Sir...i am a process engineer with just 2 years experience.....now i have come across a work in blowdown during fire exposure...........................i have some basic questions:

1)Why do we need to put an restriction orifice in the downstream of the blowdown.........isnt this opposite to the purpose why we have given a blowdown valve(to decrease the pressure in a very little time)?

2)Why cant we have a PCV instead of a blowdwon valve?

3)Isnt this a dynamic condition whereby you are relieving the pressure and the inlet pressure keeps on decreasing and the flowrate keeps changing.....then how to fix the RO size?

Are there any literatures published.

Plz help me out


Vinod

vinod,

I am sure many Che Jedi can answer these question. Let me goes first...



To limit flow rate to avoid high blowdown load (high flare capacity), high velocity leads to severe vibration, temperature get too low causing material problem, etc BUT within a reasonable time (e.g. 15 min per API Std 521).



You may subject to 2 major unwanted scenarios. PCV failed OPEN leads to high blowdown load. PCV failed CLOSE (when it is required) defeat blowdown intention.




As mentioned earlier, blowdown rate shall be fixed such that accumulated emergency blowdown time shall be limited to reasonable time (e.g. 15 min) and from initial pressure to safe pressure (e.g. 50% of design pressure or 6.9 barg subject to conditions as stated in API Std 521).



There are many previous discussion on this issue in this forum. Please conduct a search. Also try out Google.

There are many articles related to blowdowns...example :

• Rapid depressurization of pressure vessels, Journal of Loss Prevention in the Process Industries, Volume 3, Issue 1, January 1990, Pages 4-7, Afzal Haque, Stephen Richardson, Graham Saville and Geoffrey Chamberlain.
• Modelling of two-phase blowdown from pipelines—I. A hyperbolic model based on variational principles, Chemical Engineering Science, Volume 50, Issue 4, February 1995, Pages 695-713, J. R. Chen, S. M. Richardson and G. Saville.
• Modelling of two-phase blowdown from pipelines—II, A simplified numerical method for multi-component mixtures, Chemical Engineering Science, Volume 50, Issue 13, July 1995, Pages 2173-2187, J. R. Chen, S. M. Richardson and G. Saville.
• A numerical blowdown simulation incorporating cubic equations of state, Computers & Chemical Engineering, Volume 23, Issue 9, 1 November 1999, Pages 1309-1317, Haroun Mahgerefteh and Shan M. A. Wong.
  
  and many others

You may read some articles related to DEPRESSURING

JoeWong

Sir.....thanks a lot........

1)Cant we have a fail safe PCV..................

2)Also regarding the low tempertaures produced by Joule Thomson expansion............will it be in the range of sub zero temperature...............

3)How can we size the RO....since it gives a fixed pressure drop(based on an upstream pressure)........but here we are having a change in pressure continuously..................

Controller failure could leads to PCV FULL OPEN or FULL CLOSE position. It is credible scenario.



Yes. Subject to initial Pressure and temperature, backpressure, liquid inventory, composition, external heat input, latent heat in metal mass, etc...



Blowdown rate from initial condition to final condition are dynamic and will gradually drops from peak to minimum for proper designed blowdown system. This leads to different RO size from initial condition to final condition. Experience shown that RO size may be maximum during peak.

JoeWong

Great ............

Sir....still more doubts:

1)What are the temperature ranges(lower temperature)which you would try to avoid?

2)What will be the effect of the low temperature on the blowdown valve and the piping?

3)Will the flare header also be made of some low temp exotic material?

4)What is the velocity range which you will try avoid inorder to avoid vibration problems?

Thanks a lot ..................once again thanks.........................

Vinod

It subject to material embrittlement temperature of the material. Example : CS normally is -29 degC, LTCS is -46 degC...



Effect could be icing on external, ice formed results BDV not able to close, BDV/piping crack on embrittlement, piping contraction result support failure...

The blowdown valve and piping will have to design for this low temperature.


Yes if system demand for it.



Can't remember the figure. You may check API Std 521. I guess not exceeded Mach 1...you may take some margin for uncertainty.


JoeWong

Sir....

How will the tempertaure of the vessel bought down..........i mean.......will the low twmperature near the RO creep down to the vessel and cool it down.................

During the case of a fire wont the BDV system fail(i mean the controller and the wiring)........how can we ensure the reliability of the BDV....................

Thanks

Vinod

Not clear with your question.



They shall be fire-safe.

JoeWong

Sir........


What i mean how does the vessel which you are depressurising gets cooled.........you take it below the MAWT of the metal?

Thanks

Vinod

Fundamental thermodynamic...
Depressuring involve expansion and work done. Work done came from conversion of internal energy and bring the fluid temperature down...

Metal temperature will generally close to fluid temperature (liquid contacted section) and it should be higher than the minimum allowable working temperature of metal.

JoeWong

Sir....

Good morning

Regarding the BDV....and the piping are there any thumb rules to be followed for the installation......................

In the case of a compressor,why would we want to depressurise it............in this case since the pressurised gas is produced during the normal operation and during shutdown we need to depressurrise it...........willl we follow the same technique...........................because here there is no fire and the temperature of the compressor casing and assosciates will be below MAWT????????

please help me out

Vinod

Please see one of the requirements HERE.



Please read API Std 521 edition 2007 section 5.20 for better understanding of this subject.
Please tell your understanding of "same technique".
Please clarify why "NO fire" around compressor area. Fire is credible scenario.
Please clarify " because here there is no fire and the temperature of the compressor casing and assosciates will be below MAWT????????". Do you want relate MAWT with fire ? What is you MAWT here ? Maximum allowable working temperature or MINIMUM allowable working temperature? Are you trying to say NO fire scenario, there is no requirement on temperature ?

Please make specific question, shows you have done background reading and tell what's your opinion and answer to your question.

JoeWong

Sir...

sorry for being blunt

Anyway..........assume there is no possibility of any flammable liquid spilling near the compressor area.........and there is a good drainage system........................and just before shutdown i need to depressurise the compressor.........how will i do it.............wil i need to the same calculation as in the case of a fire near a vessel...........................will i connect this line to a flare header............what is the importance of settle out in this case....


Vinod

Vinod,

Answer to your queries can form a handbook.

You still failed to understand my advice.

"Please make specific question, shows you have done background reading and tell what's your opinion and answer to your question."

You probably have partially done the first part "make specific question" but not for the rest. I guess this will invite no response.


JoeWong

Sir....

Whatever u say i am still very junior to you...........and want to learn from you...could you send any article on the calculation of depressursiation

And thanx fopr all the help

vinod