

Breizh - The low temperature N2 blanket causing a PRV to freeze up is an interesting scenario, what precautions were taken for future prevention? With a properly traced PRV did you have any issues with plugging due to the cold N2? We routinely see low temps in the winter below -10°F and any heat tracing failure causes freezeups incredibly easily on phenolics and I'm curious if there are more precautions that should be taken to increase safety.

 

shvet1 - Do you know of any location I can find a copy of the Loudon reference? I have searched the web and API's webstore and have been unable to find the specific reference in API 521 (to be clear I am not asking for to avoid copyright etc, I simply cannot find this resource anywhere).

 

As for specific cases, I am working on developing a relief standard for my client to be applicable for multiple process chemistries etc and I am working on developing a safe relief template to determine safe relief conditions. This involves qualitative assessments and quantitative analysis of relief situations with more complex dispersion analysis as necessary (if API 521/ other general relief guidelines for relieving fluid and relief quantity/ velocity are not met). I reread the section of API 521 (5.8.2.3 Mist Emission) regarding mist as well and it seems to me that the mist can be treated as a vapor unless the droplets formed are of large enough size to coalesce and fall faster than a mist would be expected to. I'm imagining if I can evaluate the Loudon criteria for condensation, then evaluate coalescing properties for condensing vapors to determine if the relief should be considered as a vapor or coalescing liquid droplets.



#6 breizh

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Posted 27 November 2023 - 06:42 PM

Hi,

A few things were done after the damage of the tank, including the electrical tracing downstream the N2 Pressure reducing valve till the inlet nozzle, the replacement of standard PSV by a steam jacketed PSV.

Breizh 



#7 Bobby Strain

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Posted 27 November 2023 - 07:03 PM

You need to be sure your company lawyer reviews what will be sent to your client. She will make sure you are not leaving your company with liabilities.

 

Bobby



#8 shvet1

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Posted 27 November 2023 - 10:58 PM

shvet1 - Do you know of any location I can find a copy of the Loudon reference? 

No

 

... I can evaluate ... condensation, then evaluate coalescing properties for condensing vapors ...

No, you can't. At labaratory scale - an evaluation having low level of cinfidence is possible. At industry scale - such is unable to be predicted, only experience is relevant. Because of this fact

CCPS' Guidelines for Pressure Relief and Effluent Handling Systems 2nd ed.

Experimental validation or testing the design on a large scale should be considered for critical applications.

How are you going to conduct such validation/testing?

 

Some guidance means some responsibility and repeatability. Are you (your employer) ready to take responsibility (personal, legal, or informal) for losses caused by unchecked / unreliable / unsafe design guided by a doc you are issuing? Why does a user / designer need so much time&efforts to find methods for mist relief? Are your client ready to industry scale testing on his assets / business / reputation? Does this particular overpressure scenario require an expensive and risky industry scale testing or it is able to be treated by a cheap and industry accepted method? 

 

As for me - only industry accepted and widely spread practices are tolerable. All those things called Inherently Safer Design. 


Edited by shvet1, 27 November 2023 - 11:26 PM.


#9 Xenolithian

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Posted 28 November 2023 - 09:37 AM

Hello all,

Thank you for your guidance and advice. Point taken on liability and ensuring that any document I issue doesn't leave me/ my company responsible for over simplifying anything. I'm leaning towards a general checklist of qualitative criteria and general guidelines for acceptable (API 521, CCPS, etc) rules of thumb for safe dispersion criteria and referring to a dispersion specialist for anything more complex/ outside those criteria.

 

Thank you for your time.






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